title: Metoclopramide

text: Shortly following the Pliva decision, the FDA proposed a rule change that would allow generics manufacturers to update the label if the originating drug had been withdrawn from the market for reasons other than safety. As of May 2016 the rule, which turned out to be controversial since it would open generic companies to product liability suits, was still not finalized, and the FDA had stated the final rule would be issued in April 2017. The FDA issued a draft guidance for generic companies to update labels in July 2016.


title: Metoclopramide

text: Metoclopramide is commonly used to prevent vomiting in cats and dogs. It is also used as a gut stimulant in rabbits.


title: Avro Vulcan

text: The Avro Vulcan (later Hawker Siddeley Vulcan from July 1963) is a jet-powered, tailless, delta-wing, high-altitude, strategic bomber, which was operated by the Royal Air Force (RAF) from 1956 until 1984. Aircraft manufacturer A.V. Roe and Company (Avro) designed the Vulcan in response to Specification B.35/46. Of the three V bombers produced, the Vulcan was considered the most technically advanced, hence the riskiest option. Several reduced-scale aircraft, designated Avro 707s, were produced to test and refine the delta-wing design principles.


title: Avro Vulcan

text: The Vulcan B.1 was first delivered to the RAF in 1956; deliveries of the improved Vulcan B.2 started in 1960. The B.2 featured more powerful engines, a larger wing, an improved electrical system, and electronic countermeasures, and many were modified to accept the Blue Steel missile. As a part of the V-force, the Vulcan was the backbone of the United Kingdom's airborne nuclear deterrent during much of the Cold War. Although the Vulcan was typically armed with nuclear weapons, it could also carry out conventional bombing missions, which it did in Operation Black Buck during the Falklands War between the United Kingdom and Argentina in 1982.


title: Avro Vulcan

text: The Vulcan had no defensive weaponry, initially relying upon high-speed, high-altitude flight to evade interception. Electronic countermeasures were employed by the B.1 (designated B.1A) and B.2 from around 1960. A change to low-level tactics was made in the mid-1960s. In the mid-1970s, nine Vulcans were adapted for maritime radar reconnaissance operations, redesignated as B.2 (MRR). In the final years of service, six Vulcans were converted to the K.2 tanker configuration for aerial refuelling.


title: Avro Vulcan

text: After retirement by the RAF, one example, B.2 XH558, named "The Spirit of Great Britain", was restored for use in display flights and air shows, whilst two other B.2s, XL426 and XM655, have been kept in taxiable condition for ground runs and demonstrations. B.2 XH558 flew for the last time in October 2015 and is also being kept in taxiable condition.


title: Avro Vulcan

text: The origin of the Vulcan and the other V bombers is linked with early British atomic weapon programme and nuclear deterrent policies. Britain's atom bomb programme began with Air Staff Operational Requirement OR.1001 issued in August 1946. This anticipated a government decision in January 1947 to authorise research and development work on atomic weapons, the U.S. Atomic Energy Act of 1946 (McMahon Act) having prohibited exporting atomic knowledge, even to countries that had collaborated on the Manhattan Project. OR.1001 envisaged a weapon not to exceed in length, in diameter and in weight. The weapon had to be suitable for release from .


title: Avro Vulcan

text: In January 1947, the Ministry of Supply distributed Specification B.35/46 to UK aviation companies to satisfy Air Staff Operational Requirement OR.229 for "a medium range bomber landplane capable of carrying one bomb to a target from a base which may be anywhere in the world." A cruising speed of at altitudes between was specified. The maximum weight when fully loaded should not exceed . Alternatively, the aircraft was to be capable of carrying a conventional bomb load of . The similar OR.230 required a "long-range bomber" with a radius of action with a maximum weight of when fully loaded; this requirement was considered too exacting. Six companies submitted technical brochures to this specification, including Avro.


title: Avro Vulcan

text: Required to tender by the end of April 1947, work began on receipt of Specification B.35/46 at Avro, led by technical director Roy Chadwick and chief designer Stuart Davies; the type designation was "Avro 698". As was obvious to the design team, conventional aircraft could not satisfy the specification. No worthwhile information about high-speed flight was available from the Royal Aircraft Establishment (RAE) or the US. Avro were aware that Alexander Lippisch had designed a delta-wing fighter and considered the same delta configuration would be suitable for their bomber. The team estimated that an otherwise conventional aircraft, with a swept wing of 45°, would have doubled the weight requirement. Realising that swept wings increase longitudinal stability, the team deleted the tail (empennage) and the supporting fuselage, it thus became a swept-back flying wing with only a rudimentary forward fuselage and a fin (vertical stabilizer) at each wingtip. The estimated weight was now only 50% over the requirement; a delta shape resulted from reducing the wingspan and maintaining the wing area by filling in the space between the wingtips, which enabled the specification to be met. Although Alexander Lippisch is generally credited as the pioneer of the delta wing, Chadwick's team had followed its own logical design process. The initial design submission had four large turbojets stacked in pairs buried in the wings on either side of the centreline. Outboard of the engines were two bomb bays.


title: Avro Vulcan

text: In August 1947, Chadwick was killed in the crash of the Avro Tudor 2 prototype, and was succeeded by Sir William Farren. Reductions in wing thickness made incorporating the split bomb bays and stacked engines impossible, thus the engines were placed side by side in pairs on either side of a single bomb bay, with the fuselage growing somewhat. The wingtip fins gave way to a single fin on the aircraft's centreline. Rival manufacturer Handley Page received a prototype contract for its crescent-winged HP.80 B.35/46 tender in November 1947. Although considered the best option, the contract award for Avro's design was delayed while its technical strength was established. Instructions to proceed with the construction of two Avro 698 prototypes were received in January 1948. As an insurance measure against both radical designs failing, Short Brothers received a contract for the prototype SA.4 to the less-stringent Specification B.14/46. The SA.4, later named Sperrin, was not required. In April 1948, Vickers also received authority to proceed with their Type 660, which, although falling short of the B.35/46 Specification, but being of a more conventional design, would be available sooner. This plane entered service as the Valiant.


title: Avro Vulcan

text: As Avro had no flight experience of the delta wing, the company planned two smaller experimental aircraft based on the 698, the one-third scale model 707 for low-speed handling and the one-half scale model 710 for high-speed handling. Two of each were ordered. The 710 was cancelled when it was considered too time-consuming to develop; a high-speed variant of the 707 was designed in its place, the 707A. The first 707, VX784, flew in September 1949, but crashed later that month, killing the Aeroplane and Armament Experimental Establishment's chief test pilot Squadron Leader Samuel Eric Esler, DFC, AE. The second low-speed 707, VX790, built with the still uncompleted 707A's nose section (containing an ejection seat) and redesignated 707B, flew in September 1950 piloted by Avro test pilot Wg Cdr Roland "Roly" Falk. The high-speed 707A, WD480, followed in July 1951.


title: Avro Vulcan

text: Due to the delay of the 707 programme, the contribution of the 707B and 707A towards the basic design of the 698 was not considered significant, though it did highlight a need to increase the length of the nosewheel to give a ground incidence of 3.5°, the optimum take-off attitude. The 707B and 707A proved the design's validity and gave confidence in the delta planform. A second 707A, WZ736, and a two-seat 707C, WZ744, were also constructed, but they played no part in the 698's development.


title: Avro Vulcan

text: More influential than the Avro 707 in the 698's design was wind-tunnel testing performed at RAE Farnborough. This necessitated a wing redesign incorporating a cranked and drooped leading edge and vortex generators to avoid the onset of compressibility drag, which would have restricted the maximum speed. This wing modification resulted in the "phase 2" wing which was first investigated on Avro 707A WD480. This modification was too late to be incorporated on the two prototype 698s and the first three B.1 aircraft before their first flights. (The B.1s were quickly retrofitted).


title: Avro Vulcan

text: Painted gloss white, the 698 prototype VX770, with its pure delta wing, flew for the first time on 30 August 1952 piloted by Roly Falk flying solo. VX770, fitted with only the first pilot's ejection seat and a conventional control wheel, was powered by four Rolls-Royce RA.3 Avon engines of thrust, its intended Bristol Olympus engines not being available. The prototype had fuselage fuel tanks but no wing tanks, so temporary additional tankage was carried in the bomb bay. VX770 made an appearance at the 1952 Society of British Aircraft Constructors' (SBAC) Farnborough Air Show the next month when Falk demonstrated an almost vertical bank.


title: Avro Vulcan

text: After its Farnborough appearance, the future name of the Avro 698 was a subject of speculation. Avro had strongly recommended the name "Ottawa", in honour of the company's connection with Avro Canada. The weekly magazine "Flight" suggested "Albion" after rejecting "Avenger", "Apollo", and "Assegai". The chief of the air staff preferred a V-class of bombers, and the Air Council announced the following month that the 698 would be called "Vulcan" after the Roman god of fire and destruction.


title: Avro Vulcan

text: In January 1953, VX770 was grounded for the installation of wing fuel tanks, Armstrong Siddeley ASSa.6 Sapphire engines of thrust and other systems; it flew again in July 1953.


title: Avro Vulcan

text: The second prototype, VX777, first flew on 3 September 1953. More representative of production aircraft, it was lengthened to accommodate a longer nose undercarriage leg to increase the angle of attack of the wing, shortening the take-off run. It featured a visual bomb-aiming blister under the cabin and was fitted with Bristol Olympus 100 engines of thrust. At Falk's suggestion, a fighter-style control stick had replaced the control wheel. Like VX770, VX777 had the original wing with straight leading edges. VX777 was joined in formation by the first prototype VX770 and four Avro 707s at the 1953 Farnborough Air Show. During trials in July 1954, VX777 was substantially damaged in a heavy landing at Farnborough. It was repaired, fitted with Olympus 101 engines of thrust before resuming trials with Avro and the Aeroplane and Armament Experimental Establishment (A&AEE) at Boscombe Down.


title: Avro Vulcan

text: While exploring VX777's high-speed and high-altitude flight envelope at the A&AEE, mild buffeting and other undesirable flight characteristics were experienced while approaching the limiting Mach number, including an alarming tendency to enter an uncontrollable dive. This was judged unacceptable for an unarmed bomber. Fitting the phase 2 wing removed the buffeting and an auto-mach trimmer countered the high-speed dive. The latter applied up-elevator as the speed critically increased. This up-elevator force was greater than the force required to counter the dive. Consequently, as speed increased, the control column had to be pushed rather than pulled to maintain level flight. This artificial pitch-up made the Vulcan handle more like other aircraft as its speed increased.


title: Avro Vulcan

text: The first production B.1 XA889 first flew in February 1955 with the original wing and joined the trials in June. In September 1955, Falk, flying the second production B.1 XA890 (which had remained at Woodford as part of the MoS's Air Fleet on radio trials), amazed crowds at the Farnborough Air Show by executing a barrel roll on his second flypast in front of the SBAC president's tent. After two days of flying, he was called in front of service and civil aviation authorities and ordered to refrain from carrying out this "dangerous" manoeuvre. Now fitted with a phase 2 wing, XA889 was delivered in March 1956 to the A&AEE for trials for the type's initial Certificate of Airworthiness which it received the following month.


title: Avro Vulcan

text: The first 15 production B.1s were powered by the Olympus 101. Many of these early examples in a metallic finish remained the property of the Ministry of Supply, being retained for trials and development purposes. Those entering RAF service were delivered to No 230 Operational Conversion Unit (OCU), the first in July 1956. Later aircraft, painted in anti-flash white and powered by the Olympus 102 with thrust, began to enter squadron service in July 1957. The Olympus 102s were modified during overhaul to the Olympus 104 standard, ultimately rated at thrust.


title: Avro Vulcan

text: Rebuilding B.1s as B.2s was considered but rejected over cost. Nevertheless, to extend the B.1's service life, 28 (the surviving B1 aircraft fitted with Olympus 102/104 engines) were upgraded by Armstrong Whitworth between 1959 and 1963 to the B.1A standard, including features of the B.2 such as ECM equipment, in-flight refuelling receiving equipment, and UHF radio. However, the B.1As were not strengthened for low-level operations and all were withdrawn by 1968.


title: Avro Vulcan

text: As far back as 1952, Bristol Aero Engines had begun development of the BOl.6 (Olympus 6) rated at thrust but if fitted to the B.1, this would have reintroduced the buffet requiring further redesign of the wing.


title: Avro Vulcan

text: The decision to proceed with the B.2 versions of the Vulcan was made in May 1956, being developed by Avro's chief designer Roy Ewans. The first B.2 was anticipated to be around the 45th aircraft of the 99 then on order. As well as being able to achieve greater heights over targets, operational flexibility was believed to be extended by the provision of in-flight refuelling equipment and tanker aircraft. The increasing sophistication of Soviet air defences required the fitting of electronic countermeasure (ECM) equipment, and vulnerability could be reduced by the introduction of the Avro Blue Steel stand-off missile, then in development. To develop these proposals, the second Vulcan prototype VX777 was rebuilt with the larger and thinner phase-2C wing, improved flying control surfaces, and Olympus 102 engines, first flying in this configuration in August 1957. Several Vulcan B.1s were used for the development of the B.2: development of the BOl.6 (later Olympus 200), XA891; a new AC electrical system, XA893; ECM including jammers within a bulged tail cone and a tail warning radar, XA895: and for Blue Steel development work, XA903.


title: Avro Vulcan

text: The 46th production aircraft and first B.2, XH533, first flew in September 1958 using Olympus 200 engines, six months before the last B.1 XH532 was delivered in March 1959. The second B.2, XH534, flew in January 1959. Powered by production Olympus 201s with thrust, it was more representative of a production aircraft, being fitted with an in-flight refuelling probe and a bulged ECM tail cone. Some subsequent B.2s were initially lacking probes and ECM tail cones, but these were retrofitted. The first 10 B.2s outwardly showed their B.1 ancestry, retaining narrow engine air intakes. Anticipating even more powerful engines, the air intakes were deepened on the 11th (XH557) and subsequent aircraft. Many of the early aircraft were retained for trials, and the 12th B.2, XH558, was the first to be delivered to the RAF in July 1960. Coincidentally, XH558 was also the last Vulcan in service with the RAF, before being retired in 1992.


title: Avro Vulcan

text: The 26th B.2, XL317, the first of a production batch ordered in February 1956, was the first Vulcan, apart from development aircraft, capable of carrying the Blue Steel missile; 33 aircraft were delivered to the RAF with these modifications. When the Mk.2 version of Blue Steel was cancelled in favour of the Douglas GAM-87 Skybolt air-launched ballistic missile in December 1959, fittings were changed in anticipation of the new missile, one under each wing. Though Skybolt was cancelled in November 1962, many aircraft were delivered or retrofitted with "Skybolt" blisters. Later aircraft were delivered with Olympus 301 engines with thrust. Two earlier aircraft were re-engined (XH557 and XJ784) for trials and development work; another seven aircraft were converted around 1963.


title: Avro Vulcan

text: The last B.2 XM657 was delivered in 1965 and the type served until 1984. Whilst in service, the B.2 was continuously updated with modifications, including rapid engine starting, bomb-bay fuel tanks, wing strengthening to give the fatigue life to enable the aircraft to fly at low level (a tactic introduced in the mid-1960s), upgraded navigation equipment, terrain-following radar, standardisation on a common weapon (WE.177) and improved ECM equipment. Nine B.2s were modified for a maritime radar reconnaissance role and six for an airborne tanker role. An updated bomb rack assembly allowing the carriage of 30 1,000 lb bombs, up from 21 was demonstrated by Avro but was not introduced. The updated wing profile increased range to 4,000 nm (7,400 km).


title: Avro Vulcan

text: The Avro 718 was a 1951 proposal for a delta-winged military transport based on the Type 698 to carry 80 troops or 110 passengers. It would have been powered by four Bristol Olympus BOl.3 engines.


title: Avro Vulcan

text: The Avro Type 722 Atlantic was a 1952 proposal (announced in June 1953) for a 120-passenger delta-winged airliner based on the Type 698.


title: Avro Vulcan

text: The Avro 732 was a 1956 proposal for a supersonic development of the Vulcan and would have been powered by 8 de Havilland Gyron Junior engines. Unlike the proposed Avro 721 low-level bomber of 1952 or the Avro 730 supersonic stainless steel canard bomber dating from 1954 (cancelled in 1957 before completion of the prototype), the Type 732 showed its Vulcan heritage.


title: Avro Vulcan

text: In 1960, the Air Staff approached Avro with a request into a study for a patrol missile carrier armed with up to six Skybolt missiles capable of a mission length of 12 hours. Avro's submission in May 1960 was the Phase 6 Vulcan, which would have been the Vulcan B.3. The aircraft was fitted with an enlarged wing of span with increased fuel capacity; additional fuel tanks in a dorsal spine; a new main undercarriage to carry an all-up-weight of ; and reheated Olympus 301s of thrust. An amended proposal of October 1960 inserted a plug into the forward fuselage with capacity for six crew members including a relief pilot, all facing forwards on ejection seats, and aft-fan versions of the Olympus 301.


title: Avro Vulcan

text: Other countries expressed interest in purchasing Vulcans, but as with the other V-bombers, no foreign sales materialised.


title: Avro Vulcan

text: As early as 1954, the Royal Australian Air Force (RAAF) recognised that the English Electric Canberra might soon become outdated. Potential replacements, such as the Boeing B-47E, Handley-Page Victor and Vulcan were considered.


title: Avro Vulcan

text: Political pressure for a Canberra replacement came to a head in 1962, by which time agile, supersonic bombers/strategic strike aircraft, such as the North American A-5 Vigilante, BAC TSR-2, General Dynamics F-111, had become available. Had the Australian government pre-ordered the TSR-2, several V-bombers, including Vulcans, would have been made available, for interim use by the RAAF; however, the F-111C was ordered. (The UK government almost followed that decision – after the cancellation of the TSR-2 – it was offered the similar F-111K.)


title: Avro Vulcan

text: In the early 1980s, Argentina approached the UK with a proposal to buy a number of Vulcans. An application, made in September 1981, requested the 'early availability' of a 'suitable aircraft'. With some reluctance, British ministers approved the export of a single aircraft but emphasised that clearance had not been given for the sale of a larger number. A letter from the British Foreign and Commonwealth Office to the Ministry of Defence in January 1982 stated that little prospect was seen of this happening without ascertaining the Argentine interest and whether such interest was genuine: 'On the face of it, a strike aircraft would be entirely suitable for an attack on the Falklands.' Argentina invaded the Falkland Islands less than three months later, after which a British embargo on the sale of any military equipment was quickly imposed.


title: Avro Vulcan

text: Despite its radical and unusual shape, the airframe was built along traditional lines. Except for the most highly stressed parts, the whole structure was manufactured from standard grades of light alloy. The airframe was broken down into a number of major assemblies: The centre section, a rectangular box containing the bomb bay and engine bays bounded by the front and rear spars and the wing transport joints; the intakes and centre fuselage; the front fuselage, incorporating the pressure cabin; the nose; the outer wings; the leading edges; the wing trailing edge and rear end of the fuselage; and a single swept tail fin with a single rudder was on the trailing edge.


title: Avro Vulcan

text: A five-man crew was accommodated within the pressure cabin on two levels; the first pilot and co-pilot sitting on Martin-Baker 3K (3KS on the B.2) ejection seats whilst on the lower level the navigator radar, navigator plotter, and air electronics officer (AEO) sat facing rearwards and would abandon the aircraft via the entrance door. The original B35/46 specification sought a jettisonable crew compartment, but this requirement was removed in a subsequent amendment; the rear crew's escape system was often an issue of controversy, such as when a practical refit scheme was rejected. A rudimentary sixth seat forward of the navigator radar was provided for an additional crew member; the B.2 had an additional seventh seat opposite the sixth seat and forward of the AEO. The visual bomb-aimer's compartment could be fitted with a T4 (Blue Devil) bombsight, in many B.2s, this space housed a vertically mounted Vinten F95 Mk.10 camera for assessing simulated low-level bombing runs.


title: Avro Vulcan

text: Fuel was carried in 14 bag tanks, four in the centre fuselage above and to the rear of the nosewheel bay, and five in each outer wing. The tanks were split into four groups of almost equal capacity, each normally feeding its respective engine, though cross-feeding was possible. The centre of gravity was automatically maintained by electric timers, which sequenced the booster pumps on the tanks. B.2 aircraft could be fitted with one or two additional fuel tanks in the bomb bay.


title: Avro Vulcan

text: Despite being designed before a low radar cross-section and other stealth factors were ever a consideration, an RAE technical note of 1957 stated that of all the aircraft so far studied, the Vulcan appeared by far the simplest radar-echoing object, due to its shape; only one or two components contributed significantly to the echo at any aspect, compared with three or more on most other types.


title: Avro Vulcan

text: The two prototype Vulcans were finished in gloss white. Early Vulcan B.1s left the factory in a natural metal finish; the front half of the nose radome was painted black, the rear half painted silver. Front-line Vulcan B.1s had a finish of anti-flash white and RAF "type D" roundels. Front-line Vulcan B.1As and B.2s were similar, but with pale roundels.


title: Avro Vulcan

text: With the adoption of low-level attack profiles in the mid-1960s, B.1As and B.2s were given a glossy sea grey medium and dark green disruptive pattern camouflage on the upper surfaces, white undersurfaces, and "type D" roundels. (The last 13 Vulcan B.2s, XM645 onwards, were delivered thus from the factory). In the mid-1970s, Vulcan B.2s received a similar scheme with matte camouflage, light aircraft grey undersides, and "low-visibility" roundels. B.2(MRR)s received a similar scheme in gloss; and the front halves of the radomes were no longer painted black. Beginning in 1979, 10 Vulcans received a wrap-around camouflage of dark sea grey and dark green because, during Red Flag exercises in the US, defending SAM forces had found that the grey-painted undersides of the Vulcan became much more visible against the ground at high angles of bank.


title: Avro Vulcan

text: The original Vulcan B.1 radio fit was: two 10-channel VHF transmitter/receivers (TR-1985/TR-1986) and a 24-channel HF transmitter-receiver (STR-18). The Vulcan B.1A also featured a UHF transmitter-receiver (ARC-52). The initial B.2 radio fit was similar to the B.1A though it was ultimately fitted with the ARC-52, a V/UHF transmitter/receiver (PTR-175), and a single-sideband modulation HF transmitter-receiver (Collins 618T).


title: Avro Vulcan

text: The navigation and bombing system comprised an H2S Mk9 radar and a navigation bombing computer Mk1. Other navigation aids included a Marconi radio compass (ADF), GEE Mk3, Green Satin Doppler radar to determine the groundspeed and drift angle, radio and radar altimeters, and an instrument landing system. TACAN replaced GEE in the B.1A and B.2 in 1964. Decca Doppler 72 replaced Green Satin in the B.2 around 1969<ref name='PB&E102/3'>Price, Blackman and Edmonson 2010, pp. 102, 103.</ref> A continuous display of the aircraft's position was maintained by a ground position indicator.


title: Avro Vulcan

text: Vulcan B.2s were eventually fitted with the free-running dual-gyroscopic heading reference system (HRS) Mk.2, based upon the inertial platform of the Blue Steel missile, which had been integrated into the system when the missile had been carried. With the HRS a navigator's heading unit was provided, which enabled the navigator plotter to adjust the aircraft heading, through the autopilot, by as little as 0.1 degrees. The B.2 (MRR) was additionally fitted with the LORAN C navigation system.


title: Avro Vulcan

text: The original ECM fit of the B.1A and B.2 was one Green Palm voice communications jammer; two Blue Diver metric jammers; three Red Shrimp S-band jammers; a Blue Saga passive warning receiver with four aerials; a Red Steer tail warning radar; and chaff dispensers. The bulk of the equipment was carried in a large, extended tail cone, and a flat ECM aerial counterpoise plate was mounted between the starboard tailpipes. Later equipment on the B.2 included: an L band jammer (replacing a Red Shrimp); the ARI 18146 X-band jammer; replacing the Green Palm; the improved Red Steer Mk.2; infra-red decoys (flares); and the ARI 18228 PWR with its aerials that gave a squared top to the fin.


title: Avro Vulcan

text: The aircraft was controlled by a fighter-type control stick and rudder bar, which operated the powered flying controls, which each had a single electrohydraulic-powered flying control unit, except the rudder, which had two, one running as a back-up. Artificial feel and auto stabilisation in the form of pitch and yaw dampers were provided, as well as an auto Mach trimmer.


title: Avro Vulcan

text: The flight instruments in the B.1 were traditional and included "G4B" compasses; Mk.4 artificial horizons; and zero reader flight display instruments. The B.1 had a Smiths Mk10 autopilot. In the B.2, these features were incorporated into the Smiths Military Flight System (MFS), the pilots' components being: two beam compasses; two director-horizons; and an Mk.10A or Mk.10B autopilot. From 1966, B.2s were fitted with the "ARI 5959" TFR, built by General Dynamics, its commands being fed into the director-horizons.


title: Avro Vulcan

text: The B.1 had four elevators (inboard) and four ailerons (outboard). In the B.2, these were replaced by eight elevons. The Vulcan was also fitted with six electrically operated three-position (retracted, medium drag, high drag) airbrakes, four in the upper centre section and two in the lower. Originally, four lower airbrakes were used, but the outboard two were deleted before the aircraft entered service. A brake parachute was installed inside the tail cone.


title: Avro Vulcan

text: The main electrical system on the B.1/B.1A was 112 V DC supplied by four 22.5kW engine-driven starter-generators. Backup power was provided by four 24 V 40 Ah batteries connected in series providing 96 V. Secondary electrical systems were 28 V DC, single-phase 115 V AC at 1600 Hz, and three-phase 115 V AC at 400 Hz, driven by transformers and inverters from the main system. The 28 V DC system was backed up by a single 24 V battery.


title: Avro Vulcan

text: For greater efficiency and higher reliability, the main system on the B.2 was changed to three-phase 200 V AC at 400 Hz supplied by four 40 kVA engine-driven constant-speed alternators. Engine starting was then by air-starters supplied from a Palouste compressor on the ground. Standby supplies in the event of a main AC failure were provided by a ram air turbine driving a 17 kVA alternator that could operate from high altitudes down to , and an airborne auxiliary power plant, a Rover gas turbine driving a 40kVA alternator, which could be started once the aircraft was below an altitude of . Secondary electrical supplies were by transformer-rectifier units for 28 V DC and rotary frequency converters for the 115 V 1600 Hz single-phase supplies.


title: Avro Vulcan

text: The change to an AC system was a significant improvement. Each PFCU had a hydraulic pump that was driven by an electric motor, in modern terminology, this is an electro-hydraulic actuator. Because no manual reversion existed, a total electrical failure would result in a loss of control. The standby batteries on the B.1 were designed to give enough power for 20 minutes of flying time, but this proved to be optimistic and two aircraft, XA891 and XA908, crashed as a result.


title: Avro Vulcan

text: The main hydraulic system provided pressure for undercarriage raising and lowering and bogie trim; nosewheel centring and steering; wheel brakes (fitted with Maxarets); bomb doors opening and closing; and (B.2 only) AAPP air scoop lowering. Hydraulic pressure was provided by three hydraulic pumps fitted to Nos. 1, 2 and 3 engines. An electrically operated hydraulic power pack (EHPP) could be used to operate the bomb doors and recharge the brake accumulators. A compressed air (later nitrogen) system was provided for emergency undercarriage lowering.


title: Avro Vulcan

text: The Rolls-Royce Olympus, originally known as the "Bristol BE.10 Olympus", is a two-spool, axial-flow turbojet that powered the Vulcan. Each Vulcan had four engines buried in the wings, positioned in pairs close to the fuselage. The engine's design began in 1947, intended to power the Bristol Aeroplane Company's own rival design to the Vulcan.


title: Avro Vulcan

text: As the prototype Vulcan VX770 was ready for flight prior to the Olympus being available, it first flew using Rolls-Royce Avon RA.3 engines of thrust. These were quickly replaced by Armstrong Siddeley Sapphire ASSa.6 engines of thrust. VX770 later became a flying test bed for the Rolls-Royce Conway. The second prototype VX777 first flew with Olympus 100s of thrust. It was subsequently re-engined with Olympus 101 engines. When VX777 flew with a Phase 2C (B.2) wing in 1957, it was fitted with Olympus 102 engines of thrust.


title: Avro Vulcan

text: Early B.1s were equipped with the Olympus 101. Later aircraft were delivered with Olympus 102s. All Olympus 102s became the Olympus 104 on overhaul and ultimately thrust on uprating. The first B.2 flew with the second-generation Olympus 200, design of which began in 1952. Subsequent B.2s were engined with either the uprated Olympus 201 or the Olympus 301. The Olympus 201 was designated 202 on being fitted with a rapid air starter. The engine would later be developed into a reheated (afterburning) powerplant for the cancelled TSR-2 strike/reconnaissance aircraft and the supersonic passenger transport Concorde.


title: Avro Vulcan

text: Around 90% power, the engines in the Vulcan would emit a distinctive "howl"-like noise due to the air intake arrangement, which became an attraction at public airshows.


title: Avro Vulcan

text: In September 1956, the RAF received its first Vulcan B.1, XA897, which immediately embarked upon a round-the-world tour. The tour was to be an important demonstration of the range and capabilities of the aircraft, but it also had other benefits in the form of conducting goodwill visits in various countries; during their service, Vulcans routinely visited various nations and distant parts of the Commonwealth as a show of support and military protection. This first tour, however, was struck by misfortune; on 1 October 1956, while landing in bad weather at London Heathrow Airport at the completion of the world tour, XA897 was destroyed in a fatal accident.


title: Avro Vulcan

text: The first two aircraft were delivered to 230 OCU in January 1957 and the training of crews started on 21 February 1957. The first OCU course to qualify was No. 1 Course, on 21 May 1957, and they went on to form the first flight of No. 83 Squadron. No. 83 Squadron was the first operational squadron to use the bomber, at first using borrowed Vulcans from the OCU, and on 11 July 1956 it received the first aircraft of its own. By September 1957, several Vulcans had been handed over to No. 83 Squadron. The second OCU course also formed a Flight of 83 Squadron, but subsequent trained crews were also used to form the second bomber squadron, 101 Squadron. The last aircraft from the first batch of 25 aircraft had been delivered by the end of 1957 to 101 Squadron.


title: Avro Vulcan

text: To increase the mission range and flight time for Vulcan operations, in-flight refuelling capabilities were added from 1959 onwards; several Valiant bombers were refurbished as tankers to refuel the Vulcans. Continuous airborne patrols proved untenable, however, and the refuelling mechanisms across the Vulcan fleet fell into disuse in the 1960s. Both Vulcans and the other V-force aircraft routinely visited the Far East, in particular Singapore, where a fully equipped nuclear-weapons storage facility had been constructed in 1959. These deployments were part of the UK's contribution to SEATO operations, often to test the defences of friendly nations in joint exercises. During the Indonesia–Malaysia confrontation, Britain planned to deploy three squadrons of V-bomber aircraft and 48 Red Beard tactical nuclear weapons to the region, although this was ultimately decided against. Vulcans trained in the region for both conventional and nuclear missions. In the early 1970s, the RAF decided to permanently deploy two squadrons of Vulcans overseas in the Near East Air Force Bomber Wing, based at RAF Akrotiri in Cyprus. The Vulcans were withdrawn in the mid-1970s, however, as Cypriot intercommunal violence intensified.


title: Avro Vulcan

text: Vulcans flew some very long-range missions. In June 1961, one flew 18,507 km from RAF Scampton to Sydney in just over 20 hours, facilitated by three air refuellings. Vulcans frequently visited the United States during the 1960s and 1970s to participate in air shows and static displays, as well as to participate in the Strategic Air Command's (SAC) Annual Bombing and Navigation Competition at such locations as Barksdale AFB, Louisiana, and the former McCoy AFB, Florida. Vulcans also took part Operation Skyshield exercises in 1960, 1961, and 1962, in which NORAD defences were tested against possible Soviet air attack, the Vulcans simulating Soviet fighter/bomber attacks against New York, Chicago, and Washington, DC. The results of the tests were classified until 1997. The Vulcan proved quite successful during the 1974 "Giant Voice" exercise, in which it managed to avoid USAF interceptors.


title: Avro Vulcan

text: As part of Britain's independent nuclear deterrent, the Vulcan initially carried Britain's first nuclear weapon, the "Blue Danube" gravity bomb. "Blue Danube" was a low-kiloton yield fission bomb designed before the United States detonated the first hydrogen bomb. These were supplemented by U.S.-owned "Mk 5" bombs (made available under the Project E programme) and later by the British "Red Beard" tactical nuclear weapon. The UK had already embarked on its own hydrogen bomb programme, and to bridge the gap until these were ready the V-bombers were equipped with an Interim Megaton Weapon based on the "Blue Danube" casing containing "Green Grass", a large pure-fission warhead of yield. This bomb was known as "Violet Club". Only five were deployed before the "Green Grass" warhead was incorporated into a developed weapon as "Yellow Sun Mk.1."


title: Avro Vulcan

text: The later "Yellow Sun Mk 2", was fitted with "Red Snow", a British-built variant of the U.S. W28 warhead. "Yellow Sun Mk 2" was the first British thermonuclear weapon to be deployed, and was carried on both the Vulcan and Handley Page Victor. The Valiant retained U.S. nuclear weapons assigned to SACEUR under the dual-key arrangements. "Red Beard" was positioned in Singapore for use by Vulcan and Victor bombers. From 1962, three squadrons of Vulcan B.2s and two squadrons of Victor B.2s were armed with the "Blue Steel" missile, a rocket-powered stand-off bomb, which was also fitted with the yield "Red Snow" warhead.


title: Avro Vulcan

text: Operationally, RAF Bomber Command and the SAC cooperated in the Single Integrated Operational Plan to ensure coverage of all major Soviet targets from 1958; 108 of the RAF's V-bombers were assigned targets under the plan by the end of 1959. From 1962 onwards, two jets in every RAF bomber base were armed with nuclear weapons and on standby permanently under the principle of Quick Reaction Alert (QRA). Vulcans on QRA were to be airborne within four minutes of receiving an alert, as this was identified as the amount of time between warning of a USSR nuclear strike being launched and it arriving in Britain. The closest the Vulcan came to taking part in potential nuclear conflict was during the Cuban Missile Crisis in October 1962, where Bomber Command was moved to Alert Condition 3, an increased state of preparedness from normal operations; however, it stood down in early November.


title: Avro Vulcan

text: The Vulcans were intended to be equipped with the Skybolt missile to replace the Blue Steel, with Vulcan B.2s carrying two Skybolts under the wings. The last 28 B.2s were modified on the production line to fit pylons to carry the Skybolt. A B.3 variant with increased wingspan to carry up to six Skybolts was proposed in 1960. When the Skybolt missile system was cancelled by U.S. President John F. Kennedy on the recommendation of his Secretary of Defense, Robert McNamara in 1962, precipitating the Skybolt Crisis, "Blue Steel" was retained. To supplement it until the Royal Navy took on the deterrent role with Polaris SLBM-equipped submarines, the Vulcan bombers adopted a new mission profile of flying high during clear transit, dropping down low to avoid enemy defences on approach, and deploying a parachute-retarded bomb, the WE.177B. However, since the aircraft had been designed for high-altitude flight, at low altitudes it could not exceed 350 knots. RAF Air Vice Marshal Ron Dick, a former Vulcan pilot, said "it is [thus] questionable whether it could have been effective flying at low level in a war against ... the Soviet Union."


title: Avro Vulcan

text: After the British Polaris submarines became operational and Blue Steel was taken out of service in 1970, the Vulcan continued to carry WE.177B in a tactical nuclear strike role as part of the British contribution to Europe's standing NATO forces, although they no longer held aircraft at 15 minutes' readiness in peacetime. Two squadrons were also stationed in Cyprus as part of the Near East Air Force and assigned to Central Treaty Organization in a strategic strike role. With the eventual demise of the WE.177B and the Vulcan bombers, the Blackburn Buccaneer, SEPECAT Jaguar, and Panavia Tornado continued with the WE.177C until its retirement in 1998. While not a like-for-like replacement, the multi-role Tornado interdictor/strike bomber is the successor for the roles previously filled by the Vulcan.


title: Avro Vulcan

text: Although in operational use, the Vulcan typically carried various nuclear armaments, the type also had a secondary conventional role. While performing conventional combat missions, the Vulcan could carry up to 21  bombs inside its bomb bay. From the 1960s, the various Vulcan squadrons routinely conducted conventional training missions; the aircrews were expected to be able to perform conventional bombing missions, in addition to the critical nuclear strike mission.


title: Avro Vulcan

text: The Vulcan's only combat missions took place towards the end of the type's service in 1982. During the Falklands War, the Vulcan was deployed against Argentinian forces which had occupied the Falkland Islands. The missions performed by the Vulcan became known as the "Black Buck" raids, each aircraft had to fly from Ascension Island to reach Stanley on the Falklands. Victor tankers conducted the necessary air-to-air refuelling for the Vulcan to cover the distance involved; approximately of fuel was used in each mission.


title: Avro Vulcan

text: Engineering work to prepare the five Vulcans that would conduct the missions began on 9 April. Each aircraft required modifications to the bomb bay, the reinstatement of the long out-of-use in-flight refuelling system, the installation of a new navigational system derived from the Vickers VC10, and the updating of several onboard electronics. Underneath the wings, new pylons were fitted to carry an ECM pod and Shrike antiradar missiles at wing hardpoint locations.


title: Avro Vulcan

text: On 1 May, the first mission was conducted by a single Vulcan (XM607) that flew over Port Stanley and dropped its bombs on the airfield, concentrating on the single runway, with one direct hit, making it unsuitable for fighter aircraft. The Vulcan's mission was quickly followed up by strikes against anti-air installations, flown by British Aerospace Sea Harriers from Royal Navy aircraft carriers. A further two missions saw missiles launched against radar installations and two additional missions were cancelled. At the time, these missions held the record for the world's longest-distance raids. The Vulcans' ECM systems proved to be effective at jamming Argentine radars; while a Vulcan was within the theatre, other British aircraft in the vicinity had a reduced chance of coming under effective fire.


title: Avro Vulcan

text: On 3 June 1982, Vulcan B.2 XM597 of No. 50 Squadron took part in the "Black Buck 6" mission against Argentinian radar sites at Stanley airfield on the Falkland Islands. While attempting to refuel for its return journey to Ascension Island, the probe broke, leaving the Vulcan with insufficient fuel, forcing a diversion to Galeão Air Force Base, Rio de Janeiro, in neutral Brazil. En route, secret papers were dumped along with the two remaining AGM-45 Shrike missiles, although one failed to launch. After a mayday call, the Vulcan, escorted by Brazilian Air Force Northrop F-5 fighters, was permitted an emergency landing at Rio with very little fuel left on board. The Vulcan and her crew were detained until the end of hostilities nine days later.


title: Avro Vulcan

text: In November 1973, as a result of the planned closure of the Victor SR.2 equipped No. 543 Squadron, No. 27 Squadron reformed at RAF Scampton equipped with the Vulcan as a replacement in the maritime radar reconnaissance role. The squadron carried out patrols of the seas around the British Isles, including the strategically important GIUK gap between Iceland and the United Kingdom, flying at high level and using the Vulcan's H2S radar to monitor shipping. In peacetime, this could be followed up by visual identification and photography of targets of interest at low level. In the event of war, a Vulcan would leave visual identification of potential targets to Buccaneers or Canberras and could coordinate attacks by Buccaneers against hostile shipping. Though initially equipped with a number of B.2 aircraft, the Squadron eventually operated nine B.2 (MRR) aircraft (also known by the unofficial designation SR.2). The aircraft were modified for the role by removing the TFR (and its thimble radome) and adding the LORAN C radio navigation aid. The main external visual difference was the presence of a gloss paint finish, with a light grey undersurface, to protect against sea spray.


title: Avro Vulcan

text: The squadron also inherited its secondary role of air sampling from No. 543 Squadron. This involved flying through plumes of airborne contamination and using onboard equipment to collect fallout released from both above ground and underground nuclear tests for later analysis at the Atomic Weapons Research Establishment at Aldermaston. Five aircraft had small pylons fitted to the redundant Skybolt hardpoints, which could be used to carry sampling pods modified from drop tanks. These pods would collect the needed samples on a filter, while an additional smaller "localiser" pod was fitted to the port wing, inboard of the main pylons.


title: Avro Vulcan

text: The squadron disbanded at Scampton in March 1982, passing on its radar reconnaissance duties to the RAF's Nimrods.


title: Avro Vulcan

text: After the end of the Falklands War in 1982, the Vulcan B.2 was due to be withdrawn from RAF service that year. The Falklands campaign, however, had consumed much of the airframe fatigue life of the RAF's Victor tankers. While Vickers VC10 tanker conversions had been ordered in 1979 and Lockheed TriStar tankers would be ordered after the conflict, as a stopgap measure six Vulcans were converted into single-point tankers. The Vulcan tanker conversion was accomplished by removing the jammers from the ECM bay in the tail of the aircraft and replacing them with a single hose drum unit. An additional cylindrical bomb-bay tank was fitted, giving a fuel capacity of almost .


title: Avro Vulcan

text: The go-ahead for converting the six aircraft was given on 4 May 1982. Just 50 days after being ordered, the first Vulcan tanker, "XH561", was delivered to RAF Waddington. The Vulcan K.2s were operated by No. 50 Squadron, along with three Vulcan B.2s, in support of UK air defence activities until it was disbanded in March 1984.


title: Avro Vulcan

text: After the disbandment of No. 50 Squadron, two Vulcans continued flying with the RAF in air displays as part of the Vulcan Display Flight, based at Waddington but administered through No. 55 Squadron, based at RAF Marham. Initially displaying using XL426, in 1986 that aircraft was sold, having been replaced by XH558, which began displays in 1985. The VDF continued with XH558 until 1992, finishing operations after the Ministry of Defence determined it was too costly to run in light of budget cuts. Both aircraft subsequently entered preservation and survived, although a third, XH560, kept in reserve in the first years, was later scrapped.


title: Avro Vulcan

text: A total of 134 production Vulcans were assembled at Woodford Aerodrome, 45 to the B.1 design and 89 were B.2 models, the last being delivered to the RAF in January 1965.


title: Avro Vulcan

text: In the event of transition to war, the V Bomber squadrons were to deploy four aircraft at short notice to each of 26 pre-prepared dispersal airfields around the United Kingdom. In the early 1960s the RAF ordered 20 Beagle Basset communication aircraft to move the crews to dispersal airfields; the importance of these aircraft was only brief, diminishing when the primary nuclear deterrent switched to the Royal Navy's Polaris Missile.


title: Avro Vulcan

text: Several Vulcans survive, housed in museums in both the United Kingdom and North America (USA and Canada). One Vulcan, XH558 (G-VLCN) "Spirit of Great Britain", was used as a display aircraft by the RAF as part of the Vulcan Display Flight until 1993. After being grounded, it was later restored to flight by the Vulcan To The Sky Trust and displayed as a civilian aircraft from 2008 until 2015, before being retired a second time for engineering reasons. In retirement, XH558 is to be retained at its base at Doncaster Sheffield Airport as a taxiable aircraft, a role already performed by two other survivors, XL426 (G-VJET) based at Southend Airport, and XM655 (G-VULC), based at Wellesbourne Mountford Airfield. An XJ823 (Avro Vulcan B2) can also be seen at the Solway Aviation Museum at Carlisle Lake District Airport.


title: Chronology of the universe

text: The chronology of the universe describes the history and future of the universe according to Big Bang cosmology.


title: Chronology of the universe

text: Research published in 2015 estimates the earliest stages of the universe's existence as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.


title: Chronology of the universe

text: For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated, into five parts. It is generally considered meaningless or unclear whether time existed before this chronology:


title: Chronology of the universe

text: The first picosecond (10) of cosmic time. It includes the Planck epoch, during which currently established laws of physics may not apply; the emergence in stages of the four known fundamental interactions or forces—first gravitation, and later the electromagnetic, weak and strong interactions; and the expansion of space itself and supercooling of the still immensely hot universe due to cosmic inflation.


title: Chronology of the universe

text: Tiny ripples in the universe at this stage are believed to be the basis of large-scale structures that formed much later. Different stages of the very early universe are understood to different extents. The earlier parts are beyond the grasp of practical experiments in particle physics but can be explored through other means.


title: Chronology of the universe

text: This period lasted around 370,000 years. Initially, various kinds of subatomic particles are formed in stages. These particles include almost equal amounts of matter and antimatter, so most of it quickly annihilates, leaving a small excess of matter in the universe.


title: Chronology of the universe

text: At about one second, neutrinos decouple; these neutrinos form the cosmic neutrino background (CνB). If primordial black holes exist, they are also formed at about one second of cosmic time. Composite subatomic particles emerge—including protons and neutrons—and from about 2 minutes, conditions are suitable for nucleosynthesis: around 25% of the protons and all the neutrons fuse into heavier elements, initially deuterium which itself quickly fuses into mainly helium-4.


title: Chronology of the universe

text: By 20 minutes, the universe is no longer hot enough for nuclear fusion, but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaque plasma.


title: Chronology of the universe

text: The recombination epoch begins at around 18,000 years, as electrons are combining with helium nuclei to form . At around 47,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. (Much later, hydrogen and helium hydride react to form molecular hydrogen (H2) the fuel needed for the first stars.) At about 370,000 years, neutral hydrogen atoms finish forming ("recombination"), and as a result the universe also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium—quickly reach their lowest energy state (ground state) by releasing photons ("photon decoupling"), and these photons can still be detected today as the cosmic microwave background (CMB). This is the oldest direct observation we currently have of the universe.


title: Chronology of the universe

text: This period measures from 370,000 years until about 1 billion years. After recombination and decoupling, the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies, so there were no new sources of light. The only photons (electromagnetic radiation, or "light") in the universe were those released during decoupling (visible today as the cosmic microwave background) and 21 cm radio emissions occasionally emitted by hydrogen atoms. The decoupled photons would have filled the universe with a brilliant pale orange glow at first, gradually redshifting to non-visible wavelengths after about 3 million years, leaving it without visible light. This period is known as the cosmic Dark Ages.


title: Chronology of the universe

text: At some point around 200 to 500 million years, the earliest generations of stars and galaxies form (exact timings are still being researched), and early large structures gradually emerge, drawn to the foam-like dark matter filaments which have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been huge (100–300 solar masses) and non-metallic, with very short lifetimes compared to most stars we see today, so they commonly finish burning their hydrogen fuel and explode as highly energetic pair-instability supernovae after mere millions of years. Other theories suggest that they may have included small stars, some perhaps still burning today. In either case, these early generations of supernovae created most of the everyday elements we see around us today, and seeded the universe with them.


title: Chronology of the universe

text: Galaxy clusters and superclusters emerge over time. At some point, high-energy photons from the earliest stars, dwarf galaxies and perhaps quasars leads to a period of reionization that commences gradually between about 250–500 million years and finishes by about 1 billion years (exact timings still being researched). The Dark Ages only fully came to an end at about 1 billion years as the universe gradually transitioned into the universe we see around us today, but denser, hotter, more intense in star formation, and more rich in smaller (particularly unbarred) spiral and irregular galaxies, as opposed to giant elliptical galaxies.


title: Chronology of the universe

text: While early stars have not been observed, some galaxies have been observed from about 400 million years cosmic time (GN-z11 at redshift z≈11.1, just after the start of reionization); these are currently the early observations of stars and galaxies. The James Webb Space Telescope, launched in 2021, is intended to push this back to z≈20 (180 million years cosmic time), enough to see the first galaxies (≈270 My) and early stars (≈100 to 180 My).


title: Chronology of the universe

text: From 1 billion years, and for about 12.8 billion years, the universe has looked much as it does today and it will continue to appear very similar for many billions of years into the future. The thin disk of our galaxy began to form at about 5 billion years (8.8 Gya), and the Solar System formed at about 9.2 billion years (4.6 Gya), with the earliest traces of life on Earth emerging by about 10.3 billion years (3.5 Gya).


title: Chronology of the universe

text: The thinning of matter over time reduces the ability of gravity to decelerate the expansion of the universe; in contrast, dark energy (believed to be a constant scalar field throughout the visible universe) is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an inflection point about five or six billion years ago, when the universe entered the modern "dark-energy-dominated era" where the universe's expansion is now accelerating rather than decelerating. The present-day universe is understood quite well, but beyond about 100 billion years of cosmic time (about 86 billion years in the future), uncertainties in current knowledge mean that we are less sure which path the universe will take.


title: Chronology of the universe

text: At some time the Stelliferous Era will end as stars are no longer being born, and the expansion of the universe will mean that the observable universe becomes limited to local galaxies. There are various scenarios for the far future and ultimate fate of the universe. More exact knowledge of the present day universe may allow these to be better understood.


title: Chronology of the universe

text: The Standard Model of cosmology is based on a model of spacetime called the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. A metric provides a measure of distance between objects, and the FLRW metric is the exact solution of Einstein field equations (EFE) if some key properties of space such as homogeneity and isotropy are assumed to be true. The FLRW metric very closely matches overwhelming other evidence, showing that the universe has expanded since the Big Bang.


title: Chronology of the universe

text: If the FLRW metric equations are assumed to be valid all the way back to the beginning of the universe, they can be followed back in time, to a point where the equations suggest all distances between objects in the universe were zero or infinitesimally small. (This does not necessarily mean that the universe was physically small at the Big Bang, although that is one of the possibilities.) This provides a model of the universe which matches all current physical observations extremely closely. This initial period of the universe's chronology is called the "Big Bang". The Standard Model of cosmology attempts to explain how the universe physically developed once that moment happened.


title: Chronology of the universe

text: The singularity from the FLRW metric is interpreted to mean that current theories are inadequate to describe what actually happened at the start of the Big Bang itself. It is widely believed that a correct theory of quantum gravity may allow a more correct description of that event, but no such theory has yet been developed. After that moment, all distances throughout the universe began to increase from (perhaps) zero because the FLRW metric itself changed over time, affecting distances between all non-bound objects everywhere. For this reason, it is said that the Big Bang "happened everywhere".


title: Chronology of the universe

text: During the earliest moments of cosmic time, the energies and conditions were so extreme that current knowledge can only suggest possibilities, which may turn out to be incorrect. To give one example, eternal inflation theories propose that inflation lasts forever throughout most of the universe, making the notion of "N seconds since Big Bang" ill-defined. Therefore, the earliest stages are an active area of research and based on ideas that are still speculative and subject to modification as scientific knowledge improves.


title: Chronology of the universe

text: Although a specific "inflationary epoch" is highlighted at around 10 seconds, observations and theories both suggest that distances between objects in space have been increasing at all times since the moment of the Big Bang, and are still increasing (with the exception of gravitationally bound objects such as galaxies and most clusters, once the rate of expansion had greatly slowed). The inflationary period marks a specific period when a very rapid change in scale occurred, but does not mean that it stayed the same at other times. More precisely, during inflation, the expansion accelerated. After inflation, and for about 9.8 billion years, the expansion was much slower and became slower yet over time (although it never reversed). About 4 billion years ago, it began slightly speeding up again.


title: Chronology of the universe

text: The Planck epoch is an era in traditional (non-inflationary) Big Bang cosmology immediately after the event which began the known universe. During this epoch, the temperature and average energies within the universe were so high that everyday subatomic particles could not form, and even the four fundamental forces that shape the universe gravitation, electromagnetism, the weak nuclear force, and the strong nuclear force were combined and formed one fundamental force. Little is understood about physics at this temperature; different hypotheses propose different scenarios. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on the theory of general relativity, which is thought to break down for this epoch due to quantum effects.


title: Chronology of the universe

text: In inflationary models of cosmology, times before the end of inflation (roughly 10 seconds after the Big Bang) do not follow the same timeline as in traditional big bang cosmology. Models that aim to describe the universe and physics during the Planck epoch are generally speculative and fall under the umbrella of "New Physics". Examples include the Hartle–Hawking initial state, string theory landscape, string gas cosmology, and the ekpyrotic universe.


title: Chronology of the universe

text: As the universe expanded and cooled, it crossed transition temperatures at which forces separated from each other. These phase transitions can be visualized as similar to condensation and freezing phase transitions of ordinary matter. At certain temperatures/energies, water molecules change their behavior and structure, and they will behave completely differently. Like steam turning to water, the fields which define the universe's fundamental forces and particles also completely change their behaviors and structures when the temperature/energy falls below a certain point. This is not apparent in everyday life, because it only happens at far higher temperatures than we usually see in the present day universe.


title: Chronology of the universe

text: These phase transitions in the universe's fundamental forces are believed to be caused by a phenomenon of quantum fields called "symmetry breaking".


title: Chronology of the universe

text: In everyday terms, as the universe cools, it becomes possible for the quantum fields that create the forces and particles around us, to settle at lower energy levels and with higher levels of stability. In doing so, they completely shift how they interact. Forces and interactions arise due to these fields, so the universe can behave very differently above and below a phase transition. For example, in a later epoch, a side effect of one phase transition is that suddenly, many particles that had no mass at all acquire a mass (they begin to interact differently with the Higgs field), and a single force begins to manifest as two separate forces.


title: Chronology of the universe

text: Assuming that nature is described by a so-called Grand Unified Theory (GUT), the grand unification epoch began with a phase transition of this kind, when gravitation separated from the universal combined gauge force. This caused two forces to now exist: gravity, and an electrostrong interaction. There is no hard evidence yet, that such a combined force existed, but many physicists believe it did. The physics of this electrostrong interaction would be described by a Grand Unified Theory.


title: Chronology of the universe

text: The grand unification epoch ended with a second phase transition, as the electrostrong interaction in turn separated, and began to manifest as two separate interactions, called the strong and the electroweak interactions.


title: Chronology of the universe

text: Depending on how epochs are defined, and the model being followed, the electroweak epoch may be considered to start before or after the inflationary epoch. In some models it is described as including the inflationary epoch. In other models, the electroweak epoch is said to begin after the inflationary epoch ended, at roughly 10 seconds.


title: Chronology of the universe

text: According to traditional Big Bang cosmology, the electroweak epoch began 10 seconds after the Big Bang, when the temperature of the universe was low enough (10 K) for the electronuclear force to begin to manifest as two separate interactions, the strong and the electroweak interactions. (The electroweak interaction will also separate later, dividing into the electromagnetic and weak interactions.) The exact point where electrostrong symmetry was broken is not certain, owing to speculative and as yet incomplete theoretical knowledge.


title: Chronology of the universe

text: At this point of the very early universe, the metric that defines distance within space suddenly and very rapidly changed in scale, leaving the early universe at least 10 times its previous volume (and possibly much more). This is equivalent to a linear increase of at least 10 times in every spatial dimension—equivalent to an object 1 nanometre (10 m, about half the width of a molecule of DNA) in length, expanding to one approximately long in a tiny fraction of a second. This change is known as inflation.


title: Chronology of the universe

text: Although light and objects within spacetime cannot travel faster than the speed of light, in this case it was the metric governing the size and geometry of spacetime itself that changed in scale. Changes to the metric are not limited by the speed of light.


title: Chronology of the universe

text: Based on strong observational evidence, it is widely accepted that it did take place. However, the reasons why it occurred are a matter of conjecture. Although models exist that explain why and how it took place, it is uncertain which model is correct.


title: Chronology of the universe

text: In several of the more prominent models, it is thought to have been triggered by the separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field. As this field settled into its lowest energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of the metric that defines space itself. Inflation explains several observed properties of the current universe that are otherwise difficult to account for, including explaining how today's universe has ended up so exceedingly homogeneous (similar) on a very large scale, even though it was highly disordered in its earliest stages.


title: Chronology of the universe

text: It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10 and 10 seconds after the Big Bang. The rapid expansion of space meant that elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe. However, the huge potential energy of the inflaton field was released at the end of the inflationary epoch, as the inflaton field decayed into other particles, known as "reheating". This heating effect led to the universe being repopulated with a dense, hot mixture of quarks, anti-quarks and gluons. In other models, reheating is often considered to mark the start of the electroweak epoch, and some theories, such as warm inflation, avoid a reheating phase entirely.


title: Chronology of the universe

text: In non-traditional versions of Big Bang theory (known as "inflationary" models), inflation ended at a temperature corresponding to roughly 10 seconds after the Big Bang, but this does "not" imply that the inflationary era lasted less than 10 seconds. To explain the observed homogeneity of the universe, the duration in these models must be longer than 10 seconds. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the "end" of inflation.


title: Chronology of the universe

text: After inflation ended, the universe continued to expand, but at a much slower rate. About 4 billion years ago the expansion gradually began to speed up again. This is believed to be due to dark energy becoming dominant in the universe's large-scale behavior. It is still expanding today.


title: Chronology of the universe

text: On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-modes power spectrum which was interpreted as clear experimental evidence for the theory of inflation. However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported and finally, on 2 February 2015, a joint analysis of data from BICEP2/Keck and the European Space Agency's" Planck" microwave space telescope concluded that the statistical "significance [of the data] is too low to be interpreted as a detection of primordial B-modes" and can be attributed mainly to polarized dust in the Milky Way.


title: Chronology of the universe

text: If supersymmetry is a property of the universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak scale. The masses of particles and their superpartners would then no longer be equal. This very high energy could explain why no superpartners of known particles have ever been observed.


title: Chronology of the universe

text: After cosmic inflation ends, the universe is filled with a hot quark–gluon plasma, the remains of reheating. From this point onwards the physics of the early universe is much better understood, and the energies involved in the Quark epoch are directly accessible in particle physics experiments and other detectors.


title: Chronology of the universe

text: Some time after inflation, the created particles went through thermalization, where mutual interactions lead to thermal equilibrium.


title: Chronology of the universe

text: The earliest stage of which we are quite confident about is some time before the electroweak symmetry breaking, at a temperature of around 10 K, approximately 10 seconds after the Big Bang. The electromagnetic and weak interaction have not yet separated, and as far as we know all particles were massless, as the Higgs mechanism had not operated yet. However exotic massive particle-like entities, sphalerons, are thought to have existed.


title: Chronology of the universe

text: This epoch ended with electroweak symmetry breaking; according to the standard model of particle physics, baryogenesis also happened at this stage, creating an imbalance between matter and anti-matter (though in extensions to this model this may have happened earlier). Little is known about the details of these processes.


title: Chronology of the universe

text: Since the interaction was strong, the cross section formula_3 was approximately the particle wavelength squared, which is roughly formula_4. The rate of collisions per particle species can thus be calculated from the mean free path, giving approximately:


title: Chronology of the universe

text: For comparison, since the cosmological constant was negligible at this stage, the Hubble parameter was:


title: Chronology of the universe

text: Thus "H" is orders of magnitude lower than the rate of collisions per particle species. This means there was plenty of time for thermalization at this stage.


title: Chronology of the universe

text: At this epoch, the collision rate is proportional to the third root of the number density, and thus to formula_7, where formula_8 is the scale parameter. The Hubble parameter, however, is proportional to formula_9. Going back in time and higher in energy, and assuming no new physics at these energies, a careful estimate gives that thermalization was first possible when the temperature was:


title: Chronology of the universe

text: As the universe's temperature continued to fall below 159.5±1.5 GeV, electroweak symmetry breaking happened. So far as we know, it was the penultimate symmetry breaking event in the formation of the universe, the final one being chiral symmetry breaking in the quark sector. This has two related effects:


title: Chronology of the universe

text: After electroweak symmetry breaking, the fundamental interactions we know of—gravitation, electromagnetic, weak and strong interactions—have all taken their present forms, and fundamental particles have their expected masses, but the temperature of the universe is still too high to allow the stable formation of many particles we now see in the universe, so there are no protons or neutrons, and therefore no atoms, atomic nuclei, or molecules. (More exactly, any composite particles that form by chance, almost immediately break up again due to the extreme energies.)


title: Chronology of the universe

text: The quark epoch began approximately 10 seconds after the Big Bang. This was the period in the evolution of the early universe immediately after electroweak symmetry breaking, when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction had taken their present forms, but the temperature of the universe was still too high to allow quarks to bind together to form hadrons.


title: Chronology of the universe

text: During the quark epoch the universe was filled with a dense, hot quark–gluon plasma, containing quarks, leptons and their antiparticles. Collisions between particles were too energetic to allow quarks to combine into mesons or baryons.


title: Chronology of the universe

text: The quark epoch ended when the universe was about 10 seconds old, when the average energy of particle interactions had fallen below the mass of lightest hadron, the pion.


title: Chronology of the universe

text: Baryons are subatomic particles such as protons and neutrons, that are composed of three quarks. It would be expected that both baryons, and particles known as antibaryons would have formed in equal numbers. However, this does not seem to be what happened—as far as we know, the universe was left with far more baryons than antibaryons. In fact, almost no antibaryons are observed in nature. It is not clear how this came about. Any explanation for this phenomenon must allow the Sakharov conditions related to baryogenesis to have been satisfied at some time after the end of cosmological inflation. Current particle physics suggests asymmetries under which these conditions would be met, but these asymmetries appear to be too small to account for the observed baryon-antibaryon asymmetry of the universe.


title: Chronology of the universe

text: The quark–gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form.


title: Chronology of the universe

text: Initially, hadron/anti-hadron pairs could form, so matter and antimatter were in thermal equilibrium. However, as the temperature of the universe continued to fall, new hadron/anti-hadron pairs were no longer produced, and most of the newly formed hadrons and anti-hadrons annihilated each other, giving rise to pairs of high-energy photons. A comparatively small residue of hadrons remained at about 1 second of cosmic time, when this epoch ended.


title: Chronology of the universe

text: Theory predicts that about 1 neutron remained for every 6 protons, with the ratio falling to 1:7 over time due to neutron decay. This is believed to be correct because, at a later stage, the neutrons and some of the protons fused, leaving hydrogen, a hydrogen isotope called deuterium, helium and other elements, which can be measured. A 1:7 ratio of hadrons would indeed produce the observed element ratios in the early and current universe.


title: Chronology of the universe

text: At approximately 1 second after the Big Bang neutrinos decouple and begin travelling freely through space. As neutrinos rarely interact with matter, these neutrinos still exist today, analogous to the much later cosmic microwave background emitted during recombination, around 370,000 years after the Big Bang. The neutrinos from this event have a very low energy, around 10 times the amount of those observable with present-day direct detection. Even high-energy neutrinos are notoriously difficult to detect, so this cosmic neutrino background (CνB) may not be directly observed in detail for many years, if at all.


title: Chronology of the universe

text: However, Big Bang cosmology makes many predictions about the CνB, and there is very strong indirect evidence that the CνB exists, both from Big Bang nucleosynthesis predictions of the helium abundance, and from anisotropies in the cosmic microwave background (CMB). One of these predictions is that neutrinos will have left a subtle imprint on the CMB. It is well known that the CMB has irregularities. Some of the CMB fluctuations were roughly regularly spaced, because of the effect of baryonic acoustic oscillations. In theory, the decoupled neutrinos should have had a very slight effect on the phase of the various CMB fluctuations.


title: Chronology of the universe

text: In 2015, it was reported that such shifts had been detected in the CMB. Moreover, the fluctuations corresponded to neutrinos of almost exactly the temperature predicted by Big Bang theory ( compared to a prediction of 1.95K), and exactly three types of neutrino, the same number of neutrino flavors predicted by the Standard Model.


title: Chronology of the universe

text: Primordial black holes are a hypothetical type of black hole proposed in 1966, that may have formed during the so-called radiation-dominated era, due to the high densities and inhomogeneous conditions within the first second of cosmic time. Random fluctuations could lead to some regions becoming dense enough to undergo gravitational collapse, forming black holes. Current understandings and theories place tight limits on the abundance and mass of these objects.


title: Chronology of the universe

text: Typically, primordial black hole formation requires density contrasts (regional variations in the universe's density) of around formula_11 (10%), where formula_12 is the average density of the universe. Several mechanisms could produce dense regions meeting this criterion during the early universe, including reheating, cosmological phase transitions and (in so-called "hybrid inflation models") axion inflation. Since primordial black holes didn't form from stellar gravitational collapse, their masses can be far below stellar mass (~2×10 g). Stephen Hawking calculated in 1971 that primordial black holes could have a mass as low as 10 g. But they can have any size, so they could also be large, and may have contributed to the formation of galaxies.


title: Chronology of the universe

text: The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving leptons (such as the electron, muons and certain neutrinos) and antileptons, dominating the mass of the universe.


title: Chronology of the universe

text: The lepton epoch follows a similar path to the earlier hadron epoch. Initially leptons and antileptons are produced in pairs. About 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton–antilepton pairs are no longer created and most remaining leptons and antileptons quickly annihilated each other, giving rise to pairs of high-energy photons, and leaving a small residue of non-annihilated leptons.


title: Chronology of the universe

text: After most leptons and antileptons are annihilated at the end of the lepton epoch, most of the mass-energy in the universe is left in the form of photons. (Much of the rest of its mass-energy is in the form of neutrinos and other relativistic particles.) Therefore, the energy of the universe, and its overall behavior, is dominated by its photons. These photons continue to interact frequently with charged particles, i.e., electrons, protons and (eventually) nuclei. They continue to do so for about the next 370,000 years.


title: Chronology of the universe

text: Between about 2 and 20 minutes after the Big Bang, the temperature and pressure of the universe allowed nuclear fusion to occur, giving rise to nuclei of a few light elements beyond hydrogen ("Big Bang nucleosynthesis"). About 25% of the protons, and all the neutrons fuse to form deuterium, a hydrogen isotope, and most of the deuterium quickly fuses to form helium-4.


title: Chronology of the universe

text: Atomic nuclei will easily unbind (break apart) above a certain temperature, related to their binding energy. From about 2 minutes, the falling temperature means that deuterium no longer unbinds, and is stable, and starting from about 3 minutes, helium and other elements formed by the fusion of deuterium also no longer unbind and are stable.


title: Chronology of the universe

text: The short duration and falling temperature means that only the simplest and fastest fusion processes can occur. Only tiny amounts of nuclei beyond helium are formed, because nucleosynthesis of heavier elements is difficult and requires thousands of years even in stars. Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and are quickly lost again. A small amount of deuterium is left unfused because of the very short duration.


title: Chronology of the universe

text: Therefore, the only stable nuclides created by the end of Big Bang nucleosynthesis are protium (single proton/hydrogen nucleus), deuterium, helium-3, helium-4, and lithium-7. By mass, the resulting matter is about 75% hydrogen nuclei, 25% helium nuclei, and perhaps 10 by mass of lithium-7. The next most common stable isotopes produced are lithium-6, beryllium-9, boron-11, carbon, nitrogen and oxygen ("CNO"), but these have predicted abundances of between 5 and 30 parts in 10 by mass, making them essentially undetectable and negligible.


title: Chronology of the universe

text: The amounts of each light element in the early universe can be estimated from old galaxies, and is strong evidence for the Big Bang. For example, the Big Bang should produce about 1 neutron for every 7 protons, allowing for 25% of all nucleons to be fused into helium-4 (2 protons and 2 neutrons out of every 16 nucleons), and this is the amount we find today, and far more than can be easily explained by other processes. Similarly, deuterium fuses extremely easily; any alternative explanation must also explain how conditions existed for deuterium to form, but also left some of that deuterium unfused and not immediately fused again into helium. Any alternative must also explain the proportions of the various light elements and their isotopes. A few isotopes, such as lithium-7, were found to be present in amounts that differed from theory, but over time, these differences have been resolved by better observations.


title: Chronology of the universe

text: Until now, the universe's large-scale dynamics and behavior have been determined mainly by radiation—meaning, those constituents that move relativistically (at or near the speed of light), such as photons and neutrinos. As the universe cools, from around 47,000 years (redshift "z" = 3600), the universe's large-scale behavior becomes dominated by matter instead. This occurs because the energy density of matter begins to exceed both the energy density of radiation and the vacuum energy density. Around or shortly after 47,000 years, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) become equal, the Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by free streaming radiation, can begin to grow in amplitude.


title: Chronology of the universe

text: According to the Lambda-CDM model, by this stage, the matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. There is overwhelming evidence that dark matter exists and dominates the universe, but since the exact nature of dark matter is still not understood, the Big Bang theory does not presently cover any stages in its formation.


title: Chronology of the universe

text: From this point on, and for several billion years to come, the presence of dark matter accelerates the formation of structure in the universe. In the early universe, dark matter gradually gathers in huge filaments under the effects of gravity, collapsing faster than ordinary (baryonic) matter because its collapse is not slowed by radiation pressure. This amplifies the tiny inhomogeneities (irregularities) in the density of the universe which was left by cosmic inflation. Over time, slightly denser regions become denser and slightly rarefied (emptier) regions become more rarefied. Ordinary matter eventually gathers together faster than it would otherwise do, because of the presence of these concentrations of dark matter.


title: Chronology of the universe

text: The properties of dark matter that allow it to collapse quickly without radiation pressure, also mean that it cannot "lose" energy by radiation either. Losing energy is necessary for particles to collapse into dense structures beyond a certain point. Therefore, dark matter collapses into huge but diffuse filaments and haloes, and not into stars or planets. Ordinary matter, which "can" lose energy by radiation, forms dense objects and also gas clouds when it collapses.


title: Chronology of the universe

text: About 370,000 years after the Big Bang, two connected events occurred: the ending of recombination and photon decoupling. Recombination describes the ionized particles combining to form the first neutral atoms, and decoupling refers to the photons released ("decoupled") as the newly formed atoms settle into more stable energy states.


title: Chronology of the universe

text: Just before recombination, the baryonic matter in the universe was at a temperature where it formed a hot ionized plasma. Most of the photons in the universe interacted with electrons and protons, and could not travel significant distances without interacting with ionized particles. As a result, the universe was opaque or "foggy". Although there was light, it was not possible to see, nor can we observe that light through telescopes.


title: Chronology of the universe

text: Starting around 18,000 years, the universe has cooled to a point where free electrons can combine with helium nuclei to form atoms. Neutral helium nuclei then start to form at around 100,000 years, with neutral hydrogen formation peaking around 260,000 years. This process is known as recombination. The name is slightly inaccurate and is given for historical reasons: in fact the electrons and atomic nuclei were combining for the first time.


title: Chronology of the universe

text: At around 100,000 years, the universe had cooled enough for helium hydride, the first molecule, to form. In April 2019, this molecule was first announced to have been observed in interstellar space, in NGC 7027, a planetary nebula within this galaxy. (Much later, atomic hydrogen reacted with helium hydride to create molecular hydrogen, the fuel required for star formation.)


title: Chronology of the universe

text: Directly combining in a low energy state (ground state) is less efficient, so these hydrogen atoms generally form with the electrons still in a high-energy state, and once combined, the electrons quickly release energy in the form of one or more photons as they transition to a low energy state. This release of photons is known as photon decoupling. Some of these decoupled photons are captured by other hydrogen atoms, the remainder remain free. By the end of recombination, most of the protons in the universe have formed neutral atoms. This change from charged to neutral particles means that the mean free path photons can travel before capture in effect becomes infinite, so any decoupled photons that have not been captured can travel freely over long distances (see Thomson scattering). The universe has become transparent to visible light, radio waves and other electromagnetic radiation for the first time in its history.


title: Chronology of the universe

text: The photons released by these newly formed hydrogen atoms initially had a temperature/energy of around ~ 4000 K. This would have been visible to the eye as a pale yellow/orange tinted, or "soft", white color. Over billions of years since decoupling, as the universe has expanded, the photons have been red-shifted from visible light to radio waves (microwave radiation corresponding to a temperature of about 2.7 K). Red shifting describes the photons acquiring longer wavelengths and lower frequencies as the universe expanded over billions of years, so that they gradually changed from visible light to radio waves. These same photons can still be detected as radio waves today. They form the cosmic microwave background, and they provide crucial evidence of the early universe and how it developed.


title: Chronology of the universe

text: Around the same time as recombination, existing pressure waves within the electron-baryon plasma—known as baryon acoustic oscillations—became embedded in the distribution of matter as it condensed, giving rise to a very slight preference in distribution of large-scale objects. Therefore, the cosmic microwave background is a picture of the universe at the end of this epoch including the tiny fluctuations generated during inflation (see 9-year WMAP image), and the spread of objects such as galaxies in the universe is an indication of the scale and size of the universe as it developed over time.


title: Chronology of the universe

text: After recombination and decoupling, the universe was transparent and had cooled enough to allow light to travel long distances, but there were no light-producing structures such as stars and galaxies. Stars and galaxies are formed when dense regions of gas form due to the action of gravity, and this takes a long time within a near-uniform density of gas and on the scale required, so it is estimated that stars did not exist for perhaps hundreds of millions of years after recombination.


title: Chronology of the universe

text: This period, known as the Dark Ages, began around 370,000 years after the Big Bang. During the Dark Ages, the temperature of the universe cooled from some 4000 K to about 60 K (3727 °C to about −213 °C), and only two sources of photons existed: the photons released during recombination/decoupling (as neutral hydrogen atoms formed), which we can still detect today as the cosmic microwave background (CMB), and photons occasionally released by neutral hydrogen atoms, known as the 21 cm spin line of neutral hydrogen. The hydrogen spin line is in the microwave range of frequencies, and within 3 million years, the CMB photons had redshifted out of visible light to infrared; from that time until the first stars, there were no visible light photons. Other than perhaps some rare statistical anomalies, the universe was truly dark.


title: Chronology of the universe

text: The first generation of stars, known as Population III stars, formed within a few hundred million years after the Big Bang. These stars were the first source of visible light in the universe after recombination. Structures may have begun to emerge from around 150 million years, and early galaxies emerged from around 380 to 700 million years. (We do not have separate observations of very early individual stars; the earliest observed stars are discovered as participants in very early galaxies.) As they emerged, the Dark Ages gradually ended. Because this process was gradual, the Dark Ages only fully ended around 1 billion years, as the universe took its present appearance.


title: Chronology of the universe

text: At present, the oldest observations of stars and galaxies are from shortly after the start of reionization, with galaxies such as GN-z11 (Hubble Space Telescope, 2016) at about z≈11.1 (about 400 million years cosmic time). Hubble's successor, the James Webb Space Telescope, launched December 2021, is designed to detect objects up to 100 times fainter than Hubble, and much earlier in the history of the universe, back to redshift z≈20 (about 180 million years cosmic time). This is believed to be earlier than the first galaxies, and around the era of the first stars.


title: Chronology of the universe

text: There is also an observational effort underway to detect the faint 21 cm spin line radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe.


title: Chronology of the universe

text: For about 6.6 million years, between about 10 to 17 million years after the Big Bang (redshift 137–100), the background temperature was between , a temperature compatible with liquid water and common biological chemical reactions. Abraham Loeb (2014) speculated that primitive life might in principle have appeared during this window, which he called the "habitable epoch of the early Universe". Loeb argues that carbon-based life might have evolved in a hypothetical pocket of the early universe that was dense enough both to generate at least one massive star that subsequently releases carbon in a supernova, and that was also dense enough to generate a planet. (Such dense pockets, if they existed, would have been extremely rare.) Life would also have required a heat differential, rather than just uniform background radiation; this could be provided by naturally occurring geothermal energy. Such life would likely have remained primitive; it is highly unlikely that intelligent life would have had sufficient time to evolve before the hypothetical oceans freeze over at the end of the habitable epoch.


title: Chronology of the universe

text: The matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. Since the start of the matter-dominated era, dark matter has gradually been gathering in huge spread-out (diffuse) filaments under the effects of gravity. Ordinary matter eventually gathers together faster than it would otherwise do, because of the presence of these concentrations of dark matter. It is also slightly more dense at regular distances due to early baryon acoustic oscillations (BAO) which became embedded into the distribution of matter when photons decoupled. Unlike dark matter, ordinary matter can lose energy by many routes, which means that as it collapses, it can lose the energy which would otherwise hold it apart, and collapse more quickly, and into denser forms. Ordinary matter gathers where dark matter is denser, and in those places it collapses into clouds of mainly hydrogen gas. The first stars and galaxies form from these clouds. Where numerous galaxies have formed, galaxy clusters and superclusters will eventually arise. Large voids with few stars will develop between them, marking where dark matter became less common.


title: Chronology of the universe

text: The exact timings of the first stars, galaxies, supermassive black holes, and quasars, and the start and end timings and progression of the period known as reionization, are still being actively researched, with new findings published periodically. As of 2019, the earliest confirmed galaxies date from around 380–400 million years (for example GN-z11), suggesting surprisingly fast gas cloud condensation and stellar birth rates, and observations of the Lyman-alpha forest and other changes to the light from ancient objects allows the timing for reionization, and its eventual end, to be narrowed down. But these are all still areas of active research.


title: Chronology of the universe

text: Structure formation in the Big Bang model proceeds hierarchically, due to gravitational collapse, with smaller structures forming before larger ones. The earliest structures to form are the first stars (known as Population III stars), dwarf galaxies, and quasars (which are thought to be bright, early active galaxies containing a supermassive black hole surrounded by an inward-spiralling accretion disk of gas). Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, "N"-body simulations with billions of particles. The Bolshoi Cosmological Simulation is a high precision simulation of this era.


title: Chronology of the universe

text: These Population III stars are also responsible for turning the few light elements that were formed in the Big Bang (hydrogen, helium and small amounts of lithium) into many heavier elements. They can be huge as well as perhaps small—and non-metallic (no elements except hydrogen and helium). The larger stars have very short lifetimes compared to most Main Sequence stars we see today, so they commonly finish burning their hydrogen fuel and explode as supernovae after mere millions of years, seeding the universe with heavier elements over repeated generations. They mark the start of the Stelliferous Era.


title: Chronology of the universe

text: As yet, no Population III stars have been found, so the understanding of them is based on computational models of their formation and evolution. Fortunately, observations of the cosmic microwave background radiation can be used to date when star formation began in earnest. Analysis of such observations made by the "Planck" microwave space telescope in 2016 concluded that the first generation of stars may have formed from around 300 million years after the Big Bang.