ethod to use them took hold. In the late 1960s, a group led by Yakov Zel'Dovich in the Soviet Union, published a short paper showing that an accurate map of CMB radiation would have very distinct features: it should consist of randomly distributed hot and cold spots with a characteristic size.
Zel'Dovich's group calculated how large the spots were when they formed 370,000 years after the big bang, an era in cosmic history known as recombination. How large the spots appear to us today depends on how fast the universe has expanded since then (see diagram below). Under reasonably simple assumptions about what the universe is made of, we can determine the distance to recombination with some accuracy. Knowing this, and assuming the universe is flat, we can use standard trigonometry to work out the angular size of the spots in the sky.
I said "assuming the universe is flat". If the universe is not flat, then we have to use a different set of rules. The typical size of hot and cold spots in a flat universe should be about 1 degree across the sky, approximately twice the angular size of the moon as seen from the ground. If the hot and cold spots look bigger to us, then space has positive curvature. If they look smaller, then the curvature is negative.
In 1992, NASA's COBE satellite provided the first all-sky map of the hot and cold spots. The pictures were simply too blurred to pin down the universe's geometry, but they triggered a gold rush as teams vied with each to take clearer pictures of the CMB, hoping to home in on that magical angular scale of about 1 degree.
In 1995, I moved to the University of California, Berkeley, where cosmologists were hard at work on the coalface of research into the CMB. I had started life as a theorist, but with COBE and the potential for true discovery I decided to roll up my sleeves and get my hands dirty. For me, this involved preparing for the launch and then analysing the signals collected by two experiments carried aloft by balloons: Maxima and Boomerang.
Both experiments carried a new breed of microwave detectors that were far more sensitive than any used before. They were also each equipped with telescopes that were over an order of magnitude more accurate than those on the COBE satellite. Both of these features meant that they were able to accurately map out the fine details of the CMB.
There had already been strong hints of the coveted spots in 1997 from an experiment called MAT/Toco. But it was only after a few years of whittling away at the signals in early 2000 that we saw our own clear evidence of hot and cold spots with a typical size of 1 degree. This meant that the geometry of the universe was almost exactly flat. We were seeing direct and unambiguous evidence that we lived in a very special universe.
Together with later measurements from NASA's WMAP satellite, the results nailed down the geometry of the universe to within a few per cent. Suddenly life became much simpler for cosmologists who had been working on different theoretical models with different geometries. From then on there was one less free parameter to fiddle with and almost all the papers and textbooks stated up front that the universe was flat. Finally we had some certainty about the state of the universe.
Except that another dramatic narrative was unfolding in parallel. For decades, we had known that there simply was not enough stuff in the universe to balance the cosmic books. All the known atoms in stars, gas and dust spread out in the cosmos, add up to less than 4 per cent of the total matter and energy we knew had to exist for the universe to have developed to its current state.
Even taking into account the elusive dark matter which we believe bolsters galaxies and allows them to spin at breakneck speeds without flying apart, there still is not enough matter and energy to make it flat; the shortfall in the total energy of the universe seems to be about 70 per cent. And if this were so, the universe would have negative curvature. There must be something else out there to make up the deficit.
The answer came i