119254658
Deviation from the Cosmological Constant or Systematic Errors
In the work of Qi, Wang & Lu, gamma-ray bursts (GRBs) are used together with an earlier Type Ia supernova (SN Ia) data set to constrain the dark energy equation of state (EOS) in a nearly model-independent way. The improvements made by including GRBs show a slight shift of the dark energy EOS toward w > -1 at redshifts z greater than or similar to 0.5. It is interesting that, when we have more SNe Ia, SNe Ia themselves also show the same trend. Motivated by the fact that both SNe Ia and GRBs seem to prefer a dark energy EOS greater than -1 at redshifts z greater than or similar to 0.5, we perform a careful investigation of this situation, including more careful treatments of measurement errors of GRBs and cross-checking the results by using different ways of including GRBs. We find that the deviation of dark energy from the cosmological constant at redshifts z greater than or similar to 0.5 is large enough that we should pay close attention to it with future observational data. Such a deviation may arise from some biasing systematic errors in the handling of SNe Ia and/or GRBs, or more interestingly from the nature of the dark energy itself.
118676242
Gravitational effects of condensate dark matter on compact stellar objects
We study the gravitational effect of non-self-annihilating dark matter on compact stellar objects. The self-interaction of condensate dark matter can give high accretion rate of dark matter onto stars. Phase transition to condensation state takes place when the dark matter density exceeds the critical value. A compact degenerate dark matter core is developed and alter the structure and stability of the stellar objects. Condensate dark matter admixed neutron stars is studied through the two-fluid TOV equation. The existence of condensate dark matter deforms the mass-radius relation of neutron stars and lower their maximum baryonic masses and radii. The possible effects on the Gamma-ray Burst rate in high redshift are discussed.
119235128
Gamma-ray burst contributions to constraining the evolution of dark energy
Aims. We explore the gamma-ray bursts' (GRBs') contributions in constraining the dark energy equation of state (EOS) at high (1.8 < z < 7) and at middle redshifts (0.5 < z < 1.8) and estimate how many GRBs are needed to get substantial constraints at high redshifts.
11226958
Constraining the cosmological parameters and transition redshift with gamma-ray bursts and supernovae
A new method of measuring cosmology with gamma-ray bursts (GRBs) has been proposed by Liang and Zhang recently. In this method, only observable quantities including the rest-frame peak energy of the νF ν spectrum (E' p ), the isotropic energy of GRB (E γ,iso ) and the break time of the optical afterglow light curves in the rest frame (t' b ) are used. By considering this method, we constrain the cosmological parameters and the redshift at which the Universe expanded from the deceleration to acceleration phase. We add five recently detected GRBs to the sample and derive E γ,iso /10 52 erg = (0.93 ± 0.25) (E' p /100 keV) 1.91±0.32 (t'b/1d) -0.93±0.38 for a flat universe with Ω M = 0.28 and H 0 = 71.0km s -1 Mpc -1 . This relation is independent of the medium density around bursts and the efficiency of conversion of the explosion energy to gamma-ray energy. We consider the E γ,iso (E' p , t' b ) relationship as a standard candle and find 0.05 < Ω M < 0.48 and Ω Λ < 1.15 (at the la confidence level). In a flat universe with the cosmological constant, we obtain 0.25 < Ω M < 0.46 and 0.54 < Ω Λ < 0.78 at the la confidence level. The transition redshift is z T = 0.69 +0.11 -0.12 . Combining 20 GRBs with 157 Type Ia supernovae (SNe Ia), we find Ω M = 0.29 ± 0.03 for a flat universe and the transition redshift is z T = 0.61 +0.06 -0.05 , which is slightly larger than the value found by considering SNe Ia alone. In particular, we also discuss several dark-energy models in which the equation of state w(z) is parametrized, and investigate constraints on the cosmological parameters in detail.
127622395
Ward identity of two-loop diagrams in SUSY
14838430
Progress in L-Band Power Distribution System R&D at SLAC
We report on the L-band RF power distribution system (PDS) developed at SLAC for Fermilab's NML superconducting test accelerator facility. The makeup of the system, which allows tailoring of the power distribution to cavities by pairs, is briefly described. Cold test measurements of the system and the results of high power processing are presented. We also investigate the feasibility of eliminating the expensive, lossy circulators from the PDS by pair-feeding cavities through custom 3-dB hybrids. A computational model is used to simulate the impact on cavity field stability due to the reduced cavity-to-cavity isolation.
14538301
Measuring dark energy with gamma-ray bursts and other cosmological probes
It has been widely shown that cosmological parameters and dark energy can be constrained by using data from Type Ia supernovae (SNe Ia), the cosmic microwave background (CMB) anisotropy, the baryon acoustic oscillation (BAO) peak from Sloan Digital Sky Survey (SDSS), the X-ray gas mass fraction in clusters, and the linear growth rate of perturbations at z = 0.15 as obtained from the 2dF Galaxy Redshift Survey. Recently, gamma-ray bursts (GRBs) have also been argued to be promising standard candles for cosmography. In this paper, we present constraints on the cosmological parameters and dark energy by combining a recent GRB sample including 69 events with the other cosmological probes. First, we find that for the ΛCDM cosmology this combination makes the constraints stringent, and the best fit is close to the flat universe. Second, we fit the flat Cardassian expansion model and find that this model is consistent with the ΛCDM cosmology. Third, we present constraints on several two-parameter dark energy models and find that these models are also consistent with the ΛCDM cosmology. Finally, we reconstruct the dark energy equation of state parameter w(z) and the deceleration parameter q(z). We see that the acceleration could have started at a redshift from zt = 0.40 to 0.65. This difference in the transition redshift is due to different dark energy models that we adopt. The most stringent constraint on w(z) lies in the redshift range z ~ 0.3-0.6.
93891215
Gamma-ray spectra of methane in the positron–electron annihilationprocess
Abstract Bound electron contribution to the Doppler-shift of gamma-ray spectra in the positron–electron annihilation process of molecular methane has been studied in gas phase. Two accurate ab initio quantum mechanical schemes, i.e. the delocalised molecular orbital (MO) and the localised natural bond orbital (NBO) schemes, are applied to study the multi-centred methane molecule. The present ab initio calculations of methane indicate that the C–H bonds are polarised with the partial negative charge of −0.36 a.u on the carbon atom and the partial positive charge of +0.09 a.u. on each of the hydrogen atoms. The positively charged hydrogen atoms produce repulsive Coulomb potentials to a positron. Both the MO and NBO schemes further reveal that the 2a 1 electrons of methane, that is, the 2a 1 electron component of the CH bonds rather than the whole CH bonds of methane, predominates the positron–electron annihilation gamma-ray spectra of the molecule. Electrons of a molecule which are dominant the positron–electron annihilation processes are called “positrophilic” electrons in the present study. It is further shown that the negative electrostatic potential (ESP) of methane facilitates with the density of the “positrophilic” 2a 1 electrons of methane. Other valence electrons (e.g. 1t 2 ) in the CH bonds play a minor “spectator” role in the annihilation process of methane.