Cosmology

About: Cosmology is a research topic. Over the lifetime, 18004 publications have been published within this topic receiving 631028 citations. The topic is also known as: physical cosmology & cosmologies.

H0LiCOW - IV. Lens mass model of HE 0435-1223 and blind measurement of its time-delay distance for cosmology

Abstract: Strong gravitational lenses with measured time delays between the multiple images allow a direct measurement of the time-delay distance to the lens, and thus a measure of cosmological parameters, particularly the Hubble constant, H-0. We present a blind lens model analysis of the quadruply imaged quasar lens HE 0435-1223 using deep Hubble Space Telescope imaging, updated time-delay measurements from the COSmological MOnitoring of GRAvItational Lenses (COSMOGRAIL), a measurement of the velocity dispersion of the lens galaxy based on Keck data, and a characterization of the mass distribution along the line of sight. HE 0435-1223 is the third lens analysed as a part of the H-0 Lenses in COSMOGRAIL's Wellspring (HOLiCOW) project. We account for various sources of systematic uncertainty, including the detailed treatment of nearby perturbers, the parametrization of the galaxy light and mass profile, and the regions used for lens modelling. We constrain the effective time delay distance to be D-Delta t = 2612(191)(+208) Mpc, a precision of 7.6 per cent. From HE 0435-1223 alone, we infer a Hubble constant of H-0 = 73.1(6.0)(+5.7) km s(-1) Mpc(-1) assuming a flat ACDM cosmology. The cosmographic inference based on the three lenses analysed by HOLiCOW to date is presented in a companion paper (HOLiCOW Paper V).

An inflationary universe in Brans-Dicke theory: a hopeful sign of theoretical estimation of the gravitational constant

Abstract: A revised model of the inflationary universe under the framework of Brans-Dicke theory is proposed. It is shown that the scale factor of the universe during the inflation varies as R approximately tw+1/2 and not exponentially, but still it gives rise to enough inflation to solve the cosmological puzzles. It is found that the quantum gravity effects come into play in the same scale as that of grand unified theories (i.e. at T approximately 1014 GeV) rather than at T approximately 1019 GeV, hence providing a natural solution to the 'milder version' of the flatness problem inherent in the conventional inflationary scenarios. This scenario also provides a simple way to estimate theoretically the present value of the gravitational constant. The estimated value agrees well with the actual value despite many approximations and the ignorance of quantum gravity; the difference is only by a factor of the order of 10-2.

Generalized Chaplygin gas as a unified scenario of dark matter/energy: Observational constraints

Abstract: Although various cosmological observations congruously suggest that our universe is dominated by two dark com- ponents, the cold dark matter without pressure and the dark energy with negative pressure, the nature and origin of these components is yet unknow. The generalized Chaplygin gas (gCg), parametrized by an equation of state, p = −A/ρ α ,w as recently proposed to be a candidate of the unified dark matter/energy (UDME) scenarios. In this work, we investigate some observational constraints on it. We mainly focus our attention on the constraints from recent measurements of the X-ray gas mass fractions in clusters of galaxies published by Allen et al. (2002, MNRAS, 334, L11; 2003, 342, 257) and the dimensionless coordinate distances to type Ia supernovae and Fanaroff-Riley type IIb radio galaxies compiled by Daly & Djorgovski (2003, ApJ, 597, 9). We obtain the confidence region on the two parameters fully characterizing gCg, As ≡ A/ρ (1+α) gCg0 and α, from a combined analysis of these databases, where ρgCg0 is the energy density of gCg at present. It is found that As = 0.70 +0.16 −0.17 and α = −0.09 +0.54 −0.33 , at a 95% confidence level, which is consistent within the errors with the standard dark matter + dark energy model, i.e., the case of α = 0. Particularly, the standard Chaplygin gas (α = 1) is ruled out as a feasible UDME by the data at a 99% confidence level.

The case for a hierarchical cosmology.

Abstract: Once upon a time philosophers and cosmographers insisted that the motions of the planets must be circular and uniform. An irrelevant aesthetic concept of ‘perfection’ and a more valid mathematical need for simplicity were at the root of this long-held error. Nowadays, theoretical cosmologists insist that the large-scale distribution of galaxies most be homogeneous and isotropic, and most astonomers believe that the expansion of the universe is linear and isotropic and that it proceeds at a uniform rate measured by the Hubble ‘constant’ H.1, 2

Magnetic Field Generation during the Cosmological QCD Phase Transition

Abstract: If the QCD phase transition is first-order, then a thermoelectric mechanism generates magnetic fields in the early universe. For reasonable transition parameters, it is found that 5-G magnetic fields could form on scales of 1 m when the temperature of the universe was around 150 MeV. These primordial fields, a consequence of the QCD phase transition, have a magnitude of about 2 x 10 to the -17th G at the time when galaxy formation begins. 43 refs.