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.

The cosmic horizon

Abstract: The cosmological principle, promoting the view that the Universe is homogeneous and isotropic, is embodied within the mathematical structure of the Robertson‐Walker (RW) metric. The equations derived from an application of this metric to the Einstein Field Equations describe the expansion of the Universe in terms of comoving coordinates, from which physical distances may be derived using a time-dependent expansion factor. These coordinates, however, do not explicitly reveal the properties of the cosmic space‐time manifested in Birkhoff’s theorem and its corollary. In this paper, we compare two forms of the metric ‐ written in (the traditional) comoving coordinates, and a set of observer-dependent coordinates ‐ first for the well-known de Sitter universe containing only dark energy, and then for a newly derived form of the RW metric, for a universe with dark energy and matter. We show that Rindler’s event horizon ‐ evident in the comoving system ‐ coincides with what one might call the ‘curvature horizon’ appearing in the observer-dependent frame. The advantage of this dual prescription of the cosmic space‐time is that with the latest Wilkinson Microwave Anisotropy Probe results, we now have a much better determination of the Universe’s mass-energy content, which permits us to calculate this curvature with unprecedented accuracy. We use it here to demonstrate that our observations have probed the limit beyond which the cosmic curvature prevents any signal from having ever reached us. In the case of de Sitter, where the mass-energy density is a constant, this limit is fixed for all time. For a universe with a changing density, this horizon expands until de Sitter is reached asymptotically, and then it too ceases to change.

Strong lensing optical depths in a ΛCDM universe

Abstract: We investigate strong gravitational lensing in the concordance ΛCDM cosmology bycarrying out ray-tracing along past light cones through the Millennium Simulation,the largest simulation of cosmic structure formation ever carried out. We extend pre-vious ray-tracing methods in order to take full advantage of the large volume and theexcellent spatial and mass resolution of the simulation. As a function of source red-shift we evaluate the probability that an image will be highly magnified, will be highlyelongated or will be one of a set of multiple images. We show that such strong lensingevents can almost always be traced to a single dominant lensing object and we studythe mass and redshift distribution of these primary lenses. We fit analytic models tothe simulated dark halos in order to study how our optical depth measurements areaffected by the limited resolution of the simulation and of the lensing planes that weconstruct from it. We conclude that such effects lead us to underestimate total strong-lensing cross sections by about 15%. This is smaller than the effects expected from ourneglect of the baryonic components of galaxies. Finally we investigate whether stronglensing is enhanced by material in front of or behind the primary lens. Although stronglensing lines-of-sight are indeed biased towards higher than average mean densities,this additional matter typically contributes only a few percent of the total surfacedensity.Keywords: gravitationallensing –darkmatter–large-scalestructure ofthe Universe– cosmology: theory – methods: numerical

Cosmology confronts particle physics

Abstract: Production of hadrons, leptons, and quarks in the early universe is described in terms of departure from thermal equilibrium. (AIP)

Difficulties with inflationary cosmology

Abstract: The problems posed for inflationary cosmological models by (1) the large-scale homogeneity of the universe and (2) the second law of thermodynamics are considered theoretically. The analyses of Penrose (1979, 1981, 1988, and 1989) are summarized and illustrated with graphs and diagrams. Particular attention is given to the definition of entropy in the cosmological context, gravitational entropy, the low entropy of the big bang, and the Weyl curvature hypothesis. 23 refs.

The evolution of the cosmic microwave background temperature Measurements of TCMB at high redshift from carbon monoxide excitation

Abstract: A milestone of modern cosmology was the prediction and serendipitous discovery of the cosmic microwave background (CMB), the radiation leftover after decoupling from matter in the early evolutionary stages of the Universe. A prediction of the standard hot Big-Bang model is the linear increase with redshift of the black-body temperature of the CMB (TCMB). This radiation excites the rotational levels of some interstellar molecules, including carbon monoxide (CO), which can serve as cosmic thermometers. Using three new and two previously reported CO absorption-line systems detected in quasar spectra during a systematic survey carried out using VLT/UVES, we constrain the evolution of TCMB to z ∼ 3. Combining our precise measurements with previous constraints, we obtain TCMB(z) = (2.725 ± 0.002) × (1 + z) 1−β K with β = −0.007 ± 0.027, a more than two-fold improvement in precision. The measurements are consistent with the standard (i.e. adiabatic, β = 0) Big-Bang model and provide a strong constraint on the effective equation of state of decaying dark energy (i.e. weff = −0.996 ± 0.025).