1,462 research outputs found
The first second of the Universe
The history of the Universe after its first second is now tested by high
quality observations of light element abundances and temperature anisotropies
of the cosmic microwave background. The epoch of the first second itself has
not been tested directly yet; however, it is constrained by experiments at
particle and heavy ion accelerators. Here I attempt to describe the epoch
between the electroweak transition and the primordial nucleosynthesis.
The most dramatic event in that era is the quark--hadron transition at 10
s. Quarks and gluons condense to form a gas of nucleons and light mesons,
the latter decay subsequently. At the end of the first second, neutrinos and
neutrons decouple from the radiation fluid. The quark--hadron transition and
dissipative processes during the first second prepare the initial conditions
for the synthesis of the first nuclei.
As for the cold dark matter (CDM), WIMPs (weakly interacting massive
particles) -- the most popular candidates for the CDM -- decouple from the
presently known forms of matter, chemically (freeze-out) at 10 ns and
kinetically at 1 ms. The chemical decoupling fixes their present abundances and
dissipative processes during and after thermal decoupling set the scale for the
very first WIMP clouds.Comment: review to appear in Annalen der Physik (51 pages, 16 figures);
references added (v2); typos corrected, resembles published version (v3
Evolution of gravitational waves through the cosmological QCD transition
The spectrum of gravitational waves that have been produced in inflation is
modified during cosmological transitions. Large drops in the number of
relativistic particles, like during the QCD transition or at
annihilation, lead to steps in the spectrum of gravitational waves. We
calculate the transfer function for the differential energy density of
gravitational waves for a first-order and for a crossover QCD transition.Comment: 10 pages, LaTeX2e, 1 figure; analytic estimate for the modification
of the spectral slope near f_* added, minor changes to improve the
presentation; accepted for publication in Mod. Phys. Lett.
Analytic Solutions for Cosmological Perturbations in Multi-Dimensional Space-Time
We obtain analytic solutions for the density contrast and the anisotropic
pressure in a multi-dimensional FRW cosmology with collisionless, massless
matter. These are compared with perturbations of a perfect fluid universe. To
describe the metric perturbations we use manifest gauge invariant metric
potentials. The matter perturbations are calculated by means of (automatically
gauge invariant) finite temperature field theory, instead of kinetic theory.
(Talk given at the Journ\'ees Relativistes '93, 5 -- 7 April, Brussels,
Belgium)Comment: 6 pages (incl. 3 figures), LaTeX (epsf), TUW-93-07, two misprints
corrected (one formula, one reference
Accelerated expansion without dark energy
The fact that the LambdaCDM model fits the observations does not necessarily
imply the physical existence of `dark energy'. Dropping the assumption that
cold dark matter (CDM) is a perfect fluid opens the possibility to fit the data
without dark energy. For imperfect CDM, negative bulk pressure is favoured by
thermodynamical arguments and might drive the cosmic acceleration. The
coincidence between the onset of accelerated expansion and the epoch of
structure formation at large scales might suggest that the two phenomena are
linked. A specific example is considered in which effective (anti-frictional)
forces, which may be due to dissipative processes during the formation of
inhomogeneities, give rise to accelerated expansion of a CDM universe.Comment: 5 pages, Talk at ``On the nature of dark energy: Observational and
theoretical results on the accelerating universe'', Institut d'Astrophysique
de Paris, France, July 1 -- 5, 2002 (v1); one reference updated (v2
Cosmological and astrophysical aspects of finite-density QCD
The different phases of QCD at finite temperature and density lead to
interesting effects in cosmology and astrophysics. In this work I review some
aspects of the cosmological QCD transition and of astrophysics at high baryon
density.Comment: 13 pages, 4 figures. Invited talk at 'QCD at Finite Baryon Density',
Bielefeld (Germany), April 199
The precision of slow-roll predictions for the CMBR anisotropies
Inflationary predictions for the anisotropy of the cosmic microwave
background radiation (CMBR) are often based on the slow-roll approximation. We
study the precision with which the multipole moments of the temperature
two-point correlation function can be predicted by means of the slow-roll
approximation. We ask whether this precision is good enough for the forthcoming
high precision observations by means of the MAP and Planck satellites. The
error in the multipole moments due to the slow-roll approximation is
demonstrated to be bigger than the error in the power spectrum. For power-law
inflation with the error from the leading order slow-roll
approximation is for the amplitudes and for the
quadrupoles. For the next-to-leading order the errors are within a few percent.
The errors increase with . To obtain a precision of 1% it is
necessary, but in general not sufficient, to use the next-to-leading order. In
the case of power-law inflation this precision is obtained for the spectral
indices if and for the quadrupoles if
only. The errors in the higher multipoles are even larger than those for the
quadrupole, e.g. for l=100, with at the
next-to-leading order. We find that the accuracy of the slow-roll approximation
may be improved by shifting the pivot scale of the primordial spectrum (the
scale at which the slow-roll parameters are fixed) into the regime of acoustic
oscillations. Nevertheless, the slow-roll approximation cannot be improved
beyond the next-to-leading order in the slow-roll parameters.Comment: 3 important additions: 1. discussion of higher multipoles, 2.
comparison of error from the slow-roll approximation with the error from the
cosmic variance, 3. suggestion for improvement of slow-roll approximation;
two figures and a table added; 15 pages, 14 figures, RevTeX; accepted for
publication in Phys. Rev.
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