911,395 research outputs found
Inhomogeneity and the foundations of concordance cosmology
The apparent accelerating expansion of the Universe is forcing us to examine
the foundational aspects of the standard model of cosmology -- in particular,
the fact that dark energy is a direct consequence of the homogeneity
assumption. We discuss the foundations of the assumption of spatial
homogeneity, in the case when the Copernican Principle is adopted. We present
results that show how (almost-) homogeneity follows from (almost-) isotropy of
various observables. The analysis requires the fully nonlinear field equations
-- i.e., it is not possible to use second- or higher-order perturbation theory,
since one cannot assume a homogeneous and isotropic background. Then we
consider what happens if the Copernican Principle is abandoned in our Hubble
volume. The simplest models are inhomogeneous but spherically symmetric
universes which do not require dark energy to fit the distance modulus. Key
problems in these models are to compute the CMB anisotropies and the features
of large-scale structure. We review how to construct perturbation theory on a
non-homogeneous cosmological background, and discuss the complexities that
arise in using this to determine the growth of large-scale structure.Comment: 26 pages and 1 figure. Invited review article for the CQG special
issue on nonlinear cosmological perturbations. v2 has additional refs and
comments, minor errors corrected, version in CQ
Inhomogeneity effects in Cosmology
This article looks at how inhomogeneous spacetime models may be significant
for cosmology. First it looks at how the averaging process may affect large
scale dynamics, with backreaction effects leading to effective contributions to
the averaged energy-momentum tensor. Secondly it considers how local
inhomogeneities may affect cosmological observations in cosmology, possibly
significantly affecting the concordance model parameters. Thirdly it presents
the possibility that the universe is spatially inhomogeneous on Hubble scales,
with a violation of the Copernican principle leading to an apparent
acceleration of the universe. This could perhaps even remove the need for the
postulate of dark energy.Comment: 29 pages. For special issue of CQG on inhomogeneous cosmologie
Non-Archimedean character of quantum buoyancy and the generalized second law of thermodynamics
Quantum buoyancy has been proposed as the mechanism protecting the
generalized second law when an entropy--bearing object is slowly lowered
towards a black hole and then dropped in. We point out that the original
derivation of the buoyant force from a fluid picture of the acceleration
radiation is invalid unless the object is almost at the horizon, because
otherwise typical wavelengths in the radiation are larger than the object. The
buoyant force is here calculated from the diffractive scattering of waves off
the object, and found to be weaker than in the original theory. As a
consequence, the argument justifying the generalized second law from buoyancy
cannot be completed unless the optimal drop point is next to the horizon. The
universal bound on entropy is always a sufficient condition for operation of
the generalized second law, and can be derived from that law when the optimal
drop point is close to the horizon. We also compute the quantum buoyancy of an
elementary charged particle; it turns out to be negligible for energetic
considerations. Finally, we speculate on the significance of the absence from
the bound of any mention of the number of particle species in nature.Comment: RevTeX, 16 page
Relativistic quantum theories and neutrino oscillations
Neutrino oscillations are examined under the broad requirements of
Poincar\'e-invariant scattering theory in an S-matrix formulation.
This approach can be consistently applied to theories with either field or
particle degrees of freedom. The goal of this paper is to use this general
framework to identify all of the unique physical properties of this problem
that lead to a simple oscillation formula. We discuss what is in principle
observable, and how many factors that are important in principle end up being
negligible in practice.Comment: 21 pages, no figure
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