23 research outputs found
Do primordial Lithium abundances imply there's no Dark Energy?
Explaining the well established observation that the expansion rate of the
universe is apparently accelerating is one of the defining scientific problems
of our age. Within the standard model of cosmology, the repulsive 'dark energy'
supposedly responsible has no explanation at a fundamental level, despite many
varied attempts. A further important dilemma in the standard model is the
Lithium problem, which is the substantial mismatch between the theoretical
prediction for 7-Li from Big Bang Nucleosynthesis and the value that we observe
today. This observation is one of the very few we have from along our past
worldline as opposed to our past lightcone. By releasing the untested
assumption that the universe is homogeneous on very large scales, both apparent
acceleration and the Lithium problem can be easily accounted for as different
aspects of cosmic inhomogeneity, without causing problems for other
cosmological phenomena such as the cosmic microwave background. We illustrate
this in the context of a void model.Comment: 14 pages, 4 figures. v2: minor rearrangements in the text, comments
and references expanded, results unchange
Primordial black hole production due to preheating
During the preheating process at the end of inflation the amplification of
field fluctuations can lead to the amplification of curvature perturbations. If
the curvature perturbations on small scales are sufficiently large, primordial
black holes (PBHs) will be overproduced. In this paper we study PBH production
in the two-field preheating model with quadratic inflaton potential. We show
that for many values of the inflaton mass m, and coupling g, small scale
perturbations will be amplified sufficiently, before backreaction can shut off
preheating, so that PBHs will be overproduced during the subsequent radiation
dominated era.Comment: 5 pages, 3 eps figures. Minor changes to match version to appear in
PRD as a rapid communicatio
Testing homogeneity with the fossil record of galaxies
The standard Friedmann model of cosmology is based on the Copernican
Principle, i.e. the assumption of a homogeneous background on which structure
forms via perturbations. Homogeneity underpins both general relativistic and
modified gravity models and is central to the way in which we interpret
observations of the CMB and the galaxy distribution. It is therefore important
to probe homogeneity via observations. We describe a test based on the fossil
record of distant galaxies: if we can reconstruct key intrinsic properties of
galaxies as functions of proper time along their worldlines, we can compare
such properties at the same proper time for our galaxy and others. We achieve
this by computing the lookback time using radial Baryon Acoustic Oscillations,
and the time along galaxy world line using stellar physics, allowing us to
probe homogeneity, in principle anywhere inside the past light cone. Agreement
in the results would be an important consistency test -- although it would not
in itself prove homogeneity. Any significant deviation in the results however
would signal a breakdown of homogeneity.Comment: Accepted for publication in JCAP. Matches published version. Minor
changes: ref. added and longer discussion on performing the test
observationally. Results unchange
Cosmological background solutions and cosmological backreactions
The cosmological backreaction proposal, which attempts to account for
observations without a primary dark energy source in the stress-energy tensor,
has been developed and discussed by means of different approaches. Here, we
focus on the concept of cosmological background solutions in order to develop a
framework to study different backreaction proposals.Comment: 14 pages, 5 figures; major changes, replaced to match the version
published in General Relativity and Gravitatio
Dark energy as a mirage
Motivated by the observed cosmic matter distribution, we present the
following conjecture: due to the formation of voids and opaque structures, the
average matter density on the path of the light from the well-observed objects
changes from Omega_M ~ 1 in the homogeneous early universe to Omega_M ~ 0 in
the clumpy late universe, so that the average expansion rate increases along
our line of sight from EdS expansion Ht ~ 2/3 at high redshifts to free
expansion Ht ~ 1 at low redshifts. To calculate the modified observable
distance-redshift relations, we introduce a generalized Dyer-Roeder method that
allows for two crucial physical properties of the universe: inhomogeneities in
the expansion rate and the growth of the nonlinear structures. By treating the
transition redshift to the void-dominated era as a free parameter, we find a
phenomenological fit to the observations from the CMB anisotropy, the position
of the baryon oscillation peak, the magnitude-redshift relations of type Ia
supernovae, the local Hubble flow and the nucleosynthesis, resulting in a
concordant model of the universe with 90% dark matter, 10% baryons, no dark
energy, 15 Gyr as the age of the universe and a natural value for the
transition redshift z_0=0.35. Unlike a large local void, the model respects the
cosmological principle, further offering an explanation for the late onset of
the perceived acceleration as a consequence of the forming nonlinear
structures. Additional tests, such as quantitative predictions for angular
deviations due to an anisotropic void distribution and a theoretical derivation
of the model, can vindicate or falsify the interpretation that light
propagation in voids is responsible for the perceived acceleration.Comment: 33 pages, 2 figs; v2: minor clarifications, results unchanged; v3:
matches the version published in General Relativity and Gravitatio
Density correlations and dynamical Casimir emission of Bogoliubov phonons in modulated atomic Bose-Einstein condensates
We present a theory of the density correlations that appear in an atomic
Bose-Einstein condensate as a consequence of the dynamical Casimir emission of
pairs of Bogoliubov phonons when the atom-atom scattering length is modulated
in time. Different regimes as a function of the temporal shape of the
modulation are identified and a simple physical picture of the phenomenon is
discussed. Analytical expressions for the density correlation function are
provided for the most significant limiting cases. This theory is able to
explain some unexpected features recently observed in numerical calculations of
Hawking radiation from analog black holes