93 research outputs found
Improved constraints on the primordial power spectrum at small scales from ultracompact minihalos
For a Gaussian spectrum of primordial density fluctuations, ultracompact
minihalos (UCMHs) of dark matter are expected to be produced in much greater
abundance than, e.g., primordial black holes. Forming shortly after
matter-radiation equality, these objects would develop very dense and spiky
dark matter profiles. In the standard scenario where dark matter consists of
thermally-produced, weakly-interacting massive particles, UCMHs could thus
appear as highly luminous gamma-ray sources, or leave an imprint in the cosmic
microwave background by changing the reionisation history of the Universe. We
derive corresponding limits on the cosmic abundance of UCMHs at different
epochs, and translate them into constraints on the primordial power spectrum.
We find the resulting constraints to be quite severe, especially at length
scales much smaller than what can be directly probed by the cosmic microwave
background or large-scale structure observations. We use our results to provide
an updated compilation of the best available constraints on the power of
density fluctuations on all scales, ranging from the present-day horizon to
scales more than 20 orders of magnitude smaller.Comment: 7 figures, 14 pages + appendices. v2 matches version accepted for
publication in PRD; updated to WMAP normalisation, updated reionisation
limits, various other small changes. v3 slightly corrects the normalisation
used for displaying past data in Fig 6, as well as a sign typo picked up in
proof in Eq 26. All results and conclusions completely unchange
Uncertainties in primordial black-hole constraints on the primordial power spectrum
The existence (and abundance) of primordial black holes (PBHs) is governed by
the power spectrum of primordial perturbations generated during inflation. So
far no PBHs have been observed, and instead, increasingly stringent bounds on
their existence at different scales have been obtained. Up until recently, this
has been exploited in attempts to constrain parts of the inflationary power
spectrum that are unconstrained by cosmological observations. We first point
out that the simple translation of the PBH non-observation bounds into
constraints on the primordial power spectrum is inaccurate as it fails to
include realistic aspects of PBH formation and evolution. We then demonstrate,
by studying two examples of uncertainties from the effects of critical and
non-spherical collapse, that even though they may seem small, they have
important implications for the usefulness of the constraints. In particular, we
point out that the uncertainty induced by non-spherical collapse may be much
larger than the difference between particular bounds from PBH non-observations
and the general maximum cap stemming from the condition on the
dark-matter density in the form of PBHs. We therefore make the cautious
suggestion of applying only the overall maximum dark-matter constraint to
models of early Universe, as this requirement seems to currently provide a more
reliable constraint, which better reflects our current lack of detailed
knowledge of PBH formation. These, and other effects, such as merging,
clustering and accretion, may also loosen constraints from non-observations of
other primordial compact objects such as ultra-compact minihalos of dark
matter.Comment: 6 pages, 2 figures; v4: revised version to match published versio
The landscape, the swampland and the era of precision cosmology
We review the advanced version of the KKLT construction and pure de
Sitter supergravity, involving a nilpotent multiplet, with regard to various
conjectures that de Sitter state cannot exist in string theory. We explain why
we consider these conjectures problematic and not well motivated, and why the
recently proposed alternative string theory models of dark energy, ignoring
vacuum stabilization, are ruled out by cosmological observations at least at
the level, i.e. with more than confidence.Comment: 48 pages, 10 figures. v2: Improved version; discussions added, typos
fixed, structure modified, appendix added on two-field scenarios, note added
in response to arXiv:1809.00154. v3: Published versio
On nonlocally interacting metrics, and a simple proposal for cosmic acceleration
We propose a simple, nonlocal modification to general relativity (GR) on
large scales, which provides a model of late-time cosmic acceleration in the
absence of the cosmological constant and with the same number of free
parameters as in standard cosmology. The model is motivated by adding to the
gravity sector an extra spin-2 field interacting nonlocally with the physical
metric coupled to matter. The form of the nonlocal interaction is inspired by
the simplest form of the Deser-Woodard (DW) model, ,
with one of the Ricci scalars being replaced by a constant , and gravity
is therefore modified in the infrared by adding a simple term of the form
to the Einstein-Hilbert term. We study cosmic expansion
histories, and demonstrate that the new model can provide background expansions
consistent with observations if is of the order of the Hubble expansion
rate today, in contrast to the simple DW model with no viable cosmology. The
model is best fit by and . We also compare the
cosmology of the model to that of Maggiore and Mancarella (MM),
, and demonstrate that the viable cosmic histories
follow the standard-model evolution more closely compared to the MM model. We
further demonstrate that the proposed model possesses the same number of
physical degrees of freedom as in GR. Finally, we discuss the appearance of
ghosts in the local formulation of the model, and argue that they are
unphysical and harmless to the theory, keeping the physical degrees of freedom
healthy.Comment: 47 pages in JCAP style, 7 figures. Some discussions extended in
response to referee's comments. Version accepted for publication in JCA
Dark energy, -attractors, and large-scale structure surveys
Over the last few years, a large family of cosmological attractor models has
been discovered, which can successfully match the latest inflation-related
observational data. Many of these models can also describe a small cosmological
constant , which provides the most natural description of the present
stage of the cosmological acceleration. In this paper, we study
-attractor models with dynamical dark energy, including the
cosmological constant as a free parameter. Predominantly, the models
with converge to the asymptotic regime with the equation of state
. However, there are some models with , which are compatible
with the current observations. In the simplest models with , one
has the tensor to scalar ratio and the asymptotic
equation of state (which in general differs from its
present value). For example, in the seven disk M-theory related model with
one finds and the asymptotic equation of state
is . Future observations, including large-scale structure surveys
as well as B-mode detectors will test these, as well as more general models
presented here. We also discuss gravitational reheating in models of
quintessential inflation and argue that its investigation may be interesting
from the point of view of inflationary cosmology. Such models require a much
greater number of -folds, and therefore predict a spectral index
that can exceed the value in more conventional models by about . This
suggests a way to distinguish the conventional inflationary models from the
models of quintessential inflation, even if they predict .Comment: 61 pages, 27 figures. v3: Improved version in response to referee's
comments; added references, expanded discussion, moved some results to an
appendix; conclusions unchange
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