39 research outputs found
Cosmological Limits on Hidden Sector Dark Matter
We explore the model-independent constraints from cosmology on a dark-matter
particle with no prominent standard model interactions that interacts and
thermalizes with other particles in a hidden sector. Without specifying
detailed hidden-sector particle physics, we characterize the relevant physics
by the annihilation cross section, mass, and temperature ratio of the hidden to
visible sectors. While encompassing the standard cold WIMP scenario, we do not
require the freeze-out process to be nonrelativistic. Rather, freeze-out may
also occur when dark matter particles are semirelativistic or relativistic. We
solve the Boltzmann equation to find the conditions that hidden-sector dark
matter accounts for the observed dark-matter density, satisfies the
Tremaine-Gunn bound on dark-matter phase space density, and has a
free-streaming length consistent with cosmological constraints on the matter
power spectrum. We show that for masses <1.5 keV no region of parameter space
satisfies all these constraints. This is a gravitationally-mediated lower bound
on the dark-matter mass for any model in which the primary component of dark
matter once had efficient interactions -- even if it has never been in
equilibrium with the standard model.Comment: 8 pages, 6 figures, 1 table; References added, Eq. 16 corrected, and
appendix with surface of allowed dark-matter abundance adde
Dark Matter Decaying into a Fermi Sea of Neutrinos
We study the possible decay of a coherently oscillating scalar field,
interpreted as dark matter, into light fermions. Specifically, we consider a
scalar field with sub-eV mass decaying into a Fermi sea of neutrinos. We
recognize the similarity between our scenario and inflationary preheating where
a coherently oscillating scalar field decays into standard model particles.
Like the case of fermionic preheating, we find that Pauli blocking controls the
dark matter decay into the neutrino sea. The radius of the Fermi sphere depends
on the expansion of the universe leading to a time varying equation of state of
dark matter. This makes the scenario very rich and we show that the decay rate
might be different at different cosmological epochs. We categorize this in two
interesting regimes and then study the cosmological perturbations to find the
impact on structure formation. We find that the decay may help alleviating some
of the standard problems related to cold dark matter.Comment: 8 pages, 3 figures, accepted for publication in Phys. Rev.
How CMB and large-scale structure constrain chameleon interacting dark energy
We explore a chameleon type of interacting dark matter-dark energy scenario
in which a scalar field adiabatically traces the minimum of an effective
potential sourced by the dark matter density. We discuss extensively the effect
of this coupling on cosmological observables, especially the parameter
degeneracies expected to arise between the model parameters and other
cosmological parameters, and then test the model against observations of the
cosmic microwave background (CMB) anisotropies and other cosmological probes.
We find that the chameleon parameters and , which determine
respectively the slope of the scalar field potential and the dark matter-dark
energy coupling strength, can be constrained to and using CMB data alone. The latter parameter in particular is constrained
only by the late Integrated Sachs-Wolfe effect. Adding measurements of the
local Hubble expansion rate tightens the bound on by a factor of
two, although this apparent improvement is arguably an artefact of the tension
between the local measurement and the value inferred from Planck data in
the minimal CDM model. The same argument also precludes chameleon
models from mimicking a dark radiation component, despite a passing similarity
between the two scenarios in that they both delay the epoch of matter-radiation
equality. Based on the derived parameter constraints, we discuss possible
signatures of the model for ongoing and future large-scale structure surveys.Comment: 25 pages, 6 figure
Early Dark Energy beyond slow-roll: implications for cosmic tensions
In this work, we explore the possibility that Early Dark Energy (EDE) is
dynamical in nature and study its effect on cosmological observables. We
introduce a parameterization of the equation of state allowing for an equation
of state differing considerably from cosmological constant (cc, )
and vary both the initial as well final equation of state of the
EDE fluid. This idea is motivated by the fact that in many models of EDE, the
scalar field may have some kinetic energy when it starts to behave like EDE
before the CMB decoupling. We find that the present data have a mild preference
for non-cc early dark energy using Planck+BAO+Pantheon+SES
data sets, leading to improvement of -2.5 at the
expense of one more parameter. However, is only weakly constrained, with
at . We argue that allowing for can play a
role in decreasing the parameter. Yet, in practice the decrease is
only and is still larger than weak lensing
measurements. We conclude that while promising, a dynamical EDE cannot resolve
both and tensions simultaneously.Comment: 8 figures, Comments are most welcom
Small scale clustering of late forming dark matter
We perform a study of the nonlinear clustering of matter in the late-forming
dark matter (LFDM) scenario in which dark matter results from the transition of
a nonminimally coupled scalar field from radiation to collisionless matter. A
distinct feature of this model is the presence of a damped oscillatory cutoff
in the linear matter power spectrum at small scales. We use a suite of
high-resolution N-body simulations to study the imprints of LFDM on the
nonlinear matter power spectrum, the halo mass and velocity functions and the
halo density profiles. The model largely satisfies high-redshift matter power
spectrum constraints from Lyman- forest measurements, while it predicts
suppressed abundance of low-mass halos ( h
M) at all redshifts compared to a vanilla CDM model. The
analysis of the LFDM halo velocity function shows a better agreement than the
CDM prediction with the observed abundance of low-velocity galaxies in
the local volume. Halos with mass h M show
minor departures of the density profiles from CDM expectations, while
smaller-mass halos are less dense, consistent with the fact that they form
later than their CDM counterparts.Comment: 13 pages, 7 figures, 1 table, added analysis from higher resolution
simulation