47 research outputs found
Quartic Chameleons: Safely Scale-Free in the Early Universe
In chameleon gravity, there exists a light scalar field that couples to the
trace of the stress-energy tensor in such a way that its mass depends on the
ambient matter density, and the field is screened in local, high-density
environments. Recently it was shown that, for the runaway potentials commonly
considered in chameleon theories, the field's coupling to matter and the
hierarchy of scales between Standard Model particles and the energy scale of
such potentials result in catastrophic effects in the early Universe when these
particles become nonrelativistic. Perturbations with trans-Planckian energies
are excited, and the theory suffers a breakdown in calculability at the
relatively low temperatures of Big Bang Nucleosynthesis. We consider a
chameleon field in a quartic potential and show that the scale-free nature of
this potential allows the chameleon to avoid many of the problems encountered
by runaway potentials. Following inflation, the chameleon field oscillates
around the minimum of its effective potential, and rapid changes in its
effective mass excite perturbations via quantum particle production. The
quartic model, however, only generates high-energy perturbations at comparably
high temperatures and is able remain a well-behaved effective field theory at
nucleosynthesis.Comment: 13 pages, 5 figures. Updated to match published versio
Comment on "Kinetic decoupling of WIMPs: Analytic expressions"
Visinelli and Gondolo (2015, hereafter VG15) derived analytic expressions for
the evolution of the dark matter temperature in a generic cosmological model.
They then calculated the dark matter kinetic decoupling temperature
and compared their results to the Gelmini and Gondolo (2008,
hereafter GG08) calculation of in an early matter-dominated
era (EMDE), which occurs when the Universe is dominated by either a decaying
oscillating scalar field or a semistable massive particle before Big Bang
nucleosynthesis. VG15 found that dark matter decouples at a lower temperature
in an EMDE than it would in a radiation-dominated era, while GG08 found that
dark matter decouples at a higher temperature in an EMDE than it would in a
radiation-dominated era. VG15 attributed this discrepancy to the presence of a
matching constant that ensures that the dark matter temperature is continuous
during the transition from the EMDE to the subsequent radiation-dominated era
and concluded that the GG08 result is incorrect. We show that the disparity is
due to the fact that VG15 compared in an EMDE to the decoupling
temperature in a radiation-dominated universe that would result in the same
dark matter temperature at late times. Since decoupling during an EMDE leaves
the dark matter colder than it would be if it decoupled during radiation
domination, this temperature is much higher than in a standard
thermal history, which is indeed lower than in an EMDE, as
stated by GG08.Comment: 4 pages, 1 figure; comment on arXiv: 1501.0223
Large-scale anomalies in the cosmic microwave background as signatures of non-Gaussianity
We derive a general expression for the probability of observing deviations
from statistical isotropy in the cosmic microwave background (CMB) if the
primordial fluctuations are non-Gaussian and extend to superhorizon scales. The
primary motivation is to properly characterize the monopole and dipole
modulations of the primordial power spectrum that are generated by the coupling
between superhorizon and subhorizon perturbations. Unlike previous proposals
for generating the hemispherical power asymmetry, we do not assume that the
power asymmetry results from a single large superhorizon mode. Instead, we
extrapolate the observed power spectrum to superhorizon scales and compute the
power asymmetry that would result from a specific realization of non-Gaussian
perturbations on scales larger than the observable universe. Our study
encompasses many of the scenarios that have been put forward as possible
explanations for the CMB hemispherical power asymmetry. We confirm our analytic
predictions for the probability of a given power asymmetry by comparing them to
numerical realizations of CMB maps. We find that non-local models of
non-Gaussianity and scale-dependent local non-Gaussianity produce
scale-dependent modulations of the power spectrum, thereby potentially
producing both a monopolar and a dipolar power modulation on large scales. We
then provide simple examples of finding the posterior distributions for the
parameters of the bispectrum from the observed monopole and dipole modulations.Comment: 21 pages, 11 figures; v2: minor changes to match the PRD accepted
versio
Bringing Isolated Dark Matter Out of Isolation: Late-time Reheating and Indirect Detection
In standard cosmology, the growth of structure becomes significant following
matter-radiation equality. In non-thermal histories, where an effectively
matter-dominated phase occurs due to scalar oscillations prior to Big Bang
Nucleosynthesis, a new scale at smaller wavelengths appears in the matter power
spectrum. Density perturbations that enter the horizon during the
matter-dominated phase grow linearly with the scale factor prior to the onset
of radiation domination, which leads to enhanced inhomogeneity on small scales
if dark matter thermally and kinetically decouples during the matter-dominated
phase. The microhalos that form from these enhanced perturbations significantly
boost the self-annihilation rate for dark matter. This has important
implications for indirect detection experiments: the larger annihilation rate
will result in observable signals from dark matter candidates that are usually
deemed untestable. As a proof of principle, we consider Binos in heavy
supersymmetry with an intermediate extended Higgs sector and all other
superpartners decoupled. We find that these isolated Binos, which lie under the
neutrino floor, can account for the dark matter relic density while also
leading to observable predictions for Fermi-LAT. Current limits on the
annihilation cross section from Fermi-LAT's observations of dwarf spheroidal
galaxies may already constrain Bino dark matter up to masses
GeV, depending on the internal structure of the microhalos. More extensive
constraints are possible with improved gamma-ray bounds and boost calculations
from -body simulations
Rolling in the Modulated Reheating Scenario
In the modulated reheating scenario, the field that drives inflation has a
spatially varying decay rate, and the resulting inhomogeneous reheating process
generates adiabatic perturbations. We examine the statistical properties of the
density perturbations generated in this scenario. Unlike earlier analyses, we
include the dynamics of the field that determines the inflaton decay rate. We
show that the dynamics of this modulus field can significantly alter the
amplitude of the power spectrum and the bispectrum, even if the modulus field
has a simple potential and its effective mass is smaller than the Hubble rate.
In some cases, the evolution of the modulus amplifies the non-Gaussianity of
the perturbations to levels that are excluded by recent observations of the
cosmic microwave background. Therefore, a proper treatment of the modulus
dynamics is required to accurately calculate the statistical properties of the
perturbations generated by modulated reheating.Comment: 27 pages, 11 figures: minor changes made to match version in JCA
A new probe of the small-scale primordial power spectrum: astrometric microlensing by ultracompact minihalos
The dark matter enclosed in a density perturbation with a large initial
amplitude (delta-rho/rho > 1e-3) collapses shortly after recombination and
forms an ultracompact minihalo (UCMH). Their high central densities make UCMHs
especially suitable for detection via astrometric microlensing: as the UCMH
moves, it changes the apparent position of background stars. A UCMH with a mass
larger than a few solar masses can produce a distinctive astrometric
microlensing signal that is detectable by the space astrometry mission Gaia. If
Gaia does not detect gravitational lensing by any UCMHs, then it establishes an
upper limit on their abundance and constrains the amplitude of the primordial
power spectrum for k~2700 Mpc^{-1}. These constraints complement the upper
bound on the amplitude of the primordial power spectrum derived from limits on
gamma-ray emission from UCMHs because the astrometric microlensing signal
produced by an UCMH is maximized if the dark-matter annihilation rate is too
low to affect the UCMH's density profile. If dark matter annihilation within
UCMHs is not detectable, a search for UCMHs by Gaia could constrain the
amplitude of the primordial power spectrum to be less than 1e-5; this bound is
three orders of magnitude stronger than the bound derived from the absence of
primordial black holes.Comment: 17 pages, 6 figures, references added and minor changes made to match
version published in PR
Supermassive Black Hole Merger Rates: Uncertainties from Halo Merger Theory
The merger of two supermassive black holes is expected to produce a
gravitational-wave signal detectable by the satellite LISA. The rate of
supermassive-black-hole mergers is intimately connected to the halo merger
rate, and the extended Press-Schechter formalism is often employed when
calculating the rate at which these events will be observed by LISA. This
merger theory is flawed and provides two rates for the merging of the same pair
of haloes. We show that the two predictions for the LISA
supermassive-black-hole-merger event rate from extended Press-Schechter merger
theory are nearly equal because mergers between haloes of similar masses
dominate the event rate. An alternative merger rate may be obtained by
inverting the Smoluchowski coagulation equation to find the merger rate that
preserves the Press-Schechter halo abundance, but these rates are only
available for power-law power spectra. We compare the LISA event rates derived
from the extended Press-Schechter merger formalism to those derived from the
merger rates obtained from the coagulation equation and find that the extended
Press-Schechter LISA event rates are thirty percent higher for a power spectrum
spectral index that approximates the full Lambda-CDM result of the extended
Press-Schechter theory.Comment: 9 pages, 9 figures. More concise treatment, accepted for publication
in MNRA
New constraints on dark matter production during kination
Our ignorance of the period between the end of inflation and the beginning of
Big Bang Nucleosynthesis limits our understanding of the origins and evolution
of dark matter. One possibility is that the Universe's energy density was
dominated by a fast-rolling scalar field while the radiation bath was hot
enough to thermally produce dark matter. We investigate the evolution of the
dark matter density and derive analytic expressions for the dark matter relic
abundance generated during such a period of kination. Kination scenarios in
which dark matter does not reach thermal equilibrium require to generate the observed
dark matter density while allowing the Universe to become radiation dominated
by a temperature of . Kination scenarios in which dark
matter does reach thermal equilibrium require in order to generate the observed
dark matter abundance. We use observations of dwarf spheroidal galaxies by the
Fermi Gamma-Ray Telescope and observations of the Galactic Center by the High
Energy Stereoscopic System to constrain these kination scenarios. Combining the
unitarity constraint on with these observational
constraints sets a lower limit on the temperature at which the Universe can
become radiation dominated following a period of kination if . This lower limit is
between and , depending on the
dark matter annihilation channel.Comment: 12 pages, 7 figure
Quartic chameleons: Safely scale-free in the early Universe
In chameleon gravity, there exists a light scalar field that couples to the trace of the stress-energy tensor in such a way that its mass depends on the ambient matter density, and the field is screened in local, high-density environments. Recently it was shown that, for the runaway potentials commonly considered in chameleon theories, the field's coupling to matter and the hierarchy of scales between Standard Model particles and the energy scale of such potentials result in catastrophic effects in the early Universe when these particles become nonrelativistic. Perturbations with trans-Planckian energies are excited, and the theory suffers a breakdown in calculability at the relatively low temperatures of Big Bang Nucleosynthesis. We consider a chameleon field in a quartic potential and show that the scale-free nature of this potential allows the chameleon to avoid many of the problems encountered by runaway potentials. Following inflation, the chameleon field oscillates around the minimum of its effective potential, and rapid changes in its effective mass excite perturbations via quantum particle production. The quartic model, however, only generates high-energy perturbations at comparably high temperatures and is able remain a well-behaved effective field theory at nucleosynthesis
The dark matter annihilation boost from low-temperature reheating
The evolution of the Universe between inflation and the onset of big bang nucleosynthesis is difficult to probe and largely unconstrained. This ignorance profoundly limits our understanding of dark matter: we cannot calculate its thermal relic abundance without knowing when the Universe became radiation dominated. Fortunately, small-scale density perturbations provide a probe of the early Universe that could break this degeneracy. If dark matter is a thermal relic, density perturbations that enter the horizon during an early matter-dominated era grow linearly with the scale factor prior to reheating. The resulting abundance of substructure boosts the annihilation rate by several orders of magnitude, which can compensate for the smaller annihilation cross sections that are required to generate the observed dark matter density in these scenarios. In particular, thermal relics with masses less than a TeV that thermally and kinetically decouple prior to reheating may already be ruled out by Fermi-LAT observations of dwarf spheroidal galaxies. Although these constraints are subject to uncertainties regarding the internal structure of the microhalos that form from the enhanced perturbations, they open up the possibility of using gamma-ray observations to learn about the reheating of the Universe