48 research outputs found
Constraining dark matter decays with cosmic microwave background and weak lensing shear observations
From observations at low and high redshifts, it is well known that the bulk
of dark matter (DM) has to be stable or at least very long-lived. However, the
possibility that a small fraction of DM is unstable or that all DM decays with
a half-life time () significantly longer than the age of the Universe is
not ruled out. One-body decaying dark matter (DDM) consists of a minimal
extension to the CDM model. It causes a modification of the cosmic
growth history as well as a suppression of the small-scale clustering signal,
providing interesting consequences regarding the tension, which is the
observed difference in the clustering amplitude between weak-lensing (WL) and
cosmic microwave background (CMB) observations. In this paper, we investigate
models in which a fraction or all DM decays into radiation, focusing on the
long-lived regime, that is, ( being the
Hubble time). We used WL data from the Kilo-Degree Survey (KiDS) and CMB data
from Planck. First, we confirm that this DDM model cannot alleviate the
difference. We then show that the most constraining power for DM decay does not
come from the nonlinear WL data, but from CMB via the integrated Sachs-Wolfe
effect. From the CMB data alone, we obtain constraints of ~Gyr
if all DM is assumed to be unstable, and we show that a maximum fraction of
is allowed to decay assuming the half-life time to be comparable to
(or shorter than) one Hubble time. The constraints from the KiDS-1000 WL data
are significantly weaker, ~Gyr and . Combining the CMB
and WL data does not yield tighter constraints than the CMB alone, except for
short half-life times, for which the maximum allowed fraction becomes .
All limits are provided at the 95% confidence level
Cosmological forecast of the 21-cm power spectrum using the halo model of reionization
The 21-cm power spectrum of reionization is a promising probe for cosmology
and fundamental physics. Exploiting this new observable, however, requires fast
predictors capable of efficiently scanning the very large parameter space of
cosmological and astrophysical uncertainties. In this paper, we introduce the
halo model of reionization (HMreio), a new analytical tool that combines the
halo model of the cosmic dawn with the excursion-set bubble model for
reionization, assuming an empirical correction factor to deal with overlapping
ionization bubbles. First, HMreio is validated against results from the
well-known semi-numerical code 21cmFAST, showing a good overall agreement for
wave-modes of h/Mpc. Based on this result, we perform a
Monte-Carlo Markov-Chain (MCMC) forecast analysis assuming mock data from
1000-hour observations with the low-frequency part of the Square Kilometre
Array (SKA) observatory. We simultaneously vary the six standard cosmological
parameters together with seven astrophysical nuisance parameters quantifying
the abundance and spectral properties of sources. Depending on the assumed
theory error, we find very competitive constraints on cosmological parameters.
In particular, it will be possible to conclusively test current cosmological
tensions related to the Hubble parameter (-tension) and the matter
clustering amplitude (-tension). Furthermore, the sum of the neutrino
masses can be strongly constrained, making it possible to determine the
neutrino mass hierarchy at the percent confidence level. However,
these goals can only be achieved if the current modelling uncertainties are
substantially reduced to below percent.Comment: 18 pages, 8 figures, comments welcom
BEoRN: A fast and flexible framework to simulate the epoch of reionisation and cosmic dawn
In this study, we introduce BEoRN (Bubbles during the Epoch of Reionisation
Numerical Simulator), a publicly available Python code that generates
three-dimensional maps of the 21-cm signal from the cosmic dawn and the epoch
of reionisation. Built upon N-body simulation outputs, BEoRN populates haloes
with stars and galaxies based on a flexible source model. It then computes the
evolution of Lyman- coupling, temperature, and ionisation profiles as a
function of source properties, and paints these profiles around each source
onto a three-dimensional grid. The code consistently deals with the overlap of
ionised bubbles by redistributing photons around the bubble boundaries, thereby
ensuring photon conservation. It accounts for the redshifting of photons and
the source look-back effect for the temperature and Lyman- coupling
profiles which extend far into the intergalactic medium to scales of order 100
cMpc. We provide a detailed description of the code and compare it to results
from the literature. After validation, we run three different benchmark models
based on a cosmological N-body simulation. All three models agree with current
observations from UV luminosity functions and estimates of the mean ionisation
fraction. Due to different assumptions regarding the small-mass stellar-to-halo
relation, the X-ray flux emission, and the ionising photon escape fraction, the
models produce unique signatures ranging from a cold reionisation with deep
absorption trough to an emission-dominated 21-cm signal, broadly encompassing
the current uncertainties at cosmic dawn. The code BEoRN is publicly available
at https://github.com/cosmic-reionization/BEoRN
Halo model approach for the 21-cm power spectrum at cosmic dawn
Prior to the epoch of reionization, the 21-cm signal of the cosmic dawn is dominated by the Lyman-α coupling and gas temperature fluctuations caused by the first sources of radiation. While early efforts to model this epoch relied on analytical techniques, the community quickly transitioned to more expensive seminumerical models. Here, we reassess the viability of simpler approaches that allow for rapid explorations of the vast astrophysical parameter space. We propose a new analytical method to calculate the 21-cm power spectrum based on the framework of the halo model. Both the Lyman-α coupling and temperature fluctuations are described by overlapping radiation flux profiles that include spectral redshifting and source attenuation due to look-back (light-cone) effects. The 21-cm halo model is compared to the seminumerical code 21 cmFAST exhibiting generally good agreement, i.e., the power spectra differ by less than a factor of 3 over a large range of k-modes and redshifts. We show that the remaining differences between the two methods are comparable to the expected variations from modeling uncertainties associated with the abundance, bias, and accretion rates of haloes. While these current uncertainties must be reduced in the future, our work suggests that inference at acceptable accuracy will become feasible with very efficient halo models of the cosmic dawn
Probing the two-body decaying dark matter scenario with weak lensing and the cosmic microwave background
Decaying dark matter (DDM) scenarios have recently re-gained attention due to
their potential ability to resolve the well-known clustering (or ) tension
between weak lensing (WL) and cosmic microwave background (CMB) measurements.
In this paper, we investigate a well-established model, where the original dark
matter (DM) particle decays into a massless and a massive daughter particles.
The latter obtains a velocity kick during the decay process resulting in a
suppression of the matter power spectrum at scales that are observable with WL
shear observations. We perform the first fully nonlinear WL analysis of this
two-body decaying dark matter (DDM) scenario including intrinsic
alignment and baryonic feedback processes. We thereby use the cosmic shear band
power spectra from the KiDS-1000 data combining it with temperature and
polarization data from Planck to constrain the DDM model. We report
new limits on the decay rate and mass splitting parameters that are
significantly stronger than previous results, especially for the case of low
mass splittings. We also investigate the tension only finding a marginal
improvement of 0.3 for DDM compared to the CDM case.
The improvement is not caused by a shift but a slight bloating of the posterior
contours caused by the additional free model parameters. We therefore conclude
that the two-body DDM model does not provide a convincing solution to
the tension. Our emulator to model the nonlinear DDM power
spectrum is published as part of the publicly available code DMemu at
https://github.com/jbucko/DMemu.Comment: 16 pages, 13 figure
Starbursts in low-mass haloes at Cosmic Dawn. I. The critical halo mass for star formation
The first stars, galaxies, star clusters, and direct-collapse black holes are
expected to have formed in low-mass () haloes
at Cosmic Dawn () under conditions of efficient gas cooling,
leading to gas collapse towards the centre of the halo. The halo mass cooling
threshold has been analyzed by several authors using both analytical models and
numerical simulations, with differing results. Since the halo number density is
a sensitive function of the halo mass, an accurate model of the cooling
threshold is needed for (semi-)analytical models of star formation at Cosmic
Dawn. In this paper the cooling threshold mass is calculated
(semi-)analytically, considering the effects of H-cooling and formation (in
the gas phase and on dust grains), cooling by atomic metals, Lyman-
cooling, photodissociation of H by Lyman-Werner photons (including
self-shielding by H), photodetachment of H by infrared photons,
photoevaporation by ionization fronts, and the effect of baryon streaming
velocities. We compare the calculations to several high-resolution cosmological
simulations, showing excellent agreement. We find that in regions of typical
baryon streaming velocities, star formation is possible in haloes of mass
for . By , the
expected Lyman-Werner background suppresses star formation in all minihaloes
below the atomic-cooling threshold (). The
halo mass cooling threshold increases by another factor of following
reionization, although this effect is slightly delayed () because
of effective self-shielding.Comment: 21 pages, 10 figures. Comments are welcom
Position-dependent power spectra of the 21-cm signal from the epoch of reionization
The 21-cm signal from the epoch of reionization is non-Gaussian. Current
radio telescopes are focused on detecting the 21-cm power spectrum, but in the
future the Square Kilometre Array is anticipated to provide a first measurement
of the bispectrum. Previous studies have shown that the position-dependent
power spectrum is a simple and efficient way to probe the squeezed-limit
bispectrum. In this approach, the survey is divided into subvolumes and the
correlation between the local power spectrum and the corresponding mean density
of the subvolume is computed. This correlation is equivalent to an integral of
the bispectrum in the squeezed limit, but is much simpler to implement than the
usual bispectrum estimators. It also has a clear physical interpretation: it
describes how the small-scale power spectrum of tracers such as galaxies and
the 21-cm signal respond to a large-scale environment. Reionization naturally
couples large and small scales as ionizing radiation produced by galactic
sources can travel up to tens of Megaparsecs through the intergalactic medium
during this process. Here we apply the position-dependent power spectrum
approach to fluctuations in the 21-cm background from reionization. We show
that this statistic has a distinctive evolution in time that can be understood
with a simple analytic model. We also show that the statistic can easily
distinguish between simple "inside-out" and "outside-in" models of
reionization. The position-dependent power spectrum is thus a promising method
to validate the reionization signal and to extract higher-order information on
this process.Comment: 24 pages, 10 figures, accepted in JCA