DOI: 10.1103/PhysRevD.105.063525
Résumé:

Recent weak lensing surveys have revealed that the direct measurement of the parameter combination S8≡σ8(Ωm/0.3)0.5S8​≡σ8​(Ωm​/0.3)0.5-- where σ8σ8​ is a measure of the amplitude of matter fluctuations on 8 h−1h−1Mpc scales -- is ∼3σ∼3σ discrepant with the value reconstructed from cosmic microwave background (CMB) data assuming the ΛΛCDM model. In this article, we show that it is possible to resolve the tension if dark matter (DM) decays with a lifetime of Γ−1≃55 GyrsΓ−1≃55 Gyrs into one massless and one massive product, and transfers a fraction ε≃0.7 %ε≃0.7 % of its rest mass energy to the massless component. The velocity-kick received by the massive daughter leads to a suppression of gravitational clustering below its free-streaming length, thereby reducing the σ8σ8​ value as compared to that inferred from the standard ΛΛCDM model, in a similar fashion to massive neutrino and standard warm DM. Contrarily to the latter scenarios, the time-dependence of the power suppression and the free-streaming scale allows the 2-body decaying DM scenario to accommodate CMB, baryon acoustic oscillation, growth factor and un-calibrated supernova Ia data. We briefly discuss implications for DM model building, galactic small-scale structure problems and the recent Xenon-1T excess. Future experiments measuring the growth factor to high accuracy at 0≲z≲10≲z≲1 can further test this scenario.

DOI: 10.1088/1475-7516/2022/10/021
Résumé:

Gamma-ray observations have long been used to constrain the properties of dark matter (DM), with a strong focus on weakly interacting massive particles annihilating through velocity-independent processes. However, in the absence of clear-cut observational evidence for the simplest candidates, the interest of the community in more complex DM scenarios involving a velocity-dependent cross-section has been growing steadily over the past few years. We present the first systematic study of velocity-dependent DM annihilation (in particular $p$-wave annihilation and Sommerfeld enhancement) in a variety of astrophysical objects, not only including the well-studied Milky Way dwarf satellite galaxies, but nearby dwarf irregular galaxies and local galaxy clusters as well. Particular attention is given to the interplay between velocity dependence and DM halo substructure. Uncertainties related to halo mass, phase-space and substructure modelling are also discussed in this velocity-dependent context. We show that, for $s$-wave annihilation, extremely large subhalo boost factors are to be expected, up to $10^{11}$ in clusters and up to $10^6-10^7$ in dwarf galaxies where subhalos are usually assumed not to play an important role. Boost factors for $p$-wave annihilation are smaller but can still reach $10^3$ in clusters. The angular extension of the DM signal is also significantly impacted, with e.g. the cluster typical emission radius increasing by a factor of order 10 in the $s$-wave case. We also compute the signal contrast of the objects in our sample with respect to annihilation happening in the Milky Way halo. Overall, we find that the hierarchy between the brightest considered targets depends on the specific details of the assumed particle-physics model.

Ref HAL: hal-03625359_v1
Ref Arxiv: 2203.16491
Ref INSPIRE: 2060145
DOI: 10.1088/1475-7516/2023/02/004
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote

Ref Arxiv: 2201.02521
DOI: 10.1103/PhysRevD.105.083520
Ref. & Cit.: NASA ADS
Résumé:

The discovery of black-hole-binary mergers through their gravitational wave (GW) emission has reopened the exciting possibility that dark matter is made, at least partly, of primordial black holes (PBHs). However, this scenario is challenged by many observational probes that set bounds on the relative PBH abundance across a broad range of viable PBH masses. Among these bounds, the ones coming from microlensing surveys are particularly severe in the mass range from $\sim 10^{-10}$ to a few M$_{\odot}$. The upper part of this range precisely corresponds to the mass window inside which the formation of PBHs should be boosted due to the QCD phase transition in the early Universe, which makes the microlensing probes particularly important. However, it has been argued that taking into account the inevitable clustering of PBH on small scales can significantly relax or entirely remove these bounds. While the impact of PBH clustering on the GW event rate has been studied in detail, its impact on the microlensing event rate has not yet been fully assessed. In this Letter, we address this issue, and show that clusters arising from PBH formed from Gaussian initial curvature perturbations do not alter the current microlensing constraints, as they are not sufficiently dense nor massive.

Commentaires: 9 pages, 1 figure. Comments are welcome!

Résumé:

The cold dark matter (CDM) scenario predicts that galactic halos should host a huge amount of subhalos possibly as light as or lighter than planets, depending on the nature of dark matter. Predicting their abundance and distribution on such small scales has important implications for dark matter searches and searches for subhalos themselves, which could provide a decisive test of the CDM paradigm. A major difficulty in subhalo population model building is to account for the gravitational stripping induced by baryons, which strongly impact on the overall dynamics within the scale radii of galaxies. In this paper, we focus on these ``baryonic'' tides from analytical perspectives, summarizing previous work on galactic disk shocking, and thoroughly revisiting the impact of individual encounters with stars. For the latter, we go beyond the reference calculation of Gerhard and Fall (1983) to deal with penetrative encounters, and provide new analytical results. Based upon a full statistical analysis of subhalo energy change during multiple stellar encounters possibly occurring during disk crossing, we show how subhalos lighter than $\sim 1$~\msun\ are very efficiently pruned by stellar encounters, and how that modifies their mass function in a stellar environment. If reasonably resilient, surviving subhalos have lost all their mass but the inner cusp, with a tidal mass function strongly departing from the cosmological one; otherwise, their number density can drop by an order of magnitude at the solar position in the Milky Way with respect to disk-shocking effects only. For illustration, we integrate these results into our analytical subhalo population model. They can easily be incorporated to any other analytical or numerical approach. This study complements those using cosmological simulations, which cannot resolve dark matter subhalos on such small scales.

DOI: 10.1088/1475-7516/2021/08/031
Résumé:

Signal predictions for galactic dark matter (DM) searches often rely on assumptions regarding the DM phase-space distribution function (DF) in halos. This applies to both particle (e.g. p-wave suppressed or Sommerfeld-enhanced annihilation, scattering off atoms, etc.) and macroscopic DM candidates (e.g. microlensing of primordial black holes). As experiments and observations improve in precision, better assessing theoretical uncertainties becomes pressing in the prospect of deriving reliable constraints on DM candidates or trustworthy hints for detection. Most reliable predictions of DFs in halos are based on solving the steady-state collisionless Boltzmann equation (e.g. Eddington-like inversions, action-angle methods, etc.) consistently with observational constraints. One can do so starting from maximal symmetries and a minimal set of degrees of freedom, and then increasing complexity. Key issues are then whether adding complexity, which is computationally costy, improves predictions, and if so where to stop. Clues can be obtained by making predictions for zoomed-in hydrodynamical cosmological simulations in which one can access the true (coarse-grained) phase-space information. Here, we test an axisymmetric extension of the Eddington inversion to predict the full DM DF from its density profile and the total gravitational potential of the system. This permits to go beyond spherical symmetry, and is a priori well suited for spiral galaxies. We show that axisymmetry does not necessarily improve over spherical symmetry because the (observationally unconstrained) angular momentum of the DM halo is not generically aligned with the baryonic one. Theoretical errors are similar to those of the Eddington inversion though, at the 10-20% level for velocity-dependent predictions related to particle DM searches in spiral galaxies. We extensively describe the approach and comment on the results.

DOI: 10.1088/1475-7516/2021/08/053
Résumé:

Primordial black holes may have been produced in the early stages of the thermal history of the Universe after cosmic inflation. If so, dark matter in the form of elementary particles can be subsequently accreted around these objects, in particular when it gets non-relativistic and further streams freely in the primordial plasma. A dark matter mini-spike builds up gradually around each black hole, with density orders of magnitude larger than the cosmological one. We improve upon previous work by carefully inspecting the computation of the mini-spike radial profile as a function of black hole mass, dark matter particle mass and temperature of kinetic decoupling. We identify a phase-space contribution that has been overlooked and that leads to changes in the final results. We also derive complementary analytical formulae using convenient asymptotic regimes, which allows us to bring out peculiar power-law behaviors for which we provide tentative physical explanations.