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Register a new userLinear scale bounds on dark matter--dark radiation interactions and connection with the small scale crisis of cold dark matter
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One of the open questions in modern cosmology is the small scale crisis of the cold dark matter paradigm. Increasing attention has recently been devoted to self-interacting dark matter models as a possible answer. However, solving the so-called missing satellites problem requires in addition the presence of an extra relativistic particle (dubbed dark radiation) scattering with dark matter in the early universe. Here we investigate the impact of different theoretical models devising dark matter dark radiation interactions on large scale cosmological observables. We use cosmic microwave background data to put constraints on the dark radiation component and its coupling to dark matter. We find that the values of the coupling allowed by the data imply a cut-off scale of the halo mass function consistent with the one required to match the observations of satellites in the Milky Way.
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Dark matter interactions with electrons or protons during the early Universe leave imprints on the cosmic microwave background and the matter power spectrum, and can be probed through cosmological and astrophysical observations. We explore these interactions using a diverse suite of data: cosmic microwave background anisotropies, baryon acoustic oscillations, the Lyman-$alpha$ forest, and the abundance of Milky-Way subhalos. We derive constraints using model-independent parameterizations of the dark matter--electron and dark matter--proton interaction cross sections and map these constraints onto concrete dark matter models. Our constraints are complementary to other probes of dark matter interactions with ordinary matter, such as direct detection, big bang nucleosynthesis, various astrophysical systems, and accelerator-based experiments.
Cosmological perturbations of sufficiently long wavelength admit a fluid dynamic description. We consider modes with wavevectors below a scale $k_m$ for which the dynamics is only mildly non-linear. The leading effect of modes above that scale can be accounted for by effective non-equilibrium viscosity and pressure terms. For mildly non-linear scales, these mainly arise from momentum transport within the ideal and cold but inhomogeneous fluid, while momentum transport due to more microscopic degrees of freedom is suppressed. As a consequence, concrete expressions with no free parameters, except the matching scale $k_m$, can be derived from matching evolution equations to standard cosmological perturbation theory. Two-loop calculations of the matter power spectrum in the viscous theory lead to excellent agreement with $N$-body simulations up to scales $k=0.2 , h/$Mpc. The convergence properties in the ultraviolet are better than for standard perturbation theory and the results are robust with respect to variations of the matching scale.
Several interesting Dark Matter (DM) models invoke a dark sector leading to two types of relic particles, possibly interacting with each other: non-relativistic DM, and relativistic Dark Radiation (DR). These models have interesting consequences for cosmological observables, and could in principle solve problems like the small-scale cold DM crisis, Hubble tension, and/or low $sigma_8$ value. Their cosmological behaviour is captured by the ETHOS parametrisation, which includes a DR-DM scattering rate scaling like a power-law of the temperature, $T^n$. Scenarios with $n=0$, $2$, or $4$ can easily be realised in concrete dark sector set-ups. Here we update constraints on these three scenarios using recent CMB, BAO, and high-resolution Lyman-$alpha$ data. We introduce a new Lyman-$alpha$ likelihood that is applicable to a wide range of cosmological models with a suppression of the matter power spectrum on small scales. For $n=2$ and $4$, we find that Lyman-$alpha$ data strengthen the CMB+BAO bounds on the DM-DR interaction rate by many orders of magnitude. However, models offering a possible solution to the missing satellite problem are still compatible with our new bounds. For $n=0$, high-resolution Lyman-$alpha$ data bring no stronger constraints on the interaction rate than CMB+BAO data, except for extremely small values of the DR density. Using CMB+BAO data and a theory-motivated prior on the minimal density of DR, we find that the $n=0$ model can reduce the Hubble tension from $4.1sigma$ to $2.7sigma$, while simultaneously accommodating smaller values of the $sigma_8$ and $S_8$ parameters hinted by cosmic shear data.
A scenario for the cosmological evolution of self-interacting Bose-Einstein condensed (SIBEC) dark matter (DM) as the final product of a transition from an initial cold DM (CDM)-like phase is considered, motivated by suggestions in the literature that a cold DM gas might have undergone a Bose-Einstein condensate phase transition. The phenomenological model employed for the cold-SIBEC transition introduces three additional parameters to those already present in $Lambda$CDM; the strength of the DM self-interaction in the SIBEC phase, the time of the transition, and the rate of the transition. Constraints on these extra parameters are obtained from large-scale observables, using the cosmic microwave background (CMB), baryonic acoustic oscillations (BAO) and growth factor measurements, and type Ia supernovae (SNIa) distances. The standard cosmological parameters are found to be unchanged from $Lambda$CDM, and upper bounds on the SIBEC-DM self-interaction for the various transition times and rates are obtained. If, however, SIBEC-DM is responsible for the tendency of low-mass halos to be cored rather than cuspy, then cold-SIBEC transition times around matter-radiation equality and earlier are ruled out.
We examine the capability of pulsar timing arrays (PTAs) to detect very small-scale clumps of dark matter (DM), which are a natural outcome of the standard cold dark matter (CDM) paradigm. A clump streaming near the Earth or a pulsar induces an impulsive acceleration to encode residuals on pulsar timing data. We show that, assuming the standard abundance of DM clumps predicted by the CDM model, small-scale DM clumps with masses from $sim 10^{-11} M_odot$ to $sim 10^{-8} M_odot$ can be detectable by a PTA observation for a few decades with ${cal O}(100)$ of pulsars with a timing noise of ${cal O}(10)$ ns located at $gtrsim 3$ kpc away from the Galactic center, as long as these mass scales are larger than the cutoff scale of the halo mass function that is determined by the particle nature of DM. Our result suggests that PTAs can provide a unique opportunity for testing one of the most fundamental predictions of the CDM paradigm. In addition, the detections and non-detections can constrain the cutoff mass scale inherent to the DM model.
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