Astrophysics

Increasing evidence for a stochastic gravitational background is being collected at pulsar timing arrays. The most plausible origin of the signal is the cumulative strain from the mergers of supermassive black holes at the center of galaxies across the history of the universe. I will discuss how the impact of dark matter dynamical friction on the black hole binary evolution can address some of the questions that this discovery raises. This includes the solution of the “final parsec problem” by which mergers would otherwise stall before gravitational wave emission can drive the coalesce. I will argue that the observational data favor the existence of dark matter self interactions with a cross-section and velocity dependence consistent with the ones capable of solving the small-scale structure problems of collisionless cold dark matter.

The absence of dark matter signals in direct detection experiments and collider searches has prompted interest in models in which dark matter belongs to a hidden sector minimally coupled to the Standard Model. In these scenarios, a long-lived massive particle might come to dominate the energy density of the early universe temporarily, causing an early matter-dominated era (EMDE) prior to the onset of nucleosynthesis. During an EMDE, matter perturbations grow more rapidly than they would in a period of radiation domination, which leads to the formation of microhalos as early as a redshift of ~5000. These microhalos generate distinct observable signatures, but the constraints on these signatures are highly sensitive to the small-scale cut-off in the matter power spectrum. We discuss the effects of an EMDE on the matter power spectrum, focusing on cases where the particle that dominates the Universe during the EMDE was initially relativistic, and the small-scale cut-off in the power spectrum is set by its pressure support. In addition, we present N-body simulations of the formation and dissipation of microhalos due to an EMDE, which imposes a free-streaming cut-off on the power spectrum after the EMDE. We discuss the implications of this gravitational heating on the (re)formation of microhalos close to the epoch of matter-radiation equality. We constrain these EMDE cosmologies using the observations of the Isotropic Gamma Ray Background and the boosted annihilation rates from the early bound structures resulting from an EMDE. In addition, we discuss prospects for observing these microhalos through pulsar timing arrays and microlensing.

The fundamental nature of dark matter (DM) so far eludes direct detection experiments, but it has left its imprint in the cosmic web. The standard cold DM model is remarkably well tested by cosmic microwave background and low-redshift galaxy surveys, but well-motivated particle candidates like ultra-light axions will leave signatures on small cosmic scales. These signatures are stronger at earlier times. Future 21 cm observations will transform our view of the primordial Universe, but we are already observing some of the first visible tracers of cosmic structure in the high-redshift galaxy population through the Hubble and James Webb Space Telescopes. I will present calculations of the effects of DM candidates like ultra-light axions on the high-z galaxy UV luminosity function. I will then present Hubble and Webb constraints on the allowed fraction of ULAs, accounting for uncertainties in how early galaxies trace the halo population, and discuss the implications for DM solutions to discrepancies in the late-time clustering of matter (S_8 tension).

Primordial black holes (PBHs) have long been considered a promising candidate or an important component of dark matter (DM). Recent gravitational wave (GW) observations of binary black hole (BH) mergers have triggered renewed interest in PBHs in the stellar-mass (∼ 10 − 100 Msun) and supermassive regimes (∼ 10^7 − 10^11 Msun). Although only a small fraction (≲ 1%) of dark matter in the form of PBHs is required to explain observations, these PBHs may play important roles in early structure/star formation. We use cosmological zoom-in simulations and semi-analytical models to explore the possible impact of stellar-mass PBHs on first star formation, taking into account two effects of PBHs: acceleration of structure formation and gas heating by BH accretion feedback. We find that the standard picture of first star formation is not changed by stellar-mass PBHs (allowed by existing observational constraints), and their global impact on the cosmic star formation history is likely minor. However, PBHs do alter the properties of the first star-forming halos and can potentially trigger the formation of direct-collapse BHs in atomic cooling halos. On the other hand, supermassive PBHs may play more important roles as seeds of massive structures that can explain the apparent overabundance of massive galaxies in recent JWST observations. Our tentative models and results call for future studies with improved modeling of the interactions between PBHs, particle DM, and baryons to better understand the effects of PBHs on early structure/star formation and their imprints in high-redshift observations.

Current and upcoming James Webb Space Telescope (JWST) imaging surveys hold enormous potential for uncovering the faint low-mass galaxy population, which could provide constraints on alternative DM (altDM) models. In this talk I will investigate the impact of altDM models that exhibit small-scale suppression of the matter power spectrum, namely warm dark matter (WDM), fuzzy dark matter (FDM), and interacting dark matter (IDM) with strong dark acoustic oscillations (sDAO) on the properties of galaxies in the EoR. In altDM scenarios, both the halo mass functions and the ultraviolet luminosity functions at z ≳ 6 are suppressed at the low-mass/faint end, leading to delayed global star formation and reionization histories. However, strong non-linear effects enable altDM models to 'catch up' with cold dark matter (CDM) in terms of star formation and reionization. The specific star formation rates are enhanced in halos below the half-power mass in altDM models. This enhancement coincides with increased gas abundance, reduced gas depletion times, more compact galaxy sizes, and steeper metallicity gradients at the outskirts of the galaxies. These changes in galaxy properties can help disentangle altDM signatures from a range of astrophysical uncertainties. However, we uncover significant systematic uncertainties in reionization assumptions on the faint-end luminosity function. This underscores the necessity of accurately modeling the small-scale morphology of reionization in making predictions for the low-mass galaxy population.

Cosmology can give insights on the self-interactions of Dark Matter. In this talk we analyze how long range forces acting solely in the Dark Sector imprint on the distribution of galaxies, the so-called Large Scale Structure (LSS). First we show how BOSS data can complement CMB information and give the strongest constraint on the strength of the self interaction. In particular we discuss how the long range force affects the galaxy power spectrum and how the theory of LSS is changed in its presence. Finally we forecast that data from future surveys (Euclid, MegaMapper) can improve the bound by roughly one order of magnitude. We also mention how higher point statistics can potentially contain relevant information on the enhanced clustering.

I describe an analytic, timescale-based model for the formation of the first stars in the center of collapsing primordial gas clouds. Despite its simplicity, the model reproduces the stellar mass scale and its parameter dependences observed in state-of-the-art cosmological zoom-in simulations, while clarifying the essential underlying physics. The model provides an inexpensive tool for studying the influence of exotic dark matter on early star formation.