Cosmology

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.

Recent work on the formation of the first dark matter halos suggests they may have had systematically different density profiles, with stronger cusps and higher densities than those predicted at low redshift. I will review recent work on the first generation(s) of dark matter halos, and discuss the implications for dark matter annihilation and structure formation in general.

The 21-cm signal provides a novel avenue to measure the thermal state of the universe during cosmic dawn and reionization, and thus a probe of exotic energy injection such as those from decaying or annihilating dark matter (DM). These DM processes are inherently inhomogeneous: both decay and annihilation are density dependent, and furthermore the fraction of injected energy that is deposited at each point depends on the gas ionization and density, leading to further inhomogeneities in absorption and propagation. In this talk, I will present a new framework for modeling the impact of spatially inhomogeneous energy injection and deposition, accounting for ionization and baryon density dependence, as well as the attenuation of propagating photons. Our simulation code, DM21cm, is the first complete inhomogeneous treatment of the effects on the 21-cm power spectrum under exotic energy injection. With our pipeline, I will present the sensitivity forecast of the upcoming HERA 21-cm power spectrum measurements to DM decays to photons and electron/positron pairs.

We will present our study of the interaction of dark matter (DM) annihilation with primordial gas during cosmic dawn, emphasizing its impact on the thermal environment of nearby dark matter halos. Utilizing a semi-analytic model, we analyze DM annihilation across redshifts $z=20 to 40$, uncovering its role in altering baryon history and impeding gas collapse and cooling in mini-halos, which affects PopIII star formation. Our study also integrates streaming velocity effects to simulate the 21-cm hydrogen line signal using the 21cmvFAST code. This approach offers a detailed view of astrophysical processes in the early universe. We will present findings demonstrating DM annihilation's significant influence on early star formation and the observable characteristics of the neutral hydrogen line. Our presentation aims to enhance our understanding of baryon physics in the cosmic dawn and provide insights into dark matter models and early galactic and stellar formation.

Causal discovery aims at learning causal relations among variables from data. It is an emerging field at the interface of machine learning and statistics that found ample application in several disciplines. From a physicist's point of view it can be seen as an operational definition of the concept of causal relation between variables, especially in an observational context where experimental manipulation is precluded. Interestingly, causal discovery has not yet been applied to Astronomy, despite it being the observational science par excellence. Here I will present the first application of causal discovery to Astronomy with the goal of addressing a debated issue in galaxy formation: the origin of the observed scaling relations between supermassive black hole (SMBH) and host galaxy properties. I apply three causal discovery algorithms to a state-of-the-art dataset of SMBH host galaxies with dynamical mass measurements, with the goal of learning a causal structure in terms of a directed acyclic graph. The results are consistent between methods and are amenable to physical interpretation, showing that across the multiplicity of possible causal structures, in elliptical galaxies SMBH mass is predominantly an effect of galaxy properties while in spiral galaxies the reverse holds. I offer an explanation of this finding in terms of the physics of galaxy merging, and address the limitations and the theoretical implications of this new method for galaxy formation in some detail.

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