Cosmology

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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The first stars are expected to form through molecular-hydrogen (H2) cooling, a channel that is especially sensitive to the thermal and ionization state of gas, and can thus act as a probe of exotic energy injection from decaying or annihilating dark matter (DM). I will discuss using a toy halo model to study the impact of DM-sourced energy injection on the H2 content of the first galaxies, and thus estimate the threshold mass required for a halo to form stars at high redshifts. I will show that currently allowed DM models can significantly change this threshold, producing both positive and negative feedback and estimate how this can impact the timing of 21cm signals at cosmic dawn.

We present a simple dark matter model where resonant annihilation can dissociate molecular hydrogen and induce direct collapse black holes in proto-galaxies. In these models, O(10 MeV) dark matter annihilates into electron-positron pairs which, in turn, inverse Compton scatter CMB light to produce a flux of Lyman-Werner radiation. This mechanism could help explain observed supermassive black holes at high redshift.

Quasars observed at redshifts z∼6−7.5 are powered by supermassive black holes which are too large to have grown from early stellar remnants without efficient super-Eddington accretion. A proposal for alleviating this tension is for dust and metal-free gas clouds to have undergone a process of direct collapse, producing black hole seeds of mass Mseed∼105M⊙ around redshift z∼17. For direct collapse to occur, a large flux of UV photons must exist to photodissociate molecular hydrogen, allowing the gas to cool slowly and avoid fragmentation. We investigate the possibility of sub-keV mass dark matter decaying or annihilating to produce the UV flux needed to cause direct collapse. We find that annihilating dark matter with a mass in the range of 13.6 eV≤mdm≤20 eV can produce the required flux while avoiding existing constraints. A non-thermally produced dark matter particle which comprises the entire dark matter abundance requires a thermally averaged cross section of ⟨σv⟩∼10−35 cm3/s. Alternatively, the flux could originate from a thermal relic which comprises only a fraction ∼10−9 of the total dark matter density. Decaying dark matter models which are unconstrained by independent astrophysical observations are unable to sufficiently suppress molecular hydrogen, except in gas clouds embedded in dark matter halos which are larger, cuspier, or more concentrated than current simulations predict. Lastly, we explore how our results could change with the inclusion of full three-dimensional effects. Notably, we demonstrate that if the H2 self-shielding is less than the conservative estimate used in this work, the range of both annihilating and decaying dark matter models which can cause direct collapse is significantly increased.

We present the νφMTH, a Mirror Twin Higgs (MTH) model realizing asymmetric reheating, baryogenesis and twin-baryogenesis through the out-of-equilibrium decay of a right-handed neutrino without any hard Z2 breaking. The MTH is the simplest Neutral Naturalness solution to the little hierarchy problem and predicts the existence of a twin dark sector related to the Standard Model (SM) by a Z2 symmetry that is only softly broken by a higher twin Higgs vacuum expectation value. The asymmetric reheating cools the twin sector compared to the visible one, thus evading cosmological bounds on ∆Neff. The addition of (twin-)colored scalars allows for the generation of the visible baryon asymmetry and, by the virtue of the Z2 symmetry, also results in the generation of a twin baryon asymmetry. We identify a unique scenario with top-philic couplings for the new scalars that can satisfy all cosmological, proton decay and LHC constraints; yield the observed SM baryon asymmetry; and generate a wide range of possible twin baryon DM fractions, from negligible to unity. Implications of predicted atomic DM fractions will be discussed, as well as model-independent asymmetric reheating implications on the abundance of twin helium. These results motivate the search for the rich cosmological and astrophysical signatures of twin baryons, and atomic dark matter more generally, at cosmological, galactic and stellar scales.

The launch of the James Webb Space Telescope (JWST) has ignited a revolution in our understanding of the early universe. Its exquisite infrared capabilities have allowed observers to find galaxies at higher redshifts than before and to measure their stellar masses. I will describe how we can use these observations to shed light on the nature of dark matter. For the JWST galaxies to form they ought to reside in dark-matter halos, allowing us to measure the clustering of dark matter in an unexplored region. I will discuss the JWST observations of ultra-massive galaxies recently argued to “break LCDM”, and how we recently disfavored a cosmological solution using HST data at the same redshifts. If time allows, I will review the path forward to measuring dark-matter clustering down to the first galaxies through 21-cm observations.

Atomic dark matter (ADM) is a simple extension to the Standard Model that is motivated by considerations in both particle and astrophysics. ADM can alter structure formation on subgalactic scales due to its ability to dissipate energy through cooling mechanisms, but is also one realisation of a possible complex dark sector. These dark sectors have been previously studied as a solution to the little hierarchy problem. Recently the first N-body simulations were completed, studying the effects of cold dark matter with a ADM subcomponent (6%) and are only beginning to be analysed. In this talk I present how the dissipative nature of ADM affects both the distribution and structure of subhalos in a Milky Way analogue, and outline how we may hope to constrain and probe the ADM parameter space.

Self interacting dark matter can form halos and compact objects in an early matter dominated era before Big Bang Nucleosynthesis. This talk explores how an asymmetric dark fermion which self interacts via a heavy vector can undergo halo formation in an early matter dominated era. These halos then cool via bremsstrahlung until they either collapse into black holes or fragment into compact pressure support objects. Radiation domination is restored via a phase transition. This provides a simple new mechanism to produce both primordial black holes and dark compact objects.

A dissipative dark sector can result in the formation of compact objects with masses comparable to stars and planets. In this work, we investigate the formation of such compact objects from a subdominant inelastic dark matter model, and study the resulting distributions of these objects. In particular, we consider cooling from dark Bremsstrahlung and a rapid decay process that occurs after inelastic upscattering. Inelastic transitions introduce an additional radiative processes which can impact the formation of compact objects via multiple cooling channels. We find that having multiple cooling processes changes the mass and abundance of compact objects formed, as compared to a scenario with only one cooling channel. The resulting distribution of these astrophysical compact objects and their properties can be used to further constrain and differentiate between dark sectors.