Dark Matter, First Light

Feb 26, 2024, 8:00 a.m. → Feb 28, 2024, 8:00 p.m. America/Toronto

PI/4-400 - Space Room (Perimeter Institute for Theoretical Physics)

Aaron Vincent (Queen's University), Katie Mack (Perimeter Institute)

New observational programs and techniques are opening a window to the first galaxies in the universe and bringing surprises along the way. In this workshop, we'll explore how dark matter phenomenology may have impacted the first stars and galaxies, focusing on how improved modeling and simulations can allow us to use new and upcoming high-redshift data to gain insight into dark matter's fundamental nature.

Sponsored in part by:

Monday, February 26

8:30 a.m. Registration

1 Opening Remarks

Speakers: Katie Mack (Perimeter Institute), Aaron Vincent (Queen's University)

2 Dark and visible structures with dissipative dark matter

Speaker: Sarah Shandera (Pennsylvania State University)

3 The Effect of Multiple Cooling Channels on the Formation of 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.

Speaker: Leo Kim (Queen's University)

4 Formation of Dark Compact Objects in an Early Matter Dominated Era

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.

Speaker: Melissa Diamond

10:30 a.m. Break

5 The First Stars in the Universe as Dark Matter Laboratories

Speaker: Cosmin Ilie (Colgate University)

ILIE First Stars as DM Probes PI 24.pdf

6 Probing Atomic Dark Matter using Simulated Galactic Subhalo Populations

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.

Speaker: Mr Caleb Gemmell (University of Toronto)

12:00 p.m. Lunch and Discussions

7 Dark matter at high redshifts with JWST

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.

Speaker: Julian Munoz (The University of Texas at Austin)

8 (Dark) Baryogenesis through Asymmetric Reheating in the Mirror Twin Higgs.

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.

Speaker: Andrija Rasovic (University of Toronton)

9 Panel Discussion: Theory motivations - why are we here?

Speakers: Cosmin ilie, Julian Munoz (The University of Texas at Austin), Sarah Shandera (Pennsylvania State University)

3:00 p.m. Break

3:30 p.m. Breakout Groups

4:30 p.m. Social Time

Tuesday, February 27

10 Supermassive black hole seeds from sub-keV dark matter

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.

Speaker: Aaron Vincent (Queen's University)

11 Direct Collapse Black Holes from Dark Matter Annihilation

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.

Speaker: Flip Tanedo (University of California, Riverside)

10:30 a.m. Break

12 Forming the first stars amidst decaying and annihilating dark matter

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.

Speaker: Wenzer Qin (MIT)

13 Dark Matter Annihilation in 21cm Signal at Cosmic Dawn

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.

Speaker: Mr Liqiang Hou (Perimeter Institute & NC State University)

12:00 p.m. Lunch and Discussions

14 Probing Dark Matter Energy Injection in the Cosmic Dawn with the 21-cm Power Spectrum

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.

Speaker: Yitian Sun (Massachusetts Institute of Technology (MIT))

15 How Dense is Early Dark Matter Structure?

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.

Speaker: James Taylor (Waterloo Centre for Astrophysics)

DMFL_Feb24_pb.pdf

16 Zero Metallicity with Zero CPU Hours

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.

Speaker: James Gurian (Perimeter Institute)

17 Large scale structures and dark fifth forces

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.

Speaker: Marco Costa

3:00 p.m. Break

3:30 p.m. Breakout Groups

18 Panel Discussion: Theory/Observations/Simulations - what will save us?

Speakers: Aaron Vincent (Queen's University), Flip Tanedo (University of California, Riverside), Wenzer Qin (MIT), Yitian Sun (Massachusetts Institute of Technology (MIT))

6:00 p.m. Banquet

Wednesday, February 28

19 Impact of alternative dark matter (altDM) models on galaxies during the Epoch of Reionization (EoR)

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.

Speaker: Rahul Kannan (York University)

20 Effects of primordial black holes on early star formation.

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.

Speaker: Boyuan Liu (University of Cambridge)

10:30 a.m. Break

21 Kinetic decoupling and the matter distribution at high redshift

Speaker: Benjamin Lehmann (Massachusetts Institute of Technology)

22 Ultra-light axions, the high-redshift galaxy population and implications for the S_8 tension

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).

Speaker: Dr Keir Rogers (University of Toronto)

12:00 p.m. Lunch & Discussions

23 Illuminating Hidden Sector Dark Matter With Early-Forming Microhalos

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.

Speaker: Mr Himanish Ganjoo (Perimeter Institute for Theoretical Physics)

24 The role of dark matter in supermassive black hole mergers

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.

Speaker: Gonzalo Alonso-Alvarez (University of Toronto)

2:00 p.m. Colloquium - Junwu Huang

3:00 p.m. Break

25 Panel Discussion: What are we missing?

Speakers: Benjamin Lehmann (Massachusetts Institute of Technology), Boyuan Liu (University of Cambridge), Rahul Kannan (York University)

26 Conference Summary

27 Closing Remarks

Speakers: Aaron Vincent (Queen's University), Katie Mack (Perimeter Institute)