arXiv subjects

M. Grant Roberts

Publications and source records attributed to M. Grant Roberts.

12 recordsLinked to original sources

arXiv2026

Ultra-Strongly Self-Interacting Dark Matter: From Phenomenology to Astrophysical Observables

We develop a minimal, testable framework for two-component self-interacting dark matter (SIDM) in which a dominant, moderately self-interacting species coexists with an ultra-strongly self-interacting subcomponent (uSIDM). A light vector mediator induces velocity-dependent self-scattering, while early-universe dynamics - standard $2 \to 2$ annihilation supplemented by interconversion $χ_1χ_1 \to χ_2χ_2$ - determine the relic abundance analytically. From observations of dwarf and low surface brightness galaxy rotation curves, as well as strong cluster lensing, we place constraints on the microphysics parameters. From these constrained regions, we map the microphysics to effective \texttt{ETHOS} parameters and evolve the linear power spectrum in \texttt{CLASS}. We identify a region where: (1) the SIDM dominant component attains $σ_{\rm{eff}}/m = 20 - 40~\text{cm}^{2}\text{g}^{-1}$ at dwarf velocities while satisfying cluster upper bounds $σ_{\rm{eff}}/m < 0.13~\rm{cm}^{2}\rm{g}^{-1}$; (2) a subpercent uSIDM fraction drives accelerated gravothermal collapse in early halos, providing seeds relevant to high-redshift quasar formation and ``little red dots''; and (3) the small-scale cutoff in the matter power spectrum remains consistent with Lyman-$α$ and satellite counts, but exhibits non-standard features, potentially discernible with future observations. The allowed space can be organized by the mediator-to-DM mass ratio and the late-time uSIDM fraction, with a narrow window singled out by the combined cosmological and astrophysical requirements.

hep-ph↗

arXiv2026

What Are Pulsar Companions Made of? Using Gravitational Tides to Probe Their Compositions

Low eccentricity, short orbital period pulsar companions may provide a probe to study novel dense and stable exoplanet internal compositions due to the potentially significant orbital evolution they experience caused by strong gravitational tides. We model the tidal characteristics such as apsidal motion constants, orbital precession, and tidal deformability for a variety of equations of state to be compared with values recovered via pulsar timing for a sample of four systems: PSR J1719-1438b, PSR J0636+5128b, PSR J2322+2650b, and PSR J1807-2459A b. With this method, we hope to place stringent limits on the chemical and structural composition of these objects. Through limiting the internal composition of pulsar companions, we aim to elucidate their unique history and formation.

astro-ph.HE↗

arXiv2026

The Cosmic Horizon of Neutrinos

The persistent discrepancy between the experimental measurement and the Standard Model (SM) prediction of the muon's anomalous magnetic moment $(g-2)_μ$ remains one of the most intriguing hints of physics beyond the SM. A well-motivated explanation involves a light $Z'$ gauge boson associated with a broken $U(1)_{L_μ- L_τ}$ symmetry. Such a boson not only resolves the $(g-2)_μ$ anomaly, but also induces resonant interactions between high-energy cosmic neutrinos and the cosmic neutrino background (C$ν$B), potentially shaping the observable neutrino flux at Earth. In this work, we explore the implications of such interactions for the cosmic propagation of high-energy neutrinos. We compute the optical depth for neutrino attenuation via $Z'$-mediated scattering, accounting for neutrino masses, hierarchies, and thermal distributions. We delineate the regions in $(m_{Z'}, m_ν)$ space where the optical depth exceeds unity, defining a ``neutrino cosmic horizon'' beyond which high-energy neutrinos are significantly attenuated. We confront these results with the parameter space required to simultaneously explain the muon $g-2$ anomaly and ease the Hubble tension via an additional contribution to the effective number of relativistic degrees of freedom, $ΔN_{\mathrm{eff}} \simeq 0.2-0.5$. Our analysis reveals a consistent region in parameter space where all three phenomena-$(g-2)_μ$, $N_{\mathrm{eff}}$, and high-energy neutrino attenuation-can be explained by the same light mediator. These findings motivate future searches for spectral features in IceCube and its next-generation successors as indirect probes of new physics in the neutrino sector.

hep-ph↗

arXiv2026

Structure Formation with Dark Magnetohydrodynamics

Long-range interactions in the dark sector can give rise to collective plasma phenomena that are capable of modifying the evolution of dark matter halos. We present the first study of gravitational collapse in a secluded dark $U(1)_D$ model using a magnetohydrodynamic description of the dark matter. We show that dark magnetic fields generate an anisotropic pressure that alters the Jeans scale and suppresses small-scale power in a direction-dependent manner. For a range of primordial magnetic spectral indices, this effect produces distinctive modifications to the linear matter power spectrum. We find that current observations cannot yet constrain viable dark magnetic fields, as CMB tensor modes mostly provide more stringent constraints. Nevertheless, forthcoming high-resolution probes of the matter power spectrum (CMB-HD lensing, HERA, and EDGES) will be able to test these predictions and are sensitive to dark charge-to-mass ratios in the range $10^{-20}\,\text{GeV}^{-1}\lesssim q_χ/m_χ\lesssim 10^{-14}\,\text{GeV}^{-1}$.

hep-ph↗

arXiv2026

Little Red Dots from Ultra-Strongly Self-Interacting Dark Matter

We investigate the possibility that the recently identified population of high-redshift, obscured quasars - known as "Little Red Dots" (LRDs) - originates from early black hole seed formation driven by ultra-strongly self-interacting dark matter (uSIDM). In this framework, dark matter halos undergo gravothermal core collapse due to large self-interaction cross sections, resulting in the rapid formation of massive black hole (BH) seeds with masses $\gtrsim 10^{5} M_\odot$ at redshifts $z \gtrsim 5$. We develop a semi-analytic model that tracks the evolution of the dark matter halo population, the redshift of collapse $z_{\rm coll}$, and the corresponding BH mass function. Black hole growth is modeled stochastically via a log-normal Eddington ratio distribution and a finite duty cycle. We find that the uSIDM scenario naturally reproduces key observed properties of LRDs, including their abundance, compactness, and characteristic BH masses, while offering a mechanism for early, obscured black hole formation that is difficult to achieve in standard CDM-based models. The predicted SMBH mass function at $z \sim 5$ shows excellent agreement with LRD observational data and SIDM merger-tree simulations, particularly at the high-mass end $(m_{\rm BH} \gtrsim 10^{7} M_\odot)$. These results suggest that LRDs may serve as powerful observational tracers of exotic dark sector physics and that SMBH formation in the early universe could be significantly shaped by non-gravitational dark matter interactions.

astro-ph.GA↗

arXiv2026

A Constraint on Dark Matter Self-Interaction from Combined Strong Lensing and Stellar Kinematics in MACS J0138-2155

Self-Interacting Dark Matter (SIDM) represents a compelling alternative to collisionless dark matter, with diverse phenomenological signals from dwarf galaxy to galaxy cluster scales. We present new constraints on the SIDM cross section from the galaxy cluster MACS J0138-2155, host to the strongly lensed supernovae Requiem and Encore. Our analysis combines strong gravitational lensing with spatially resolved stellar kinematics of the central galaxy, employing several methodological advances over previous cluster-scale SIDM studies. The result is a self-consistent measurement of the density profile of MACS J0138-2155 across two orders of magnitude in radius. Our lensing and kinematics analyses individually yield highly consistent results, and from their combination we report a 95% confidence upper limit on the SIDM cross section of $σ/m < 0.613$ cm$^2$/g, at an interaction velocity of $\langle v_\text{pair}\rangle < 2090$ km/s. This constraint, derived from the most detailed single-system analysis to date, is competitive with previous cluster-scale limits while demonstrating the power of combining complementary gravitational probes. The methodology developed here advances precision cluster lens modeling and will inform future studies of dark matter physics, as well as time-delay cosmography in this unique strong lensing system. Additionally, our results imply SN Requiem will reappear sooner than previously reported, with a 1$σ$ CL between January 2027 and November 2028 at H$_0 = 67.7$ km s$^{-1}$ Mpc$^{-1}$.

astro-ph.CO↗

arXiv2025

The Clustering of Little Red Dots from Ultra-Strongly Self-Interacting Dark Matter

We predict the effective clustering bias parameter, $b_{\rm{eff}}$, at $z\sim5$ for Little Red Dots (LRDs) seeded by Ultra-Strongly Self-Interacting Dark Matter (uSIDM). From our model, we find that $b_{\rm{eff}}\sim4.5$, thus we infer that LRDs seeded by uSIDM would populate halos of typical masses $\sim 8\times10^{10}~M_{\odot}$; this bias factor is consistent with LRDs being a distinct population from high redshift quasars. To the extent that we are aware, this is the first formation-based theoretical prediction of LRD clustering from a model consistent with the LRD mass function. We find that this bias and clustering is insensitive to a wide range of the underlying uSIDM microphysics parameters, including the uSIDM cross-section $σ/m$ and uSIDM fraction $f$. This is therefore a robust prediction from the uSIDM model, and will allow for direct probes of the uSIDM paradigm as the origin of LRDs in the next few years. Upcoming \texttt{JWST} observations will constrain the population of LRDs, including directly measuring their clustering.

astro-ph.CO↗

arXiv2025

Spectroscopic and X-ray Modeling of the Strong Lensing Galaxy Cluster MACS J0138.0-2155

We model the total mass and galactic substructure in the strong lensing galaxy cluster MACS J0138.0-2155 using a combination of Chandra X-ray data, Multi-Unit Spectroscopic Explorer (MUSE) spectroscopy, and Hubble Space Telescope imaging. MACS J0138.0-2155 lenses a source galaxy at $z=1.95$ which hosts two strongly lensed supernovae, Requiem and Encore. We find MACS J0138.0-2155 to have an X-ray temperature of $6.7\pm0.4$ keV and a velocity dispersion of cluster member galaxies of $718^{+132}_{-182}$ km s$^{-1}$. These lead to the mass estimates for the cluster of $M_{500} = 5.2^{+1.5}_{-1.2} \times 10^{14} M_\odot$ from the X-ray results and $M_{200} \approx 3.6^{+2.0}_{-2.7} \times 10 ^{14} M_{\odot}$ from the velocity dispersion results. The round morphology of the X-ray emission indicates that this cluster is relaxed with an ellipticity within the lensing region of $e=0.12\pm0.03$. Using 18 of the brightest, non-blended, quiescent galaxies, we fit the cluster specific Faber-Jackson relation, including a set of 81 variations in the analysis choices to estimate the systematic uncertainties in our results. We find a slope of $α= 0.26 \pm 0.06 (\mathrm{stat.}) \pm 0.03 (\mathrm{sys.})$ with an intrinsic scatter of $Δσ= 31^{+8}_{-6} (\mathrm{stat.}) \pm 4 (\mathrm{sys.})$ km s$^{-1}$ at a reference velocity dispersion of $\sim 220$ km s$^{-1}$. We also report on significant galaxies along the line-of-sight potentially impacting the lens modeling, including a massive galaxy with stellar velocity dispersion of $236 \pm 3$ km s$^{-1}$ which lies close in projection to the central cluster galaxy. This galaxy is part of a small group at a slightly higher redshift than the cluster.

astro-ph.CO↗

arXiv2025

Gravothermal collapse and the diversity of galactic rotation curves

The rotation curves of spiral galaxies exhibit a great diversity that challenge our understanding of galaxy formation and the nature of dark matter. Previous studies showed that in self-interacting dark matter (SIDM) models with a cross section per unit mass of $σ/m\approx{\cal O}(1)~{\rm cm^2/g}$, the predicted dark matter central densities are a good match to the observed densities in galaxies. In this work, we explore a regime with a larger cross section of $σ/m\approx20\text{-}40~{\rm cm^2/g}$ in dwarf galactic halos. We will show that such strong dark matter self-interactions can further amplify the diversity of halo densities inherited from their assembly history. High concentration halos can enter the gravothermal collapse phase within $10~{\rm Gyr}$, resulting in a high density, while low concentration ones remain in the expansion phase and have a low density. We fit the rotation curves of $14$ representative low surface brightness galaxies and demonstrate how the large range of observed central densities are naturally accommodated in the strong SIDM regime of $σ/m\approx20\text{-}40~{\rm cm^2/g}$. Galaxies that are outliers in the previous studies due to their high halo central densities, are no longer outliers in this SIDM regime as their halos would be in the collapse phase. For galaxies with a low density, the SIDM fits are robust to the variation of the cross section. Our findings open up a new window for testing gravothermal collapse, the unique signature of strong dark matter self-interactions, and exploring a broader SIDM model space. As an example, we illustrate how the larger cross sections favored by our fits, together with upper limits from strong lensing observations in clusters, pick out the preferred SIDM model space for a dark matter particle coupled to a light gauge boson in the Born regime.

astro-ph.GA↗

arXiv2025

Early formation of supermassive black holes from the collapse of strongly self-interacting dark matter

Evidence for high-redshift supermassive black holes challenges standard scenarios for how such objects form in the early universe. Here, we entertain the possibility that a fraction of the cosmological dark matter could be ultra-strongly self interacting. This would imply that gravothermal collapse occur at early times in the cores of dark matter halos, followed by accretion. We study under which conditions on the abundance and interaction strength and structure of such ultra self-interacting dark matter the black holes resulting from the end-point of gravothermal core collapse can seed the observed, early-forming supermassive black holes. We find, depending on the velocity dependence of the self-interaction cross section, a bimodal structure in the favored parameter space, where data points to either a small collapsing dark matter fraction with a large cross section, or a large fraction and a relatively small cross section. While self-interaction cross sections with different velocity dependence can explain observations, we find that the best, self-consistent results correspond to a Rutherford-like self-interaction, typical of long-range dark-sector forces with light mediators. We discuss complementary observational probes if this scenario is realized in nature, focusing especially on the expected intermediate mass black holes predicted to exist in smaller galaxies.

astro-ph.GA↗

arXiv2024

On the Impact of Dark Matter Scattering on the Trajectory of High-Energy Cosmic Rays

We study the impact on the trajectory of high-energy cosmic-ray protons of scattering off the cosmic dark matter. We compute the scattering angle as a function of the cosmic-ray energy, of the dark matter mass, and of the interaction strength for a few representative choices for the relevant interaction cross section. We find that the typical deflection angle over the cosmic ray path is largely independent of the dark matter mass. Given existing limits on the interaction strength, we compute the average deflection angle. We find that for large interaction cross sections and low cosmic ray energies, the predicted deflection angle is much larger than the angular resolution of very high-energy cosmic-ray observatories such as Pierre Auger.

astro-ph.HE↗

arXiv2021

A Stringent Upper Limit on Dark Matter Self-Interaction Cross Section from Cluster Strong Lensing

We analyze strongly lensed images in 8 galaxy clusters to measure their dark matter density profiles in the radial region between 10 kpc and 150 kpc, and use this to constrain the self-interaction cross section of dark matter (DM) particles. We infer the mass profiles of the central DM halos, bright central galaxies, key member galaxies, and DM subhalos for the member galaxies for all 8 clusters using the Qlens code. The inferred DM halo surface densities are fit to a self-interacting dark matter (SIDM) model, which allows us to constrain the self-interaction cross section over mass $σ$/m. When our full method is applied to mock data generated from two clusters in the Illustris-TNG simulation, we find results consistent with no dark matter self-interactions as expected. For the eight observed clusters with average relative velocities of $1458_{-81}^{+80}$ km/s, we infer $σ$/m = $0.082_{-0.021}^{+0.027}$ cm$^2$/g and $σ$/m < 0.13 cm$^2$/g at the 95% confidence level.

astro-ph.CO↗