arXiv subjects

Kevork N. Abazajian

Publications and source records attributed to Kevork N. Abazajian.

At least 19 recordsLinked to original sources

arXiv2026

Comment on "Fermi-LAT Galactic Center Excess Morphology of Dark Matter in Simulations of the Milky Way Galaxy"

Muru et al. [Phys. Rev. Lett. 135, 161005 (2025)] conclude from the HESTIA constrained simulations that the $γ$-ray morphologies of the Galactic Center Excess predicted by dark matter annihilation and by an old stellar population are essentially indistinguishable. Their "quadratic dark matter" maps, however, show the square of the mass projected along the line of sight, $\bigl[\intρ\,s^{2}\,ds\bigr]^{2}$, rather than the annihilation integral $\intρ^{2}\,ds$. Squaring a nonnegative map preserves its isophote shapes at corresponding intensity levels, as confirmed by the reported contour axis ratios. The corrected calculation on the same simulations does not support their conclusion.

astro-ph.HE↗

arXiv2026

Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics

Neutrinos and other light relics leave characteristic imprints in the cosmic microwave background anisotropies, making their observation a sensitive probe of the particle content and thermal history of the early universe. The energy density in these relativistic species is parameterized by their effective number $N_\mathrm{eff}$. Measuring this parameter at the percent level, which is a long-standing science goal of CMB-S4 and other experiments, would test a wide range of well-motivated physics within and beyond the Standard Model of particle physics. In this paper, we present Fisher-matrix forecasts of the projected sensitivity to $N_\mathrm{eff}$ of several CMB-S4 survey configurations considered during its extensive design phase. The conceptual design reaches $σ(N_\mathrm{eff}) < 0.03$ over its seven-year observing period, while the revised configuration achieves the same precision over a longer timescale. We complement these results with a cosmic-variance-limited survey over the same multipole range to quantify the room for improvement accessible with additional instrumental, observational, and theoretical efforts. Finally, we discuss the broad implications of precise $N_\mathrm{eff}$ measurements for the radiation sector, big bang nucleosynthesis, light thermal relics, and other early-universe physics. The forecasts presented in this work are performed with the publicly released DRAFT (Dark Radiation Anisotropy Flowdown Team) tool. It provides an end-to-end pipeline from simulated foreground maps and component separation to delensing and projected sensitivities for any cosmological parameter, and it can be directly applied to other cosmic microwave background survey designs.

astro-ph.CO↗

arXiv2026

Statistics Meet Systematics: Resolution of the Massive Early JWST Galaxy Tension

The discovery of massive, high redshift galaxies with the James Webb Space Telescope (JWST) has been argued to challenge $Λ$CDM (cold dark matter): such systems would require extremely rare halos and baryon-to-stellar-mass conversion efficiencies unphysically approaching -- or exceeding -- $100\%$. If confirmed at galaxy-formation--forbidden efficiencies, these galaxies could signal new physics beyond standard cosmological structure formation. We develop a galaxy model framework that ties the linear power spectrum to the inferred efficiencies of galaxy growth while incorporating multiple sources of uncertainties in order to test the structure formation models. The sources of error include (i) observational sample variance, (ii) asymmetric scatter induced by the steepness of the high-mass halo tail, and (iii) systematic uncertainties in stellar mass estimates. We find that the inferred star-formation efficiency is largely controlled by systematic uncertainties in the stellar mass estimates derived from spectral energy distribution modeling of JWST-detected galaxies. Because of the inherent Eddington-like bias, systematic uncertainties amplify the asymmetry of the scatter, in some cases by orders of magnitude, thereby bringing the inferred efficiencies into closer agreement with expectations from early galaxy formation models. We also present how these uncertainties can be applied to the inferred UV luminosity function. Our framework can be used to test $Λ$CDM as errors are reduced and further detections are made.

astro-ph.GA↗

arXiv2025

Return of the Lepton Number: Sterile Neutrino Dark Matter Production and the Revival of the Shi-Fuller Mechanism

We explore resonant production of sterile neutrino dark matter via the Shi-Fuller (SF) mechanism, revisiting its cosmological viability in light of recent results demonstrating that lepton-number asymmetries $L_α\gtrsim 1$ at temperatures $T > 20\rm\,MeV$ are consistent with big bang nucleosynthesis (BBN). Using a quasiclassical Boltzmann transport calculation of the dark matter production, we compute the nonthermal phase space distributions of sterile neutrinos across a broad range of particle mass $m_s$ and mixing angle $\sin^2{(2θ)}$ parameter space. We then evolve the resulting distributions through linear structure formation using CLASS and fit the resulting matter power spectra to thermal warm dark matter (WDM) transfer functions, enabling a direct mapping between SF models and equivalent thermal WDM particle masses $m_{\mathrm{th}}$. This allows us to reinterpret existing structure formation limits and Lyman-$α$ forest preferences in the context of SF production. We find that lepton asymmetries $L \gtrsim 0.5$ at high temperatures open significant viable parameter space in the $m_s \gtrsim 10\,\mathrm{keV}$ and $\sin^2 (2θ) \lesssim 10^{-14}$ regime, compatible with both x-ray constraints from NuSTAR and INTEGRAL/SPI and recent Lyman-$α$ inferences of $m_{\mathrm{th}} \approx 4.1\,\mathrm{keV}$. Following lepton number evolution below 20 MeV, we also specifically show that this lepton asymmetry parameter space is compatible with BBN and cosmic microwave background constraints. We present updated constraints, a refined $m_{\mathrm{th}}$ fitting function, and power-law approximations for $L$ across the parameter space. Our results motivate future x-ray observations targeting the $\sim\! 20\,\mathrm{keV}$ photon regime and testing of the $m_\mathrm{th} \gtrsim 10\,\mathrm{keV}$ WDM region.

hep-ph↗

arXiv2025

Cosmic Structure Strikes Back: The Elimination of Vector-Mediated Nonstandard Interaction Models as a Mechanism for Sterile Neutrino Dark Matter Production

We revisit sterile-neutrino production enabled by nonstandard interactions (NSI) among active neutrinos mediated by new bosons. We focus on vector mediators, including neutrinophilic, gauged $L_μ\!-\!L_τ$, and $B\!-\!L$ realizations that modify in-medium dispersion and scattering, thereby altering the active-sterile conversion history. Building on a novel production framework with NSI thermal potentials and collision integrals, we compute nonthermal phase-space distributions across sterile neutrino mixing and NSI parameters and map each point to an equivalent thermal warm dark matter particle mass $m_\mathrm{th}$ via linear-theory transfer function fitting with cosmological structure-formation Boltzmann solver. This enables a direct reinterpretation of state-of-the-art structure-formation limits from Milky Way satellites, strong lensing, and the Lyman-$α$ forest. These limits, in conjunction with X-ray decay searches, as well as results from a wide variety of particle physics experiments allow for a more complete examination of these models. We find that these vector-mediated models are ruled out when the full combination of current constraints, listed above, are taken into account. NSI scalar-mediated models and models with low-reheating temperatures remain viable.

hep-ph↗

arXiv2025

Extra Radiation Cosmologies: Implications of the Hubble Tension for eV-scale Neutrinos

We present a new analysis on sterile neutrino cosmologies using the Dark Energy Spectroscopic Instrument (DESI) second data release (DR2) baryon acoustic oscillation (BAO) measurements in combination with cosmic microwave background (CMB), CMB lensing, and supernova data. We show that BAO observables are intrinsically less sensitive to the combined effects of relativistic energy density, $N_{\rm eff}$, and the sum of neutrino masses, $Σm_ν$, which are both augmented in sterile neutrino cosmologies. With SH0ES local expansion rate, $H_0$, data, we find $N_{\rm eff} = 3.43 \pm 0.13$, reducing the Hubble tension to $2.4σ$. For a 0.1 eV sterile neutrino, we find $N_{\rm eff}=3.50$ as the best fit. For this representative $N_{\rm eff}$, we find an upper limit of $m_s < 0.17$ eV (95% CL), greater than a factor of four weaker than standard constraints on $Σm_ν$. When SH0ES is included, light sterile neutrinos with masses $m_s\simeq0.1$-$0.2$ eV are favored at $\gtrsim 3σ$, whereas eV-scale sterile masses remain strongly excluded by the data in the cosmologies we study. Our findings confirm our previous results that partially thermalized sub-eV sterile neutrinos are preferred by the SH0ES $H_0$ data. The preferred $m_s$ mass scale overlaps with, but is not identical to, that favored in neutrino oscillation solutions to short-baseline anomalies.

hep-ph↗

arXiv2025

The Preference for Evolving Dark Energy from Cosmological Distance Measurements and Possible Signatures in the Growth Rate of Perturbations

In this study, we use a flexible parametrization of the equation of state of dark energy to explore its possible evolution with datasets from the Dark Energy Spectroscopic Instrument (DESI), Planck cosmic microwave background, and either the 5-year Dark Energy Survey (DES) or the Pantheon+ (PP) supernova (SN) compilation. This parametrization, called transitional dark energy, allows for rapid changes in the equation of state but also changes like that in the Chevallier-Polarski-Linder parametrization. We find a 3.8σ preference for evolving dark energy over ΛCDM with the DES dataset and a weaker 2.4σ preference when using the PP dataset. This corroborates the finding of the DESI Collaboration, who found that their baryon acoustic oscillation data preferred evolving dark energy when fit with the CPL parametrization of the equation of state. Our analysis reveals no significant outliers in the DESI data around the TDE best-fit, while the data is asymmetrically distributed around the ΛCDM best-fit model such that the measured distances are on average smaller. The DESI and SN data both prefer an expansion history that implies a higher dark energy density around z=0.5 than in the Planck-ΛCDM model, with the inferred equation of state being greater than -1 around z=0 and close to or below -1 at z>0.5. We show that when the expansion rate is greater than that in the Planck-ΛCDM model (around z=0.5), the growth rate calculated assuming General Relativity is suppressed relative to the Planck-ΛCDM model, and it rebounds as the expansion rate differences between the models become smaller closer to the present time. The resulting flattening of the $fσ_8(z)$ curve compared to the ΛCDM model could be an independent signature of the temporal evolution of dark energy.

astro-ph.CO↗

arXiv2025

The Status of Neutrino Cosmology: Standard $Λ$CDM, Extensions, and Tensions

We examine the performance of the six-parameter $Λ$CDM model and its extensions in light of recent cosmological observations, with particular focus on neutrino properties inferred from cosmology. Using a broad suite of nine combinations of datasets, with three separate analyses of the Planck Cosmic Microwave Background (CMB) data, and three separate supernovae (SNe) survey data, plus the recent DESI baryon acoustic oscillation (BAO) scale results, we derive constraints on the sum of neutrino masses ($Σm_ν$). Our results show upper limits in the range of $Σm_ν< 76.9\,\mathrm{meV}$ to $Σm_ν< 108\,\mathrm{meV}$ (95\% CL). The variation in the limits on $Σm_ν$ arises from the separate analyses of the Planck CMB data and the separate supernovae datasets, as they relate to the inferred matter density and its relation to the sensitivity of the BAO scale and CMB lensing to $Σm_ν$. In the context of hierarchical mass models in $Λ$CDM, we find a $1.47σ$ preference for normal ordering (NO) over inverted ordering (IO), with similar values of preference across all datasets. Despite the strong constraints, an inclination towards massless neutrinos over NO remains weak at $1.36σ$. We find that a ``negative'' neutrino mass, inferred from the shape of the likelihood in the physical regime, $Σm_ν> 0$, is only present at less than $2σ$. We confirm that models allowing extra relativistic degrees of freedom, with $N_{\rm eff} \approx 3.5$, alleviate the Hubble tension. Significantly, we find a $3.3σ$ preference for a 0.1 eV partially thermalized sterile neutrino when the SH0ES $H_0$ measurement is included, a scale of interest in short-baseline oscillation experiment results. [abridged]

astro-ph.CO↗

arXiv2024

Robust inference of the Galactic centre gamma-ray excess spatial properties

The gamma-ray Fermi-LAT Galactic centre excess (GCE) has puzzled scientists for over 15 years. Despite ongoing debates about its properties, and especially its spatial distribution, its nature remains elusive. We scrutinize how the estimated spatial morphology of this excess depends on models for the Galactic diffuse emission, focusing particularly on the extent to which the Galactic plane and point sources are masked. Our main aim is to compare a spherically symmetric morphology - potentially arising from the annihilation of dark matter (DM) particles - with a boxy morphology - expected if faint unresolved sources in the Galactic bulge dominate the excess emission. Recent claims favouring a DM-motivated template for the GCE are shown to rely on a specific Galactic bulge template, which performs worse than other templates for the Galactic bulge. We find that a non-parametric model of the Galactic bulge derived from the VVV survey results in a significantly better fit for the GCE than DM-motivated templates. This result is independent of whether a GALPROP-based model or a more non-parametric ring-based model is used to describe the diffuse Galactic emission. This conclusion remains true even when additional freedom is added in the background models, allowing for non-parametric modulation of the model components and substantially improving the fit quality. When adopted, optimized background models provide robust results in terms of preference for a boxy bulge morphology of the GCE, regardless of the mask applied to the Galactic plane.

astro-ph.GA↗

arXiv2023

Synergy between cosmological and laboratory searches in neutrino physics

The intersection of the cosmic and neutrino frontiers is a rich field where much discovery space still remains. Neutrinos play a pivotal role in the hot big bang cosmology, influencing the dynamics of the universe over numerous decades in cosmological history. Recent studies have made tremendous progress in understanding some properties of cosmological neutrinos, primarily their energy density. Upcoming cosmological probes will measure the energy density of relativistic particles with higher precision, but could also start probing other properties of the neutrino spectra. When convolved with results from terrestrial experiments, cosmology can become even more acute at probing new physics related to neutrinos or even Beyond the Standard Model (BSM). Any discordance between laboratory and cosmological data sets may reveal new BSM physics and/or suggest alternative models of cosmology. We give examples of the intersection between terrestrial and cosmological probes in the neutrino sector, and briefly discuss the possibilities of what different laboratory experiments may see in conjunction with cosmological observatories.

hep-ph↗

arXiv2023

Visible in the laboratory and invisible in cosmology: decaying sterile neutrinos

The expansion history and thermal physical process that happened in the early Universe before big bang nucleosynthesis (BBN) remains relatively unconstrained by observations. Low reheating temperature universes with normalcy temperatures of $T_\mathrm{RH}\sim 2\,\mathrm{MeV}$ remain consistent with primordial nucleosynthesis, and accommodate several new physics scenarios that would normally be constrained by high-temperature reheating models, including massive sterile neutrinos. We explore such scenarios' production of keV scale sterile neutrinos and their resulting constraints from cosmological observations. The parameter space for massive sterile neutrinos is much less constrained than in high-$T_\mathrm{RH}$ thermal histories, though several cosmological constraints remain. Such parameter space is the target of several current and upcoming laboratory experiments such as TRISTAN (KATRIN), HUNTER, MAGNETO-$ν$, and PTOLEMY. Cosmological constraints remain stringent for stable keV-scale sterile neutrinos. However, we show that sterile neutrinos with a dark decay to radiation through a $Z^\prime$ or a new scalar are largely unconstrained by cosmology. In addition, this mechanism of sterile neutrinos with large mixing may provide a solution to the Hubble tension. We find that keV-scale sterile neutrinos are therefore one of the best probes of the untested pre-BBN era in the early Universe and could be seen in upcoming laboratory experiments.

hep-ph↗

arXiv2023

Constraints on sterile neutrino models from strong gravitational lensing, Milky Way satellites, and Lyman-$α$ forest

The nature of dark matter is one of the most important unsolved questions in science. Some dark matter candidates do not have sufficient nongravitational interactions to be probed in laboratory or accelerator experiments. It is thus important to develop astrophysical probes which can constrain or lead to a discovery of such candidates. We illustrate this using state-of-the-art measurements of strong gravitationally-lensed quasars to constrain four of the most popular sterile neutrino models, and also report the constraints for other independent methods that are comparable in procedure. First, we derive effective relations to describe the correspondence between the mass of a thermal relic warm dark matter particle and the mass of sterile neutrinos produced via Higgs decay and GUT-scale scenarios, in terms of large-scale structure and galaxy formation astrophysical effects. Second, we show that sterile neutrinos produced through the Higgs decay mechanism are allowed only for mass $>26$ keV, and GUT-scale scenario $>5.3$ keV. Third, we show that the single sterile neutrino model produced through active neutrino oscillations is allowed for mass $>92$ keV, and the 3 sterile neutrino minimal standard model ($ν$MSM) for mass $>16$ keV. These are the most stringent experimental limits on these models.

hep-ph↗

arXiv2023

Entering the Era of Measuring Sub-Galactic Dark Matter Structure: Accurate Transfer Functions for Axino, Gravitino & Sterile Neutrino Thermal Warm Dark Matter

We examine thermal warm dark matter (WDM) models that are being probed by current constraints, and the relationship between the particle dark matter spin and commensurate thermal history. We find significant corrections to the linear matter power spectrum for given thermal WDM particle masses. Two primary classes are examined: spin-1/2 particles (e.g., thermalized sterile neutrinos, axinos) and thermal spin-3/2 particles (e.g., gravitinos or non-supersymmetric particles). We present new transfer function fits for thermal WDM candidates in particle mass regimes beyond the range of previous work, and at the scales of current and upcoming constraints. Importantly, we find that the standard, predominantly used, spin-1/2, thermal WDM particle produces a colder transfer function than that determined in previous work. We also analyze the entropy requirements for these WDM models to successfully produce observed dark matter densities. We explore the early Universe physics of gravitinos as either partially thermalized or fully thermalized species, which considerably changes the particle dark matter candidates' thermalization history and effects on structure formation. For the first time, we also calculate the transfer function for thermal spin-3/2 WDM.

hep-ph↗

arXiv2022

Early or phantom dark energy, self-interacting, extra, or massive neutrinos, primordial magnetic fields, or a curved universe: An exploration of possible solutions to the $H_0$ and $σ_8$ problems

In this paper we explore the existing tensions in the local cosmological expansion rate, $H_0$, and amplitude of the clustering of large-scale structure at $8\, h^{-1}\mathrm{Mpc}$, $σ_8$, as well as models that claim to alleviate these tensions. We consider seven models: evolving dark energy ($w$CDM), extra radiation ($N_\mathrm{eff}$), massive neutrinos, curvature, primordial magnetic fields (PMF), self-interacting neutrino models, and early dark energy (EDE). We test these models against three data sets that span the full range of measurable cosmological epochs, have significant precision, and are well-tested against systematic effects: the Planck 2018 cosmic microwave background data, the Sloan Digital Sky Survey baryon acoustic oscillation scale measurements, and the Pantheon catalog of Type Ia supernovae. We use the recent SH0ES $H_0$ measurement and several measures of $σ_8$ (and its related parameter $S_8=σ_8\sqrt{Ω_\mathrm{m}/0.3}$). We find that four models are above the "strong" threshold in Bayesian model selection, $w$CDM, $N_\mathrm{eff}$, PMF, and EDE. However, only EDE also relieves the $H_0$ tension in the full data sets to below 2$σ$. Contrarily, no model alleviates the $S_8/σ_8$ tension in the full data set, nor does better than $Λ$CDM in the combined case of both $H_0$ and $S_8/σ_8$ tensions.

astro-ph.CO↗

arXiv2022

Snowmass2021 Cosmic Frontier: Cosmic Microwave Background Measurements White Paper

This is a solicited whitepaper for the Snowmass 2021 community planning exercise. The paper focuses on measurements and science with the Cosmic Microwave Background (CMB). The CMB is foundational to our understanding of modern physics and continues to be a powerful tool driving our understanding of cosmology and particle physics. In this paper, we outline the broad and unique impact of CMB science for the High Energy Cosmic Frontier in the upcoming decade. We also describe the progression of ground-based CMB experiments, which shows that the community is prepared to develop the key capabilities and facilities needed to achieve these transformative CMB measurements.

astro-ph.CO↗

arXiv2021

Neutrinos in Astrophysics and Cosmology: Theoretical Advanced Study Institute (TASI) 2020 Lectures

I introduce the consequences of neutrino mass and mixing in the dense environments of the early Universe and in astrophysical environments. Thermal and matter effects are reviewed in the context of a two-neutrino formalism, with methods of extension to multiple neutrinos. The observed large neutrino mixing angles place the strongest constraint on cosmological lepton (or neutrino) asymmetries, while new sterile neutrinos provide a wealth of possible new physics, including lepton asymmetry generation as well as candidates for dark matter. I also review cosmic microwave background and large-scale structure constraints on neutrino mass and energy density. Lastly, I review how X-ray astronomy has become a branch of neutrino physics in searches for keV-scale sterile neutrino dark matter radiative decay.

hep-ph↗

arXiv2021

Strong constraints on thermal relic dark matter from Fermi-LAT observations of the Galactic Center

The extended excess toward the Galactic Center (GC) in gamma rays inferred from Fermi-LAT observations has been interpreted as being due to dark matter (DM) annihilation. Here, we perform new likelihood analyses of the GC and show that, when including templates for the stellar galactic and nuclear bulges, the GC shows no significant detection of a DM annihilation template, even after generous variations in the Galactic diffuse emission models and a wide range of DM halo profiles. We include Galactic diffuse emission models with combinations of three-dimensional inverse Compton maps, variations of interstellar gas maps, and a central source of electrons. For the DM profile, we include both spherical and ellipsoidal DM morphologies and a range of radial profiles from steep cusps to kiloparsec-sized cores, motivated in part by hydrodynamical simulations. Our derived upper limits on the dark matter annihilation flux place strong constraints on DM properties. In the case of the pure $b$-quark annihilation channel, our limits on the annihilation cross section are more stringent than those from the Milky Way dwarfs up to DM masses of approximately TeV and rule out the thermal relic cross section up to approximately 300 GeV. Better understanding of the DM profile, as well as the Fermi-LAT data at its highest energies, would further improve the sensitivity to DM properties.

hep-ph↗

arXiv2020

Technical Comment on "The dark matter interpretation of the 3.5-keV line is inconsistent with blank-sky observations"

I show that model dependencies in the analysis by Dessert, Rodd & Safdi (2020) relax their claimed constraint by a factor of ~20. After including conservative model choices, the derived limits are comparable to or slightly better than limits from previous searches. Further model tests and expansion of the data energy may enhance or relax sensitivity of the methodology.

astro-ph.HE↗