arXiv subjects

Nashwan Sabti

Publications and source records attributed to Nashwan Sabti.

18 recordsLinked to original sources

arXiv2025

The First Billion Years in Seconds: An Effective Model for the 21-cm Signal with Population III Stars

Observations of the 21-cm signal are opening a window to the cosmic-dawn epoch, when the first stars formed. These observations are usually interpreted with semi-numerical or hydrodynamical simulations, which are often computationally intensive and inflexible to changes in cosmological or astrophysical effects. Here, we present an effective, fully analytic model for the impact of the first stars on the 21-cm signal, using the modular code Zeus21. Zeus21 employs an analytic prescription of the star formation rate density (SFRD) to recover the fully nonlinear and nonlocal correlations of radiative fields that determine the 21-cm signal. We introduce the earliest Population III (Pop III) stars residing in low-mass molecular-cooling galaxies in Zeus21, with distinct spectra from later Pop II stars. We also self-consistently model feedback in the form of $H_2$-dissociating Lyman-Werner (LW) radiation, as well as dark matter-baryon relative velocities, both of which suppress star formation in the lowest-mass halos. LW feedback produces a scale-dependence on the SFRD fluctuations, due to the long mean free path of LW photons. Relative velocities give rise to "wiggles" in the spatial distribution of the 21-cm signal; we present an improved calculation of the shape of these velocity-induced acoustic oscillations, showing they remain a standard ruler at cosmic dawn. Our improved version of Zeus21 predicts the 21-cm global signal and power spectra in agreement with simulations at the $\sim 10\%$ level, yet is at least three orders of magnitude faster. This public code represents a step towards efficient and flexible parameter inference at cosmic dawn, allowing us to predict the first billion years of the universe in mere seconds.

astro-ph.CO↗

arXiv2024

Rapid Methods for Modeling Overdensities of Massive Neutrinos and Other Non-Cold Relics

Recent work has highlighted the potentially detectable gravitational-lensing effect of neutrino halos on cosmic-microwave-background (CMB) fluctuations with upcoming instruments like SO, CMB-S4, and CMB-HD. Accurate modeling of neutrino-halo density profiles are essential for making theory predictions of their cosmological effects. Yet, they are computationally intensive, particularly in the nonlinear regime. In this work, we present an efficient numerical framework for computing neutrino profiles based on N-1-body simulations within a flat FRW Universe. Our approach enables highly parallelized, rapid calculations of neutrino trajectories near spherically symmetric dark-matter halos, delivering results within seconds. In addition to neutrinos, we demonstrate an application to model the clustering of other non-cold relics, such as a freeze-in dark-matter component. The framework is flexible in its definitions of cosmological and dark-matter-halo parameters, which can be particularly valuable for rapid-scanning tasks. It can also seamlessly incorporate new physics, as we demonstrate with examples of a time-varying gravitational constant and nonstandard neutrino phase-space distributions.

astro-ph.CO↗

arXiv2024

A Generative Modeling Approach to Reconstructing 21-cm Tomographic Data

Analyses of the cosmic 21-cm signal are hampered by astrophysical foregrounds that are far stronger than the signal itself. These foregrounds, typically confined to a wedge-shaped region in Fourier space, often necessitate the removal of a vast majority of modes, thereby degrading the quality of the data anisotropically. To address this challenge, we introduce a novel deep generative model based on stochastic interpolants to reconstruct the 21-cm data lost to wedge filtering. Our method leverages the non-Gaussian nature of the 21-cm signal to effectively map wedge-filtered 3D lightcones to samples from the conditional distribution of wedge-recovered lightcones. We demonstrate how our method is able to restore spatial information effectively, considering both varying cosmological initial conditions and astrophysics. Furthermore, we discuss a number of future avenues where this approach could be applied in analyses of the 21-cm signal, potentially offering new opportunities to improve our understanding of the Universe during the epochs of cosmic dawn and reionization.

astro-ph.CO↗

arXiv2024

Unveiling Neutrino Halos with CMB Lensing

The existence of a cosmic neutrino background has been inferred indirectly from cosmological surveys through its effect on the linear-theory evolution of primordial density perturbations, as well as from measurements of the primordial abundances of light elements. Constraints on the masses of the three neutrino species imply that at least two of them move non-relativistically today. As a consequence, non-linear evolution of density perturbations results in the formation of neutrino halos around dark-matter halos. We study whether these neutrino halos can be detected in the foreseeable future through measurements of weak gravitational lensing of the cosmic microwave background, thus providing, possibly, the first beyond-linear-theory signature of cosmic neutrinos.

astro-ph.CO↗

arXiv2024

Insights from HST into Ultra-Massive Galaxies and Early-Universe Cosmology

The early-science observations made by the James Webb Space Telescope (JWST) have revealed an excess of ultra-massive galaxy candidates that appear to challenge the standard cosmological model ($Λ$CDM). Here, we argue that any modifications to $Λ$CDM that can produce such ultra-massive galaxies in the early Universe would also affect the UV galaxy luminosity function (UV LF) inferred from the Hubble Space Telescope (HST). The UV LF covers the same redshifts ($z\approx 7-10$) and host-halo masses $(M_\mathrm{h}\approx 10^{10}-10^{12}\, M_\odot$) as the JWST candidates, but tracks star-formation rate rather than stellar mass. We consider beyond-$Λ$CDM power-spectrum enhancements and show that any departure large enough to reproduce the abundance of ultra-massive JWST candidates is in conflict with the HST data. Our analysis, therefore, severely disfavors a cosmological explanation for the JWST abundance problem. Looking ahead, we determine the maximum allowable stellar-mass function and provide projections for the high-$z$ UV LF given our constraints on cosmology from current HST data.

astro-ph.CO↗

arXiv2023

Breaking degeneracies in the first galaxies with clustering

The high-redshift galaxy UV luminosity function (UVLF) has become essential for understanding the formation and evolution of the first galaxies. Yet, UVLFs only measure galaxy abundances, giving rise to a degeneracy between the mean galaxy luminosity and its stochasticity. Here, we show that upcoming clustering measurements with the James Webb Space Telescope (JWST), as well as with Roman, will be able to break this degeneracy, even at redshifts $z \gtrsim 10$. First, we demonstrate that current Subaru Hyper Suprime-Cam (HSC) measurements of the galaxy bias at $z\sim 4-6$ point to a relatively tight halo-galaxy connection, with low stochasticity. Then, we show that the larger UVLFs observed by JWST at $z\gtrsim 10$ can be explained with either a boosted average UV emission or an enhanced stochasticity. These two models, however, predict different galaxy biases, which are potentially distinguishable in JWST and Roman surveys. Galaxy-clustering measurements, therefore, will provide crucial insights into the connection between the first galaxies and their dark-matter halos, and identify the root cause of the enhanced abundance of $z \gtrsim 10$ galaxies revealed with JWST during its first year of operations.

astro-ph.CO↗

arXiv2023

Snowmass2021 Cosmic Frontier White Paper: Dark Matter Physics from Halo Measurements

The non-linear process of cosmic structure formation produces gravitationally bound overdensities of dark matter known as halos. The abundances, density profiles, ellipticities, and spins of these halos can be tied to the underlying fundamental particle physics that governs dark matter at microscopic scales. Thus, macroscopic measurements of dark matter halos offer a unique opportunity to determine the underlying properties of dark matter across the vast landscape of dark matter theories. This white paper summarizes the ongoing rapid development of theoretical and experimental methods, as well as new opportunities, to use dark matter halo measurements as a pillar of dark matter physics.

hep-ph↗

arXiv2022

GALLUMI: A Galaxy Luminosity Function Pipeline for Cosmology and Astrophysics

Observations of high-redshift galaxies have provided us with a rich tool to study the physics at play during the epoch of reionisation. The luminosity function (LF) of these objects is an indirect tracer of the complex processes that govern galaxy formation, including those of the first dark-matter structures. In this work, we present an extensive analysis of the UV galaxy LF at high redshifts to extract cosmological and astrophysical parameters. We provide a number of phenomenological approaches in modelling the UV LF and take into account various sources of uncertainties and systematics in our analysis, including cosmic variance, dust extinction, scattering in the halo-galaxy connection, and the Alcock-Paczyński effect. Using UV LF measurements from the Hubble Space Telescope together with external data on the matter density, we derive the large-scale matter clustering amplitude to be $σ_8=0.76^{+0.12}_{-0.14}$, after marginalising over the unknown astrophysical parameters. We find that with current data this result is only weakly sensitive to our choice of astrophysical modelling, as well as the calibration of the underlying halo mass function. As a cross check, we run our analysis pipeline with mock data from the IllustrisTNG hydrodynamical simulations and find consistent results with their input cosmology. In addition, we perform a simple forecast for future space telescopes, where an improvement of roughly 30% upon our current result is expected. Finally, we obtain constraints on astrophysical parameters and the halo-galaxy connection for the models considered here. All methods discussed in this work are implemented in the form of a versatile likelihood code, GALLUMI, which we make public.

astro-ph.CO↗

arXiv2022

New Roads to the Small-Scale Universe: Measurements of the Clustering of Matter with the High-Redshift UV Galaxy Luminosity Function

The epochs of cosmic dawn and reionisation present promising avenues for understanding the role of dark matter (DM) in our cosmos. The first galaxies that populated the Universe during these eras resided in DM halos that were much less massive than their counterparts today. Consequently, observations of such galaxies can provide us with a handle on the clustering of DM in an otherwise currently inaccessible regime. In this work, we use high-redshift UV galaxy luminosity-function (UV LF) data from the Hubble Space Telescope to study the clustering properties of DM at small scales. In particular, we present new measurements of the matter power spectrum at wavenumbers $0.5\,\mathrm{Mpc}^{-1} < k < 10\,\mathrm{Mpc}^{-1}$ to roughly 30% precision, obtained after marginalising over the unknown astrophysics. These new data points cover the uncharted redshift range $4\leq z\leq 10$ and encompass scales beyond those probed by Cosmic-Microwave-Background and large-scale-structure observations. This work establishes the UV LF as a powerful tool to probe the nature of DM in a different regime than other cosmological and astrophysical data sets.

astro-ph.CO↗

arXiv2022

What can CMB observations tell us about the neutrino distribution function?

Cosmic Microwave Background (CMB) observations have been used extensively to constrain key properties of neutrinos, such as their mass. However, these inferences are typically dependent on assumptions about the cosmological model, and in particular upon the distribution function of neutrinos in the early Universe. In this paper, we aim to assess the full extent to which CMB experiments are sensitive to the shape of the neutrino distribution. We demonstrate that Planck and CMB-S4-like experiments have no prospects for detecting particular features in the distribution function. Consequently, we take a general approach and marginalise completely over the form of the neutrino distribution to derive constraints on the relativistic and non-relativistic neutrino energy densities, characterised by $N_\mathrm{eff} = 3.0 \pm 0.4$ and $ρ_{ν,0}^{\rm NR} < 14 \, \mathrm{eV}\,\mathrm{cm}^{-3}$ at 95% CL, respectively. The fact that these are the only neutrino properties that CMB data can constrain has important implications for neutrino mass limits from cosmology. Specifically, in contrast to the $Λ$CDM case where CMB and BAO data tightly constrain the sum of neutrinos masses to be $\sum m_ν< 0.12 \, \mathrm{eV}$, we explicitly show that neutrino masses as large as $\sum m_ν\sim 3 \, \mathrm{eV}$ are perfectly consistent with this data. Importantly, for this to be the case, the neutrino number density should be suitably small such that the bound on $ρ_{ν,0}^\mathrm{NR} = \sum m_νn_{ν,0}$ is still satisfied. We conclude by giving an outlook on the opportunities that may arise from other complementary experimental probes, such as galaxy surveys, neutrino mass experiments and facilities designed to directly detect the cosmic neutrino background.

astro-ph.CO↗

arXiv2022

Cosmic Neutrino Background Detection in Large-Neutrino-Mass Cosmologies

The Cosmic Neutrino Background (CNB) is a definite prediction of the standard cosmological model and its direct discovery would represent a milestone in cosmology and neutrino physics. In this work, we consider the capture of relic neutrinos on a tritium target as a possible way to detect the CNB, as aimed for by the PTOLEMY project. Crucial parameters for this measurement are the absolute neutrino mass $m_ν$ and the local neutrino number density $n_ν^{\rm loc}$. Within the $Λ$CDM model, cosmology provides a stringent upper limit on the sum of neutrino masses of $\sum m_ν< 0.12\,{\rm eV}$, with further improvements expected soon from galaxy surveys by DESI and EUCLID. This makes the prospects for a CNB detection and a neutrino mass measurement in the laboratory very difficult. In this context, we consider a set of non-standard cosmological models that allow for large neutrino masses ($m_ν\sim 1\,{\rm eV}$), potentially in reach of the KATRIN neutrino mass experiment or upcoming neutrinoless double-beta decay searches. We show that the CNB detection prospects could be much higher in some of these models compared to those in $Λ$CDM, and discuss the potential for such a detection to discriminate between cosmological scenarios. Moreover, we provide a simple rule to estimate the required values of energy resolution, exposure, and background rate for a PTOLEMY-like experiment to cover a certain region in the $(m_ν,\, n_ν^{\rm loc})$ parameter space. Alongside this paper, we publicly release a code to calculate the CNB sensitivity in a given cosmological model.

hep-ph↗

arXiv2022

Addendum: Refined bounds on MeV-scale thermal dark sectors from BBN and the CMB

Very recently, the LUNA collaboration has reported a new measurement of the $d+p\to {}^{3}\text{He}+γ$ reaction rate, which plays an important role in the prediction of the primordial deuterium abundance at the time of BBN. This new measurement has triggered a new set of global BBN analyses within the context of the Standard Model. In this addendum to JCAP 01 (2020) 004 (arXiv:1910.01649), we consider the implications of these new results for our constraints on MeV-scale dark sectors. Importantly, we find that our bounds in the BBN-only and Planck-only analyses are insensitive to these updates. Similarly, we find that our constraints derived using BBN and CMB data simultaneously are not significantly modified for neutrinophilic particles. The bounds on electrophilic dark sector states, however, can vary moderately when combining BBN and CMB observations. We present updated results for all the relevant light dark sector states, calculated using the rates obtained by the leading groups performing standard BBN analyses.

hep-ph↗

arXiv2021

When FIMPs Decay into Neutrinos: The $N_\mathrm{eff}$ Story

The existence of feebly interacting massive particles (FIMPs) could have significant implications on the effective number of relativistic species $N_\mathrm{eff}$ in the early Universe. In this work, we investigate in detail how short-lived FIMPs that can decay into neutrinos affect $N_\mathrm{eff}$ and highlight the relevant effects that govern its evolution. We show that even if unstable FIMPs inject most of their energy into neutrinos, they may still decrease $N_{\mathrm{eff}}$, and identify neutrino spectral distortions as the driving power behind this effect. As a case study, we consider Heavy Neutral Leptons (HNLs) and indicate which regions of their parameter space increase or decrease $N_{\mathrm{eff}}$. Moreover, we derive bounds on the HNL lifetime from the Cosmic Microwave Background and comment on the possible role that HNLs could play in alleviating the Hubble tension.

hep-ph↗

arXiv2021

First Constraints on Small-Scale Non-Gaussianity from UV Galaxy Luminosity Functions

UV luminosity functions provide a wealth of information on the physics of galaxy formation in the early Universe. Given that this probe indirectly tracks the evolution of the mass function of dark matter halos, it has the potential to constrain alternative theories of structure formation. One of such scenarios is the existence of primordial non-Gaussianity at scales beyond those probed by observations of the Cosmic Microwave Background. Through its impact on the halo mass function, such small-scale non-Gaussianity would alter the abundance of galaxies at high redshifts. In this work we present an application of UV luminosity functions as measured by the Hubble Space Telescope to constrain the non-Gaussianity parameter $f_\mathrm{NL}$ for wavenumbers above a cut-off scale $k_{\rm cut}$. After marginalizing over the unknown astrophysical parameters and accounting for potential systematic errors, we arrive at a $2σ$ bound of $f_{\rm NL}=71^{+426}_{-237}$ for a cut-off scale $k_{\rm cut}=0.1\,\mathrm{Mpc}^{-1}$ in the bispectrum of the primordial gravitational potential. Moreover, we perform forecasts for the James Webb Space Telescope and the Nancy Grace Roman Space Telescope, finding an expected improvement of a factor $3-4$ upon the current bound.

astro-ph.CO↗

arXiv2020

New Constraints on the Mass of Fermionic Dark Matter from Dwarf Spheroidal Galaxies

Dwarf spheroidal galaxies are excellent systems to probe the nature of fermionic dark matter due to their high observed dark matter phase-space density. In this work, we review, revise and improve upon previous phase-space considerations to obtain lower bounds on the mass of fermionic dark matter particles. The refinement in the results compared to previous works is realised particularly due to a significantly improved Jeans analysis of the galaxies. We discuss two methods to obtain phase-space bounds on the dark matter mass, one model-independent bound based on Pauli's principle, and the other derived from an application of Liouville's theorem. As benchmark examples for the latter case, we derive constraints for thermally decoupled particles and (non-)resonantly produced sterile neutrinos. Using the Pauli principle, we report a model-independent lower bound of $m \geq 0.18\,\mathrm{keV}$ at 68% CL and $m \geq 0.13\,\mathrm{keV}$ at 95% CL. For relativistically decoupled thermal relics, this bound is strengthened to $m \geq 0.59\,\mathrm{keV}$ at 68% CL and $m \geq 0.41\,\mathrm{keV}$ at 95% CL, whilst for non-resonantly produced sterile neutrinos the constraint is $m \geq 2.80\,\mathrm{keV}$ at 68% CL and $m \geq 1.74\,\mathrm{keV}$ at 95% CL. Finally, the phase-space bounds on resonantly produced sterile neutrinos are compared with complementary limits from X-ray, Lyman-$α$ and Big Bang Nucleosynthesis observations.

hep-ph↗

arXiv2020

An Extended Analysis of Heavy Neutral Leptons during Big Bang Nucleosynthesis

Heavy Neutral Leptons (HNLs) are strongly motivated by theory due to their capability of simultaneously explaining the observed phenomena of dark matter, neutrino oscillations and the baryon asymmetry of the Universe. The existence of such particles would affect the expansion history of the Universe and the synthesis of primordial abundances of light elements. In this work we review, revise and extend the phenomenology of HNLs during the Big Bang Nucleosynthesis (BBN) epoch for masses up to 1 GeV. This is of great importance, as BBN is able to provide complementary bounds to those from upcoming and proposed laboratory experiments. To this end we have developed a high-precision Boltzmann code that simulates BBN in the presence of HNLs and takes into account all relevant HNL decay channels, as well as subsequent interactions of decay products (thermalization and decay showers), dilution due to QCD phase transition, active neutrino oscillations and corrections to the weak reaction rates. We present robust bounds on the lifetime and mixing angles of HNLs for masses $3\,\mathrm{MeV}\leq m_N \leq 1\,\mathrm{GeV}$ and show that BBN is able to constrain HNL lifetimes down to $0.03 - 0.05$ s, depending on the mixing pattern. Moreover, combining our results with current experimental searches, we can exclude HNLs that mix purely with electron neutrinos up to ${\sim}$450 MeV and those that mix purely with muon neutrinos up to ${\sim}$360 MeV, in both cases for lifetimes up to at least a few tens of seconds. Finally, we compare the BBN constraints with those obtained from Cosmic Microwave Background observations and explore how our results will be improved by a number of upcoming and proposed laboratory experiments.

hep-ph↗

arXiv2020

Improved BBN Constraints on the Variation of the Gravitational Constant

Big Bang Nucleosynthesis (BBN) is very sensitive to the cosmological expansion rate. If the gravitational constant $G$ took a different value during the nucleosynthesis epoch than today, the primordial abundances of light elements would be affected. In this work, we improve the bounds on this variation using recent determinations of the primordial element abundances, updated nuclear and weak reaction rates and observations of the Cosmic Microwave Background (CMB). When combining the measured abundances and the baryon density from CMB observations by Planck, we find $G_\mathrm{BBN}/G_0 = 0.99^{+0.06}_{-0.05}$ at $2σ$ confidence level. If the variation of $G$ is linear in time, we find $\dot{G}/G_0 = 0.7^{+3.8}_{-4.3}\times 10^{-12} \, \mathrm{yr}^{-1}$, again at $2σ$. These bounds are significantly stronger than those from previous primordial nucleosynthesis studies, and are comparable and complementary to CMB, stellar, solar system, lunar laser ranging, pulsar timing and gravitational wave constraints.

astro-ph.CO↗

arXiv2019

Refined Bounds on MeV-scale Thermal Dark Sectors from BBN and the CMB

New light states thermally coupled to the Standard Model plasma alter the expansion history of the Universe and impact the synthesis of the primordial light elements. In this work, we carry out an exhaustive and precise analysis of the implications of MeV-scale BSM particles in Big Bang Nucleosynthesis (BBN) and for Cosmic Microwave Background (CMB) observations. We find that, BBN observations set a lower bound on the thermal dark matter mass of $m_χ> 0.4\,\text{MeV}$ at $2σ$. This bound is independent of the spin and number of internal degrees of freedom of the particle, of the annihilation being s-wave or p-wave, and of the annihilation final state. Furthermore, we show that current BBN plus CMB observations constrain purely electrophilic and neutrinophilic BSM species to have a mass, $m_χ> 3.7\,\text{MeV}$ at $2σ$. We explore the reach of future BBN measurements and show that upcoming CMB missions should improve the bounds on light BSM thermal states to $m_χ> (10-15)\,\text{MeV}$. Finally, we demonstrate that very light BSM species thermally coupled to the SM plasma are highly disfavoured by current cosmological observations.

hep-ph↗