arXiv subjects

Itamar J. Allali

Publications and source records attributed to Itamar J. Allali.

13 recordsLinked to original sources

arXiv2026

Axion Perturbations: A General Analytical Treatment

Cosmological data provides us two key constraints on dark matter (DM): it must have a particular abundance, and it must have an adiabatic spectrum of density perturbations in the early universe. Many different cosmological scenarios have been proposed that establish the abundance of axion DM in qualitatively different ways. In this paper we emphasize that, despite this variety of backgrounds, the perturbations in axion DM can be understood from universal principles. How does a feebly interacting axion field acquire perturbations proportional to those of photons? How do the isocurvature power spectrum and non-Gaussianity depend on the background evolution of the universe? We answer these questions for a completely general choice of cosmological background and temperature-dependent axion potential. We show that the most general solution to the axion field equation on super-horizon scales is entirely determined by the family of background solutions for different initial field values $θ_{\rm ini}$. This holds for both the component in the field perturbation solution contributing to the DM isocurvature perturbation (enhanced at late times by the sensitivity of the DM abundance to the initial condition, $\partial Ω_a / \partial θ_{\rm ini}$, which can be large for initial conditions near the hilltop), and the other component that contributes to the DM curvature perturbation. In particular, we explain that an unperturbed axion field in the early universe evolving into one with nontrivial adiabatic perturbations is guaranteed by Weinberg's theorem on adiabatic modes. These results have been derived before with various assumptions, such as a radiation dominated background or a quadratic potential. Our aim is to give a clear, simple derivation that is manifestly independent of those assumptions, and thus can be applied to any cosmological axion scenario.

hep-ph↗

arXiv2025

Cosmic $τ$ensions Indirectly Correlate with Reionization Optical Depth

The reionization optical depth $τ_{\rm reio}$ has interesting connections to existing cosmological anomalies. As first studied in the context of the Hubble tension in our previous paper, a larger $τ_{\rm reio}$, which could be achieved by removing the Planck low-$\ell$ polarization data, could boost $H_0$ slightly, resulting in a mild reduction of the tension between the early- and late-universe determinations of $H_0$. It has been shown later that a larger $τ_{\rm reio}$ could also relieve other anomalies including: the tension between BAO and CMB data, the neutrino mass tension, and the latest DESI plus supernovae data's tension with the standard cosmological constant scenario. In this paper, we systematically analyze the correlations between $τ_{\rm reio}$ and relevant cosmological parameters in the existing cosmic observation anomalies. In addition to Pearson correlation coefficients extracted directly from the covariance matrix, we also study partial correlation coefficients which measure intrinsic relationships between pairs of parameters removing the influence of other parameters. We show that $τ_{\rm reio}$ has weak intrinsic correlations with the parameters responsible for the tensions and anomalies discussed. The large direct Pearson correlations that allow larger $τ_{\rm reio}$ inferences to alleviate the cosmological tensions each arise from complicated networks through multiple parameters. As a result, the relationships between $τ_{\rm reio}$ and each anomaly are not independent of each other. We also employ our method of computing correlations to clarify the impact of large scale polarization data, and comment also on the effects of CMB observations from ACT and SPT.

astro-ph.CO↗

arXiv2025

Reionization and the Hubble Constant: Correlations in the Cosmic Microwave Background

Recently, the James Webb Space Telescope (JWST) has found early galaxies producing photons from more efficient ionization than previously assumed. This may suggest a reionization process with a larger reionization optical depth, $τ_{\rm reio}$, in some mild disagreement with that inferred from measurements of cosmic microwave background (CMB). Intriguingly, the CMB would prefer larger values of $τ_{\rm reio}$, more consistent with the recent JWST hint, if the large-scale measurements (i.e. $\ell <30$) of E-mode polarization are removed. In addition, $τ_{\rm reio}$ has an indirect correlation with today's Hubble constant $H_0$ in $Λ$CDM. Motivated by these interesting observations, we investigate and reveal the underlying mechanism for this correlation, using the CMB dataset without the low-$\ell$ polarization data as a proxy for a potential cosmology with a larger $τ_{\rm reio}$. We further explore how this correlation may impact the Hubble tension between early and late universe measurements of $H_0$, in $Λ$CDM as well as two proposals to alleviate the Hubble tension: the dark radiation (DR) and early dark energy (EDE) models. We find that the Hubble tension gets further reduced mildly for almost all cases due to the larger $τ_{\rm reio}$ and its positive correlation with $H_0$, with either the Baryon Acoustic Oscillations (BAO) data before those from the Dark Energy Spectroscopic Instrument (DESI) or the DESI data.

astro-ph.CO↗

arXiv2025

Reduced Hubble Tension in Dark Radiation Models after DESI 2024

We investigate the presence of extra relativistic degrees of freedom in the early Universe, contributing to the effective number of neutrinos $N_\text{eff}$, as $ΔN_\text{eff}\equiv N_\text{eff}-3.044\geq 0$, in light of the recent measurements of Baryon Acoustic Oscillations (BAO) by the DESI collaboration. We analyze one-parameter extensions of the $Λ$CDM model where dark radiation (DR) is free streaming or behaves as a perfect fluid, due to self-interactions. We report a significant relaxation of upper bounds on $ΔN_\text{eff}$, with respect to previous BAO data from SDSS+6dFGS, when additionally employing Planck data (and supernovae data from Pantheon+), setting $ΔN_\text{eff}\leq 0.39$ ($95\%$ C.L.) for free streaming DR, and a very mild preference for fluid DR, $ΔN_\text{eff} = 0.221^{+0.088}_{-0.18}$ ($\leq 0.46$, $95\%$ C.L.). Applying constraints from primordial element abundances leads to slightly tighter constraints on $ΔN_\text{eff}$, but they are avoided if DR is produced after Big Bang Nucleosynthesis (BBN). For fluid DR we estimate the tension with the SH$_0$ES determination of $H_0$ to be less than $3σ$ and as low as $2σ$, and for free-streaming DR the tension is below $3σ$ if production occurs after BBN. This lesser degree of tension motivates a combination with SH$_0$ES in these cases, resulting in a $4.4σ-5σ$ evidence for dark radiation with $ΔN_\text{eff}\simeq 0.6$ and large improvements in $χ^2$ over $Λ$CDM, $-18\lesssim Δχ^2\lesssim -25$. Upcoming data releases by DESI and other CMB and LSS surveys will decisively confirm or disfavour this conclusion.

astro-ph.CO↗

arXiv2024

Neutrino mass bounds from DESI 2024 are relaxed by Planck PR4 and cosmological supernovae

The recent DESI 2024 Baryon Acoustic Oscillations (BAO) measurements combined with the CMB data from the Planck 18 PR3 dataset and the Planck PR4+ACT DR6 lensing data, with a prior on the sum of the neutrino masses $\sum m_ν>0$, leads to a strong constraint, $\sum m_ν<0.072$ eV, which would exclude the inverted neutrino hierarchy and put some tension on even the standard hierarchy. We show that actually this bound gets significantly relaxed when combining the new DESI measurements with the HiLLiPoP+LoLLiPoP likelihoods, based on the Planck 2020 PR4 dataset, and with supernovae datasets. We note that the fact that neutrino masses are pushed towards zero, and even towards negative values, is known to be correlated with the so-called $A_L$ tension, a mismatch between lensing and power spectrum measurements in the Planck PR3 data, which is reduced by HiLLiPoP+LoLLiPoP to less than 1$σ$. We find $\sum m_ν<0.1$ eV and $\sum m_ν<0.12$ eV, with the supernovae Pantheon+ and DES-SN5YR datasets respectively. The shift caused by these datasets is more compatible with the expectations from neutrino oscillation experiments, and both the normal and inverted hierarchy scenarios remain now viable, even with the $\sum m_ν>0$ prior. Finally, we analyze neutrino mass bounds in an extension of $Λ$CDM that addresses the $H_0$ tension, with extra fluid Dark Radiation, finding that in such models bounds are further relaxed and the posterior probability for $\sum m_ν$ begins to exhibit a peak at positive values.

astro-ph.CO↗

arXiv2024

The Juno Mission as a Probe of Long-Range New Physics

Orbits of celestial objects, especially the geocentric and heliocentric ones, have been well explored to constrain new long-range forces beyond the Standard Model (SM), often referred to as fifth forces. In this paper, for the first time, we apply the motion of a spacecraft around Jupiter to probe fifth forces that don't violate the equivalence principle. The spacecraft is the Juno orbiter, and ten of its early orbits already allow a precise determination of the Jovian gravitational field. We use the shift in the precession angle as a proxy to test non-gravitational interactions between Juno and Jupiter. Requiring that the contribution from the fifth force does not exceed the uncertainty of the precession shift inferred from data, we find that a new parameter space with the mass of the fifth-force mediator around $10^{-14}$ eV is excluded at 95% C.L.

hep-ph↗

arXiv2024

Cosmological probes of Dark Radiation from Neutrino Mixing

Models of stepped dark radiation have recently been found to have an important impact on the anisotropies of the cosmic microwave background, aiding in easing the Hubble tension. In this work, we study models with a sector of dark radiation with a step in its abundance, which thermalizes after big bang nucleosynthesis by mixing with the standard model neutrinos. For this, we extend an earlier work which has focused on the background evolution only until the dark sector thermalizes by deriving the full background and perturbation equations of the model and implementing them in an Einstein-Boltzmann solving code. We expound on the behavior of this model, discussing the wide range of parameters that result in interesting and viable cosmologies that dynamically generate dark radiation during a range of epochs. We find that for the strongly self-coupled regime, there is no large cosmological impact for a tight prior on the mass, whereas larger mass ranges allow a smooth interpolation between a behavior close to the $Λ$CDM cosmological standard model and close to an additional component of strongly self-interacting dark radiation. In the weakly self-coupled regime we find that we can accommodate a parameter space relevant for the neutrino anomalies as well as one relevant to easing the Hubble tension.

astro-ph.CO↗

arXiv2023

Dark Sectors with Mass Thresholds Face Cosmological Datasets

Interacting dark sectors may undergo changes in the number of their relativistic species during the early universe, due to a mass threshold $m$ (similar to changes in the Standard Model bath), and in doing so affect the cosmic history. When such changes occur close to recombination, i.e., for $m\sim (0.1-10)~\text{eV}$, the stringent bound on the effective number of neutrino species, $N_{\text{eff}}$, can be relaxed and the value of the Hubble expansion rate $H_0$ inferred from Cosmic Microwave Background (CMB) observations raised. We search for such sectors (with and without mass thresholds) in the latest cosmological datasets, including the full-shape (FS) of BOSS DR12 galaxy power spectrum. We perform a detailed analysis, accounting for the choice of prior boundaries and additionally exploring the possible effects of dark sector interactions with (a fraction of) the dark matter. We find $ΔN_{\text{eff}}\leq 0.55\, (0.46)$ at 95% C.L. with (without) a mass threshold. While a significantly larger Hubble rate is achieved in this scenario, $H_0=69.01^{+0.66}_{-1.1}$, the overall fit to CMB+FS data does not provide a compelling advantage over the $Λ$CDM model. Furthermore, we find that dark matter interactions with the dark sector do not significantly improve the (matter fluctuations) $S_8$ tension with respect to the $Λ$CDM model. Our work provides model-independent constraints on (decoupled) dark sectors with mass thresholds around the eV scale.

astro-ph.CO↗

arXiv2023

Reduced Axion Abundance from an Extended Symmetry

In recent work we showed that the relic dark matter abundance of QCD axions can be altered when the Peccei-Quinn (PQ) field is coupled to very light scalar/s, rendering the effective axion mass dynamical in the early universe. In this work we develop this framework further, by introducing a new extended symmetry group to protect the new particles' mass. We find that with a new global $SO(N)$ symmetry, with large $N$, we can not only account for the lightness of the new scalars, but we can reduce the axion relic abundance in a technically natural way. This opens up the possibility of large PQ scales, including approaching the GUT scale, and still naturally producing the correct relic abundance of axions. Also, in these models the effective PQ scale is relatively small in the very early universe, and so this can help towards alleviating the isocurvature problem from inflation. Furthermore, instead of possible over-closure from cosmic strings, the extended symmetry implies the formation of non-topological textures which provide a relatively small abundance.

hep-ph↗

arXiv2022

Altered Axion Abundance from a Dynamical Peccei-Quinn Scale

We build a model in which the relic abundance of axions is altered from the standard misalignment mechanism, either increased or decreased, due to the presence of a new light scalar that couples to the radial part of the Peccei-Quinn (PQ) field. The light scalar makes the effective PQ symmetry-breaking scale dynamical, altering the early-time dynamics for the axion and affecting its late-time dark matter abundance. We analyze this new mechanism semianalytically and numerically, showing that we can accommodate both lighter or heavier axion dark matter, compared to the standard treatments. We discuss implications of the model for axion searches and fundamental physics.

hep-ph↗

arXiv2022

Dark Sector to Restore Cosmological Concordance

We develop a new phenomenological model that addresses current tensions between observations of the early and late Universe. Our scenario features: (i) a decaying dark energy fluid (DDE), which undergoes a transition at $z \sim 5,000$, to raise today's value of the Hubble parameter -- addressing the $H_0$ tension, and (ii) an ultra-light axion (ULA), which starts oscillating at $z\gtrsim 10^4$, to suppress the matter power spectrum -- addressing the $S_8$ tension. Our Markov Chain Monte Carlo analyses show that such a Dark Sector model fits a combination of Cosmic Microwave Background (CMB), Baryon Acoustic Oscillations, and Large Scale Structure (LSS) data slightly better than the $Λ$CDM model, while importantly reducing both the $H_0$ and $S_8$ tensions with late universe probes ($\lesssim 3σ$). Combined with measurements from cosmic shear surveys, we find that the discrepancy on $S_8$ is reduced to the $1.4σ$ level, and the value of $H_0$ is further raised. Adding local supernovae measurements, we find that the $H_0$ and $S_8$ tensions are reduced to the $1.4σ$ and $1.2σ$ level respectively, with a significant improvement $Δχ^2\simeq -18$ compared to the $Λ$CDM model. With this complete dataset, the DDE and ULA are detected at $\simeq 4σ$ and $\simeq2σ$, respectively. We discuss a possible particle physics realization of this model, with a dark confining gauge sector and its associated axion, although embedding the full details within microphysics remains an urgent open question. Our scenario will be decisively probed with future CMB and LSS surveys.

astro-ph.CO↗

arXiv2021

General Relativistic Decoherence with Applications to Dark Matter Detection

Quantum mechanics allows for states in macroscopic superpositions, but they ordinarily undergo rapid decoherence due to interactions with their environment. A system that only interacts gravitationally, such as an arrangement of dark matter (DM), may exhibit slow decoherence. In this Letter, we compute the decoherence rate of a quantum object within general relativity, focusing on superposed metric oscillations; a rare quantum general relativistic result. For axion DM in a superposition of the field's phase, we find that DM in the Milky Way is robust against decoherence, while a spatial superposition is not. This novel phase behavior may impact direct detection experiments.

gr-qc↗

arXiv2021

Decoherence from General Relativity

It is of great interest to explore matter in nontrivial quantum arrangements, including Schrodinger cat-like states. Such states are sensitive to decoherence from their environment. Recently, in Ref. [1] we computed the rate of decoherence of a piece of superposed matter that primarily only interacts gravitationally, a dark-matter-Schrodinger-cat-state (DMSCS), within the nonrelativistic approximation. In this work we improve this to a general relativistic analysis. We firstly derive a single particle relativistic Schrodinger equation for a probe particle that passes through the DMSCS; the interaction is provided by the weak field metric of general relativity from the source. For a static DMSCS we find a neat generalization of our previous results. We then turn to the interesting new case of a time dependent DMSCS, which can be provided by a coherently oscillating axion field leading to superposed time dependent oscillations in the metric; a truly quantum-general relativistic phenomenon. We use scattering theory to derive the decoherence rate in all these cases. When the DMSCS is in a superposition of distinct density profiles, we find that the decoherence rate can be appreciable. We then consider the novel special case in which the density is not in a superposition, but the phase of its field oscillation is; this is a property that cannot be decohered within the nonrelativistic framework. We find that if the probe particle and/or the DMSCS's velocity dispersion is slow, then the rate of decoherence of the phase is exponentially suppressed. However, if both the probe and the DMSCS's velocity dispersion are relativistic, then the phase can decohere more rapidly. As applications, we find that diffuse galactic axions with superposed phases are robust against decoherence, while dense boson stars and regions near black hole horizons are not, and we discuss implications for experiment.

gr-qc↗