arXiv subjects

Levon Pogosian

Publications and source records attributed to Levon Pogosian.

At least 19 recordsLinked to original sources

arXiv2026

Cosmological Impact of Redshift-Dependent Type Ia Supernovae Calibration

Type Ia supernovae (SNIa) play a central role in constraining the late-time expansion history of the Universe and are directly implicated in current cosmological tensions. Motivated by the possibility of unaccounted redshift-dependent calibration systematics or new physics, we investigate the impact of a phenomenological correction to SNIa magnitudes that scales with cosmic look-back time. We parameterize this effect with a free amplitude and constrain it using a combination of cosmic microwave background, baryon acoustic oscillation, and SNIa data, considering both $Λ$CDM and dynamical dark energy models. Importantly, our parameterization is not intended to serve as a proxy for SNIa progenitor age, as current observations show no significant difference in standardized SNIa brightness between young and old progenitor populations at low redshift. We find no evidence for a redshift-dependent calibration effect when fitting uncalibrated SNIa data, and its inclusion has a negligible impact on cosmological parameters within $Λ$CDM, nor does it qualitatively change the inferred dynamics of evolving dark energy. When incorporating a prior on the SNIa absolute magnitude from SH0ES, a nonzero calibration parameter is weakly preferred within $Λ$CDM. Interestingly, with dynamical dark energy, the preference of a nonzero calibration parameter increases to $4.3σ$, and it can accommodate both the distance ladder and early-Universe constraints, reducing the Hubble tension to $1.5σ$, with the best-fit model effectively corresponding to a constant equation of state with $w < -1$. Overall, our results indicate that redshift-dependent SNIa calibration effects, as parameterized here, are not supported by current data within $Λ$CDM, but can play a role in reconciling cosmological datasets when combined with extensions to the late-time expansion history.

astro-ph.CO↗

arXiv2026

Sound-Horizon-Agnostic Inference of the Hubble Constant and Neutrino Mass from BAO, CMB Lensing, and Galaxy Weak Lensing and Clustering

We present a sound-horizon-agnostic determination of the Hubble constant, $H_0$, by combining DESI DR2 baryon acoustic oscillation (BAO) data with the latest cosmic microwave background (CMB) lensing measurements from Planck, ACT, and SPT-3G, the angular size of the CMB acoustic scale, Dark Energy Survey Year-3 ($3\times2$-pt) galaxy weak lensing and clustering correlations, and the Pantheon+ supernova sample. In this analysis, The sound horizon at the drag epoch, $r_d$, is treated as a free parameter. By combining uncalibrated comoving distances from BAO and supernovae with constraints on the matter density $Ω_m h^2$ from CMB and galaxy lensing/clustering, we break the $r_d$-$H_0$ degeneracy and obtain $H_0 = 70.0 \pm 1.7$ km/s/Mpc when the sum of the neutrino masses is fixed at $Σm_ν= 0.06$ eV. With an informative prior on the amplitude of primordial fluctuations, $A_s$, we find $H_0 = 70.03 \pm 0.97$ km/s/Mpc. Allowing $Σm_ν$ to vary, we find that the neutrino mass is weakly constrained and strongly prior-dependent. Consequently, the inferred $H_0$ is sensitive to the choice of the $Σm_ν$ prior, with a uniform prior biasing results toward larger neutrino masses and higher $H_0$, while a logarithmic prior reduces this bias significantly. Forecasts for the completed DESI BAO program, combined with Simons-Observatory-like CMB lensing, next-generation $3\times2$-pt data, and expanded supernova samples predict $σ(H_0) \simeq 0.67$ km/s/Mpc with fixed $Σm_ν$, and $σ(H_0) \simeq 1.1$ km/s/Mpc with $Σm_ν< 0.133$ ($<0.263$) eV at 68% (95%) CL when $Σm_ν$ is varied.

astro-ph.CO↗

arXiv2025

Is Dynamical Dark Energy Necessary? DESI BAO and Modified Recombination

Recent measurements of baryon acoustic oscillations (BAO) by the Dark Energy Spectroscopic Instrument (DESI) exhibit a mild-to-moderate tension with cosmic microwave background (CMB) and Type Ia supernova (SN) observations when interpreted within the $Λ$CDM framework. This discrepancy has been cited as evidence for dynamical dark energy (DDE). Given the profound implications of DDE for fundamental physics, we explore whether the tension can instead be resolved by modifying the physics of recombination. We find that a phenomenological model of modified recombination can effectively reconcile the BAO and CMB datasets and, unlike DDE, also predicts a higher Hubble constant $H_0$, thereby partially alleviating the Hubble tension. A global fit to BAO, CMB, and calibrated SN data favors modified recombination over DDE.

astro-ph.CO↗

arXiv2025

Hints of Primordial Magnetic Fields at Recombination and Implications for the Hubble Tension

Primordial Magnetic Fields (PMFs), long studied as relics of the early Universe, accelerate recombination and have been proposed as a way to relieve the Hubble tension. However, previous studies relied on simplified toy models. Here we use recent evaluations of recombination with PMFs, incorporating full magnetohydrodynamic (MHD) simulations and detailed Lyman-alpha radiative transfer, to test PMF-enhanced recombination ($bΛ$CDM) against observational data from the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), and Type Ia supernovae (SN). Focusing on non-helical PMFs with a Batchelor spectrum, we find a preference for present-day total field strengths of approximately 5-10 pico-Gauss. Depending on the dataset combination, this preference ranges from mild ($\sim 1.8σ$ with Planck + DESI) to moderate ($\sim 3σ$ with Planck + DESI + SH0ES-calibrated SN) significance. The $bΛ$CDM has Planck + DESI $χ^2$ values equal to or better than $Λ$CDM while predicting a higher Hubble constant. Future high-resolution CMB temperature and polarization measurements will be crucial for confirming or further constraining PMFs at recombination. Field strengths of 5-10 pico-Gauss align closely with those required for cluster magnetic fields to originate entirely from primordial sources, without the need for additional dynamo amplification.

astro-ph.CO↗

arXiv2025

Dynamical Dark Energy in light of the DESI DR2 Baryonic Acoustic Oscillations Measurements

Understanding whether cosmic acceleration arises from a cosmological constant or a dynamical component is a central goal of cosmology, and the Dark Energy Spectroscopic Instrument (DESI) enables stringent tests with high-precision distance measurements. We analyze baryon acoustic oscillation (BAO) measurements from DESI Data Release 1 (DR1) and Data Release 2 (DR2), combined with Type Ia supernovae and a cosmic microwave background (CMB) distance prior. With the larger statistical power and wider redshift coverage of DR2, the preference for dynamical dark energy does not diminish relative to DR1. Using both a shape-function reconstruction and non-parametric approaches with a Horndeski-motivated correlation prior, we find that the dark-energy equation of state $w(z)$ varies with redshift. BAO data alone yield modest constraints, but in combination with independent supernova compilations and the CMB prior they strengthen the evidence for dynamics. Bayesian model comparison shows moderate support for departures from $Λ$CDM when multiple degrees of freedom in $w(z)$ are allowed, corresponding to $\approx3σ$ tension with $Λ$CDM (and higher for some data sets). Despite methodological differences, our results are consistent with companion DESI papers, underscoring the complementarity of approaches. Possible systematics remain under study; forthcoming DESI, \emph{Euclid}, and next-generation CMB data will provide decisive tests.

astro-ph.CO↗

arXiv2025

Cosmic strings in the complex symmetron model

We study cosmic strings in the complex symmetron model, a scalar-tensor theory with a spontaneously broken local $U(1)$ symmetry in low matter density regions. Using numerical simulations, we show that these strings preferentially attach to matter halos, leading to the stabilization of string loops. While the requirement for screening of fifth-force interactions in the solar system limits observable signatures in theories with universal coupling to matter, analogous topological defects in the dark sector may still influence cosmic structure formation, offering a novel avenue to constrain dark-sector interactions.

hep-ph↗

arXiv2025

Modified recombination and the Hubble tension

We investigate the extent to which modifying the ionization history at cosmological recombination can relieve the Hubble tension, taking into account all relevant datasets and considering the implications for the galaxy clustering parameter $S_8$ and the matter density fraction $Ω_m$. We use the linear response approximation to systematically search for candidate ionization histories parameterized with a cubic-spline that provide good fits to the Planck CMB and DESI BAO data while relieving the $H_0$ tension, followed by MCMC fits of the most promising candidate models to the data. We also fit to the data a physically motivated phenomenological model of ionization history that has four parameters. Our main result is that models of modified recombination can reduce the Hubble tension to below 2$σ$ while improving the fit to the current CMB and BAO data and reducing the $S_8$ tension. The promising candidate ionization histories have simple shapes, with no need for an oscillatory dependence on redshift. Our study also demonstrates the importance of the high-resolution CMB temperature and polarization anisotropies for constraining modified recombination, with the candidate models in this study showing varying levels of agreement with the current ACT DR4 and SPT-3G data.

astro-ph.CO↗

arXiv2024

Disentangling modified gravity from a dark force with gravitational redshift

The standard approach to test for deviations from general relativity on cosmological scales is to combine measurements of the growth rate of structure with gravitational lensing. In this study, we show that this method suffers from an important limitation with regard to these two probes: models of dark matter with additional interactions can lead to the very same observational signatures found in modified gravity and vice versa. Using synthetic data of redshift-space distortions, weak lensing, and cosmic microwave background, we demonstrate that this degeneracy is inevitable between modifications of gravity and a dark fifth force. We then show that the coming generation of surveys, in particular the Square Kilometre Array, will allow us to break the degeneracy between such models through measurements of gravitational redshift. Performing a Markov chain Monte Carlo analysis of the synthetic dataset, we quantify the extent to which gravitational redshift can distinguish between two representative classes of models, generalized Brans-Dicke (modified gravity) and coupled quintessence (fifth force).

astro-ph.CO↗

arXiv2024

Extending MGCAMB tests of gravity to nonlinear scales

Modified Growth with {\tt CAMB} ({\tt MGCAMB}) is a patch for the Einstein-Boltzmann solver {\tt CAMB} for cosmological tests of gravity. Until now, {\tt MGCAMB} was limited to scales well-described by linear perturbation theory. In this work, we extend the framework with a phenomenological model that can capture nonlinear corrections in a broad range of modified gravity theories. The extension employs the publicly available halo model reaction code {\tt ReACT}, developed for modeling the nonlinear corrections to cosmological observables in extensions of the $Λ$CDM model. The nonlinear extension makes it possible to use a wider range of data from large scale structure surveys, without applying a linear scale cut. We demonstrate that, with the 3$\times$2pt Dark Energy Survey data, we achieve a stronger constraint on the linear phenomenological functions $μ$ and $Σ$, after marginalzing over the additional nonlinear parameter $p_1$, compared to the case without the nonlinear extension and using a linear cut. The new version of {\tt MGCAMB} is now forked with {\tt CAMB} on GitHub allowing for compatibility with future upgrades.

astro-ph.CO↗

arXiv2024

A consistency test of the cosmological model at the epoch of recombination using DESI BAO and Planck measurements

The value of the Hubble constant determined from CMB and BAO measurements is directly dependent on the sound horizon at the photon-baryon decoupling. There has been significant interest in the possibility of new physics at the epoch around recombination that could reduce the sound horizon and increase the inferred value of $H_0$, thus helping to relieve the Hubble tension. One way to determine if new physics is required would be to measure $H_0$ from BAO and CMB without assuming any model for computing the sound horizon. In this study, we use the recently released DESI Year 1 BAO data combined with the CMB acoustic scale and the Planck $Λ$CDM prior on $Ω_{\rm m} h^2$ to determine $H_0$ while treating the sound horizon at baryon decoupling $r_{\rm d}$ as a free parameter. We find $H_0=69.48 \pm 0.94$ km/s/Mpc, which is $\sim2σ$ larger than $H_0 = 67.44 \pm 0.47$ km/s/Mpc in the Planck-best-fit $Λ$CDM where $r_{\rm d}$ is derived using the standard recombination model. For comparison, we perform the same analysis using the pre-DESI BAO data with the CMB acoustic scale and the same prior on $Ω_{\rm m} h^2$, finding $H_0= 68.05 \pm 0.94$ km/s/Mpc. This difference derives from the notably larger value of the product $r_{\rm d}h$ measured by DESI. We compare results obtained with and without including the Pantheon Plus sample of uncalibrated supernovae magnitudes in our analysis. Future BAO data from DESI will help determine if the cosmological model at the epoch of recombination model requires a modification.

astro-ph.CO↗

arXiv2023

New MGCAMB tests of gravity with CosmoMC and Cobaya

We present a new version of MGCAMB, a patch for the Einstein-Boltzmann solver CAMB for cosmological tests of gravity. New features include a new cubic-spline parameterization allowing for a simultaneous reconstruction of $μ$, $Σ$ and the dark energy density fraction $Ω_X$ as functions of redshift, the option to work with a direct implementation of $μ$, $Σ$ (instead of converting to $μ$, $γ$ first), along with the option to test models with a scalar field coupled only to dark matter, and the option to include dark energy perturbations when working with $w\ne -1$ backgrounds, to restore consistency with CAMB in the GR limit. This version of MGCAMB comes with a Python wrapper to run it directly from the Python interface, an implementation in the latest version of CosmoMC, and can be used with Cobaya.

astro-ph.CO↗

arXiv2023

On validity of the quasi-static approximation in scalar-tensor theories

The discovery of cosmic acceleration motivated extensive studies of dynamical dark energy and modified gravity models. Of particular interest are the scalar-tensor theories, with a scalar field dark energy non-minimally coupled to matter. Cosmological constraints on these models often employ the quasi-static approximation (QSA), in which the dynamics of the scalar field perturbations is proportional to the perturbation in the matter density. Using the QSA simplifies the physical interpretation of the phenomenology of scalar-tensor theories, and results in substantial savings of computing time when deriving parameter constraints. Focusing on the symmetron model, which is a well-motivated scalar-tensor theory with a screening mechanism, we compare the exact solution of the linearly perturbed field equations to those obtained under the QSA and identify the range of the model parameters for which the QSA is valid. We find that the evolution of background scalar field is most important, namely, whether it is dominated by the Hubble friction or the scalar field potential. This helps us derive a criterion for the symmetron model, but same argument can be applied to other scalar-tensor theories of generalized Brans-Dicke type. We consider two scenarios, one where the scalar field is only coupled to dark matter and where it couples to all of the matter.

astro-ph.CO↗

arXiv2023

Primordial magnetic fields and the Hubble tension

Magnetic fields appear to be present in essentially all astrophysical environments, including galaxies, clusters of galaxies and voids. There are both observational and theoretical motives for considering the possibility of their origin tracing back to the events in the very early universe, such as the electroweak phase transition or Inflation. Such a primordial magnetic field (PMF) would remain embedded in the plasma and evolve to persist through the radiation and matter eras, and to the present day. As described in this Chapter, a PMF present in the primordial plasma prior to recombination could help relieve the Hubble tension. A stochastic magnetic field would induce inhomogeneities, pushing the baryons into regions of lower magnetic energy density and speeding up the recombination process. As a consequence, the sound horizon at last scattering would be smaller, which is a necessary ingredient for relieving the Hubble tension. Intriguingly, the strength of the magnetic field required to alleviate the tension is of the right order to also explain the observed magnetic fields in galaxies, clusters of galaxies and voids. These findings motivate further detailed studies of recombination in the presence of PMFs and observational tests of this hypothesis.

astro-ph.CO↗

arXiv2023

Modified Einstein versus Modified Euler for Dark Matter

Modifications of General Relativity generically contain additional degrees of freedom that can mediate forces between matter particles. One of the common manifestations of a fifth force in alternative gravity theories is a difference between the gravitational potentials felt by relativistic and non-relativistic particles, also known as "the gravitational slip". In contrast, a fifth force between dark matter particles, due to dark sector interaction, does not cause a gravitational slip, making the latter a possible smoking gun of modified gravity. In this article, we point out that a force acting on dark matter particles, as in models of coupled quintessence, would also manifest itself as a measurement of an effective gravitational slip by cosmological surveys of large-scale structure. This is linked to the fact that redshift-space distortions due to peculiar motion of galaxies do not provide a measurement of the true gravitational potential if dark matter is affected by a fifth force. Hence, it is extremely challenging to distinguish a dark sector interaction from a modification of gravity with cosmological data alone. Future observations of gravitational redshift from galaxy surveys can help to break the degeneracy between these possibilities, by providing a direct measurement of the distortion of time. We discuss this and other possible ways to resolve this important question.

astro-ph.CO↗

arXiv2022

Imprints of cosmological tensions in reconstructed gravity

There has been a significant interest in modifications of the standard $Λ$ Cold Dark Matter ($Λ$CDM) cosmological model prompted by tensions between certain datasets, most notably the Hubble tension. The late-time modifications of the $Λ$CDM model can be parametrized by three time-dependent functions describing the expansion history of the Universe and gravitational effects on light and matter in the Large Scale Structure. We perform the first joint Bayesian reconstruction of these three functions from a combination of recent cosmological observations, utilizing a theory-informed prior built on the general Horndeski class of scalar-tensor theories. This reconstruction is interpreted in light of the well-known $H_0$, the $S_8$ and the $A_L$ tensions. We identify the phenomenological features that alternative theories would need to have in order to ease some of the tensions, and deduce important constraints on broad classes of modified gravity models. Among other things, our findings suggest that late-time dynamical dark energy and modifications of gravity are not likely to offer a solution to the Hubble tension, or simultaneously solve the $A_L$ and $S_8$ tensions.

astro-ph.CO↗

arXiv2022

Principal reconstructed modes of dark energy and gravity

Recently, in [1], we presented the first combined non-parametric reconstruction of the three time-dependent functions that capture departures from the standard cosmological model, $Λ$CDM, in the expansion history and gravitational effects on matter and light from the currently available combination of the background and large scale structure data. The reconstruction was performed with and without a theory-informed prior, built on the general Horndeski class of scalar-tensor theories, that correlates the three functions. In this work, we perform a decomposition of the prior and posterior covariances of the three functions to determine the structure of the modes that are constrained by the data relative to the Horndeski prior. We find that the combination of all data can constrain 15 combined eigenmodes of the three functions with respect to the prior. We examine and interpret their features in view of the well-known tensions between datasets within the $Λ$CDM model. We also assess the bias introduced by the simplistic parameterizations commonly used in the literature for constraining deviations from GR on cosmological scales.

astro-ph.CO↗

arXiv2022

Report of the Topical Group on Cosmic Probes of Fundamental Physics for for Snowmass 2021

Cosmic Probes of Fundamental Physics take two primary forms: Very high energy particles (cosmic rays, neutrinos, and gamma rays) and gravitational waves. Already today, these probes give access to fundamental physics not available by any other means, helping elucidate the underlying theory that completes the Standard Model. The last decade has witnessed a revolution of exciting discoveries such as the detection of high-energy neutrinos and gravitational waves. The scope for major developments in the next decades is dramatic, as we detail in this report.

hep-ph↗

arXiv2021

Consistency of Planck, ACT and SPT constraints on magnetically assisted recombination and forecasts for future experiments

Primordial magnetic fields can change the recombination history of the universe by inducing clumping in the baryon density at small scales. They were recently proposed as a candidate model to relieve the Hubble tension. We investigate the consistency of the constraints on a clumping factor parameter $b$ in a simplistic model, using the latest CMB data from Planck, ACT DR4 and SPT-3G 2018. For the combined CMB data alone, we find no evidence for clumping being different from zero, though when adding a prior on $H_0$ based on the latest distance-ladder analysis of the SH0ES team, we report a weak detection of $b$. Our analysis of simulated datasets shows that ACT DR4 has more constraining power with respect to SPT-3G 2018 due to the degeneracy breaking power of the TT band powers (not included in SPT). Simulations also suggest that the TE,EE power spectra of the two datasets should have the same constraining power. However, the ACT DR4 TE,EE constraint is tighter than expectations, while the SPT-3G 2018 one is looser. While this is compatible with statistical fluctuations, we explore systematic effects which may account for such deviations. Overall, the ACT results are only marginally consistent with Planck or SPT-3G, at the $2-3σ$ level within $Λ$CDM+$b$ and $Λ$CDM, while Planck and SPT-3G are in good agreement. Combining the CMB data together with BAO and SNIa provides an upper limit of b<0.4 at 95\% c.l. (b<0.5 without ACT). Adding a SH0ES-based prior on the Hubble constant gives $b = 0.31^{+0.11}_{-0.15}$ and $H_0=69.28 \pm 0.56$ km/s/Mpc ($b = 0.41^{+0.14}_{-018}$ and $H_0=69.70 \pm 0.63$ km/s/Mpc without ACT). Finally, we forecast constraints on $b$ for the full SPT-3G survey, Simons Observatory, and CMB-S4, finding improvements by factors of 1.5 (2.7 with Planck), 5.9 and 7.8, respectively, over Planck alone.

astro-ph.CO↗