arXiv subjects

Alain Blanchard

Publications and source records attributed to Alain Blanchard.

At least 19 recordsLinked to original sources

arXiv2026

Growth, geometry, and early-universe split of the matter density parameter $\Omega_{\rm m}$

While the $\Lambda$ cold dark matter ($\Lambda$CDM) model can successfully reproduce the measurements of many cosmological probes, some discrepancies have recently emerged. Therefore, it is necessary to test the standard cosmological model for consistency. An important stress test is to separate the influence of different cosmological regimes on the parameter inference. We treat three regimes separately here: geometry, growth, and the early universe. The geometrical regime concerns the expansion and curvature history, while the growth regime governs structure formation and the early-universe regime affects physics prior to recombination. Previous analyses have performed the split between geometry and growth, whereas we also consider the early universe influence separately. We perform this split for the present day matter density parameter $\Omega_{\rm m}$ using multiple cosmological observables. The used data are galaxy clustering and weak lensing statistics (3x2pt) from the Dark Energy Survey (DES), cosmic microwave background (CMB) data from Planck, spectroscopic baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI), type-Ia supernovae (SNe Ia) samples from Pantheon+, and redshift-space distortions (RSD) from a collection of galaxy surveys. For each of these probes, we introduce a phenomenological split into these three regimes. This work shows a strong correlation between the geometric and the early regime for the matter density, but no strong correlation between the growth regime and the others. All regimes are compatible in the posterior distribution, however the difference between the geometry and the early regimes, $\Delta\Omega_{\rm m}^{\rm geo,early}$, is 2$\sigma$ apart from 0.

astro-ph.CO↗

arXiv2026

Cosmology since the first Astro/Cosmo Moriond meeting// The emergence of the Big Bang 2.0

This paper presents a necessarily incomplete review of the evolution of cosmology since the first Astro/Cosmo Moriond meeting in 1981. I trace the journey from the classical Big Bang model based on three pillars -- universe expansion, primordial nucleosynthesis, and the cosmic microwave background -- to the modern $\Lambda$CDM paradigm and the discovery of cosmic acceleration. I discuss major observational milestones: the COBE discovery of CMB fluctuations, the CMB measurements of the flat universe, the pivotal discovery of accelerated expansion through Type Ia supernovae and the emergence of precision cosmology with Planck. I review current tensions in cosmological parameters, particularly the Hubble tension and $\s8$ discrepancies, and discuss future prospects from large-scale structure surveys like DESI. The emergence of ``Big Bang 2.0'' reflects the profound paradigm shift from a model based on standard physics to a dynamical cosmos dominated by dark matter and dark energy, the description of which requires a physics that has yet to be developed and validated.

astro-ph.CO↗

arXiv2026

Polarization of the CMB in the Standard Model Extension

In standard cosmology, Cosmic Microwave Background photons near the last scattering surface exhibit only linear polarization due to Compton scattering, leading to the assumption that primordial circular polarization is negligible. However, the physics of Lorentz violation (LV), associated with specific operators, can influence these polarization characteristics. This study employs the Boltzmann equation within the framework of the Standard Model Extension (SME) to explore how the background LV tensor $ K_{AF} $ can induce circular polarization in CMB radiation. By computing the transformation of linear polarization into circular polarization and utilizing the Faraday conversion angle, we derive a bound for $ K_{AF} $ on the order of $ 10^{-41} \, \text{GeV} $, aligning with recent findings. Additionally, we consider the total pure photon terms within the SME, demonstrating that LV in the presence of scalar perturbations can also generate cosmic birefringence (CB) in the CMB radiation. Through analysis of best-fitting CB angles, we establish a more stringent bound of approximately $ 10^{-32} \, \text{GeV} $ for $ K_{F} $.

astro-ph.CO↗

arXiv2025

The optical analogy between a Lorentz-violating cosmos and a Magneto-Electric medium

The goal of our study is to investigate the effects of Lorentz symmetry violation by examining the behavior of photons within the framework of the Standard Model Extension (SME). We show that, from an optical point of view, the Lorentz-violating cosmos is analogous to a Magneto-Electric medium like the quantum vacuum in the presence of the magnetic and electric field. This analogy provides a formulation for exploring experimental evidence of Lorentz symmetry violation in the propagation of light in a vacuum, in particular in the case of a radiation background observed like the CMB.

hep-ph↗

arXiv2025

Phase Diffusion of Light Immersed In Quantum Tides: Open Quantum System Approach

The interaction between quantum gravitational waves (GWs) and electromagnetic (EM) fields is investigated within the open quantum system formalism, where GWs are considered as a heat bath reservoir occupying a generic state $\hat{\rho}_{\text{gws}}$. Following the quantum Langevin equations, it turns out that the correlations of the Langevin noise operator associated with the GW background directly determine the statistical properties of the EM phasor $\phi(t)$. We apply this formalism to the background of inflationary-generated primordial gravitational waves (PGW). Since this background has an astronomically large correlation time, of the order of the Hubble time $H_0^{-1}$, we show that it leads to a non-Markovian dynamics of the EM field, which causes memory effects. As a result of the Gaussianity of PGW, it turns out that the EM phasor goes through a stochastic process, which is a manifestation of the fluctuation-dissipation in EM-GW system. The variance of the EM phase smears out as $\Delta^2\varphi(t)= \Delta^2\varphi_0+ 4(t/\tau_c)^4$, where the characteristic time scale $\tau_c$ is associated with the diffusion rate caused by PGWs. The specific quartic growth of the phase noise is thus attributed to the two-mode squeezed nature of PGWs, which is inherently different from the phase diffusion induced by vacuum fluctuations of spacetime or a thermal heat bath of gravitons.

gr-qc↗

arXiv2025

The fallacies of LCDM falsifications

In recent years, numerous arguments have emerged suggesting that the LCDM (Lambda Cold Dark Matter) model may be inconsistent with observational data, requiring more or less radical revisions. Notable examples include the Hubble tension, the discrepancy between early and late-universe measurements of the Hubble constant, as well as tensions in measurements of cosmic structure growth. These issues have led some to question the validity of the LCDM framework and consider possible modifications or alternative models. However, upon closer inspection, many of these critiques stem from methodological or interpretive disagreements rather than from clear falsifications in the strict Popperian sense. Karl Popper proposed that scientific theories must be testable and falsifiable; in other words, a theory should be rejected if it fails a specific, reproducible test. Yet, many of the alleged inconsistencies within LCDM, while statistically significant, are not definitive falsifications but rather indicators of areas needing refinement or more complex modeling within the same framework. Thus, I review the recent claims about LCDM's limitations and analyze why they often reflect individual biases or philosophical preferences, rather than rigorous scientific falsifications. For example, alternative cosmological models such as MOND (Modified Newtonian Dynamics) or models incorporating new physics like quintessence or modified gravity are sometimes advocated based on theoretical appeal rather than direct evidence from critical tests. In many cases, these arguments for falsifying LCDM reveal more about subjective interpretations of data than about concrete observational contradictions.

physics.hist-ph↗

arXiv2025

Revisiting van Citter-Zernike correlations in the presence of primordial gravitational waves

In this paper, we develop a quantum field theory framework to describe the interaction between a gravitational wave (GW) background and an electromagnetic (EM) field emitted from a distant celestial source, such as a star. We demonstrate that a background of primordial gravitational waves (PGWs), as predicted by the inflationary scenario, induces a loss of spatial coherence in the EM field as it propagates over cosmological distances. This effect leads to the degradation of van Cittert-Zernike correlations, ultimately rendering them unobservable - a phenomenon referred to as blurring. Since spatial coherence is observed in very long baseline interferometry (VLBI) measurements of distant quasars, this places constraints on the amplitude of the PGW background. We quantitatively evaluate the blurring effect caused by PGWs in a two-mode squeezed state, which represents the standard quantum state predicted by the simplest inflationary models. However, due to the weak coupling between GWs and the EM field, we find that the induced incoherence is too small to be detected in current VLBI observations.

gr-qc↗

arXiv2025

Probing the Distance Duality Relation with Machine Learning and Recent Data

The distance duality relation (DDR) relates two independent ways of measuring cosmological distances, namely the angular diameter distance and the luminosity distance. These can be measured with baryon acoustic oscillations (BAO) and Type Ia supernovae (SNe Ia), respectively. Here, we use recent DESI DR1, Pantheon+, SH0ES and DES-SN5YR data to test this fundamental relation. We employ a parametrised approach and also use model-independent Generic Algorithms (GA), which are a machine learning method where functions evolve loosely based on biological evolution. When we use DESI and Pantheon+ data without Cepheid calibration or big bang nucleosynthesis (BBN), there is a $2\sigma$ violation of the DDR in the parametrised approach. Then, we add high-redshift BBN data and the low-redshift SH0ES Cepheid calibration. This reflects the Hubble tension since both data sets are in tension in the standard cosmological model $\Lambda$CDM. In this case, we find a significant violation of the DDR in the parametrised case at $6\sigma$. Replacing the Pantheon+ SNe Ia data by DES-SN5YR, we find similar results. For the model-independent approach, we find no deviation in the uncalibrated case and a small deviation with BBN and Cepheids which remains at 1$\sigma$. This shows the importance of considering model-independent approaches for the DDR.

astro-ph.CO↗

arXiv2024

Cosmological measurement of the gravitational constant $G$ using the CMB, the BAO and the BBN

Recent cosmological observations have provided numerous new observations with increasing precision that have led to the era of precision cosmology. The exquisite quality of these observations opens new possibilities towards measuring fundamental constants with good precision and at scales which are complementary to the laboratory ones. In particular, the cosmic microwave background (CMB) temperature and polarization spectra contain a wealth quantity of information, well beyond the basic cosmological parameters. In this paper, we update the precision on a cosmological determination of $G$ by using the latest Planck data release (PR4) in combination with the latest baryon acoustic oscillation (BAO) from the Dark Energy Spectroscopic Instrument (DESI) data release 1 and the primordial Helium fraction from BBN. We demonstrate a precision of $1.8\%$, corresponding to a $\sim25\%$ improvement compared to the literature. This measurement is compatible with laboratory ones within one standard deviation. Finally, we show that this cosmological measurement of $G$ is robust against several assumptions made on the cosmological model, in particular when considering a non-standard dark energy fluid or non-flat models.

astro-ph.CO↗

arXiv2024

Generation of the CMB cosmic Birefringence through Axion-like particles, Sterile and Active neutrinos

The cosmic birefringence (CB) angle refers to the rotation of the linear polarization plane of Cosmic Microwave Background (CMB) radiations when parity-violating theories are considered. We analyzed the Quantum Boltzmann equation for an ensemble of CMB photons interacting with the right-handed sterile neutrino dark matter (DM) and axion-like particles (ALPs) DM in the presence of the scalar metric perturbation. We used the birefringence angle of CMB to study those probable candidates of DM. It is shown that the CB angle contribution of sterile neutrino is much less that two other sources considered here. Next, we combined the results of the cosmic neutrinos' contribution and the contribution of the ALPs to producing the CMB birefringence and discussed the uncertainty on the parameter space of axions caused by the share of CMB-cosmic neutrino interaction in generating this effect. Finally, we plotted the EB power spectrum of the CMB and showed that this spectrum behaves differently in the presence of cosmic neutrinos and ALPs interactions in small $l$. Hence, future observed data for $C^{l}_{EB}$, will help us to distinguish the CB angle value due to the various sources of its production.

hep-ph↗

arXiv2024

Muon anomalous magnetic moment and Right handed sterile neutrino

The muon's magnetic moment is a fundamental quantity in particle physics and the deviation of its value from quantum electrodynamics (QED), motivates research beyond the standard models (SM). In this study, we utilize the effective coupling of right-handed sterile neutrinos with SM gauge bosons to calculate the muon anomalous magnetic moment ($\boldsymbol{\mu}$AMM) at one-loop level. The contribution of the sterile neutrino interactions on the $\boldsymbol{\mu}$AMM is calculated by considering the standard and non-standard neutrino interactions. Our results show that the standard sterile neutrino interactions give a negligible contribution to $\Delta a_{\boldsymbol{\mu}}$ while the non-standard neutrino interactions can play a significant role in explaining the muon $(g-2)$ anomaly. In the context of the non-standard neutrino interaction, our calculation shows that a Dirac mass scale $M_D$ around $100\,\text{GeV}$ could explain the muon anomaly if the right handed sterile neutrino's coupling with SM particles is about $\mathcal{G}_R\approx 10^{-3}$. We have also plotted the allowed region of the model parameters that satisfy the experimental data on $\Delta a_{{\boldsymbol{\mu}}}^{SN}$ and discuss the percentage of the ${\boldsymbol{\mu}}$ anomaly compensation in terms of the coupling constant $\mathcal{G}_R$.

hep-ph↗

arXiv2024

Cosmological implications of the Gaia Milky Way declining rotation curve

Although the existence of dark matter has been widely acknowledged in the cosmology community, it is as yet unknown in nature, despite decades of research, which questions its very existence. This never-ending search for dark matter leads to consider alternatives. Since increasing the enclosed mass is the only way to explain the flat appearance of galaxies' rotation curves in a Newtonian framework, the MOND theory proposed to modify Newton's dynamics when the acceleration is around or below a threshold value, $a_0$. Observed rotation curves, generally flat at large distances, are then usually well reproduced by MOND with $a_0 \sim 1.2 10^{-10}$ m/s$^{2}$. However, the recent Gaia evidence of a decline in the Milky Way rotation curve is a distinct behavior. Therefore, we examine whether MOND can accommodate the Gaia declining rotation curve of the Milky Way. We first depict a standard model to describe the Milky Way's baryonic components. Secondly, we show that a NFW (Navarro, Frenk, \& White ) model is able to fit the decline, assuming a scale radius $R_s$ of the order of $4$ kpc. In a third step, we show that the usual MOND paradigm is not able to reproduce the declining part for a standard baryonic model. Finally, we examine whether the MOND theory can accommodate the declining part of the rotation curve when relaxing the characteristics of the baryonic components. To do so we use a MCMC method on the characteristics of the stellar and the HI disk, including their mass. We found that the stellar disk should be massive, of the order of $10^{11}$ M$_{\odot}$. The HI disk mass is capped at nearly 1.8 $ 10^{11}$ M$_{\odot}$ but could also be negligible. Finally, $a_0$ is consistent with 0, with an upper limit of $0.53 10^{-10}$ m/s$^{2}$ (95\%), a value much lower than the above mentioned value usually advocated to explain standard flat rotation curves in MOND theory.

astro-ph.CO↗

arXiv2024

$Λ$CDM is alive and well

The $Λ$CDM model faces several tensions with recent cosmological data and their increased accuracy. The mismatch between the values of the Hubble constant $H_0$ obtained from direct distance ladder measurements and from the cosmic microwave background (CMB) is the most statistically significant, but the amplitude of the matter fluctuations is also regarded as a serious concern, leading to the investigation of a plethora of models. We first show that the combination of several recent measurements from local probes leads to a tight constraint on the present-day matter density $Ω_M$ as well as on the amplitude of the matter fluctuations, both acceptably consistent with the values inferred from the CMB. Secondly, we show that the data on cosmic chronometers allow to derive an accurate value of the Hubble constant $H_0$ for $Λ$CDM models: $H_0 = 67.4 \pm 1.34$ km/s/Mpc. This implies that, within $Λ$CDM, some determinations of $H_0$ are biased. Considering a bias on the Hubble constant as a nuisance parameter within $Λ$CDM, we examine such a $Λ$CDM$+ H_0$ bias model on the same statistical grounds as alternative cosmological models. We show that the former statistically supersede most existing extended models proposed up to now. In a third step, we show that the value of $Ω_M$ we obtained, combined with $H_0$ from SH0ES, leads to an accurate measurement of $ω_M$, providing an additional low-redshift test for cosmological models. From this test, most extensions seem to be confronted with a new tension, whereas the $Λ$CDM with $H_0 \sim 67 $ has none. We conclude that a standard $Λ$CDM model with an unknown bias in the Cepheids distance calibration represents a model that reaches a remarkable agreement, statistically better than previously proposed extensions with $H_0 \sim 73 $ for which such a comparison can be performed. (abridged)

astro-ph.CO↗

arXiv2022

Cluster counts III. $Λ$CDM extensions and the cluster tension

In this work, we examine whether further extensions to the $Λ$CDM model could alleviate the $σ_8$ tension. For that, we derive constraints on the parameters subject of the discrepancy, using CMB $C_{\ell}$ combined with cluster counts SZ sample with a free dark energy equation of state parameter while allowing the clusters mass calibration parameter $(1-b)$ to vary. The latter is degenerate with $σ_8$, which translates the discrepancy within $Λ$CDM framework into one between $(1-b) \sim0.6$, corresponding to constraints on $σ_8$ obtained from CMB, and $(1-b)\sim0.8$, the value adopted for the SZ sample calibration. We find that a constant $w$, when left free to vary along with large priors on the matter density ([0.1,1.0]) and Hubble parameters ([30,200]), can reduce the discrepancy to less than 2$σ$ for values far below its fiducial $w$ = -1. However, the latter were not allowed when we additionally combine with other probes like BAO feature angular diameter distance measured in galaxies clustering surveys. We found also, when we allow to vary, in addition to $w$, a modification of the growth rate through the growth index $γ$, that the tension is alleviated, with $(1-b)$ likelihood now centered around the Planck calibration value of $\sim$ 0.8. However, here again, combining with geometrical distance probes restores the discrepancy, with $(1-b)$ preferred value reverting back to the $Λ$CDM one of $\sim$ 0.6. The same situation is observed when introducing, along with $w$ and $γ$, further extensions to $Λ$CDM like massive neutrinos, although the latter allows to reduce the tension to 2$σ$, even when combining with BAO datasets. We conclude that none of these common extensions to $Λ$CDM is able to fix the discrepancy and a misdetermination of the calibration factor is the most preferred explanation... (Abridged)

astro-ph.CO↗

arXiv2022

Can Dark Energy Emerge from a Varying $G$ and Spacetime Geometry?

The accelerated expansion of the universe implies the existence of an energy contribution known as dark energy. Associated with the cosmological constant in the standard model of cosmology, the nature of this dark energy is still unknown. We will discuss an alternative gravity model in which this dark energy contribution emerges naturally, as a result of allowing for a time-dependence on the gravitational constant, $G$, in Einstein's field equations. With this modification, Bianchi's identities require an additional tensor field to be introduced so that the usual conservation equation for matter and radiation is satisfied. The equation of state of this tensor field is obtained using additional constraints, coming from the assumption that this tensor field represents the space-time response to the variation of $G$. We will also present the predictions of this model for the late-universe data, and show that the energy contribution of this new tensor is able to explain the accelerated expansion of the universe without the addition of a cosmological constant. Unlike many other alternative gravities with varying gravitational strength, the predicted $G$ evolution is also consistent with local observations and therefore this model does not require screening. We will finish by discussing possible other implications this approach might have for cosmology and some future prospects.

gr-qc↗

arXiv2021

Closing up the cluster tension?

The excellent measurements of the cosmic microwave background (CMB) fluctuations by Planck allow us to tightly constrain the amplitude of matter fluctuations at redshift $\sim 1100$ in the $Λ$-cold dark matter ($Λ$CDM) model. This amplitude can be extrapolated to the present epoch, yielding constraints on the value of the $σ_8$ parameter. On the other hand, the abundance of Sunyaev-Zeldovich (SZ) clusters detected by Planck, with masses inferred using a hydrostatic equilibrium assumption, leads to a significantly lower value of the same parameter. This discrepancy is often dubbed the $σ_8$ tension in the literature and is sometimes regarded as a possible sign of new physics. Here, we examine a direct determination of $σ_8$ at the present epoch in $Λ$CDM, and thereby the cluster mass calibrations using cosmological data at low redshift, namely the measurements of $fσ_8$ from the analysis of the completed Sloan Digital Sky Survey (SDSS). We combined redshift-space distortion measurements with Planck CMB constraints, X-ray, and SZ cluster counts within the $Λ$CDM framework, but leaving the present-day amplitude of matter fluctuations as an independent parameter (i.e. no extrapolation is made from high-redshift CMB constraints). The calibration of X-ray and SZ masses are left as free parameters throughout the whole analysis. Our study yields tight constraints on the aforementioned calibrations, with values entirely consistent with results obtained from the full combination of CMB and cluster data only. Such an agreement suggests an absence of tension in the $Λ$CDM model between CMB-based estimates of $σ_8$ and constraints from low-redshift on $fσ_8$; however, it also indicates tension with the standard calibration of clusters masses.

astro-ph.CO↗

arXiv2021

Gravitation And the Universe from large Scale-Structures: The GAUSS mission concept

Today, thanks in particular to the results of the ESA Planck mission, the concordance cosmological model appears to be the most robust to describe the evolution and content of the Universe from its early to late times. It summarizes the evolution of matter, made mainly of dark matter, from the primordial fluctuations generated by inflation around $10^{-30}$ second after the Big-Bang to galaxies and clusters of galaxies, 13.8 billion years later, and the evolution of the expansion of space, with a relative slowdown in the matter-dominated era and, since a few billion years, an acceleration powered by dark energy. But we are far from knowing the pillars of this model which are inflation, dark matter and dark energy. Comprehending these fundamental questions requires a detailed mapping of our observable Universe over the whole of cosmic time. The relic radiation provides the starting point and galaxies draw the cosmic web. JAXA's LiteBIRD mission will map the beginning of our Universe with a crucial test for inflation (its primordial gravity waves), and the ESA Euclid mission will map the most recent half part, crucial for dark energy. The mission concept, described in this White Paper, GAUSS, aims at being a mission to fully map the cosmic web up to the reionization era, linking early and late evolution, to tackle and disentangle the crucial degeneracies persisting after the Euclid era between dark matter and inflation properties, dark energy, structure growth and gravitation at large scale.

astro-ph.CO↗

arXiv2020

Low-redshift tests of Newtonian cosmologies with a time-varying gravitational constant

In this work, we investigate Newtonian cosmologies with a time-varying gravitational constant, $G(t)$. We examine whether such models can reproduce the low-redshift cosmological observations without a cosmological constant, or any other sort of explicit dark energy fluid. Starting with a modified Newton's second law, where $G$ is taken as a function of time, we derive the first Friedmann--Lema{î}tre equation, where a second parameter, $G^*$, appears as the gravitational constant. This parameter is related to the original $G$ from the second law, which remains in the acceleration equation. We use this approach to reproduce various cosmological scenarios that are studied in the literature, and we test these models with low-redshift probes: type-Ia supernovae (SNIa), baryon acoustic oscillations, and cosmic chronometers, taking also into account a possible change in the supernovae intrinsic luminosity with redshift. As a result, we obtain several models with similar $χ^2$ values as the standard $Λ$CDM cosmology. When we allow for a redshift-dependence of the SNIa intrinsic luminosity, a model with a $G$ exponentially decreasing to zero while remaining positive (model 4) can explain the observations without acceleration. When we assume no redshift-dependence of SNIa, the observations favour a negative $G$ at large scales, while $G^*$ remains positive for most of these models. We conclude that these models offer interesting interpretations to the low-redshift cosmological observations, without needing a dark energy term.

astro-ph.CO↗