arXiv subjects

Cyril Pitrou

Publications and source records attributed to Cyril Pitrou.

At least 19 recordsLinked to original sources

arXiv2026

Beyond-eikonal diffraction integral in gravitational lensing

We revisit the derivation of the diffraction integral, which is an approximate evaluation of the Kirchhoff integral, widely used in the literature for phenomenological applications in gravitational lensing. We propose a systematic approach to evaluate the Kirchhoff integral within a beyond-eikonal expansion, carefully tracking all approximations involved in its standard derivation. In this framework, we recover the usual diffraction integral as the leading contribution, together with a correction term that is typically small in realistic lensing scenarios. We further show that our evaluation of the Kirchhoff integral is equivalent to a beyond-eikonal expansion of wave propagation, after summing over all geometric-optics images. This result clarifies the physical origin of diffraction in gravitational lensing and demonstrates that diffraction effects and beyond-eikonal corrections are not distinct phenomena, but rather different but equivalent descriptions of the same underlying physics.

gr-qc↗

arXiv2026

What could an emerging Big Bang Nucleosynthesis discrepancy be hinting at?

The latest measurement of the primordial deuterium abundance is in $\sim 2\sigma$ tension with several state-of-the-art predictions of standard Big Bang nucleosynthesis (BBN), when using the baryon density inferred from the $\Lambda$CDM model fit to cosmic microwave background (CMB) data. This tension increases to $\sim 3\sigma$ for models attempting to solve the Hubble tension, such as early dark energy (EDE), which generally predict a larger baryon density than in $\Lambda$CDM. We test whether this discrepancy could be pointing to a non-standard expansion history during BBN. We compute light-element abundances with PRIMAT and compare $\Lambda$CDM, a $\Delta N_{\rm eff}$ extension, and a very early dark energy (vEDE) component. For vEDE, we sample $\Delta H/H$, the fractional increase of the expansion rate while deuterium burning is freezing out and helium-4 fusion is mostly over. The Bayesian analysis using BBN plus the CMB baryon-density constraint in the EDE cosmology gives $\Delta H/H = 0.087^{+0.036}_{-0.037}$ during the deuterium burning epoch, i.e. at a temperature $T_{\rm D}\simeq0.03\,{\rm MeV}$, and no residual tension. The vEDE component preserves the observed deuterium abundance at the larger CMB baryon density while only mildly affecting helium-4. By contrast, extra radiation raises the helium-4 abundance too efficiently and does not reconcile the baryon density determinations. Together with inflation, dark energy, and EDE, our results hint at the presence of another light scalar field in cosmology.

astro-ph.CO↗

arXiv2026

The Status of Gravitational Vector Perturbations with Recent CMB Data

We present new constraints on gravitational vector perturbations ($\mathcal{V}$-modes) using Cosmic Microwave Background (CMB) data, including temperature and $E$-mode polarization from SPT-3G D1, ACT-DR6, and $Planck$, as well as $B$-mode data from BICEP/Keck and SPTpol, which provide the strongest constraints on $\mathcal{V}$-modes. We consider three initial conditions (ICs) that source $\mathcal{V}$-modes: neutrino isocurvature (ISO), neutrino octupole (OCT), and a sourced mode (SMD) generated by an anisotropic stress before matter-radiation equality. We also consider including tensor modes along with $\mathcal{V}$-modes for each of these ICs. Combining all datasets, we obtain 95\% confidence level upper limits of $r_\mathrm{v} < 1.3\times10^{-4}$ (ISO), $r_\mathrm{v} < 6.8$ (OCT), and $r_\mathrm{v} < 4.2$ (SMD), with slightly tighter bounds when tensors are included, at a pivot scale $k_p\ =\ 0.05$ Mpc$^{-1}$. Interestingly, for SMD without tensors, using SPTpol $B$-modes alone yields $r_\mathrm{v} = 4.7 \pm 2.1$, consistent with zero at $2.2\sigma$. Similar result is found for SMD when including tensor perturbations. No statistically significant deviation from $\Lambda$CDM is found. However, $\mathcal{V}$-modes are not fully excluded by current $B$-mode data and should be considered when interpreting primordial signals.

astro-ph.CO↗

arXiv2026

Probing the Kinematic Dipole with LISA: an analytical treatment

The motion of the Solar System with respect to the cosmic rest frame induces a kinematic dipole in the stochastic gravitational-wave background (GWB). Detecting this signal with space-based interferometers would provide an independent measurement of our peculiar velocity and a GW probe of cosmic anisotropies. We present a fully analytic derivation of the response of the \emph{Laser Interferometer Space Antenna} (LISA) to a kinematic dipole, and construct an optimal estimator for its detection. We show that the dipolar response is governed by a single frequency-dependent function fixed by symmetry, and we compute its behaviour across the LISA band. Using Fisher forecasts, we find that for a scale-invariant background detectability requires $h^2\Omega_{\rm GW} \gtrsim 5\times 10^{-8}$ for \emph{fiducial} LISA, and $h^2\Omega_{\rm GW} \gtrsim 5\times 10^{-10}$ for a detector with characteristic instrumental-noise amplitudes improved by an order of magnitude. Prospects are more favorable for signals with richer frequency profile. We also explore the potential of the kinematic dipole to break degeneracies, particularly in the presence of strong galactic foregrounds or noise features that closely mimic the primordial signal.

gr-qc↗

arXiv2026

Gravitational lensing beyond the eikonal approximation

Waves propagating through a gravitational potential exhibit wave-optics effects when their wavelength is not significantly smaller than the lensing scales. We study the propagation of a scalar wave, governed by the Klein-Gordon equation in curved spacetime, to focus on effects on amplitude and phase, while leaving aside the issue of wave polarization which affects electromagnetic and gravitational waves. Using the Newman-Penrose formalism, we obtain the first corrections beyond the geometric optics in the expansion in the inverse frequency. In vacuum, that is for Weyl tensor lensing, there is no wave effect at first order in $G$ and wave effects start at order $G^2$. Conversely, if the wave travels through a non-vanishing matter density, the first corrections start at order $G$. We check these analytic results by solving numerically the equations dictating the evolution of the corrections either in the vicinity of a Schwarzschild black hole or through a transparent star.

gr-qc↗

arXiv2025

Fifty shades of grayness: parametrizations of spectral distortions and applications in cosmology

Thermal distribution functions can only be of the Fermi-Dirac or Bose-Einstein types, whereas distorted spectra encompass any possible deviations from these shapes. It is fruitful to devise parametrizations of these distortions with only a few parameters which depend on the physical system considered. A method proposed by Stebbins consists in describing a distorted spectrum as a sum of thermalized spectra with a distribution of temperatures, the moments of which are the parameters of interest. After revisiting and extending this approach by working at the level of the number density distribution instead of the standard spectrum, we build another method which consists in describing the distorted spectrum by a polynomial modulating a reference thermalized spectrum. The distortion parameters are then the coefficients of a decomposition on a suitable orthonormal polynomial basis. We advocate that the latter is computationally easier and allows to describe a wide range of distortions. With this formalism, we efficiently describe the standard distortions of the cosmological backgrounds of neutrinos and photons, and we obtain model-independent constraints on nonstandard distortions of these cosmological relics.

hep-ph↗

arXiv2025

CMB line-of-sight integrators for nearly-isotropic cosmological models

Homogeneous and nearly-isotropic cosmological models are natural extensions of standard Friedmann cosmologies. Constraining their features is crucial, as any detection of their properties would impact our understanding of inflation and the cosmological principle. Since these models evolve as a set of non-interacting scalar, vector, and tensor modes on top of homogeneous and isotropic spacetimes, their imprints on cosmological observables, particularly the CMB, can be obtained using standard line-of-sight methods. This requires (1) that one resorts on Laplacian eigenmodes on spatially curved spaces and (2) that radial functions for these modes are analytically continued to accommodate complex (i.e., supercurvature) wavenumbers. We introduce two line-of-sight integrators implementing the evolution of the CMB anisotropies in these models: \texttt{AniLoS}, a user-friendly and easy to modify \texttt{Python} package, and \texttt{AniCLASS}, an advanced and efficient extension of the Boltzmann solver \texttt{CLASS}. We discuss possible initial conditions that could generate such fluctuations and provide illustrative examples using our codes. This work offers a pathway for leveraging diverse cosmological datasets to constrain superhorizon anisotropies of the late-time universe.

astro-ph.CO↗

arXiv2025

Primordial neutrinos fade to gray: constraints from cosmological observables

We investigate the effect of potentially large distortions of the relic neutrino spectra on cosmological observables. To that end, we consider a phenomenological model of "gray" spectral distributions, described by a single parameter, which generalizes the traditional $y$-distortions to possibly large negative values. Implementing these distortions in the primordial nucleosynthesis code PRIMAT, we can constrain the distortion parameter along with the presence of extra radiation, exploiting the complementarity of big bang nucleosynthesis and cosmic microwave background measurements to disentangle gravitational and non-thermal effects. These constraints rule out a distortion where more than $\sim 1/2$ of the neutrinos' energy density is replaced by dark radiation. Nonetheless, we find that large distortions, accompanied with extra radiation, are allowed-and even slightly preferred in some cases-by current cosmological observations. As this scenario would require substantial modifications to the physics of neutrino decoupling in the early Universe, these observational constraints call for a renewed attention on the possibility of large deviations from the standard cosmological model in the neutrino sector.

hep-ph↗

arXiv2025

Measuring anisotropies in the PTA band with cross-correlations

The astrophysical gravitational wave background in the nanohertz (nHz) band is expected to be primarily composed of the superposition of signals from binaries of supermassive black holes. The spatial discreteness of these sources introduces shot noise, which, in certain regimes, would overwhelm efforts to measure the anisotropy of the gravitational wave background. Moreover, the fact the time-residual map has a finite angular resolution and the presence of pulsar noise, affects our ability to construct the angular power spectrum of the anisotropy from a time-residual map (finite resolution noise). In this work, we explicitly demonstrate, starting from first principles, that cross-correlating a gravitational wave background map with a sufficiently dense galaxy survey can mitigate this issue. This approach could potentially reveal underlying properties of the gravitational wave background that would otherwise remain obscured. We quantify both the shot noise and the finite resolution noise level and show that cross-correlating the gravitational wave background with a galaxy catalog improves by more than one order of magnitude the prospects for a first detection of the background anisotropy by a gravitational wave observatory operating in the nHz frequency range. In particular, we find that with a futuristic scenario with an effective number of frequencies equal to $N_f=10$, the detection of the spectral amplitude can be achieved combining the first $20$ multipoles, with a threshold to resolve single events SKA-like. Increasing observation time, pulsar number or reducing the pulsar white noise considerably improves the detection significance.

gr-qc↗

arXiv2024

Mitigating cosmic variance in the Hellings-Downs curve: a Cosmic Microwave Background analogy

The Hellings-Downs (HD) correlation, which characterizes the signature of a stochastic gravitational wave background measured via Pulsar Timing Arrays (PTA), is derived using a harmonic formalism. This approach closely follows the framework traditionally employed to compute correlations of temperature fluctuations in the CMB. This parallel enables a direct comparison between the correlations observed in PTA and those in CMB. After providing analytic estimates of the transmission functions, we show that the covariance matrix in frequency space becomes very non-diagonal. We then build formally the quadratic estimator for the HD correlation in multipolar space, for both a perfect experiment, and for a realistic pulsar noise model. For a perfect experiment, we show that the SNR grows with the observation time and the number of frequency bins, in turn determined by the cadence of observation. For an imperfect experiment, the behaviour is similar, with an effective multipole-dependent number of frequency, obtained after weighting with noise. We predict that with $\sim 200$ pulsars monitored for $25$ years, multipoles of the HD correlation up to $\ell=4$ can be measured. Our findings clarify that is called \emph{cosmic variance} in previous literature is not an intrinsic limitation for PTA measurements. Instead, with optimal estimators, it can be mitigated by accumulating more observation time or improving the cadence of pulsar monitoring. Therefore, unlike CMB angular correlations, where cosmic variance represents an irreducible constraint, it can be reduced in PTA measurements. Finally, we show that if the primordial power spectrum of tensor fluctuations was very blue with $n_T>4$, the CMB angular correlation due to these tensor modes would also exhibit a HD correlation. We also discuss the case in which the graviton distribution function is anisotropic.

gr-qc↗

arXiv2024

Resurrecting Gravitational Vector Modes and their Magnetogenesis

We revisit the presence of primordial gravitational vector modes (V-modes) and their sourcing of primordial magnetic fields (PMF), i.e. magnetogenesis. As the adiabatic vector mode generically decays with expansion, we consider exotic initial conditions which circumvent this issue and lead to observational imprints. The first initial condition is an isocurvature mode between photons and neutrinos vorticities, and the second one is a non-trivial initial condition on the neutrino octupole. Both types of conditions sustain a constant vector mode on super Hubble scales at early times. We also consider a third scenario in which the adiabatic vector modes are rapidly sourced, at a given early but finite time, by an exotic component which develops an anisotropic stress. We find the best fitting parameters in these three cases to CMB and BAO data. We compare the resulting $B$-mode spectra of the CMB to data from BICEP/Keck and SPTpol. We find that none of the proposed initial conditions can produce large enough PMFs to seed every type of magnetic fields observed. However, V-modes are still consistent with the data and ought to be constrained for a better understanding of the primordial Universe before its hot big-bang phase.

astro-ph.CO↗

arXiv2024

Constraints on primordial lepton asymmetries with full neutrino transport

Primordial neutrino-antineutrino asymmetries can be constrained through big-bang nucleosynthesis (BBN) relic abundances and cosmic microwave background (CMB) anisotropies, both observables being sensitive to neutrino properties. The latter constraint, which is due to gravitational effects from all neutrino flavors, is very minute since it is at least quadratic in the asymmetries. On the contrary, the constraints from primordial abundances presently dominate, although these abundances are almost only sensitive to the electron flavor asymmetry. It is generally assumed that neutrino asymmetries are sufficiently averaged by flavor oscillations prior to BBN, which allows one to constrain a common primordial neutrino asymmetry at the epoch of BBN. This simplified approach suffers two caveats that we deal with in this article, combining a neutrino evolution code and BBN calculation throughout the MeV era. First, flavor "equilibration" is not true in general, therefore an accurate dynamical evolution of asymmetries is needed to connect experimental observables to the primordial asymmetries. Second, the approximate averaging of asymmetries through flavor oscillations is associated to a reheating of the primordial plasma. It is therefore crucial to correctly describe the interplay between flavor equilibration and neutrino decoupling, as an energy redistribution prior to decoupling does not significantly alter the final effective number of neutrino species' value. Overall, we find that the space of allowed initial asymmetries is generically unbound when using currently available primordial abundances and CMB measurements. We forecast constraints using future CMB experiment capabilities, which should reverse this experimental misfortune.

hep-ph↗

arXiv2024

Boosting gravitational waves: a review of kinematic effects on amplitude, polarization, frequency and energy density

We review the kinematic effects on a gravitational wave due to either a peculiar motion of the astrophysical source emitting it or a local motion of the observer. Working in the context of general relativity, we show at fully non-linear order in velocity, that the amplitude of the wave is amplified by the Doppler factor in the case in which the source moves with respect to a reference frame, while it is invariant if the observer moves (with respect to a reference observer). However, the observed specific intensity transforms in the same way under a boost of the source or of the observer. We also show at fully non-linear order that under a boost (of either source or observer), the polarization tensor is rotated in the same way the wave direction is rotated by aberration, such that the only net effect of a boost on polarization is to change the phase of the helicity components. We apply these results to a wave emitted by a binary system of compact objects in the cosmological context.

gr-qc↗

arXiv2024

Wave optics lensing of gravitational waves: theory and phenomenology of triple systems in the LISA band

We study lensing of gravitational waves by a black hole in the deep wave optics regime, i.e. when the wavelength is much larger than the black hole Schwarzschild radius. We apply it to triple systems, with a binary of stellar mass objects in the inspiraling phase orbiting around a central massive black hole. We describe the full polarisation structure of the wave and derive predictions for the polarisation modes of the scattered wave measured by the observer. We show that lensing in the wave optics regime is not helicity preserving, as opposed to lensing in the geometric optics regime. The amplitude of the total wave is modulated due to interference between the directly transmitted and lensed components. The relative amplitude of the modulation is fixed by the lensing geometry and can reach unity in the most favourable settings. This indicates that wave optics lensing is potentially detectable by LISA for sufficiently high SNR systems. Our findings show that in the wave optics regime it is necessary to go beyond the usual lensing description where the amplification factor is assumed to be the same for both helicity modes. While motivated by GW190521 and the AGN formation scenario, our results apply more broadly to stellar-mass binaries orbiting a third body described as a Schwarzschild black hole, with a period comparable to the GW observation time.

gr-qc↗

arXiv2023

Hubble tension as a window on the gravitation of the dark matter sector: Exploration of a family of models

A family of simple and minimal extensions of the standard cosmological $\Lambda$CDM model in which dark matter experiences an additional long-range scalar interaction is demonstrated to alleviate the long lasting Hubble-tension while letting primordial nucleosynthesis predictions unaffected and passing by construction all current local tests of general relativity. This article describes their theoretical formulation and their implications for dark matter. Then, it investigates their cosmological signatures, both at the background and perturbation levels. A detailed comparison to astrophysical data is performed to discuss their ability to fit existing data. A thorough discussion of the complementarity of the low- and high-redshift data and on their constraining power highlights how these models improve the predictions of the $\Lambda$CDM model whatever the combination of datasets used and why they can potentially resolve the Hubble tension. Being fully predictive in any environment, they pave the way to a better understanding of gravity in the dark matter sector.

astro-ph.CO↗

arXiv2023

Hubble tension as a window on the gravitation of the dark matter sector

A simple and minimal extension of the standard cosmological $\Lambda$CDM model in which dark matter experiences an additional long-range scalar interaction is demonstrated to alleviate the long lasting Hubble-tension while letting primordial nucleosynthesis predictions unaffected and passing by construction all current local tests of general relativity. The theoretical formulation of this $\Lambda\beta$CDM model and its comparison to astrophysical observations are presented to prove its ability to fit existing data and potentially resolve the tension.

astro-ph.CO↗

arXiv2022

Rutherford scattering of quantum and classical fields

Quantum Rutherford scattering and scattering of classical waves off Coulomb-like potentials have similar formal structures and can be studied using the same mathematical techniques. In both contexts, the long-range nature of the interaction leads to a divergent total cross-section, which has been interpreted and regularized in various ways in the past literature. We review in detail the origin of this divergence, in both real and multipole spaces, and show that it arises from incorrectly using approximations out of their domain of validity. We also stress that although classical and quantum Rutherford scattering share the same formalism, the natures of the associated physical observables differ. We comment on the role of interference: while interference can be safely neglected in a quantum context (due to the fact that the observable quantity is a flux, and the incoming flux is collimated), in a classical context one expects to see a superposition of transmitted and scattered waves in a broad region downstream of the target, hence a cross-section is not connected to any physically observable quantity.

math-ph↗

arXiv2022

Aberration of gravitational waveforms by peculiar velocity

One key prediction of General Relativity is that gravitational waves are emitted with a pure spin-2 polarisation. Any extra polarisation mode, spin-1 or spin-0, is consequently considered a smoking gun for deviations from General Relativity. In this paper, we show that the velocity of merging binaries with respect to the observer gives rise to spin-1 polarisation in the observer frame even in the context of General Relativity. These are pure projection effects, proportional to the plus and cross polarisations in the source frame, hence they do not correspond to new degrees of freedom. We demonstrate that the spin-1 modes can always be rewritten as pure spin-2 modes coming from an aberrated direction. Since gravitational waves are not isotropically emitted around binary systems, this aberration modifies the apparent orientation of the binary system with respect to the observer: the system appears slightly rotated due to the source velocity. Fortunately, this bias does not propagate to other parameters of the system (and therefore does not spoil tests of General Relativity), since the impact of the velocity can be fully reabsorbed into new orientation angles.

gr-qc↗