arXiv subjects

Jose María Ezquiaga

Publications and source records attributed to Jose María Ezquiaga.

At least 37 records · Page 2Linked to original sources

arXiv2024

LISA Definition Study Report

The Laser Interferometer Space Antenna (LISA) is the first scientific endeavour to detect and study gravitational waves from space. LISA will survey the sky for Gravitational Waves in the 0.1 mHz to 1 Hz frequency band which will enable the study of a vast number of objects ranging from Galactic binaries and stellar mass black holes in the Milky Way, to distant massive black-hole mergers and the expansion of the Universe. This definition study report, or Red Book, presents a summary of the very large body of work that has been undertaken on the LISA mission over the LISA definition phase.

astro-ph.CO↗

arXiv2023

Primordial black holes and their gravitational-wave signatures

In the recent years, primordial black holes (PBHs) have emerged as one of the most interesting and hotly debated topics in cosmology. Among other possibilities, PBHs could explain both some of the signals from binary black hole mergers observed in gravitational wave detectors and an important component of the dark matter in the Universe. Significant progress has been achieved both on the theory side and from the point of view of observations, including new models and more accurate calculations of PBH formation, evolution, clustering, merger rates, as well as new astrophysical and cosmological probes. In this work, we review, analyse and combine the latest developments in order to perform end-to-end calculations of the various gravitational wave signatures of PBHs. Different ways to distinguish PBHs from stellar black holes are emphasized. Finally, we discuss their detectability with LISA, the first planned gravitational-wave observatory in space.

astro-ph.CO↗

arXiv2023

Identifying strongly lensed gravitational waves through their phase consistency

Strongly lensed gravitational waves (GWs) from binary coalescence manifest as repeated chirps from the original merger. At the detectors, the phase of the lensed GWs and its arrival time differences will be consistent modulo a fixed constant phase shift. We develop a fast and reliable method to efficiently reject event pairs that are not-lensed copies and appropriately rank the most interesting candidates. Our method exploits that detector phases are the best measured GW parameter, with errors only of a fraction of a radian and differences across the frequency band that are better measured than the chirp mass. The arrival time phase differences also avoid the shortcomings of looking for overlaps in highly non-Gaussian sky maps. Our basic statistic determining the consistency with lensing is the distance between the phase posteriors of two events and it directly provides information about the lens-source geometry which helps inform electromagnetic followups. We demonstrate that for simulated signals of not-lensed binaries specifically chosen with many coincident properties so as to trigger false lensing alarms none of the pairs have phases closer than $3σ$, and most cases reject the lensing hypothesis by $5σ$. Looking at the latest catalog, GWTC3, we find that only $6\%$ of the pairs are consistent with lensing at 99\% confidence level. Moreover, we reject about half of the pairs that would otherwise favor lensing by their parameter overlaps and demonstrate good correlation with detailed joint parameter estimation results. This reduction of the false alarm rate will be of paramount importance in the upcoming observing runs and the eventual discovery of lensed GWs. Our code is publicly available and could be applied beyond lensing to test possible deviations in the phase evolution from modified theories of gravity and constrain GW birefringence.

astro-ph.CO↗

arXiv2023

Lensing or luck? False alarm probabilities for gravitational lensing of gravitational waves

Strong gravitational lensing of gravitational waves (GWs) has been forecasted to become detectable in the upcoming observing runs. However, definitively distinguishing pairs of lensed sources from random associations is a challenging problem. We investigate the degree to which unlensed events mimic lensed ones because of the overlap of parameters due to a combination of random coincidence and errors in parameter estimation. We construct a mock catalog of lensed and unlensed events. We find that the false alarm probability (FAP) based on coincidental overlaps of the chirp mass, sky location, and coalescence phase are approximately $9\%$, $1\%$, and $10\%$ per pair, respectively. Combining all three, the overall FAP per pair is $\sim10^{-4}$. As the number of events, $N$, in the GW catalogs increases, the number of random pairs of events increases as $\sim N^2$. Meanwhile, the number of lensed events will increase linearly with $N$, implying that for sufficiently high $N$, the false alarms will always dominate over the true lensing events. This issue can be compensated for by placing higher thresholds on the lensing candidates (e.g., selecting a higher signal-to-noise ratio (SNR) threshold), which will lead to better parameter estimation and, thus, lower FAP per pair -- at the cost of dramatically decreasing the size of the lensing sample (by $\sim 1/\mbox{SNR}^3$). We show that with our simple overlap criteria for current detectors at design sensitivity, the false alarms will dominate for realistic lensing rates ($\lesssim10^{-3}$) even when selecting the highest SNR pairs. These results highlight the necessity to design alternative identification criteria beyond simple waveform and sky location overlap. Future GW detectors Cosmic Explorer and Einstein Telescope may provide sufficient improvement in parameter estimation, allowing for the conclusive detection of strong lensing of GWs.

astro-ph.CO↗

arXiv2023

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $Λ$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

gr-qc↗

arXiv2023

Massive galaxy clusters like "El Gordo'' hint at primordial quantum diffusion

It is generally assumed within the standard cosmological model that initial density perturbations are Gaussian at all scales. However, primordial quantum diffusion unavoidably generates non-Gaussian, exponential tails in the distribution of inflationary perturbations. These exponential tails have direct consequences for the formation of collapsed structures in the universe, as has been studied in the context of primordial black holes. We show that these tails also affect the very-large-scale structures, making heavy clusters like "El Gordo", or large voids like the one associated with the cosmic microwave background cold spot, more probable. We compute the halo mass function and cluster abundance as a function of redshift in the presence of exponential tails. We find that quantum diffusion generically enlarges the number of heavy clusters and depletes subhalos, an effect that cannot be captured by the famed $f_{\mathrm{NL}}$ corrections. These late-universe signatures could thus be fingerprints of quantum dynamics during inflation that should be incorporated in $N$-body simulations and checked against astrophysical data.

astro-ph.CO↗

arXiv2022

Spectral sirens: cosmology from the full mass distribution of compact binaries

We explore the use of the mass spectrum of neutron stars and black holes in gravitational-wave compact binary sources as a cosmological probe. These standard siren sources provide direct measurements of luminosity distance. In addition, features in the mass distribution, such as mass gaps or peaks, will redshift, and thus provide independent constraints on their redshift distribution. We argue that the entire mass spectrum should be utilized to provide cosmological constraints. For example, we find that the mass spectrum of LIGO--Virgo--KAGRA events introduces at least five independent mass ``features'': the upper and lower edges of the pair instability supernova (PISN) gap, the upper and lower edges of the neutron star--black hole gap, and the minimum neutron star mass. We find that although the PISN gap dominates the cosmological inference with current detectors (2G), as shown in previous work, it is the lower mass gap that will provide the most powerful constraints in the era of Cosmic Explorer and Einstein Telescope (3G). By using the full mass distribution, we demonstrate that degeneracies between mass evolution and cosmological evolution can be broken, unless an astrophysical conspiracy shifts all features of the full mass distribution simultaneously following the (non-trivial) Hubble diagram evolution. We find that this self-calibrating ``spectral siren'' method has the potential to provide precision constraints of both cosmology and the evolution of the mass distribution, with 2G achieving better than $10\%$ precision on $H(z)$ at $z\lesssim1$ within a year, and 3G reaching $\lesssim1\%$ at $z\gtrsim2$ within one month.

astro-ph.CO↗

arXiv2021

Breaking the mass-sheet degeneracy with gravitational wave interference in lensed events

The mass-sheet degeneracy is a well-known problem in gravitational lensing which limits our capability to infer astrophysical lens properties or cosmological parameters from observations. As the number of gravitational wave observations grows, detecting lensed events will become more likely, and to assess how the mass-sheet degeneracy may affect them is crucial. Here we study both analytically and numerically how the lensed waveforms are affected by the mass-sheet degeneracy computing the amplification factor from the diffraction integral. In particular, we differentiate between the geometrical optics, wave optics and interference regimes, focusing on ground-based gravitational waves detectors. In agreement with expectations of gravitational lensing of electromagnetic radiation, we confirm how, in the geometrical optics scenario, the mass-sheet degeneracy cannot be broken with only one lensed image. However, we find that in the interference regime, and in part in the wave-optics regime, the mass-sheet degeneracy can be broken with only one lensed waveform thanks to the characteristic interference patterns of the signal. Finally, we quantify, through template matching, how well the mass-sheet degeneracy can be broken. We find that, within present GW detector sensitivities and considering signals as strong as those which have been detected so far, the mass-sheet degeneracy can lead to a $1σ$ uncertainty on the lens mass of $\sim 12\%$. With these values the MSD might still be a problematic issue. But in case of signals with higher signal-to-noise ratio, the uncertainty can drop to $\sim 2\%$, which is less than the current indeterminacy achieved by dynamical mass measurements.

astro-ph.CO↗

arXiv2021

Hearing gravity from the cosmos: GWTC-2 probes general relativity at cosmological scales

Gravitational-wave (GW) catalogs are rapidly increasing in number, allowing for robust statistical analyses of the population of compact binaries. Nonetheless, GW inference of cosmology has typically relied on additional electromagnetic counterparts or galaxy catalogs. I present a new probe of cosmological modifications of general relativity with GW data only. I focus on deviations of the GW luminosity distance constrained with the astrophysical population of binary black holes (BBHs). The three key observables are 1) the number of events as a function of luminosity distance, 2) the stochastic GW background of unresolved binaries and 3) the location of any feature in the source mass distribution, such as the pair instability supernova (PISN) gap. Despite a priori degeneracies between modified gravity and the unknown evolution of the merger rate and source masses, a large damping of the GW amplitude could be falsifiable since as redshift grows it reduces the events and lowers the edges of the PISN gap, which is against standard astrophysical expectations. Applying a hierarchical Bayesian analysis to the current LIGO--Virgo catalog (GWTC-2), the strongest constraints to date are placed on deviations from the GW luminosity distance, finding $c_{_M}=-3.2^{+3.4}_{-2.0}$ at $68\%$ C.L., which is $\sim10$ times better than multi-messenger GW170817 bounds. These modifications also affects the determination of the BBH masses, which is crucial to accommodate the high-mass binary GW190521 away from the PISN gap. In this analysis it is found that the maximum mass of $99\%$ of the population shifts to lower masses with increased uncertainty, $m_{99\%}=46.2^{+11.4}_{-9.1}M_\odot$ at $68\%$ C.L. Testing gravity at large scales with the population of BBHs will become increasingly relevant with future catalogs, providing an independent and self-contained test of the standard cosmological model.

astro-ph.CO↗

arXiv2021

Phase effects from strong gravitational lensing of gravitational waves

Assessing the probability that two or more gravitational waves (GWs) are lensed images of the same source requires an understanding of the image properties, including their relative phase shifts in strong lensing (SL). For non-precessing, circular binaries dominated by quadrupole radiation these phase shifts are degenerate with either a shift in the coalescence phase or a detector and inclination dependent shift in the orientation angle. This degeneracy is broken by the presence of higher harmonic modes with $|m|\ne 2$ in the former and $|m| \ne l$ in the latter. Precession or eccentricity will also break this degeneracy. This implies that lensed GWs will not necessarily be consistent with (unlensed) predictions from general relativity (GR). Therefore, unlike EM lensing, GW SL can lead to images with an observable modified phase evolution. However, for a wide parameter space, the lensed waveform is similar enough to an unlensed waveform that detection pipelines will still find it. For present detectors, templates with a shifted detector-dependent orientation angle have SNR differences of less than $1\%$ for mass ratios up to 0.1, and less than $5\%$ for precession parameters up to 0.5 and eccentricities up to 0.4 at 20Hz. The mismatch is lower than $10\%$ with the alternative detector-independent coalescence phase shift. Nonetheless, for a loud enough source, even with one image it may be possible to directly identify it as a SL image from its non-GR waveform. In more extreme cases, lensing may lead to considerable distortions, and the lensed GWs may be undetected with current searches. Nevertheless, an exact template with a phase shift in Fourier space can always be constructed to fit any lensed GW. We conclude that an optimal search strategy would incorporate phase information in all stages, with an exact treatment in the final assessment of the probability of multiple lensed events.

gr-qc↗

arXiv2021

Jumping the gap: searching for LIGO's biggest black holes

Gravitational wave (GW) detections of binary black holes (BBHs) have shown evidence for a dearth of component black holes with masses above $\sim50M_\odot$. This is consistent with expectations of a mass gap due to the existence of pair-instability supernovae (PISN). We argue that ground-based GW detectors will be sensitive to BBHs with masses above this gap, $\gtrsim120\,M_\odot$. With no detections, two years at upgraded sensitivity (A+) would constrain the local merger rate of these BBHs on the "far side" of the PISN gap to be lower than $0.01\,\mathrm{yr}^{-1}\mathrm{Gpc}^{-3}$. Alternatively, with a few tens of events we could constrain the location of the upper edge of the gap to the percent level. We consider the potential impact of "interloper" black holes within the PISN mass gap on this measurement. Far side BBHs would also be observed by future instruments such as Cosmic Explorer (CE), Einstein Telescope (ET) and LISA, and may dominate the fraction of multi-band events. We show that by comparing observations from ground and space it is possible to constrain the merger rate history. Moreover, we find that the upper edge of the PISN mass gap leaves an imprint on the spectral shape of the stochastic background of unresolved binaries, which may be accessible with A+ sensitivity. Finally, we show that by exploiting the upper edge of the gap, these high-mass BBHs can be used as standard sirens to constrain the cosmic expansion at redshifts of $\sim0.4$, $0.8$, and~$1.5$ with LISA, LIGO-Virgo, and CE/ET, respectively. These far-side binaries would be the most massive BBHs LIGO-Virgo could detect.

astro-ph.HE↗

arXiv2020

Gravitational wave lensing beyond general relativity: birefringence, echoes and shadows

Gravitational waves (GW), as light, are gravitationally lensed by intervening matter, deflecting their trajectories, delaying their arrival and occasionally producing multiple images. In theories beyond general relativity (GR), new gravitational degrees of freedom add an extra layer of complexity and richness to GW lensing. We develop a formalism to compute GW propagation beyond GR over general space-times, including kinetic interactions with new fields. Our framework relies on identifying the dynamical propagation eigenstates (linear combinations of the metric and additional fields) at leading order in a short-wave expansion. We determine these eigenstates and the conditions under which they acquire a different propagation speed around a lens. Differences in speed between eigenstates cause birefringence phenomena, including time delays between the metric polarizations (orthogonal superpositions of $h_+,h_\times$) observable without an electromagnetic counterpart. In particular, GW echoes are produced when the accumulated delay is larger than the signal's duration, while shorter time delays produce a scrambling of the wave-form. We also describe the formation of GW shadows as non-propagating metric components are sourced by the background of the additional fields around the lens. As an example, we apply our methodology to quartic Horndeski theories with Vainshtein screening and show that birefringence effects probe a region of the parameter space complementary to the constraints from the multi-messenger event GW170817. In the future, identified strongly lensed GWs and binary black holes merging near dense environments, such as active galactic nuclei, will fulfill the potential of these novel tests of gravity.

gr-qc↗

arXiv2020

The exponential tail of inflationary fluctuations: consequences for primordial black holes

The curvature perturbations produced during an early era of inflation are known to have quasi-Gaussian distribution functions close to their maximum, where they are well constrained by measurements of the cosmic microwave background anisotropies and of the large-scale structures. In contrast, the tails of these distributions are poorly known, although this part is the relevant one for rare, extreme objects such as primordial black holes. We show that these tails are highly non-Gaussian, and cannot be described with standard non-Gaussian expansions, that are designed to approximate the distributions close to their maximum only. Using the stochastic-$δN$ formalism, we develop a generic framework to compute the tails, which are found to have an exponential, rather than Gaussian, decay. These exponential tails are inevitable, and do not require any non-minimal feature as they simply result from the quantum diffusion of the inflaton field along its potential. We apply our formalism to a few relevant single-field, slow-roll inflationary potentials, where our analytical treatment is confirmed by comparison with numerical results. We discuss the implications for the expected abundance of primordial black holes in these models, and highlight that it can differ from standard results by several orders of magnitude. In particular, we find that potentials with an inflection point overproduce primordial black holes, unless slow roll is violated.

astro-ph.CO↗

arXiv2020

Apparent Superluminality of Lensed Gravitational Waves

We consider gravitational wave (GW) sources with an associated electromagnetic (EM) counterpart, and analyze the time delay between both signals in the presence of lensing. If GWs have wavelengths comparable to the Schwarzschild radius of astrophysical lenses, they must be treated with wave optics, whereas EM waves are typically well within the approximation of geometric optics. With concrete examples, we confirm that the GW signal never arrives before its EM counterpart, if both are emitted at the same time. However, during the inspiral of a binary, peaks of the GW waveform can arrive before their EM counterpart. We stress this is only an apparent superluminality since the GW waveform is both distorted and further delayed with respect to light. In any case, measuring the multi-messenger time delay and correctly interpreting it has important implications for unveiling the distribution of lenses, testing the nature of gravity, and probing the cosmological expansion history.

astro-ph.CO↗

arXiv2020

Probing cosmological fields with gravitational wave oscillations

Gravitational wave (GW) oscillations occur whenever there are additional tensor modes interacting with the perturbations of the metric coupled to matter. These extra modes can arise from new spin-2 fields (as in e.g. bigravity theories) or from non-trivial realisations of the cosmological principle induced by background vector fields with internal symmetries (e.g. Yang-Mills, gaugids or multi-Proca). We develop a general cosmological framework to study such novel features due to oscillations. The evolution of the two tensor modes is described by a linear system of coupled second order differential equations exhibiting friction, velocity, chirality and mass mixing. We follow appropriate schemes to obtain approximate solutions for the evolution of both modes and show the corresponding phenomenology for different mixings. Observational signatures include modulations of the wave-form, oscillations of the GW luminosity distance, anomalous GW speed and chirality. We discuss the prospects of observing these effects with present and future GW observatories such as LIGO/VIRGO and LISA.

astro-ph.CO↗

arXiv2018

Dark Energy in light of Multi-Messenger Gravitational-Wave astronomy

Gravitational waves (GWs) provide a new tool to probe the nature of dark energy (DE) and the fundamental properties of gravity. We review the different ways in which GWs can be used to test gravity and models for late-time cosmic acceleration. Lagrangian-based gravitational theories beyond general relativity (GR) are classified into those breaking fundamental assumptions, containing additional fields and massive graviton(s). In addition to Lagrangian based theories we present the effective theory of DE and the $μ$-$Σ$ parametrization as general descriptions of cosmological gravity. Multi-messenger GW detections can be used to measure the cosmological expansion (standard sirens), providing an independent test of the DE equation of state and measuring the Hubble parameter. Several key tests of gravity involve the cosmological propagation of GWs, including anomalous GW speed, massive graviton excitations, Lorentz violating dispersion relation, modified GW luminosity distance and additional polarizations, which may also induce GW oscillations. We summarize present constraints and their impact on DE models, including those arising from the binary neutron star merger GW170817. Upgrades of LIGO-Virgo detectors to design sensitivity and the next generation facilities such as LISA or Einstein Telescope will significantly improve these constraints in the next two decades.

astro-ph.CO↗

arXiv2018

Quantum diffusion beyond slow-roll: implications for primordial black-hole production

Primordial black-holes (PBH) can be produced in single-field models of inflation with a quasi-inflection point in the potential. In these models, a large production of PBHs requires a deviation from the slow-roll (SR) trajectory. In turn, this SR violation can produce an exponential growth of quantum fluctuations. We study the back-reaction of these quantum modes on the inflationary dynamics using stochastic inflation in the Hamilton-Jacobi formalism. We develop a methodology to solve quantum diffusion beyond SR in terms of the statistical moments of the probability distribution. We apply these techniques to a toy model potential with a quasi-inflection point. We find that there is an enhancement of the power spectrum due to the dominance of the stochastic noise in the phase beyond SR. Moreover, non-Gaussian corrections become as well relevant with a large positive kurtosis. Altogether, this produces a significant boost of PBH production. We discuss how our results extend to other single-field models with similar dynamics. We conclude that the abundance of PBHs in this class of models should be revisited including quantum diffusion.

astro-ph.CO↗

arXiv2017

Dark Energy after GW170817: dead ends and the road ahead

Multi-messenger gravitational wave (GW) astronomy has commenced with the detection of the binary neutron star merger GW170817 and its associated electromagnetic counterparts. The almost coincident observation of both signals places an exquisite bound on the GW speed $|c_g/c-1|\leq5\cdot10^{-16}$. We use this result to probe the nature of dark energy (DE), showing that a large class of scalar-tensor theories and DE models are highly disfavored. As an example we consider the covariant Galileon, a cosmologically viable, well motivated gravity theory which predicts a variable GW speed at low redshift. Our results eliminate any late-universe application of these models, as well as their Horndeski and most of their beyond Horndeski generalizations. Three alternatives (and their combinations) emerge as the only possible scalar-tensor DE models: 1) restricting Horndeski's action to its simplest terms, 2) applying a conformal transformation which preserves the causal structure and 3) compensating the different terms that modify the GW speed (to be robust, the compensation has to be independent on the background on which GWs propagate). Our conclusions extend to any other gravity theory predicting varying $c_g$ such as Einstein-Aether, Hořava gravity, Generalized Proca, TeVeS and other MOND-like gravities.

astro-ph.CO↗