Stochastic Background Research Articles

Inflationary initial conditions for the cosmological gravitational wave background

The initial conditions on the anisotropies of the stochastic gravitational-wave background of cosmological origin (CGWB) largely depend on the mechanism that generates the gravitational waves. Since the CGWB is expected to be non-thermal, the computation of the initial conditions could be more challenging w.r.t. the Cosmic Microwave Background (CMB), whose interactions with other particles in the early Universe lead to a blackbody spectrum. In this paper, we show that the initial conditions for the cosmological background generated by quantum fluctuations of the metric during inflation deviate from adiabaticity. These primordial gravitational waves are indeed generated by quantum fluctuations of two independent degrees of freedom (the two polarization states of the gravitons). Furthermore, the CGWB plays a negligible role in the Einstein's equations, because its energy density is subdominant w.r.t. ordinary matter. Therefore, the only possible way to compute the initial conditions for inflationary gravitons is to perturb the energy-momentum tensor of the gravitational field defined in term of the small-scale tensor perturbation of the metric. This new and self-consistent approach shows that a large, non-adiabatic initial condition is present even during the single-field inflation. Such a contribution enhances the total angular power spectrum of the CGWB compared to the standard adiabatic case, increasing also the sensitivity of the anisotropies to the presence of relativistic and decoupled particles in the early Universe. In this work we have also proved that our findings are quite general and apply to both single-field inflation and other scenarios in which the CGWB is generated by the quantum fluctuations of the metric, like the curvaton.

Development of generic no-scale inflation

We develop generalized no-scale supergravity models of inflation, and then study the corresponding cosmological predictions as well as the formation of primordial black holes (PBHs) and scalar-induced gravitational waves (SIGWs). With a new parameter 0 < a ≤ 1, the generalized no-scale supergravity provides the continuous connections among the generic no-scale supergravity from string theory compactifications. The resulting prediction of the CMB, spectrum index ns , and tensor-to-scalar ratio r can be highly consistent with the latest Planck/BICEP/Keck Array observations. Notably, the models with a ≠ 1 give a smaller ratio r ≤ 10-3, which is flexible even under the anticipated tighter observational constraints at the future experiments. Additionally, these models have the potential to generate a broad-band stochastic gravitational wave background, and thus explain the NANOGrav 15yr signal. Furthermore, they predict the formation of PBHs with various mass scales, which could account for a significant portion of dark matter relic density in the Universe.

Chiral gravitational wave background in millihertz from axion-like fields

Chiral gravitational wave background in millihertz from axion-like fields

Neural networks unveiling the properties of gravitational wave background from supermassive black hole binaries

Supermassive black hole binaries (SMBHBs) are binary systems formed by black holes with masses exceeding millions of solar masses, and are expected to form and evolve in the nuclei of galaxies. The extremely compact nature of these objects leads to the intense and efficient emission of gravitational waves (GWs), which can be detected by the Pulsar Timing Array (PTA) experiment in the form of a gravitational wave background (GWB); that is, a superposition of GW signals coming from different sources. The modelling of the GWB requires some assumptions as to the binary population, and exploration of the whole parameter space involved is hindered by the great computational cost involved. We trained two neural networks (NN) on a semi-analytical modelling of the GWB generated by an eccentric population of MBHBs that interact with the stellar environment. We then used the NN to predict the characteristics of the GW signal in regions of the parameter space that we did not sample analytically. The developed framework allows us to quickly predict the amplitude, shape, and variance of the GWB signals produced in different realisations of the universe.

Stochastic gravitational wave background from binary black hole mergers dynamically assembled in dense star clusters

Stochastic gravitational wave background from binary black hole mergers dynamically assembled in dense star clusters

Eccentricity evolution of PTA sources from cosmological initial conditions

ABSTRACT Recent results from pulsar timing arrays (PTAs) show evidence for a gravitational wave background (GWB) consistent with a population of unresolved supermassive black hole (SMBH) binaries (BHBs). While the data do not yet constrain the slope of the spectrum, this appears to flatten at the lowest frequencies, deviating from the power-law shape expected for circular binaries evolving solely due to gravitational wave (GW) emission. Interestingly, such flattening can be explained with a population of eccentric rather than circular binaries. The eccentricity of BHBs is notoriously difficult to predict based simply on the parameters of the host galaxies and the initial galactic orbit, as it is subject to stochastic effects. We study the evolution of the eccentricity of BHBs formed in galactic mergers with cosmological initial conditions from pairing to coalescence, with a focus on potential PTA sources. We select galactic mergers from the IllustrisTNG100-1 simulation and re-simulate them at high resolution with the N-body code griffin down to binary separations of the order of a parsec. We then estimate coalescence time-scales with a semi-analytical model of the evolution under the effects of GW emission and stellar hardening. We find that most mergers in IllustrisTNG100-1 occur on highly eccentric orbits, and that the eccentricity of BHBs at binary formation correlates with the initial eccentricity of the merger, if this is no larger than approximately 0.9. For extremely eccentric mergers, the binaries tend to form with modest eccentricities. We discuss the implications of these results on the interpretation of the observed GWB.

Impact of a local CP -odd domain in hot QCD on the axionic domain-wall interpretation of NANOGrav 15-year data

We argue that the axionic domain-wall with a QCD bias may be incompatible with the NANOGrav 15-year data on a stochastic gravitational wave (GW) background, when the domain wall network collapses in the hot-quantum chromodynamics (QCD) induced local CP-odd domain. This is due to the drastic suppression of the QCD bias set by the QCD topological susceptibility in the presence of the CP-odd domain with nonzero θ parameter of order one which the QCD sphaleron could generate. We quantify the effect on the GW signals by working on a low-energy effective model of Nambu-Jona-Lasinio type in the mean field approximation. We find that only at θ=π, the QCD bias tends to get significantly large enough due to the criticality of the thermal CP restoration, which would, however, give too big signal strengths to be consistent with the NANOGrav 15-year data and would also be subject to the strength of the phase transition at the criticality. Published by the American Physical Society 2024

Detecting dark domain walls through their impact on particle trajectories in tailored ultrahigh vacuum environments

Light scalar fields, with double well potentials and direct matter couplings, undergo density driven phase transitions, leading to the formation of domain walls. Such theories could explain dark energy or dark matter or source the nanohertz gravitational-wave background. We describe an experiment that could be used to detect such domain walls in a laboratory environment, solving for the scalar field profile and showing how the domain wall affects the motion of a test particle. We find that, in currently unconstrained regions of parameter space, the domain walls leave detectable signatures. Published by the American Physical Society 2024

Impact of spin in compact binary foreground subtraction for estimating the residual stochastic gravitational-wave background in ground-based detectors

Impact of spin in compact binary foreground subtraction for estimating the residual stochastic gravitational-wave background in ground-based detectors

Gravitational waves produced by domain walls during inflation

We study the properties of the stochastic gravitational-wave background (SGWB) produced by domain walls (DWs) during inflation without forming a network. We numerically simulate the DW production caused by a second-order phase transition and calculate the SGWB spectrum using a 1000×1000×1000 lattice. We show that the SGWB can be observed directly by future terrestrial and space gravitational-wave detectors and through the B-mode spectrum in the cosmic microwave background. With numerical simulations, we derive an empirical formula for the strength and qualitative features of the SGWB spectrum. The details of the SGWB spectrum also contain information about the later evolution of the Universe. Published by the American Physical Society 2024

Probing the galactic and extragalactic gravitational wave backgrounds with space-based interferometers

We employ the formalism developed in [1] and [2] to study the prospect of detecting an anisotropic Stochastic Gravitational Wave Background (SGWB) with the Laser Interferometer Space Antenna (LISA) alone, and combined with the proposed space-based interferometer Taiji. Previous analyses have been performed in the frequency domain only. Here, we study the detectability of the individual coefficients of the expansion of the SGWB in spherical harmonics, by taking into account the specific motion of the satellites. This requires the use of time-dependent response functions, which we include in our analysis to obtain an optimal estimate of the anisotropic signal. We focus on two applications. Firstly, the reconstruction of the anisotropic galactic signal without assuming any prior knowledge of its spatial distribution. We find that both LISA and LISA with Taiji cannot put tight constraints on the harmonic coefficients for realistic models of the galactic SGWB. We then focus on the discrimination between a galactic signal of known morphology but unknown overall amplitude and an isotropic extragalactic SGWB component of astrophysical origin. In this case, we find that the two surveys can confirm, at a confidence level ≳ 3σ, the existence of both the galactic and extragalactic background if both have amplitudes as predicted in standard models. We also find that, in the LISA-only case, the analysis in the frequency domain (under the assumption of a time average of data taken homogeneously across the year) provides a nearly identical determination of the two amplitudes as compared to the optimal analysis.

Constraints on inflation with null energy condition violation from advanced LIGO and advanced Virgo's first three observing runs

The null energy condition (NEC) is a cornerstone of general relativity, and its violation could leave observable imprints in the cosmic gravitational wave spectrum. Theoretical models suggest that NEC violations during inflation can amplify the primordial tensor power spectrum, leading to distinct features in the stochastic gravitational wave background (SGWB). In this work, we search for these NEC-violating signatures in the SGWB using data from Advanced LIGO and Advanced Virgo's first three observing runs. Our analysis reveals no statistically significant evidence of such signals, allowing us to place stringent upper limits on the tensor power spectrum amplitude, P T,2, during the second inflationary stage. Specifically, we find that P T,2 ≲ 0.15 at a 95% confidence level. Notably, this upper limit is consistent with constraints derived from pulsar timing array observations, reinforcing the hypothesis that NEC violations during inflation could explain the signal detected by pulsar timing arrays. Our findings contribute to a deeper understanding of the early Universe and highlight the potential of current and future gravitational wave experiments in probing the physics of inflation and NEC violations.

The PTA Hellings and Downs correlation unmasked by symmetries

The Hellings and Downs correlation curve describes the correlation of the timing residuals from pairs of pulsars as a function of their angular separation on the sky and is a smoking-gun signature for the detection of an isotropic stochastic background of gravitational waves. We show that it can be easily obtained fromrealizing that Lorentz transformations are conformal transformationson the celestial sphere and from the conformal properties of the two-point correlation of the timing residuals. This result allows several generalizations, e.g. the calculation of the three-point correlator of the time residuals and the inclusion of additional polarization modes (vector and/or scalar) arising in alternativetheories of gravity.

Stochastic gravitational wave background: birth from string-wall death

We study a new source of stochastic gravitational wave background (SGWB) from the final collapse of a network of topological defects. Typically, the final collapse is considered negligible for generating gravitational waves (GWs) due to its subdominance compared with the network's long-term evolution in the scaling regime. However, in some cases, a network can be driven outside of horizon by inflation and later re-enter horizon. Then, the network's final collapse after re-entering horizon becomes the dominant GW source and therefore cannot be neglected. We demonstrate this phenomenon in the context of N DW = 1 string-wall networks which naturally arise in axion models, although the framework can be generalized to other types of topological networks. The final collapse of walls bounded by strings releases GWs. Our calculation of the corresponding GW spectrum suggests it could be related to the first few bins of the nano-Hertz SGWB signal possibly detected by various Pulsar Timing Array (PTA) collaborations. However, it is important to note that such GW spectrum falls within a relatively narrow frequency range, which may not completely account for the PTA signal that spans more than one order of magnitude in frequency. Furthermore, with different parameter choices, the resultant GWs generated in this mechanism could be probed by various GW interferometry experiments.

Primordial black holes and induced gravitational waves in non-singular matter bouncing cosmology

We present a novel model-independent generic mechanism for primordial black hole formation within the context of non-singular matter bouncing cosmology. In particular, considering a short transition from the matter contracting phase to the Hot Big Bang expanding Universe, we find naturally enhanced curvature perturbations on very small scales which can collapse and form primordial black holes. Interestingly, the primordial black hole masses that we find can lie within the observationally unconstrained asteroid-mass window, potentially explaining the totality of dark matter. Remarkably, the enhanced curvature perturbations, collapsing to primordial black holes, can induce as well a stochastic gravitational-wave background, being potentially detectable by future experiments, in particular by SKA, PTAs, LISA and ET, hence serving as a new portal to probe the bouncing nature of the initial conditions prevailing in the early Universe.

Pulsar Timing Noise from Brownian Motion of the Sun

Recently, pulsar timing arrays (PTAs) reported a signal at nanohertz frequencies consistent with a stochastic gravitational-wave background. Here, I show that the Brownian motion of the Sun as a result of its random gravitational interactions with the cluster of thousands of unmodeled main-belt asteroids of diameters ≲80 km, combined with small inaccuracies in the estimated masses of larger asteroids, introduces timing noise for pulsars, which is up to 20% of the reported signal at frequencies of ∼1 few years–1. The asteroid contribution needs to be modeled better in order to obtain accurate inferences from the PTA signal.

Upper bound on thermal gravitational wave backgrounds from hidden sectors

Hot viscous plasmas unavoidably emit a gravitational wave background, similar to electromagnetic black body radiation.We study the contribution from hidden particles to the diffuse background emitted by the primordial plasma in the early universe.While this contribution can easily dominate over that from Standard Model particles, we find that both are capped by a generic upper bound that makes them difficult to detect with interferometers in the foreseeable future.We illustrate our results for axion-like particles and heavy neutral leptons.Finally, our results suggest that previous works overestimated the gravitational wave background from particle decays out of thermal equilibrium.

Connecting low-redshift LISA massive black hole mergers to the nHz stochastic gravitational wave background

Pulsar Timing Array (PTA) experiments worldwide recently reported evidence of a nHz stochastic gravitational wave background (sGWB) compatible with the existence of slowly inspiralling massive black hole (MBH) binaries (MBHBs). The shape of the signal contains valuable information about the evolution of z &lt; 1 MBHs above 108 M⊙, suggesting a faster dynamical evolution of MBHBs towards the gravitational-wave-driven inspiral or a larger MBH growth than usually assumed. In this work, we investigate if the nHz sGWB could also provide constraints on the population of merging lower-mass MBHBs (&lt; 107 M⊙) detectable by LISA. To this end, we use the L-Galaxies semi-analytical model applied to the Millennium suite of simulations. We generate a population of MBHs compatible simultaneously with current electromagnetic and nHz sGWB constraints by including the possibility that, in favourable environments, MBHs can accrete gas beyond the Eddington limit. The predictions of this new model for the sGWB show that the global (integrated up to high-z) LISA detection rate is not significantly affected when compared to a fiducial model whose nHz sGWB signal is ∼2 times smaller. In both cases, the global rate yields ∼12 yr−1 and is dominated by systems of 105 − 6 M⊙. The main differences are limited to low-z (z &lt; 3), high-mass (&gt; 106 M⊙) LISA MBHBs. The model compatible with the latest PTA results predicts up to ∼1.6 times more detections, with a rate of ∼1 yr−1. We find that these LISA MBHB systems have 50% probability of shining with bolometric luminosities &gt; 1043 erg s−1. Hence, in case PTA results are confirmed and given the current MBH modelling, our findings suggest there will be higher chances to perform multimessenger studies with LISA MBHB than previously expected.

Exploring the NANOGrav signal and planet-mass primordial black holes through Higgs inflation

The data recently released by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) provides compelling evidence supporting the existence of a stochastic signal that aligns with a gravitational-wave background. We show that the scalar-induced gravitational waves from the Higgs inflation model with the parametric amplification mechanism can explain this signal. Such a gravitational-wave background naturally predicts the substantial existence of planet-mass primordial black holes, which can be planet 9 in our solar system and the lensing objects for the ultrashort-timescale microlensing events observed by the Optical Gravitational Lensing Experiment. Therefore, the NANOGrav signal, the potential Planet 9 in our solar system, and the Optical Gravitational Lensing Experiment can be explained within the framework of Higgs inflation.

Complete analysis of the background and anisotropies of scalar-induced gravitational waves: primordial non-Gaussianity f NL and g NL considered

Investigation of primordial non-Gaussianity holds immense importance in testing the inflation paradigm and shedding light on the physics of the early Universe. In this study, we conduct the complete analysis of scalar-induced gravitational waves (SIGWs) by incorporating the local-type non-Gaussianity f NL and g NL. We develop Feynman-like diagrammatic technique and derive semi-analytic formulas for both the energy-density fraction spectrum and the angular power spectrum. For the energy-density fraction spectrum, we analyze all the relevant Feynman-like diagrams, determining their contributions to the spectrum in an order-by-order fashion. As for the angular power spectrum, our focus lies on the initial inhomogeneities, giving rise to anisotropies in SIGWs, that arise from the coupling between short- and long-wavelength modes due to primordial non-Gaussianity. Our analysis reveals that this spectrum exhibits a typical multipole dependence, characterized by C̃ ℓ ∝ [ℓ(ℓ + 1)]-1, which plays a crucial role in distinguishing between different sources of gravitational waves. Depending on model parameters, significant anisotropies can be achieved. We also show that the degeneracies in model parameters can be broken. The findings of our study underscore the angular power spectrum as a robust probe for investigating primordial non-Gaussianity and the physics of the early Universe. Moreover, our theoretical predictions can be tested using space-borne gravitational-wave detectors and pulsar timing arrays.