Stochastic Gravitational Wave Research Articles

Phases of Pseudo-Nambu-Goldstone bosons

We study the vacuum dynamics of pseudo-Nambu-Goldstone bosons (pNGBs) for SO(N + 1) → SO(N) spontaneous and explicit symmetry breaking. We determine the magnitude of explicit symmetry breaking consistent with an EFT description of the effective potential at zero and finite temperatures. We expose and clarify novel additional vacuum transitions that can arise for generic pNGBs below the initial scale of SO(N + 1) → SO(N) spontaneous symmetry breaking, which may have phenomenological relevance. In this respect, two phenomenological scenarios are analyzed: thermal and supercooled dark sector pNGBs. In the thermal scenario the vacuum transition is first-order but very weak. For a supercooled dark sector we find that, depending on the sign of the explicit symmetry breaking, one can have a symmetry-restoring vacuum transition SO(N – 1) → SO(N) which can be strongly first-order, with a detectable stochastic gravitational wave background signal.

Dissecting the stochastic gravitational wave background with astrometry

Astrometry, the precise measurement of star motions, offers an alternative avenue to investigate low-frequency gravitational waves through the spatial deflection of photons, complementing pulsar timing arrays reliant on timing residuals. Upcoming data from Gaia, Theia, and Roman can not only cross-check pulsar timing array findings but also explore the uncharted frequency range bridging pulsar timing arrays and LISA. We present an analytical framework to evaluate the feasibility of detecting a gravitational wave background, considering measurement noise and the intrinsic variability of the stochastic background. Furthermore, we highlight astrometry's crucial role in uncovering key properties of the gravitational wave background, such as spectral index and chirality, employing information-matrix analysis. Finally, we simulate the emergence of quadrupolar correlations, commonly referred to as the generalized Hellings-Downs curves.

Observing gravitational waves with solar system astrometry

The subtle influence of gravitational waves on the apparent positioning of celestial bodies offers novel observational windows [1,2,3,4]. We calculate the expected astrometric signal induced by an isotropic Stochastic Gravitational Wave Background (SGWB) in the short distance limit. Our focus is on the resultant proper motion of Solar System objects, a signal on the same time scales addressed by Pulsar Timing Arrays (PTA). We derive the corresponding astrometric deflection patterns, finding that they manifest as distinctive dipole and quadrupole correlations or, in some cases, may not be present. Our analysis encompasses both Einsteinian and non-Einsteinian polarisations. We estimate the upper limits for the amplitude of SGWBs that could be obtained by tracking the proper motions of large numbers of solar system objects such as asteroids. We find that for SGWBs with negative spectral indices, such as that generated by Super Massive Black Hole Binaries (SMBHB), the constraints from these observations could rival those from PTAs. With the Gaia satellite and the Vera C. Rubin Observatory poised to track an extensive sample of asteroids — ranging from 𝒪(105) to 𝒪(106), we highlight the significant future potential for similar surveys to contribute to our understanding of the SGWB.

Percolating Cosmic String loops from evaporating primordial black holes

The Pulsar timing data from NANOGrav Collaboration has regenerated interest in the possibility of observing stochastic gravitational wave background arising from cosmic strings. Standard theory of formation of cosmic strings is based on a spontaneous symmetry breaking (SSB) phase transition in the early universe, with a string network forming via the so called Kibble mechanism. This string network rapidly evolves and reaches a scaling solution with a given spectrum of string loops and long strings at any given time. This scenario necessarily requires that the entire observable Universe goes through the SSB phase transition. This would not be possible, e.g., in models of low energy inflation, where the reheat temperature is much lower than the energy scale of cosmic strings. We point out a very different possibility, where a network of even high energy scale cosmic strings can form when the temperature of the Universe is much lower. We consider local heating of plasma in the early universe by evaporating primordial black holes (PBHs). It is known that for suitable masses of PBHs, Hawking radiation of evaporating primordial black holes may re-heat the surrounding plasma to high temperatures, restoring certain symmetries locally which are broken at the ambient temperature of the Universe at that stage. Expansion of the hot plasma cools it so that the locally restored symmetry is spontaneously broken again. If this SSB supports formation of cosmic strings, then string loops will form in this region around the PBH. Further, resulting temperature gradients lead to large pressure gradients such that plasma will develop radial flow with the string loops getting stretched as they get dragged by the flow. For a finite density of PBHs of suitable masses, one will get local hot spots, each one contributing to expanding cosmic string loops. For suitable PBH density, the loops from different regions may intersect. If that happens, then intercommutation of strings can lead to percolation, leading to the possibility of formation of infinite string network, even when the entire universe never goes through the respective SSB phase transition.

Probing primordial black holes at high redshift with future gravitational wave detectors

Abstract We analyze the detection prospects for potential Primordial Black Hole Binary (PBHB) populations buried in the Stellar-Origin Black Hole Binary (SOBHB) population inferred by the LVK collaboration. We consider different PBHB population scenarios and several future Gravitational Wave (GW) detectors. To separate the PBHB component from the SOBHB one, we exploit the prediction that the PBHB merger rate does not decline as fast as the SOBHB one at high redshift. However, only a tiny fraction of PBHB events may be resolved individually, and the sub-threshold events may yield an undetectable Stochastic GW Background (SGWB). For this reason, we determine the statistical significance of the PBHB contributions in the number of resolvable events seen in future Earth-based detectors and the SGWB measured at LISA. We quantify them in the limit that SOBHB population uncertainties are small, as one may optimistically expect at the time that future detectors will operate. In general, we find the synergy between these probes will consistently help assess whether or not a sizeable PBHB population is present.

Probing parity violation in the stochastic gravitational wave background with astrometry

Probing parity violation in the stochastic gravitational wave background with astrometry

Constraints on supermassive black hole binaries from JWST and NANOGrav

We use the recent statistics of dual active galactic nuclei (AGN) in the James Webb Space Telescope (JWST) data at $z 3.4$ to address two aspects of the feedback and evolution scenarios of supermassive black hole binaries (SMBHBs). We find that the JWST data provide evidence for the members of a binary BH being `lit' at the same time, rather than independently -- a scenario which is consistent with gas-rich mergers being responsible for concurrent AGN activity. This conclusion is supported by the recent NANOGrav Pulsar Timing Array (PTA) measurements, whose upper limits on the stochastic gravitational wave strain amplitude lie below those expected from extrapolating the dual AGN fraction. The results indicate either a `stalling' of the binaries at the separations probed by NANOGrav, or rapid gas-driven inspirals.

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 $z<1$ MBHs above $ odot $, suggesting a faster dynamical evolution of MBHBs towards the gravitational-wave-driven inspiral or a larger MBH growth than usually assumed. In this work, we investigate if the nHz sGWB could also provide constraints on the population of merging lower-mass MBHBs ($ odot $) detectable by LISA. To this end, we use the L-Galaxies semi-analytical model applied to the Millennium suite of simulations. We generate a population of MBHs compatible simultaneously with current electromagnetic and nHz sGWB constraints by including the possibility that, in favourable environments, MBHs can accrete gas beyond the Eddington limit. The predictions of this new model for the sGWB show that the global (integrated up to high-$z$) LISA detection rate is not significantly affected when compared to a fiducial model whose nHz sGWB signal is $ sim \,2$ times smaller. In both cases, the global rate yields sim $ and is dominated by systems of $ odot $. The main differences are limited to low-$z$ ($z<3$), high-mass ($ odot $) LISA MBHBs. The model compatible with the latest PTA results predicts up to $ sim \,1.6$ times more detections, with a rate of sim erg/s$. 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.

A Superconducting Tensor Detector for Mid-Frequency Gravitational Waves: Its Multichannel Nature and Main Astrophysical Targets

Abstract Mid-frequency band gravitational-wave detectors will be complementary to the existing Earth-based detectors (sensitive above 10 Hz or so) and the future space-based detectors such as the Laser Interferometer Space Antenna (LISA), which will be sensitive below around 10 mHz. A ground-based superconducting omnidirectional gravitational radiation observatory (SOGRO) has recently been proposed along with several design variations for the frequency band of 0.1–10 Hz. For two conceptual designs of SOGRO (i.e. SOGRO and advanced SOGRO [aSOGRO]), we examine their multichannel natures, sensitivities, and science cases. One of the key characteristics of the SOGRO concept is its six detection channels. The response functions of each channel are calculated for all possible gravitational wave (GW) polarizations including scalar and vector modes. Combining these response functions, we also confirm the omnidirectional nature of SOGRO. Hence, even a single SOGRO detector will be able to determine the position of a source and polarizations of GWs, if detected. Taking into account SOGRO’s sensitivity and technical requirements, two main targets are most plausible: GWs from compact binaries and stochastic backgrounds. Based on assumptions we consider in this work, detection rates for intermediate-mass binary black holes (in the mass range of hundreds up to $10^{5}\, M_\odot$) are expected to be 0.0065–8.1 yr−1. In order to detect the stochastic GW background, multiple detectors are required. Two aSOGRO detector networks may be able to put limits on the stochastic background beyond the indirect limit from cosmological observations.

BCS in the sky: signatures of inflationary fermion condensation

We consider a Bardeen-Cooper-Schrieffer (BCS)-like model in the inflationary background. We show that with an axial chemical potential, the attractive quartic fermion self-interaction can lead to a BCS-like condensation. In the rigid-de Sitter (dS) limit of inflation where backreaction from the inflaton and graviton is neglected, we perform the first computation of the non-perturbative effective potential that includes the full spacetime curvature effects in the presence of the chemical potential, subject to the mean-field approximation whose validity has been checked via the Ginzburg criterion. The corresponding BCS phase transition is always first-order, when the varying Hubble is interpreted as an effective Gibbons-Hawking temperature of dS spacetime. In the condensed phase, the theory can be understood from UV and IR sides as fermionic and bosonic, respectively. This leads to distinctive signatures in the primordial non-Gaussianity of curvature perturbations. Namely, the oscillatory cosmological collider signal is smoothly turned off at a finite momentum ratio, since different momentum ratios effectively probe different energy scales. In addition, such BCS phase transitions can also source stochastic gravitational waves, which are feasible for future experiments.

Correlations for an anisotropic polarized stochastic gravitational wave background in pulsar timing arrays

The recent compelling observation of the nanohertz stochastic gravitational wave background has brought to light a new galactic arena to test gravity. In this paper, we derive a formula for the most general expression of the stochastic gravitational wave background correlation that could be tested with pulsar timing and future square kilometer arrays. Our expressions extend the harmonic space analysis, also often referred to as the power spectrum approach, to predict the correlation signatures of an anisotropic polarized stochastic gravitational wave background with subluminal tensor, vector, and scalar gravitational degrees of freedom. We present the first few nontrivial anisotropy and polarization signatures in the correlation and discuss their dependence on the gravitational wave speed and pulsar distances. Our results set up tests that could potentially be used to rigorously examine the isotropy of the stochastic gravitational wave background and strengthen the existing constraints on possible non-Einsteinian polarizations in the nanohertz gravitational wave regime.

Gravitational wave probe of primordial black hole origin via superradiance

In this article we have used stochastic gravitational wave background as a unique probe to gain insight regarding the creation mechanism of primordial black holes. We have considered the cumulative gravitational wave background which consists of the primary part coming from the creation mechanism of the primordial black holes and the secondary part coming from the different mechanisms the primordial black holes go through. We have shown that in the presence of light or ultra light scalar bosons, superradiant instability generates the secondary part of the gravitational wave background which is the most detectable. In order to show the unique features of the cumulative background, we have consdiered the delayed vacuum decay during a first order phase transition as the origin of primordial black holes. We have shown the dependence of the features of the cumulative background, such as the mass of the relevant light scalars, peak frequencies, etc. on the transition parameters. We have also generated the cumulative background for a few benchmark cases to further illustrate our claim.

Supermassive primordial black holes from inflation

There is controversy surrounding the origin and evolution of our universe's largest supermassive black holes (SMBHs). In this study, we consider the possibility that some of these black holes formed from the direct collapse of primordial density perturbations. Since the mass of a primordial black hole is limited by the size of the cosmological horizon at the time of collapse, these SMBHs must form rather late, and are naively in conflict with constraints from CMB spectral distortions. These limits can be avoided, however, if the distribution of primordial curvature perturbations is highly non-Gaussian. After quantifying the departure from Gaussianity needed to evade these bounds, we explore a model of multi-field inflation — a non-minimal, self-interacting curvaton model — which has all the necessary ingredients to yield such dramatic non-Gaussianities. We leave the detailed model building and numerics to a future study, however, as our goal is to highlight the challenges associated with forming SMBHs from direct collapse and to identify features that a successful model would need to have. This study is particularly timely in light of recent observations of high-redshift massive galaxy candidates by the James Webb Space Telescope as well as evidence from the NANOGrav experiment for a stochastic gravitational wave background consistent with SMBH mergers.

Probing ultralight tensor dark matter with the stochastic gravitational-wave background from advanced LIGO and Virgo's first three observing runs

Ultralight bosons are attractive dark-matter candidates and appear in various scenarios beyond standard model. They can induce superradiant instabilities around spinning black holes (BHs), extracting the energy and angular momentum from BHs, and then dissipate through monochromatic gravitational radiation, which become promising sources of gravitational wave detectors. In this letter, we focus on massive tensor fields coupled to BHs and compute the stochastic gravitational wave backgrounds emitted by these sources. We then undertake a search for this background within the data from LIGO/Virgo O1∼O3 runs. Our analysis reveals no discernible evidence of such signals, allowing us to impose stringent limits on the mass range of tensor bosons. Specifically, we exclude the existence of tensor bosons with masses ranging from 4.0 × 10-14 to 2.0 × 10-12 eV at 95% confidence level.

Primordial Black Holes from Spatially Varying Cosmological Constant Induced by Field Fluctuations in Extra Dimensions

The origin and evolution of supermassive black holes (SMBHs) in our universe have sparked controversy. In this study, we explore the hypothesis that some of these black holes may have seeded from the direct collapse of dark energy domains with density significantly higher than the surrounding regions. The mechanism of the origin of such domains relies on the inflationary evolution of a scalar field acting in D dimensions, which is associated with the cosmological constant in our four-dimensional spacetime manifold. Inner space quantum fluctuations of the field during inflation are responsible for the spatial variations of the dark energy density in our space. This finding holds particular significance, especially considering recent evidence from pulsar timing array observations, which supports the existence of a stochastic gravitational wave background consisting of SMBH mergers.

Detectability of stochastic gravitational wave background from weakly hyperbolic encounters

We compute the stochastic gravitational wave (GW) background generated by black hole–black hole (BH–BH) hyperbolic encounters with eccentricities close to one and compare them with the respective sensitivity curves of planned GW detectors. We use the Keplerian potential to model the orbits of the encounters and the quadrupole formula to compute the emitted GWs. We take into account hyperbolic encounters that take place in clusters up to redshift 5 and with BH masses spanning from 5 M⊙ to 55 M⊙. We assume the clusters to be virialized and study several cluster models with different mass and virial velocity, and finally obtain an accumulative result, displaying the background as an average. Using the maxima and minima of our accumulative result for each frequency, we provide analytical expressions for both optimistic and pessimistic scenarios. Our results suggest that the background from these encounters is likely to be detected by the third-generation detectors Cosmic explorer and Einstein telescope, while the tail section at lower frequencies intersects with DECIGO, making it a potential target source for both ground- and space-based future GW detectors.

Wave-optics limit of the stochastic gravitational wave background

The stochastic gravitational wave background (SGWB) is a rich resource of cosmological information, encoded both in its source statistics and anisotropies induced by propagation effects. We provide a theoretical description of it, without employing tools which rely on the geometric optics assumption. Our formalism is based on the so-called classical matter approximation and it is able to capture wave-optics effects, such as interference and diffraction. We show that the interaction between the gravitational waves and the cosmic structures along the line-of-sight produce observable scalar and vector polarization modes, on top of modulating the tensorial ones. We build the two point correlation function describing the statistics of the SGWB, and introduce the intensity and gravitational Stokes parameters for all its components. In the case of an unpolarized, Gaussian and statistically homogeneous SGWB, we show that the interaction with matter modulates its intensity and does not generate a net difference between left- and right-helicity tensor and vector modes, as expected. We demonstrate that, in order to produce QT- and UT-tensor polarization modes the background must possess a hexadecapole anisotropy, while a quadrupole anisotropy can source the vector QV- and UV-Stokes parameters.

Gravitational waves from domain wall collapse, and application to nanohertz signals with QCD-coupled axions

We study for the first time the gravitational waves generated during the collapse of domain walls, incorporating the potential bias in the lattice simulations. The final stages of domain wall collapse are crucial for the production of gravitational waves, but have remained unexplored due to computational difficulties. As a significant application of this new result, we show that the observed NANOGrav, EPTA, PPTA, and CPTA data, which indicate stochastic gravitational waves in the nanohertz regime, can be attributed to axion domain walls coupled to QCD. In our model, non-perturbative effects of QCD induce a temperature-dependent bias around the QCD crossover, inducing the rapid collapse of the domain walls. We use sophisticated lattice simulations that account for the temperature-dependent bias to measure the gravitational waves resulting from the domain wall annihilation. We also discuss the future prospects for accelerator-based searches for the axion and the potential for the formation and detection of primordial black holes.

Ability of LISA, Taiji, and their networks to detect the stochastic gravitational wave background generated by cosmic strings

Ability of LISA, Taiji, and their networks to detect the stochastic gravitational wave background generated by cosmic strings

Probing the speed of gravitational waves beyond general relativity from CMB observations

In modified gravity theories, gravitational waves can propagate differently from general relativity and their propagating speed can be either constant or acquire a time dependence. We consider the constant models first and update the constraints on cosmological parameters from the combinations of [Formula: see text] datasets. In this case, excluding superluminal propagation, we obtain the lower limit on the speed [Formula: see text] at 95% C.L. A non-trivial propagating speed impacts the amplitude of tensor spectrum by adding a factor [Formula: see text], where [Formula: see text] is the tensor tilt. We find that the value of [Formula: see text] has positive correlation with the tensor-to-scalar ratio and anti-correlation with the factor [Formula: see text]. Then we explore a time-dependent speed which contains the resonance of the stochastic gravitational wave background. If the speed of gravitational waves oscillates at primordial era, resonance continuously enhances stochastic gravitational wave background which produces observable effects on tensor power spectra. We derive the constraints on the amplitude of oscillatory speed and tensor parameters from the combinations of [Formula: see text] datasets. The numerical results show that the speed resonance of the stochastic gravitational wave background is sensitive to CMB observations.