Stochastic Gravitational Wave Research Articles

Gravitational waves from axion wave production

AbstractWe consider a scenario with axions/axion-like particles Chern–Simons gravity coupling, such that gravitational waves can be produced directly from axion wave parametric resonance in the early universe after inflation. This axion gravity term is less constrained compared to the well-searched axion photon coupling and can provide a direct and efficient production channel for gravitational waves. Such stochastic gravitational waves can be detected by either space/ground-based gravitational wave detectors or pulsar timing arrays for a broad range of axion masses and decay constants.

Gravitational wave spectrum of chain inflation

Chain inflation is an alternative to slow-roll inflation in which the inflaton tunnels along a large number of consecutive minima in its potential. In this work we perform the first comprehensive calculation of the gravitational wave (GW) spectrum of chain inflation. In contrast to slow-roll inflation the latter does not stem from quantum fluctuations of the gravitational field during inflation, but rather from the bubble collisions during the first-order phase transitions associated with vacuum tunneling. Our calculation is performed within an effective theory of chain inflation which builds on an expansion of the tunneling rate capturing most of the available model space. The effective theory can be seen as chain inflation’s analog of the slow-roll expansion in rolling models of inflation. The near scale-invariance of the scalar power spectrum translates to a quasiperiodic shape of the inflaton potential in chain inflation, with the tunneling rate changing very slowly during the e-folds leading to cosmic microwave background observables. We show that chain inflation produces a very characteristic double-peak GW spectrum: a faint high-frequency peak associated with the gravitational radiation emitted during inflation, and a strong low-frequency peak associated with the graceful exit from chain inflation (marking the transition to the radiation-dominated epoch). There exist very exciting prospects to test the gravitational wave signal from chain inflation at the aLIGO-aVIRGO-KAGRA network, at LISA and /or at pulsar timing array experiments. A particularly intriguing possibility we point out is that chain inflation could be the source of the stochastic gravitational wave background recently detected by NANOGrav, PPTA, EPTA, and CPTA. We also show that the gravitational wave signal of chain inflation is often accompanied by running/ higher running of the scalar spectral index to be tested at future cosmic microwave background experiments. Published by the American Physical Society 2024

Measuring the circular polarization of gravitational waves with pulsar timing arrays

The circular polarization of the stochastic gravitational wave background (SGWB) is a key observable for characterizing the origin of the signal detected by Pulsar Timing Array (PTA) collaborations. Both the astrophysical and the cosmological SGWB can have a sizeable amount of circular polarization, due to Poisson fluctuations in the source properties for the former, and to parity violating processes in the early universe for the latter. Its measurement is challenging since PTA are blind to the circular polarization monopole, forcing us to turn to anisotropies for detection. We study the sensitivity of current and future PTA datasets to circular polarization anisotropies, focusing on realistic modelling of intrinsic and kinematic anisotropies for astrophysical and cosmological scenarios respectively. Our results indicate that the expected level of circular polarization for the astrophysical SGWB should be within the reach of near future datasets, while for cosmological SGWB circular polarization is a viable target for more advanced SKA-type experiments. Published by the American Physical Society 2024

Oscillations of Ultralight Dark Photon into Gravitational Waves

The discovery of gravitational waves (GWs) opens a new window for exploring the physics of the early universe. Identifying the source of GWs and their spectra at present turns out to be important tasks so as to assist the experimental detection of stochastic GW signal. In this paper, we investigate oscillations of the ultralight dark photon (ULDP) into GWs in the dark halo. Assuming dark matter is composed of the ULDP and there are primordial dark magnetic fields (PDMFs) arising from the axion inflation and/or the dark phase transition, then the ULDP can oscillate into the GW when it passes through an environment of PDMFs. We derive the local energy density of GWs in the galaxy cluster induced by the instaneous oscillation of ULDP in the PDMFs. These stochastic local GWs exhibit a pulse-like spectrum, with frequency depending on the mass of the ULDP, and can be detected in Pulsar Timing Arrays (PTAs) or future space-based interferometers. We also find that the low-frequency GW signal observed by the NANOGrav collaboration and other PTA experiments can be addressed by the oscillation of the ULDP in the PDMFs in the early universe.

Beyond the Hellings–Downs curve: Non-Einsteinian gravitational waves in pulsar timing array correlations

Pulsar timing arrays (PTAs) have revealed galaxy-size gravitational waves (GWs) in the form of a stochastic gravitational wave background (SGWB), correlating the radio pulses emitted by millisecond pulsars. This discovery naturally leads to the question of the origin and the nature of the SGWB; the latter is synonymous to testing how quadrupolar the inter-pulsar spatial correlation is. In this paper, we investigate the nature of the SGWB by considering correlations beyond the Hellings–Downs (HD) curve of Einstein’s general relativity. We scrutinize the HD and non-Einsteinian GW correlations with the North American Nanohertz Observatory for Gravitational Waves and the Chinese PTA data, and find that both data sets allow a graviton mass of mg ≲ 1.04 × 10−22 eV/c2 and subluminal traveling waves. We discuss gravitational physics scenarios beyond general relativity that could host non-Einsteinian GW correlations in the SGWB and highlight the importance of the cosmic variance inherited from stochastic variations across realizations in interpreting PTA observations.

Primordial gravitational waves of big bounce cosmology in light of stochastic gravitational wave background

Primordial gravitational waves of big bounce cosmology in light of stochastic gravitational wave background

Memory burden effect mimics reheating signatures on SGWB from ultra-low mass PBH domination

Ultra-low mass primordial black holes (PBH), briefly dominating the expansion of the universe, would leave detectable imprints in the secondary stochastic gravitational wave background (SGWB). Such a scenario leads to a characteristic doubly peaked spectrum of SGWB and strongly depends on the Hawking evaporation of such light PBHs. However, these observable signatures are significantly altered if the memory burden effect during the evaporation of PBHs is taken into account. We show that for the SGWB induced by PBH density fluctuations, the memory burden effects on the Hawking evaporation of ultra-low mass PBHs can mimic the signal arising due to the non-standard reheating epoch before PBH domination. Finally, we point out that this degeneracy can be broken by the simultaneous detection of the first peak in the SGWB, which is typically induced by the inflationary adiabatic perturbations.

Uncertainty of the white dwarf astrophysical gravitational wave background

The astrophysical gravitational wave background (AGWB) is a stochastic gravitational wave (GW) signal emitted by different populations of in-spiralling binary systems containing compact objects throughout the Universe . In the frequency range between 10$^ $ and 10$^ $ hertz (Hz), it will be detected by future space-based gravitational wave detectors, such as Laser Interferometer Space Antenna (LISA). In a recent work, we concluded that the white dwarf (WD) contribution to the AGWB dominates that of black holes (BHs) and neutron stars (NSs) We aim to investigate the uncertainties of the WD AGWB that arise from the use of different stellar metallicities, star formation rate density (SFRD) models, and binary evolution models. We used the code we previously developed to determine the WD component of the AGWB. We used a metallicity-dependent SFRD based on an earlier work to construct five different SFRD models. We used four different population models based on a range of common-envelope treatments and six different metallicities for each model. For all possible combinations, the WD component of the AGWB is dominant over other populations of compact objects. The effects of metallicity and population model are less significant than the effect of a (metallicity dependent) SFRD model. We find a range of about a factor of 5 in the level of the WD AGWB around a value of $ WD $ at 1 mHz and a shape that is weakly dependent on the model. We find the uncertainty for the WD component of the AGWB to be about a factor of 5. We note that there are other uncertainties that have an effect on this signal as well. We discuss whether the turnover of the WD AGWB at 10 mHz will be detectable by LISA and find it to be likely. We confirm our previous findings asserting that the WD component of the AGWB dominates over other populations, in particular, BHs.

Efficient Computation of Overlap Reduction Functions for Pulsar Timing Arrays.

Pulsar timing arrays seek and study gravitational waves (GWs) through the angular two-point correlation function of timing residuals they induce in pulsars. The two-point correlation function induced by the standard transverse-traceless GWs is the famous Hellings-Downs curve, a function only of the angle between the two pulsars. Additional polarization modes (vector or scalar) that may arise in alternative-gravity theories have different angular correlation functions. Furthermore, anisotropy, linear, or circular polarization in the stochastic GW background gives rise to additional structure in the two-point correlation function that cannot be written simply in terms of the angular separation of the two pulsars. In this Letter, we provide a simple formula for the most general two-point correlation function-or overlap reduction function (ORF)-for a gravitational-wave background with an arbitrary polarization state, possibly containing anisotropies in its intensity and polarization (linear and/or circular). We provide specific expressions for the ORFs sourced by the general-relativistic transverse-traceless GW modes as well as vector (or spin-1) modes that may arise in alternative-gravity theories.

Ultralight dark matter explanation of NANOGrav observations

The angular correlation of pulsar residuals observed by NANOGrav and other pulsar timing array collaborations show evidence in support of the Hellings-Downs correlation expected from stochastic gravitational wave background (SGWB). In this paper, we offer a nongravitational wave explanation of the observed pulsar timing correlations as caused by an ultralight Lμ−Lτ gauge boson dark matter (ULDM). ULDM can affect the pulsar correlations in two ways. The gravitational potential of vector ULDM gives rise to a Shapiro time delay of the pulsar signals and a nontrivial angular correlation (as compared to the scalar ULDM case). In addition, if the pulsars have a nonzero charge of the dark matter gauge group, then the electric field of the local dark matter causes an oscillation of the pulsar and a corresponding Doppler shift of the pulsar signal. We point out that pulsars carry a significant charge of muons, and thus the Lμ−Lτ vector dark matter contributes to both the Doppler oscillations and the time delay of the pulsar signals. The synergy between these two effects provides a better fit to the shape of the angular correlation function, as observed by the NANOGrav collaboration, compared to the standard SGWB explanation or the SGWB combined with time delay explanations. Our analysis shows that, in addition to the SGWB signal, there may potentially be excess timing residuals attributable to the Lμ−Lτ ULDM. Published by the American Physical Society 2024

Theoretical probes of Higgs boson-axion nonperturbative couplings

In this work we investigate the phenomenological implications of several nontrivial Higgs-boson- axion couplings, which cover most of the possible nonperturbative scenarios. Specifically we consider the combination of having higher-order nonrenormalizable Higgs-boson-axion couplings originating from a weakly coupled effective theory combined with nonperturbative couplings of the form ∼εΛc2|H|2cos(ϕfa). Since we consider the misalignment axion, the nonperturbative couplings can be expanded in the form of a perturbation expansion in powers of ϕ/fa, thus after the electroweak symmetry breaking, the effective potential of the axion is drastically affected by these terms. We investigate the phenomenological implications of these terms for various values of the mass scale Λc, and some scenarios are theoretically disfavored, while other scenarios with nonperturbative Higgs-boson-axion couplings of the form ∼εΛc2|H|2cos(ϕfa) with Λc∼10−10×ma and ma∼10−10 eV, lead to a characteristic pattern in the stochastic gravitational wave background via the deformation of the background equation of state parameter occurring at frequencies probed by the Einstein Telescope. We also consider loop effects from the Higgs sector caused by the term ∼εΛc2|H|2cos(ϕfa) if we close the Higgs in one loop. Published by the American Physical Society 2024

Explaining PTA results by metastable cosmic strings from SO(10) GUT

In a recent paper ( Phys. Rev. D 108 (2023) 095053), we have demonstrated that the 2023 PTA results, which hint at a stochastic gravitational wave (GW) background at nanohertz frequencies, point towards a promising model-building route for realizing SO(10) Grand Unification with embedded inflation. The proposed supersymmetric scenario solves the doublet-triplet splitting without fine-tuning, accounts for charged fermion and neutrino masses, avoids conflicts with current proton decay bounds, and includes only representations no larger than the adjoint. It features multi-step breaking of SO(10) to the Standard Model gauge symmetry, with inflation embedded such that metastable cosmic strings are produced at the end of inflation. This cosmic string network generates a stochastic GW background that can explain the PTA results. In this paper, we provide a detailed analysis of the singled out GUT model class, focusing on how the gauge coupling unification condition affects the scales of multi-step SO(10) breaking and the preferred GW spectra. The lowest breaking scale, linked to inflation, the generation of right-handed neutrino masses for the seesaw mechanism, and metastable cosmic string production, coincides with the range suggested by the PTA results.

Induced domain walls of QCD axion, and gravitational waves

We show that heavy axion domain walls induce domain walls of the QCD axion through a mixing between the heavy axion and the QCD axion, even when the pre-inflationary initial condition is assumed for the QCD axion. The induced domain walls arise because the effective θ parameter changes across the heavy axion domain walls, shifting the potential minimum of the QCD axion. When the heavy axion domain walls collapse, the induced QCD axion domain walls collapse as well. This novel mechanism for producing the QCD axions can explain dark matter even with the axion decay constant as small as 𝒪(109) GeV. In particular, this scenario requires domain wall collapse near the QCD crossover, potentially accounting for the stochastic gravitational wave background suggested by recent pulsar timing array observations, including NANOGrav. Using this mechanism, it is also possible to easily create induced domain walls for string axions or axions with a large decay constant, which would otherwise be challenging. We also comment on the implications for cosmic birefringence using induced axion domain walls.

Gravitational waves from first-order phase transitions in LISA: reconstruction pipeline and physics interpretation

We develop a tool for the analysis of stochastic gravitational wave backgrounds from cosmological first-order phase transitions with LISA: we initiate a template databank for these signals, prototype their searches, and forecast their reconstruction.The templates encompass the gravitational wave signals sourced by bubble collisions, sound waves and turbulence. Accounting for Galactic and extra-Galactic foregrounds, we forecast the region of the parameter space that LISA will reconstruct with better than ∼ 10% accuracy, if certain experimental and theoretical uncertainties are solved by the time LISA flies.We illustrate the accuracy with which LISA can reconstruct the parameters on a few benchmark signals, both in terms of the template parameters and the phase transition ones.To show the impact of the forecasts on physics beyond the Standard Model, we map the reconstructed benchmark measurements into the parameter spaces of the singlet extension of the Standard Model and of the classically conformal invariant U(1) B-L model.

Gravitational waves from a curvature-induced phase transition of a Higgs-portal dark matter sector

The study of interactions between dark matter and the Higgs field opens an exciting connection between cosmology and particle physics, since such scenarios can impact the features of dark matter as well as interfering with the spontaneous breaking of the electroweak symmetry. Furthermore, such Higgs-portal models of dark matter should be suitably harmonised with the various epochs of the universe and the phenomenological constraints imposed by collider experiments. At the same time, the prospect of a stochastic gravitational wave background offers a promising new window into the primordial universe, which can complement the insights gained from accelerators. In this study, we examined whether gravitational waves can be generated from a curvature-induced phase transition of a non-minimally coupled dark scalar field with a portal coupling to the Higgs field. The main requirement is that the phase transition is of first order, which can be achieved through the introduction of a cubic term on the scalar potential and the sign change of the curvature scalar. This mechanism was investigated in the context of a dynamical spacetime during the transition from inflation to kination, while also considering the possibility for inducing electroweak symmetry breaking in this manner for a sufficiently low reheating temperature when the Higgs-portal coupling is extremely weak. We considered a large range of inflationary scales and both cases of positive and negative values for the non-minimal coupling, while taking into account the bound imposed by Big Bang Nucleosythesis. The resulting gravitational wave amplitudes are boosted by kination and thus constrain the parameter space of the couplings significantly. Even though the spectra lie at high frequencies for the standard high inflationary scales, there are combinations of parameter space where they could be probed with future experiments.

Constraining the Origin of the Nanohertz Gravitational-wave Background by Pulsar Timing Array Observations of Both the Background and Individual Supermassive Binary Black Holes

The gravitational waves (GWs) from supermassive binary black holes (BBHs) have long been sought by pulsar timing array (PTA) experiments, in the forms of both a stochastic GW background (GWB) and individual sources. Evidence for a GWB was reported recently by several PTAs with origins to be determined. Here we use a BBH population synthesis model to investigate the detection probability of individual BBHs by the Chinese PTA (CPTA) and the constraint on the GWB origin that may be obtained by PTA observations of both GWB and individual BBHs. If the detected GWB signal is entirely due to BBHs, a significantly positive redshift evolution (∝ (1 + z)2.07) of the mass scaling relation between supermassive black holes and their host galaxies is required. In this case, we find that the detection probability of individual BBHs is ∼85% or 64% if using a period of 3.4 yr of CPTA observation data, with an expectation of ∼1.9 or 1.0 BBHs detectable with a signal-to-noise ratio ≥3 or 5, and it is expected to increase to >95% if the observation period is extended to 5 yr or longer. Even if the contribution from BBHs to the GWB power signal is as small as ∼10%, a positive detection of individual BBHs can still be expected within an observation period of ∼10 yr. A nondetection of individual BBHs within several years from now jointly with the detected GWB signal can put a strong constraint on the upper limit of the BBH contribution to the GWB signal and help identify/falsify a cosmological origin.

No Pulsar Timing Noise from Brownian Motion of the Sun

It was recently claimed that evidence for a stochastic gravitational wave background observed by pulsar timing arrays can be attributed instead to random perturbations of the Sun’s motion by transiting asteroids. I show that that this would lead to a large dipole component accompanying a much smaller quadrupolar perturbation of pulsar timing signals, which would not be confused with a gravitational wave signal. Such an anomalous dipole would have been detected and identified as a spurious background by the PTA collaborations, if it existed.

Stochastic gravitational wave background due to core collapse resulting in neutron stars

Stochastic gravitational wave background due to core collapse resulting in neutron stars

Dual inflation and bounce cosmologies interpretation of pulsar timing array data

We explore a dual scenario of generalized inflation and bounce cosmologies, producing a scale-invariant curvature perturbation spectrum. Bayesian analysis with pulsar timing array data identifies, for the first time, viable regions from inflation and bounce that simultaneously explain stochastic gravitational wave background (SGWB) signals and CMB anisotropies. Bayes factor calculations strongly favor this dual scenario over conventional sources and provide initial evidence of a duality between inflation and bounce regarding SGWB, offering new insights for early universe model-building and future observations.

Signal identification of the stochastic gravitational wave background in the Mock LISA Data Challenge

Signal identification of the stochastic gravitational wave background in the Mock LISA Data Challenge