3D simulations in an expanding background show cosmic expansion drives nonlinear growth that amplifies gravitational-wave spectra from slow phase transitions by factors of 10 to 100.
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The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals
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abstract
We present the results of the search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies using the second data release of the European Pulsar Timing Array (EPTA) for 25 millisecond pulsars and a combination with the first data release of the Indian Pulsar Timing Array (InPTA). We analysed (i) the full 24.7-year EPTA data set, (ii) its 10.3-year subset based on modern observing systems, (iii) the combination of the full data set with the first data release of the InPTA for ten commonly timed millisecond pulsars, and (iv) the combination of the 10.3-year subset with the InPTA data. These combinations allowed us to probe the contributions of instrumental noise and interstellar propagation effects. With the full data set, we find marginal evidence for a GWB, with a Bayes factor of four and a false alarm probability of $4\%$. With the 10.3-year subset, we report evidence for a GWB, with a Bayes factor of $60$ and a false alarm probability of about $0.1\%$ ($\gtrsim 3\sigma$ significance). The addition of the InPTA data yields results that are broadly consistent with the EPTA-only data sets, with the benefit of better noise modelling. Analyses were performed with different data processing pipelines to test the consistency of the results from independent software packages. The inferred spectrum from the latest EPTA data from new generation observing systems is rather uncertain and in mild tension with the common signal measured in the full data set. However, if the spectral index is fixed at 13/3, the two data sets give a similar amplitude of ($2.5\pm0.7)\times10^{-15}$ at a reference frequency of $1\,{\rm yr}^{-1}$. By continuing our detection efforts as part of the International Pulsar Timing Array (IPTA), we expect to be able to improve the measurement of spatial correlations and better characterise this signal in the coming years.
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representative citing papers
Future microhertz detections combined with nanohertz pulsar terms can serve as gravity echoes to measure supermassive black hole binary inspiral rates from hundreds to thousands of years in the past.
First search for high-frequency gravitational waves via inverse Gertsenshtein conversion in Earth's magnetic field with VLA and ALMA sets new upper limits h_c ≲ 10^{-18} from 1 GHz to 1 THz.
Implements full-Stokes SGWB map-making for ground-based networks, applies to LVK O3 data, and constrains polarized angular spectra while showing intensity-only models can be biased.
Calypso is a parameter-conditioned stochastic surrogate model for circumbinary accretion flows using PCA and multivariate Gaussian modeling, released as open-source software with a closed-form likelihood for parameter inference from time series.
Hardening binaries experience deterministic self-acceleration of their center of mass, induced precession, and plane rotation in uniform isotropic media, driving outward spiraling and eccentricity growth in all cases rather than circularization.
A full-covariance formalism for PTA-astrometry ORF estimators forecasts graviton-mass upper limits of 4.41e-24 eV/c2 for current-like setups and 0.48e-24 eV/c2 for SKA/Theia-like future setups, with astrometry adding significant power in the latter case.
Pulsar timing arrays can probe supermassive black hole binaries that merged prior to observations via the pulsar term, with SKA potentially detecting a few such zombie binaries at SNR > 3.
A dipole pulsar timing array detects chiral nanohertz gravitational waves and extends PTA sensitivity into the microhertz regime.
High-resolution lunar simulations reveal up to tenfold amplification of deci-hertz gravitational-wave signals in thick-crust regions via mode coupling.
Cross-correlating pulsar timing and polarimetry isolates the circular polarization of isotropic stochastic GW backgrounds and shares the Hellings-Downs angular pattern.
In pure axion inflation, detectable gravitational wave signals arise only in parameter regions with strong backreaction that violate the upper bound on ΔN_eff.
Scalar metric perturbations after inflation break conformal invariance and induce quantum production of gravitons, generating a GW spectrum that peaks near GHz frequencies for standard primordial scalar power spectra.
First astrometric constraints on parity-violating SGWB amplitude are reported as h70²ΩV = -0.020 ± 0.025 (Gaia) and -0.004 ± 0.010 (VLBA) at 2σ, consistent with zero, over 4.2e-18 Hz to 1.1e-8 Hz.
Axion-like fields coupled to the Nieh-Yan term generate a chiral GW background during radiation domination, with parameter space explored for detectability in PTA and space-based observatories.
Proposes APTA with 6 satellites and 10^{-18} relative clock uncertainty at 1s averaging to achieve sensitivity for observing 10^3-10^4 solar-mass black hole mergers in the decihertz band.
Scaling monopoles generate PBHs via stochastic overdensities and GWs with correlated spectra, potentially with magnetically charged PBHs as a signature if the scaling ends via gauge boson mass.
PPO reinforcement learning accelerates identification of gravitational wave signals from supercooled phase transitions in a minimal dark U(1)_x sector compared to Monte Carlo sampling.
Scalar-induced tensor modes inherit anomalous coherence from a decohered two-mode Gaussian scalar state, generating nonzero tensor discord and connected power covariance as probes of primordial quantum memory.
Monte Carlo and ML surrogate framework projects PTA sensitivity to compact DM substructures and shows SGWB weakens it, with only Shapiro searches retaining sensitivity in optimistic cases.
A joint model of GWB and resolvable SMBHBs for PTA data proposes N_c as astrophysical detection statistic and applies it to NANOGrav 15-year simulations, finding tensions with 21 of 114 AGN candidates and low (2-5%) detection probabilities for isolated sources.
DarkAgents is an LLM-powered multi-agent framework for model building, pipeline computation, and assumption auditing in astroparticle physics, demonstrated on first-order phase transitions fitting NANOGrav gravitational wave data.
Bayesian multiband analysis shows LISA and Taiji reconstruct PTA-compatible domain wall parameters in the strong-signal regime, with joint PTA priors reducing 10D degeneracies.
Self-consistent thermal regulation in circumbinary disks permits long-lived non-accretion phases that suppress binary feeding rates toward the Eddington limit while leaving optical/near-IR detectability intact.
citing papers explorer
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Nonlinear growth and amplification of phase-transition gravitational waves induced by cosmic expansion
3D simulations in an expanding background show cosmic expansion drives nonlinear growth that amplifies gravitational-wave spectra from slow phase transitions by factors of 10 to 100.
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Gravity Echoes from Supermassive Black Hole Binaries
Future microhertz detections combined with nanohertz pulsar terms can serve as gravity echoes to measure supermassive black hole binary inspiral rates from hundreds to thousands of years in the past.
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Search for High-Frequency Gravitational Waves via Geomagnetic Conversion with Radio Telescopes
First search for high-frequency gravitational waves via inverse Gertsenshtein conversion in Earth's magnetic field with VLA and ALMA sets new upper limits h_c ≲ 10^{-18} from 1 GHz to 1 THz.
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Polarized Anisotropic Stochastic Gravitational Wave Background Search with Ground-Based Detector Networks
Implements full-Stokes SGWB map-making for ground-based networks, applies to LVK O3 data, and constrains polarized angular spectra while showing intensity-only models can be biased.
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\texttt{calypso}: a Parameter-Conditioned Stochastic Surrogate Model for Circumbinary Accretion Time-Series
Calypso is a parameter-conditioned stochastic surrogate model for circumbinary accretion flows using PCA and multivariate Gaussian modeling, released as open-source software with a closed-form likelihood for parameter inference from time series.
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Self-acceleration of Hardening Binaries
Hardening binaries experience deterministic self-acceleration of their center of mass, induced precession, and plane rotation in uniform isotropic media, driving outward spiraling and eccentricity growth in all cases rather than circularization.
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Forecasting graviton-mass constraints from the full covariance of PTA-astrometry ORF estimators
A full-covariance formalism for PTA-astrometry ORF estimators forecasts graviton-mass upper limits of 4.41e-24 eV/c2 for current-like setups and 0.48e-24 eV/c2 for SKA/Theia-like future setups, with astrometry adding significant power in the latter case.
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Probing Supermassive Black Hole Mergers with Pulsar Timing Arrays
Pulsar timing arrays can probe supermassive black hole binaries that merged prior to observations via the pulsar term, with SKA potentially detecting a few such zombie binaries at SNR > 3.
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Detecting Chiral Gravitational Wave Background with a Dipole Pulsar Timing Array
A dipole pulsar timing array detects chiral nanohertz gravitational waves and extends PTA sensitivity into the microhertz regime.
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Thick Lunar Crust Amplifies Deci-Hertz Gravitational-Wave Signal
High-resolution lunar simulations reveal up to tenfold amplification of deci-hertz gravitational-wave signals in thick-crust regions via mode coupling.
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Detecting Parity-Violating Gravitational Wave Backgrounds with Pulsar Polarization Arrays
Cross-correlating pulsar timing and polarimetry isolates the circular polarization of isotropic stochastic GW backgrounds and shares the Hellings-Downs angular pattern.
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Gravitational waves from axion inflation in the gradient expansion formalism. Part I. Pure axion inflation
In pure axion inflation, detectable gravitational wave signals arise only in parameter regions with strong backreaction that violate the upper bound on ΔN_eff.
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Quantum production of gravitational waves after inflation
Scalar metric perturbations after inflation break conformal invariance and induce quantum production of gravitons, generating a GW spectrum that peaks near GHz frequencies for standard primordial scalar power spectra.
-
First astrometric constraints on parity-violation in the gravitational wave background
First astrometric constraints on parity-violating SGWB amplitude are reported as h70²ΩV = -0.020 ± 0.025 (Gaia) and -0.004 ± 0.010 (VLBA) at 2σ, consistent with zero, over 4.2e-18 Hz to 1.1e-8 Hz.
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Chiral Gravitational Wave Background from Audible Axion via Nieh-Yan Term
Axion-like fields coupled to the Nieh-Yan term generate a chiral GW background during radiation domination, with parameter space explored for detectability in PTA and space-based observatories.
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Artificial Precision Timing Array: bridging the decihertz gravitational-wave sensitivity gap with clock satellites
Proposes APTA with 6 satellites and 10^{-18} relative clock uncertainty at 1s averaging to achieve sensitivity for observing 10^3-10^4 solar-mass black hole mergers in the decihertz band.
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PBHs and GWs from Scaling Monopoles
Scaling monopoles generate PBHs via stochastic overdensities and GWs with correlated spectra, potentially with magnetically charged PBHs as a signature if the scaling ends via gauge boson mass.
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Supercool with PPO: Exploring Supercooled Phase Transitions via Reinforcement Learning
PPO reinforcement learning accelerates identification of gravitational wave signals from supercooled phase transitions in a minimal dark U(1)_x sector compared to Monte Carlo sampling.
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Quantum Memory in Scalar-Induced Gravitational Waves
Scalar-induced tensor modes inherit anomalous coherence from a decohered two-mode Gaussian scalar state, generating nonzero tensor discord and connected power covariance as probes of primordial quantum memory.
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Projecting the ultimate pulsar timing sensitivity to dark matter substructure in a stochastic gravitational wave background
Monte Carlo and ML surrogate framework projects PTA sensitivity to compact DM substructures and shows SGWB weakens it, with only Shapiro searches retaining sensitivity in optimistic cases.
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A Joint Optimal Search for Gravitational Waves from Resolved and Unresolved Supermassive Binary Black Holes with Pulsar Timing Arrays
A joint model of GWB and resolvable SMBHBs for PTA data proposes N_c as astrophysical detection statistic and applies it to NANOGrav 15-year simulations, finding tensions with 21 of 114 AGN candidates and low (2-5%) detection probabilities for isolated sources.
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DarkAgents
DarkAgents is an LLM-powered multi-agent framework for model building, pipeline computation, and assumption auditing in astroparticle physics, demonstrated on first-order phase transitions fitting NANOGrav gravitational wave data.
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PTA-Compatible Domain Walls at LISA and Taiji: Bayesian Reconstruction and Multiband Inference
Bayesian multiband analysis shows LISA and Taiji reconstruct PTA-compatible domain wall parameters in the strong-signal regime, with joint PTA priors reducing 10D degeneracies.
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Dynamics and detectability of long-lived non-accretion phases for massive black hole binaries in cold, thermally regulating disks
Self-consistent thermal regulation in circumbinary disks permits long-lived non-accretion phases that suppress binary feeding rates toward the Eddington limit while leaving optical/near-IR detectability intact.
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Searching for a waveform-agnostic gravitational wave signal in pulsar timing arrays
Presents a new Fourier-expansion Bayesian hierarchical model with Lorentzian hyperprior for waveform-agnostic searches of nanohertz gravitational wave sources in pulsar timing array data.
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Higher-order statistics of the stochastic gravitational wave background from supermassive black hole binaries
With a physically motivated z_min cutoff, higher-order moments of the SGWB from SMBH binaries depend on the mass function solely via <M^{10/3}>, giving a variance-to-mean ratio for <M^{10/3}>/<M^{5/3}> and a kurtosis-skewness consistency relation independent of redshift evolution.
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Primordial Black Hole from Tensor-induced Density Fluctuation: First-order Phase Transitions and Domain Walls
Tensor perturbations from first-order phase transitions and domain wall annihilation induce curvature fluctuations at second order that form primordial black holes, allowing asteroid-mass PBHs to comprise all dark matter for specific parameter ranges with associated gravitational wave peaks in LISA,
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Phase-resolved field-space distance criteria in ekpyrotic, bouncing and cyclic cosmologies
Proposes phase-resolved invariant path-length criteria and a master formula for lower bound on ε_ek in ekpyrotic cosmologies, using BKL suppression and conversion windows as constraints.
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Testing General Relativity with Individual Supermassive Black Hole Binaries
A framework is developed to test beyond-GR effects in nanohertz continuous waves from individual SMBHBs, deriving modified inter-pulsar correlations, antenna responses, and phase delays for three deviation classes, validated by injection-recovery simulations showing parameter recovery and no GR bias
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Are PTA measurements sensitive to gravitational wave non-Gaussianities?
PTA statistical tests cannot distinguish Gaussian and non-Gaussian GWB amplitude distributions in a model-agnostic way after decorrelation.
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High-Power AM-CW Lunar Laser Ranging as a $\mu$Hz SGWB Detector
AM-CW lunar laser ranging achieves μHz SGWB sensitivity of 5.29×10^{-9} D_cov (80 μm range uncertainty) or 2.07×10^{-9} D_cov (50 μm) over 5 years, with discovery possible if covariance degradation stays below ~3.6-13.7.
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Exploring the statistical anisotropy of primordial curvature perturbations with pulsar timing arrays
A phenomenological dipole anisotropy in primordial perturbations induces dipolar and quadrupolar anisotropies in SIGW energy density spectra, producing frequency-dependent PTA overlap reduction functions that depend on pulsar sky distribution, but NANOGrav 15-year data yields no significant evidence
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The Heavy Tailed Non-Gaussianity of the Supermassive Black Hole Gravitational Wave Background
The gravitational wave background from supermassive black hole binaries has a universal heavy-tailed amplitude distribution with power-law index -4, causing divergent higher moments and dominance of the strongest signals by few loud sources.
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Stochastic gravitational-wave background search using data from five pulsar timing arrays
Combined five-PTA dataset yields posterior on SGWB power-law amplitude and index consistent with nonzero signal but below 5-sigma significance, with reconstructed angular correlations matching the Hellings-Downs prediction.
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Addressing prior dependence in hierarchical Bayesian modeling for PTA data analysis II: Noise and SGWB inference through parameter decorrelation
A reparametrized hierarchical Bayesian approach using normalizing flows and orthogonal projection of hyperparameters yields tighter noise constraints and partially breaks the red-noise-SGWB degeneracy in a minimal 3-pulsar PTA analysis.
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Finding Supermassive Black Hole Binary Mergers in Pulsar Timing Array Data
A complete SMBHB waveform model enables unified PTA searches for mergers and memory signals, with parameter recovery shown on simulated data for 10^8-10^10 solar mass systems.
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Running into tension: primordial black holes from ultra-slow-roll inflation, spectral running, and the Hubble tension
EDE models increase inferred α_s from CMB data, strengthening tension with USR PBH models that predict negative running.
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TOA_SP: A Multi-Strategy Framework for Single-Pulse Timing
toa_sp applies multiple single-pulse timing strategies to 688 pulses from RRAT J1913+1330, yielding 1.33 ms weighted RMS residuals (24% better than PSRCHIVE) while keeping all pulses.
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The NANOGrav 15 yr Data Set: Customized Chromatic Noise Models
Customized chromatic noise models for 67 pulsars detect non-dispersive delays in 21 cases, alter achromatic noise inferences in 19, and enable solar wind density estimates over 1.5 cycles.
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Cancellation of one-loop time dependence in superhorizon curvature perturbations from all scales
One-loop time dependence in superhorizon curvature perturbations cancels when all-scale contributions and boundary terms are included consistently via the δN formalism.
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Natural Supercooling and Reheating along Supersymmetric Flat Directions and Observable Gravitational Waves at the Einstein Telescope and the Cosmic Explorer
Radiative barriers in SUSY flat directions enable supercooled PTs yielding Ω_GW h² up to ~3e-10 for M_λ̃/v_X in 0.05-0.23, with the hidden sector also reproducing Ω_CDM h²=0.12 for m_q ~30-800 keV.
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Fixing the Renormalization of Inflationary Loops via Ward Identities
Ward identities from large gauge symmetry impose model-independent constraints on renormalizing inflationary loops and non-perturbatively govern the infrared power spectrum evolution.
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Purely Quadratic Non-Gaussianity from Tachyonic Instability: Primordial Black Holes and Scalar-Induced Gravitational Waves
Purely quadratic non-Gaussianity from tachyonic instability allows narrow curvature spectra to exponentially suppress primordial black hole overproduction via correlation coefficient ρ approaching -1 while retaining sizable scalar-induced gravitational waves.
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Hawking area law in quantum gravity
Exact Hawking area law from black hole mergers restricts quantum gravity to singular Ricci-flat or specific regular black holes in Stelle and nonlocal theories, derives the standard entropy-area law, and realizes Barrow fractal black holes.
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Dark QCD Origin of the NANOGrav Signal and Self-Interacting Dark Matter
A dark QCD model with a first-order phase transition at 5-6 MeV produces the NANOGrav SGWB amplitude while supplying self-interacting dark matter via a 40 GeV baryon and 20-50 MeV dilaton, linked by entropy dilution.
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LISA as a probe of pre-big-bang physics: a nested sampling analysis
Nested sampling analysis indicates LISA could constrain H1, m, sigma_i and beta in the pre-big-bang model to relative uncertainties of about 18 percent under favorable conditions when including foregrounds.
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Expectations for the first supermassive black-hole binary resolved by PTAs II: Milestones for binary characterization
Simulations of continuous-wave searches show that PTA data first constrain GW frequency and strain amplitude together, then sky location, with chirp mass and inclination following later for evolving sources, with precision depending on source frequency and sky position.
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Thermodynamical uncertainties for primordial black holes from cosmological phase transitions
A state-of-the-art thermodynamic analysis of supercooled phase transitions yields a universal lower bound β/H_* ≃ 5 and shows that viable PBH dark-matter parameter space in classically conformal gauge-Higgs theories is severely limited by percolation and QCD constraints.
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Relic gravitational waves from primordial gravitational collapses
Sound shell collisions from Hubble-scale primordial density perturbations generate a stochastic GW background whose peak frequency and amplitude scale with the Hubble horizon and shell abundance.
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Environmental effects vs. modified gravity in the LISA massive black hole binary population
Population-level hierarchical analysis shows environmental effects from circumbinary disks are unlikely to bias LISA tests of general relativity for massive black hole binaries in realistic scenarios.