The first search for scalar-induced gravitational waves via pulsar parameter drifts yields f_PBH < 10^{-10} (95% CL) for PBH masses 0.3 to 4e4 solar masses, strongly disfavoring a primordial black hole origin for LVK binary black holes.
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Scalar induced gravitational waves review
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abstract
We provide a review on the state-of-the-art of gravitational waves induced by primordial fluctuations, so-called induced gravitational waves. We present the intuitive physics behind induced gravitational waves and we revisit and unify the general analytical formulation. We then present general formulas in a compact form, ready to be applied. This review places emphasis on the open possibility that the primordial universe experienced a different expansion history than the often assumed radiation dominated cosmology. We hope that anyone interested in the topic will become aware of current advances in the cosmology of induced gravitational waves, as well as becoming familiar with the calculations behind.
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representative citing papers
Accounting for the minimal mass spread of primordial black holes from gravitational collapse suppresses the Poltergeist GW background to the level of generic scalar-induced signals and reopens ultra-light PBH parameter space.
For infinitely differentiable effective potentials describing the post-inflation transition, the regularized power spectrum of primary gravitational waves exhibits exponential suppression at small scales.
Dissipation of small-scale primordial perturbations after neutrino decoupling cools relic neutrinos and reduces their abundance, enabling PTOLEMY to constrain the primordial curvature power spectrum to O(0.1) on scales k ≲ 3×10^5 Mpc^{-1}.
The conventional truncation in stochastic inflation is inconsistent because quadratic-noise contributions are the same perturbative order as the deterministic non-Markovian corrections.
A quenched-disorder approach with Schwinger-Keldysh path integrals produces an averaged density matrix for gravitational waves that separates phase-suppressing exponential terms from oscillatory corrections to coherent propagation.
The work shows that free-streaming dark radiation isocurvature produces a qualitatively different gravitational wave spectrum than cold dark matter isocurvature and derives constraints on isocurvature power spectra around 10^6 Mpc^{-1} from NANOGrav data.
Lattice simulations directly calculate SIGW spectra with non-Gaussianity to all orders and show that modest non-Gaussianity alters ultraviolet spectral behavior.
In pure axion inflation, detectable gravitational wave signals arise only in parameter regions with strong backreaction that violate the upper bound on ΔN_eff.
Incorporating the general-relativity mass tail df_PBH/d ln M ∝ M^3.78 smooths PBH evaporation, suppresses the scalar-induced GW signal by orders of magnitude, and reopens the ultra-light PBH window for the hot Big Bang.
Domain wall annihilation imprints a two-peaked spectrum on induced gravitational waves via an early matter-dominated phase and entropy dilution.
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
Derives stochastic equations from Schwinger-Keldysh formalism that include quantum diffusion and classical metric perturbations for non-perturbative ultra-slow-roll inflation, validated on Starobinsky and critical Higgs models.
Multi-phase inflation with chiral vector interactions generates amplified primordial magnetic fields that induce a detectable circularly polarized gravitational-wave background.
Induced gravitational waves during inflation obey a universal tensor spectral index formula that yields near scale-invariance for slow-roll expansion regardless of the source field's original spectrum.
Extends diagrammatic approach for scalar-induced gravitational waves to arbitrary-order local PNG, deriving semi-analytic spectra for energy density, anisotropies, bispectrum and trispectrum up to quartic terms.
Covariant analysis of curvature perturbations from first-order phase transitions reveals gauge-dependent overestimation of primordial black holes and gravitational waves in prior non-covariant calculations, leading to strong suppression of both signals.
Hybrid inflation produces enhanced curvature perturbations with a broad power spectrum peak featuring k^3 infrared growth and positive f_NL fixed by tachyonic waterfall geometry, potentially accounting for PBH dark matter and LISA-detectable SGWB.
Classical part of 1-loop tensor power spectrum in de Sitter is IR divergent but cancels with vacuum part, enabling non-perturbative renormalization to extract unaffected physical information.
A transient parity-violating phase during inflation generates a robust blue-tilted (n_T ≃ 2) primordial gravitational wave spectrum at small scales with nearly maximal helicity coherence and linear polarization, offering a cosmological template for PTA data distinct from astrophysical sources.
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.
Clustered primordial black holes may constitute all dark matter and produce a flat stochastic gravitational wave background detectable by the Einstein Telescope.
A spectator scalar field with strong portal coupling to the inflaton sources a stochastic gravitational wave background reaching Ω_GW h² ∼ 10^{-11} at frequencies 10^7-10^8 Hz for benchmark parameters σ/λ ≃ 10^4 and T_reh = 2×10^{14} GeV.
Adiabatic regularization combined with smoothed transitions suppresses the high-frequency oscillations in the power spectrum of primary gravitational waves about a zero mean.
citing papers explorer
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Constraints on the Primordial Black Hole Abundance using Pulsar Parameter Drifts
The first search for scalar-induced gravitational waves via pulsar parameter drifts yields f_PBH < 10^{-10} (95% CL) for PBH masses 0.3 to 4e4 solar masses, strongly disfavoring a primordial black hole origin for LVK binary black holes.
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Primary gravitational waves at high frequencies II: Emergence of the exponential cut-off in the power spectrum
For infinitely differentiable effective potentials describing the post-inflation transition, the regularized power spectrum of primary gravitational waves exhibits exponential suppression at small scales.
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A consistent formulation of stochastic inflation I: Non-Markovian effects and issues beyond linear perturbations
The conventional truncation in stochastic inflation is inconsistent because quadratic-noise contributions are the same perturbative order as the deterministic non-Markovian corrections.
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Gravitational-wave lensing beyond rays: a disordered-system approach
A quenched-disorder approach with Schwinger-Keldysh path integrals produces an averaged density matrix for gravitational waves that separates phase-suppressing exponential terms from oscillatory corrections to coherent propagation.
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Isocurvature Induced Gravitational Waves at Pulsar Timing Arrays
The work shows that free-streaming dark radiation isocurvature produces a qualitatively different gravitational wave spectrum than cold dark matter isocurvature and derives constraints on isocurvature power spectra around 10^6 Mpc^{-1} from NANOGrav data.
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Scalar-induced gravitational waves with non-Gaussianity up to all orders
Lattice simulations directly calculate SIGW spectra with non-Gaussianity to all orders and show that modest non-Gaussianity alters ultraviolet spectral behavior.
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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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Nonperturbative stochastic inflation in perturbative dynamical background
Derives stochastic equations from Schwinger-Keldysh formalism that include quantum diffusion and classical metric perturbations for non-perturbative ultra-slow-roll inflation, validated on Starobinsky and critical Higgs models.
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Chiral gravitational waves from multi-phase magnetogenesis
Multi-phase inflation with chiral vector interactions generates amplified primordial magnetic fields that induce a detectable circularly polarized gravitational-wave background.
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A universal scaling law for gravitational waves induced during inflation
Induced gravitational waves during inflation obey a universal tensor spectral index formula that yields near scale-invariance for slow-roll expansion regardless of the source field's original spectrum.
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Isotropy, anisotropies and non-Gaussianity in the scalar-induced gravitational-wave background: diagrammatic approach for primordial non-Gaussianity up to arbitrary order
Extends diagrammatic approach for scalar-induced gravitational waves to arbitrary-order local PNG, deriving semi-analytic spectra for energy density, anisotropies, bispectrum and trispectrum up to quartic terms.
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Superhorizon curvature perturbations in hybrid inflation revisited
Hybrid inflation produces enhanced curvature perturbations with a broad power spectrum peak featuring k^3 infrared growth and positive f_NL fixed by tachyonic waterfall geometry, potentially accounting for PBH dark matter and LISA-detectable SGWB.
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Transient Parity Violation during Inflation: Implications for PTA Gravitational Waves
A transient parity-violating phase during inflation generates a robust blue-tilted (n_T ≃ 2) primordial gravitational wave spectrum at small scales with nearly maximal helicity coherence and linear polarization, offering a cosmological template for PTA data distinct from astrophysical sources.
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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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Hunting Dark Matter with the Einstein Telescope
Clustered primordial black holes may constitute all dark matter and produce a flat stochastic gravitational wave background detectable by the Einstein Telescope.
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Primary gravitational waves at high frequencies I: Origin of suppression in the power spectrum
Adiabatic regularization combined with smoothed transitions suppresses the high-frequency oscillations in the power spectrum of primary gravitational waves about a zero mean.
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Stochastic Gravitational Waves from Modulated Reheating
A spectator scalar in modulated reheating with large Higgs-like couplings generates detectable scalar-induced stochastic gravitational waves for BBO and DECIGO, but only outside perturbative low-energy extrapolations.
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Exploring Gravitational Wave Signatures Due to Primordial Non-gaussianity and Large Scale Structure Using SKAO
Explores SKAO detection of scalar-induced GW backgrounds as probes of primordial non-Gaussianity and parity violation, with LSS cross-correlation to improve SNR.
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Gravitational Waves from Post-Inflationary Magnetism: Direct and Scalar-Induced Contributions
In a post-inflationary magnetogenesis scenario with time-dependent gauge couplings, magnetic anisotropic stress dominates peak GW amplitude while scalar-induced terms matter on larger scales, both showing f^3 infrared scaling for blue spectra and potentially reaching PTA frequencies.
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Precision Analysis for $\boldsymbol{H_0}$ Using Upcoming Multi-band Gravitational Wave Observations
Multi-band GW observations of PBHs can reduce H0 uncertainty to ≲2 km/s/Mpc (conservative) or O(0.1) km/s/Mpc (optimistic) via Fisher forecasts on M_PBH and f_PBH.
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Probing non-Gaussianity during reheating with SIGW in the LISA band
Non-standard reheating imprints detectable features on SIGW spectra via non-Gaussianity, with dynamics that can suppress or boost the signal amplitude for LISA.
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Probing the scalar-induced gravitational waves with the Five-hundred-meter Aperture Spherical radio Telescope and the Square Kilometer Array
Forecasts show that adding FAST/SKA upper or lower limits on scalar-induced GWs to CMB+BAO data shifts the scalar spectral index ns by several sigma in LambdaCDM+r and extended models, offering a potential detection indicator.