Search arXiv⌕ Search
Home/Archive/arXiv

arXiv · 2011.01605

Nanohertz gravitational waves from NEC violation in the early universe

Hiroaki W. H. Tahara·Tsutomu Kobayashi
View original source ↗Read source document ↗Export metadata ↓

Abstract

We study nanohertz gravitational waves relevant to pulsar timing array experiments from quantum fluctuations in the early universe with null energy condition (NEC) violation. The NEC violation admits accelerated expansion with the scale factor $a\propto (-t)^{-p}$ ($p>0$), which gives the tensor spectral index $n_t=2/(p+1)>0$. To evade the constraint from Big Bang nucleosynthesis (BBN), we connect the NEC-violating phase to a subsequent short slow-roll inflationary phase which ends with standard reheating, and thereby reduce the high frequency part of the spectrum. An explicit model is constructed within the cubic Horndeski theory which allows for stable violation of the NEC. We present numerical examples of the background evolution having the different maximal Hubble parameters (which determine the peak amplitude of gravitational waves), the different inflationary Hubble parameters (which determine the amplitudes of high frequency gravitational waves), and different durations of the inflationary phase (which essentially determine the peak frequency of the spectrum). We display the spectra with $n_t=0.8$, $0.9$, and $0.95$ for $f\lesssim 1/{\rm yr}$, which are consistent with the recent NANOGrav result. We also check that they do not contradict the BBN constraint. We discuss how the nearly scale-invariant spectrum of curvature perturbations is produced in the NEC-violating phase.

Explore related subjects

gr-qcastro-ph.CO

Keep this discovery

BibTeXRIS

Hiroaki W. H. Tahara, Tsutomu Kobayashi. 2020-11-03. Nanohertz gravitational waves from NEC violation in the early universe. https://doi.org/10.1103/physrevd.102.123533

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Electrovacuum Black Hole Uniqueness

We prove the black hole uniqueness conjecture in the axially symmetric, stationary, electrovacuum setting, subject to the refined asymptotic analysis of the associated singular harmonic maps, which includes an analyticity hypothesis at the axes. More precisely, it is shown that any asymptotically flat solution of the Einstein--Maxwell equations in this class, with more than one black hole horizon component is either: Majumdar--Papapetrou, up to a duality rotation, in which case all logarithmic angle defects vanish, or every finite axis rod logarithmic angle defect is strictly negative and hence every interaction force is strictly attractive. The proof extends the singular harmonic map method used for vacuum Kerr uniqueness in [18].

gr-qc↗
arXiv2026

Constraining Modified Mass-to-Horizon Cosmology Through Primordial Inflationary Observables

We investigate slow-roll inflation in a modified cosmological framework inspired by a generalized mass-to-horizon relation (MHR), $M=\gamma {c^2 L^n}/{G}$, where $n$ is a real parameter and $\gamma$ a dimensional constant. Using Padmanabhan's emergence paradigm, we derive the modified Friedmann equations for a flat FRW universe and analyze the dynamics of a canonical scalar field (inflaton) under the slow-roll approximation. We study the resulting inflationary phenomenology for power-law and Starobinsky potentials. For power-law potentials, the MHR modification fails to reconcile these models with current CMB constraints on $r$ and $n_s$. In contrast, Starobinsky inflation exhibits significant sensitivity to deviations from $n=1$. A perturbative analysis ($n=1+\Delta$) yields corrections to inflationary observables. We observe that the scalar power-spectrum normalization, under a fixed-Starobinsky prescription, imposes the stringent constraint $0.960 \lesssim n \lesssim 1.040$ for $N=60$ efolds. This is considerably tighter than spectral-index bounds. Our results establish inflation, particularly Starobinsky-like models, as a sensitive probe of generalized horizon thermodynamics and departures from standard MHR scaling.

gr-qc↗
arXiv2026

Improving the Sensitivity of Gravitational Wave Detection with Weighted Conformal Prediction

In the last decade, kilometre-scale interferometric gravitational-wave detectors have observed hundreds of compact binary mergers, the majority of which are binary black holes. However, the data are noise-dominated, and multiple independent search algorithms (pipelines) are used to enhance sensitivity and improve robustness. Rather than the standard approach of selecting the most significant pipeline output, we combine the outputs from all pipelines using a conformal prediction-based framework to provide statistically rigorous confidence estimates for candidate events. While combining pipelines improves sensitivity and ranking robustness, it requires a principled statistical framework that remains valid as data properties evolve across observing runs. A key challenge is distribution shifts between simulated datasets used for training and calibration and the real, unlabelled, observations used for testing, which can invalidate coverage guarantees and bias confidence estimates. In this work, we address this challenge by incorporating likelihood-ratio reweighting into our conformal prediction framework to account for covariate shift. Using mock datasets containing simulated signals, we demonstrate that weighted conformal prediction restores well-calibrated coverage under covariate shift and increases the confidence of events near the detection threshold, recovering true signals that would otherwise be missed.

gr-qc↗