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 NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background
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We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15-year pulsar-timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. The correlations follow the Hellings-Downs pattern expected for a stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law-spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of $10^{14}$, and this same model is favored over an uncorrelated common power-law-spectrum model with Bayes factors of 200-1000, depending on spectral modeling choices. We have built a statistical background distribution for these latter Bayes factors using a method that removes inter-pulsar correlations from our data set, finding $p = 10^{-3}$ (approx. $3\sigma$) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of inter-pulsar correlations yields $p = 5 \times 10^{-5} - 1.9 \times 10^{-4}$ (approx. $3.5 - 4\sigma$). Assuming a fiducial $f^{-2/3}$ characteristic-strain spectrum, as appropriate for an ensemble of binary supermassive black-hole inspirals, the strain amplitude is $2.4^{+0.7}_{-0.6} \times 10^{-15}$ (median + 90% credible interval) at a reference frequency of 1/(1 yr). The inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from a population of supermassive black-hole binaries, although more exotic cosmological and astrophysical sources cannot be excluded. The observation of Hellings-Downs correlations points to the gravitational-wave origin of this signal.
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- abstract We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15-year pulsar-timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. The correlations follow the Hellings-Downs pattern expected for a stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law-spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of $10^{14}$, and this same model is favored over an uncorrelated common power-law-spectrum model with Baye
- background sient catalogs GWTC-1 through GWTC-3 [2-4]. These observations have transformed gravitational-wave astron- omy into a precision discipline, enabling detailed tests of strong-field gravity and compact-object population stud- ies. In parallel, pulsar timing arrays, including NANOGrav, have recently reported evidence for a stochastic nanohertz gravitational-wave background [5], opening a complementary low-frequency window onto su- permassive black hole binaries and possible cosmological sources. Ev
- background (PTAs) has provided strong evidence for a stochastic GW background [5-11]. With the fourth LIGO-Virgo- KAGRA observing run in progress and the continued PTA campaign [5, 7, 12, 13], more and more observa- tional data and, thus, scientific insight can be expected. While Earth-based detectors utilize interferometry to detect GWs, PTAs, including NANOGrav [7], the Euro- pean Pulsar Timing Array [8, 14], the Parkes Pulsar Tim- ing Array [9, 10, 12], the Chinese Pulsar Timing Array [11], and the Meer
- background ch § dschmitt@itp.uni-frankfurt.de [1] B. P. Abbott et al. (LIGO Scientific, Virgo), Phys. Rev. Lett. 116, 061102 (2016), arXiv:1602.03837 [gr-qc]. [2] G. Agazie et al. (NANOGrav), Astrophys. J. Lett. 951, L10 (2023), arXiv:2306.16218 [astro-ph.HE]. [3] G. Agazie et al. (NANOGrav), Astrophys. J. Lett. 951, L8 (2023), arXiv:2306.16213 [astro-ph.HE]. [4] G. Agazie et al. (NANOGrav), Astrophys. J. Lett. 951, L9 (2023), arXiv:2306.16217 [astro-ph.HE]. [5] G. Agazie et al. (NANOGrav), (2023), arXiv:2
- background Ever since the first detection of a gravitational wave event by LIGO [183], several large observatories are projected, e.g., LISA [184], Taiji [185], TianQin [185] and the Einstein Telescope [186]. In addi- tion to the detection of singular events, the gravitational wave background has also been recently detected by Pulsar Timing Array collaborations (e.g., NANOGrav, [187], CPTA [188], EPTA, [189]). Regarding FRBs, current radio observatories regularly detect these events, compiling ever-growing
- background be the detection of a stochastic primordial gravitational wave background, and the future gravitational wave experiments will operate in frequency bands that will probe the inflationary gravitational waves [6-14]. NANOGrav already verified a stochastic gravitational wave background back in 2023 [15], but inflation itself cannot generate such a signal [16, 17]. Thus the next ten years will be extremely important for modern theoretical physics. Recently, the Atacama Cosmology Telescope (ACT) [18,
- background turbation equations in certain higher-dimensional scenar - ios due to the influence of the bulk on the brane [55, 56]. Secondly, massive gravitons, either in explicit massive gravity theories or as effective degrees of freedom, have been argued to contribute to very long-wavelength grav- itational signals [57], which are currently being probed by Pulsar Timing Array experiments [58, 59]. Thirdly, massive fields may support arbitrarily long-lived QNMs for particular values of the field mass, leading
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