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Shallow relic gravitational wave spectrum with acoustic peak
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).ORCID iD: 0000-0002-2549-6861
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Carnegie Mellon University, USA; Ilia State University, Georgia.ORCID iD: 0000-0002-7304-021X
Number of Authors: 42023 (English)In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 12, article id 042Article in journal (Refereed) Published
Abstract [en]

We study the gravitational wave (GW) spectrum produced by acoustic waves in the early universe, such as would be produced by a first order phase transition, focusing on the low-frequency side of the peak. We confirm with numerical simulations the Sound Shell model prediction of a steep rise with wave number k of k9  to a peak whose magnitude grows at a rate (H/kp)H, where H is the Hubble rate and kp the peak wave number, set by the peak wave number of the fluid velocity power spectrum. We also show that hitherto neglected terms give a shallower part with amplitude (H/kp)2  in the range H ≲ k ≲ kp, which in the limit of small H/k rises as k. This linear rise has been seen in other modelling and also in direct numerical simulations. The relative amplitude between the linearly rising part and the peak therefore depends on the peak wave number of the velocity spectrum and the lifetime of the source, which in an expanding background is bounded above by the Hubble time H-1. For slow phase transitions, which have the lowest peak wave number and the loudest signals, the acoustic GW peak appears as a localized enhancement of the spectrum, with a rise to the peak less steep than k9. The shape of the peak, absent in vortical turbulence, may help to lift degeneracies in phase transition parameter estimation at future GW observatories.

Place, publisher, year, edition, pages
2023. no 12, article id 042
Keywords [en]
cosmological phase transitions, physics of the early universe, primordial gravi-tational waves (theory)
National Category
Fluid Mechanics and Acoustics Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:su:diva-226986DOI: 10.1088/1475-7516/2023/12/042ISI: 001156562700003Scopus ID: 2-s2.0-85181247229OAI: oai:DiVA.org:su-226986DiVA, id: diva2:1842415
Available from: 2024-03-05 Created: 2024-03-05 Last updated: 2024-03-05Bibliographically approved

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Sharma, RamkishorBrandenburg, Axel

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Sharma, RamkishorBrandenburg, AxelHindmarsh, Mark
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Nordic Institute for Theoretical Physics (Nordita)Department of AstronomyThe Oskar Klein Centre for Cosmo Particle Physics (OKC)
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Journal of Cosmology and Astroparticle Physics
Fluid Mechanics and AcousticsFusion, Plasma and Space Physics

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