arXiv : Has NANOGrav found first evidence for cosmic strings?

Blasi, Simone; Brdar, Vedran; Schmitz, Kai

doi:10.1103/PhysRevLett.126.041305

Article
Report number	arXiv:2009.06607 ; CERN-TH-2020-151
Title	Has NANOGrav found first evidence for cosmic strings?
Author(s)	Blasi, Simone (Heidelberg, Max Planck Inst.) ; Brdar, Vedran (Heidelberg, Max Planck Inst.) ; Schmitz, Kai (CERN)
Publication	2021-01-29
Imprint	2020-09-14
Number of pages	7
Note	7 pages, 3 figures. v2: revised treatment of the higher cosmic-string modes [see Eq. (6)], resulting in a few numerical but no qualitative changes. v3: matches version published in PRL
In:	Phys. Rev. Lett. 126 (2021) 041305
DOI	10.1103/PhysRevLett.126.041305 (publication)
Subject category	hep-th ; Particle Physics - Theory ; hep-ph ; Particle Physics - Phenomenology ; gr-qc ; General Relativity and Cosmology ; astro-ph.CO ; Astrophysics and Astronomy
Abstract	The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has recently reported strong evidence for a stochastic common-spectrum process affecting the pulsar timing residuals in its 12.5-year data set. We demonstrate that this process admits an interpretation in terms of a stochastic gravitational-wave background emitted by a cosmic-string network in the early Universe. We study stable Nambu-Goto strings in dependence of their tension $G\mu$ and loop size $\alpha$ and show that the entire viable parameter space will be probed by an array of future experiments.
Copyright/License	preprint: (License: arXiv nonexclusive-distrib 1.0) publication: © 2021-2025 authors (License: CC-BY-4.0)

$Scan over the cosmic-string tension $G\mu$ and loop size parameter $\alpha$ projected into the $\gamma$\,--\,$A$ plane, where $-\gamma$ represents the spectral index of the pulsar timing-residual cross-power spectrum and $A$ is the characteristic GW strain amplitude at $f = f_{\rm yr}$. The black contours denote the $1\,\sigma$ and $2\,\sigma$ posteriors in the NANOGrav analysis that allow to describe the observed stochastic common-spectrum process. Here, we use the contours based on the five lowest frequency bins in the NANOGrav data (see~\cite{Arzoumanian:2020vkk} for details). The gray vertical line indicates the theoretical prediction for a population of SMBHBs, $\gamma = 13/3$. The benchmark point at $\gamma = 5.02$ and $A = 9 \times 10^{-16}$ indicated by the black star corresponds to $\alpha = 0.01$ and $G\mu = 10^{-7}$ (see Figs.~\ref{fig:scan} and \ref{fig:spectrum}). We also note that, for $\gamma < 5$ ($\gamma > 5$), the GW spectrum is rising (decreasing) as a function of frequency. In this case, NANOGrav observes GWs at frequencies below (above) the radiation--matter--equality peak in the spectrum. At the same time, most of the points clustering around $\gamma \simeq 5$ belong to the flat plateau in the spectrum at frequencies above the peak.$ $Scan over the cosmic-string tension $G\mu$ and loop size $\alpha$ projected onto the $\gamma$\,--\,$A$ plane, where $-\gamma$ represents the spectral index of the pulsar timing-residual cross-power spectrum and $A$ is the characteristic GW strain amplitude at $f = f_{\rm yr}$. The black contours denote the $1\,\sigma$ and $2\,\sigma$ posteriors in the NANOGrav analysis that allow to describe the observed stochastic process. Here, we use the contours based on the five lowest frequency bins in the NANOGrav data (see~\cite{Arzoumanian:2020vkk} for details). The gray vertical line indicates the theoretical prediction for a population of SMBHBs, $\gamma = 13/3$. The parameter values of the benchmark points ($\smallblackstar$, $\smallblackdiamond$, $\smallblackcircle$) are listed in Tab.~\ref{tab:bp}. For $\gamma < 5$ ($\gamma > 5$), the GW spectrum is rising (decreasing) as a function of frequency. In this case, NANOGrav observes GWs at frequencies below (above) the radiation--matter-equality peak in the spectrum. At the same time, most of the points clustering around $\gamma \simeq 5$ belong to the flat plateau in the spectrum at frequencies above the peak.$ $Scan over the cosmic-string tension $G\mu$ and loop size parameter $\alpha$ projected into the $\gamma$\,--\,$A$ plane, where $-\gamma$ represents the spectral index of the pulsar timing-residual cross-power spectrum and $A$ is the characteristic GW strain amplitude at $f = f_{\rm yr}$. The black contours denote the $1\,\sigma$ and $2\,\sigma$ posteriors in the NANOGrav analysis that allow to describe the observed stochastic common-spectrum process. Here, we use the contours based on the five lowest frequency bins in the NANOGrav data (see~\cite{Arzoumanian:2020vkk} for details). The gray vertical line indicates the theoretical prediction for a population of SMBHBs, $\gamma = 13/3$. The benchmark point at $\gamma = 5.02$ and $A = 9 \times 10^{-16}$ indicated by the black star corresponds to $\alpha = 0.01$ and $G\mu = 10^{-7}$ (see Figs.~\ref{fig:scan} and \ref{fig:spectrum}). We also note that, for $\gamma < 5$ ($\gamma > 5$), the GW spectrum is rising (decreasing) as a function of frequency. In this case, NANOGrav observes GWs at frequencies below (above) the radiation--matter--equality peak in the spectrum. At the same time, most of the points clustering around $\gamma \simeq 5$ belong to the flat plateau in the spectrum at frequencies above the peak.$ Show more plots

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Record created 2020-09-16, last modified 2023-11-10

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