Did We Find First Evidence of Cosmic Strings? (Cosmology / Quantum)

Summary:

Recently, NANOGrav Collaboration presented its results of a search for an isotropic SGWB based on its 12.5-year data set. Remarkably enough, this study yields strong evidence for the presence of a stochastic process across the 45 pulsars included in the analysis. They claimed they found 1st evidence of cosmic strings.

○ Now, Simone and colleagues shown their results in favor of NANOGrav Collaboration, they demonstrated that stochastic gravitational-wave background (SGWB) could be emitted by a cosmic-string network in the early Universe.

○ They showed that NANOGrav signal points to symmetry breaking scales in the range v ∼ 10¹⁴⋯10^16 GeV.

They also argued that the current gravitational wave (GW) experiments could not able to detect the signal at the sufficient signal to noise ratio, but entire viable parameter region will be probed in future GW experiments.

According to Simone, if confirmed in the future, the NANOGrav signal will mark the beginning of a new era in GW astronomy and revolutionize our understanding of the cosmos.


Many models of new physics beyond the Standard Model predict cosmological phase transitions in the early Universe that lead to the spontaneous breaking of an Abelian symmetry. An exciting phenomenological consequence of such phase transitions is the generation of a network of cosmic strings, vortex-like topological defects that restore the broken symmetry at their core. Cosmic strings can form closed loops that lose energy and shrink via the emission of gravitational waves (GWs). Indeed, numerical simulations of cosmic strings based on the Nambu–Goto action showed that this is the dominant energy loss mechanism of cosmic-string loops, if the underlying broken symmetry corresponds to a local gauge symmetry. The primordial GW signal from a cosmic-string network, which encodes crucial information on ultraviolet physics far beyond the reach of terrestrial experiments, is therefore a major target of ongoing and upcoming searches for a stochastic GW background (SGWB).

FIG. 1: Scan over the cosmic-string tension Gµ and loop size α projected onto the γ – A plane, where −γ represents the spectral index of the pulsar timing-residual cross-power spectrum and A is the characteristic GW strain amplitude at f = fyr. The black contours denote the 1 σ and 2 σ posteriors in the NANOGrav analysis that allow to describe the observed stochastic process.. Here, they use the contours based on the five lowest frequency bins in the NANOGrav data. The gray vertical line indicates the theoretical prediction for a population of SMBHBs, γ = 13/3. The parameter values of the benchmark points (⋆, ⬩, •) are listed in Tab. I. For γ < 5 (γ > 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 γ ≃ 5 belong to the flat plateau in the spectrum at frequencies above the peak. © Simone et al.

A cosmic-string-induced SGWB is expected to stretch across a vast range of GW frequencies, making it an ideal signal for multifrequency GW astronomy. At high frequencies in the milli- to kilohertz range, the signal can be searched for in space- and ground-based GW interferometers, while at low frequencies in the nanohertz range, pulsar timing array (PTA) experiments are sensitive to the signal. Now, Simon Blassi and colleagues investigated the latter possibility, a cosmic-string-induced GW signal at Nanohertz frequencies, in light of the recent results reported by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA experiment.

FIG. 2: NANOGrav 1 σ and 2 σ posterior contours projected onto the α – Gµ plane. They do not consider α values larger than α = 0.1, which is the maximal value found in simulations. At α < 10^−8 , they quickly cease to find viable points because α > lk/t in Eq. (5) given in paper is no longer satisfied when evaluated in the first NANOGrav frequency bin, i.e., f ≃ 2 × 10−9 Hz. The ⋆ benchmark point saturates the CMB limit on Gµ (see Tab. I). The diagonal black line labeled α =Γ Gµ distinguishes between the small-loop and the large-loop regime © Simone et al.

Recently in Dec 2020, the NANOGrav Collaboration presented its results of a search for an isotropic SGWB based on its 12.5-year data set. Remarkably enough, this study yields strong evidence for the presence of a stochastic process across the 45 pulsars included in the analysis. The interpretation of the observed signal in terms of a common-spectrum process is strongly preferred over independent red-noise processes (a Bayesian model comparison yields a log10 Bayes factor larger than 4); however, a conclusive statement on the physical origin of the signal is currently not yet feasible.

FIG. 3: GW spectra for the benchmark points (⋆, ⬩, •) in Tab. I alongside the power-law-integrated sensitivity curves of various present (solid boundaries) and future (dashed boundaries) GW experiments. The EPTA, PPTA, and NANOGrav curves at low frequencies represent the status of PTA constraints on the GW spectrum prior to the new NANOGrav result! Their benchmark spectra therefore illustrate that the NANOGrav signal exceeds previous PTA constraints © Simone et al.

Now, Simone Blasi and colleagues investigated the results of the NANOGrav analysis based on the assumption that this stochastic process corresponds to a primordial SGWB emitted by cosmic strings in the early Universe.

They studied stable Nambu–Goto strings in dependence of their tension Gµ and loop size α and identified the viable cosmic-string parameter space and argued that the current GW experiments could not able to detect the signal at the sufficient signal to noise ratio, but entire viable parameter region will be probed in future GW experiments.

©Simone et al.

They noted that the height of the flat plateau in the GW spectrum roughly scales as follows in dependence of α and Gµ,

where ¯α  max {α, 9/4 Γ Gµ}. © Simone et al.

According to this relation, all viable points in Fig. 2 predict a plateau that is at most suppressed by a factor of O(10−³) compared to their benchmark spectrum. As evident from Fig. 3, all viable points will therefore be probed in future experiments.


PTA experiments will be able to improve our understanding on the current NANOGrav analysis and confirm (or refute) the presence of the signal at increasingly higher significance.

— said Simone Blasi, lead author of the study.

They also commented on the relation between the cosmic-string tension Gµ and the underlying energy scale v of spontaneous U(1) symmetry breaking,

implying that the NANOGrav signal points to symmetry breaking scales in the range v ∼ 10¹⁴⋯10^16 GeV.


This is an exciting result that may indicate a connection between the observed signal and spontaneous symmetry breaking close to the energy scale of grand unification.

— said Kai Schmitz, co-author of the study

If confirmed in the future, the NANOGrav signal will mark the beginning of a new era in GW astronomy and revolutionize our understanding of the cosmos.


References: (1) Zaven Arzoumanian, Paul T. Baker et al., “The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background”, The Astrophysical Journal Letters, Volume 905, Number 2, Dec 2020. https://iopscience.iop.org/article/10.3847/2041-8213/abd401/meta (2) Simone Blasi, Vedran Brdar, and Kai Schmitz, “Has NANOGrav Found First Evidence for Cosmic Strings?”, Phys. Rev. Lett. 126, 041305 – Published 28 January 2021. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.041305


Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us.

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