Metastable cosmic strings

Abstract

Many symmetry breaking patterns in grand unified theories (GUTs) give rise to cosmic strings that eventually decay when pairs of GUT monopoles spontaneously nucleate along the string cores. These strings are known as metastable cosmic strings and have intriguing implications for particle physics and cosmology. In this article, we discuss the current status of metastable cosmic strings, with a focus on possible GUT embeddings and connections to inflation, neutrinos, and gravitational waves (GWs). The GW signal emitted by a network of metastable cosmic strings in the early universe differs, in particular, from the signal emitted by topologically stable strings by a suppression at low frequencies. Therefore, if the underlying symmetry breaking scale is close to the GUT scale, the resulting GW spectrum can be accessible at current ground-based interferometers as well as at future space-based interferometers, such as LISA, and at the same time account for the signal in the most recent pulsar timing data sets. Metastable cosmic strings thus nourish the hope that future GW observations might shed light on fundamental physics close to the GUT scale.

Inflation, superheavy metastable strings and gravitational waves in non-supersymmetric flipped SU(5)

“…The gravitational wave spectra exhibit an f 2 power law at low frequencies, followed by an almost scaleinvariant plateau, and an f −1/3 tail in the high-frequency region depending on tF , and agree with refs [10,44]. that employ a self-consistent method for computing these spectra.…”

“…We provide an inflation model with the gauge symmetry being SU(5) × U(1) X (flipped SU(5)), one of the maximal subgroups of the SO(10) gauge group. In order to realize metastable strings in flipped SU(5), we consider the following symmetry breaking chain (for a recent discussion of topological structures in SO (10) and its various subgroups see ref. [18], whose notation we follow here):…”

“…where Γ ≃ 50 is a numerical factor [42,43]. The loop distribution at time t > t s is given by [10,44] n r (l, t) = 0.18 Θ(0.18t − l − ΓGµ(t − t s ))…”

“…Walls bounded by superheavy strings provide another scenario [6,9] in good agreement with the NANOGrav data. An alternative scenario is based on metastable strings with the dimensionless string tension parameter Gµ ≈ 10 −7 [1,[10][11][12]. 1 The latter parameter is carefully chosen to explain the NANOGrav observations, without running into (serious) conflict with the LIGO-VIRGO results.…”

Enhancement of gravitational waves at Q-ball decay including non-linear density perturbations

“…Recently, the pulsar timing array (PTA) experiments, NANOGrav [1], EPTA [2] with InPTA data, PPTA [3], and CPTA [4], have reported the signal of a stochastic gravitational wave background in the nHz range. While it may indicate inspiraling supermassive black hole binaries [5][6][7][8][9][10][11][12], 1 various possibilities for its origin have been discussed such as cosmic strings [17][18][19][20][21][22][23][24][25][26][27][28], domain walls [21,[29][30][31][32][33][34][35][36][37][38][39][40][41], a first-order phase transition [42][43][44][45][46][47][48][49][50][51]…”