Gravitational waves from metastable cosmic strings in the delayed scaling scenario (2501.18380v1)

Abstract: Recent observations by pulsar timing arrays (PTAs) such as NANOGrav, EPTA, PPTA, and CPTA suggest the presence of nanohertz stochastic gravitational wave background (GWB). While such signals could be explained by gravitational waves from a network of metastable cosmic strings (CSs), standard scenarios involving the Kibble-Zurek mechanism triggered by a thermal potential face significant challenges. Specifically, these scenarios predict a GWB spectrum inconsistent with the non-detection at higher frequencies by LIGO-Virgo-KAGRA (LVK) for CSs with relatively large string tension. It is also difficult to prevent the monopole forming phase transition just before the CS forming symmetry breaking, which spoils the CS network formation. In contrast, a delayed scaling scenario, where the CSs start to emit GWs at a later time due to the dilution during inflation, alleviates these issues. This scenario allows for a larger string tension while monopoles are sufficiently diluted such that the CS network safely forms. In this study, we clarify the spectrum of stochastic GWB from metastable CSs in the delayed scaling scenario, consistent with the PTA observations while satisfying the LVK constraints. Furthermore, we explore its potential signatures at frequencies accessible to other detectors such as LVK as well as LISA, Taiji, and TianQin or DECIGO and BBO. We also discuss the implications on inflation and underlying UV theories, such as the grand unified theories.

• The paper introduces a delayed scaling model that allows metastable cosmic strings to produce detectable nanohertz gravitational waves.
• The analysis uses numerical simulations to reveal a distinctive spectrum transitioning from f² scaling to a plateau and then decaying as f⁻¹/³.
• The study’s predictions support future tests by GW observatories like LISA and Taiji, providing new constraints on high-energy physics models.

Gravitational Waves from Metastable Cosmic Strings: An Analysis of the Delayed Scaling Scenario

The paper of cosmic strings and their implications for gravitational wave (GW) astronomy offers a significant avenue for understanding early Universe cosmology. In the paper "Gravitational waves from metastable cosmic strings in the delayed scaling scenario," the authors propose a model in which a stochastic gravitational wave background (GWB), potentially observed by pulsar timing arrays (PTAs), is sourced by metastable cosmic strings (CSs) under a delayed scaling scenario.

Context and Motivation

Recent observations from PTAs, such as NANOGrav and EPTA, suggest the presence of a nanohertz stochastic GWB. While conventional proposals attribute these observations to cosmic strings formed through the Kibble-Zurek mechanism, such scenarios face notable challenges. Specifically, they are inconsistent with the non-detection of GWs at higher frequencies by LIGO-Virgo-KAGRA (LVK) for strings with significant tension. Additionally, these conventional scenarios struggle to prevent monopole formation, which undermines the formation of a cosmic string network.

To circumvent these issues, the paper explores a delayed scaling scenario, wherein cosmic strings form during inflation and only enter a scaling regime later in cosmic history. This alternative approach allows for the use of cosmic strings with higher tension, as it curtails early GW emissions and monopole-related disruptions.

Framework and Methodology

The authors provide a detailed analysis of the GW spectrum resulting from cosmic strings in the delayed scaling regime. Initially, they review the scaling evolution of conventional cosmic string networks before extending these models to account for metastable strings subject to decay through monopole-antimonopole pair creation. The delayed scaling scenario is modelled by introducing a period wherein the cosmic string network experiences dilution during inflation and eventually transitions into a scaling regime.

The paper employs the framework of loop production in cosmic string networks to evaluate the contribution to the stochastic GWB. Leveraging numerical simulations and analytical expressions, they characterize the spectrum across a broad range of frequencies. A central aspect of their approach is the stipulation that the emission of GWs by these loops is delayed, thus addressing constraints imposed by LVK on high-frequency observations.

Key Findings and Implications

• Consistency with Observations: By considering a delay in the scaling regime, the paper finds that larger string tensions (up to $G\mu \sim 10^{-5}$) are permissible while still satisfying LVK's observational constraints. This is a significant finding given previous limitations on string tension in the context of PTA observations.
• Characteristic Spectrum: The paper identifies distinctive characteristics in the GW spectrum. At low frequencies, the spectrum initially scales as $f^{2}$ and then transitions to a plateau before decreasing as $f^{-1/3}$ at higher frequencies. This $f^{-1/3}$ decay is unique to cosmic string networks and differentiates it from other potential stochastic GW sources.
• Future Observations: The predicted spectral features at intermediate frequencies provide a viable target for upcoming GW observatories such as LISA and Taiji. These instruments could validate the specific signatures of delayed scaling cosmic strings and potentially help determine underlying physics parameters.

Outlook and Further Research

The implications of this research extend to identifying the conditions for the onset of scaling evolution and understanding the symmetry breaking patterns associated with grand unified theories (GUTs). With insights from the PTA and ongoing and future GW surveys, this work illustrates a refined pathway for probing the cosmic landscape and testing high-energy physics models beyond the Standard Model.

Building on this paper, future work should aim to integrate detailed inflationary dynamics with cosmic string evolution, further refining the constraints on the delayed scaling scenario. Additionally, it remains crucial to explore the interplay of cosmic defects with other inflationary relics to form a cohesive picture of the early Universe's thermal history. Such integrations may yield broader astronomical and theoretical insights, advancing our understanding of fundamental physics through cosmological observations.