Has NANOGrav found first evidence for cosmic strings? (2009.06607v3)

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.

• The paper investigates NANOGrav’s 12.5-year dataset to assess cosmic strings as the origin of the stochastic gravitational-wave background.
• It employs the Nambu-Goto framework to map key parameters such as string tension (Gμ) and loop size (α), aligning theoretical predictions with data.
• Findings suggest an extensive viable cosmic string parameter space, indicating potential detection in future GW experiments like LISA and Cosmic Explorer.

Analysis of Cosmic Strings as the Source of NANOGrav's Observational Data

The paper by Blasi, Brdar, and Schmitz examines the intriguing possibility that the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) may have uncovered evidence for cosmic strings, reinforced by their recent data indicating a stochastic gravitational-wave background (SGWB). This analysis is grounded on NANOGrav's 12.5-year dataset evidencing a stochastic process that potentially aligns with SGWB from cosmic strings. The authors leverage the Nambu-Goto framework to model cosmic strings and meticulously trod through the parameter space defined by string tension $G\mu$ and loop size $\alpha$.

Summary of the Analytical Approach

The researchers articulate that cosmic strings are a consequence of symmetry-breaking phase transitions in the early universe, resulting in a network of vortex-like objects. These cosmic strings, particularly Nambu-Goto strings considered in the paper, emit gravitational waves as they lose energy, primarily through loop formation and subsequent radiation. The parameter space concerning string tension $G\mu$ and loop size $\alpha$ is critical as it determines the viability of cosmic strings as an explanation for the observations. The analysis hinges on comparing the predicted GW energy density $\Omega_{\rm gw}(f)$ with the observational data fitted by a power-law characterized by amplitude $A$ and spectral index $\gamma$.

Findings and Parameter Space

The NANOGrav data shows strong evidence for a stochastic process with characteristics compatible with a cosmic string origin, yet distinct from other potential sources like supermassive black hole binaries, which are tightly constrained by known merger rates and cosmic background observations. The findings suggest significant coverage of the viable cosmic string parameter space, with potential overlap within the $1\sigma$ confidence interval of NANOGrav's analysis.

Noticeably, the permissible region for $G\mu$, even under conservative estimates, is extensive given the marked relaxation in bounds from previous analyses of the Cosmic Microwave Background (CMB). This parameter range infers the feasibility of detecting similar signals across environments beyond PTAs, including space-based detectors like LISA and perhaps ground-based facilities like Cosmic Explorer (CE).

Implications and Future Directions

Should the NANOGrav observations be indeed attributed to cosmic strings, the implications extend across cosmology and high-energy physics, providing insights into the universe's formative symmetries potentially at grand unified theory scales. Future GW experiments, both ongoing and conceptual, are well-positioned to further probe the cosmic string GW footprint across varied frequency regimes, thereby reinforcing or refuting these provisional findings.

Furthermore, the interplay between the cosmic strings' GW emissions and their broader astrophysical and cosmological context opens avenues for multidisciplinary inquiry, encompassing particle physics phenomenology and early universe cosmology. It may also precipitate refinements in modeling the evolution and manifestation of cosmic strings themselves, potentially leveraging both observational and theoretical advancements.

In conclusion, the investigation underlines that with enhanced observational data, multifrequency GW astronomy promises comprehensive explorations through and beyond the presently understood parameter spaces. This paper is just a seminal step towards unpacking the potential linkage between newfound GW signals and cosmic topologies indicative of primordial phenomena, laying the groundwork for future exploration and theoretical framing.