[Paper Review] Cosmic Superstrings Revisited in Light of NANOGrav 15-Year Data

John Ellis, Marek Lewicki+2 more|arXiv (Cornell University)|Jun 29, 2023

Cosmology and Gravitation Theories8 citations

TL;DR

The paper reanalyzes cosmic string and cosmic superstring models against NANOGrav 15-year data, finding good fits for Gμ ~ 10^-12–10^-11 with intercommutation probability p ~ 10^-3–10^-1, and explores robustness under nonstandard early-Universe expansion scenarios.

ABSTRACT

We analyze cosmic superstring models in light of NANOGrav 15-year pulsar timing data. A good fit is found for a string tension $G μ\sim 10^{-12} - 10^{-11}$ and a string intercommutation probability $p \sim 10^{-3} - 10^{-1}$. Extrapolation to higher frequencies assuming standard Big Bang cosmology is compatible at the 68\% CL with the current LIGO/Virgo/KAGRA upper limit on a stochastic gravitational wave background in the 10 to 100 Hz range. The superstring interpretation of the NANOGrav data would be robustly testable by future experiments even in modified cosmological scenarios.

Motivation & Objective

• Assess whether cosmic strings or cosmic superstrings can explain the NANOGrav 15-year gravitational wave data.
• Extend previous models to accommodate updated spectral shape and higher-frequency constraints.
• Investigate how nonstandard early-Universe expansion (early matter domination, inflation) impacts the SGWB predictions and testability.
• Compare cosmic string predictions with LVK O3 limits and future detectors across frequencies.

Proposed method

• Model the GW spectrum from a string network using the velocity-dependent one-scale (VOS) framework with correlation length L and velocity v̄.
• Compute GW emission from large loops with initial size l_i = α_L L(t_i) and include higher string modes via Omega_GW^(k)(f) with q = 4/3.
• Sum contributions over loops and modes to obtain Omega_GW(f) with a practical truncation (N=10^3 for modes, extending to k up to 10^12).
• Incorporate a 0.1 energy transfer factor into loops and a f_r = √2 reduction for loop peculiar velocities.
• Explore nonstandard cosmologies by modifying the Hubble rate to model early matter domination ending before BBN and scenarios with dilution by inflation/reheating.

Experimental results

Research questions

• RQ1 Can cosmic (super)strings with variable intercommutation probability p reproduce the NANOGrav 15-year SGWB signal without violating higher-frequency constraints?
• RQ2 How do nonstandard early-Universe expansion histories (early matter domination, inflationary dilution) modify the SGWB spectrum and the detectability by LVK, ET, LISA, and other future experiments?
• RQ3 What ranges of Gμ and p remain viable when considering LVK limits and higher-frequency prospects under standard and modified cosmologies?
• RQ4 Do cosmic strings provide robust predictions for higher-frequency GW detectors that could validate or invalidate the model independently of PTA data?

Key findings

• Cosmic superstrings can fit the NANOGrav 15-year data for Gμ ~ 10^-12–10^-11 and p ~ 10^-3–10^-1.
• Under standard cosmology, LVK O3 data excludes much of the 68–99% CL regions, and LVK design sensitivity cannot fully exclude the NANOGrav-fit region.
• Extrapolations to 10–100 Hz show compatibility with LVK upper limits at 68% CL for standard expansion, while modified cosmologies shift spectra.
• An early period of matter domination ending before BBN can push the GW signal to evade some LVK exclusions but remains testable by ET, AION-km, AEDGE, and LISA.
• Inflationary dilution lowers the SGWB signal and can modify the spectrum at PTA frequencies; longer inflation reduces detectability at low frequencies but cannot hide it from all future experiments.
• Overall, cosmic superstrings remain testable by a broad suite of future GW detectors even with nonstandard early-Universe expansion.