[Paper Review] Prospects for Taiji to detect a gravitational-wave background from cosmic strings

Zu-Cheng Chen, Qing-Guo Huang+7 more|arXiv (Cornell University)|Sep 30, 2023

Pulsars and Gravitational Waves Research148 references4 citations

TL;DR

This paper investigates Taiji’s capability to detect a stochastic gravitational-wave background (SGWB) from cosmic strings, using Bayesian parameter estimation on simulated Taiji data. It demonstrates that Taiji can measure cosmic string parameters with 20–70 times greater precision than NANOGrav 15-year data, significantly enhancing constraints on the string tension $G\mu$ and reconnection probability $p$. This positions Taiji as a powerful complementary tool to pulsar timing arrays for probing early-Universe physics.

ABSTRACT

Recently, multiple pulsar timing array collaborations have presented compelling evidence for a stochastic signal at nanohertz frequencies, potentially originating from cosmic strings. Cosmic strings are linear topological defects that can arise during phase transitions in the early Universe or as fundamental strings in superstring theory. This paper focuses on investigating the detection capabilities of Taiji, a planned space-based gravitational wave detector, for the gravitational wave background generated by cosmic strings. By analyzing simulated Taiji data and utilizing comprehensive Bayesian parameter estimation techniques, we demonstrate a significant improvement in precision compared to the NANOGrav 15-year data, surpassing it by an order of magnitude. This highlights the enhanced measurement capabilities of Taiji. Consequently, Taiji can serve as a valuable complementary tool to pulsar timing arrays in validating and exploring the physics of cosmic strings in the early Universe.

Motivation & Objective

• To assess Taiji’s detection potential for a stochastic gravitational-wave background (SGWB) from cosmic strings.
• To quantify the precision of parameter estimation for cosmic string models using simulated Taiji data.
• To compare Taiji’s measurement accuracy with that of NANOGrav 15-year data, highlighting improvements in uncertainty.
• To validate the role of space-based detectors like Taiji in verifying pulsar timing array signals and probing early-Universe physics.

Proposed method

• Simulated Taiji data were generated using a realistic noise model, foregrounds from double white dwarfs (DWDs), and extragalactic compact binaries (ECBs).
• The gravitational wave background from cosmic strings was modeled using the energy spectrum from relativistic loop oscillations and reconnection dynamics.
• Bayesian parameter estimation was performed using the dynesty sampler within the Bilby package to explore the full posterior distribution.
• The likelihood function combined Gaussian and log-normal components to model data fidelity across frequency channels.
• Priors were assigned to model parameters including $G\mu$, $p$, and foreground amplitudes, with constraints informed by NANOGrav 15-year data.
• Posterior distributions were analyzed to assess recovery accuracy and uncertainty, with injected values compared to median estimates and credible intervals.

Experimental results

Research questions

• RQ1 Can Taiji detect a stochastic gravitational-wave background from cosmic strings with higher precision than pulsar timing arrays?
• RQ2 What level of uncertainty can Taiji achieve in measuring the cosmic string parameters $G\mu$ and $p$?
• RQ3 How do the foreground contributions from DWDs and ECBs affect the parameter estimation accuracy in Taiji data?
• RQ4 To what extent does Taiji improve on the NANOGrav 15-year data in constraining the cosmic string model?
• RQ5 How well can Taiji disentangle the cosmic string SGWB from astrophysical foregrounds?

Key findings

• Taiji achieves a relative uncertainty of 0.3% for $\log_{10}G\mu$ and 0.6% for $\log_{10}p$, representing a 70× and 20× improvement over NANOGrav 15-year data, respectively.
• All model parameters, including noise amplitudes $A$ and $P$, were recovered within 2σ credible intervals, confirming robust parameter estimation.
• The noise parameters $A$ and $P$ were measured with relative uncertainties of 0.4% and 0.03%, respectively, indicating high instrumental calibration fidelity.
• Foreground parameters from DWDs and ECBs showed larger uncertainties due to weaker amplitudes, but remained well-constrained.
• The posterior distributions from Taiji showed significantly narrower credible intervals than those from NANOGrav, confirming enhanced sensitivity.
• The study confirms that Taiji can serve as a complementary and highly precise probe for validating cosmic string signals detected by pulsar timing arrays.