cjc5

 NYTimes article (popular overview)

Nature article (paper on observations/chemistry/biology)

Daniel J. D'Orazio, Abraham Loeb

We demonstrate how pulsar timing arrays (PTAs) yield a purely gravitational wave (GW) measurement of the luminosity distance and co-moving distance to a supermassive black hole binary source, hence providing an estimate of the source redshift and the Hubble constant. The luminosity distance is derived through standard measurement of the chirp mass, which for the slowly evolving binary sources in the PTA band can be found by comparing the frequency of GW-timing residuals at the Earth compared to those at distant pulsars in the array. The co-moving distance can be measured from GW-timing parallax caused by the curvature of the GW wavefronts. This can be detected for single sources at the high-frequency end of the PTA band out to ~10 Gpc with a future PTA containing well-timed pulsars out to ~10 kpc. We estimate that for a future PTA with ~100 pulsars between 1 and 20 kpc and 1% pulsar-distance errors, the Hubble constant can be measured to better than 30% for a single source at $0.1 \lesssim z \lesssim 2$. At $z \lesssim 0.1$, the luminosity and co-moving distances are too similar to disentangle. At $z\gtrsim 2$, this measurement will be restricted by a signal-to-noise ratio threshold.

John Ellis, Marek Lewicki

Pulsar timing data used to provide upper limits on a possible stochastic gravitational wave background (SGWB). However, the NANOGrav Collaboration has recently reported strong evidence for a stochastic common-spectrum process, which we interpret as a SGWB in the framework of cosmic strings. The possible NANOGrav signal would correspond to a string tension $G\mu \in (4, 9) \times 10^{-11}$ at the 68% confidence level, with a different frequency dependence from supermassive black hole mergers. The SGWB produced by cosmic strings with such values of $G\mu$ would be beyond the reach of LIGO, but could be measured by other planned and proposed detectors such as SKA, LISA, TianQin, AION-1km, AEDGE, Einstein Telescope and Cosmic Explorer.

Simone Blasi, Vedran Brdar, Kai Schmitz

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 local 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.