NANOGrav: First Detection of Gravitational Wave Background
Are high frequency Gravitational Waves (GWs) drowned by noise?
Links: astrobites is always the first website I’ll go to get a general idea of what happened. These two YouTube videos are interesting: Gravitational Waves | Cosmic Symphony, NANOGrav - 15 Years of Gravitational Wave Research. Here’s some recommended review articles: [1901.06785] Probing supermassive black hole mergers and stalling with pulsar timing arrays, [2211.05148] Seeing the gravitational wave universe. Well, NANOGrav released a lot of papers on the same day: 2306.16223, 2306.16222, 2306.16221, 2306.16220, 2306.16219, 2306.16218, 2306.16217, 2306.16215, 2306.16213. The most important of which is probably [2306.16213] The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background, in which the dataset, method, validation are explained. I’ll charge into all the technical details when I have the time. And there’re more papers from different observatories, including Parkes Pulsar Timing Array, European Pulsar Timing Array (combined with Indian Pulsar Timing Array) and Chinese Pulsar Timing Array.
The general idea is:
- Pulsars are accurate.
- The pulse delays (caused by GW) between two pulsars should be correlated according to their angular separation in the sky, which is called the Hellings and Downs correlation.
LIGO observes Gravitational Wave (GW) with the wavelength of $30 \text{Hz} - 7 \text{kHz}$, caused by merges of Block Holes few times heavier than the sun, while NANOGrav observes GW with the wavelength of $10^{-9} \text{Hz}$ (thus the name), caused by source unknown, probably merges of Supermassive Black Holes (SMBH). LIGO is few kilometers long, while pulsars can be few lightyears away, which is in accordance to the fact that GW can only be detected by things close to its magnitude of wavelength.
It could be a window to the early universe, earlier than Cosmic Microwave Background. Because the current SMBH theory does not explain it very well. The higher frequency region of Figure 1. (a) of [2306.16213] looks like system error.
Detecting GW with radio telescope through the observation of Pulsars Timing Array (PTA) is like how pregnant women start to see other pregnant women everywhere — Different people see different aspects of the whole picture: GW people see GW, radio telescope people see radio telescope. (This is called attentional bias by the way.)