What is a Pulsar Timing Array?

Question

What is this ? And why it is blowing up on Twitter with astrophysicists? Are we doomed ?

In: 2

Answer 1

This question got asked a couple times earlier today. It got removed both times, since ELI5 is not supposed to be about current events, but the question “What is a PTA?” is pretty broad and not strictly a current event. I’ll answer this again in the hopes it stays up (this is largely copied and pasted from my other answers)

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Pulsars are neutron stars that spin very quickly. They give off a beam of radio waves in a particular direction, and this beam spins around with the star. If we’re in the path of the beam, we see it as short pulses of radio light. Pulsar rotation is extremely stable; we should always measure the same time interval between any two adjacent pulses.

Gravitational waves stretch and compress space itself. The kind of gravitational waves that were detected with [LIGO](https://en.m.wikipedia.org/wiki/LIGO) were *high-frequency* gravitational waves caused by a merger between two massive objects (I forget if it was two neutron stars or two black holes). The arms of the LIGO arrays are 4 km long. To detect *low-frequency* gravitational waves, you need something like LIGO with arms that are millions of times longer.

We can combine both of these ideas: Since pulsars should be extremely stable, by measuring tiny variations in pulsar times-of-arrival over the course of several years, we can (in theory) detect the effects low-frequency gravitational waves. This is what a pulsar timing array is—many research groups studying many pulsars at different locations in the sky whose times-of-arrival are used in an effort to detect GWs. These would be GWs that were emitted billions of years ago by mergers between “primordial” black holes. Understanding primordial black hole mergers can potentially tell us about the early universe.

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As for why Twitter is so interested in PTAs right now, one PTA group called NANOGrav tweeted that they have some announcement to make on Thursday. I actually worked with NANOGrav for a bit, but I haven’t worked with them for about a year now, so I’m not sure what news they’re gearing up to announce (also, IIRC members of NANOGrav are not allowed to disclose research, so you probably won’t be able to find out until Thursday). My guess would be that they have strong evidence of low-frequency gravitational waves to publish