NANOGrav results on Thursday (June 29)

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TL;DR Summary

NANOGrav is studying pulsar timing to look for low-frequency gravitational waves. They announced to provide an update in a major press conference.

Supermassive black hole in close orbits should produce powerful gravitational waves, but their frequency is too low to be measured with gravitational wave detectors we can build. Luckily nature has built something we can use. The NANOGrav collaboration is studying the arrival time of signals from pulsars. Some of them are extremely reliable. A gravitational wave passing through our line of sight should induce periodic changes in the timing. If the same signal can be seen for multiple pulsars, we can determine that it must have been caused by a gravitational wave. This method is most sensitive in the range of Nanohertz, leading to the name of the collaboration.

After 15 years of measurements, and some exclusion limits published in January, they now announced a press conference for Thursday. You don't do that for 10% better exclusion limits - they must have found something. Supermassive black holes in a very close orbit or even merging black holes would be the most interesting result (short of something so weird we didn't even think of it).

At the moment there seems to be no official webpage or other reference for the announcement event, but here is a tweet.

I don't expect a relation, but it's interesting that IceCube has some news the same day.
There is also the upcoming launch of Euclid (July 1).

 

[George Jones]

Supermassive black hole in close orbits should produce powerful gravitational waves, but their frequency is too
After 15 years of measurements, and some exclusion limits published in January, they now announced a press conference for Thursday. You don't do that for 10% better exclusion limits - they must have found something. Supermassive black holes in a very close orbit or even merging black holes would be the most interesting result (short of something so weird we didn't even think of it).

I hope that NANOGrav has seen individual events(s), but I think it is more likely that they will announce that they have detected the "rumble" of many events, i.e., a stochastic background of long wavelength gravitational waves.

I don't expect a relation, but it's interesting that IceCube has some news the same day.

A member of NANOGrav recently commented that there is no relationship. I wonder what IceCube has that warrants a public announcement.

 

[Madeleine Birchfield]

The paper is out:

The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background​

 

[Madeleine Birchfield]

Is the gravitational-wave background mentioned here a gravitational wave analogue of the CMB?

 

[Ibix]

Is the gravitational-wave background mentioned here a gravitational wave analogue of the CMB?

"Origins remain indeterminate" according to the paper. I think their main observation is that it's consistent with a population of binary super-massive black holes with orbits decaying by gravitational wave emission, but it could be primordial too.

 

[Hornbein]

It seems to me that the news is that a signal was detected. The measurement system works. As to where that signal came from...

 

[Ibix]

I don't think eliminating relatively mundane sources (noise from the pulsars themselves) is trivial, and nor is showing that there's a signal there to detect. And they've tried modelling their detection but are not yet able to rule anything out. So I think you may be rather underselling the achievement, but broadly I agree.

 

[Tom.G]

Here is a link to the 55 minute long Public Briefing of June 29, 2023 1:00pm Eastern US Time:


Their home page is at: https://nanograv.org/news/15yrDataSet

 

[ohwilleke]

By carefully observing ultra-precise pulsar timing data over fifteen years and correlating irregularities from different sources, they have observed a shared random background gravitational wave signal that follows a power law.

The signal seen is what would be expected from the gravitational waves created by multiple binary supermassive black hole systems.

The main paper and its abstract are as follows:

We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves.

The correlations follow the Hellings–Downs pattern expected for a stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of 10^14, and this same model is favored over an uncorrelated common power-law spectrum model with Bayes factors of 200–1000, depending on spectral modeling choices.

We have built a statistical background distribution for the latter Bayes factors using a method that removes interpulsar correlations from our data set, finding p = 10^−3 (≈3σ) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of interpulsar correlations yields p = 5 × 10^−5 to 1.9 × 10^−4 (≈3.5σ–4σ).

Assuming a fiducial f−2/3 characteristic strain spectrum, as appropriate for an ensemble of binary supermassive black hole inspirals, the strain amplitude is

(median + 90% credible interval) at a reference frequency of 1 yr^−1. The inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from a population of supermassive black hole binaries, although more exotic cosmological and astrophysical sources cannot be excluded. The observation of Hellings–Downs correlations points to the gravitational-wave origin of this signal.

Gabriella Agazie, et al., "The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background" 951(1) The Astrophysical Journal Letters L8 (June 29, 2023) DOI 10.3847/2041-8213/acdac6