NANOGrav waves support string theory?

[apostolosdt]

NANOGrav waves are real observational data, and now this: https://arxiv.org/abs/2307.08601. I don't know much in this area of research, except for the basics on LIGO and the like. Any comment from the knowledgeable members here?

 

[mitchell porter]

There are a lot of steps in the argument here. The string theory compactification has to produce supergravity with a "no-scale" potential, then that has to produce Starobinsky-type inflation, that lasts for a few "e-folds" so that a population of primordial black holes with a particular spread of masses can form and then evaporate before primordial nucleosynthesis begins; and the claim is that the gravitational waves arising from these transient black holes, should arise within the frequency range detected by NANOGrav. The journey from "string theory" to "nanohertz gravitational waves" is so long, that there seems to be a lot of room for optimistic fudge factors to enter the calculation and steer its outcome towards the right order of magnitude.

Meanwhile, if I look at the NANOGrav paper, what does it say are the possible causes of this gravitational-wave background? Supermassive black hole binaries are the favorite, but "more exotic cosmological sources such as inflation, cosmic strings, phase transitions, domain walls, and curvature-induced GWs can also produce detectable GWBs in the nHz range". Given that you can probably realize each of these possibilities within some string model, the specific scenario of primordial black holes doesn't seem particularly compelling.

 

[ohwilleke]

if I look at the NANOGrav paper, what does it say are the possible causes of this gravitational-wave background? Supermassive black hole binaries are the favorite, but "more exotic cosmological sources such as inflation, cosmic strings, phase transitions, domain walls, and curvature-induced GWs can also produce detectable GWBs in the nHz range". Given that you can probably realize each of these possibilities within some string model, the specific scenario of primordial black holes doesn't seem particularly compelling.

Indeed, any time that you can explain an observation without new physics, any new physics explanation for the same observation should come with a giant warning label.

Supermassive black hole binaries involve no new physics and are something that ought to exist in any theory that approximates reality including any workable string theory. After all, stellar sized black hole binaries and intermediate sized black hole binaries have both been observed many times and produce well understood gravitational wave signals, so supermassive black hole binaries should be just a matter of scale and frequency. And, the signal observed matches the ex ante prediction for the gravitational wave signal that supermassive black hole binaries would create that they were looking for in the first place because it was predicted by existing mainstream astrophysics.

Admittedly, predicting exactly how common binary super massive black holes are is tricky. But the NANOgrav observation still has significant uncertainty in it, so there is a significant window of uncertainty in both the measurement and the prediction. As a result, every existing physics and new physics conclusion needs to be taken with a grain of salt, which the NANOgrav experimentalists did in their paper.

The string theory explanation is not very credible at all.