Thursday, 11 February 2016

LIGO: what's in it for us?

I mean us theoretical particle physicists. With this constraint, the short answer is not much.  Of course, every human being must experience shock and awe when picturing the phenomenon observed by LIGO. Two black holes spiraling into each other and merging in a cataclysmic event which releases energy equivalent to 3 solar masses within a fraction of a second.... Besides, one can only  admire the ingenuity that allows us to detect here on Earth a disturbance of the gravitational field created in a galaxy 1.3 billion light years away. In more practical terms, the LIGO announcement starts the era of gravitational wave astronomy and thus opens a new window on the universe. In particular, LIGO's discovery is a first ever observation of a black hole binary, and we should soon learn more about the ubiquity of astrophysical systems containing one or more black holes. Furthermore, it is possible that we will discover completely new objects whose existence we don't even suspect. Still, all of the above is what I fondly call dirty astrophysics on this blog,  and it does not touch upon any fundamental issues. What are the prospects for learning something new about those?

In the long run, I think we can be cautiously optimistic. While we haven't learned anything unexpected from today's LIGO announcement, progress in gravitational wave astronomy should eventually teach us something about fundamental physics. First of all, advances in astronomy, inevitably brought by this new experimental technique,  will allow us to better measure the basic parameters of the universe. This in turn will provide us information about aspects of fundamental physics that can affect the entire universe, such as e.g. the dark energy. Moreover, by observing phenomena occurring in strong gravitational fields and of which signals propagate over large distances, we can place constraints on modifications of Einstein gravity such as the graviton mass (on the downside,  often there is no consistent alternative theory that can be constrained).

Closer to our hearts, one potential source of gravitational waves is a strongly first-order phase transition. Such an event may have occurred as the early universe was cooling down.  Below a certain critical temperature a symmetric phase of the high-energy theory may no longer be energetically preferred, and the universe enters a new phase where the symmetry is broken. If the transition is violent (strongly first-order in the physics jargon), bubbles of the new phase emerge, expand, and collide, until they fill the entire visible universe. Such a dramatic event produces gravitational waves with the amplitude that may be observable by future experiments.   Two examples of phase transitions we suspect to have occurred are the QCD phase transition  around T=100 MeV, and the electroweak phase transition around T=100 GeV. The Standard Model predicts that neither is first order, however new physics beyond the Standard Model may change that conclusion. Many examples of required  new physics have been proposed to modify the electroweak phase transition, for example models with additional Higgs scalars, or with warped extra dimensions.  Moreover, the phase transition could be related to symmetry breaking in a hidden sector that is very weakly or not at all coupled (except via gravity) to ordinary matter.  Therefore, by observing or putting limits on phase transitions in the early universe we will obtain complementary information about the fundamental theory at high energies.    

Gravitational waves from phase transitions are typically predicted to peak at frequencies much smaller than the ones probed by LIGO (35 to 250 Hz). The next generation of gravitational telescopes will be more equipped to detect such a signal thanks to a much larger arm-length (see figure borrowed from here). This concerns especially the eLISA space interferometer which will probe millihertz frequencies. Even smaller frequencies can be probed by pulsar timing arrays which search for signals of gravitational waves using stable pulsars for an antenna.  The worry is that the  interesting signal may be obscured by astrophysical backgrounds, such as (oh horror) gravitational wave emission from white dwarf binaries. Another  interesting beacon for future experiments is to detect gravitational waves from inflation (almost discovered 2 years ago via another method by the BICEP collaboration).  However, given the constraints from the CMB observations,  the inflation signal may well be too weak even for the future  giant space interferometers like DECIGO or BBO.

To summarize, the importance of the LIGO discovery for the field  of particle physics is mostly the boost it gives to  further experimental efforts in this direction.  Hopefully, the eLISA project will now take off, and other ideas will emerge. Once gravitational wave experiments become sensitive to sub-Hertz frequencies, they will start probing the parameter space of interesting theories beyond the Standard Model.  

Thanks YouTube! It's the first time I see a webcast of a highly anticipated event running smoothly in spite of 100000 viewers. This can be contrasted with Physical Review Letters who struggled to make one damn pdf file accessible ;) 

27 comments:

Anonymous said...

Did they at least show that gravity waves move at the speed of light?

Carla

David said...

I also wonder whetheventually grav wave observations of neutron stars etc could place bounds on the statistics of the population of these stellar objects, which can help understand astro backgrounds for eg indirect DM detection searches?

Jester said...

Yes, it's not impossible to imagine that, in the future, we may be e.g. mapping the galactic center with the help of gravitational waves astronomy.

Ervin Goldfain said...

Anonymous @ 21:36

Yes, LIGO detection sets an upper bound on the graviton mass, consistent with zero (m < 1.2 x 10^(- 22) eV), see:

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102

Robert L. Oldershaw said...

One cannot overestimate how amazing Einstein was as an old school natural philosopher, as well as a mathematical physicist. First came the empirically-driven intuitive insights about fundamental principles, and only then did he bring in the tools of mathematical physics (with lots of help from others in the case of General Relativity) to give rigor, details and predictive capacity to his conceptual discoveries.

Maybe it is time to reassess the conventional scoring of the Einstein-Bohr debates and to reconsider the reservations Einstein had about the incompleteness and opaque quality of Quantum Mechanics. His intuition was legendary, and well deserved.

StevieB said...

Jester,

As a longtime devotee of the Jester Exclusion Principle (although I worry I may have referenced it one too many times in the past...), could you please comment on what the implications of today's announcement are for the JEP?

Fingers and toes crossed that there is no tension!

Steve

Rastus Odinga Odinga said...

What's in it for us? To answer this question, compare today's gr-qc [electrifying!] with today's hep-ph [Yawn!] or today's hep-th [YAWN!!!!].

Xezlec said...

Does anyone happen to know whether the recorded signal is available for download anywhere? It's in the same frequency range as audible sound, so I really want to turn it into a sound file.

Jester said...

StevieB: the principle applies only to fundamental particles. LIGO observed a completely classical phenomenon, and there's no problem that, at a fundamental level, the signal is a superposition of spin-2 gravitons.
On the other hand, if the 750 GeV signal is true it would be another triumphant confirmation of JEP ;)

Thierry said...

Thanks for the review. Do you know what value could represent a power of "50 times all the pwoer of stars visible in the universe" as stated by Kip Thorne during the Press conference ?
cf. http://www.youtube.com/watch?v=vy5vDtviIz0

Jake said...

@Xezlec, check out the full data and many related files and informative links at the LIGO Open Science Center here:

https://losc.ligo.org/events/GW150914/

HD said...

@Thierry

The emitted power from a merging is of order 0.1 M c^2 (maybe the half here, it depends on the BH mass ratio and spins), and the duration equals to a few innermost stable orbits, i.e. 100 G M / c^3. This gives

Power \simeq O(10^-3) c^5 / G, which gives around 3.10^49 Watts, which corresponds to a bit less than 10^23 Solar luminosities.

Compare this with the fact that there are ~ 10^80 baryons in the observable Universe, and that the Sun has 10^57 of these. You have therefore 10^23 Solar masses of baryonic matter, 10% of which at most are into stars, most of which have lower mass and hence a higher mass-to luminosity ratio than the Sun (L_star \propto M_star^3, typically). Therefore, this event is clearly more than one order of magnitude brighter than the integrated starlight of the whole observable universe.

Robert L. Oldershaw said...

Gravitons?

How about extraterrestrial farts?

Martin said...

Jester,

you mentioned BICEPS. How are they getting on? I thought they only needed to do a few more measurements to confirm their original findings?

Jester said...

Nope, their original findings were refuted. It is possible that BICEP or another similar experiment will find a signal in the future when sensitivity is improved, but for the moment there is nothing.

Anonymous said...

BICEP3 together with other data should be able to get upper limit to r<=0.03 (see 1601.00125). I think results should be coming this year, but I don't know when.

Xezlec said...

Thanks, Jake!

Alex Small said...

There is a certain sense in which any experimental verification of GR or its Newtonian limit is significant for particle physics: One significant motivation for searching for particles not in the standard model is that we're pretty sure dark matter is real, and we are pretty sure of this because observational astronomers see gravitational phenomena that can't be accounted for with the matter described in the Standard Model. The hidden assumption there is that we understand gravity well enough to say that when we see an apparent gravitational effect of is in fact a gravitational effect and hence there must be some matter to produce it.

Obviously the LIGO result is not a direct test of the dark matter concept, but every time gravitational theory passes a test we are justified in saying "yep, there's a very high Bayesian prior probability that dark matter is producing those gravitational effects."

Mathew Binkley said...
This comment has been removed by the author.
Mathew Binkley said...

Now that we live in an age where Occam's Razor tells us that two 30 solar mass black holes merging at half the speed of light is the simplest, most straightforward answer, let's speculate a bit. :-)

Astrophysicists have made estimates of the probability of black hole mergers, but LIGO hints that the actual rate may be significantly higher than that. Which means that A) their estimate was off or B) collisions may also be occurring in a hidden sector. Think mirror matter black holes, or something even weirder.

Could we discriminate between a classical black hole merger, and that of some sort of degenerate bodies existing in a hidden sector, strictly by their gravity wave signature?

mfb said...

Dark matter as 30 solar mass black holes would have been noted via gravitational lensing.

1 event in 16 days is within the range of prior estimates of the frequency of visible events. Even if the rate turns out to be as low as once every few years: well, then they were a bit lucky.

Anonymous said...

The frequency rate of black hole mergers has not been measured yet, they need to run the detector and keep taking data for at least a few years before any estimate on the rate can be given. Detection of one event doesn't tell you anything about their frequency.


Anonymous said...

They already have other (possible) detections, just not as strong as the one announced.

Felix said...

The detected GW came from black holes collision some 1.3 billion light-year. What does this say about the dimensions of the universe? If the universe is >4 of space-time, some of the wave would 'leak' into the extra dimensions. But none apparently did, its intensity arrived as predicted. This rules out extra dimensions?

Ervin Goldfain said...

Felix,

An interesting observation, worthy of further scrutiny. It may be especially relevant for the Randall-Sundrum and the ADD models.

mfb said...

I'm not sure if the conclusion is so easy. The estimate of 1.3 billion light years was probably made assuming GR in 3+1 dimensions. Different models could lead to different estimates, and I'm not sure if the distance can be estimated with reasonable precision if you do not include the amplitude in the fit. They have ~30% uncertainty even taking the amplitude into account.

@Anonymous 06:04: A single event in 16 days does tell you something about the frequency. Even zero events do. You can rule out (xx% CL) a rate below ~1 per year, and also a rate above several per week. Didn't calculate it exactly and it depends a bit on what exactly you define as event, but there is certainly an estimate you can make.

RBS said...

What if we combine gravitational and traditional astronomy by observing the same object in both gravitational and electromagnetic light? Shouldn't it give us an idea of distance, and as a result, scattering of gravitational power into higher dimensions? Let's say that no such scattering is observed eventually - would it mean the end of string theories?