Standard Sirens

Everyone is rightly excited about the latest gravitational-wave discovery. The LIGO observatory, recently joined by its European partner VIRGO, had previously seen gravitational waves from coalescing black holes. Which is super-awesome, but also a bit lonely — black holes are black, so we detect the gravitational waves and little else. Since our current gravitational-wave observatories aren’t very good at pinpointing source locations on the sky, we’ve been completely unable to say which galaxy, for example, the events originated in.

This has changed now, as we’ve launched the era of “multi-messenger astronomy,” detecting both gravitational and electromagnetic radiation from a single source. The event was the merger of two neutron stars, rather than black holes, and all that matter coming together in a giant conflagration lit up the sky in a large number of wavelengths simultaneously.

Look at all those different observatories, and all those wavelengths of electromagnetic radiation! Radio, infrared, optical, ultraviolet, X-ray, and gamma-ray — soup to nuts, astronomically speaking.

A lot of cutting-edge science will come out of this, see e.g. this main science paper. Apparently some folks are very excited by the fact that the event produced an amount of gold equal to several times the mass of the Earth. But it’s my blog, so let me highlight the aspect of personal relevance to me: using “standard sirens” to measure the expansion of the universe.

We’re already pretty good at measuring the expansion of the universe, using something called the cosmic distance ladder. You build up distance measures step by step, determining the distance to nearby stars, then to more distant clusters, and so forth. Works well, but of course is subject to accumulated errors along the way. This new kind of gravitational-wave observation is something else entirely, allowing us to completely jump over the distance ladder and obtain an independent measurement of the distance to cosmological objects. See this LIGO explainer.

The simultaneous observation of gravitational and electromagnetic waves is crucial to this idea. You’re trying to compare two things: the distance to an object, and the apparent velocity with which it is moving away from us. Usually velocity is the easy part: you measure the redshift of light, which is easy to do when you have an electromagnetic spectrum of an object. But with gravitational waves alone, you can’t do it — there isn’t enough structure in the spectrum to measure a redshift. That’s why the exploding neutron stars were so crucial; in this event, GW170817, we can for the first time determine the precise redshift of a distant gravitational-wave source.

Measuring the distance is the tricky part, and this is where gravitational waves offer a new technique. The favorite conventional strategy is to identify “standard candles” — objects for which you have a reason to believe you know their intrinsic brightness, so that by comparing to the brightness you actually observe you can figure out the distance. To discover the acceleration of the universe, for example,  astronomers used Type Ia supernovae as standard candles.

Gravitational waves don’t quite give you standard candles; every one will generally have a different intrinsic gravitational “luminosity” (the amount of energy emitted). But by looking at the precise way in which the source evolves — the characteristic “chirp” waveform in gravitational waves as the two objects rapidly spiral together — we can work out precisely what that total luminosity actually is. Here’s the chirp for GW170817, compared to the other sources we’ve discovered — much more data, almost a full minute!

So we have both distance and redshift, without using the conventional distance ladder at all! This is important for all sorts of reasons. An independent way of getting at cosmic distances will allow us to measure properties of the dark energy, for example. You might also have heard that there is a discrepancy between different ways of measuring the Hubble constant, which either means someone is making a tiny mistake or there is something dramatically wrong with the way we think about the universe. Having an independent check will be crucial in sorting this out. Just from this one event, we are able to say that the Hubble constant is 70 kilometers per second per megaparsec, albeit with large error bars (+12, -8 km/s/Mpc). That will get much better as we collect more events.

So here is my (infinitesimally tiny) role in this exciting story. The idea of using gravitational-wave sources as standard sirens was put forward by Bernard Schutz all the way back in 1986. But it’s been developed substantially since then, especially by my friends Daniel Holz and Scott Hughes. Years ago Daniel told me about the idea, as he and Scott were writing one of the early papers. My immediate response was “Well, you have to call these things `standard sirens.'” And so a useful label was born.

Sadly for my share of the glory, my Caltech colleague Sterl Phinney also suggested the name simultaneously, as the acknowledgments to the paper testify. That’s okay; when one’s contribution is this extremely small, sharing it doesn’t seem so bad.

By contrast, the glory attaching to the physicists and astronomers who pulled off this observation, and the many others who have contributed to the theoretical understanding behind it, is substantial indeed. Congratulations to all of the hard-working people who have truly opened a new window on how we look at our universe.

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43 Responses to Standard Sirens

  1. James Goetz says:

    This is fascinating. I have a tangent question, if I may. For example, I hear some skepticism of the cosmic distance ladder and likewise the expansion of the universe. Does this independent measurement of gravitational waves in any way support the calculations of the cosmic distance ladder?

  2. As the gravitational waves are much stronger near a black whole merger wouldn’t we expect some radiation from them shaking the matter around the pair?
    Of course, it might be quite weak and hard to detect with the poor localization given by gravitational waves only.

    Arthur Snyder, SLAC

  3. Sean Carroll says:

    James– There is no good reason to be skeptical of the basic idea of the cosmic distance ladder or the expansion of the universe.

    Arthur– The shaking from gravitational waves won’t induce much radiation, and there might not be that much matter around coalescing black holes.

  4. James Goetz says:

    Sean, I in no way support skepticism of the cosmic distance latter, but I have seen the skepticism at moderated physics newsgroups. Regardless, my main inquiry here asks if the “independent measurement of gravitational waves in any way support the calculations of the cosmic distance ladder.” If there is not yet enough information on that, then no problem for me. However, I find it more interesting if the independent measurements also support the cosmic distance ladder.

  5. Sean Carroll says:

    The fact that the standard sirens give H0 = 70 is independent support that the cosmic distance ladder is correct.

  6. alexandra moffat says:

    ‘the event produced an amount of gold equal to several times the mass of the Earth.”

    so Fort Knox is now completely irrelevant?

  7. Mike Rinaldi says:

    With VIRGO now in place and soon the Japanese and then Indian LIGO’s coming on line we should see many Gamma Ray Bursters (GRB’s) possibly associated with GW’s. GRB’s have been seen many time by multiple instruments and many theories have been proposed about what causes them. It now looks like at least some of these GRB’s are associated with NS-NS merges. Exciting times in High Energy Astrophysics!!

  8. Scott Hughes says:

    Plot of the posterior PDF for Hubble now decorating my office door 😉

  9. If position resolution on black hole mergers we good enough to identify what galaxy they occur in, would that allow the red-shift of the galaxy to be used to measure H0 w/o any need to ‘see’ the merger it self?

    Arthur Snyder, SLAC

  10. anon says:

    Could you elaborate a little more on the statement
    “by looking at the precise way in which the source evolves — the characteristic “chirp” waveform in gravitational waves as the two objects rapidly spiral together — we can work out precisely what that total luminosity actually is.”?

    I tried looking at the relevant papers, but found them a bit too dense. In particular, what’s special about gravitational waves that makes this possible? (I’m assuming such a measurement can’t be done by just looking at the electromagnetic waves.)

  11. Eric Shumard says:

    What’s the sound of two neutron stars colliding?

  12. James Goetz says:

    “Sean Carroll says: The fact that the standard sirens give H0 = 70 is independent support that the cosmic distance ladder is correct.”

    Excellent, thank you, you implied that your OP, but I needed a little extra prodding to see that. I suppose more gravitational wave discoveries and all time-elapsed observations of individual candles in the cosmic distance ladder will help to narrow down the value of the Hubble Constant. Perhaps, give or take 10% is the best we can hope for any calculation in astrophysics 🙂

  13. Steven Mellemans says:

    I wonder why there is a delay of about 2 sec between gamma-ray burst and the GW.

  14. Mitch C says:

    Sean. As you said, your contribution to this particular observation may be somewhat small. However, you can claim to be affiliated with the same university as the recent Nobel laureate Kip Thorne!!!! In fact, since he is the Feynman professor of theoretical physics, pehaps you should give him your desk!!! LOL.

    All jesting aside, please keep up your excellent work on this blog. As a humble nuclear & electrical engineer, I thoroughly enjoy reading your blog. I also buy all your books which I devour the moment they go on sale.

  15. William Adam says:

    Has the information gathered so far said anything about the nature of space i.e. stretchable fabric/particulate/loops etc ?

  16. Chandresh Patel says:

    I’m not a Physics graduate (practising Chemical Engineer) but ever since I saw the movie Interstellar couple years ago,I got pulled into Physics and Astronomy.I have read few books on Physics (like History of time,The Big Picture and From Etrnity to here).

    LIGO success of detecting GW due to black hole collisions and now Neutron star collisions is super exciting.It will allow humans to understand the universe in more detail.

  17. Scott Hughes says:

    Arthur Snyder: Yes. In fact, the original idea that Holz and I considered was close to what you discuss. We originally considered a situation in which LISA measurements of merging black holes could pin down the position well enough to determine the galaxy hosting the merger. It wasn’t until later papers we wrote (with a couple of other colleagues joining us) that we really started digging into the idea of binary neutron star mergers as standard sirens.

  18. Scott Hughes says:

    anon: The way standard candles normally work in astronomy is to measure a source whose luminosity is “known” (more on that in a moment). The brightness we measure falls off as distance squared from the source. From the “known” luminosity and the measured brightness, we infer the distance.

    Of course, we never “know” the luminosity precisely — no astronomical objects come with “N trillion trillion Watts” written on the side. However, there are objects whose luminosity can be determined by a sequence of calibrations. As such, even if true “standard” candles are hard to come by, there are objects which we can regard as “standardizable” candles.

    The key thing that makes standard sirens work (and the bit that Bernard Schutz nailed in 1986) is that they are effectively self calibrating. The GW signal that we measure has an amplitude that depends on the source’s intrinsic “loudness,” and falls as off as one over distance. It also has a frequency that changes with time, chirping up from low frequency to high frequency. The cool bit is that the rate at which this frequency changes directly encodes the intrinsic “loudness” of the signal. When we measure the full waveform, we get both the signal’s amplitude in the detector (which depends on distance and the source’s intrinsic loudness) and the rate of change of the signal’s frequency (which depends on the intrinsic loudness). The distance to the source then basically pops out.

    (Actually, in practice there are a lot more details that need to be nailed down. In particular, the measured amplitude doesn’t JUST depend on the source’s intrinsic loudness and distance, but also on the source’s position on the sky and how the binary is oriented at that position. To really get the distance, we need to learn the position and orientation too. It turns out one of the best ways to do that is to measure the GWs in multiple widely separated detectors, and to have an electromagnetic event that goes “boom” in conjunction with the GWs. If I were to order the perfect standard siren, it would look pretty much just like GW170817!)

  19. anon says:

    Scott Hughes: Thanks for the response. I would like to get some (vague) sense of the “cool bit” that you mention, i.e. the fact that the rate at which the frequency of the GW signal changes encodes the intrinsic loudness. In particular, why isn’t there some analog of this phenomenon in the EM spectrum?

  20. John Joseph says:

    Thank you for the detail explanation Dr. Sean Carroll. Enjoyed the whole writing and also congratulations to all involved in detecting the difficult gravitational wave.

  21. Scott Hughes says:

    anon: A binary’s gravitational wave amplitude and the the rate at which its orbital frequency evolves are both determined by the rate of change of its quadrupole moment. (When I say “loudness” I really mean “intrinsic GW amplitude,” which in turn really means “rate of change of the source quadrupole moment.”) Measuring the frequency evolution determines the rate at which the mass quadrupole changes, which then fixes what the waves’ amplitude should be. The fact that two measurables encode this piece of information is key to the idea that standard sirens are self calibrating.

    There doesn’t appear to be any electromagnetic emitter that similarly has such a self calibrating nature. In presenting the standard siren idea at a talk a few weeks ago, I asked the audience to imagine a hypothetical standard candle that consisted of a single coherent electric dipole emitter, with the dipole moment varying in time in a predictable way. That would be the electromagnetic equivalent of what the standard siren does. IF such a thing existed, it would be amazing … but the closest we’ve been able to find to this ideal is gravitational, not electromagnetic.

    Cepheid variable stars come close. They are variable stars in which the period of variability is proportional to the stars’ intrinsic luminosity — the brighter the star, the longer the period. And indeed they are some of the most powerful standard candles we know. Type Ia supernovae also come close. In both cases, though, there’s a calibration that, although it works well, depends on a sequence of empirical measurements. The virtue of standard sirens is that their self calibrating nature does an end run around this sequence of empirical calibrations.

  22. Mike Scott says:

    Alexandra Moffat …
    On a side note … Fort Knox is probably irrelevent anyway … because the widely held belief is that there isn’t actually any gold there any more ! 😉

  23. daniel holz says:

    Sean, very nice post, as always. It is indeed amazing that this has all worked out. The very first gravitational-waves from a binary neutron star. The very first gamma-ray burst in coincidence with a gravitational-wave source. The very first optical counterpart from a gravitational-wave source, enabling the very first host galaxy for a gravitational-wave source, enabling the very first standard siren measurement. And because it’s the loudest and closest gravitational wave source ever, we get a pretty nice measurement of the Hubble constant!! Even now, I’m still finding it hard to believe that this all has really happened. You write the papers, but it’s still shocking to find that sometimes Nature cooperates…I guess if I were braver I would get it tattooed somewhere, but instead I’ll take Scott’s approach.

  24. Frances Day says:

    Hi Sean – I am so interested in this discovery, particularly since I wrote a short paper on measuring the Hubble constant for my final year’s study in Astrophysics with the Open University a couple of years ago. The prospect of another independent measurement is very exciting. I should very much like to study this in more detail – could you tell me whether there is a particular field of study I would need to get under my belt in order to try to understand the physics involved with this? I want to understand how the information from gravitational waves differs from EM. I am not a university student – I study simply for my own enjoyment.
    Maybe I’m not asking the right questions here?