Interview with Mike Turner

0:00 I should probably just speak into it and say that it's the 2nd of September, 1997. It's about 11am and I'm speaking with Mike Turner. So, well, I suppose maybe I should follow on from what you said and ask about what kind of gravitational waves are of interest to you and what are the prospects you think of projects like LIGO and LISA, if they're detecting anything. So the kind of gravitational waves that come from the early universe, which is my major interest, are even speculative by gravitational wave standards because they will be the noise where the astrophysical sources are the signal. So once you get the signal and then dig under to find noise that can't be explained by the detector, that's the kind of source from the early universe. So even by gravitational wave standards, it's a bit speculative. So I can think of three very interesting possibilities. One are cosmic strings. which maybe you talked to Bruce Allen about. And they have a very distinctive signature, and they're probably the easiest to detect, although, you know, in terms of signal strength. One of the advantages of some of the early universe sources is they're very broad band, so cosmic strengths are extraordinarily broad band. Okay. And the amplitude is relatively high, but on other grounds, many of us believe that there aren't cosmic strings out there. Another possibility are the gravitational waves produced by inflation. They're very broadband. The amplitude is extremely model dependent. So even if you believe inflation happened, explains where all the structure is in the universe, the amplitude depends very much on the model of inflation. would have relatively, well, none of them are particularly high, but, you know, high. Some are just going to be indetectably small. And the other source of gravitational waves from the early universe, which is actually probably the most accurately calculated,

2:30 is from phase transitions. If you had a strongly first-order phase transition, or even a not-so-strongly first-order phase transition, then the energy, there are bubbles, the phase transition is carried out by bubbles, and these bubbles carry an enormous amount of energy, and they get very big and collide, and they're efficient producers of gravitational waves. The problem here is that, so the amplitude is quite high, higher than the other two. The problem here is that it's narrowband. In high frequency or low frequency? High, low. It depends upon when the phase transition took place. It's possible. So the best prospect, at least for LIGO, would be a phase transition that took place at a temperature of 10 to the 7 GeV. And that is not a familiar scale. I mean, for example, 10 to the 15 is 10 to the 16 GeV is the gut scale, and 10 to the 3 GeV is the electroweak. This is not a familiar scale, but that doesn't mean that there wasn't something interesting going on. Where would LISA come in? Well, LISA, of course, goes to lower frequencies, and lower frequencies would go to lower vase transition temperatures. So, you mentioned that one of the problems would be that a lot of these ways would be basically the noise to the type of signal, the astrophysical signal, that LIGO is primarily, I suppose, CURIDA to look for. So, given that, for instance, in your paper, I think you were, it seemed, well, with your paper about the inflation noise, that it seemed fairly pessimistic that, for instance, LIGO would be able to see anything. Has anyone given much thought to, as it were, detectives, well, signal processing strategies, you know, how you would go about really identifying whether some of your background is really... Yeah. Well, Bruce Allen has spent a lot of time. You must know Bruce from... I do know Bruce, yeah, and I haven't...

5:00 Obviously, I'll have to interview him specifically for this. But LIGO misses by quite a bit from even the most optimistic estimates. So I think signal processing is not like you need another factor of three and you might get one. But with inflation, you know, it's worth thinking about, you know, beyond LIGO, beyond LISA. If inflation is correct and there's some evidence that inflation is not on the right track, it has three primary predictions. Number one, a flat universe. That will test. Number two, a certain spectrum of density and homogeneities. The first two, I think, will test best with the microwave background. And then the third are gravity waves. And let's suppose we verify the first two, which I think we definitely have the possibility of doing with the microwave background. I don't think that really proves inflation took place, partly because those two, although predicted by inflation, were things that were already rattling around. any sensible model of the universe must have a flat universe. Any sensible model of the universe must have the scale and variant density perturbations. So for other philosophical reasons, well, and practical reasons, those were both people were saying, you know, that any sensible cosmology would predict those. I mean, for example, the scale and variant spectrum is called the Harrison-Zeldovich spectrum. Neither one had anything to do with showing that inflation gave that spectrum. It was that they pointed out the scale-invariant spectrum had particularly nice properties. So it's not going to be easy to find these gravity waves left over from inflation, but it will be extraordinarily important. So it's, in some sense, the smoking gun test of inflation, since its other two primary predictions were sort of around before inflation was around. So it would be a very important result if you could get direct detection of these gravitational waves.

7:30 And then not only that, the gravity waves provide an important means of not only testing inflation, but learning about the model of inflation. So by measuring the amplitude of the gravity waves, you're measuring the scale of the vacuum energy that drove inflation. Right, sure, I guess you could learn very interesting things, especially if you were lucky enough to see waves from a phase transition. Well, and then phase transition is sort of a separate issue. So this was inflation. Now, it could be that inflation involved a phase transition. Most of the models of inflation right now don't have that being the case. But we still don't really understand inflation all that well, except that it's rapid expansion driven by false vacuum energy. One way of getting out of a false vacuum is by a phase transition with the nucleation of bubbles and then the bubbles colliding. So it could be that inflation not only produces this stochastic background that originated as quantum fluctuations, but it also produces a higher background of narrowband gravitational waves from the event that ended inflation, that converted the vacuum energy to radiation. So, given the importance of potentially detecting early universe gravitation waves, is there any particular type of experiment that you look to with some hope? I mean, if LIGO seems like not a very good candidate, do you think there's any hope for any of the other possibilities, LISA or spacecraft tracking? Well, I think LISA's, I mean, first of all, you know, we shouldn't be so arrogant as to think that we, you know, that the predictions that we make are the best that we can, and they're very sharp predictions, but it could well be that inflation operates very differently than we think, and so it could be that this pessimistic estimate that I made actually is too pessimistic. Sure. But beyond that, I think that, you know, Lisa would be the next step.

10:00 Getting to lower frequencies, right, Lisa goes to lower frequencies, yeah, makes it more favorable for detection. because for a given strain amplitude as you lower the frequency you lower omega gravity wave and so therefore if you can maintain strain sensitivity while going to lower frequency which is not a given but if you can then you're becoming more sensitive right what about spacecraft tracking I mean, are you thinking about, what was it called, Sagittarius? Yeah. I don't think they were as sensitive as Lisa. In general, they're not. I just wondered if, again, given the low frequency still. Yeah. Well, I mean, Lisa's spacecraft tracking, right? Yeah, essentially. Yeah, sure. I was curious about any other aspects of possible uses of LIGO that would be interesting to cosmologists. that there's a possibility of using an advanced LIGO for estimating the Hubble constant through looking and seeing neutron star binaries out to, say, gigaparsec distances. Unfortunately, it's been a while since then. No, that's right. In principle, if you saw a neutron star coalescence and you also had a regi... Well, then you can get a distance for the object. The system is sufficiently over-determined that you can get a distance. So if you saw an optical counterpart, if you knew that that coalescence happened in a certain galaxy... For instance, there was a gamma ray burst. For example, now with a gamma ray burst, You know, it's definitely, given how undirectional the gravity wave antennas are, having the gamma ray bursts would make a very big difference. Then you could have the distance to the object and its velocity, and therefore you'd get the Hubble constant, maybe even the deceleration parameter.

12:30 So that presumably would be a major triumph for LIGO, for instance, on the cosmology point? Yeah, it would be, although my suspicion on that is that it will play an important but more supporting role. I think that the Hubble constant is probably going to be measured to very high precision from anisotropy of the microwave background. There I think you can probably do better than 1%. Other techniques are really improving. For example, using... I'll come down. No, that's okay. Okay. Okay. Welcome. Hello. Using the SZ, the Soniaev-Zeldovich effect, measuring the effect of a hot cluster gas That technique is really improving very rapidly. Gravitational lenses. So I don't know what precision you could get with this. So it might play a supporting role. It might be useful in the deceleration parameter. At the very least, it will be useful in a test of the standard cosmology. See, in the end, all the methods that you measure the Hubble constant with better give the same number. Yeah, sure. So you feel it's likely that by the time LIGO would be at that level, other methods would have already proven it. That would be my guess, but it doesn't mean that it won't be valuable. And I don't know what precision, were they talking about 10%? I'm trying to remember. I can't remember. But the name of the game, I mean one of the things is the stakes, I mean, You know, Hubble constant, measuring the Hubble constant has become science. Okay? People give error bars, all the measurements agree within their error bars, and the error bars are rapidly shrinking. And so to get in the Hubble constant game, to be a real player, one is going to have to do much better than 10%. Probably, potentially sooner than, say, for instance, 10 years from now, which might be... That's right. I mean, the microwave background, I think, is going to probably get the Hubble constant to a few percent by the year 2002.

15:00 And other methods, I believe, will have it down to maybe not quite as good, 4 or 5 percent by the same time. The Soniaev-Zeldovich, gravitational lensing, the empirical standard candles. But, you know, it still will be important to have an independent confirmation. Well, that is kind of interesting because occasionally from astronomers, for instance, I'm married to one, so occasionally I hear this, one hears them say, well, look how many telescopes you can build with the money for LIGO. From the cosmologist's point of view, does LIGO seem like a worthwhile project? I, yes, but not even just for the cosmology. You know, this is, uh, NSF has a budget line for big projects, um, and so they spend a quarter of a billion dollars building, you know, Gemini, or building, uh, uh, what are some of the other things that they've built, LIGO, B-factories, things like that, and when you spend that amount of money, every once in a while you have to take a big swing, and, Sometimes you build bread and butter, which is Gemini, okay? So eight-meter telescopes will be very useful. They'll discover a lot of things, but it's kind of bread and butter. Every once in a while you have to take a big cut, take a big gamble. Detecting gravity waves would be fantastic. It would open a new window to the universe. It would provide a confirmation of a fundamental prediction of general relativity. And it might do some interesting science. Yeah. I support it because often on these big projects, people tend to be too conservative. We have to have guaranteed results. I'm not saying every project should be taking it. I haven't heard anything, no. I have heard that they probably will come in, and I don't know what we'll do. I have a reservation at the Q Club. Okay, great. Thanks.

17:30 So every once in a while, you should swing for the fences. And that's what I see LIGO as. And I think that astronomers have been very unfair in their criticism. I don't understand it, quite frankly. Because number one, it hasn't come from their budget. Number two, it was not their turn. They just got the Gemini telescopes. And we should be excited about each other's science. Sure. I'm always struck by this English sociologist that I've been working with, I noticed he has a tendency to say, boy, I like these physicists, they always support each other's work and feel that everybody should support each other when they get funded sort of for public consumption. He seems to think sociologists should... Well, astronomers have behaved very badly on this LIGO thing, And it's been simply stupid. It would be one thing if they had something to gain. I was on the NSF Astronomy Advisory Committee, and we were in the midst. NSF was very, very nice to astronomy on the Gemini. The Gemini was a project that was going south fast, and NSF could have easily pulled the plug on that project. It was having deep, deep problems. And NSF stood with it, in spite of the fact that many eminent astronomers were criticizing LIGO. Well, as a final question, I guess, from the cosmology point of view, do you see LIGO as primarily, and other projects like this, if they come to fruition, Do you see them mostly as providing tests or confirmation of existing theories in cosmology, or would you look potentially at least to... Discover something new? Yeah. I think, you know, whenever you... I mean, this sounds like motherhood and apple pie, but whenever you open a new window, there are real opportunities for surprises. And people will give the example of opening the X-ray window. that's a good example where there were things that were just undreamed of and so I think there could be some real surprises and certainly gravity waves

20:00 allow us to probe the universe way way back because the universe is transparent to gravity waves so anything that happened during the entire history of the universe that produces gravity waves we can see we can't see that in photons even neutrinos only go back So gravity waves do significantly better than the gravity waves? Yeah, they go back to the plank time in principle. So, potentially at least, projects such as IGO could be very fruitful. Well, interesting. Thanks very much. You're welcome.