LIGO Webcast Transcript
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LISA-JOY ZGORSKI: Examining gravitational waves is the only way to directly probe the universe at the moment of its birth. They're unique in that regard; detecting them is the enormous challenge. Today, with LIGO, the Laser Interferometer Gravitational Wave Observatory, we're beginning to tackle that challenge. The LIGO scientific collaboration, with nearly 700 scientists from over 60 institutions in 11 countries is making great gains. Its findings are detailed in the first major paper to result from LIGO research on the early universe. It will appear in the journal Nature tomorrow. I'm Lisa-Joy Zgorski with the National Science Foundation. Joining me today is one of the lead authors of that paper, physicist, Vuk Mandic. Vuk is an assistant professor at the University of Minnesota. He serves as co-chair of the stochastic working group of the LIGO scientific collaboration.
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Vuk, thanks for joining us.
VUK MANDIC: Thank you for having me.
LISA-JOY ZGORSKI: Beverly Berger, an NSF program manager for gravitational physics, is excited by this paper. She asserts that LIGO is making real astronomical measurements and looking at the universe in a completely new way. But it's my understanding that LIGO actually has not yet detected gravitational waves. Is this much ado about nothing?
VUK MANDIC: Absolutely not. I would share Beverly's excitement and I think this result is a hint of what is to come in the future of the gravitational wave field. You're absolutely right; we have not observed gravitational waves. We have also not observed the stochastic background of these waves. But even this non-detection allows us to rule out some of the models of the stochastic background that predicts relatively loud or strong stochastic backgrounds of gravitational waves, and we can, in this way, start essentially learning how the universe is not like or what the universe is not like. So, in this sense, this is a major milestone in this field. It is important to appreciate the fact that this is really the first time that a direct search for gravitational waves of this type has reached sensitivity sufficient to start making statements about cosmological models and about models of the early universe and evolution.
LISA-JOY ZGORSKI: For those listening in, they may not have a science background. You mentioned stochastic waves and, in fact, you're the co-chair of the stochastic task group or working group. Can you kind of disassemble that term and tell the audience what, in fact, that entails?
VUK MANDIC: Sure. So maybe I can say a few words what the gravitational waves are first. So, general relativity predicts existence of these waves. They don't exist in Newtonian physics and essentially, they're produced by mass distributions which are sufficiently complicated and in the physics jargon, we say that they have quadruple moment. If such distributions accelerate, they produce gravitational waves. An example of this would be, say, two stars or two black holes which revolve around each other and eventually merge. As they do that, they perturb the spacetime around them, and these perturbations or small ripples in the fabric of spacetime travel through space much like the usual electromagnetic waves, like the light. So, for example, they travel at the speed of light and so on. Now, if you have many sources of these gravitational waves, then you get a mixture, which we call a stochastic background and a good analogy is to imagine, for example, a surface of the pond on which you have many, many waves, different amplitudes, different directions and so on and the mixture of them creates some sort of a mess on the surface. This is exactly what we're looking for.
LISA-JOY ZGORSKI: Earlier you mentioned incoherent sources of these waves. Can you say a little more about that?
VUK MANDIC: So, these are sources which are not correlated. They're producing gravitational waves which are of different amplitudes. They're not necessarily going in phase. They may be in different polarizations and so on. Contrary to, for example, having a single source which may be, say, periodic and very well defined. In this case, what we get is a stochastic or a random signal which, in our detectors, appear essentially as an elevated noise.
LISA-JOY ZGORSKI: The result rules out some Big Bang models. In your estimation, what existed before the Big Bang?
VUK MANDIC: Well, this is a very difficult question to answer because we really have no observational data for what the universe was like before the Big Bang or even if it makes sense to talk about it. There are some theoretical models, for example, the pre-Big Bang models, which are alternatives to inflation and we can constrain some of those models as well through as they also make predictions of what the stochastic background would be like. So, we could, in particular, rule out some values of the parameters of those models but we certainly couldn't rule all of them at this point.
LISA-JOY ZGORSKI: At this point, I want to remind journalists that if you'd like to enter the queue to ask a question, please dial *1 on your touchtone phone. You may also send questions via email at firstname.lastname@example.org.
This is the first time that a gravitational wave measurement was used to make a statement about cosmology. Is that right?
VUK MANDIC: That is correct.
LISA-JOY ZGORSKI: And what does that mean?
VUK MANDIC: So, this result, this is the first time, as I mentioned earlier, that the sensitivity of this kind of research is sufficient that we could start probing some of the models of cosmology in the early universe. Intuitively, the picture is something like this. Starting from about one minute after the Big Bang, which is the time of the Big Bang nucleosynthesis where the lightest nuclei were formed and all the way to today, we understand the evolution, at least, we believe we understand it, the evolution of the universe very well and it follows very well known laws of physics and nuclear physics, particle physics, cosmology and so on. However, when the universe was less than one minute old, in that very first minute after the Big Bang, we have very little observational data and, moreover it's very difficult to reproduce those conditions in the lab because very large energies are required and we simply don't have them or cannot reproduce them in accelerators, for example, and so on. On the other hand, gravitational waves that may have been produced in that very early universe, they would have traveled until today essentially unaltered so they would carry the information of what the universe was like in those very early times, and this is very unique. This is one of the reasons which makes this result and this kind of approach very, very exciting. This may really be the only way for us to probe such early times in the evolution of the universe.
LISA-JOY ZGORSKI: Thanks. We have a question that's been submitted from Dan Rogano at USA Today. So what cosmological models can you rule out from this result or non-result, as it were? Did you expect to see gravitational waves at this point?
VUK MANDIC: So, we have ruled out some of the theoretical models, which are perfectly viable so we could have gotten lucky in some sense and observed the stochastic background already. However, it's probably not a surprise that we have not observed them yet. What we can do at this point, what our sensitivities allow us to say is that, for example, what we call stiff energy components in the very early universe are not very likely. Or, to put it in other words, we can constrain parameters that describe the stiffness of the energy components in the early universe based on our measurement. And by stiff here, I should clarify. The stiff energy component would be such that a small change in density would create a large change in the pressure at the time which, at the moment, we are not aware of any existence of such a component.
LISA-JOY ZGORSKI: And what would the process be like of eventually using LIGO to detect gravitational waves? How does this further that process?
VUK MANDIC: As I mentioned, this is really a hint of what we hope that the field will give us in the future. First of all, I should say that this is a result of work of, as you mentioned in the introduction, of hundreds of people, scientists, engineers, technicians and so on working over several decades and also it's a result of a tremendous investment from the side of the NSF into this field. All of this effort has essentially led to the current sensitivities of our detectors, which are, as I said, sufficient to start making some of these interesting statements about cosmology and about astrophysical sources such as pulsars, theoretic sources and others, but we’re also expecting tremendous advances in the coming years. For example, advanced LIGO is already funded and the sensitivity of this detector or these detectors will be at least a factor of ten better than what we have now because, for example, if you're looking for transient sources such as pulsars or merges of neutron star pairs or black hole pairs and so on, a factor of ten improvement allows you to explore a factor of 1,000 larger volume of the universe. Similarly, for the stochastic background, we except advanced LIGO to give us another factor of 1,000 in sensitivity, and all of this will go a long way in probing various models of stochastic background and of many different types of gravitational wave sources. So, we really expect that with advanced LIGO, a new era will begin in which we will be probing astrophysics and cosmology and astronomy in completely new way to a larger sense complimentary to what we currently do with the optical and other electromagnetic observations.
LISA-JOY ZGORSKI: I've heard the term cosmic string theory brought up in this regard. Is this something that will be furthered or can you explain -- put that into context?
VUK MANDIC: So cosmic strings are objects which hypothetically may have formed in the very early universe as well and, moreover, in recent years, there has been advance in the string theory which suggests that the fundamental strings, the string theory strings could have been blown up to cosmological scales and they could exist, essentially flow through the universe today. Such strings could also emit gravitational waves. For example, if they have kinks or cusps on them which are moving at very high speeds close to the speed of light, they could emit bursts of gravitational waves, or if we sum up over all of the bursts in the entire universe, then we would get a stochastic background. And again, our result is constraining these models as well. We have already ruled out some of the possible parameter values, which means that if these objects exist, their parameters that describe them, such as the string tension, for example, will have to conform with our observations. And again, with advanced LIGO, we are expecting tremendous improvements and we will explore a large fraction of the parameter space in these models.
LISA-JOY ZGORSKI: Yes, to that point, Graham Collins from the Scientific American has a question. Could you briefly outline plans to increase LIGO sensitivity in the future and the status of work on carrying that out?
VUK MANDIC: Sure. So, advanced LIGO will essentially use the existing LIGO facility but all of the subsystems of LIGO will be significantly improved. This includes, for example, using a much more powerful laser which will give us an increase by the factor of ten over most of the frequency range. In addition, there will be a much more sophisticated active and passive seismic and other vibrational isolations who will have quadruple pendula suspensions instead of just single pendula that we have now and all of this will allow us to suppress the vibrations either due to the seismic noise around the detectors or the motion of people or trucks and so on, any other local disturbances, and all of this will not just push the sensitivity but it will also increase the sensitive band. For example, at the moment, LIGO is sensitive to frequencies above 40 hertz. With advanced LIGO, we hope to go down to 10 hertz or so, which will open up a window to a larger number of gravitational wave sources as well. Advanced LIGO has been funded, last year I believe, and we are, at the moment, essentially finalizing all of the designs and starting to make purchase orders and pulling all of the pieces together. In 2011, we're hoping to start the construction or the installation of advanced LIGO and the first data is expected to arrive some time in 2014.
LISA-JOY ZGORSKI: Great. I just want to remind the audience that this webcast will be posted on the NSF Web site after its conclusion. I also want to, anyone lingering on the phone, *1 on your touchtone phone in order to ask a question is the quickest, most efficient way.
You mentioned gravitational waves are ripples in space and time. That's provocative. What exactly does it mean?
VUK MANDIC: So, gravitational waves are essentially exactly a ripple in spacetime. More specifically, they're a perturbation in the metric which allows us to calculate distances between two points in space and time. More specifically, the physical manifestation of gravitational waves is that they will stretch and shrink the dimensions of space, which are ortogonal to the propagation of the wave and they will do that in an alternating manner. So, while stretching one ortogonal dimension, the other one will be stretched a little bit. So, if you think about it, an ideal machine, if you like, to search for such an effect is an interferometer. So, what we do is we take a laser beam and we split it into two halves and each half goes through an arm of the interferometer which has an L shape, and at the ends of the arms we have mirrors which then reflect the two beams back and they get superposed at the vertex of the interferometer. So, if the gravitational wave fact is true, it will slightly stretch one arm and slightly shrink the other, which will mean that the light in the two arms travel slightly different distances and when the two beams come back, they don't perfectly superpose anymore. There is slightly less, slightly smaller light intensity coming out of the interferometer and this fluctuation of the intensity is what we measure. So that's what tells us that our gravitational wave is passing through or not.
LISA-JOY ZGORSKI: Interesting. Here's a question. Gravitational waves manifest in shrinking and stretching distances between free-falling objects. How so, and do they somehow warp speed?
VUK MANDIC: I wouldn't quite go as far as warping speed or space. They are really doing exactly what I just described. They stretch one ortogonal dimension to the propagation of the wave and shrink the other and this effect is essentially the effect that we are trying to measure.
LISA-JOY ZGORSKI: This has been terrific. Is there anything else you'd like to add about your research or findings about LIGO in general?
VUK MANDIC: Just that we are very excited about this and that we are looking forward and expecting a lot more exciting news in the coming decade.
LISA-JOY ZGORSKI: Thank you. Again, the full broadcast will be posted on the NSF Web site. Thank you for joining us.
VUK MANDIC: Thank you.