Everyone has been pretty excited by the recent observation of gravitational waves. I know that I am prone to exaggeration, but gravitational waves really do open up a new way to observe the Universe.

At the moment, when we observe the night sky, the farther into the distance we look, the further back in time we see. But relationship is based on an assumption: the light we see has not bounced off anything in between us and its origin. Normally, this is a pretty safe assumption, because space is pretty big, and most of the material in it (like dust, etc) doesn't do much.

But in the very early Universe, before atoms had formed, things were very dense, so light scattered a lot. The scattering means that the information that a photon carried about its origin was lost. As a result, we can't really see much beyond the time when all the charged particles all agreed to stick together and create the first three elements of the periodic table.

This is not the case for gravitational waves. Gravitational waves will generally pass through the early Universe and not notice the electrons and protons that we think of as ordinary matter. So the waves will carry information about the events that created them. And those rare moments when they scatter? We might get information from that, too.

The matter of the early Universe is just one playground, though. The other exciting possibility is to use gravitational waves to observe objects that do not emit light. These events and objects will be visible due to how they scatter gravitational waves. What might these objects be? In addition to the usual suspects, like black holes, are exotic things like cosmic strings. Yes, strings... in space.

String theory is not something I wish to discuss much, except to say that it is one approach to providing a unified description of quantum mechanics and gravity. Unfortunately, the strings of string theory are not easily observable—doing so would require energies much higher than our current colliders are capable of. But the early Universe is exactly when the energy scales that are relevant to string theory were available. At this point in time, there may well have been strings floating free in space, doing what strings do (vibrating).

Most of these early strings shuffled off their mortal coils and joined the choir invisible, but the more hopeful string theorists think that a few might have survived. Under this hypothesis, the survivors have gone on to become cosmic strings, floating through space feeling very lonely and wondering what they did wrong (and probably wishing that their name didn't make it sound like it belonged to a cult).

Since strings represent a little piece of space-time that is tightly coiled up on itself, a gravitational wave will scatter off of it. But what would this look like, and could we observe it? This is exactly what a pair of Spanish physicists have tried to calculate.

They pictured a string as a sharp crease in space-time and calculated the propagation of a gravitational wave through the crease. They showed that the wave pattern far away from the defect looks exactly the same as you would observe from a light beam that scattered off a thin wire. This meant that the researchers could use all the tools of classical optics to calculate what the spatial patterns of the gravitational waves would look like and how these patterns would spread out in space and time.

From our point of view, what we would see is a characteristic intensity pattern of gravitational waves. Essentially, the waves that passed to the left and the right of the string are bent so that they pass through each other and interfere. This results in strong gravitational waves propagating off at some angles and no gravitational waves at other angles. By measuring this pattern, we would learn a lot about strings. And, indeed, to observe such a pattern would be a fairly good indication that there might be such a thing as a cosmic string.

Unfortunately, it's not going to happen any time soon. The big problem is that for strings from the early Universe, the pattern is spread out over the entire Universe, so we won't see any intensity fluctuations; instead, all we'd see is a complicated mixture of scattering from many, many strings.

For cosmic strings there are two problems. We need to be lucky—if the string has just the right properties, we might just resolve the intensity fluctuations as the Earth orbits the Sun, but otherwise, the pattern is either too closely or too widely spaced. Unfortunately, the authors don't really give any hint of where this sweet spot might lie in terms of actual string dimensions.

The second problem is that, at present, we detect gravitational waves from transient events. We don't have six months to observe from different locations, and we can't resolve any patterns. Even when more gravitational wave detectors come online, we won't have the resolution to see these patterns in transient signals.

The authors acknowledge this by doing what seems to be a common thing in cosmology now, suggesting that their calculations can be used to help us understand solid state physics. However, I'm not convinced that this particular calculation adds anything compared to standard wave theory.

String theory and its hip cousin M-theory are notorious for making predictions that we can't actually test. While that is not due to the lack of desire on the part of theoreticians, some would argue that this places string theory outside of the realms of science. I'm not that strongly opinionated on the subject, because I think a multitude of approaches to a problem is best, which means you have to accept some waste in the process.

This paper, though, shows that some types of observations may offer a dim ray of hope to those of us who think observational evidence is important. Why? Current instrumentation and techniques may make this observation out of reach, but that doesn't mean that it will always be out of reach.

More importantly, this is just the beginning of gravitational wave astronomy. Last year, calculating the gravitational wave signature of stuff that we might observe was largely of academic interest. Now, however, it is the stuff that will drive our observatories.

Quelle: ars technica