Listening to our universe with gravitational waves - From ComSciCon

ComSciCon is a workshop series organized by graduate students, for graduate students, focused on science communication skills. This is the third in a series of posts which showcases talent from the ComSciCon 2015, the national meeting in Cambridge, MA. You can find more details about the meeting here. We hope this can give an example of actual output that can come from conferences, the subject of upcoming blogposts. This piece is by Béatrice Bonga, a graduate student at Pennsylvania State University.

Imagine watching a movie with the sound off. How much would you enjoy and understand this movie? Surprisingly, this is what scientists who study stars, galaxies and other objects in the sky have been doing until now! Basically everything we know about our universe is through light: not just light in the visible range, but also through infrared light, x-rays and radio waves. But imagine that we could also "listen" to what happens in the universe, wouldn't that be fantastic? Scientists are at the verge of detecting gravitational waves, which just like sound waves are qualitatively different from light. This detection would "open our ears" and be an entirely new window to learn more about our universe.

Gravitational waves: ripples in space-time

So what are these gravitational waves? Space and time are kinda fluid, therefore scientists combine them in one object: space-time. Now light rides as waves on space-time, whereas gravitational waves are waves of space-time itself! These gravitational waves cause small fluctuations in the force of gravity that in principle one should be able to measure directly, but so far scientists have been unsuccessful in doing so. Why is that? In order to measure their effects, we need to be able to measure distances extremely precisely. The minimum precision required is like measuring the distance to the nearest star outside our solar system, Proxima Centauri, down to 10 micrometers (roughly one tenth of a newspaper page)!

Exciting new science on the horizon

Visionary scientists started this endeavor in the 60s, eventually resulting in aLIGO (advanced Laser Interferometer Gravitational wave-Observatory) in 2002. aLIGO is the biggest science project ever funded by the National Science Foundation. Currently, more than 650 scientists from 59 institutions in 11 countries are working together on this ambitious project. The first test runs are complete. We can expect the official science runs in September 2015, next month! Hopefully soon, gravitational waves will be detected.

My research: the influence of the expansion of our universe on gravitational waves

In 2011, the Nobel Prize for physics was given to a group of astronomers who showed in 1998 that our universe is expanding in an accelerated fashion. I study the effects of this expansion on gravitational waves together with my advisor Professor Abhay Ashtekar and fellow grad student Aruna Kesavan. Understanding gravitational waves mathematically is quite tricky, even without incorporating the expansion of our universe. Physicists argued for years whether gravitational waves are real or mathematical artifacts. In fact, even Einstein himself, after he predicted the existence of gravitational waves in 1917, was not so sure any more about their reality in the 1930s. Only in the 1960s was this resolved with the development of a new mathematical framework.

Unfortunately, this mathematical framework is not valid when you take the accelerated expansion into account. So far, physicists have done simple calculations to estimate the effect of this expansion on gravitational waves. However, the relevant equations are extremely intricate and it can be quite difficult to separate physics and mathematical abstractions in these equations. It is a little bit like your friend telling you that she earned 1000 points in a game - without telling you which game she played. In this analogy, the 1000 points is the mathematical abstraction, and determining what this number means is the physics part. Maybe she played a game in which the maximum score is 1000 points, then her result is fantastic! Or, she played a game with a maximum number of 3 billion points , then her 1000 points are not such a good outcome. Likewise, the equations can fool you too: sometimes it appears as if there are large gravitational waves, but in reality they are small or even non-existent! The new framework being developed removes all these ambiguities. This is essential for a better theoretical understanding of gravitational waves in our expanding universe. in turn it will help experimenters better interpret their data.

Conclusion

The sources that aLIGO expects to see gravitational waves from are relatively close by on cosmological scales and are therefore not expected to be affected much by the expansion of our universe. However, the development of the new framework that takes the expansion of our universe into account is crucial for a solid theoretical understanding of gravitational waves now. In the future, it might also be important for experiments. Since gravitational waves are an entirely new way of experiencing the universe - much like listening is completely different from seeing - this new theoretical framework may help us "listen better" to our universe.