Research

Cosmic Strings

Consider an ice cube, and you will still see defects in it: bubbles, cracks. These defects are not due to impurities in the water, but instead formed as a result of the water cooling into ice, what we call a phase transition. The formation of defects at phase transitions is seen in many systems in physics, including the early universe. I am interested in a particular class of defects called cosmic strings.

Cosmic strings are so named because they are long, thin objects. By "long", I mean "anywhere from a few meters to larger than galaxies", and by "thin", I mean "thinner than an atom"! We believe that a network of these strings formed very soon after the Big Bang, and has existed since then until the present day. This network consists of "infinite" strings, which stretch across the entire observable universe, and loops of strings. While we have not yet detected cosmic strings, it is this network of strings which represents our best chance at observing them.

These strings are less like shoelaces and more like rubber bands. By this I mean that they have a tension, and this tension causes them to move and oscillate, similar to how a stretched rubber band oscillates when you pluck it. And, similar to how that plucked rubber band creates waves in the air that you hear as sound, the string loops create gravitational waves in space that we hope to "hear" in experiments such as the LISA gravitational wave observatory.

How do we do this? The loops in the network have been oscillating for a very long time, and so they have emitted many, many waves at many, many different frequencies. In a crowded room, everyone's voices combine to form a background "buzz"; in a loop network, all of the individual loops' waves combine to create what we call a stochastic gravitational wave background. It is this "buzz" of strings that we hope to detect in LISA.

Why care about strings? Two major reasons. Firstly, when strings formed and what properties they have are determined by the underlying physical theory that created them. Not all theories create the same type of strings, and so we could use information about strings to constrain or rule out different models of our universe. Secondly, because strings have existed for a long time, we can use them to conduct cosmic archaeology: as different things happened in our universe, they might have left an imprint in the string stochastic background. By carefully studying the sound of strings, we can learn more about the history of our universe.

The majority of my work in cosmic strings has been focused on how strings self-interact via the gravitational waves that they produce. These waves carry energy, and so when one part of a string loop wiggles and makes a wave, that wave could travel to another part of the same loop and cause that second part to move in response. Over time, this self-interaction could change the shape of loops significantly enough that the stochastic background would be different than what we otherwise predict. This is important because when we do detect strings, we will need to know precisely what the network sounds like in order to understand what mechanisms could have created the strings!

Landscapes

When I say "the landscape", I mean the broad range of physics theories that try to explain the origin of our universe by using higher dimensions. This is "string theory", but we're not talking about the same strings as cosmic strings! It just so happens that the theories in the landscape also contain long, thin objects, and physicists like to name things in a straightforward fashion. We are, after all, the same people who called matter that doesn't reflect light "dark matter".

In the landscape, I am mostly concerned with statistics and characterization. What are all the different universes a given model could predict? Are those universes similar to ours or not? If, for example, a particular theory would typically lead to a universe in which the charge of an electron were a hundred times stronger than it is in ours, well, that's not likely to be the theory that led to the world we live in.

Related to these questions are questions of numerics and efficiency. Even low-dimensional models have huge numbers of possible solutions, depending on many different initial choices, and so to characterize them accurately requires us to solve many equations. Of course, we want to learn things sooner rather than later, and so we need to have both faster methods for solving these equations as well as efficient approximations to a full theory, which allows us to get almost the same amount of information from solving a much simpler problem.

String theory is a theory of quantum gravity, and so by better understanding the range of possible string theoretic models, we can gain a better understanding of the fundamental theory of physics. We can then better understand physical systems such as black holes and other strongly-gravitating objects.

Cosmic strings, Up-goer Five version

Here's an alternate explanation of my strings research, using only the ten hundred most common words.

I think about at things in space that are long but not wide, like how hairs are. But these things can be much much shorter than the hairs on your head, or much much longer. They can be so short that you can't see them, or so long that they could go from here to the sun and back hundreds of times, or anything in between. Let's call them space hairs.

The space hairs move around in space, and when they move, they make waves. These waves run into the space hairs and shake them, which changes what the space hairs look like. If the waves change what the space hairs look like, then the space hairs make new and different waves. So a space hair and the waves it makes are always changing.

We are trying to see the waves, but we haven't been able to yet. The waves are very small, so they are hard to see, but there are a lot of different space hairs, so we think we can see the waves as long as we look at all of the waves from all of the space hairs and add them together.

My job is to think about how the space hairs change when they shake, and also make pictures of them after they have been shaking for a while so we know what they look like now.