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Researcher likens gravitational waves to a cosmic thunderstorm in an effort to unravel the universe’s strange expansion
Erin Broberg

“How does it feel knowing that 95 percent of the universe you inhabit is utterly unknown?”

When this question was posed to physicist Marcelle Soares-Santos, she paused, considering its implications.

Today, just 5 percent of the universe is composed of matter we can see, touch or measure. Water droplets, eyelashes, computer screens, distant planets and supernovae—all infinitely complex in their own right—are often unimpressively categorized by physicists as “ordinary matter.” What remains to be explained is often—ominously—called the “dark sector.” Dark matter, an unknown substance that exerts gravitational force on clumps of matter in the universe, makes up about 27 percent of the universe. The remaining 68 percent, called dark energy, is the force responsible for propelling the ever-expanding universe.

Soares-Santos, like the rest of us, exists in this curious reality. But she is also part of the global effort to better understand the largest missing piece of that reality: dark energy.

“On one hand,” Soares-Santos began, “it could be a bit overwhelming to think that we only know what is going on in 5 percent of the universe. But, if we look at how we are able to transform the world around us with knowledge of only 5 percent, I think that creates a very strong motivation to expand our knowledge into the remaining 95 percent.”

Soares-Santos is a Brandeis University researcher and a member of the Dark Energy Survey (DES) Collaboration and the Large Synoptic Survey Telescope Dark Energy Science Collaboration (LSST/DESC). Last week, she visited the Sanford Underground Research Facility (Sanford Lab) and spoke with us about her research into the dark sector. The interview has been condensed and edited for clarity.

If you don’t know exactly what dark energy is, how do you know it exists?

After the Big Bang, the universe rapidly expanded, structures formed, and everything cooled down. Today, the universe naturally continues to expand; you don't need dark energy to explain that. That is expected.

What we cannot explain is the fact that the universe’s expansion is speeding up. The expansion rate of the universe is growing, not slowing down. To explain this, you need to be adding some extra amount of energy to keep feeding this expansion.

Dark energy is the label that we have for that extra energy. It's some form of energy—completely unexplained in the framework of physics that we have right now—that causes the expansion rate of the universe to accelerate. 

How are researchers attempting to study this “unexplainable” force?

We cannot replicate dark energy in the lab or even make a detector that might capture some sort of dark energy particles. The only way that we can study dark energy right now is by observing astrophysical objects.

Can you explain the Hubble Constant, or how fast the universe is expanding, and why it is vital to understanding dark energy?

The Hubble Constant is the anchor, or reference point. If you know how quickly the universe is expanding today, then you can start measuring how the rate of expansion is changing over time. The Hubble Constant is a number that we already have, but one that we want to know better.

One way to measure the Hubble Constant is to observe large areas of the sky with galaxy surveys. We look at the spatial distribution and the evolution of individual galaxies and of clumps of galaxies. If dark energy is having an effect on those galaxies, we would be able to notice it at that large scale.

Another way is by studying supernova explosions of stars at the end of their lives. These cataclysmic events have a standardized brightness. That is great for us, because we see it far away, and we can tell the distance by measuring the observable brightness.

What is also fantastic is that these different methods produce results that disagree with each other, within a few percent.

Wait, why are you excited that your measurements disagree?

Often times this disagreement is framed as a problem or even a crisis, but I actually see that as a big opportunity. Assuming the disagreement is not a problem in the way the measurement is made, which I don’t think it is, then it is really a clue. When there are tensions between what you expect to see and what you do see, it usually means unknown physics are in action.

How are you working toward a better measurement of the Hubble Constant?

One problem with using supernovas, for example, is that there are tiny variations in the environment where each supernova explodes, causing the light to appear a bit brighter or fainter. Because we don't control those environments, because we don't build them in the lab, we have to constantly be mitigating those systematic effects.

The best way of bypassing that problem is having independent methods, such as the new method that I'm working on with DES. We are trying to combine sky survey information with new information from gravitational wave detectors (Laser Interferometer Gravitational-Wave Observatory and Virgo) as they detect collisions between neutron stars and black holes. 

For 30 years, people have been suggesting gravitational waves as such a method, but only now do we have the sensitivity to pursue it.

Just last week, news broke that LIGO and Virgo picked up on something really exciting. What exactly did they see?

It was the first time the gravitational wave detectors were able to capture a collision of a black hole and a neutron star. So far, we had seen black hole/black hole collisions or neutron star/neutron star collisions, but this was the first mixed event. We did expect that this type of collision should exist, but it's really exciting to see for the first time. 

Other than being an incredible thing to imagine, a black hole consuming a neutron star in distant space, what does this detection tell us about dark energy?

The principle is comparable to a thunderstorm. If you hear thunder, you can tell approximately how far away it is. That's because we intuitively have an idea of how loud a lightning strike is.

In deep space, these massive objects—black holes and neutron stars—collide. While a lightning strike produces thunder in the form of sound waves, this collision will cause gravitational waves. By analyzing the shape of the gravitational waves, we can tell how “loud” it was. Knowing how intrinsically strong these events are and how faint the signal is when detected on Earth, we can determine how far away it occurred.

What do you hope this field looks like 50 years from now? 

Assuming we nail down the rate of expansion of the universe with a very, very tiny error bar, then we will be able to study the physical mechanisms that are making those measurements appear to be discrepant.

Perhaps dark energy is not constant in time and space. Maybe it is slowly evolving as the universe changes, becoming stronger over time. Or maybe dark energy behaves differently in different environments. I think that we are getting to a point that we can finally start addressing those questions.