## The fate of the universe rests on the Hubble constant, so why can't we figure out its value?

#### In this episode of our podcast, assistant professor of physics Marcelle Soares-Santos explains why the constant is so difficult to calculate and how her new approach might finally get it right.

Below is a transcript of the episode.

LAWRENCE GOODMAN, HOST

Hello, and welcome to this edition of the Brandeis University podcast, "The Take: Big Ideas Explained in Under 5 Minutes," where professors explain core concepts of their research in under five minutes.

Our topic today is, "The fate of the universe rests on the Hubble constant, so why can't we figure out its value?"

I'm Lawrence Goodman with the Office of Communications, and my guest is assistant professor of physics Marcelle Soares-Santos.

What is the Hubble constant?

ASSISTANT PROFESSOR OF PHYSICS MARCELLE SOARES-SANTOS

The Hubble constant is a measure of the rate of growth of the universe.

HOST: Currently, there are two ways of measuring the constant.

SOARES-SANTOS: One method is using objects that we call standard candles. Supernova explosions are the prime standard candle that we use. The observed brightness is going to tell us how far away the object is. We also observe the colors of these objects. And as the light from the object travels to Earth, spacetime is expanding, so the waveform of the photons traveling through time is getting stretched out. So the object will appear redder. This redshift effect is proportional to the distance. So an object that is twice farther away will be twice more redshifted than an object that is closer.

The second method is to look at the universe as it was in the very, very early stages. We are measuring today the cosmic microwave background radiation, which is a signal that propagates through the universe since 300,000 years after the Big Bang. So what we do is to compare the shape of that signal with what we would expect given a certain rate of expansion. Different rates of expansion will result in a different signature for the observed radiation.

HOST: And yet both of these methods aren't quite right.

SOARES-SANTOS: If you take the number of the Hubble constant measured via supernova and compare with the number measured via the cosmic microwave background, they disagree significantly. It's only a few percent of disagreement.

HOST: The future of the universe rests on what the value of the Hubble constant is, correct?

SOARES-SANTOS: Correct. The value of the Hubble constant will set what is the rate of expansion today, what is the fate of the universe in the future. For example, if the rate of expansion is too high, that would mean that our universe would tend to continue expanding forever. The rate of expansion will become so large that the gravitational forces that keep our solar system together would not be sufficient to keep it together, so we would be flying away in the solar system.

If that number is too low, then we would have evidence for a future in which the expansion could actually halt and reverse, so that we would have what some people call sometimes the Big Crunch, the opposite of the Big Bang,

HOST: You and a team of researchers recently reported a new way to calculate the Hubble constant using gravitational waves.

First, what are gravitational waves?

SOARES-SANTOS: Gravitational waves are ripples in space that propagate through spacetime.

HOST: Now how can you use these waves to get at the Hubble constant?

SOARES-SANTOS: Take for example, when we hear the sound of thunder. Depending on how loud it is, we get an idea of whether it was far or nearby. In the case of gravitational waves, the idea is similar. We detect the sound of the gravitational waves on Earth. That analysis of the waveform tells us how massive the system was, how loud that signal should be. Now we can compare what is our estimate of how loud it should be with how loud it actually is. And with that, we estimate how far the object is located.

HOST: So you have at this point a value for the Hubble constant.

SOARES-SANTOS: The value that we measure using this method is actually halfway between the value that we got with supernova versus the cosmic microwave background.

HOST: So there's still a lot of work left to be done.

SOARES-SANTOS: So far, we did this measurement using only one event, and by that I mean one collision of two neutron stars that generated gravitational waves and we detected here. Now we want to repeat that with many, many more events. And as we speak, the gravitational wave detectors are up and running and we are hoping to accumulate more of those events, more neutron stars in collisions, so that we can make a more precise measurement and determine whether the supernova or the cosmic microwave background is the correct answer.

HOST: And there you have it, "The fate of the universe rests on the value of the Hubble constant, so why can't we figure it out?" explained in under five minutes. You can find this podcast on iTunes, Spotify and SoundCloud, so we hope you'll subscribe and keep listening to "The Take: Big Ideas Explained in Under 5 Minutes," brought to you by Brandeis University.

Categories: Research, Science and Technology