A 60-year-old prediction just helped astronomers measure the expansion of the universe.
Astronomers recently used the light of a distant supernova to calculate how fast the universe is expanding. But one of the biggest challenges for modern astronomy is that we seem to have too many answers to that question. Eventually, though, all those seemingly contentious answers may help astronomers close in on the real number, called the Hubble Constant, which holds the keys to calculating the age of the universe and its eventual fate.
University of Minnesota astronomer Patrick Kelly and his colleagues published their findings in the journal Science.
How Fast is the Universe Expanding?
Somewhere in a galaxy far, far away, a massive star exploded in a supernova so bright that the Hubble Space Telescope could see it from Earth orbit. As the light from that distant galaxy traveled through space, it got warped, curved, and duplicated thanks to the gravitational lens of a massive galaxy cluster, and its image appeared in four different spots on the other side.
Gravitational lenses often create multiple images of an object because the object’s light might follow several different curving paths around the galaxy cluster (a different group of astronomers recently used this quirk of physics to watch how a supernova remnant evolved in a distant galaxy). In this case, Kelly and his colleagues saw four images of the galaxy but only three of them contained the supernova — so they kept watching the last one for news of the star’s explosive demise to arrive. And it did, about a year after the astronomers first spotted the supernova.
“This method involves predicting that we will see a supernova appear on the sky before it happens! It is incredible that our understanding of astronomy has gotten to the point where we can do this,” University of Chicago astronomer Daniel Holtz, who wasn’t involved in the study, tells Inverse.
By precisely measuring the time between the supernova’s first appearance and when it showed up in the last galaxy image, Kelly and his colleagues could calculate how far the supernova’s light had traveled from its distant galaxy, through the gravitational lens, and all the way to Earth.
“You can measure both sides of the triangle, and then figure out ‘How big is the universe?’” Kelly tells Inverse.
And by measuring the redshift of the galaxy cluster (the lens) and the supernova's home galaxy, Kelly and his colleagues were also able to measure how quickly both the cluster and the supernova are moving away from Earth as the universe expands.
"Together, that gives you a way to measure the expansion rate of the universe," says Kelly. It's a simplified explanation; in reality, the computer models and calculations took Kelly and his colleagues nearly seven years to complete. Eventually, they concluded that the universe is expanding at about 66.6 kilometers per second per megaparsec. In other words, everything is moving away from us, and it speeds up about 66.6 kilometers per second for every megaparsec it travels (give or take about 4 km/s/Mpc).
It’s Complicated, Though
The problem is that we know how fast the universe is expanding – in pretty much the same way that a person with five or six old-fashioned watches knows what time it is.
Over the last few decades, astronomers have used several other methods to measure the same thing — the shape of gravitational waves, the distance to far-off stars, and even reconstructions of the Big Bang based on the last echoes of radiation it left behind. And they’ve all come up with different — but close — answers.
Scientists’ best estimates for how fast the universe is expanding range from about 73 km/s/Mpc (by measuring the distance to certain stars called Cepheids, which dim and brighten at regular intervals, which help astronomers to measure how far away the star is), to Kelly and his colleagues’ new figure of 66.6 km/s/Mpc. Modeling what happened in the early universe by studying the cosmic microwave background — leftover radiation from the Big Bang — tells us the universe is expanding at 67.8 km/s/Mpc. And measuring the way gravitational waves disperse as they travel through space suggests something closer to 70 km/s/Mpc.
“The fact that different measurements of this number don't agree is troubling […] and keeps lots of astronomers awake at night,” says Holtz. “It could be problems with the measurement methods and observational data we're using. Or it could be a sign that there is something fundamentally flawed with our understanding of the universe.”
In other words, there may be something missing — or wrong — in the equations physicists use to describe how the universe works. One possibility, according to Kelly, could be that we need more precise measurements and better models of how dark matter is scattered throughout galaxy clusters like MACSJ1149.
Closing in on the Answer
Even so, physicists are narrowing it down.
“Our old arguments about the Hubble constant were the equivalent of not knowing whether someone was under 25 years old or over 50 years old,” says Holtz. “Now we're arguing about whether their age is 23 or 25. This is great progress! As our methods have improved, we're now in the situation where we have multiple precise birth certificates, and they disagree.”
Of course, the universe isn’t 23 or 25 years old — it’s more like 13.8 billion, and pinning down the Hubble Constant will help us calculate that age more exactly. And that’s where new calculations using new methods — like Kelly and his colleagues’ supernova measurement — will help.
“What you really want are totally independent measurements, but don't rely on the same assumptions or tools,” says Kelly. His team’s supernova calculations tackle the problem in a completely different way than Holtz’s work with gravitational waves or University of Chicago astronomer Wendy Freedman’s measurements of the distance to Cepheid stars or very bright red giant stars. With enough different approaches, astronomers can rule out uncertainties from one method by checking with another, and eventually, they can close in on a precise answer.
Predicting the Future
Kelly and his colleagues say that the fact that their new calculation is relatively close to others, obtained with other types of measurement, suggests that the supernova method is a useful new tool for astronomers. That’s pretty impressive for a method that was first suggested 60 years ago.
Back in 1964, Norwegian astronomer Sjur Refsdal proposed the idea that watching for a delay between different gravitationally-lensed images of the same supernova could give astronomers a way to calculate how fast the universe is expanding. But the method requires a supernova to happen on the far side of a galaxy cluster massive enough to create a gravitational lens — and that’s not exactly something you can just order from a lab equipment catalog.
“No one was able to find one for the next fifty years, and so this is the first example,” says Kelly. “But we named [the supernova] after him with the idea in mind that we could we could carry out this experiment.”
Kelly and his colleagues have recently found another supernova, magnified and duplicated by the gravitational lens of another galaxy cluster, that could also yield a measurement of the Hubble Constant. And new telescopes like the upcoming Vera Rubin Observatory could help them find more.
“I'm hoping that there will be quite a number of these. We should be able to find more of these events with the Rubin telescope, which is starting up in Chile,” says Kelly. “It'd be great to have a whole bunch of independent measurements and see if they agree or not.”
