Two Stars Slammed Into Each Other And Solved Half Of Astronomy’s Problems. What Comes Next?
Filed under Space
Progress, as they say, is slow. In science, this is often true even for major breakthroughs; rarely is an entire field of research remade in a single swoop. The Human Genome Project took a decade. Finding the first gravitational waves took multiple decades. So it’s hard to overstate the enormous leap forward that astronomy took on Aug. 17, 2017.
On that day, astronomers bore witness to the titanic collision of two neutron stars, the densest things in the universe besides black holes. In the collision’s wake, astronomers answered multiple major questions that have dominated their field for a generation. They solved the origin of gamma-ray bursts, mysterious jets of hardcore radiation that could potentially roast Earth. They glimpsed the forging of heavy metals, like gold and platinum. They measured the rate at which the expansion of the universe is accelerating. They caught light at the same time as gravitational waves, confirmation that waves move at the speed of light. And there was more, and there is much more yet to come from this discovery. It all happened so quickly and revealed so much that astronomers are already facing a different type of question: Now what?
“Even people like me, who have been waiting for this for a long time and preparing for this, I don’t think we’re ready,” said Edo Berger, an astronomer at Harvard who studies explosive cosmic events. “Now it’s a question of, do we have the right instrumentation for doing all the follow-up work? Do we have the right telescopes? What’s going to happen when we have not just one event, but one a month, or one a week — how do we deal with that flood?”
From Copernicus and Kepler to Hubble and Einstein, astronomy has experienced plenty of tectonic shifts. The discovery of GW170817, as the August event is known, will be another of these. Astronomers often describe the detection of gravitational waves — which happened for the first time just last year, and was awarded a Nobel prize in October — as a new form of perception, as though we can now hear as well as see. The neutron-star merger event was like seeing and hearing at the same time, and with a dictionary to make sense of it all.
The Aug. 17 gravitational wave gave astronomers a glimpse at an entirely different universe. For most of history, they’ve studied stars and galaxies, which seem static and unchanging from the vantage point of human timescales. “You can look at them today and look at them 10 years from now, and they will be the same,” Berger said. But GW170817 revealed a universe alive, pulsating with creation and destruction on human timescales. Think about that: GW170817 was a relatively close 130 million light years from Earth, meaning its gravitational waves and light were emitted while the first flowering plants were busy evolving on Earth, around the time stegosauruses roamed the plains. But the event itself unfolded in less than three human-designated weeks. This faster timescale is “pushing the way astronomy is done,” Berger said.
When the wave crashed through Earth, it caused a tiny shift in the path of laser beams traveling down long corridors in observatories called the Laser Interferometer Gravitational-Wave Observatory (LIGO), in the U.S., and the Virgo interferometer, in Italy. On Aug. 17, LIGO’s twin detectors and Virgo each felt the wave, which allowed astronomers to roughly triangulate from which direction it rolled in. They swung every bit of glass they had, both on Earth and in the heavens, in that general direction. In space, the Fermi space telescope glimpsed a burst of gamma radiation. Within an hour, astronomers made six independent discoveries of a bright, fast-fading flash: A new phenomenon called a kilonova. Astronomers saw the telltale sign of gold being forged, a major discovery by itself. Nine days later, X-rays streamed in, and after 16 days, radio waves arrived, too. Each type of information tells astronomers something different. Richard O’Shaughnessy, an astronomer at the Rochester Institute of Technology, describes the discovery as a “Rosetta stone for astronomy.” “What this has done is provide one event that unites all these different threads of astronomy at once,” he said. “Like, all our dreams have come true, and they came true now.”
As O’Shaughnessy put it, every discovery eventually becomes a tool. Astronomers hope to use neutron-star mergers to test general relativity, the mind-bending conceit that what we perceive as gravity is actually a curving of space and time. Binary neutron stars and black holes may deviate from the gravitational fields predicted by general relativity, which could put Einstein — and alternative theories for gravity in extreme systems — to the test, said Jacqueline Hewitt, a physicist who directs MIT’s Kavli Institute for Astrophysics and Space Research.
Gravitational waves aren’t blocked by dark matter, dust or other space objects, so they can serve as messengers from the insides of stars, Hewitt said. When LIGO upgrades are finished next year, astronomers will be able to investigate how the waves form and reconstruct the violent smashups.
Eleonora Troja, an astronomer at NASA’s Goddard Space Flight Center who studies X-rays, had hoped for years to detect the light from a neutron-star merger, but many people thought she was dreaming. “I had a lot of proposals rejected because they were considered too visionary, too advanced,” she said. But even Troja never imagined witnessing what happened this summer. “Sometimes, I am still like, ‘Did that really happen?’”
Troja says that the information gathered in August could eventually serve as a template for finding other neutron-star collisions and gamma-ray jets. We may have already unwittingly captured evidence of many such events, but the record is likely buried in a decade’s worth of data from the Fermi and Swift gamma-ray space telescopes, waiting to be uncovered. Those observatories, and new ones under construction now, will allow humanity to see even more violent, rapidly changing astronomical phenomena. The Large Synoptic Survey Telescope, for example, is currently under construction and will eventually photograph the whole sky every three nights. “In the future, when we digest all this information, it will be a drastic change in the way we study these cosmic objects,” Troja said.
This event that unfolded across a couple of weeks will also inform our deepest experience of time, the beginning and the end of our cosmology. Combinations of light and gravitational waves, like those detected after the neutron-star merger, can be used to measure the rate at which the expansion of the universe is accelerating.
“It’s totally new,” Troja said. “Comparing the two independent measurements, the one from light and the one from gravitational waves, you can measure the rate of the expansion of the universe.” All our futures are wrapped up in this question.
Thanks to the Aug. 17 event, astronomers now know what to look for. Soon, they will be able to sift through an embarrassment of neutron-star mergers and other phenomena. And as with any disruption, there will be a period of adjustment. Huge telescopes in space and on Earth are few and far between, and on Earth, most of them can only work when it’s dark and the skies are clear. That means thousands of people vie for limited time at the proverbial eyepiece. Telescope committees are set up to review proposals and grant that time, and assignments are often allotted months in advance. That will have to change as astronomers chase events in real time.
“In our case, for the telescopes we were using in Chile, people traveled to Chile to use the telescopes, and we asked them to give up their time [to track the Aug. 17 event]. And everybody did it with so much enthusiasm,” Berger said, adding that anyone who sacrificed hard-won telescope time was credited in the scientific literature. “But we need better mechanisms. You can’t call up every individual person and negotiate and explain, especially when these objects are fading away so quickly, while you’re on the phone with them.”
Hewitt is chair of a committee that develops 10-year plans for astronomy, known as the decadal surveys, and said the detection of gravitational waves — and neutron-star mergers — were listed as goals for the next several years in the most recent report in 2010 and in the mid-point report in 2015. We got there early, and now astronomers are talking about how to prioritize their time, where to focus, and how to pivot to the next big thing, she said. Many are now hoping that the U.S. rejoins a space-based gravitational wave experiment called LISA. And they are talking about how to turn their eyes to the sky, at a moment’s notice, the next time the universe throws something big their way.
“It’s a wonderful time, it’s a terrifying time,” O’Shaughnessy said. “I can’t really capture the wonder and the horror and glee and happiness.”