September 23, 2021, 4:19

Gravitational waves just led us to the incredible origin of gold in the universe

Gravitational waves just led us to the incredible origin of gold in the universe

The Nobel Prize–winning LIGO observatory has already changed the world of astronomy. When the scientists in the LIGO collaboration announced the first detection of gravitational waves in 2016, it meant they’d discovered a new way to observe the universe. For the first time, scientists could “listen” to ripples in spacetime created by the collision of massive objects like black holes.

But that was just the beginning. The dream, all along, was to combine gravitational wave detections with observations from more traditional telescopes.

On Monday, a team of thousands of LIGO scientists around the globe published an incredible finding spread throughout several papers in the journal Physical Review Letters. Not only did these scientists detect, for the first time, the gravitational waves produced from two colliding neutron stars, but they were able to pinpoint their location in the sky and witness the event with optical and electromagnetic telescopes.

“It’s one of the most complete stories of an astrophysical event that you could possibly imagine,” says LIGO physicist Peter Saulson at Syracuse University.

Each data source tells a different part of the story.

The gravitational waves tell physicists how large and how far away the objects are, and allow scientists to recreate the moments before they collided. Then the observations in optical light and electromagnetic waves fill in the blanks that gravitational waves can’t answer. They help astronomers nail down exactly what the objects were made out of, and which elements their collisions produced. In this case, the scientists were able to conclude that the resulting explosion from a neutron star merger produces heavy elements like gold, platinum, and uranium (which has been previously theorized but not confirmed by direct observation).

These scientists were able to witness, directly, the alchemy of the universe in action.

“I think the scientific impact of this discovery is actually going to be bigger than the first detection of black holes from gravitational waves,” Duncan Brown, another LIGO collaborator also at Syracuse, says. “There is so much more physics and astronomy involved.”

And it’s all the result of an amazing worldwide treasure hunt among the stars.

A race against the clock. A cosmic treasure hunt.

On August 17 at 8:41 am, LIGO detected gravitational waves — literal distortions in space and time — passing through Earth. LIGO is a pair of L-shaped observatories in Washington state and Louisiana that can detect when these waves temporarily squish and stretch the fabric of spacetime around us. In the past two years, LIGO had detected gravitational waves generated by black holes that had crashed into one another.

But this detection was very different.

For one, the signal was much stronger than the ones from the black hole discovery, which suggested it was much closer to Earth. It lasted 100 seconds, whereas the black hole signals lasted just a few.

When LIGO detects gravitational waves, it automatically sends out alerts to hundreds of scientists across the world. Brown was one of them. “We got on the phone very quickly, and we realized this was a very loud gravitational wave signal. It blew our socks off,” he says.

Immediately apparent: This was no black hole merger. The initial analysis revealed that the waves were generated by the collision of two neutron stars — extraordinarily dense, strange objects thought to be the cauldrons in which heavy elements are alchemized. Brown’s heart started to race.

When LIGO detects gravitational waves from colliding black holes, there’s nothing to see in the sky. Black holes are, as their name implies, dark. But a neutron star collision? That should unleash some visible fireworks.

X marks the spot

On the day of the gravitational wave detection, the scientists immediately got another clue that something big was happening. Two seconds after LIGO detected the gravitational waves, Fermi, a NASA satellite, detected a gamma-ray burst, one of the most powerful explosions of energy we know of in the universe.

It had long been theorized that neutron star mergers could create gamma-ray bursts. This couldn’t be a coincidence.

But light from the neutron star merger and subsequent explosion would soon dim. And so the LIGO collaboration scientists were suddenly under intense pressure to move quickly. “The sooner you get telescopes on this thing, the more information you get,” Brown says. Studying that light, and how it changes, would teach scientists a huge amount about neutron stars and how their collisions transform matter.

Brown and his team roared into gear, teleconferencing with dozens of scientists across the globe. The LIGO team, along with their counterparts VIRGO (a gravitational wave observing station in Italy), worked furiously to produce a star map showing the location of the gravitational waves. They narrowed it down to an area in the sky roughly the size of a fist held at arm’s length. (Even so, that’s a huge area, astronomically speaking. Even an area the size of a pinhead at arm’s length can contain thousands of galaxies.) The VIRGO detector in Italy actually did not pick up on the signal, but this helped zero in on the location. VIRGO has known blind spots, so the location of these neutron stars had to be near one of them.

Here’s how data from Fermi, LIGO, VIRGO, and another gamma-ray detector called Integral were combined to create the star map. Each detector produced an area where the signal could have possibly originated. Where they all overlapped was the “X marks the spot” on this cosmic treasure hunt.

With the map in hand, the LIGO team sent out an email alert to a global network of astronomers that could scan that region of sky once night fell.

And eureka! Several ground-based observatories nailed down the location of the kilonova, or the explosion after two neutron stars collide, that very night. On the left, you can see what astronomers captured on the night of the discovery. On the right is what it looked like a few days later. It had already dimmed greatly.

Here’s what the galaxy looked like a few weeks before the kilonova explosion (bottom image). The top image shows the kilonova.

These images may look fuzzy, but they’re bursting with information.

With the exact coordinates in hand, scientists could then focus the Hubble Space Telescope and the Chandra X-ray Observatory on the kilonova. And with these instruments, scientists were able to witness a slice of creation.

How colliding neutron stars make gold

Neutron stars are very strange objects. They’re the leftovers of stars that have collapsed in on themselves (i.e., gone supernova). They’re extremely dense. Imagine an object that has the same mass as the sun but is only 15 miles in diameter. That’s 333,000 times the mass of the entire Earth squished into a ball roughly the size of Manhattan. The pressure inside this object is so immense, the only things that can exist inside it are neutrons (protons fused with electrons).

In a galaxy 130 million light-years away, two of these objects were dancing around one another in orbit, growing closer and closer. Each was so dense and generated so much gravity that it caused tidal bulges on the other. The two collided, and the energy from the impact sent a wave of distorted spacetime across the universe, as well as a massive jet of particles out into space (this was the gamma-ray burst detected alongside the gravitational waves). Both the gravitational waves and the gamma rays traveled at the speed of light, which is yet another proof of Albert Einstein’s theory of general relativity. It’s possible the neutron stars were massive enough to form a new black hole after they merged. But there’s not yet enough evidence to say this conclusively.

Here’s what there is evidence to conclude: After the explosion, many of the remaining neutrons merged together to form elements.

All of us, and every element on planet Earth, are made from stars. The Big Bang at the beginning of time created the very light elements — hydrogen and helium. Those elements came together and formed stars, whose fusion reactions formed elements with higher and higher masses.

When those stars went supernova (collapsed in on themselves and exploded), even heavier elements were created. But it’s been “a mystery for a long time where gold and platinum come from,” Brown explains. Even supernovae are not powerful enough to create those.

It had been theorized that a kilonova — the explosion after the merger of two neutron stars — could. And because astronomers were able to so quickly locate the merger, they confirmed this. The color and quality of the light coming from the afterglow of the explosion confirmed the creation of gold and platinum. It was like witnessing alchemy in action.

“The gold that we see on Earth was once created in the nuclear fire of a binary [neutron star] merger,” Brown says. “I’m wearing a wedding ring right now; it’s made of platinum. I’m like holy shit, this was made in a neutron star collision.”

Scientists are convinced this is the beginning of a new age in astronomy

This discovery is so exciting because it means we’re truly in a new age of astronomy. It means scientists can study celestial objects not just in terms of the light or radiation they emit — they can also combine those observations with data from gravitational waves. It means scientists have data on the entirety of this collision. They have data on how the two neutron stars danced around each other, they have data on the moment of impact, and they have extensive data on the aftermath.

Combining all these sources of data is called “multi-messenger” astronomy. And it’s been a dream of LIGO scientists since the observatory’s inception.

“Imagine if we lived in a windowless room and all we could hear is thunder and never see the lightning,” Vicky Kalogera, a Northwestern astrophysicist and LIGO collaborator, explains. “And then imagine we put ourselves in a room with a window. And not only do we hear the thunder but we see the lightning. Seeing the lightning gives you a whole new opportunity to study a thunderstorm and understand what is really going on.”

The gravitational waves are the thunder. The telescope observations of the explosions are the lightning.

Just a few weeks ago, three founding members of LIGO won the Nobel Prize in physics for their pioneering efforts. As Ed Yong at The Atlantic explained, awarding just three people out of the hundreds who made significant contributions to LIGO is awkward and problematic. But the latest results show the prize for the scientific work was well deserved.

The fun of gravitational wave observing is that it works passively. LIGO and VIRGO will “hear” whatever gravitational waves happen to be passing through the Earth on a given day. Each detection starts its own treasure hunt, as scientist have to discover what created the ripples in spacetime.

Scientists expect to observe more black hole mergers, more neutron star mergers. But stranger, cooler observations may come through as well. If LIGO and VIRGO continue to be upgraded, it’s possible they could detect gravitational waves still rippling away from the Big Bang. Or, more excitingly, they could detect sources of gravitational waves that have never been predicted or observed.

“I was a little sad I was not alive for the first moon landing,” Thomas Corbitt, a physicist and LIGO collaborator at Louisiana State University, says. “But when you see things like this, which are a testament to what people can do when they work together, it really is inspiring, and it teaches us about the universe.”

Correction: An earlier version of this post stated LIGOs gravitational wave detectors are buried underground. They are not. You can see them above ground right here.


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