The pair of black holes is called PG 1302-102. Earlier this year, ground-based telescopes spotted the two black holes 3.5 billion light-years away in the Virgo constellation. What makes the pair so special? They are separated by just a light-week. The next closest black hole pair is separated by 20 light-years.

Zoltan Haiman, a senior author of a study published today, explains the significance of this black hole pair. “This is the closest we’ve come to observing two black holes on their way to a massive collision,” said Haiman. “Watching this process reach its culmination can tell us whether black holes and galaxies grow at the same rate, and ultimately test a fundamental property of space-time: its ability to carry vibrations called gravitational waves, produced in the last, most violent, stage of the merger.”

Let’s take a step back for a second. Pg 1302-102 was discovered earlier this year by scientists at the California Institute of Technology. What caught their attention was a curious light signal coming from the center of a galaxy. Using ground-based telescopes, they were able to show the varying light signal is likely caused by two black holes orbiting each other every five years.

The latest study provides extra evidence that it is indeed the two black holes causing this light signal. Scientists from Columbia University used data from NASA’s Galaxy Evolution Explorer (GALEX) and NASA’s Hubble Space Telescope.

“We were lucky to have GALEX data to look through,” said co-author David Schiminovich of Columbia University. “We went back into the GALEX archives and found that the object just happened to have been observed six times.”

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The Hubble observations were handy because it can see in ultraviolet light. Using ultraviolet light data, the scientists were able to test their theory. In the image below, you’ll see how the black hole pair creates a cyclical light pattern.

varying light signal in black hole pair

Scientists believe that one of the black holes is generating more light. It’s ‘eating’ more material than the other one and producing more light. As the brighter black hole orbits its partner every five years, the light changes and appears brighter to us.

“It’s as if a 60-Watt light bulb suddenly appears to be 100 Watts,” says lead author Daniel D’Orazio, also from Columbia University. “As the black hole light speeds away from us, it appears as a dimmer 20-Watt bulb.”

Why the changes in light? In this case, one of the reasons is ‘blueshift.’ You know how the Doppler effect works with a fire truck coming towards you? The sirens blare at a higher frequency when it comes toward you then when it moves away. This is a similar effect, except with light. I wrote about the opposite of blueshift, redshift, earlier this month.

Another reason for the varying light is the insane speed of the black hole. The brighter black hole is tearing through space at seven percent the speed of light! Speeds like this are known as relativistic, meaning the light becomes brighter.

Scientists predicted if the changes in light are due to the relativistic boosting effect, then the same variations in light brightness should be present in ultraviolet wavelengths – except amplified 2.5 times. That’s where the archived data from GALEX and Hubble comes in. And sure enough, the data from GALEX and Hubble lined up with their prediction.

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“We are strengthening our ideas of what’s going on in this system and starting to understand it better,” said Haiman.

When will the black hole pair collide?

Don’t hold your breath. They are close, but not that close. Today’s best estimates put a collision occurring somewhere between 20,000 and 350,000 years from now.

But, astronomers are hopeful that progress made in detecting black hole binaries will lead to witnessing a collision as it happens in the next decade or two.

NASA explains the excitement surrounding merging black holes:

The results will also help researchers understand how to find even closer-knit merging black holes in the future, what some consider the holy grail of physics and the search for gravitational waves. In the final moments before the ultimate union of two black holes, when they are tightly spinning around each other like ice skaters in a “death spiral,” they are predicted to send out ripples in space and time. These so-called gravitational waves, whose existence follows from Albert Einstein’s gravity theory published 100 years ago, hold clues about the fabric of our universe.

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