LIGO Scientists Detect Another Cosmic Collision

10:12 minutes

An artist’s conception of two merging black holes similar to those detected by LIGO. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

This week, scientists using the Laser Interferometer Gravitational-Wave Observatory, or LIGO, announced that they had detected another gravitational wave—the third ripple observed since September 2015. The findings were published in the journal Physical Review Letters.

[A century-long search leads to a landmark gravitational wave discovery.]

The source of this most recent gravitational wave is a black hole 49 times larger than our sun that was formed by two colliding black holes located 3 billion light-years away. The data indicates that the spin of one or both of the black holes may have a tilted orbit, which can reveal clues to their origins. Theoretical astrophysicist Priyamvada Natarajan explains how this finding sheds light on black hole formation, and how it affects our understanding of general relativity and dark matter.

[The road to LIGO.]

Segment Guests

Priyamvada Natarajan

Priyamvada Natarajan is a theoretical astrophysicist and author of Mapping the Heavens: The Radical Scientific Ideas The Reveal The Cosmos (Yale University Press, 2016). She’s a professor in the departments of physics and astronomy at Yale University in New Haven, Connecticut.

Segment Transcript

IRA FLATOW: You may remember last February, scientists revealed the chirp heard around the world. That is the sound of the first gravitational wave detected by the LIGO detector in 2015. Scientists are starting to amass a bit of a collection of chirps. The second gravitational wave was detected last June, and this week they announced a third wave. So what are these chirps telling us?

My next guest is here to help decipher the sound in the data. Priyamvada Natarajan is a theoretical physicist who studies black holes. And she is a professor in the Department of Physics and Astronomy at Yale University. She’s not a part of the LIGO team, but we’re always welcome and happy to have Priya back with us. Welcome back to Science Friday.

PRIYAMVADA NATARAJAN: Happy to be back IRA. And it’s so exciting. In the last 24 hours, we have had human beings really disappoint us. But the universe has not disappointed us. That’s what is so magical about it.

IRA FLATOW: You have a way with words, Priya. So tell us what is so– OK, so it’s the third time. Is the third time a charm in this case? What would make it a charm in this case? What is so exciting about having it the third time?

PRIYAMVADA NATARAJAN: Well, first of all, it’s a another combination of black holes doing the tango and hooking up. So this one was an event detected on the 4th of January this year. And you had two black holes that were about 31 solar masses, and about 19 times the mass of the sun, kind of in this death spiral, and hooking up, and then merging and producing these gravitational waves.

So the first thing is that, of course, it ratifies the earlier detection. And truth be told, this is not the most massive pair. The first one that was detected was the most massive pair. If you remember, that was like 36 and 29 times the mass of the sun. So the issue here is that we never expected this mass range of black holes to be all that common. They’re sort of bulkier than you would expect from the end states of stars.

So that’s what is kind of special, that we found a second one with roughly the same mass range as the first detection. And we never believed that these were going to be the common kinds of merging events. So the interesting thing about it, these events, one shouldn’t think of these events as just sort of sitting there slowly counting numbers.

I was thinking about it, and you know, its like a kaleidoscope. Like every event is like a little piece in the pattern. And once the whole sort of pieces fill in, we really understand something quite fundamental about how black holes are made, and how they evolve, and how they end up actually merging.

So one exciting thing about this particular event is that it occurred farther away, three billion light years away. The other events occurred a billion light years away. So this is much farther away. And what is interesting about that is that we have a been pushed to a new sort of distance, which allows us to do a new fundamental test of general relativity.

So when you have light, optical light, and you put it through a prism, it disperses and it gives you rainbows. So gravitational waves are not like light. So according to Einstein’s theory of general relativity, they should not disperse. There’s no dispersion. And so because this is farther away, we can put some tighter constraints on whether there is dispersion.

There’s actually no dispersion detected. So we are still very consistent with Einstein’s idea of general relativity. So this event ratifies that. So the other fun thing about this event is that from the in-spiral, from the template in-spiral, and the data match, you can actually figure out the spin alignment or misalignment. So black holes can spin, and they’ll have a spin vector. And the plane that contains the orbit of the two that are tangoing, the two black holes that are tangoing, also has a spin vector.

And the question is are they all aligned? If they’re all aligned, there’s a lot of angular momentum that has to be lost before these chaps can actually hook up. And so the event would last longer. And so you would see sort of these signatures. So what they found with this event is that actually it was quite short. And it suggested that the spins were actually misaligned.

So the deputy spokesperson of the LIGO collaboration, Laura Cadonati, had a very nice analogy. It’s like if you have these two tornadoes, and they are not, if they line up, and then they line up with the orbit, then you would have a much more massive tornado, so you’d take a lot more time to dissipate that energy. It’s like dissipating the spin. So the event was shorter. And so that tells you that the spin is a constraint on the fact that the spins were likely misaligned.

And why is this cool? This is cool because then it offers a clue to how these black holes might have ended up as a binary pair. So you know, there are a bunch of different scenarios. There are many scenarios on how you can make these black old binaries. And one of them is, of course, they could hook up after becoming black holes. Or they could start out as binary stars, and then one of them becomes a black hole, and then they could hook up when the second one becomes a black hole.

So they could start as stars, or they could hook up after becoming black holes. And the thing is, if they hook up stars first, then because the binary star orbits are large, there’s enough time for their spins to line up. They would be lined up, aligned. Whereas if they hook up after becoming black holes, their spins could be random, because this is what’s called sort of a dynamical formation.

So what this suggests, this event is very suggestive that these guys hooked up after becoming black holes. And that then points to the fact that this happens when you have close encounters in very crowded places. So you have a nuclear star cluster, or globular cluster, and you can have a bunch of black holes. And that’s– it’s very interesting that we’re getting a new window, new sets of clues to how these black holes could have ended up in this configuration.

IRA FLATOW: Let me interrupt to say I’m Ira Flatow. This is Science Friday from PRI, Public Radio International. Talking about black holes with CNN Priya Natarajan. You know why are scientists so interested in black holes? I mean, OK, maybe there’s a black hole, they found a black hole. They spin, they collide, they put out gravitational waves. Why is so much interest concentrated? What will they tell us that we don’t know, or what could they explain?

PRIYAMVADA NATARAJAN: Well, first of all, look, they’re the most enigmatic objects in the universe. I mean, they’re crazy cool objects. Given what they do to light, what they do to matter around them, and so on. First, there’s just that, that they are the strangest objects.

IRA FLATOW: I’m good with strange. Strange is good in my book.

PRIYAMVADA NATARAJAN: You’re good with that?

IRA FLATOW: Yeah, I’m good with that.

PRIYAMVADA NATARAJAN: And then, we also know that the end states of massive stars would give you this kind of dense compact state. So we know that they correspond in a very real way to physics. So stars would burn up all their fuel, and if they’re about 10 to 15 times the mass of sun, they would end up as a black hole. So the other use– and then we see the super-massive black holes everywhere.

So the question is what’s the link, how do these black holes grow? They seem to be a very important participant in the universe.

IRA FLATOW: But let me interrupt, because we’re running out of time, and I have a fundamental question. And a listener is asking this. Will studying them get us any closer to uniting the gravity, the intense gravity they have with the quantum side of physics? Will we find a gravitational particle that corresponds? And is there a predicted one that corresponds with quantum– should be graviton or something, right? That carries the gravity.

PRIYAMVADA NATARAJAN: Right, so there are constraints now from this test of Einstein’s theory of general relativity, but as I mentioned with the dispersion, the fact that no dispersion is seen, puts a limit on the mass of the graviton. But I think there are other– there have been some other interesting suggestions. Your favorite one and mine, the dream, that what if these black holes, 30 times the mass of the sun, could account for all of that dark matter in the universe.

Alas, they can’t quite account for all of the dark matter. But I think what black holes are going to teach us is test the limits of this theory of Einstein’s theory of general relativity. We know it’s not a complete theory. And that there has to be another covering theory. So any anomalies that we might find between the expected properties of black holes and what we can measure from LIGO events, is going to bring us closer to what the covering model is.

IRA FLATOW: OK, I’m very happy with that. Because even Einstein, when he talked about quantum mechanics, he said it’s OK for now, but it’s going to be something bigger and better that covers everything. And that’s basically what you’re saying about–

PRIYAMVADA NATARAJAN: The only disappointing thing at the moment is because the LIGO arms in India and Italy have not switched on, the we can’t quite find electromagnetic counterparts to this merger and gravitational waves. Looking for other signatures that would signal the environment. Because the outer box is still too big, the swath of sky is just too big. So we just have to wait for more events.

IRA FLATOW: Go over there and kick it on. Go ahead Craig, just turn on those machines. All right, thank you. It’s always a pleasure to have you on. Please come back and talk with us soon. OK?

PRIYAMVADA NATARAJAN: Thank you so much for inviting me.

IRA FLATOW: You’re welcome. Dr. Priya Natarajan, theoretical astrophysicist and professor in the Departments of Physics and Astronomy at good old Yale University in New Haven.

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