Prepare for an Explosion of Gravitational Wave Detections

By Eric Betz | April 11, 2016 4:20 pm

An illustration of merging black holes. (Credit: NASA)

In the time it takes you to finish your lunch break, several pairs of black holes will merge somewhere in the universe. That’s the incredible picture emerging from early insights by the Laser Interferometer Gravitational-wave Observatory.

In February, LIGO announced the first detection of gravitational waves, confirming a key prediction of Albert Einstein’s theory of general relativity. That historic wave reached Earth at light speed on September 14, 2015, from a pair of black holes that collided 1.3 billion light-years away.

But LIGO heard another suspect gravitational wave signal that got less attention. Though it wasn’t as strong, it looked promising.

The Other Collision

An analysis of that event, labeled LVT151012, has shown with 90 percent certainty that it also came from a pair of colliding black holes. That’s not sufficient for scientists to deem it a “detection,” but the LIGO team is confident enough that they’re using it to start piecing together a picture of black holes in the universe.

“The best guess we have is that binary black holes merge in our universe at the rate of a few per hour,” says LIGO scientist Jolien Creighton of the University of Wisconsin-Milwaukee.

Assuming LIGO’s early data are not exceptional, scientists will soon piece together the first black hole census. Extrapolating from those two mergers in 16 days to the larger universe beyond what LIGO can see, the team calculates that a few binary black holes should merge every hour in the cosmos.

“It does imply that we should have tens of detections over the next few years, and hundreds through the end of the decade,” says Hanna. “That’s enough to do some pretty significant astronomy. That’s a big population.”

Based on the signals seen so far and the sensitivity of LIGO’s detectors, scientists estimate that they’ll see between 10 and 100 black hole mergers during the instrument’s next observing run, which begins in late summer.

“When the second science run turns on, we’ll be seeing more systems at rates of once every few days or weeks,” Creighton adds. “And the run will also last longer, so we will be collecting more and more events.”

The First Census

Black holes are well suited to their names. They emit no light. So, before LIGO, astronomers could only infer a black hole’s existence by watching it interact with objects caught in their gravitational grip. Astronomers spotted most of the known stellar mass black holes indirectly: they observed X-rays that were emitted as they feed on a stellar companion.

“In some ways, LIGO provides the first real direct way of probing black holes,” says LIGO scientist Chad Hanna of Pennsylvania State University. Physicists knew they existed, but the instrument allowed them to actually measure a black hole’s space-time and show it’s consistent with theories.


(Credit: Roen Kelly/Astronomy)

LIGO accomplished this because it isn’t bound by sight. Its twin detectors can see tiny stretches and squeezes of space-time from large objects merging. These gravitational waves carry information about a black hole’s mass, spin and location.

To date, only some 19 stellar mass black holes are known in the Milky Way. And considering our galaxy has hundreds of billions of stars, that number is certainly incorrect. However, the true black hole population size remains unknown.

In fact, before LIGO, astronomers weren’t positive that nature could create binary black holes at all.

“We’d never seen a binary black hole, so we didn’t know for sure that they existed,” says Creighton. “And, if they did exist, we didn’t know if they would ever merge.”

Instead, most experts thought LIGO’s first gravitational wave observations would come from merging binary neutron stars. Astronomers had already seen these collapsed supernova cores orbiting each other. Theoretical estimates predicted LIGO would catch around 40 of these neutron star mergers, and between 10 and 20 black hole mergers, each year.

Rewriting Stellar Evolution

One of the first things this black hole census can do is rewrite the textbook version of stellar evolution.

Scientists have a pretty good idea of how single stars will live and die. Stars like our sun will grow into behemoths called red giants before they shed their outer layers and become a planetary nebula. Larger stars — those with more than about eight solar masses — will explode as supernovas. And, theoretically, any star at least 25 times bigger than the sun will end its life as a black hole.

But most stars in the Milky Way are actually binaries. So, understanding such stellar pairs is fundamental to understanding stellar evolution as a whole.

“Binary evolution is more complicated than single star evolution,” says Creighton. “There’s a lot more processes that can happen — mass exchange between the binary companions, winds, kicks during supernovae, all sorts of things.”

Interestingly, many binary evolution models didn’t predict binary black holes as big as the ones LIGO announced in February. That first gravitational wave signal came from the merger of black holes 36 times and 29 times more massive than the sun.

These stars likely formed in low-metal environments — a term astronomers use for anything more complicated than hydrogen or helium — during the early universe. But it’s also possible these binary black holes are born in dense clusters where stars interact more often.

As more detections stream in from LIGO, astronomers can begin to get an idea for the size of most stellar mass black holes. The likely second detection, LVT151012, had black holes of 23 and 13 solar masses — more in line with what astronomers expected to detect.

“When we see more and more of these black holes, and we get this mass distribution, we should be able to distinguish between the different scenarios for where these stars are coming from,” says Creighton.

CATEGORIZED UNDER: Space & Physics, top posts
  • Uncle Al

    LIGO was the first detection of gravitational radiation as spacetime strain (as opposed to observed inspiraling orbital energy loss re the Hulse-Taylor pulsar). LIGO was the first observation confirming black hole existence and models. LIGO demands general relativity is operationally flawless in its most extreme predictions. To break GR one must venture testably outside physics’ founding postulates (re Euclid and mapping the Earth’s surface). Good luck telling physics it is testably fundamentally incomplete. Experiment can fail but theory can be published.

  • OWilson

    Its relatively easy to theorize these days.

    You just invoke certain convenient “constants”, Invoke Dark Energy and Dark Mass, or a strange “Inflation” that miraculously speeds up at the right time and slows down again to explain what is observed.

    You can postulate any weird phenomena, as long as it covers the observed reality.

    You can even violate the sacrosant rule that the fermion Electron cannot be divided (anti-particle aside) and as long as it temporarily answers questions, you are home free (maybe with a grant).

    If physics was this easy when I was growing up, OMG!

    Bohr, Einstein and Hoyle were not as easy to convince as NASA/NOAA/IPCC, and they weren’t constrained by the PC necessity to sign on to a “Muslim Outreach Program”, to get funding.

    • Uncle Al

      The fundamental bankruptcy of contemporary physical theory is revealed by the OPERA experiment timing glitch that implied superluminal muon neutrinos, violating special relativity. (Supernova 1987a had all its stuff arrive SOP.) How many learned physicists pulled theory out of their bottoms?

      Superluminal neutrinos, explained: (* = repeat first four digits) arXiv:1312.4837, 1304.0038, 1210.5248, 1205.0145, 1204.0484, 1203.4052; 1202.3319, *.0469; 1201.6496, *.5847, *.4147, *.2085, *.1368, *.1322, *.0915; 1112.6217, *.4779, *.4714, *.3753, *.3050, *.2689, *.1222, *.0815, *.0527, *.0353, *.0300; 1111.7181, *.6579, *.6330, *.4994, *.4532, *.3888, *.2271, *.1574, *.0805, *.0093; 1110.6673, *.6577, *.6571, *.6408, *.4754, *.3581, *.3540, *.3071, *.2685, *.2463, *.2236, *.2219, *.2170, *.2146, *.2060, *.2015, *.1943, *.1875, *.1790, *.1330, *.1253, *.0931, *.0762, *.0521, *.0644, *.0456, *.0451, *.0449, *.0430, *.0392, *.0351, *.0245, *.0243, *.0239, *.0234; 1109.6312, *.6308, *.6296, *.6282, *.4897.

      Theory seeks monopoly through yelling. Any attempt to empirically falsify theory outside its postulates (where incomplete theory will fail) cannot be funded – for it contradicts theory.

      • TLongmire

        It’s that sutle huff that sees us thru

      • OWilson

        My respectful, on topic and most insightful comment was deleted:)

  • Mike Richardson

    Looking forward to further findings from LIGO, which has one of its detectors nearby in Livingston, Louisiana. Einstein of course, theorized about things that LIGO is now proving. I think he’d be pleased to see vindication from LIGO, and from other tests that have been conducted over the years to verify the predictions of his theory of relativity. Probably not as pleased to see that cosmic inflation, rather than his favored theory of a steady state universe, is gaining more and more evidence. He was truly an extraordinary mind, one of the greatest theorists that ever lived. But theories need to be tested, and LIGO has done that for Einstein, as it will likely continue to do for the current generation of theoretical physicists and cosmologists.

    • OWilson

      Yeah, me too :)

    • OWilson

      Me too!

      Einstein was awesome, he really did “theorize about things”.

      But as usual, your science is not :)

      Einstein’s General Theory of Relativity, predicted a dynamic universe. When confronted with the “consensus” of the astronomers of the day, who observed it was stable he had to invoke a kludge fudge, his infamous “cosmological constant” to explain the lack of expansion or contraction.

      He was subsequently happy when new telescopes revealed his calculations had been correct all along and the kludge fudge was no longer needed.

      It was my fellow Yorkshireman, Fred Hoyle who was the leading proponent of what he called, the “Steady State” theory, and remained mistakenly wedded to it all his life. He also coined the term. “Big Bang”, to describe what he thought was a foolish notion.

      • Mike Richardson

        Actually, Einstein did initially favor a steady state universe, but abandoned it. I stand humbly corrected on that count. Still glad to have LIGO, though.

        • OWilson

          A little more respectful to the great man, but still not correct.

          His General theory which incorporated 4 dimensional space time with his new theory of gravity was that the universe must be expanding or contracting.

          As a theorist he depended on depended on observational evidence for confirmation. It was the astronomers of the day who told him the universe was stable, so he tried to account for that. (100% peer pressure, see how powerful, and wrong, a scientific “consensus” can be?)

  • Neil Crabtree

    What I can never understand in these conversations, is that: 13.8 billion years ago there was an occasion when infinite density and infinite temperature inspired to produce the known universe – a universe that was created at all places in all space at the same point in time.
    So why are we looking out there when a ‘sleight’ is at hand?

    • Uncle Al

      The observed cosmic microwave background must be explained. The Big Bang does that to decimal places where nothing else works at all. Observed matter (you, for instance) requires Big Bang baryogenesis (6.1×10^(-10) bias, hadrons less antihadrons versus photons) breaks a boatload of conservation laws. All is well if physics admits geometric chirality exists (re Sakharov conditions) and tests for it. Physics denies physical left and right hands exist (e.g., Green’s function – everything is squared). There’s your problem.

      • OWilson

        The Big Bang is today’s version of Ptolemy’s “epicycles”, but it no more more “explains” the universe we see, than a magician’s wand. Or Gore’s movie.

        We don’t even know what it’s major constituents are. Dark Matter, Dark Energy. Or how it started up then slowed down, then started up again “Guth’s Inflaltion”, then slows down again. Now they tell us it is speeding up again.

        Alan Guth, who made it work for a while, says “we have more work to do”.

        Dark Magic :)

        You can’t sell a racehorse, if you can’t show it’s parentage.

        You can’t sell a phenomena, if you can’t show a “cause”.

        Not in science, anyway :)


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