GRAVITATIONAL-WAVE ASTRONOMY TAKES OFF WITH THIRD DETECTION OF MERGING BLACK HOLES

Researchers at LIGO, the Laser Interferometry Gravitational-Wave Observatory, announced today their third detection of space-time vibrations from the collision of of a pair of black holes, in this case nearly three billion light-years from Earth.

In terms of mass, this new black hole merger--which created a black hole with a mass 49 times heavier than our Sun--fell neatly between the two earlier detections, in September and December of 2015. Those two cosmic crashes resulted in black holes with 62 and 21 times the Sun's mass respectively, and took place much closer to Earth.

The current crash was between black holes with estimated masses of 31.2 and 19.4  times that of the Sun. That means that a mass equal to two Suns was converted into gravitational waves in a fraction of a second as the two black holes completed their death spiral.

Simulated black-hole merger

Credit: LIGO Lab Caltech

This third detection tells the scientists that the unique new window on the cosmos provided by gravitational-wave detectors is now wide open. With improved sensitivity of LIGO's existing laser interferometers and the addition later this year of a third detector, called Virgo, the researchers hope to be able to detect black hole mergers and other space-time-shaking events on a daily basis.

Aerial view of the Virgo interferometer near Pisa, Italy
Credit: The Virgo Consortium

Besides further proof of the existence of black holes in this mass range, the detection allowed researchers to test one of the predictions of Einstein's General Relativity--that gravitational waves travel through space at the same speed regardless of their frequency. Once again, General Relativity passed the test.

“It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us," says MIT's David Shoemaker, LIGO's spokesperson.

The number of black holes in this mass range raises the intriguing possibility that they may at least partly explain dark matter, which is known to exist because of its gravitational effects on galaxies and galaxy clusters, but whose nature remains mysterious.

"There's this intriguing indication that with the size of black holes, 10-100 solar masses, in about the quantity that we see them or expect to see them, might account for dark matter," says LIGO researcher Mike Landry. "It's not impossible."

For the first time, researchers were able to suss out information about how the black holes were spinning before they collided. It's more likely than not that the black holes were not spinning in the same plane as their orbit. That, in turn, implies that they may have formed far apart and only later fell into each other's gravitational thrall.

As gravitational-wave astronomers detect more of these incredibly powerful cosmic events, and gain the ability to match them with observations using visible light, infrared, X-rays and gamma rays, astrophysicists, nuclear physicists and cosmologists all expect exciting new findings that will cast light on black holes, neutron stars, dark matter and the nature of space-time itself.

The new findings appear in this week's edition of Physical Review Letters.

You can view some great video simulations at LIGO's YouTube channel.

You can become a citizen scientist helping to sift actual gravitational-wave signals from the noise by participating in GravitySpy or Einstein@Home.

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The "Big Splash" that formed the Moon was ten times bigger than we thought

Astronomers think that the Moon was formed when a Mars-sized planet smashed into the still-growing Earth some 4.5 billion years ago--a titanic collision often called the "Big Splash."

New high-precision comparisons of the ratio of potassium isotopes in rocks from the Earth and Moon make that collision even more catastrophic than previously thought—releasing so much energy that all of the impactor and most of Earth's mantle were splashed into orbit, forming a hot, dense rapidly-rotating disc from which the Moon quickly condensed.

Artist's conception of Moon-forming impact

Credit: NASA/JPL-Caltech

“Our results provide the first hard evidence that the impact really did—largely--vaporize Earth,” says Kun Wang, a geoscientist at Washington University in St. Louis, Missouri. He and Harvard University researcher Stein Jacobsen detailed their findings in a Nature article published today.

Wang and Jacobsen developed analytic techniques that let them measure minute differences between Earth and Moon rocks for the first time. It turns out that Moon rocks have significantly more heavy isotopes of potassium than Earth rocks, which is best explained by partial condensation from a superheated disc with a high internal pressure. That, in turn, requires a collision ten times more powerful than previously estimated.

Old and new collision models: In the lower-energy collision (top), the Moon has no more heavy potassium than Earth. In the higher-energy collision (bottom), the Moon has more heavy potassium than Earth, as the new study found. Credit: Kun Wang

What was earlier hypothesized to be a Mars-sized object--named Theia after the mythological mother of Selena, the Moon--crashing into the proto-Earth might have been hurtling through space much more rapidly than thought, or it might have been much more massive, perhaps as large as Earth. “It does not have to be Mars-sized anymore,” says Wang.

Many scientists think that our unusually large Moon—one quarter of Earth's diameter—has played a vital role by stabilizing and slowing Earth's rotation, making it easier for life to develop here. If they're right, we may owe our existence to a 4-billion-year-old smashup that almost vaporized Earth.

Gravitational wave astronomy leaps forward with second detection of merging black holes

--Second detection of colliding black holes

--Suggests that there are more black hole pairs than expected

--Confirms that we've entered the era of gravitational wave astronomy 

Researchers representing the LIGO and Virgo consortiums announced today that their twin gravitational wave detectors recorded a second black hole merger on December 26, 2015. The discovery was announced at the annual meeting of the American Astronomical Society, in San Diego, California, and will be detailed in the journal Physical Review Letters.

Simulation of spacetime distortion as two black holes merge

This event differed significantly from the first detected black hole merger, which took place on September 14, 2015 (see zerospinzone, February 11, 2016, below). These two black holes were very light compared to the first pair, which weighed in at 36 and 29 times the mass of our sun. The researchers calculate that the second pair of collapsed stars weighed just 14 and 8 times the mass of the sun. 

Crucially, because they were less massive, their death spiral took far longer, providing the researchers with much more information. Instead of a "chirp" lasting just one-fifth of a second, this merger warped and roiled spacetime for a full second as the black holes whirled around each other 57 times before merging and falling silent. The radiating ripples through spacetime carried away as much energy as if the entire mass of the sun were instantly transformed into pure energy.

Spacetime seismograms recorded at LIGO's gravitational-wave detectors

It's easy to see the increase in amplitude and frequency as 

the two black holes whirled closer and closer together

You can compare how the two mergers sound here.

Detecting two black hole mergers during the short first run of the LIGO interferometers--September 12, 2015 through January 19, 2016--implies that there are more relatively low-mass black hole pairs than expected lurking in the nearby universe. The LIGO team is currently making their twin detectors even more sensitive. So when they start their second round of observations this fall, they expect to detect many more gravitational events, eventually hundreds or even thousands per year.

A team of theorists at Johns Hopkins University, in Baltimore, Maryland, is excited by the hint that there may be many black holes in this mass range. They are exploring the possibility that these are primordial black holes, formed directly from gas clouds in the early history of the universe. If there are enough of them, they may explain dark matter, whose nature remains a major mystery despite decades of efforts to pin it down.

The Virgo team hopes to have a third Earth-based gravitational wave observatory, near Pisa, Italy, up and running later this year. The three detectors working together will allow researchers to pinpoint the location in space of black hole mergers and other sources of gravitational waves such as rapidly spinning neutron stars or colliding neutron stars.

Researchers in the now burgeoning field of gravitational astronomy hope one day to be able to listen in on the gravitational relics of the Big Bang itself--the birth cry of our universe.