Third Detection of Gravitational Waves Confirmed
(Inside Science) -- A major international collaboration between the Laser Interferometer Gravitational-Wave Observatory in the U.S. and a similar project in Europe named Virgo has confirmed the successful detection of gravitational waves for the third time in human history.
Scientists from the LIGO and Virgo collaboration detected the astronomical event known as GW170104 on January 4, 2017. The observed gravitational waves were generated when two black holes with a combined mass 49 times that of our own sun slammed into each other, jostling the fabric of space and time. The collision, which occurred roughly 3 billion light-years away from Earth, is the most remote source of gravitational waves yet detected.
Third time's the charm
Gravitational waves are ripples in space-time triggered by gravitational interactions between massive objects such as black holes and neutron stars. The theory behind gravitational waves was first established by Einstein's theory of general relativity in 1916, but the definite proof remained elusive until a century later, when the LIGO collaboration announced the first detection of gravitational waves in 2016.
"A key thing to take away from this third highly confident event is that we're really moving from novelty to new observational science -- a new astronomy of gravitational waves," said David Shoemaker of MIT and the newly elected spokesperson for the LIGO Scientific Collaboration, during a May 31 press conference.
The massive collision is estimated to be more than twice the distance from Earth than both previous detections, which were 1.3 and 1.4 billion light-years away, respectively. The great distance of this merger also provides the most rigorous test to date for a specific part of Einstein's general theory of relativity -- the lack of dispersion in gravitational waves.
Dispersion refers to the effect when waves travel at different velocities depending on their wavelengths. For instance, when a beam of white light is shone through a prism, the difference in wavelengths will cause the light waves to move at slightly different speeds, which results in slightly different paths and produces a band of rainbow.
However, according to Einstein's theory, gravitational waves must remain undispersed when travelling through the fabric of space-time. This latest detection, due to the great distance between the source and Earth, provides the most precise measurement of whether gravitational waves disperse or not.
"We made a very careful measurement of that effect, and our measurements are very sensitive to minute differences in those speeds of different frequencies, but we did not discover any dispersion, once again failing to prove that Einstein was wrong," said Bangalore Sathyaprakash of both Penn State and Cardiff Universities, one of the editors of the new paper, during the press conference.
The ballet of two black holes
When two black holes spiral toward each other, they also spin on their own axes. Scientists are interested in observing how these paired black holes spin relative to each other, because that comparison contains clues about how the pair was formed in the first place.
Before merging together, two black holes can either spin in the same direction as they orbit each other -- known as aligned spins -- or they can spin in the opposing direction -- known as non-aligned spins. Scientists think the aligned black hole pairs are born together when two circling stars become two black holes -- since the stars would have been spinning in alignment, the black holes should do the same.
For the non-aligned pairs, scientists instead propose that the duos form when two originally separated black holes converge toward the center of a dense cluster of stars. In this case, the black holes can spin in any direction relative to their orbital motion. The latest detection provides clues that at least one of the black holes are spinning non-aligned when compared to that direction of orbital motion -- something that hasn't been observed before.
"How we can tell this is from the gravitational wave fingerprints," said Laura Cadonati of Georgia Tech and the Deputy Spokesperson of the LIGO Scientific Collaboration, during the press conference. "If the black hole spins are aligned with the direction of the orbit, they need to get closer and shed some of the total rotational energy in the system before a final black hole can form. This means they take longer to merge than if the spins are non-aligned."
According to Scott Ransom, an astronomer from the National Radio Astronomy Observatory and University of Virginia, who was not part of either research group, both aligned and non-aligned black hole pairs should exist in our universe. However, scientists aren't sure which scenario is more likely to happen yet. This is an important detail that can teach us more about the dynamics of our universe, and so far we can only detect the spins of black holes using the brand new technique of gravitational wave measurements.
"In order to figure out which is the dominant case, we're probably going to need hundreds of these observations, so hopefully when the sensitivity improves we'll start seeing these things all the time," said Ransom in an interview. "Right now, we only have three."
The European Space Agency, and potentially in collaboration with NASA, is currently developing a space-based gravitational wave observatory. The ambitious project known as LISA, which stands for Laser Interferometer Space Antenna, will orbit above the Earth. Without the need to build physical vacuum tunnels in space, the three satellites of LISA will form a interferometer with arms each a million kilometers long. At that size, the instrument will be able to measure gravitational waves at lower frequencies previously not accessible by LIGO and Virgo. LISA is projected to be launched around 2030.
"LISA could see things that were much much heavier, and at much higher distances," said Ira Thorpe from NASA's Goddard Space Flight Center in Maryland, in an interview. He did not participate in the study. "Hundreds of thousands of solar mass black holes, million solar mass black holes -- the type of black holes at the centers of galaxies, and what you expect to merge when you have galaxy mergers."
The first detection of gravitational waves in 2015 was announced on February 11, 2016, after months of careful analysis and confirmation by a team of more than 1,000 scientists. The announcement fell just after the nomination deadline for the 2016 Nobel Prize for Physics. With two more subsequent detections under their belts, the scientists behind the discovery will likely be the favorites to win the prize this year.