Scientists observe gravitational waves — again!

Continued success of LIGO experiment opens new frontiers in astronomy

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Artist's conception shows two merging black holes similar to those detected by LIGO. The black holes are spinning in a non-aligned fashion, which means they have different orientations relative to the overall orbital motion of the pair.
Courtesy of LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

LIGO observed gravitational waves for a third time Jan. 4, scientists announced at a press conference May 31. With the new detection, this field is moving from a “novelty” to a “new observational science” of gravitational waves, said LIGO Scientific Collaboration spokesperson David Shoemaker.

In a paper that has been accepted for publication in Physical Review Letters, scientists discuss this third confirmed detection of the spacetime wiggles known as gravitational waves.

Just like in the first two observations, these wiggles resulted from the merger of two black holes. This observation has enabled scientists to refine their understanding of binary black hole systems, and it has contributed more evidence in favor of Einstein’s famous theory of general relativity.

Everything produces gravitational waves, but most things produce them at orders of magnitude much too small for humans to detect with current scientific instruments. LIGO is one of the most precise scientific experiments ever devised, including cutting-edge technology for reducing noise, and it still can only detect one of the most violent, powerful collisions in the universe: the merger of black holes.

The observation and its results

The signal, which scientists detected Jan. 4, took several months to analyze fully, hence the delay between detection and publication.

The black hole system that created the gravitational waves has a mass of around 50 times the mass of our sun — in between the masses of LIGO’s first and second confirmed observations, which were 62 and 21 solar masses. It’s also twice as far as the first two observations, at a distance of 3 billion light years.

The detectors have a false alarm rate of less than 1 in 70,000 years, meaning signals that look like gravitational waves but actually aren’t occur very infrequently.

It is not currently possible to pin down the location of the black holes (i.e. their position in the sky, the galaxy they are in, etc.), though having more LIGO observatories around the globe will help with this in future observations.

Scientists also noted there is not enough data now to know how the number of collisions correlates with distance. That information, if obtained, would tell us how often these collisions have happened at different times throughout the history of the universe.

An important result from this detection has to do with how binary black hole systems form. There are two major theories. The first is that the stars are already paired up before they become black holes. The second is that the black holes form inside dense stellar clusters and then fall to the center of those clusters and pair up with each other.

In these binary systems, the black holes spin on their own axes as well as revolve around each other, much like the planets rotate and revolve in our solar system. If the black holes were already paired up when they were still stars, as in the first theory, they would have to spin in the same direction that they revolve. Scientists found this likely wasn’t the case for the binary black holes in this observation, lending support to the second theory.

About the observatories

There are two Laser Interferometer Gravitational-Wave Observatories in the U.S., one at Livingston, Louisiana, and one at Hanford, Washington.

Each observatory is a giant interferometer. It has two long arms that stick out at right angles. A laser shines down the arms. When a gravitational wave comes through, one arm is stretched and one is squeezed, so when the laser reflects back the crests and troughs of the beams from each arm will no longer be aligned. So, when the right kind of wave signal is detected, that indicates the passing of a gravitational wave.

LIGO’s current observing run started last November and will end in August. Before this run, both the observatories received updates.

At Livingston, scientists focused on reducing noise at low frequencies, which entailed efforts to reduce scattered light. This is light that reflects off chambers and tubes inside the instrument and creates noise. Scientists had to examine the instrument to find out where the light was coming from and then take measures to block that light.

At Hanford, scientists focused on reducing noise at high frequencies by increasing the power of the laser. However, a power increase does not automatically guarantee a sensitivity increase. For example, the increased power caused the mirrors in the device to ring like little bells, and scientists had to work to suppress that noise. In the end, they did not gain as much sensitivity as they hoped, but they have more plans to implement after this observing round ends.

Before the next observing run starts in 2018, each observatory will receive the updates that the other one has already received.

Testing general relativity

Using the new observation, LIGO scientists also tested Einstein’s theory of general relativity.

Dispersion is the concept that different wavelengths of a wave signal travel at different speeds through a medium. This is the phenomenon that causes light to split into a rainbow inside a prism.

In general relativity, gravitational waves are nondispersive. LIGO scientists compared the predictions of GR and the predictions of other theories of gravity in which gravitational waves could have different amounts of dispersion. The new data supported GR’s nondispersive waves.

They also performed “null tests” of GR, in which they test how far the data deviates from the theory’s predictions without comparing to any other theories of gravity. None of these tests indicated a statistically significant departure from GR, according to the paper.

What’s next for LIGO?

In the summer, LIGO is expected to be joined by Virgo, another gravitational wave observatory located in Italy. Adding Virgo’s observations to the mix will enable scientists to improve their estimate of the position of black hole collisions in the sky by one or two magnitudes.

By the mid-2020s, two more observatories will go online — one in Japan and one in India.

Scientists now estimate that one of these black hole collisions could be detected on the order of once a day to once a week.

In the future, scientists hope LIGO will be able to detect the mergers of not only black holes, but also neutron stars, especially after further updates to the detectors’ sensitivity.

According to the agreement LIGO has with the National Science Foundation, the organization that partially funds the research, after four confident detections, the LIGO team must start to share their data more publicly.