Third Gravitational Wave Event Detected

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.
Artist's impression of merging binary black holes. Credit: LIGO/A. Simonnet.

A third gravitational wave has been detected by the Laser Interferometer Gravitational-wave Observatory (LIGO). An international team announced the detection today, while the event itself was detected on January 4th, 2017. Gravitational waves are ripples in space-time predicted by Albert Einstein over a century ago.

LIGO consists of two facilities: one in Hanford, Washington and one in Livingston, Louisiana. When LIGO announced its first gravitational wave back in February 2016 (detected in September 2015), it opened up a new window into astronomy. With this gravitational wave, the third one detected, that new window is getting larger. So far, all three waves detected have been created by the merging of black holes.

The team, including engineers and scientists from Northwestern University in Illinois, published their results in the journal Physical Review Letters.

When the first gravitational wave was finally detected, over a hundred years after Einstein predicted it, it helped confirm Einstein’s description of space-time as an integrated continuum. It’s often said that it’s not a good idea to bet against Einstein, and this third detection just strengthens Einstein’s theory.

Like the previous two detections, this one was created by the merging of two black holes. These two were different sizes from each other; one was about 31.2 solar masses, and the other was about 19.4 solar masses. The combined 50 solar mass event caused the third wave, which is named GW170104. The black holes were about 3 billion light years away.

“…an intriguing black hole population…” – Vicky Kalogera, Senior Astrophysicist, LIGO Scientific Collaboration

LIGO is showing us that their is a population of binary black holes out there. “Our handful of detections so far is revealing an intriguing black hole population we did not know existed until now,” said Northwestern’s Vicky Kalogera, a senior astrophysicist with the LIGO Scientific Collaboration (LSC), which conducts research related to the twin LIGO detectors, located in the U.S.

“Now we have three pairs of black holes, each pair ending their death spiral dance over millions or billions of years in some of the most powerful explosions in the universe. In astronomy, we say with three objects of the same type you have a class. We have a population, and we can do analysis.”

The Laser Interferometer Gravitational-Wave Observatory (LIGO)facility in Livingston, Louisiana. The other facility is located in Hanford, Washington. Image: LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO)facility in Livingston, Louisiana. The other facility is located in Hanford, Washington. Image: LIGO

When we say that gravitational waves have opened up a new window on astronomy, that window opens onto black holes themselves. Beyond confirming Einstein’s predictions, and establishing a population of binary black holes, LIGO can characterize and measure those black holes. We can learn the holes’ masses and their spin characteristics.

“Once again, the black holes are heavy,“ said Shane Larson, of Northwestern University and Adler Planetarium in Chicago. “The first black holes LIGO detected were twice as heavy as we ever would have expected. Now we’ve all been churning our cranks trying to figure out all the interesting myriad ways we can imagine the universe making big and heavy black holes. And Northwestern is strong in this research area, so we are excited.”

This third finding strengthens the case for the existence of a new class of black holes: binary black holes that are locked in relationship with each other. It also shows that these objects can be larger than thought before LIGO detected them.

“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.” – David Shoemaker, MIT

“We have further confirmation of the existence of black holes that are heavier than 20 solar masses, objects we didn’t know existed before LIGO detected them,” said David Shoemaker of MIT, spokesperson for the LIGO Scientific Collaboration . “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.”

An artist's impression of two merging black holes. Image: NASA/CXC/A. Hobart
An artist’s impression of two merging black holes. Image: NASA/CXC/A. Hobart

“With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe,” said David Reitze of Caltech, executive director of the LIGO Laboratory and a Northwestern alumnus. “While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars.”

A tell-tale chirping sound confirms the detection of a gravitational wave, and you can hear it described and explained here, on a Northwestern University podcast.


Weekly Space Hangout – May 19, 2017: Eric Fisher of Labfundr

Host: Fraser Cain (@fcain)

Special Guest:
Eric Fisher is the head of Labfundr, a Canadian crowdsourcing platform for science research and outreach. Eric is an entrepreneur, recovering biochemist, and son of a glaciologist. He completed a PhD in Biochemistry & Molecular Biology at Dalhousie University in Halifax, Nova Scotia, Canada. At Dalhousie, Eric investigated how liver cells create and destroy “bad” cholesterol particles. Eric recently founded Labfundr, Canada’s first crowdfunding platform for science, which aims to boost public engagement and investment in research. He stays on his toes by trying to keep up with his dog Joni, who is smarter and faster than him.

Dr. Kimberly Cartier ( / @AstroKimCartier )
Dr. Morgan Rehnberg ( / @MorganRehnberg
Alessondra Springmann (@sondy)
Their stories this week:

Explaining massive black hole formation with LIGO

Discovery of a moon around large dwarf planet

More troubles for SLS and here

A Neptune-sized planet that looks like a Jupiter

Rivers on Titan look more like Mars than Earth

We use a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (, which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!


The WSH recently welcomed back Mathew Anderson, author of “Our Cosmic Story,” to the show to discuss his recent update. He was kind enough to offer our viewers free electronic copies of his complete book as well as his standalone update. Complete information about how to get your copies will be available on the WSH webpage – just visit for all the details.

If you’d like to join Fraser and Paul Matt Sutter on their Tour to Iceland in February 2018, you can find the information at

If you would like to join the Weekly Space Hangout Crew, visit their site here and sign up. They’re a great team who can help you join our online discussions!

We record the Weekly Space Hangout every Friday at 12:00 pm Pacific / 3:00 pm Eastern. You can watch us live on Universe Today, or the Universe Today YouTube page

Towards A New Understanding Of Dark Matter

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.
Artist's impression of merging binary black holes. Credit: LIGO/A. Simonnet.

Dark matter remains largely mysterious, but astrophysicists keep trying to crack open that mystery. Last year’s discovery of gravity waves by the Laser Interferometer Gravitational Wave Observatory (LIGO) may have opened up a new window into the dark matter mystery. Enter what are known as ‘primordial black holes.’

Theorists have predicted the existence of particles called Weakly Interacting Massive Particles (WIMPS). These WIMPs could be what dark matter is made of. But the problem is, there’s no experimental evidence to back it up. The mystery of dark matter is still an open case file.

When LIGO detected gravitational waves last year, it renewed interest in another theory attempting to explain dark matter. That theory says that dark matter could actually be in the form of Primordial Black Holes (PBHs), not the aforementioned WIMPS.

Primordial black holes are different than the black holes you’re probably thinking of. Those are called stellar black holes, and they form when a large enough star collapses in on itself at the end of its life. The size of these stellar black holes is limited by the size and evolution of the stars that they form from.

This artist’s drawing shows a stellar black hole as it pulls matter from a blue star beside it. Could the stellar black hole’s cousin, the primordial black hole, account for the dark matter in our Universe?
Credits: NASA/CXC/M.Weiss

Unlike stellar black holes, primordial black holes originated in high density fluctuations of matter during the first moments of the Universe. They can be much larger, or smaller, than stellar black holes. PBHs could be as small as asteroids or as large as 30 solar masses, even larger. They could also be more abundant, because they don’t require a large mass star to form.

When two of these PBHs larger than about 30 solar masses merge together, they would create the gravitational waves detected by LIGO. The theory says that these primordial black holes would be found in the halos of galaxies.

If there are enough of these intermediate sized PBHs in galactic halos, they would have an effect on light from distant quasars as it passes through the halo. This effect is called ‘micro-lensing’. The micro-lensing would concentrate the light and make the quasars appear brighter.

A depiction of quasar microlensing. The microlensing object in the foreground galaxy could be a star (as depicted), a primordial black hole, or any other compact object. Credit: NASA/Jason Cowan (Astronomy Technology Center).

The effect of this micro-lensing would be stronger the more mass a PBH has, or the more abundant the PBHs are in the galactic halo. We can’t see the black holes themselves, of course, but we can see the increased brightness of the quasars.

Working with this assumption, a team of astronomers at the Instituto de Astrofísica de Canarias examined the micro-lensing effect on quasars to estimate the numbers of primordial black holes of intermediate mass in galaxies.

“The black holes whose merging was detected by LIGO were probably formed by the collapse of stars, and were not primordial black holes.” -Evencio Mediavilla

The study looked at 24 quasars that are gravitationally lensed, and the results show that it is normal stars like our Sun that cause the micro-lensing effect on distant quasars. That rules out the existence of a large population of PBHs in the galactic halo. “This study implies “says Evencio Mediavilla, “that it is not at all probable that black holes with masses between 10 and 100 times the mass of the Sun make up a significant fraction of the dark matter”. For that reason the black holes whose merging was detected by LIGO were probably formed by the collapse of stars, and were not primordial black holes”.

Depending on you perspective, that either answers some of our questions about dark matter, or only deepens the mystery.

We may have to wait a long time before we know exactly what dark matter is. But the new telescopes being built around the world, like the European Extremely Large Telescope, the Giant Magellan Telescope, and the Large Synoptic Survey Telescope, promise to deepen our understanding of how dark matter behaves, and how it shapes the Universe.

It’s only a matter of time before the mystery of dark matter is solved.

What Happens When Black Holes Collide?

The sign of a truly great scientific theory is by the outcomes it predicts when you run experiments or perform observations. And one of the greatest theories ever proposed was the concept of Relativity, described by Albert Einstein in the beginning of the 20th century.

In addition to helping us understand that light is the ultimate speed limit of the Universe, Einstein described gravity itself as a warping of spacetime.

He did more than just provide a bunch of elaborate new explanations for the Universe, he proposed a series of tests that could be done to find out if his theories were correct.

One test, for example, completely explained why Mercury’s orbit didn’t match the predictions made by Newton. Other predictions could be tested with the scientific instruments of the day, like measuring time dilation with fast moving clocks.

Since gravity is actually a distortion of spacetime, Einstein predicted that massive objects moving through spacetime should generate ripples, like waves moving through the ocean.

The more massive the object, the more it distorts spacetime. Credit: LIGO/T. Pyle
The more massive the object, the more it distorts spacetime. Credit: LIGO/T. Pyle

Just by walking around, you leave a wake of gravitational waves that compress and expand space around you. However, these waves are incredibly tiny. Only the most energetic events in the entire Universe can produce waves we can detect.

It took over 100 years to finally be proven true, the direct detection of gravitational waves. In February, 2016, physicists with the Laser Interferometer Gravitational Wave Observatory, or LIGO announced the collision of two massive black holes more than a billion light-years away.

Any size of black hole can collide. Plain old stellar mass black holes or supermassive black holes. Same process, just on a completely different scale.

Colliding black holes. Credit: LIGO/A. Simonnet
Colliding black holes. Credit: LIGO/A. Simonnet

Let’s start with the stellar mass black holes. These, of course, form when a star with many times the mass of our Sun dies in a supernova. Just like regular stars, these massive stars can be in binary systems.

Imagine a stellar nebula where a pair of binary stars form. But unlike the Sun, each of these are monsters with many times the mass of the Sun, putting out thousands of times as much energy. The two stars will orbit one another for just a few million years, and then one will detonate as a supernova. Now you’ll have a massive star orbiting a black hole.  And then the second star explodes, and now you have two black holes orbiting around each other.

As the black holes zip around one another, they radiate gravitational waves which causes their orbit to decay. This is kind of mind-bending, actually. The black holes convert their momentum into gravitational waves.

As their angular momentum decreases, they spiral inward until they actually collide.  What should be one of the most energetic explosions in the known Universe is completely dark and silent, because nothing can escape a black hole. No radiation, no light, no particles, no screams, nothing. And if you mash two black holes together, you just get a more massive black hole.

The gravitational waves ripple out from this momentous collision like waves through the ocean, and it’s detectable across more than a billion light-years.

Arial view of LIGO Livingston. (Image credit: The LIGO Scientific Collaboration).
Arial view of LIGO Livingston. Credit: The LIGO Scientific Collaboration

This is exactly what happened earlier this year with the announcement from LIGO. This sensitive instrument detected the gravitational waves generated when two black holes with 30 solar masses collided about 1.3 billion light-years away.

This wasn’t a one-time event either, they detected another collision with two other stellar mass black holes.

Regular stellar mass black holes aren’t the only ones that can collide. Supermassive black holes can collide too.

From what we can tell, there’s a supermassive black hole at the heart of pretty much every galaxy in the Universe. The one in the Milky Way is more than 4.1 million times the mass of the Sun, and the one at the heart of Andromeda is thought to be 110 to 230 million times the mass of the Sun.

In a few billion years, the Milky Way and Andromeda are going to collide, and begin the process of merging together. Unless the Milky Way’s black hole gets kicked off into deep space, the two black holes are going to end up orbiting one another.

Just with the stellar mass black holes, they’re going to radiate away angular momentum in the form of gravitational waves, and spiral closer and closer together. Some point, in the distant future, the two black holes will merge into an even more supermassive black hole.

View of Milkdromeda from Earth "shortly" after the merger, around 3.85-3.9 billion years from now Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
View of Milkdromeda from Earth “shortly” after the merger, around 3.85-3.9 billion years from now. Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger

The Milky Way and Andromeda will merge into Milkdromeda, and over the future billions of years, will continue to gather up new galaxies, extract their black holes and mashing them into the collective.

Black holes can absolutely collide. Einstein predicted the gravitational waves this would generate, and now LIGO has observed them for the first time. As better tools are developed, we should learn more and more about these extreme events.

The 2016 Nobel Prize In Physics: It’s Complicated

This year's Nobel Prize in physics highlights the complications of awarding breakthrough achievements. Credit:

Update: This year’s Nobel Prize in Physics has been awarded to David J. Thouless (University of Washington), F. Duncan M. Haldane (Princeton University), and J. Michael Kosterlitz of Brown University for “theoretical discoveries of topological phase transitions and topological phases of matter”. One half of the prize was awarded to Thouless while the other half was jointly awarded to Haldane and Kosterlitz.

The Nobel Prize in physics is a coveted award. Every year, the prize is bestowed upon the individual who is deemed to have made the greatest contribution to the field of physics during the preceding year. And this year, the groundbreaking discovery of gravitational waves is anticipated to be the main focus.

This discovery, which was announced on February 11th, 2016, was made possible thanks to the development of the Laser Interferometer Gravitational-Wave Observatory (LIGO). As such, it is expected that the three scientists that are most responsible for the invention of the technology will receive the Nobel Prize for their work. However, there are those in the scientific community who feel that another scientist – Barry Barish – should also be recognized.

But first, some background is needed to help put all this into perspective. For starers, gravitational waves are ripples in the curvature of spacetime that are generated by certain gravitational interactions and which propagate at the speed of light. The existence of such waves has been postulated since the late 19th century.

LIGO's two facilities, located in . Credit:
LIGO’s two observatories, the located in Livingston, Louisiana; and Hanford, Washington. Credit:

However, it was not until the late 20th century, thanks in large part to Einstein and his theory of General Relativity, that gravitational-wave research began to emerge as a branch of astronomy. Since the 1960s, various gravitational-wave detectors have been built, which includes the LIGO observatory.

Founded as a Caltech/MIT project, LIGO was officially approved by the National Science Board (NSF) in 1984. A decade later, construction began on the facility’s two locations – in Hanford, Washington and Livingston, Louisiana. By 2002, it began to obtain data, and work began on improving its original detectors in 2008 (known as the Advanced LIGO Project).

The credit for the creation of LIGO goes to three scientists, which includes Rainer Weiss, a professor of physics emeritus at the Massachusetts Institute of Technology (MIT); Ronald Drever, an experimental physics who was professor emeritus at the California Institute of Technology and a professor at Glasgow University; and Kip Thorne, the Feynman Professor of Theoretical Physics at Caltech.

In 1967 and 68, Weiss and Thorne initiated efforts to construct prototype detectors, and produced theoretical work to prove that gravitational waves could be successfully analyzed. By the 1970s, using different methods, Weiss and Denver both succeeded in building detectors. In the coming years, all three men remained pivotal and influential, helping to make gravitational astronomy a legitimate field of research.

 A bird's eye view of LIGO Hanford's laser and vacuum equipment area (LVEA). The LVEA houses the pre-stabilized laser, beam splitter, input test masses, and other equipment. Credit:
LIGO Hanford’s laser and vacuum equipment area (LVEA), which houses the pre-stabilized laser, beam splitter, input test masses, and other equipment. Credit:

However, it has been argued that without Barish – a particle physicist at Caltech – the discovery would never have been made. Having become the Principal Investigator of LIGO in 1994, he inherited the project at a very crucial time. It had begun funding a decade prior, but coordinating the work of Wiess, Thorne and Drever (from MIT, Caltech and the University of Glasgow, respectively) proved difficult.

As such, it was decided that a single director was needed. Between 1987 and 1994, Rochus Vogt – a professor emeritus of Physics at Caltech – was appointed by the NSF to fill this role. While Vogt brought the initial team together and helped to get the construction of the project approved, he proved difficult when it came to dealing with bureaucracy and documenting his researchers progress.

As such, between 1989 through 1994, LIGO failed to progress technically and organizationally, and had trouble acquiring funding as well. By 1994, Caltech eased Vogt out of his position and appointed Barish to the position of director. Barish got to work quickly, making significant changes to the way LIGO was administered, expanding the research team, and developing a detailed work plan for the NSF.

Barish was also responsible for expanding LIGO beyond its Caltech and MIT constraints. This he did through the creation of the independent LIGO Scientific Collaboration (LSC), which gave access to outside researchers and institutions. This was instrumental in creating crucial partnerships, which included the UK Science and Technology Facilities Council, the Max Planck Society of Germany, and the Australian Research Council.

Artist's impression of how massive bodies (like our Sun) distort space time. Credit: T. Pyle/Caltech/MIT/LIGO Lab
Artist’s impression of how massive bodies (like our Sun) distort space time. Such bodies also create gravity waves when they accelerate through space and time. Credit: T. Pyle/Caltech/MIT/LIGO Lab

By 1999, construction had wrapped up on the LIGO observatories, and by 2002, they began taking their first bits of data. By 2004, the funding and groundwork was laid for the next phase of LIGO development, which involved a multi-year shut-down while the detectors were replaced with improved “Advanced LIGO” versions.

All of this was made possible by Barish, who retired in 2005 to head up other projects. Thanks to his sweeping reforms, LIGO got to work after an abortive start, began to produce data, procured funding, crucial partnerships, and now has more than 1000 collaborators worldwide, thanks to the LSC program he established.

Little wonder then why some scientists think the Nobel Prize should be split four-ways, awarding the three scientists who conceived of LIGO and the one scientist who made it happen. And as Barish himself was quoted as saying by Science:

“I think there’s a bit of truth that LIGO wouldn’t be here if I didn’t do it, so I don’t think I’m undeserving. If they wait a year and give it to these three guys, at least I’ll feel that they thought about it,” he says. “If they decide [to give it to them] this October, I’ll have more bad feelings because they won’t have done their homework.”

The approximate locations of the two gravitational-wave events detected so far by LIGO are shown on this sky map of the southern hemisphere. . Credit: LIGO/Axel Mellinger
The approximate locations of the two gravitational-wave events detected so far by LIGO are shown on this sky map of the southern hemisphere. . Credit: LIGO/Axel Mellinger

However, there is good reason to believe that the award will ultimately be split three ways, leaving Barish out. For instance, Weiss, Drever, and Thorne have been honored three times already this year for their work on LIGO. This has included the Special Breakthrough Prize in Fundamental Physics, the Gruber Cosmology Prize, and Kavli Prize in Astrophysics.

What’s more, in the past, the Nobel Prize in physics has tended to be awarded to those responsible for the intellectual contributions leading to a major breakthrough, rather than to those who did the leg work. Out of the last six Prizes issued (between 2010 and 2015), five have been awarded for the development of experimental methods, observational studies, and theoretical discoveries.

Only one award was given for a technical development. This was the case in 2014 where the award was given jointly to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for “the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”.

Basically, the Nobel Prize is a complicated matter. Every year, it is awarded to those who made a considerable contribution to science, or were responsible for a major breakthrough. But contributions and breakthroughs are perhaps a bit relative. Whom we choose to honor, and for what, can also be seen as an indication of what is valued most in the scientific community.

In the end, this year’s award may serve to highlight how significant contributions do not just entail the development of new ideas and methods, but also in bringing them to fruition.

Further Reading: Science, LIGO,

Weekly Space Hangout – June 17, 2016: LIGO Team

Host: Fraser Cain (@fcain)

Special Guest: LIGO Team Members:Kai Staats and Michael Landry
Kai Staats is a filmmaker, lecturer and writer working in science outreach. He is currently completing his MSc thesis for his research in machine learning applied to radio astronomy at the University of Cape Town and the Square Kilometer Array, South Africa. Staats was for ten years CEO of a Linux OS and HPC solutions provider whose systems were used to process images at NASA JPL, conduct sonar imaging on-board Navy submarines, and conduct bioinformatics research at DoE labs. In 2012 Staats engaged his passion for storytelling through film. His work includes sci-fi, human interest, wildlife conservation, and science outreach and education. “LIGO Detection” marks Staats’ 3rd film for the gravitational wave observatory that in February announced detection of merging black holes.

Mike Landry is Detection Lead Scientist at LIGO Hanford Observatory (LHO), Washington State. He began working on LIGO in 2000 as a Caltech postdoc at LHO, and has remained there since. Mike has worked on a variety of aspects of the experiment, including commissioning, calibration, and searches for gravitational waves from spinning neutron stars. From 2010 to 2015, he led the installation of Advanced LIGO at Hanford. Prior to working on LIGO, he received his Ph.D. in particle and nuclear physics from the University of Manitoba, for studies in strange hadronic physics at the Brookhaven National Laboratory’s AGS accelerator.

Paul M. Sutter ( / @PaulMattSutter)
Morgan Rehnberg ( / @MorganRehnberg)
Kimberly Cartier (@AstroKimCartier )

Their stories this week:
The discovery of a habitable zone “Tatooine” planet

Experimenting with igniting fires in space

1/3 of the world (and 80% of Americans) can’t see the Milky Way

Eight space telescopes are renewed by NASA

We’ve had an abundance of news stories for the past few months, and not enough time to get to them all. So we are now using a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (, which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!

We record the Weekly Space Hangout every Friday at 12:00 pm Pacific / 3:00 pm Eastern. You can watch us live on Google+, Universe Today, or the Universe Today YouTube page.

You can also join in the discussion between episodes over at our Weekly Space Hangout Crew group in G+!

Second Gravitational Wave Source Found By LIGO

This image depicts two black holes just moments before they collided and merged with each other, releasing energy in the form of gravitational waves. Image credit: Numerical Simulations: S. Ossokine and A. Buonanno, Max Planck Institute for Gravitational Physics, and the Simulating eXtreme Spacetime (SXS) project. Scientific Visualization: T. Dietrich and R. Haas, Max Planck Institute for Gravitational Physics.

Lightning has struck twice – maybe three times – and scientists from the Laser Interferometer Gravitational-wave Observatory, or LIGO, hope this is just the beginning of a new era of understanding our Universe. This “lightning” came in the form of the elusive, hard-to-detect gravitational waves, produced by gigantic events, such as a pair of black holes colliding. The energy released from such an event disturbs the very fabric of space and time, much like ripples in a pond. Today’s announcement is the second set of gravitational wave ripples detected by LIGO, following the historic first detection announced in February of this year.

“This collision happened 1.5 billion years ago,” said Gabriela Gonzalez of Louisiana State University at a press conference to announce the new detection, “and with this we can tell you the era of gravitational wave astronomy has begun.”

LIGO’s first detection of gravitational waves from merging black holes occurred Sept. 14, 2015 and it confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity. The second detection occurred on Dec. 25, 2015, and was recorded by both of the twin LIGO detectors.

While the first detection of the gravitational waves released by the violent black hole merger was just a little “chirp” that lasted only one-fifth of a second, this second detection was more of a “whoop” that was visible for an entire second in the data. Listen in this video:

“This is what we call gravity’s music,” said González as she played the video at today’s press conference.

While gravitational waves are not sound waves, the researchers converted the gravitational wave’s oscillation and frequency to a sound wave with the same frequency. Why were the two events so different?

From the data, the researchers concluded the second set of gravitational waves were produced during the final moments of the merger of two black holes that were 14 and 8 times the mass of the Sun, and the collision produced a single, more massive spinning black hole 21 times the mass of the Sun. In comparison, the black holes detected in September 2015 were 36 and 29 times the Sun’s mass, merging into a black hole of 62 solar masses.

The scientists said the higher-frequency gravitational waves from the lower-mass black holes hit the LIGO detectors’ “sweet spot” of sensitivity.

“It is very significant that these black holes were much less massive than those observed in the first detection,” said Gonzalez. “Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe.”

An aerial view of LIGO Hanford. (Credit:  Gary White/Mark Coles/California Institue of Technology/LIGO/NSF).
An aerial view of LIGO Hanford. (Credit: Gary White/Mark Coles/California Institue of Technology/LIGO/NSF).

LIGO allows scientists to study the Universe in a new way, using gravity instead of light. LIGO uses lasers to precisely measure the position of mirrors separated from each other by 4 kilometers, about 2.5 miles, at two locations that are over 3,000 km apart, in Livingston, Louisiana, and Hanford, Washington. So, LIGO doesn’t detect the black hole collision event directly, it detects the stretching and compressing of space itself. The detections so far are the result of LIGO’s ability to measure the perturbation of space with an accuracy of 1 part in a thousand billion billion. The signal from the lastest event, named GW151226, was produced by matter being converted into energy, which literally shook spacetime like Jello.

LIGO team member Fulvio Ricci, a physicist at the University of Rome La Sapienzaa said there was a third “candidate” detection of an event in October, which Ricci said he prefers to call a “trigger,” but it was much less significant and the signal to noise not large enough to officially count as a detection.

But still, the team said, the two confirmed detections point to black holes being much more common in the Universe than previously believed, and they might frequently come in pairs.

“The second discovery “has truly put the ‘O’ for Observatory in LIGO,” said Albert Lazzarini, deputy director of the LIGO Laboratory at Caltech. “With detections of two strong events in the four months of our first observing run, we can begin to make predictions about how often we might be hearing gravitational waves in the future. LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe.”

LIGO is now offline for improvements. Its next data-taking run will begin this fall and the improvements in detector sensitivity could allow LIGO to reach as much as 1.5 to two times more of the volume of the universe compared with the first run. A third site, the Virgo detector located near Pisa, Italy, with a design similar to the twin LIGO detectors, is expected to come online during the latter half of LIGO’s upcoming observation run. Virgo will improve physicists’ ability to locate the source of each new event, by comparing millisecond-scale differences in the arrival time of incoming gravitational wave signals.

In the meantime, you can help the LIGO team with the Gravity Spy citizen science project through Zooniverse.

Sources for further reading:
Press releases:
University of Maryland
Northwestern University
West Virginia University
Pennsylvania State University
Physical Review Letters: GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence
LIGO facts page, Caltech

For an excellent overview of gravitational waves, their sources, and their detection, check out Markus Possel’s excellent series of articles we featured on UT in February:

Gravitational Waves and How They Distort Space

Gravitational Wave Detectors and How They Work

Sources of Gravitational Waves: The Most Violent Events in the Universe

The Future of Gravitational Wave Astronomy: Pulsar Webs, Space Interferometers and Everything

A merging of two massive objects, sending ripples through the fabric of space and time. Image credit: R. Hurt/Caltech JPL

It’s the hot new field in modern astronomy. The recent announcement of the direct detection of gravitational waves by the Laser Interferometer Gravitational-wave Observatory (LIGO) ushers in a new era of observational astronomy that is completely off the electromagnetic spectrum. This detection occurred on September 14th, 2015 and later earned itself the name GW150914. This occurred shortly after Advanced LIGO turned on in early September, a great sign concerning the veracity of the equipment. Continue reading “The Future of Gravitational Wave Astronomy: Pulsar Webs, Space Interferometers and Everything”

Weekly Space Hangout – Feb. 12, 2016: Amy Shira Teitel

Host: Fraser Cain (@fcain)

Special Guest: Amy Shira Teitel, Author of “Breaking the Chains of Gravity” that was released in the States on January 12, 2016. Contributing writer for Vintage Space column on Popular Science, and host at Discovery News.

Morgan Rehnberg ( / @MorganRehnberg )
Jolene Creighton ( / @futurism)

Continue reading “Weekly Space Hangout – Feb. 12, 2016: Amy Shira Teitel”

Sources of Gravitational Waves: The Most Violent Events in the Universe

One of the most promising gravitational wave sources: Bodies orbiting each other under their own gravity

Soon, very soon, Thursday, February 11, at 10:30 Eastern time, we are likely to learn at any one of several press conferences – at the National Press Club in Washington, D.C., in Hannover, Germany, near Pisa in Italy and elswhere – that gravitational waves have been measured directly, for the first time. This would mean the first direct detection of minute distortions of spacetime, travelling at the speed of light, first postulated by Albert Einstein almost exactly 100 years ago.

Time to brush up on your gravitational wave basics: In Gravitational waves and how they distort space, we had a look at what gravitational waves do. In Gravitational wave detectors: How they work we saw how you can measure gravitational waves. Third and final step: What are typical gravitational wave sources? How are these waves produced?

Objects in orbit

The simplest situation that produces gravitational waves in the cosmos is almost ubiquitous: two or more objects orbiting around each other under their own gravity. The waves they generate are reminiscent to a very slow mixer in the middle of a pool of water: One of the most promising gravitational wave sources: Objects in orbit around each other. By Sascha Husa, Universitat de les Illes Balears This is not something you would see, of course. The wave that is pictured here represents the strength of the minute changes in distance that would be caused by the gravitational wave, just as we’ve seen in Gravitational waves and how they distort space. The animation is courtesy of Sascha Husa of the Universitat de les Illes Balears.

Indirect evidence

Gravitational waves emitted by orbiting objects carry away energy. Elementary physics tells you that if you remove energy from an orbiting system, the distance between the orbiting objects will shrink, and they will orbit each other faster than before.

In fact, gravitational waves making a binary system of neutron stars speed up was the first evidence for the existence of gravitational waves. The binary neutron star was discovered by Hulse and Taylor in 1974, and the speed-up caused by gravitational waves published by Taylor and Weisberg in 1984, after a careful analysis of seven years’ worth of data. Hulse and Taylor were awarded the Nobel prize in physics in 1993 for their discovery.

Here, in an image from an article by Weisberg 2010, is the match between general relativistic prediction and observation in all its glory (or at least in all its glory up to 2005): weisberg2010As the two neutron stars speed up, they will reach the point of closest approach within their orbit earlier and earlier. How much earlier, in seconds, is plotted on the vertical axis, year of measurement on the horizontal axis.

A matter of frequency

Today’s ground-based detectors cannot detect gravitational waves from all kinds of bodies in mutual orbit. The bodies need to be massive, compact and, crucially, orbit each other quickly enough. For bodies orbiting each other less than a few times per second (very quick, if you are talking about astronomical bodies!), the frequency of the resulting gravitational wave will be too low for ground-based detectors to measure reliably. In the low-frequency regime, below 10–100 Hertz, disturbances caused by undulating motions of the Earth’s surface (“seismic noise”) are dominant, and drown out the minute effects of gravitational waves.

When it comes to gravitational waves from supermassive black holes, or from white dwarfs, we will have to wait for future space-based gravitational wave detectors.

The most promising gravitational wave sources go “chirp”

When an orbiting system emits gravitational waves, orbital motion speeds up. And when orbital motion speeds up, the system emits even more energy in form of gravitational wave. This runaway process ends only when the orbiting objects collide and merge.

The final phase is marked by a quick increase in orbital speed, corresponding to ever higher gravitational wave frequency, and ever higher intensity. Here’s what such a signal looks like (image and audio from “Chirping Neutron Stars” on Einstein Online): chirp-enYou can see how the frequency and intensity increase right up to time 0, when the two neutron stars collide and merge.

For stellar black holes (with masses between a few and a few dozen solar masses) and neutron stars, in any combination, the frequencies of these gravitational waves are the same as the frequencies of audible sound waves. One can actually represent these changes in frequency as an audible tone, as in this example of two neutron stars merging (Audio © B. Owen, Penn State University):

Here is the same kind of audible representation for the merger of a black hole and a neutron star (© AEI/GEO600):

Sadly, what a gravitational wave detector registers is the combination of this sound plus assorted noise, which sounds like this (© AEI/GEO600):

Colleagues at Cardiff University have made this into a nice online game: Black Hole Hunter. Head over there and see if you can hear the signal beneath the noise!

(And you can hear live chirps by various astrophysicists (and others) under the hashtag #chirpForLIGO on Twitter.)

This kind of signal, from merging stellar black holes or neutron stars (in any combination) is the most promising candidate signal for today’s detectors – and going by the rumors, that is indeed what LIGO appears to have found.

The final part of the signal is interesting for a particular reason: It doesn’t follow from any simple formulae, and can only be modelled with complex computer simulations of such situations known as numerical relativity. If the detectors get a good detection of this very last bit, that will be a good test for current numerical simulations of general relativity!

Other gravitational wave sources

Chirps are comparatively simple, and likely the first signals to be found.

Another kind of signal that could be found is periodic (or nearly so), and would be produced e.g. if rapidly rotating neutron stars are less than perfectly smooth. No such luck as of yet, though.

Next would come the gravitational wave sources that are somewhat less understood, such as the processes in the interior of supernova explosions. And finally, once numerous signals have been detected, showing the scientists that their detectors are indeed working as they should, there might be the detection of completely unexpected signals. Whenever astronomers have opened a new window to the cosmos – the radio window, infrared window, x-ray window – they have found something new and unexpected. Who can tell what opening the Einstein window, the window of gravitational waves, will teach us about the universe?

Update: Gravitational Waves Discovered