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.

Sources:

Do Gravitational Waves Permanently Alter the Nature of Spacetime?

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.

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first detection of gravitational waves. This development, which confirmed a prediction made by Einstein’s Theory of General Relativity a century prior, opened new avenues of research for cosmologists and astrophysicists. It was also a watershed for researchers at Monash University, who played an important role in the discovery.

And now, a little over a year later, a team of researchers from the Monash Center for Astrophysics has announced another potential revelation. Based on their ongoing studies of gravitational waves, the team recently proposed a theoretical concept known as ‘orphan memory’. If true, this concept could revolutionize the way we think about gravitational waves and spacetime.

Researchers from Monash Center for Astrophysics are part of what is known as the LIGO Scientific Collaboration (LSC) – a group of scientists dedicated to developing the hardware and software needed to study gravitational waves. In addition to creating a system for vetting detections, the team played a key role in data analysis – observing and interpreting the data that was gathered – and were also instrumental in the design of the LIGO mirrors.

Looking beyond what LIGO and other experiments (like the Virgo Interferometer) observed, the research team sought to address how these detectors capabilities could be extended further by finding the “memory” of gravitational waves. The study that describes this theory was recently published in the Physical Review Letters under the title “Detecting Gravitational Wave Memory without Parent Signals“.

According to their new theory, spacetime does not return to its normal state after a cataclysmic event generates gravitational waves that cause it to stretch out. Instead, it remains stretched, which they refer to as “orphan memory” – the word “orphan” alluding to the fact the “parent wave” is not directly detectable. While this effect has yet to be observed, it could open up some very interesting opportunities for gravitational wave research.

At present, detectors like LIGO and Virgo are only able to discern the presence of gravitational waves at certain frequencies. As such, researchers are only able to study waves generated by specific types of events and trace them back to their source. As Lucy McNeill, a researchers from the Monash Center for Astrophysics and the lead author on the paper, said in a recent University press statement:

“If there are exotic sources of gravitational waves out there, for example, from micro black holes, LIGO would not hear them because they are too high-frequency. But this study shows LIGO can be used to probe the universe for gravitational waves that were once thought to be invisible to it.”

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

As they indicate in their study, high-frequency gravitational-wave bursts (i.e. ones that are in or below the kilohertz  range) would produce orphan memory that the LIGO and Virgo detectors would be able to pick up. This would not only increase the bandwidth of these detectors exponentially, but open up the possibility of finding evidence of gravity wave bursts in previous searches that went unnoticed.

Dr Eric Thrane, a lecturer at the Monash School of Physics and Astronomy and another a member of the LSC team, was also one of the co-authors of the new study. As he stated, “These waves could open the way for studying physics currently inaccessible to our technology.”

But as they admit in their study, such sources might not even exist and more research is needed to confirm that “orphan memory” is in fact real. Nevertheless, they maintain that searching for high-frequency sources is a useful way to probe for new physics, and it just might reveal things we weren’t expecting to find.

“A dedicated gravitational-wave memory search is desirable. It will have enhanced sensitivity compared to current burst searches,” they state. “Further, a dedicated search can be used to determine whether a detection candidate is consistent with a memory burst by checking to see if the residuals (following signal subtraction) are consistent with Gaussian noise.”

Alas, such searches may have to wait upon the proposed successors to the Advanced LIGO experiment. These include the Einstein Telescope and Cosmic Explorer, two proposed third-generation gravitational wave detectors. Depending on what future surveys find, we may discover that spacetime not only stretches from the creation of gravitational waves, but also bears the “stretch marks” to prove it!

Further Reading: Physical Review Letters

 

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: nobelprize.org

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.caltech.edu
LIGO’s two observatories, the located in Livingston, Louisiana; and Hanford, Washington. Credit: ligo.caltech.edu

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.caltech.edu
LIGO Hanford’s laser and vacuum equipment area (LVEA), which houses the pre-stabilized laser, beam splitter, input test masses, and other equipment. Credit: ligo.caltech.edu

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, Nobelprize.org

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

Beyond WIMPs: Exploring Alternative Theories Of Dark Matter

Image from Dark Universe, showing the distribution of dark matter in the universe. Credit: AMNH

The standard model of cosmology tells us that only 4.9% of the Universe is composed of ordinary matter (i.e. that which we can see), while the remainder consists of 26.8% dark matter and 68.3% dark energy. As the names would suggest, we cannot see them, so their existence has had to be inferred based on theoretical models, observations of the large-scale structure of the Universe, and its apparent gravitational effects on visible matter.

Since it was first proposed, there have been no shortages of suggestions as to what Dark Matter particles look like. Not long ago, many scientists proposed that Dark Matter consists of Weakly-Interacting Massive Particles (WIMPs), which are about 100 times the mass of a proton but interact like neutrinos. However, all attempts to find WIMPs using colliders experiments have come up empty. As such, scientists have been exploring the idea lately that dark matter may be composed of something else entirely. Continue reading “Beyond WIMPs: Exploring Alternative Theories Of Dark Matter”

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. 19, 2016: Rebecca Roth

Host: Fraser Cain (@fcain)

Special Guest: Rebecca Roth, Imaging Coordinator & Social Media Specialist at NASA’s Goddard Space Flight Center; responsible for sharing imagery with the media, as well as sharing those images with the public, mainly through social media such as Instagram and Flickr.

Guests:

Morgan Rehnberg (MorganRehnberg.com / @MorganRehnberg )
Kimberly Cartier (@AstroKimCartier )
Dave Dickinson (www.astroguyz.com / @astroguyz)
Jolene Creighton (fromquarkstoquasars.com / @futurism)

Their stories this week:
New X-ray observatory takes to the skies

Hubble Directly Measures Rotation of Cloudy “Super-Jupiter”

Possible Hidden Layer of Meteorites in Antarctica

ESA says Goodbye to Philae

China to Displace Thousands for New Radio Telescope

Visible light and gravitational waves

We’ve had an abundance of news stories for the past few months, and not enough time to get to them all. So we’ve started a new system. Instead of adding all of the stories to the spreadsheet each week, 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 (http://bit.ly/WSHVote), 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+!

Gravitational Wave Detectors: How They Work

Simplified gravitational wave detectors

It’s official: this Thursday, February 11, at 10:30 EST, there will be parallel press conferences at the National Press Club in Washington, D.C., in Hannover, Germany, and near Pisa in Italy. Not officially confirmed, but highly probable, is that people running the LIGO gravitational wave detectors will announce the first direct detection of a gravitational wave. 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. Nobel prize time.

Time to brush up on your gravitational wave basics, if you haven’t done so! In Gravitational waves and how they distort space, I had a look at what gravitational waves do. Now, on to the next step: How can we measure what they do? How do gravitational wave detectors such as LIGO work?

Recall that this is how a gravitational wave will change the distances between particles, floating freely in a circular formation in empty space: How distances change when a simple gravitational wave passes through a ring of particles. This is what gravitational wave detectors need to measure.The wave is moving at right angles to the screen, towards you. I’ve greatly exaggerated the distance changes. For a realistic wave, even the giant distance between the Earth and the Sun would only change by a fraction of the diameter of a hydrogen atom. Tiny changes indeed.

How to detect something like this?

The first unsuccessful attempts to detect gravitational waves in the 1960s tried to measure how they make aluminum cylinders ring like a very soft bell. (Tragic story; Joe Weber [1919-2000], the pioneering physicist behind this, was sure he had detected gravitational waves in this way; after thorough analysis and replication attempts, community consensus emerged that he hadn’t.)

Afterwards, physicists came up with alternative scheme. Imagine that you are replacing the black point in the center of the previous animation with a detector, and the rightmost red particle with a laser light source. Now you send light pulses (represented here by fast red dots) from the light source to the detector; let’s first look at this with the gravitational wave switched off:Simplified gravitational wave detector without gravitational wave

Every time a light pulse reaches the detector, an indicator light flashes yellow. The pulses are sent out regularly, they all travel at the same speed, hence they also reach the detector in regular intervals.

If a gravitational wave passes through this system, again from the back and coming towards you, distances will change. Let us keep our camera trained on the detector, so the detector remains where it is. The changing distance to the light source, and also the changing distances between the light pulses, and some of the changes in distance between light pulses and detector or source, are due to the gravitational wave. Here is what that would look like (again, hugely exaggerated): The same simplified gravitational wave detector, but now with a gravitational wave passing through.

Keep your eye on the blinking light, and you will see that its blinking is not so regular any more. Sometimes, the light blinks faster, sometimes slower. This is an effect of the gravitational wave. An effect by which we can hope to detect the gravitational wave.

“We” in this case are the radio astronomers working on what are known as Pulsar Timing Arrays. The sender of regular pulses are pulsars, rotating neutron stars sweeping a radio beam across our antennas like a cosmic lighthouse. The detectors are radio telescopes here on Earth. Detection is anything but easy. With a single pulsar, you’d need to track pulse arrival times with an accuracy of a few billionths of a second over half a year, and make sure you are not being fooled by various other sources of timing variations. So far, no gravitational waves have been detected in this way, although the radio astronomers are keeping at it.

To see how gravitational wave detectors like LIGO work, we need to make things a little more complex.

Interferometric gravitational wave detectors: the set-up

Here is the basic set-up: Two mirrors, a receiver (or “light detector”), a light source and what is known as a beamsplitter: Basic setup for an interferometric gravitational wave detector

Light is sent into the detector from the (laser) light source LS to the beamsplitter B which, true to its name, sends half of the light on to the mirror M1 and lets the other half through to the mirror M2. At M1 and M2, respectively, the light is reflected back to the beam splitter. There, the light arriving from M1 (or M2) is split again, with half going towards the light detector LD, the other half back in the direction of the light source LS. We will ignore the latter half and pretend, for the sake of our simplified explanation, that all the light reaching B from M1 or M2 goes on to the light detector LD.

(To avoid confusion, I will always refer to LD as the “light detector” and take the unqualified word “detector” to mean the whole setup.)

This setup, by the way, is called a Michelson Interferometer. We’ll see below why it is a good setup for gravitational wave detectors.

In what follows, we will assume that the mirrors and the beam splitter, shown as being suspended, react to the gravitational wave in the same way freely floating particles would react. The key effects are between the mirrors and the beam splitter in what are called the two arms of the detector. Arm length is huge in today’s detectors, running to a few kilometers. In comparison, light source and light detector are very close to the beamsplitter; changes of the distances between these three do not signify.

Light pulses in a gravitational wave detector

Next, let us see how light pulses run through this detector. Here is the same setup, seen from above: Simple interferometric gravitational wave detector, seen from aboveLight source LS, the two mirrors M1 and M2, the beamsplitter B and the light detector LD: all present and accounted for.

Next, we let the light source emit light pulses. For greater clarity, I will make two artificial and unrealistic changes. I will send red and green pulses into the detector, representing the light that goes into the horizontal and the vertical arm, respectively. In reality, there is no distinction, just light apportioned at the beamsplitter. Light running towards M1 will be offset a little to the left, light coming back from M1 to the right, for better clarity. Same goes for M2. This, too, is different in a real detector. That said, here come the light pulses: Simplified interferometric gravitational wave detector with light running through both armsLight starts at the light source to the left. Light that has left the source together, travels together (so green and red pulses are side by side) until the beam splitter. The beam splitter then sends the green pulses on their upward journey and lets the red pulses pass on their way towards the mirror on the right. All the particles that arrive back at the beamsplitter after reflection at M1 or M2. At the beamsplitter, they are directed towards the light detector at the bottom.

In this setup, the horizontal arm is slightly longer than the vertical arm. Red particles have to cover some extra distance. That is why they arrive at the detector a bit later, and we get an alternating rhythm: green, red, green, red, with equal distances in between. This will become important later on.

Here is a diagram, a kind of registration strip, which shows the arrival times for red and green pulses at the light detector (time is measured in “animation frames”): Arrival times at the light detector of a simplified gravitational wave detectorThe pattern is clear: red and green pulses arrive evenly spaced, one after the other.

Bring on the gravitational wave!

Next, let’s switch on our standard gravitational wave (exaggerated, passing through the screen towards you, and so on). Here is the result: Simple interferometric gravitational wave detector with a gravitational wave passing throughWe have trained our camera on the beamsplitter (so in our image, the beamsplitter doesn’t move). We ignore any slight changes in distance between beamsplitter and light source/light detector. Instead, we focus on the mirrors M1 and M2, which change their distance from the beamsplitter just as we would expect from the earlier animations.

Look at the way the pulses arrive at our light detector: sometimes red and green are almost evenly spaced, sometimes they close together. That is caused by the gravitational wave. Without the wave, we had strict regularity.

Here is the corresponding “registration strip” diagram. You can see that at some times, the light pulses of each color are closer together, at others, farther apart: Arrival times for light pulses in a gravitational wave detector

At the time I have marked with a hand-drawn arrow, red and green pulses arrive almost in unison!

The pattern is markedly different from the scenario without a gravitational wave. Detect this change in the pattern, and you have detected the gravitational wave.

Running interference

If you’ve wondered why detectors like LIGO are called interferometric gravitational wave detectors, we will need to think about waves a bit more. If not, let me just state that detectors like LIGO use the wave properties of light to measure the changes in pulse arrival rate you have seen in the last animation. To skip the details, feel free to jump ahead to the last section, “…and now for something a thousand times more complicated.”

Light is a wave, with crests and troughs corresponding to maxima and minima of the electric and of the magnetic field. While the animations I have shown you track the propagation of light pulses, they can also be used to understand what happens to a light wave in the interferometer. Just assume that each of the moving red and green dots in the detector marks the position of a wave crest.

Particles just add up. Take 2 particle and add 2 particles, and you will end up with 4 particles. But if you add up (combine, superimpose) waves, it depends. Sometimes, one wave plus another wave is indeed a bigger wave. Sometimes, it’s a smaller wave, or no wave at all. And sometimes it’s complicated.

When two waves are in perfect sync, the crests of the one aligning with the crests of the other, and the troughs aligning, too, you indeed get a bigger wave. The following diagram shows at which times the different parts of two light waves arrive at the light detector, and how they add up. (I’ve placed a dot on top of each crest; that is what the dots where meant to signify, after all.) Constructive interference of light wavesOn top, the green wave, perfectly aligned with the red wave (which, for clarity, is shown directly below the green wave). Add the two waves up, and you will get the (markedly stronger) blue wave in the bottom panel.

Not so if the two waves are maximally misaligned, the crests of each aligned with the troughs of the other. A crest and a trough cancel each other out. The sum of a wave and a maximally misaligned wave of equal strength is: no wave at all. Here is the corresponding diagram: Destructive interference of light wavesRecall that this was exactly the setup for our gravitational wave detector in the absence of gravitational waves: Red and green pulses with equal spacing; troughs of the one wave perfectly aligned with the crests of the other. The result: No light at the light detector. (For realistic gravitational wave detectors, that is almost true.)

When a gravitational wave passes through the detector, the situation changes. Here is the corresponding pattern of pulse/wave crest arrival times for the animation above: Interference pattern for a gravitational wave passing through the simplified gravitational wave detectorThe blue pattern, which is the sum of the red and the green, is complex. But it is not a flat line. There is light at the light detector where there was no light before, and the cause of the change is the gravitational wave passing through.

All in all, this makes a (highly simplified) version of how gravitational wave detectors such as LIGO work. Whatever the scientists will report this Thursday, it is based on light signals at the exit of such an interferometric detector.

And now for something a thousand times more complicated

Real gravitational wave detectors are, of course, much more complicated than that. I haven’t even started talking about the many disturbances scientists need to take into account – and to suppress as far as possible. How do you suspend the mirrors so that (at least for certain gravitational waves) they will indeed be influenced as if they were freely floating particles? How do you prevent seismic noise, cars or trains in the wider neighborhood and so on from moving your mirrors a tiny little bit (either by vibrations or by their own gravity)? What about fluctuations of the laser light?

Gravitational wave hunting is largely a hunt for noise, and for ways of suppressing that noise. The LIGO gravitational wave detectors and their kin are highly complex machines, with hundreds of control circuits, highly elaborate mirror suspensions, the most stable lasers known to physics (and some of the most high-powered). The technology has been contributed by numerous group from all over the world.

But all this is taking us too far, and I refer you to the pages of the detectors and collaborations for additional information:

LIGO pages at Caltech

Pages of the LIGO Scientific Collaboration

GEO 600 pages

VIRGO / EGO pages

You can find some further information about gravitational waves on the Einstein Online website:

Einstein Online: Spotlights on gravitational waves

Update: Gravitational Waves Discovered