Posted on by Matt Williams

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

 

Posted on by Evan Gough

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.

Posted on by Nancy Atkinson

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