̽��ֱ�� of Cambridge - Davide Gerosa

Using gravitational waves to catch runaway black holes

sc604 — Thu, 30 Jun 2016 08:47:41 +0000

Researchers have developed a new method for detecting and measuring one of the most powerful, and most mysterious, events in the Universe – a black hole being kicked out of its host galaxy and into intergalactic space at speeds as high as 5000 kilometres per second.

̽��ֱ��method, developed by researchers from the ̽��ֱ�� of Cambridge, could be used to detect and measure so-called black hole superkicks, which occur when two spinning supermassive black holes collide into each other, and the recoil of the collision is so strong that the remnant of the black hole merger is bounced out of its host galaxy entirely. Their results are reported in the journal Physical Review Letters.

Earlier this year, the LIGO Collaboration announced the first detection of gravitational waves – ripples in the fabric of spacetime – coming from the collision of two black holes, confirming a major prediction of Einstein’s general theory of relativity and marking the beginning of a new era in astronomy. As the sensitivity of the LIGO detectors is improved, even more gravitational waves are expected to be detected – the second successful detection was announced in June.

As two black holes circle each other, they emit gravitational waves in a highly asymmetric way, which leads to a net emission of momentum in some preferential direction. When the black holes finally do collide, conservation of momentum imparts a recoil, or kick, much like when a gun is fired. When the two black holes are not spinning, the speed of the recoil is around 170 kilometres per second. But when the black holes are rapidly spinning in certain orientations, the speed of the recoil can be as high as 5000 kilometres per second, easily exceeding the escape velocity of even the most massive galaxies, sending the black hole remnant resulting from the merger into intergalactic space.

̽��ֱ��Cambridge researchers have developed a new method for detecting these kicks based on the gravitational wave signal alone, by using the Doppler Effect. ̽��ֱ��Doppler Effect is the reason that the sound of a passing car seems to lower in pitch as it gets further away. It is also widely used in astronomy: electromagnetic radiation coming from objects which are moving away from the Earth is shifted towards the red end of the spectrum, while radiation coming from objects moving closer to the Earth is shifted towards the blue end of the spectrum. Similarly, when a black hole kick has sufficient momentum, the gravitational waves it emits will be red-shifted if it is directed away from the Earth, while they will be blue-shifted if it’s directed towards the Earth.

“If we can detect a Doppler shift in a gravitational wave from the merger of two black holes, what we’re detecting is a black hole kick,” said study co-author Davide Gerosa, a PhD student from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “And detecting a black hole kick would mean a direct observation that gravitational waves are carrying not just energy, but linear momentum as well.”

Detecting this elusive effect requires gravitational-wave experiments capable of observing black hole mergers with very high precision. A black hole kick cannot be directly detected using current land-based gravitational wave detectors, such as those at LIGO. However, according to the researchers, the new space-based gravitational wave detector known as eLISA, funded by the European Space Agency (ESA) and due for launch in 2034, will be powerful enough to detect several of these runaway black holes. In 2015, ESA launched the LISA Pathfinder, which is successfully testing several technologies which could be used to measure gravitational waves from space.

̽��ֱ��researchers found that the eLISA detector will be particularly well-suited to detecting black hole kicks: it will be capable of measuring kicks as small as 500 kilometres per second, as well as the much faster superkicks. Kick measurements will tell us more about the properties of black hole spins, and also provide a direct way of measuring the momentum carried by gravitational waves, which may lead to new opportunities for testing general relativity.

“When the detection of gravitational waves was announced, a new era in astronomy began, since we can now actually observe two merging black holes,” said study co-author Christopher Moore, a Cambridge PhD student who was also a member of the team which announced the detection of gravitational waves earlier this year. “We now have two ways of detecting black holes, instead of just one – it’s amazing that just a few months ago, we couldn’t say that. And with the future launch of new space-based gravitational wave detectors, we’ll be able to look at gravitational waves on a galactic, rather than a stellar, scale.”

Reference:
Davide Gerosa and Christopher J. Moore. ‘Black-hole kicks as new gravitational-wave observables.’ Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.011101

Black holes are the most powerful gravitational force in the Universe. So what could cause them to be kicked out of their host galaxies? Cambridge researchers have developed a method for detecting elusive ‘black hole kicks.’

We now have two ways of detecting black holes, instead of just one – it’s amazing that just a few months ago, we couldn’t say that.

Christopher Moore

SXS Lensing

Computer simulations motivated by GW150914

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Licence type:

Attribution-ShareAlike

New insights found in black hole collisions

sc604 — Fri, 27 Mar 2015 08:00:00 +0000

An international team of astronomers, including from the ̽��ֱ�� of Cambridge, have found solutions to decades-old equations describing what happens as two spinning black holes in a binary system orbit each other and spiral in toward a collision.

̽��ֱ��results, published in the journal Physical Review Letters, should significantly impact not only the study of black holes, but also the search for elusive gravitational waves – a type of radiation predicted by Einstein’s theory of general relativity – in the cosmos.

Unlike planets, whose average distance from the sun does not change over time, general relativity predicts that two black holes orbiting around each other will move closer together as the system emits gravitational waves.

“An accelerating charge, like an electron, produces electromagnetic radiation, including visible light waves,” said Dr Michael Kesden of the ̽��ֱ�� of Texas at Dallas, the paper’s lead author. “Similarly, any time you have an accelerating mass, you can produce gravitational waves.”

̽��ֱ��energy lost to gravitational waves causes the black holes to spiral closer and closer together until they merge, which is the most energetic event in the universe, after the big bang. That energy, rather than going out as visible light, which is easy to see, goes out as gravitational waves, which are much more difficult to detect.

While Einstein’s theories predict the existence of gravitational waves, they have not been directly detected. But the ability to ‘see’ gravitational waves would open up a new window to view and study the universe.

Optical telescopes can capture photos of visible objects, such as stars and planets, and radio and infrared telescopes can reveal additional information about invisible energetic events. Gravitational waves would provide a qualitatively new medium through which to examine astrophysical phenomena.

“Using gravitational waves as an observational tool, you could learn about the characteristics of the black holes that were emitting those waves billions of years ago, information such as their masses and mass ratios, and the way they formed” said co-author and PhD student Davide Gerosa, of Cambridge’s Department of Applied Mathematics and Theoretical Physics. “That’s important data for more fully understanding the evolution and nature of the universe.”

Later this year, upgrades to the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and VIRGO in Europe will be completed, and the first direct measurements of gravitational waves may be just around the corner. Around the same time, the LISA Pathfinder mission will be launched as a test mission for establishing a gravitational wave detector of unprecedented sensitivity in space.

“ ̽��ֱ��equations that we solved will help predict the characteristics of the gravitational waves that LIGO would expect to see from binary black hole mergers,” said co-author Dr Ulrich Sperhake, who, along with Gerosa, is also a member of Cambridge’s Centre for Theoretical Cosmology. “We’re looking forward to comparing our solutions to the data that LIGO collects.”

̽��ֱ��equations the researchers solved deal specifically with the spin angular momentum of binary black holes and a phenomenon called precession.

“Like a spinning top, black hole binaries change their direction of rotation over time, a phenomenon known as procession,” said Sperhake. “ ̽��ֱ��behaviour of these black hole spins is a key part of understanding their evolution.”

Just as Kepler studied the motion of the earth around the sun and found that orbits can be ellipses, parabola or hyperbolae, the researchers found that black hole binaries can be divided into three distinct phases according to their rotation properties.

̽��ֱ��researchers also derived equations that will allow statistical tracking of such spin phases, from black hole formation to merger, far more efficiently and quickly than was possible before.

“With these solutions, we can create computer simulations that follow black hole evolution over billions of years,” said Kesden. “A simulation that previously would have taken years can now be done in seconds. But it’s not just faster. There are things that we can learn from these simulations that we just couldn’t learn any other way.”

“With these tools, new insights into the dynamics of black holes will be unveiled,” said Gerosa. “Gravitational wave signals can now be better interpreted to unveil mysteries of the massive universe.”

Researchers from the Rochester Institute of Technology and the ̽��ֱ�� of Mississippi also contributed to the Physical Review Letters paper. ̽��ֱ��researchers were supported in part by the Science and Technology Facilities Council, the European Commission, the National Science Foundation, UT Dallas and the ̽��ֱ�� of Cambridge.

Inset image: Illustration of two rotating black holes in orbit. Both, the black hole spins (red arrows) and the orbital angular momentum (blue arrow) precess about the total angular momentum (grey arrow) in a manner that characterizes the black-hole binary system. Gravitational waves carry away energy and momentum from the system and the orbital plane (light blue) tilts and turns accordingly. Credit: Graphic by Midori Kitagawa

Adapted from ̽��ֱ�� of Texas at Dallas press release.

New research provides revelations about the most energetic event in the universe — the merging of two spinning, orbiting black holes into a much larger black hole.

̽��ֱ��behaviour of these black hole spins is a key part of understanding their evolution

Ulrich Sperhake

NASA's Marshall Space Flight Center

Black Holes Go 'Mano a Mano' (NASA, Chandra, 10/06/09)

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Licence type:

Attribution-Noncommerical