The lives of massive stars are characterized by companionship: these stars are almost always found in gravitationally bound pairs. As such massive binaries evolve further, their cores run out of nuclear fuel and the stars can explode as supernovae, leaving behind in their centers either a neutron star or a black hole. In most cases such an explosion would be fatal for the binary, and disrupt it. In some cases, however, the final phases of binary stellar evolution can produce two compact objects -either white dwarfs, neutron stars or black holes– in tight binary systems.
Compact objects in binary pairs are driven closer and closer together as their system loses energy through interactions and gravitational radiation. The resulting merger of these types of objects is thought to be responsible for some of the most energetic events in the Universe including Type Ia supernovae and short duration gamma ray bursts.
The Laser Interferometer Gravitational-wave Observatory (LIGO) has made it possible to observe the gravitational waves emitted during mergers of compact objects. Since the beginning of data collection with the advanced instrument the LIGO observatory has detected gravitational waves from three different mergers of black hole pairs. Each of these detections was surprising because they involved a population of black holes that had not been observed before : black holes with masses of a few tens of solar masses. Scientists are also anticipating that LIGO will detect merging neutron stars.
Compact binary systems are really a cornerstone of modern astrophysics. Once the merger of a neutron star binary is detected in gravitational waves and electromagnetically, this will tell us about General Relativity, give us hints on how and where such binary systems form and —maybe most surprisingly— it may answer questions about nuclear physics that cannot be answered otherwise. This is maybe the most fascinating part of this rich story. — Stephan Rosswog
Connecting the sources of gravitational waves with phenomena that scientists are already familiar with, like supernovae and gamma ray bursts, requires that we observe the electromagnetic counterpart to the gravitational wave event. This is a challenging task with LIGO in its current state because the detector isn’t able to localize an event very precisely so follow-up searches with optical (and other wavelength) telescopes must search large areas of the sky for a new transient source.
Leloudas and colleagues performed follow-up observations after the luminous event and found that with ten months of additional data the event no longer resembled that of a supernova. Co-investigator, Christoffer Fremling from the OKC, did all of the image subtraction to extract the light curve for this event. Instead, they suggest, ASSASN-15lh was a star ripped apart by a supermassive black hole – a tidal disruption event.
One interesting aspect is the suggestion that this supermassive black hole might be rotating rapidly, says co-investigator Jesper Sollerman from OKC. Maybe these kind of tidal disruption events will become a way to explore the rotation of supermassive black holes billons of light years away.
[Top image: This artist’s impression depicts a sun-like star close to a rapidly spinning supermassive black hole, with a mass of about 100 million times the mass of the sun, in the centre of a distant galaxy. Photograph: ESO, ESA/Hubble, M. Kornmesser]
Black holes leave their marks all over the observed universe. They do however also inspire new and exciting ideas about space and time itself, both in the micro-cosmos and on the large scales of the universe.
This August around 150 researchers from all of the world gathered for a Nordita program and conference devoted to “Black Holes and Emergent Spacetime”, organised in part by fellow OKC-er Larus Thoilacius and myself. In this blog post I shall try to explain part of the excitement and also how some ideas that were discussed address the dark matter and dark energy puzzles directly, as well as the details of the fluctuations in the cosmic microwave background.
The background story starts with the realization by Bekenstein, Hawking and others that black hole behaviour is captured by thermodynamics, when temperature and entropy are identified correctly. Since modern physicists associate thermodynamics to an underlying statistical description of a quantum system, many vague ideas about the nature of such a system has since been proposed.
Finally, in 1997, Juan Maldacena, who was then a first-year postdoc, wrote a paper about black holes that has now gathered more than 12 000 citations. It listed a number of quantum mechanical systems that could precisely and completely describe spacetime and gravitational physics.
Maldacena’s discovery is as close to an experimental breakthrough as theorists get. It changed our perspective on the world in a completely unexpected way, and hordes of theorists have set out to apply it and understand it on a deeper level. Recently, a large community again focuses on black holes and the spacetime as an emergent rather than fundamental concept.
Before coming to potential applications to observable physics let me highlight some other of the ideas I personally found interesting and fun. They seem distant from observation at present, but can be of conceptual importance.
• Non-singular version of black holes: In higher-dimensional gravity there is a plethora of solutions without the physical singularities that plague the standard rotating Kerr black hole solution. Could it be that actual physical black holes are nonsingular when studied in detail?
• Andy Strominger of Harvard discussed the ideas developed by Hawking, Perry and himself on how information is transported out to arbitrary distances from black holes and could be recovered by detecting so called BMS charges (although in practise many orders of magnitude more work than at LIGO would be required).
• Nobel laureate and world-renowned particle physicist Gerard ‘t Hooft explained his puzzling recent ideas about the non-classical geometry of black holes. He said that his picture would lead to (in principle) observable correlations between Hawking radiation in opposite directions from a black hole. ‘t Hooft was the first to realise some of the “holographic” properties of gravity that later was concretised in Maldacena’s conjecture.
• So called Higher Spin Black holes were clarified by Juan Jottar from ETH, Zürich. Higher spin symmetries are symmetries that may be an alternative to supersymmetry in regulating short-distance problems of field theory. Another merit of these models is that they permit precise calculations, although technically demanding. Fundamental black hole physics is such a murky subject that explicit constructions are in dire need. The connection gravity and quantum mechanical system promises to be simpler in this case than in most other.
• Jonathan Lindgren, from Brussels, had found exact solutions of particles colliding to form general black holes. Of course, this problem is beyond reach in 3 space dimensions, but his 2-dimensional solution is still interesting.
We had two talks a day in four weeks and 30 talks in the conference, so these examples of talks by a Nobel laureate, a professor, a postdoc and a PhD student cannot do justice to the scope of the program.
There were two talks in the program that focused on cosmology. Both apply the idea of holography to a time evolving universe. These ideas are most naturally applied to universe with an almost constant acceleration of its expansion, i.e. to a quasi-de Sitter universe, although other cases can be described with more effort.
Erik Verlinde described an ambitious project that aims to derive both the effects of dark energy and dark matter as consequences of holography and a spacetime that is emergent rather than fundamental. This is a setting in which spacetime is subject to thermodynamic laws and relations. As I understood it dark energy is proposed to be an elastic medium, and what we interpret as dark matter is then merely the effects of the interactions of ordinary matter with the dark energy medium. I think it would be interesting to find out whether this is really possible. There have been other suggestions on how to avoid dark matter, for example the heavily criticized MOND proposal of a modification of Newton’s laws. Verlinde got the question about how he would explain the apparent separation of dark and luminous matter in the so-called Bullet Cluster, a system of two colliding clusters. His answer was that in contrast to MOND he proposes a general dynamical and relativistic framework that may well lead to such effects, whereas his initial estimates are necessarily crude and rely on thermodynamic equilibrium. Other calculations are needed for time dependent phenomena like the collision in the Bullet Cluster.
Kostas Skenderis presented holographic descriptions of an inflationary era, which permit a direct calculation of the fluctuations of the microwave background. A holographic perspective involves a quantum field theory as in-data to the computation, but in contrast to standard inflation it does not presuppose a geometric description of the inflationary spacetime. This is a definite advantage, since a geometric spacetime is likely to clash with quantum gravity in one form or another. I think it is also exciting to have an entirely new kind of model with a straightforwardly calculable effect on the CMB.
– Bo Sundborg, professor at the Oskar Klein Centre (email@example.com)
It seems that nearly exactly 100 years after their prediction by Albert Einstein, Gravitational Waves have finally been directly detected for the first time. Speakers of the LIGO experiment announced yesterday that they have witnessed the final stages of the inspiral and merger of a massive black hole binary system. This marks the beginning of a new type of astronomy with gravitational waves that allows to explore a so-far completely unknown side of the Universe.
Einstein’s Theory of Gravity
In November 1915, nearly exactly 100 years ago, Albert Einstein
presented his new General Theory of Gravity to the Prussian Academy of Sciences. The theory was somewhat perplexing since the effect of gravity was not a force acting on massive bodies, but instead gravity was claimed to warp the four-dimensional space-time we live in.
By now, this theory has been phenomenally successful. It could explain a long-known anomaly of the planet Mercury’s orbit around the Sun, its so-called perihelion shift. The theory also predicted that a gravitational field should deflect the paths of light rays, an effect that was later confirmed experimentally.
According to Einstein’s Special Theory of relativity, however, no information should be able to travel faster than the speed of light. Therefore, if at some location in the Universe a catastrophic event heavily perturbs the space-time, the “news” of this warp can only travel at a finite speed and –according to Einstein’s theory– this must be the speed of light.
One can think of a gravitational wave as a “ripple” travelling across the otherwise smooth space-time. This is similar to throwing a stone into a calm lake: this causes ring-like perturbations that travel away from where the stone hit the water. The physical reality of gravitational waves, however, had been doubted for decades, they were often considered as mere artefacts of Einstein’s theory.
A first glimpse of the elusive waves
This only changed in 1974 when a very “exotic” stellar system was discovered: two neutron stars orbiting around each other at a good fraction of the speed the light. A neutron star is a stellar corpse that emerges when an exploding star compresses its interior to densities that are larger than those in an atomic nucleus. So in a sense, one can think of neutron stars as being gigantic atomic nuclei of about 10 km radius. The newly discovered binary system has turned out to be an excellent laboratory for relativistic gravity, many general relativistic effects predicted by Einstein’s theory could be measured in it to exquisite precision.
Probably the most spectacular effect is that the two neutron stars slowly spiral towards each other, in excellent agreement with the prediction of Einstein’s theory. Within one orbital revolution (which takes less than 8 hours) this is a tiny effect, but since its discovery in 1974 the orbital period has already changed by 40 seconds! This discovery pulverised the doubts about the reality of gravitational waves and the discoverers of the binary system, Russel Hulse and Joseph Taylor, were honoured with the Physics Nobel Prize in 1993.
Listening to the dark side of the Universe
Although convincing, this is only an indirect confirmation of gravitational waves and one would like, of course, to detect them directly. This would mean that one could not only “see” the Universe (via electromagnetic waves) but one could also “listen” to the so far dark side of the Universe by means of gravitational waves. According to all we know, only 4 % of the energy of the Universe is made of matter that we think we understand. This includes all the objects of everyday life that are made of neutrons, protons and electrons.
The remaining 96%, however, may also produce gravitational waves and detecting them directly will open a new window to a completely unknown side of the Universe. Surprises are therefore virtually guaranteed!
In the last 25 years enormous efforts have been undertaken towards a direct detection of gravitational waves. An international detector network has been built up with facilities in Germany, Italy, Japan and the United States. The American “Laser Interferometer Gravitational-wave Observatory” (LIGO) has recently undergone a major upgrade and it has started taking data in its new configuration in September 2015. Once its final design sensitivity has been reached, it will be able to listen to a 1000-times larger volume of the Universe than before.
On 2015 September 14 at 9:50 UTC, during the last stages of “engineering runs” and before the originally planned observation period, LIGO has observed a gravitational wave burst signal from two merging, massive black holes. Both LIGO detectors, separated by 3000 km, saw a so-called “chirp” signal of increasing amplitude and frequency sweeping up frequencies from 35 to 250 Hz. This signal is well explained by the merger of two black holes with 29 and 36 solar masses.If this is the correct interpretation, then 2015 September 14 marks the beginning of the era of gravitational wave astronomy!
The LIGO collaboration consists of about one thousand scientists working in more than fifteen countries. Beyond the collaboration, LIGO’s results will both rely on and inform the observations of dozens of other telescopes and satellite observatories. Researchers would like to observe such extreme events as mergers of black holes and neutron stars with as many instruments as possible, and as soon after the burst as possible. LIGO itself, however, will not be able to localise the direction of bursts in the sky to high accuracy (for the observed event the source position is only known to within 600 square degrees). The first stage of help can come from instruments that monitor large parts or all of the sky continuously. At the Oskar Klein Centre, researchers working on the Fermi Gamma-ray Space Telescope, the Intermediate Palomar Transient Factory (iPTF), and the IceCube Neutrino Observatory were alerted about the September 14 burst, and asked to check whether they had recorded anything unusual around or after the burst time and in the same general direction. No definitive excess has been reported yet from these or other observatories. Seeing no other emission would be consistent with the interpretation of two merging black holes, since it is not obvious how an electromagnetic emission would arise in such a case. On the other hand, if a neutron star were involved, one would expect to see an electromagnetic flash caused by radioactivity from freshly synthesized heavy elements, a so-called “macronova”.
An interesting aspect of the LIGO procedure has been their well-publicised plan to test themselves and their fellow observers with occasional “fake” alerts. Except for a few individuals, no one even in LIGO would know for sure if a given alert was real. For this reason, it was not obvious in the early days after the alert that something real had been observed, let alone the amazing discovery that it turned out to be. It only became gradually, increasingly clear in the last few days before the announcement that this burst, now designated as GW150914, was not only real, but spectacular!
If the black hole merger interpretation is correct, then the lack of definitive detection besides gravitational waves would not be surprising. If the black holes were not surrounded by gas and ordinary matter that could be ejected in the violent aftermath, the merger was likely to be undetectable except by the enormous energy carried away by the gravitational waves themselves (corresponding to about three times the rest mass of our Sun). With LIGO starting to run in its advanced configuration now, and continuing to improve sensitivity, it is likely that more merger events will follow soon. Some of these will involve neutron stars rather than black holes, and these events are expected to leave visible traces.
– Stephan Rosswog (firstname.lastname@example.org) and Chad Finley (email@example.com)
Stephan Rosswog is Professor of Astronomy at Stockholm University and researches on compact objects such neutron stars and black holes.
Chad Finley is Senior Researcher in Physics at Stockholm University and IceCube coordinator for the joint search with LIGO.
Read also: “High-energy Neutrino follow-up search of Gravitational Wave Event
GW150914 with ANTARES and IceCube”, ANTARES, IceCube, LIGO, and VIRGO collaborations. https://dcc.ligo.org/LIGO-P1500271/public
The cover photo shows two Black Holes merging into one. This simulation was created by the multi-university SXS (Simulating eXtreme Spacetimes) project. For more information, visit http://www.black-holes.org: Photo Credit: SXS.
About once a day, a gamma-ray burst is detected. When this happens, e-mails get sent around and scientists scramble to detect whatever few photons might have been sent our way. But sometimes things are different…
On April 27th this year, an e-mail alert was sent around signifying the detection of yet another GRB. Yet this event was like no other. Rather than fighting to catch photons, there were suddenly too many to detect! The main emission episode was so bright that the GBM instrument on Fermi became saturated. And not only that – the GeV emission lasted for more than a day! Continue reading GRB130427A – a challenge to our models→
The discovery of a black hole enjoying a feeding frenzy in our nearest neighbor galaxy, Andromeda, has provided new insights into a mysterious class of extreme astrophysical objects called “ultraluminous X-ray sources”.
It isn’t unusual for material falling into a black hole to generate copious X-ray emission, but ultraluminous X-ray sources are so bright that they sometimes outshine their entire host galaxy in the X-ray band. Astronomers have spent years debating the nature of these enigmatic objects and two main scenarios have emerged. Either ultraluminous X-ray sources are unusually massive black holes feeding steadily on gas from an orbiting companion star, or alternatively they may be black holes around ten or twenty times as massive as our Sun that are somehow being force-fed by the in-falling gas.