Tag Archives: gravitational waves

Hunting for extra dimensions with gravitational waves

Right or left, back or forth, up or down, earlier or later: the everyday world our senses experience is, at least apparently, 3+1-dimensional. One would then think that extra dimensions beyond the usual 3+1 spacetime ones, an exciting and common ingredient in many science fiction stories, should be indeed left to science fiction novelists. It turns out, however, that extra dimensions are not at all uncommon in actual physics theories. For several decades, scientists have toyed with the idea of introducing extra dimensions to solve some of the long-standing problems of physics. One such puzzle, known as the “hierarchy problem”, is why gravity is so much weaker than the other three fundamental forces of nature (the strong, weak, and electromagnetic forces).

In some of these extra dimensional theories all particles and forces, except gravity, are confined to the usual 3+1-dimensional spacetime. Gravity, on the other hand, would feel these extra dimensions and travel through them. The “leaking” of gravity into extra dimensions might even help explain why it is so much weaker than all the other known forces. Our senses would not be aware of the fifth dimension, much as the two-dimensional inhabitants of Flatland were not aware of the existence of the third dimension. This is the idea behind “brane cosmology” or “brane-world models”, where our four-dimensional spacetime is restricted to a “brane” which lives in a higher-dimensional space known as the “bulk”. Among the most popular brane-worlds we find the Randall-Sundrum models (named after Lisa Randall and Raman Sundrum, see this paper and this other paper for their original work) where our 3+1-dimensional spacetime is surrounded by an infinite 5-dimensional anti-de Sitter spacetime (or AdSfor short). This AdSspacetime is characterized by its curvature radius: the larger the curvature radius, the more AdSlooks flat (think of the Earth: the larger the Earth radius, the more it appears flat).

This all sounds exciting and – perhaps – looks like it belongs more to the science fiction realm than to reality. However, physicists have good reasons to take these types of conjectures seriously. The question is then: can we ever hope to observationally probe these extra dimensions? A lot of smart people have thought about ways of testing the existence of extra dimensions, for example by looking at particular signatures at colliders such as the LHC or searching for modifications to Newton’s law at short distances (if you are curious, have a look at this review for more information). It turns out there is another way to hunt for extra dimensions, based on an idea by Robert Caldwell and David Langlois, that makes use of a source of gravitational waves. You need to be able to detect both the gravitational waves and the light released from the event. This is the basis of what is known as multi-messenger astronomy. Since (in Einstein’s theory of General Relativity) gravitational waves and light both travel at the same speed — you guessed it, the speed of light! — one would expect the two signals to reach us at the same time. However, if brane-world extra dimensions exist, gravitational waves can take a shortcut in the fifth dimension and actually reach us before the photons do.

Sounds crazy? On August 17, 2017 LIGO and Virgo detected the gravitational waves released during the last minutes of a binary neutron star merger (GW170817) . Within two seconds, Fermi and INTEGRAL also detected a short gamma-ray burst released by the same neutron star merger event. The near-simultaneous arrival of the two signals was used to set very stringent constraints on deviations from Einstein’s general relativity (see for instance this review if you are interested).

Thanks to these two amazing detections, it has also been possible to hunt for extra dimensions using gravitational waves for the first time. OKC researchers, Katie Freese, Luca Visinelli, and Sunny Vagnozzi, in collaboration with Nadia Bolis who was visiting OKC from CEICO in Prague, used the measurements of the time-delay between the gravitational and the electromagnetic signal to put constraints on the curvature radius of the fifth dimension, assuming that gravitational waves travelled along a shortcut in the fifth dimension. They did a careful statistical analysis of the time-delay, taking into account uncertainties in the emission of the gamma-ray burst from the neutron star and the impact of large-scale structure between us and the neutron star on the propagation of the gravitational waves. They concluded that no evidence for non-zero curvature radius could be found and set a 95% confidence level upper limit of 1.997 Megaparsecs on the curvature radius of the fifth dimension. In other words, we can be 95% sure that the curvature radius of the fifth dimension is less than about 6.5 million light-years large.

The results, which appeared in November in the arXiv preprint repository were then published in March in the journal Physical Review D. The tools to reproduce the analysis are publicly available on Github.

This is the first time that researchers have been able to hunt for extra dimensions using gravitational waves.  We like to think it’s no accident that this work was done at the OKC, an institute named after the late Oskar Klein who is considered by many to be the father of extra dimensional theories.

Text by OKC graduate student researcher Sunny Vagnozzi.

Extreme-Gravity Stars

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.

A photo of Stephan Rosswog
Stephan Rosswog, Oskar Klein Centre, Stockholm University

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.

A conference on The Physics of Extreme-Gravity Stars took place in Stockholm in June 2017.

Header image is from a simulation of two neutron stars merging, credit to Stephan Rosswog.

Gravitational waves finally detected

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 (stephan.rosswog@astro.su.se) and Chad Finley (cfinley@fysik.su.se)

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.