GRAVITATIONAL WAVES*
What are they?[1]
According to Einstein’s theory of general relativity promulgated in 1915 a violent and rapid expansion of the universe would create ripples in space-time in the form of gravitational waves, and a gravitational field is a distortion in the underlying global fabric of space-time, and a time-varying source of energy – for example, the motion of a planet around its sun or of one star around another – would produce a time-varying distortion that would propagate away from the source at the speed of light.
Because gravitational waves are so weak, it takes a cataclysmic event such as the Big Bang or the merging of two black holes to produce waves that have any chance of
being measured. All objects sitting in the path of gravitational waves rhythmically move further apart and closer together as the space they exist in is stretched and squeezed. The biggest gravitational waves would only cause the equivalent of stretching and shrinking of the Australian continent by 10 millionths of the width of an atom. Any asymmetrical event, such as the orbit of the moon around the earth, should produce these waves, but they would never be detected.
Interestingly enough, although general relativity implied the existence of gravitational waves, Einstein initially rejected his own prediction, but he had applied the wrong set of coordinates. He had tried to find a solution for planar waves which oscillate in a constant direction as they move, whereas he should have applied a different coordinate system – one appropriate for cylindrical waves, whose plane of oscillation rotates as the wave moves[2].
The gravamen of the problem
When the universe was very young, before the time of inflation, it was compressed into a volume much smaller than the size of an atom. At such tiny scales, the rules of quantum mechanics reign. And yet because the amount of energy packed into each bit of that tiny space was incredibly high, this large energy requires us to use the theory of relativity to describe it. To understand the properties of the early universe, we need to use quantum field theory (QFT), which incorporates both quantum mechanics and special relativity, the theory that relates both space and time together.
QFT tells us that at very small scales, all quantum mechanical fields are wildly fluctuating, and if all other quantum fields behave similarly during the period when the inflationary energy density dominated the expansion of the universe, then the gravitational fields may have fluctuated as well. And if we can find the gravitational waves from inflation, we get not only a smoking-gun confirmation that inflation once took place but also a direct view into the quantum processes that drove inflation.
The key: CMB distortions caused by gravitational waves may be, and in fact recent events show that they are, detectable
What likely occurred was this. Inflation had the effect of expanding space by more than 25 orders of magnitude in less than 10-36 seconds, causing the significant stretching of tiny quantum fluctuations in the gravitational field. The wavelength of some of these fluctuations would get so big they would require longer than the age of the (very young) universe to oscillate, so they would “freeze” until the universe was old enough for them to oscillate again. When the CMB came into being when the universe was 380,000 years old, the free electrons would have been immersed in a radiation bath slightly more intense in one direction than another because large scale gravitational waves would have been compressing space in one direction and stretching it in another, and if the effect were large enough, it could have produced a small distortion in the CMB that might be detectable.
When inflation ended, these oscillations had grown into long-waved gravitational waves that alternately stretched and compressed space around them. The spatial distortion produced by gravitational waves could cause the electron-scattered CMB radiation to have a greater amplitude along one axis than along the perpendicular one. In other words, the CMB could be polarised and in this polarisation process, gravitational waves produce a striking twisting pinwheel pattern: in the areas where space has been compressed, the photons are packed together and the radiation hotter.
Discovery of gravitational waves [3]
In March 2014, a team from the Harvard-Smithsonian Centre for Astrophysics announced that its Background Imaging of Cosmic Extragalactic Polarisation 2 (BICEP2) experiment at the South Pole had detected very specific patterns of light, known as B-modes, which were "almost certainly" caused by very early gravitational waves – ripples in the fabric of space and time - as a result of cosmic expansion. It turned out that this was a false dawn and that the signal was entirely attributable to dust in the Milky Way rather than having a more ancient, cosmic origin[4].
However, about 18 months later, on 14 September 2015, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston Louisiana, detected gravitational waves not as a cosmic echo of the Big Bang but as the result of the merging of two black holes which occurred around 1.3 billion years ago. The ripples from this merger-collision registered as no more than a 'chirp' on LIGO's sensors. LIGO is comprised of 2 observing stations, one in Louisiana and the other in Washington. To rule out false positives, a candidate gravitational wave signal needs to be detected by both stations. The September 14 event was detected first in Louisiana and then 7 milliseconds later in Washington. The discovery was announced on 11 February 2016.
And there is another dimension to this discovery. The black holes involved were so heavy, it is hard to explain how they formed from stars at all, and as physicist Juan Garcia-Bellido and cosmologist Sébastien Clesse go on to explain, even if two such black holes did independently form from the deaths of very massive stars, they would have then had to find each other and merge—an event with an exceedingly low probability of occurring within the current age of the universe. The authors postulate that the black holes involved, denominated primordial (hence PBHs), may have predated the formation of the stars themselves, forming instead in astronomical numbers from the hot, dense plasma that filled the cosmos less than one second after the big bang, and that PBHs may constitute some, if not all, of dark matter — the invisible 85 percent of the matter in the universe that acts as gravitational glue to hold galaxies and galaxy clusters together. [5]
The technique used at LIGO uses a split laser beam to measure extremely small distances with incredible accuracy. They pick up the distortions in spacetime when the waves change the length of a detector arm by less than the diameter of a proton. Each detector uses laser beams to constantly measure the lengths of two perpendicular pipes with stunning accuracy. A single laser beam is split in two, with each beam travelling down one arm of the interferometer. Mirrors at either end of the arms bounce the beam back and forth, and it is then recombined. Because the two arms are identical in length (4 kilometres), the recombined laser beams perfectly cancel each other out. When a gravitational wave from a distant cataclysmic event reaches the detectors, the rhythmic stretching and squeezing of space and time make the pipes longer and shorter in turns, and the recombined beams no longer cancel out perfectly. Instead, a telltale pulsing signal is detected. Such a signal gives direct evidence for gravitational waves.
A gravitational wave moving through LIGO would compact and stretch the tubes so that the lasers' travel times would change by one part in 1021, meaning that the four-kilometer tube would be altered by 1/10,000th the diameter of a proton, something like changing the U.S. national debt by one millionth of a cent. [6]
At the time the 2015 discoveries took place, a third detector, the Virgo gravitational-wave detector near Pisa in Italy, was offline undergoing an extensive upgrade, fitting it with new mirrors, vacuum pumps and lasers. When all three detectors are up and running together (the LIGO detectors are about to shut down for their own upgrades), scientists hope to significantly improve efforts to determine the sources of gravitational waves – be they black hole collisions, the aftermath of a supernova or of high-energy radiation bursts emitted from near the event horizons of merging black holes, or perhaps two colliding neutron stars in a black hole’s gravitational maw, some such events even emitting light and other electromagnetic radiation that telescopes can see - by enabling them to focus on a triangulated area of sky, trace the resultant waves and hopefully spot the collisions from which they emanate. The 2015 detections narrowed the waves’ origins to an area of about 2% of the sky, still a huge area. When Advanced Virgo comes on board, this will shrink by a further factor of five, “thereby reducing the source problem from something horrific to just something terrible”. Using all three detectors together will also help to rule out local vibrations caused by sources on Earth that might mask incoming gravitational waves.[7]
Then astronomers detected evidence of a third instance of gravitational waves, a tiny ripple in space-time that swept past Earth at the speed of light in just 0.4 seconds on January 4 2017, the result of a collision and coalescence of two black holes 3 billion light years away, forming a larger black hole with the mass of nearly 50 suns. This latest discovery tells us more about the nature of black holes – and maybe dark matter – and shows gravitational-wave astronomy has arrived as a new window on the universe. LIGO scientists said the discovery provided evidence that black holes exhibit a spin property that the black holes may not be aligned, affording just a tiny hint that pairs of black holes may form in dense stellar clusters. It is possible that the merger was the product of a binary system of black holes formed in the early universe, contributing significantly to the dark matter of the cosmos [8].
The significance of all this is that we are now going to be able to look at the universe in terms of gravitational wave emissions, a completely different kind of information carrier. Most astronomy up to now – from X-rays through visual light to radio waves has been in the electromagnetic spectrum. Gravitational waves are a new kind of source and their great promise is to unlock some of the deepest secrets of the universe, and one of the holy grails of this field would be to see the gravitational wave residue of the Big Bang. [9]
Extreme environments, nano-seconds after the Big Bang or at the edges of black holes, are where the dynamical strong-field regime operates, as opposed to the weak field that governs things like the orbit of planets and falling apples. “By probing the strong-field regime using gravitational waves, we might see hints that general relativity isn’t quite right and this will point a way towards a more fundamental understanding of gravity”. [10]
In other words. gravitational wave astronomy will allow us to look further back in time and deeper inside the most extreme objects in the sky — to the earliest instant after the Big Bang. It will reveal the insides of distant objects because it will let us "see" their mass. It will let us see everything from the heart of a black hole, to the moments after the Big Bang. The pattern of movement as black holes coalesce, the changes inside a supernova, the mechanisms of a gamma ray burst will all become visible to us. And because gravitational waves only interact with gas and dust to a tiny extent, their signal is much cleaner than those from light. Our picture of the universe will come into much sharper focus. There will also be technological spin-offs in metrology (the science of measurement) and optics (the science of the behaviour and properties of light).
Gravitational waves portend radio signals [11]
It did not take long for an instance of the use of this new gravitational wave astronomy to manifest itself. On 17 August 2017, astronomers were able to observe a neutron star merger about to happen in a galaxy NGC 4993 about 130 million light years away via the gravitational waves which preceded the merger and the optical and radio signals which were the result. (Recall that neutron stars are the dense, collapsed cores of massive stars cores of massive stars that have exploded as supernovae). One of the stars was observed for roughly the last 100 seconds before the collision and apparent collapse into a black hole. Those operating the two detectors at LIGO spotted the last moments of the death spiral before the two neutron stars merged in an explosive collision known as a kilonova entitled GW 170817. Gravitational waves from the collision itself were not detected, but scientists' instruments may not yet be sensitive enough to do so.
Within 1.7 seconds of the merger, gamma rays emitted from the event were detected by NASA's Fermi space telescope, thus confirming the hypothesis that neutron star mergers may be the source of gamma rays and also Einstein’s theory that gravitational waves travel at the speed of light. 16 days later, the members of an Australian team, who by virtue of the signals already received were able to direct the Australia Telescope Compact Array at Narrabri in northern NSW to an area about 150 times the size of the moon in the night sky, became the first to confirm the radio signal from the collision on September 5. An illustration of these phenomena appears at the foot of this page.
A large fraction of the universe's heaviest elements, including platinum, uranium and a huge amount of gold is now thought to be produced in kilonova explosions, and by combining light signals with gravitational waves, scientists also hope to learn more about how gold and other elements are formed. It is already known that elements heavier than iron must be produced in supernova explosions which form neutron stars – there is nothing else about with enough energy to produce them - and with these whole new techniques of investigating the universe now at their disposal, scientist hope to be able to probe further.
Until these recent detections, gravitational waves were the last outstanding unconfirmed prediction of Einstein's theory of General Relativity, behind gravitational lensing, the perihelion of Mercury's orbit, gravitational redshift and the equivalence principle, all considered elsewhere herein. Einstein's theory explained the attraction of one mass to another—the apple to Earth—by proposing that mass curved the spacetime around it; the greater the mass, the deeper the curvature. The apple does not so much fall to Earth as it spirals down along the curvature our planet's mass has made in spacetime. The theory went on to predict that if a mass accelerates, the curvature moves outward in waves. The waves are spacetime itself compacting and stretching.[12] As we have already witnessed, the implications for science generally are exciting and may assist in the search for a grand unified theory of matter.
Multimessenger astronomy [13]
To recapitulate:
Following on from these developments, over three and a half weeks in September-October 2017, astronomers observed the same celestial event (perceived to be a flare-up from matter falling into a supermassive black hole) through multiple wavelengths of light as well as neutrinos particles. These combined observations have offered scientists much more information about these phenomena than any single measurement alone, per medium of the phenomenon now known as multimessenger astronomy.
To flesh out what occurred over these three and a half weeks:
What does all this mean?
Recall that neutrinos from beyond our solar system were first detected via Supernova 1987A in February 1987, becoming the first messenger that was not light, and that since 2015 astronomers have been able to detect the "rolling waves of gravity" in the form of gravitational waves, becoming the second form of multimessenger.
The great advantage of so-called multimessenger astronomy is that unlike light—an electromagnetic wave that can get "reflected, absorbed and misdirected, obscuring information about its source" - almost nothing stops gravitational waves or neutrinos. “The message they carry is pure; it comes in directly and at or near the speed of light”, and their sources - colliding black holes or collapsing supernovae or merging neutron stars— are “transient, unspeakably violent perplexities”.[14] The coincidence of the IceCube-170922A neutrino with the Texas source—which was eventually observed by at least 19 instruments in gamma rays and x-rays and optical and radio wavelengths - now make up the second neutrino multimessenger event following on from Supernova 1987A some thirty years before.
Meanwhile, a whole raft of new multimessenger detectors is now on the horizon: LIGO has siblings under construction in Japan and India. The Laser Interferometer Space Antenna (LISA) will be an orbiting gravitational-wave detector scheduled to launch in the 2030s; "its arms are lasers zipping among three spacecraft arranged so they form a triangle with sides extending around a million miles". Also proposed are new high-energy neutrino detectors, including a next-generation IceCube and KM3NeT, a cubic kilometer of sensors 3,500 meters down in the Mediterranean Sea. [15]
Other searches
Other endeavours involved, or to be involved, in the search for gravitational waves include the Einstein underground telescope, to be built in one of 14 candidate sites including abandoned mine sites in Poland, Hungary, Romania, France, Italy and Germany, and specifically designed to detect gravitational ripples or waves postulated to have been created by such things as black holes, neutron holes and the Big Bang itself and first predicted by Einstein in his theory of general relativity. The telescope (still in the course of design as at December 2012) may also reveal for the first time whether there were universes in existence before our own by looking for the echoes of previous Big Bangs similar to the one that created our own universe 13.7 billion years ago.
Because the waves are very weak when they reach the earth, it will be sited underground to minimise the possibility of interference. The tunnels will consist of two arms each six miles long, at the end of each will be mirrored targets suspended on the end of long pendulums that will be used to reflect a laser beam. The lasers will be fired along the six mile arms in a close to perfect vacuum at temperatures below -238 degrees F to reduce the external changes that could interfere with signals from gravitational waves. Differences in the stretching and shrinking of the waves as they interact with particles can then be used to build up a picture of what created the gravitational wave and pinpoint its source[16].
The discovery of gravitational waves stemming from the Big Bang also has the capacity to provide a solution to two apparent paradoxes of the early universe: the horizon problem and the flatness problem[17].
A large fraction of the universe's heaviest elements, including platinum, uranium and a huge amount of gold is now thought to be produced in kilonova explosions, and by combining light signals with gravitational waves, scientists also hope to learn more about how gold and other elements are formed. It is already known that elements heavier than iron must be produced in supernova explosions which form neutron stars – there is nothing else about with enough energy to produce them - and with these whole new techniques of investigating the universe now at their disposal, scientist hope to be able to probe further.
Until these recent detections, gravitational waves were the last outstanding unconfirmed prediction of Einstein's theory of General Relativity, behind gravitational lensing, the perihelion of Mercury's orbit, gravitational redshift and the equivalence principle, all considered elsewhere herein. Einstein's theory explained the attraction of one mass to another—the apple to Earth—by proposing that mass curved the spacetime around it; the greater the mass, the deeper the curvature. The apple does not so much fall to Earth as it spirals down along the curvature our planet's mass has made in spacetime. The theory went on to predict that if a mass accelerates, the curvature moves outward in waves. The waves are spacetime itself compacting and stretching.[12] As we have already witnessed, the implications for science generally are exciting and may assist in the search for a grand unified theory of matter.
Multimessenger astronomy [13]
To recapitulate:
- between September 14, 2015, and August 14, 2017, LIGO-Virgo detected five different sources of gravitational waves, each produced by the collisions of two black holes that merged into single black holes.
- then, three days later, on August 17, 2017, LIGO-Virgo detected gravitational waves coming from the galaxy NGC4993, and 1.74 seconds later the Fermi telescope saw a burst of gamma rays, an event, known as GW170817, being created not by the collision and merger of black holes but of neutron stars, graphically described as “the collapsed cores of past supernovae, so compact that all their protons and electrons have squished together to make neutrons; they are the final state of stars not quite massive enough to form black holes".
Following on from these developments, over three and a half weeks in September-October 2017, astronomers observed the same celestial event (perceived to be a flare-up from matter falling into a supermassive black hole) through multiple wavelengths of light as well as neutrinos particles. These combined observations have offered scientists much more information about these phenomena than any single measurement alone, per medium of the phenomenon now known as multimessenger astronomy.
To flesh out what occurred over these three and a half weeks:
- On 22 September 2017 at 4.54 pm, the IceCube Neutrino Observatory at the South Pole detected a high-energy neutrino, now known as IceCube-170922A, and issued an alert. IceCube's more than 5,000 sensors, which look for flashes of light made by neutrinos interacting with atoms in the ice, is capable of tracing the path of the flash back to the particle's origin in the sky.
- then, on 26 September, the orbiting Swift X-ray telescope reported finding nine sources of x-rays coming from the same area of the sky as the neutrino.
- two days later, on 28 September 2017 at 6.10 am, the Fermi space telescope identified gamma rays coming from the same position as both IceCube-170922A and Swift's second x-ray source. The gamma source was already known and named TXS 0506+056, also known as "the Texas source".on the same day at 2 pm, a network of ground-based optical telescopes called ASAS-SN (pronounced assassin) announced that this source had been brightening over the past 50 days and was the brightest it had been in several years.the next day, September 29, at 9:00 A.M., another optical telescope found that the Texas source was a blazar, a supermassive black hole at the centre of a galaxy that sporadically flares up as matter falls into it, sending out jets aimed straight at us.
- then, on 17 October, the Very Large Array in New Mexico, operating at radio wavelengths, confirmed that the light, the source of all these signals, was a jet from a blazar. Blazars were already well known but had never been observed in multiple wavelengths and simultaneously identified as the source of a neutrino, and the Texas source was also the first time a high-energy neutrino coincided in space and time with a similarly high-energy gamma-ray photon.
- which brings us to the events of the 'hushed thud' of May and August 2019.
What does all this mean?
Recall that neutrinos from beyond our solar system were first detected via Supernova 1987A in February 1987, becoming the first messenger that was not light, and that since 2015 astronomers have been able to detect the "rolling waves of gravity" in the form of gravitational waves, becoming the second form of multimessenger.
The great advantage of so-called multimessenger astronomy is that unlike light—an electromagnetic wave that can get "reflected, absorbed and misdirected, obscuring information about its source" - almost nothing stops gravitational waves or neutrinos. “The message they carry is pure; it comes in directly and at or near the speed of light”, and their sources - colliding black holes or collapsing supernovae or merging neutron stars— are “transient, unspeakably violent perplexities”.[14] The coincidence of the IceCube-170922A neutrino with the Texas source—which was eventually observed by at least 19 instruments in gamma rays and x-rays and optical and radio wavelengths - now make up the second neutrino multimessenger event following on from Supernova 1987A some thirty years before.
Meanwhile, a whole raft of new multimessenger detectors is now on the horizon: LIGO has siblings under construction in Japan and India. The Laser Interferometer Space Antenna (LISA) will be an orbiting gravitational-wave detector scheduled to launch in the 2030s; "its arms are lasers zipping among three spacecraft arranged so they form a triangle with sides extending around a million miles". Also proposed are new high-energy neutrino detectors, including a next-generation IceCube and KM3NeT, a cubic kilometer of sensors 3,500 meters down in the Mediterranean Sea. [15]
Other searches
Other endeavours involved, or to be involved, in the search for gravitational waves include the Einstein underground telescope, to be built in one of 14 candidate sites including abandoned mine sites in Poland, Hungary, Romania, France, Italy and Germany, and specifically designed to detect gravitational ripples or waves postulated to have been created by such things as black holes, neutron holes and the Big Bang itself and first predicted by Einstein in his theory of general relativity. The telescope (still in the course of design as at December 2012) may also reveal for the first time whether there were universes in existence before our own by looking for the echoes of previous Big Bangs similar to the one that created our own universe 13.7 billion years ago.
Because the waves are very weak when they reach the earth, it will be sited underground to minimise the possibility of interference. The tunnels will consist of two arms each six miles long, at the end of each will be mirrored targets suspended on the end of long pendulums that will be used to reflect a laser beam. The lasers will be fired along the six mile arms in a close to perfect vacuum at temperatures below -238 degrees F to reduce the external changes that could interfere with signals from gravitational waves. Differences in the stretching and shrinking of the waves as they interact with particles can then be used to build up a picture of what created the gravitational wave and pinpoint its source[16].
The discovery of gravitational waves stemming from the Big Bang also has the capacity to provide a solution to two apparent paradoxes of the early universe: the horizon problem and the flatness problem[17].
* Header illustration: Gravitational waves produced by two orbiting black holes. Photo: Henze/NASA/LIGO
[1] This background is drawn from Lawrence M. Krauss, “A beacon from the Big Bang”, Scientific American, October 2014, 47 ff.
[2] Lawrence M. Krauss, “What Einstein got wrong”, Scientific American, Special Issue – 100 years of General Relativity, September 2015, 41 at 42-43,44.
[3] See http://www.abc.net.au/news/2016-02-12/first-direct-evidence-of-gravitational-waves-detected/7140750 and http://news.discovery.com/space/weve-detected-gravitational-waves-so-what-160213.htm A short ABC Catalyst programme video of the detection of gravitational waves caused by the merger of two black holes and the role of LIGO in the detection is available for download, at least for viewers in Australia,
at https://www.abc.net.au/catalyst/stories/4433050.htm See also
https://www.abc.net.au/news/2019-04-07/black-hole-first-ever-photograph-could-be-unveiled-this-week/10979244
[4] http://www.nature.com/news/gravitational-waves-discovery-now-officially-dead-1.16830
[5] Katherine Wright, "Sky is the limit", Scientific American, Advances, April 2017, 10-13. The citation stems from a remark by Fulvio Ricci, Virgo's spokesperson and a physicist at Sapienza University of Rome.
[6] Ann Finkbeiner, "Messengers from the sky", Scientific American", May 2018.
[7] "Black holes from the beginning of time", Scientific American, July 2017, 31 at 31-2.
[8] http://www.smh.com.au/technology/sci-tech/australian-experts-help-ligo-open-new-eyes-on-universe-using-gravitational-waves-20170601-gwi0ho.html
[9] Professor David Reitze, executive director of LIGO, cited in Marcus Strom, “Nobel favourite hopes to chase waves in Australia”, The Sun-Herald, 16 April 2017, 19.
[10] Ibid.
[11] www.smh.com.au/technology/technology-news/revelation-of-cosmic-secrets-triggers-a-frenzy-of-global-scientific-activity-20171015-gz1hmw.html
[12] Ann Finkbeiner, "Messengers from the sky", op cit.
[13] Ibid
[14] Ibid.
[15] Ibid.
[16] “Underground telescope could peer beyond the Big Bang”, The Telegraph, 18 April 2012.
[17] It is, of course, suggested that solutions to both these paradoxes have already been provided by the inflationary cosmology model of the newly born universe.