Exploring The Mysteries Of The Cosmos With Spacetime Ripples
By Judith E Braffman-Miller
Gravitational waves are ripples in the fabric of Spacetime, and these ripples travel through the Universe at the speed of light, taking with them secrets that have long been hidden from those who are trying to solve its myriad mysteries. These propagating ripples, that have frequently been compared to the ripples in a pond, were first predicted by Albert Einstein in his General Theory of Relativity (1915), when his mathematics demonstrated that massive accelerating objects–such as neutron stars and black holes–in orbit around each other, would stir up Spacetime so furiously that “waves” of distorted Space would zoom screaming away from the source. In October 2017, scientists announced that they have made the historic detection of gravitational waves, in addition to the light being emitted, originating from the dramatic and spectacular collision of two neutron stars. This marks the first time that such a cosmic event has been observed in both gravitational waves and light.
The discovery was made using the United States-based Laser Interferometer Gravitational Wave Observatory (LIGO); the Europe-based Virgo detector; and about 70 ground and space-based observatories.
Neutron stars are the stellar ghosts, left as relics by massive stars, that have blasted themselves to smithereens in violent and catastrophic supernovae explosions–thus heralding their demise as hydrogen-burning main-sequence stars on the Hertzsprung-Russell Diagram of Stellar Evolution. Neutron stars are both the densest and smallest stars known to exist in the Universe. As the two neutron stars, that were being observed, spiraled into each other, they emitted gravitational waves that were detectable for about 100 seconds. When they collided, a burst of light in the form of high-energy gamma-rays were shot into Space, and observed by astronomers on Earth about 2 seconds after the gravitational waves. In the days and weeks after this dramatic, devastating cosmic collision, other forms of light–or electromagnetic radiation–including X-ray, ultraviolet, optical, infrared, and radio waves–were detected.
The new observations have provided astronomers with an unprecedented gift–the opportunity to investigate the collision of a doomed duo of neutron stars. Indeed, the observations performed by the U.S. Gemini Observatory, the European Very Large Telescope (VLT), and the Hubble Space Telescope (HST), display signs of recently synthesized atomic matter–including the heavy elements gold and platinum. This revelation has solved a decades-long mystery concerning where about 50% of all atomic elements heavier than iron are produced.
The Big Bang birth of the Universe, that is thought to have occurred almost 14 billion years ago, produced only the lightest of atomic elements–hydrogen, helium, and traces of beryllium and lithium. All of the atomic elements heavier than helium, called metals by astronomers, were produced in the nuclear-fusing cores of stars. This process, termed stellar nucleosynthesis, churned out increasingly heavier and heavier atomic elements–such as carbon, oxygen, neon, potassium, and calcium–all the way up to iron. The mystery, that has intrigued scientists for years, is where and how the atomic elements that are heavier than iron are produced. One of the favored explanations put forward is that these heaviest of all atomic elements are manufactured in the fiery explosion of a supernova conflagration–and the new findings certainly strengthen that theory.
The LIGO-Virgo results are published in the October 16, 2017 issue of the journal Physical Review Letters. Additional research papers from the LIGO and Virgo collaborations and the astronomical community have either been submitted or accepted for publication in a number of different scientific journals.
“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the Universe. This discovery realizes a long-standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories. Only through the National Science Foundation’s (NSF’s) four-decade investment in gravitational wave observatories, coupled with telescopes that observe from radio to gamma-ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes,” explained Dr. France A. Cordova in an October 16, 2017 NSF Press Release. Dr. Cordova is director of the NSF, which funds LIGO.
Even though Einstein predicted the existence of gravitational waves in 1915, it was not until a century later that their real existence in nature was verified. In the Fall of 2015, extremely sensitive detectors received the gravitational waves that had formed during the turbulent merging of two black holes. Gravitational waves are unlike any other waves–despite their frequent comparison to ripples in a pool of water. As gravitational waves ripple through the Universe, they alternately shrink and stretch the Spacetime continuum. This is because gravitational waves distort the geometry of the fabric of Space itself. Even though accelerating masses create gravitational waves, these can only be measured when the mass is extremely large.
Indications that it is possible for astronomers to find these Spacetime waves came back in 1974, twenty years after Einstein’s death. That year, two astronomers, Dr. Russell Alan Hulse and Dr. Joseph Hooton Taylor, Jr., using the Arecibo Radio Observatory in Puerto Rico, discovered a binary pulsar–a duo of extremely dense, massive, city-sized newborn neutron stars, in orbit around one another. Pulsars are very young, rapidly, and regularly spinning neutron stars. The pulsar binary has been named after its two discoverers (the Hulse-Taylor Binary), but is is also commonly referred to as PSR B1913+16.
Knowing that the Hulse-Taylor Binary pulsar system could be used to test Einstein’s prediction, the two astronomers began to measure how the period of the stars’ orbits evolved over time. After almost a decade of watching the duo of young neutron stars, the astronomers came to the conclusion that the two pulsars were waltzing towards one another–ever closer and closer and closer at precisely the rate predicted in General Relativity. This pulsar system has been carefully observed by astronomers for almost fifty years, and the alterations in their orbit agree so perfectly with Einstein’s predictions that there is little doubt that this dense duo emits gravitational waves.
Since then, many astrophysicists have observed the timing of pulsar radio emissions, and have had similar results. This further indicates the existence of these remarkable, mysterious ripples in the fabric of Spacetime.
However, until recently, these confirmations were always derived from indirect studies or mathematical calculations–and not through necessary direct “physical” observations. At last, on September 14, 2015, the LIGO Gravitational Wave Interferometer directly picked up the distortions in Spacetime resulting from the propagating ripples of gravitational waves. These Spacetime ripples were formed by a dancing duo of black holes far, far away. Certainly, this discovery will go down in history as one of the greatest achievements in astronomy.
The Nobel Prize In Physics 2017
LIGO is a collaborative project with over one thousand scientists, from more than twenty countries, participating. Together, the researchers realized a vision that is almost half a century old. In honor of their work, the 2017 Nobel Prize in physics has been awarded to Dr. Rainer Weiss, Dr. Kip S. Thorne, together with Dr. Barry C. Barish. In the mid-1970s, Dr. Weiss had already analyzed possible sources of background noise that would conflict with measurements, and he had also designed a gravitational wave detector, a laser-based interferometer, which would overcome this unwelcome noise. Early on, Dr. Kip Thorne and Dr. Weiss became convinced that gravitational waves could be detected and bring about a revolution in our knowledge of the Universe.
So far, different forms of electromagnetic radiation and particles, such as cosmic rays or neutrinos, have been used to investigate the myriad mysteries of the Cosmos. However, gravitational waves are direct testimony to disturbances in Spacetime itself. This is something new and different, opening up a new window on previously unseen worlds.
The gravitational waves, traveling from the duo of dancing black holes, took 1.3 billion years to arrive at the LIGO detector in the U.S. The signal was very weak by the time it reached Earth, but it is already living up to its promise of creating a revolution in astrophysics. Gravitational waves provide an entirely new way for astronomers to observe the most violent events in Space, and it tests the limits of our scientific knowledge.
Gravitational waves can arrive at our planet from where they originated in the distant corners of the Cosmos. They are the outcome of a catastrophic event, and the very first direct observation of their existence in Space, opens up an unprecedented new window into some of the best-kept secrets of the Universe. This is because the traveling Spacetime ripples carry with them important information about their violent origins that could not be obtained otherwise. The reason for this is that gravitational waves can access events that electromagnetic radiation cannot obtain. Astrophysicists can now observe the Cosmos using gravity as an important new tool–as well as light. For example, black holes cannot be observed using traditional methods, such as radio and optical telescopes.
The use of gravitational wave astronomy is especially useful for scientific cosmologists because they can use them to observe the most distant, deep, dark secrets of the baby Universe. This is not possible using conventional methods because the primordial Universe was opaque to electromagnetic radiation. Also, precise measurements of gravitational waves can be used by scientists to test Einstein’s Theory of General Relativity. By using gravitational waves, astrophysicists can gain an unprecedented peek into what really triggered the birth of the Universe almost 14 billion years ago.
Fortunately for life on our planet, while the violent origins of gravitational waves can be highly destructive, by the time they have reached us they are millions of times weaker–and much less destructive. Indeed, by the time the gravitational waves, propagating out from the dancing duo of black holes–observed for the first time by LIGO–had finally reached Earth, they were thousands of times smaller than an atomic nucleus.
Gravitational waves travel at the speed of light, and they fill the entire Universe, just as Einstein described in General Relativity. They always form when a mass accelerates–for example, the ballerina-type pirouettes of a duo of black holes, whirling around one another, as they perform their fantastic cosmic dance. Einstein was certain that scientists would never be able to measure these Spacetime ripples. However, the LIGO project’s use of two enormous laser interferometers, to measure a change thousands of times smaller than an atomic nucleus, succeeded in detecting these wandering waves as they passed the Earth. This technique is something new and different, opening up a myriad of fascinating wonders long kept in secret by unseen worlds. By capturing the ripples, scientists can now interpret their mysterious message.
Exploring The Mysteries Of The Cosmos Using Spacetime Ripples
The gravitational signal, emitted by the neutron star smash-up, has been named GW170817, and it was first detected on August 17, 2017 at 8:41 a.m. EDT. The detection was made by the duo of identical LIGO detectors located in Hanford, Washington, and Livingston, Louisiana. The signal provided by the third detector, Virgo, located in Pisa, Italy, provided an improvement in localizing the cosmic collision. At the time, LIGO was approaching the end of its second observing run since being upgraded in a program dubbed Advanced LIGO, while Virgo had begun its first run soon after completing its own upgrade called Advanced Virgo.
The NSF-funded LIGO observatories were conceived, constructed, and operated by the California Institute of Technology (Caltech) in Pasadena, and the Massachusetts Institute of Technology (MIT) in Cambridge. Virgo is funded by the Istituto Nazionale di Fisica Nucleare (INFN) in Italy and the Centre National de la Recherche Scientifique (CNRS) in France, and operated by the European Gravitational Observatory (EGO).
Each observatory is composed of two long tunnels arranged in an L shape, at the joint of which a laser beam is split in two. Light dispatched down the length of each tunnel is then reflected back in the direction it came from by a suspended mirror. In the absence of gravitational waves, the laser light in each tunnel should return to the location where the beams were split at precisely the same time. However, if a gravitational wave wanders through the observatory, it will change each laser beam’s arrival time. This creates an exquisitely tiny, barely perceptible, alteration in the observatory’s output signal.
On August 17, 2017, LIGO’s real time data analysis software captured a powerful signal of gravitational waves from space in one of the two LIGO detectors. Simultaneously, the Gamma-ray Burst Monitor aboard NASA’s Fermi Gamma-ray Space Telescope detected a burst of gamma-rays. LIGO-Virgo analysis software put the two signals together and saw that it was extremely unlikely to be a mere coincidence. Still another automated LIGO analysis suggested that there was a coincident gravitational wave signal in the other LIGO detector. Rapid gravitational wave detection by the LIGO-Virgo team, along with Fermi’s gamma-ray detection, triggered the launch of follow-up by telescopes all over Earth.
The LIGO data indicated that a duo of astrophysical objects, located at the relatively close distance of approximately 130 million light-years from Earth, had been spiraling inward towards one another. The objects were not as massive as binary black holes. Instead, the dancing duo was estimated to be about 1.1 to 1.6 times solar-mass–which is in the mass range of neutron stars. A neutron star is about 12 miles in diameter and is so extremely dense that a teaspoon full of neutron star stuff sports the impressive mass of about a billion tons.
“It immediately appeared to us the source was likely to be neutron stars, the other coveted source we were hoping to see–and promising the world we would see. From informing detailed models of the inner workings of neutron stars and the emissions they produce, to more fundamental physics such as General Relativity, this event is just so rich. It is a gift that will keep on giving,” commented Dr. David Shoemaker in the October 16, 2017 NSF Press Release. Dr. Shoemaker is spokesperson for the LIGO Scientific Collaboration and senior research scientist in MIT’s Kavli Institute for Astrophysics and Space Research.
Frequently in science, when one mystery appears to be solved, new mysteries emerge in its place. The observed short gamma-ray burst detected by Fermi was one of the closest to Earth seen so far, yet it was surprisingly weak for its distance. Scientists are beginning to propose new models for why this might be. There will probably be important new insights arising for years to come.
Fermi was able to pinpoint a location for GW170817 that was later confirmed and greatly refined with the coordinates derived by the combined LIGO-Virgo discovery. With these coordinates, observatories around the world were able, hours later, to start exploring the portion of the sky where the signal was believed to have originated. A new point of light, that resembled a new star, was first discovered by optical telescopes. Finally about 70 observatories on the ground and in space explored the event at their individual wavelengths.
“This detection opens the window of a long-awaited ‘multi-messenger’ astronomy. It’s the first time that we’ve observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves–or cosmic messengers. Gravitational-wave astronomy offers new opportunities to understand the properties of neutron stars in ways that just can’t be achieved with electromagnetic radiation alone,” explained Caltech’s Dr. David H. Reitze in the October 16, 2017 NSF Press Release.
The Dancers And Their Dance
A general picture is emerging. This picture further confirms that the first gravitational-wave signal did, indeed, originate from an inspiraling duo of doomed neutron stars.
Picture this: About 130 million years ago, the two neutron stars were caught, in their final, fatal moments, shortly before their tragic merger. At this point, the denizens of the dense duo were separated by only 200 miles. The two dancers began to gather speed, as they traveled ever closer and closer to one another, and as they did this, they closed the small distance separating them. As they met, in their final embrace, they stretched and distorted their surrounding region of Spacetime–emitting energy in the form of gravitational waves, before crashing into one another, with deadly results.
As the two neutron stars collided, the bulk of the duo merged into one single, solitary, and ultradense object–thus emitting a “fireball” of gamma-rays. The first gamma-ray measurements, combined with the gravitational-wave detection, also provide confirmation for Einstein’s General Theory of Relativity, which predicts that gravitational waves should ripple through Space at the speed of light.
Theorists have predicted that what follows the initial fireball is a kilonova–a cosmic explosion whereby the material left over from the neutron star merger, which glows with light, is blown from the immediate region far out into the space between stars. The recent light-based observations reveal that heavy elements, such as lead and gold, are born in these fiery, fatal collisions, and are ultimately distributed all over the Universe.
The first direct detection of a kilonova was in 2013, in association with the short-durationa gamma-ray burst dubbed GRB 130603B, where the faint infrared emission from the distant kilonova was spotted using the HST.
In the weeks and months ahead, telescopes around the world will continue to observe the tattle-tale afterglow of the neutron star merger–and gather additional evidence about various stages of the merger, its interaction with its environment, and the processes that create the heaviest atomic elements in the Universe.
Dr. Fred Raab of Caltech, and LIGO associate director for observatory operations, noted in the October 16, 2017 NSF Press Release that “When we were first planning LIGO back in the later 1980s, we knew that we would ultimately need an international network of gravitational-wave observatories, including Europe, to help localize gravitational-wave sources so that light-based telescopes can follow up and study the glow of events like this neutron star merger. Today we can say that our gravitational-wave network is working together brilliantly with the light-based observatories to usher in a new era in astronomy, and will improve with the planned addition of observatories in Japan and India.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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