Gravitational Waves From Long Ago And Far, Far Away
By Judith E Braffman-Miller
After the late summer storm that blasted through the starlit twilight, the warm wind still continued to rage, causing ripples to form in the lingering puddles on the pavement–while the little girl watched in wonder. Ripples on water are similar to the ripples in the fabric of Spacetime–we call these ripples gravitational waves. Albert Einstein predicted the existence of gravitational waves in 1916 in his General Theory of Relativity, and these ripples travel at the speed of light through the Universe, carrying with them strange secrets long hidden about the birth of Space and Time. In February 2017, a team of theoretical astrophysicists announced that they have calculated the signal of specific gravitational wave sources that were born only fractions of a second after the Big Bang beginning of the Universe almost 14 billion years ago. The scientists propose that the origin of the signal is a long-lost, mysterious cosmological phenomenon called oscillon.
Einstein’s mathematics demonstrated that massive accelerating objects–such as binary neutron stars and black holes, as they orbited each other–would jumble Spacetime in such a dramatic way that “waves” of distorted Space would zip away from the source–just as ripples in a lingering puddle, left by torrents of rain after a summer storm, radiate through the pooled water. The speed of light sets something a universal speed limit. No known signal can travel faster than light through a vacuum. The ripples formed from gravitational waves propagate through Spacetime at the speed of light, taking along with them valuable information about their own cataclysmic origins–along with clues about the mysterious nature of gravity itself.
The most powerful gravitational waves of all are created by catastrophic events such as the explosive collapse of stellar cores (supernovae), the coalescing of stellar ghosts like dense neutron stars or white dwarfs, the violent collision of black holes, the wobbly rotation of neutron stars that are not in the shape of perfect spheres, and the lingering relics of the primordial gravitational radiation left to tell the ancient story of the birth of the Universe itself.
Even though Einstein predicted the existence of gravitational waves in 1915, it was not until 100 years later that their true existence in nature was actually proven. In the autumn of 2015, highly sensitive detectors received the gravitational waves that had been created during the violent merging of two black holes. Gravitational waves are unlike any other known waves. As gravitational waves ripple through the Universe, they alternately shrink and stretch the Spacetime continuum. This means that gravitational waves distort the geometry of the fabric of Space itself. Even though the accelerating masses emit gravitational waves, these can only be measured when the mass is extremely large–as it is, for example, with black holes or supernovae.
Hints of the possibility of finding these Spacetime waves were discovered back in 1974, 20 years after Einstein’s death. In that year, two astronomers, Dr. Russell Alan Hulse and Dr. Joseph Hooton, Taylor, Jr., working at the Arecibo Radio Observatory in Puerto Rico detected a binary pulsar–a duo of extremely dense and massive city-sized stellar relics in orbit around each other. The pulsar binary has been named for its two discoverers (the Hulse-Taylor Binary), but it is also commonly designated PSR B1913+16.
Pulsars are new-born neutron stars–and neutron stars are the lingering ghosts of massive stars that have gone supernova, leaving these celestial souvenirs behind to haunt the Cosmos, telling the tragic story of a star-that-was that is a star-no-more. Fresh young pulsars are born from the fiery supernova funeral pyres of their progenitor stars, and they are spinning wildly–emitting beams of light that are so regular they have been likened to the beams of a lighthouse on Earth.
PSR B1913+16 was precisely the type of system that, according to General Relativity, should send the ripples formed by gravitational waves out into Space. Knowing that this discovery of the binary pulsar system could be used to test Einstein’s prediction, the astronomers began to measure how the period of the stars’ orbits changed as time went by. After almost a decade of observations, it was determined that the two pulsars were dancing ever closer and closer to each other at exactly the rate predicted by General Relativity. This pulsar system has now been carefully watched for almost half a century, and the observed alterations in the orbit agree so perfectly with the predictions of General Relativity, there is little doubt left that it is emitting gravitational waves.
Since then, many astrophysicists have observed the timing of pulsar radio emissions and have had similar results. This further confirms the existence of these ripples in the fabric of Spacetime.
However, until recently, these confirmations always came from indirect studies or mathematical calculations–and not through the necessary direct “physical” observations. At last, on September 14, 2015, the LIGO Gravitational Wave Interferomenter directly picked up the distortions in Spacetime resulting from traveling, rippling gravitational waves. These Spacetime ripples were produced by the dancing duo of colliding black holes situated at the incredible distance of almost 1.3 billion light-years away! Certainly, this discovery will go down in history as one of the greatest achievements in science.
Gravitational waves can reach Earth, from where they are created, as the result of a catastrophic event in the remote Universe. This very first observation of their real existence in nature opens up an unprecedented new window into the well-kept secrets of the Cosmos. This is because propagating Spacetime ripples carry with them, during their long journey through the Universe, important information about their violent origins that otherwise could not be obtained by scientists in any other way. The reason for this is that gravitational waves can access regions of Space that electromagnetic waves fail to reach. Astrophysicists can now observe the Universe and its many well-kept secrets using gravity as an important tool–as well as light. Gravitational waves can provide scientists with extremely important information about exotic objects in the most secretive and distant regions of the Cosmos. For example, black holes cannot be observed using more traditional methods–such as radio and optical telescopes.
Therefore, gravitational wave astronomy provides a valuable method that scientists can use to gain a better understanding of how our weird, wonderful, and mysterious Universe operates. This is particularly true for scientific cosmologists because gravitational waves can enable them to observe the deep, dark secrets of the primordial Universe. This is not possible using conventional methods because, during our Universe’s babyhood, it was opaque to electromagnetic radiation. In addition, precise measurements of gravitational waves can be used by scientists to test Einstein’s Theory of General Relativity. By using gravitational waves, astrophysicists can come to a greatly improved comprehension of exactly what happened at the initial singularity, which is what is commonly believed to have given birth to the baby Universe about 13.8 billion years ago.
Fortunately for Earthlings, while the violent origins of gravitational waves can be catastrophically destructive, by the time the wandering waves finally reach our planet they are millions of times smaller and less destructive. Indeed, by the time gravitational waves, traveling out from the dancing black hole duo, were first discovered by LIGO, the amount of Spacetime wobbling generated was literally thousands of times smaller than the nucleus of an atom.
LIGO was originally proposed as a new way to find very elusive gravitational waves back in the 1980s. This proposal was first made by Dr. Rainer Weiss, professor of physics, emeritus at the Massachusetts Institute of Technology (MIT); Dr. Kip Thorne, the California Institute Of Technology’s (Caltech’s) Richard P. Feynman Professor of Physics, emeritus; and Dr. Ronald Drever, professor physics, emeritus, also from Caltech.
Wonderful, Wandering Waves
As a wandering gravitational wave passes a distant observer, the observer will watch in wonder as Spacetime becomes distorted by the bizarre effects of that propagating ripple. The distances that exist between free objects will increase, and then decrease, in a rhythmic way as the weird wave makes its journey–and as it travels it does so at a frequency that corresponds to that of the wave itself. The magnitude of this effect decreases inversely with distance from the catastrophic source of the wandering wave–born from such violent events as doomed duos of sister neutron stars, that are in the process of dancing ever closer and closer to one another, in a strange spiral ballet. The dance ends when the ballerinas blast into one another and merge, in a final grand finale of their catastrophic cosmic ballet. Unfortunately, because of the great distances to these sources of origin, the effects when measured by astrophysicists on our own planet are predicted to be quite small.
The twin Laser Interferometer Gravitational Wave Observatory (LIGO) detectors are located in Livingston, Louisiana and Hanford, Washington. The LIGO observatories were funded by the National Science Foundation (NSF), and are operated, constructed, and invented by scientists at Caltech in Pasadena, California, and MIT in Cambridge, Massachusetts.
Based on the signals that they have detected, LIGO scientists proposed that the doomed black hole duo, that caused the catastrophic event, are 29 and 36 times the mass of our Sun–and that the dramatic collision and merger took place approximately 1.3 billion years ago. It is believed that about three times the mass of our Star was converted into rippling gravitational waves in only a fraction of a second–with the peak power output amounting to approximately 50 times that of the entire visible Universe. By observing the arrival time of the signals, the detector in Livingston recorded the event 7 milliseconds before its twin detector in Hanford. Scientists think that the event occurred in the Southern Hemisphere.
According to General Relativity, an unlucky black hole duo loses energy when it emits gravitational waves. This is the reason why the dancing black holes approach each other slowly over a lengthy time span of billions of years–and then pirouette more rapidly in their doomed danse macabre in the final moments of their fatal collision. Before the final act of this strange ballet, in the merest fraction of a second, the tragic duo blast into one another at half the speed of light. The result of this catastrophic collision is a solitary, and much more massive, black hole–converting a fraction of the combined black hole duo’s mass into energy, according to Einstein’s well-known formula E= mc squared. The energy is hurled out as a dramatic, final, and very powerful blast of gravitational waves–the very gravitational waves that LIGO detected.
Gravitational Waves From Long Ago And Far, Far Away!
Wandering gravitational waves offer scientists an insight into the birth of the Universe itself. In order to find out more about the primordial Universe, Professor Stefan Antusch and his team from the Department of Physics at the University of Basel in Switzerland are conducting research into what is known as the stochastic background of gravitational waves. This background is composed of gravitational waves traveling out from many sources that overlap with one another. When put together, the gravitational waves background provides a broad spectrum of frequencies. The Basel-based physicists calculate predicted frequency ranges and intensities for the Spacetime ripples, which can ultimately be tested in new experiments.
Soon after the Big Bang, the Universe that we observe today was very small–as well as extremely dense and searing-hot. “Picture something about the size of a football,” Dr. Antusch commented in a February 10, 2017 University of Basel Press Release. The entire primordial Universe was compressed into this extremely small space–and it was wildly turbulent. Modern scientific cosmology assumes that, at that very ancient time, the Universe was dominated by a particle known as the inflaton and its associated field.
The inflaton experienced some extremely intensive fluctuations, and also had certain special properties. For example, the inflatons merged together to create clumps that caused them to oscillate in certain localized regions of Space. These regions are termed oscillons, and they can be visualized in the mind’s eye as standing waves. “Although the oscillons have long since ceased to exist, the gravitational waves they emitted are omnipresent–and we can use them to look further into the past than ever before,” Dr. Antusch added.
Using numerical simulations, the theoretical physicist and his colleagues were able to calculate the shape of the oscillon’s signal, which was sent forth mere fractions of a second after the Universe was born in the Big Bang. This signal appears as a pronounced peak in the otherwise broad spectrum of the gravitational waves.
Dr. Antusch explained in the February 10, 2017 Basel University Press Release that “We would not have thought before our calculations that oscillons could produce such a strong signal at a specific frequency.”
Now, in a second step, the experimental physicists are planning to actually prove the real existence of the signal using detectors.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. 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.