Imagine ripples propagating through a small pond in mid-summer, spreading through the glistening sunlit water from where a little boy has just tossed a pebble into the pond. The gravitational ripples that propagate through the fabric of Spacetime are similar to those ripples spreading through the Sun-warmed water of the pond, except that the ripples spreading through Spacetime–called gravitational waves–are generated when accelerated masses propagate as waves outward from their source at the speed of light. Now imagine that the water of the pond is the fabric of the Universe itself, through which the gravitational waves ripple. The most powerful gravitational waves of all propagate as the result of catastrophic events, such as the violent collision of a pair of dense stellar relics called neutron stars. In May 2018, a team of astronomers announced they have discovered that the spectacular, brilliant merger of a duo of neutron stars had generated gravitational waves–and probably did something else, as well, because their merger likely spawned a black hole that would be the lowest mass black hole ever detected.
The new study analyzed data derived from NASA’s Chandra X-ray Observatory, that had been obtained in the days, weeks, and months following the detection of rippling gravitational waves by the Laser Interferometer Gravitational Wave Observatory (LIGO), and gamma rays by NASA’s Fermi mission, on August 17, 2017. The twin LIGO detectors are located in Hanford, Washington and Livingston, Louisiana. The two observatories are funded by the National Science Foundation (NSF), and were invented, constructed, and operated by scientists at the California Institute of Technology (Caltech) in Pasadena, California. The Fermi Gamma-ray Space Telescope was launched on June 11, 2008 aboard a Delta II rocket. Fermi is a joint NASA, U.S. Department of Energy mission that also includes agencies in France, Germany, Italy, Japan and Sweden.
Almost every telescope available to professional astronomers had been used to observe the mysterious source of the tattle-tale gravitational waves, officially dubbed GW170817. Nevertheless, X-rays obtained from Chandra proved crucial for gaining a new understanding of what had actually happened after the two neutron stars had managed to crash into one another in a horrific merging event.
Neutron stars are the lingering cores of massive stars that perished in a brilliant, multicolored supernova fireworks display, after having used up their necessary supply of nuclear-fusing fuel. In the end, harboring a hard heart of iron that cannot be used for fuel, the heavy stars must meet their explosive doom. Neutron stars are city-sized, extremely dense spheres. Indeed, a teaspoon full of neutron-star-stuff can weigh as much as a pride of lions.
From the data derived from LIGO, astronomers were able to determine a good estimate of the mass of the neonatal black hole resulting from the neutron star merger. The team of scientists calculated that the black hole’s mass would be equivalent to about 2.7 times the mass of our Sun. This places the source on a fuzzy “tightrope” of undetermined identity. That is because this mass indicates that it can be either the most massive neutron star ever discovered or the lowest mass black hole. The previous record holders for the title of smallest known black hole are no less than approximately four or five times solar-mass.
Albert Einstein predicted the existence of gravitational waves in his Theory of General Relativity (1915), and these propagating ripples through the fabric of Spacetime take along with them, for the ride, long-lost secrets about the birth of the Universe.
Einstein’s mathematics demonstrates that massive accelerating bodies, such as neutron stars and black holes–as they orbit one another–can churn up Spacetime in such a dramatic way that the resulting ripples of distorted Space would fly away from their source. This is comparable to the way ripples in a pond propagate away from their place of origin. Gravitational waves travel at the speed of light, and the speed of light sets something of a universal speed limit. No known signal in the Universe can travel faster than light in a vacuum.
“While neutron stars and black holes are mysterious, we have studied many of them throughout the Universe using telescopes like Chandra. That means we have both data and theories on how we expect such objects to behave in X-rays,” explained Dr. David Pooley in a May 31, 2018 Chandra X-ray Observatory Press Release. Dr. Pooley, who led the study, is of Trinity University in San Antonio, Texas.
Gravitational waves were first proposed to exist by the French mathematician and theoretical physicist Henri Poincare (1854-1912) in 1905. Ten years later the existence of these Spacetime ripples were predicted by Einstein on the basis of General Relativity. Gravitational waves carry along with them energy in the form of gravitational radiation, a form of radiant energy akin to electromagnetic radiation. However, Sir Isaac Newton’s law of universal gravitation, part of classical mechanics, does not predict their existence. That is because this law is based on the assumption that physical interactions propagate instantaneously (at infinite speed), thus revealing one of the ways the methods of classical physics fail to explain phenomena associated with Relativity.
As a branch of observational astronomy, gravitational wave astronomy uses gravitational waves to obtain observational information concerning sources of detectable gravitational waves. These Spacetime ripples originate, for example, in binary stellar systems that are made up of white dwarfs, neutron stars, and black holes. Gravitational wave astronomy also provides important new information about supernovae explosions, as well as the birth and evolution of the primordial Universe soon after the Big Bang.
On February 11, 2016, the LIGO and Virgo Scientific Collaboration made the important announcement that they had succeeded in making the very first observation of the predicted Spacetime ripples. The actual observation was made on September 14, 2015, using the Advanced LIGO detectors. These first-to-be-detected gravitational waves originated from a duo of merging black holes. Soon after the initial announcement, the LIGO instruments spotted two more confirmed, and one potential, gravitational wave events. In August 2017, the two LIGO instruments, along with the Virgo instrument, spotted a fourth gravitational wave originating from merging black holes, as well as a fifth gravitational wave resulting from the merger of a duo of neutron stars that had originally composed a binary system before their smash-up.
The 2017 Nobel Prize in Physics was awarded to Dr. Rainer Weiss (MIT), Dr. Kip Thorne (Caltech), and Dr. Barry Barrish (Caltech) for their work on the first detection of these Spacetime ripples.
Currently, there are several more gravitational wave detectors that are either under construction or in the planning stages.
As a traveling gravitational wave passes a faraway observer, the observer will stare in wonder as Spacetime itself becomes distorted due to the weird effects of that propagating ripple. The distances between free objects will first increase, and then decrease rhythmically, as the bizarre Spacetime ripple makes its incredible journey. As the gravitational wave travels, it does so at a frequency that corresponds to that of the wave itself. The magnitude of this strange effect decreases inversely with distance from the turbulent source of the propagating wave. The wandering Spacetime ripple formed as a result of a violent event–such as the merger of a duo of neutron stars. As a result, the two neutron stars dance ever closer–and closer–to one another, participating in a bizarre mesmerizing cosmic waltz. The weird waltz is over when the two dancers crash into one another and, as a result, merge–making their final farewell performance to the Universe. A black hole of stellar mass may be born as a result of this exotic, heavenly waltz of a doomed duo of neutron stars. Alas, as a result of the great distances that exist between Earth-bound observers and the dancing gravitational wave sources, the effects when measured by astrophysicists on our own planet are predicted to be small.
As gravitational waves ripple through the Universe, they alternately stretch and shrink the fabric of the Spacetime continuum. This means that these propagating ripples distort the geometry of the fabric of Space itself. Even though accelerating objects emit gravitational waves, these can only be measured by astrophysicists on Earth when the mass is very large.
Propagating gravitational waves provide astronomers with new insight into the mysterious birth of the Universe itself, enabling them to learn more and more about the primordial Cosmos. Soon after the inflationary Big Bang birth of the Universe, it was much smaller than what we see today–and it was also extremely hot and dense. Imagine something about the same size as a soccer ball. The entire primordial Universe was squashed into this extremely small space–and the soccer-ball-sized ancient Cosmos was a turbulent and violent place. Modern scientific cosmologists assume that, at this very ancient time, the Universe was dominated by a particle termed an inflaton and its associated field.
The first tantalizing hints of the possibility of discovering these Spacetime ripples came in 1974–twenty years after Einstein’s death. In that year, two astronomers, Dr. Russell Alan Hulse and Dr. Joseph Hootin Taylor, Jr., working at the Arecibo Radio Observatory in Puerto Rico, discovered a binary pulsar–a pair of extremely massive, dense Chicago-sized stellar relics in orbit around each other. The pulsar binary has been named after its two discoverers (the Hulse-Taylor Binary). However, it is also known by the telephone-book-sounding designation of PSR B1913+16.
Pulsars are baby neutron stars–and neutron stars are the relic cores of massive progenitor stars that blasted themselves to pieces in supernova explosions. Fresh newborn pulsars spin wildly, and send forth beams of light that are so regular that they are frequently compared to lighthouse beacons on Earth.
PSR B1913+16 was exactly the kind of stellar system that, according to General Relativity, should send ripples traveling into the space between stars. Realizing that this type of binary pulsar system could be used to test Einstein’s prediction, astronomers started to measure how the period of the stellar duo’s orbits altered over time. After almost ten years of observations, the researchers determined that the two pulsars were waltzing closer towards one another at precisely the rate predicted by Einstein in General Relativity. This pulsar binary has been studied for almost fifty years, and the observed changes in the orbit agree so well with General Relativity, that astronomers are certain that it is sending propagating ripples through Spacetime.
Since these early observations, many astrophysicists have studied the timing of pulsar radio emissions and have derived similar results, thus further confirming the existence of these waves rippling through the fabric of the Universe.
It was not until September 14, 2015, that the LIGO Gravitational Wave Interferometer directly detected the distortions in Spacetime caused by rippling gravitational waves. Up to that point, most of the evidence for their existence came from mathematical calculations or other indirect investigations. The first detected ripples were the result of a dancing duo of merging black holes located at the great distance of almost 1.3 billion light-years from Earth.
Gravitational waves can reach our planet from their distant places of origin. The very first direct observation of their real existence opens up a new vista that astronomers can use to uncover some of the Universe’s many mysteries. Gravitational waves take along with them important information about their turbulent places of birth that could otherwise not be obtained. These Spacetime ripples reveal regions of the Universe that electromagnetic waves are unable to access. Astrophysicists can now observe the Cosmos and its well-hidden mysteries using gravity as a tool–as well as light.
The Birth Of A Black Hole
If the merging neutron stars composing the GW170817 source had created a more massive neutron star, then Chandra would show it spinning rapidly and churning out an extremely powerful magnetic field. This would have then been followed by an expanding bubble composed of high-energy particles that produced a brilliant blast of X-ray emission. However, this is not what the Chandra data show. Instead, the information derived from Chandra show levels of X-rays that are a factor of a few to several hundred times lower than expected for a wildly spinning, merged neutron star duo and its assoiated bubble of high-energy particles. This indicates the birth of a black hole instead of a more massive neutron star.
If this result is confirmed, it would reveal that the secret recipe for cooking up a black hole can sometimes be rather complicated. In the case of GW170817, it would have required two supernova blasts to have left behind two neutron stars in a sufficiently close orbit for gravitational wave radiation to merge the neutron star duo together.
“We may have answered one of the most basic questions about this dazzling event: what did it make? Astronomers have long suspected that neutron star mergers would form a black hole and produce bursts of radiation, but we lacked a strong case for it until now,” explained study co-author Dr. Pawan Kumar in the May 31, 2018 Chandra Press Release. Dr. Kumar is of the University of Texas at Austin.
A Chandra observation two to three days following the merger did not detect a source. However, subsequent observations 9, 15, and 16 days following the event revealed important detections. The source traveled behind our Sun soon afterwards, but additional brightening was observed by Chandra about 110 days following the event. This brightening was then followed by comparable X-ray intensity after 160 days.
By comparing the data derived from Chandra observations to those taken by the NSF’s Jansky Very Large Array (VLA), Dr. Pooley and collaborators explain the observed X-ray emission as being caused entirely by shock waves resulting from the merger blasting into ambient gas. There is no sign of X-rays resulting from a newborn neutron star.
The claims by Dr. Pooley and team can be tested by upcoming X-ray and radio observations. If the remnant that the merger left behind does turn out to be a neutron star with a powerful magnetic field, then the source should continue to get much brighter at X-ray and radio wavelengths in about two years or so–when the bubble of high-energy particles at last catches up with the shock wave that would be slowing down. If it is indeed a baby black hole, astronomers expect it to continue to grow fainter and fainter. This has been recently observed as the shock wave weakens.
“GW170817 is the astronomical event that keeps on giving. We are learning so much about the astrophysics of the densest known objects from this one event,” commented Dr. J. Craig Wheeler in the May 31, 2018 Chandra Press Release. Dr. Wheeler, a co-author on the study, is also of the University of Texas at Austin.
If follow-up observations spot a heavy neutron star as the survivor of the merger, such a discovery would challenge theories for the structure of neutron stars and how massive they can get.
“At the beginning of my career, astronomers could only observe neutron stars and black holes in our own Galaxy, and now we are observing these exotic stars across the Cosmos. What an exciting time to be alive, to see instruments like LIGO and Chandra showing us so many thrilling things nature has to offer,” said study co-author Dr. Bruce Grossan in the Chandra Press Release. Dr. Grossan is of the University of California at Berkeley.
A paper describing this research is published in The Astrophysical Journal Letters.