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.