In 1993 Stephen Hawking proposed in Black Holes and Baby Universes that there might be “primordial black holes which were formed in the early universe that could be less than the size of the nucleus of an atom, yet their mass could be a billion tons, the mass of Mount Fuji. A black hole weighing a billion tons,” Hawking explained, “would have a radius of about 10-13 centimeter (the size of a neutron or a proton). It could be in orbit either around the sun or around the center of the galaxy, emitting hard gamma rays with an energy of about 100 million electron volts.”
“Calculations based on measurements of the cosmic background of gamma radiation,” wrote Hawking, “show that the average density of primordial black holes in the universe must be less than about two hundred per cubic light-year. The local density in our galaxy could be a million times higher than this figure if primordial black holes were concentrated in the “halo” of galaxies—the thin cloud of rapidly moving stars in which each galaxy is embedded—rather than being uniformly distributed throughout the universe. This would imply that the primordial black hole closest to the Earth is probably at least as far away as the planet Pluto.”
Primordial Black Holes –A Source of Dark Matter?
“If these black holes were initially lighter than a million kilograms or so,” says theoretical physicist Dan Hooper about the possibility that if primordial black holes exist, they’d have potential to solve a whole host of the biggest problems in cosmology, not the least being the mystery of dark matter, considered to be the backbone to the structure of the universe. “they would have evaporated in the first second after the Big Bang. In the process of this evaporation, they could have created any number of exotic forms of matter and energy, including dark matter.”
Signal Detected by NANOGrave–Pulsar Timing Array
In February 2021, a team of researchers who recently published data on a hint of a signal that scientists speculate could be evidence of gravitational waves, cosmic strings or primordial black holes. The data sent ripples of excitement through the physics community. These monumental findings are the culmination of 12 and a half years of data gathered from NANOGrav—a network of pulsars across the galaxy –pulsar timing arrays searching for the gravitational waves the Laser Interferometer Gravitational-waves Observatory (LIGO) Observatory can’t hear that are essentially gravitational wave detectors as big as the Milky Way that have transformed the Milky Way into a massive gravitational wave detector.
“Like light, however, gravitational waves span a broad range of frequencies, and just as you cannot use an infrared telescope to observe ultraviolet light or X-rays, you cannot use LIGO to look for every kind of gravitational wave,” says University of Connecticut gravitational-wave astrophysicist and study leader, Chiara Mingarelli, an associate research scientist at the Center for Computational Astrophysics (CCA) at the Flatiron Institute. “LIGO’s detectors, with arms four kilometers. long, saw (or rather, heard) waves generated by two merging black holes, each approximately 30 times as massive as the Sun. But for the very low frequency gravitational waves created by the merger of the million- to billion-solar-mass black holes at the centers of most galaxies, you need a detector as big as the Milky Way.”
Gravitational Waves –the Source?
Gravitational waves are generated when galaxies merge and supermassive black holes at their centers collide and send low-frequency gravitational waves out into the universe. The team thinks the source of the signal could be gravitational waves, but it will take about two more years of data to be sure.
The findings, reports the University of Connecticut, sparked the interest of other physicists with their own speculations about the signal, such as cosmic strings — galaxy-sized filaments of raw energy, that may be threaded through spacetime– or primordial black holes. Though still a couple of years away, Mingarelli says the final results could help test General Relativity or even open the door to entirely new physics.
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