Mysterious Primordial Black Holes
By Judith E Braffman-Miller
It is generally thought that the Universe originated as an unimaginably tiny Patch, that was smaller than a proton, about 13.8 billion years ago. Then–in the tiniest fraction of a second–it inflated exponentially to attain macroscopic size. All of Spacetime was born from a tiny primordial brew of densely packed, searing-hot particles–and it has been expanding and cooling off from that initial incandescent state ever since. Of course, there was nothing around with eyes to see that could watch the Cosmos emerge from this mysterious initial state, and so we search for our own origins long ago and far away, as the Universe keeps its myriad mysteries to itself. Primordial black holes are one of those mysteries. These very ancient, hypothetical black holes are thought to have formed during the high-energy and inhomogeneous stage of the Universe’s birth, as a result of the gravitational collapse of density fluctuations in the primeval fireball. In September 2017, physicists proposed new theories explaining how the Universe’s first black holes might have been born, and these primordial black holes–that formed shortly after the Big Bang birth of the Cosmos–might explain how heavy atomic elements like gold, platinum, and uranium are formed.
A perplexing puzzle plaguing astrophysicists concerns the very first black holes. Did primordial black holes form less than a second after the Big Bang, or were they born millions of years later as a result of the ancient cosmic fireworks heralding the explosive deaths of the first generation of stars? The first generation of stars are thought to have been very massive. The more massive the star, the shorter its hydrogen-burning “life”.
Massive stars live fast and die young. This is because they are extremely hot and burn their supply of nuclear-fusing hydrogen fuel very quickly, at least by star standards. The first stars were not like the stars that we know today–they formed from pristine hydrogen and helium, and were not “polluted” by the heavier atomic elements produced in the furnaces of stars. This is because there was no previous generation of stars to produce these heavier elements. Massive stars, when they finally deplete their necessary supply of nuclear-fusing fuel, do not go gentle into that good night, but instead blow themselves to smithereens in the fiery fury of a supernova conflagration, leaving behind either a neutron star or a black hole of stellar mass to tell the tragic story of how there was once a star, that is a star no more.
In Search Of Primordial Black Holes
The ancient Universe was filled with high-energy radiation, a turbulent and swirling sea of hot particles of light called photons. The entire beautiful, bright baby Universe was glaring with the primordial fires of its mysterious birth. What we now see, almost 14 billion years after this initial blast of formation, is the dimming, cooling, greatly expanded and still expanding aftermath of that first burst of brilliant light. As our Universe continued to expand to its current vast size, the fires of its formation faded, and so we watch from our tiny, obscure, rocky, watery blue world as our Universe relentlessly grows larger and larger, darker and darker, colder and colder, dying as it expands eerily to ash.
Georges Henri Joseph Eduard Lemaitre (1894-1966) was a Belgian astronomer, priest, and professor of physics at the Catholic University of Louvain. Lemaitre was one of the first to propose that our Universe is expanding. He also formulated the hypothesis that would come to be called the Big Bang Universe. Once Lemaitre noted that “The evolution of the world may be compared to a display of fireworks that has just ended: some few wisps, ashes, and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanished brilliance of the origins of worlds.”
In the 18th century, John Michell and Pierre-Simon Laplace pondered the possibility that there could really exist in nature weird objects like black holes. Albert Einstein, in his Theory of General Relativity (1915) predicted the existence of objects that possessed such immense gravitational fields that anything unlucky enough to wander too close to the voracious beast would be devoured. However, the concept that such genuinely bizarre objects could really exist in nature seemed so absurd at the time that Einstein doubted his own findings–even though his calculations indicated otherwise.
In 1916, Karl Schwarzschild developed the first modern solution to General Relativity that could describe a black hole. However, its interpretation as a region of Space from which no object could ever free itself from the snatching claws of the gravitational beast, was not truly developed for almost fifty years. Up until that time, black holes were considered to be mere mathematical oddities. It was not until the 1960s that theoretical work showed that black holes are a generic prediction of General Relativity.
Black holes can come in any size–pack a sufficient amount of matter into a small enough space and a black hole will form every time. It is generally thought that black holes come in two, or possibly three, primary sizes on cosmological scales: supermassive, stellar mass, and intermediate. Supermassive beasts likely inhabit the dark hearts of every large galaxy in the Universe–including our own Milky Way–and they weigh-in at millions to billions of times solar-mass. Stellar mass black holes, that form when a doomed, massive star fatally collapses in the rage of a supernova explosion, weigh considerably less than their supermassive kin. The still somewhat hypothetical, but nonetheless probable, intermediate mass objects occupy a middle-ground between their supermassive and stellar-mass cousins.
The concept of primordial black holes was first introduced in 1971 by Dr. Stephen Hawking of the University of Cambridge in the UK. Dr. Hawking proposed that black holes may exist that are smaller than those of stellar mass. This would mean that they are not born as a result of stellar gravitational collapse. Since then, several mechanisms have been put forward to explain ancient inhomogeneities as the origin of primordial black hole birth–such as cosmic inflation, phase transitions, and reheating.
Primordial black holes are also a possible dark matter candidate. The dark matter is thought to compose most of the matter in the Universe–there is much more of it than the atomic (baryonic) matter that makes up our familiar world. Even though atomic matter composes literally every element listed in the familiar Periodic Table, the mysterious non-atomic dark matter is considerably more abundant. Dark matter is composed of as yet unidentified exotic particles that do not dance with light or any other form of electromagnetic radiation–which makes it transparent and invisible. However, scientists are almost certain that the dark matter really is there because it does gravitationally interact with objects that can be seen. In this way, the ghostly dark stuff reveals its phantom-like presence.
Primordial black holes may be the “seeds” that merged together in the ancient Universe to become the enormous supermassive beasts that dwell hungrily and secretively in the hearts of massive galaxies–as well as their intermediate-mass cousins.
In March 2016, only one month after the announcement of a detection of gravitational waves by Advanced LIGO/VIRGO, three separate teams of scientists proposed independently that the gravitational waves had been emitted by two merging 30 solar-mass black holes–and that the duo had a primordial origin. Gravitational waves are ripples in the curvature of Spacetime, caused by certain gravitational interactions. The waves propagate outward from their source at the speed of light.
Two of the three teams of scientists found that the merging rate of the pair, revealed by LIGO, indicated that all of the Universe’s dark matter is composed of primordial black holes. This would happen if a sufficient number of them are somehow clustered within halos such as globular clusters or dim dwarf galaxies–as predicted by the standard theory of cosmic structure formation. The third team of astronomers claimed that these merging rates are incompatible with an all-dark-matter scenario and that primordial black holes could only account for less than one percent of the total dark matter.
However, the surprisingly large mass of the duo of merging black holes spotted by LIGO has given new life to a hunt for primordial black holes with masses that range from 1 to 100 times the mass of our Sun. Nevertheless, it is still not clear whether this range is excluded by other observations, such as the absence of microlensing of stars, the cosmic microwave background (CMB) radiation anisotropies, the size of dim dwarf galaxies, and the absence of correlation between X-ray and radio sources towards the galactic core.
In May 2016, Dr. Alexander Kashlinsky proposed that observed spatial correlations in unresolved gamma-ray and X-ray background radiations could result from primordial black holes sporting similar masses–that is, if their abundance is similar to that of the mysterious dark matter. Dr. Kashlinsky is an astronomer and cosmologist at NASA Goddard Space Flight Center in Greenbelt, Maryland.
Primordial black holes may have been born in the very ancient Universe less than 1 second after its birth in the Big Bang–during what is termed the radiation dominated era. The most important ingredient needed for a primordial black hole to form is a fluctuation in the density of the Universe, because this would trigger its gravitational collapse.
Mysterious Primordial Black Holes
Dr. Alexander Kusenko, of the University of California Los Angeles (UCLA), and Eric Cotner, a UCLA doctoral student developed an intriguingly simple new theory proposing that black holes could have been born very soon after the Big Bang–long before the first generation of stars had been born. Astronomers had already suggested that these primordial black holes could account for the puzzling, ghostly dark matter, and that these could have been responsible for “seeding” the supermassive gravitational beasts haunting the dark hearts of large galaxies. The new theory proposes that primordial black holes may also have helped to create many of the heavier elements found in nature.
The Big Bang itself gave birth to only the lightest of atomic elements–hydrogen, helium, and traces of beryllium and lithium (Big Bang nucleosynthesis). All of the heavier atomic elements were formed in the nuclear-fusing furnaces of the stars (stellar nucleosynthesis). However, some theories propose that the heaviest atomic elements of all–such as gold, uranium, and platinum–form as a result of the supernova blasts that herald the demise of massive stars (supernova nucleosynthesis). All of the atomic elements heavier than hydrogen and helium are termed metals by astronomers, and so the term metal carries a different meaning for astronomers than it does for chemists.
Kusenko and Cotner began their research by considering that there had been a uniform field of energy that had pervaded the primeval Universe soon after the Big Bang. Indeed, many scientific cosmologists think that such fields existed in the very remote past. After the Universe had expanded in the wild exponential burst of cosmic Inflation, this energy field would have fragmented into separate clumps. Gravity would then cause these clumps to dance together, and eventually collide, merging together to create larger objects. The UCLA researchers then went on to propose that some small percentage of these ever-growing clumps became sufficiently dense to collapse to primordial black holes.
Their new theory is fairly generic, Dr. Kusenko commented in a September 1, 2017 UCLA Press Release. He added that it doesn’t depend on what he called the “unlikely coincidences” that are the foundations of other theories explaining primordial black holes.
The paper describing the new research suggests that it is possible to hunt for these elusive primordial objects using astronomical observations. One technique involves making measurements of the extremely small alterations in a star’s brightness that result from the gravitational effects of a primordial black hole traveling between Earth and that star. Earlier in 2017, U.S. and Japanese astronomers published a paper describing their discovery of one star in a nearby galaxy that dimmed and brightened exactly as if a primordial black hole was wandering in front of it.
In a separate research study, Dr. Kusenko, Dr. Volodymyr Takhistov, a UCLA postdoctoral researcher, and Dr. George Fuller, a professor at the University of California, San Diego (UCSD), proposed that primordial black holes may have played a starring role in the formation of heavy elements (metals) such as gold, silver, platinum, and uranium, which could still be an ongoing process both in our own Milky Way Galaxy and in others.
The origin of those heavy metals has been an intriguing mystery for scientific cosmologists for many years.
“Scientists know that these heavy elements exist, but they’re not sure where these elements are being formed. This has been really embarrassing,” Dr. Kusenko continued to note in the September 1, 2017 UCLA Press Release.
The UCLA study indicates that a primordial black hole sometimes bumps into a neutron star–the Chicago-sized, extremely dense, spinning stellar corpse of a massive star that lingers in the Universe after some supernova blasts. Neutron stars, like black holes of stellar mass, are remnants of massive stars that have gone supernova. However, neutron stars are the lingering leftovers of stars that are not quite as massive as those that collapse into stellar mass black holes. After the primordial black hole–neutron star collision, the black hole would sink down into the neutron star’s very depths.
Dr. Kusenko went on to explain that, when this collision occurs, the primordial black hole devours the neutron star from within, a process that lasts approximately 10,000 years. As the neutron star shrinks, as a result, it spins faster, and faster, and faster. This wild spinning eventually causes small fragments to tear off and fly away. Those escaping fragments of neutron star stuff may be the sites in which neutrons fuse into heavier and heavier atomic elements.
However, the probability of a neutron star capturing a black hole is a bit low, Dr. Kusenko added, and this is consistent with observations of only some galaxies being enriched in heavy metals. The idea that primordial black holes bump into neutron stars and create heavy elements also explains the observed lack of neutron stars in our own Galaxy’s center–a nagging, long-standing mystery in astrophysics.
During the coming winter months, Dr. Kusenko and his team will collaborate with scientists at Princeton University in New Jersey, on supercomputer simulations of the heavy elements formed by a neutron star-black hole interaction. By comparing the results of those simulations with observations of heavy elements in nearby galaxies, the scientists hope to determine whether primordial black holes are really responsible for Earth’s gold, platinum, and uranium.
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.
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