In our series of posts exploring black holes we have discussed the fantastical properties of their horizons, their reaction to perturbations and triumphant lack of hair, conjectured fuzzy nature and their possible ability to glow and evaporate. If all of this sounds like deranged ramblings to you I implore you to revert to previous editions in the series on these exotic entities. Today we turn our attention to a new class of black holes, which, if proven to exist, would officially hold the title of being the oldest and most mysterious beings in the universe. The existence of such ancient relics, dubbed primordial black holes, would answer some of the universe’s biggest questions. A universal case of respecting your elders.
The dawn of the universe was flooded with radiation produced by the Big Bang. The popular cosmological picture of the beginning of the universe is that of a swirling soup of such radiation which, as the cosmos expanded, began to clump together to form matter and subsequently the first stars and planets. In this popular picture it was only millions of years later, once these stars became sufficiently dense that the strength of their own gravity caused them to collapse and form black holes. However, new models suggest that the early universe could have been a breeding ground for other beasts, primordial black holes (PBHs (the cosmic OAPs)). It is believed that black holes could have also been formed from extreme density fluctuations in the early universe, less than one second after the Big Bang. There are many cosmological phenomena that could have produced such density fluctuations such as cosmic inflation, reheating or cosmological phase transitions. From these, fluctuations (sharp points of contrast) in the matter densities, the spacetime would again be in the position to undergo gravitational collapse upon itself, forming a black hole. Such PBHs would then be able to devour the radiation surrounding them, growing as they ate.
Theoretically PBHs could have initial masses ranging from 10^(-8)kg to thousands of solar masses, however those having masses lower than 10^(11)kg would not have survived to the present day due to having evaporated entirely by now (see Glowing and Shrinking) through the process of Hawking radiation. However these limits still allow good theoretical scope for observation of some of such relics today, which in turn would resolve many unanswered cosmic questions.
Shedding light on darkness
Dark matter is a substance thought to account for approximately 85% of the matter in the universe and about a quarter of its total energy density. The fact that the main constituents of our universe remain a mystery is a sorry state of affairs for theoretical physicists. Popular candidates for the elusive constituents of dark matter are WIMPs (see The Dark Side) and MACHOs: massive compact halo objects. MACHOs are large objects that emit little to no radiation, given their nature PBHs are a possible type of such object. Taken into account their formation at the dawn of time, their supreme density and the masking properties of the horizon to direct observation, it is easily believable that PBHs could be the dominant, or even sole, component of dark matter.
A second theory is that even if PBHs are not directly the dark matter constituents, through their evaporation they could emit whatever the true dark matter particles are. The type of particles emitted during the process of Hawking radiation (during the evaporation of a black hole) crucially do not depend on what fell into reach of the black hole during its lifetime. The black hole amalgamates all it ingests and the by-products of its evaporation can take the from of whatever particle exist in nature. Dark matter treated on an equal footing.
Cosmic speed limit
The rate of the expansion of the universe has not yet been pinned down and cosmologists currently have two ways to measure the rate of acceleration. The first involves the measurement of light from supernovae, whilst the second uses the ancient radiation leftover over from the Big Bang (cosmic microwave background radiation). The trouble is, these two measurements are currently in conflict with each other. However if we account for the radiation effects from the ancient relics of PBHs alongside the ancient radiation of the CMB, this would seem equate rate calculated from the two approaches.
The mass of a black hole increases as it sucks in the matter surrounding it. However if black holes are only able to be born from the fiery deaths of stars, this puts a size limit on the what their mass upper bound should be (due to the restricted time they could have been alive and thus been able to grow). This formation theory for black holes is in direct contradiction with the size of the supermassive black holes (SBHs) we now know exist at the centres of most large galaxies. The gargantuan size of these SBHs is simply impossible based on the current understanding of standard black hole formation. Either there is another process in place by which black holes can grow, perhaps the amalgamation of black holes born from stars is much more frequent than we predict or perhaps PBHs are the answer. If black holes can exist from moments after the Big Bang, seeded instead from cosmic density fluctuations and not have to wait around for stars to form and die, it becomes plausible to reach the supermassive size necessary to agree with current day findings.
Are they out there
LIGO – the Laser Interferometer Gravitational-Wave Observatory has now detected gravitational waves signals from over ten binary black hole mergers (for more on the spectacular details of a black hole merger and what we can learn from them see Inspirational Systems). There are mutterings amongst the LIGO community that some of these detections came from PBHs rather than the standard stellar remnant black holes .
Many of the black holes detected by LIGO did not seem to be spinning fast. If the black holes were formed from resurrected-stars in a binary system, they would tend to have a degree of spin as the stars would have possessed angular moment. PBHs born in isolation in the early universe however, do not tend to have much spin.
PBHs were born from density fluctuations in the early universe and predictions on the nature of such fluctuations can then be extrapolated to tell us what the masses of such PBHs would roughly be today. The average answer is suggested to be about 30 solar masses. Remarkably most of the LIGO observed black holes fall around such a mass range. The majority of the early LIGO measurements coming in at this range is argued by some to support the case.
As the next generation of gravitational wave detectors enter the game, a resolution to the debate over PBHs may come. Whatever the answer may be one thing is for certain, gravitational waves are a revolutionary new medium with which we can explore and understand the universe. For now we continue to wonder whether primordial black holes may hold the answer to such primordial questions.