Primordial black holes: solution to the dark matter enigma?

By Leo Li

Black holes have always been regarded as stellar afterlives. When a star approaches the end of its lifetime – that is, when it runs out of fuel for fusion – it succumbs to its own central gravity and collapses inward. If the star satisfies all necessary initial conditions, after a breath-taking supernova, it compacts itself as a spacetime singularity, from which not even light cannot escape. These cosmic infants are dispersed across the Universe where their previous incarnations used to dwell, one being in the centre of our Milky Way Galaxy.

It was all until the day astronomers began questioning the youth of black holes, and realised, that they can be much older than all the starlight they engulf. These ancient entities are called primordial blackholes.

In 1966, Yakov B. Zel’dovich and Igor D. Novikov pioneered the idea of primordial black holes. Our  early, radiation-dominated Universe – roughly 1 second-old since the Big Bang – was wild, variable and patchy enough to allow for great density fluctuations. Like how matter aggregated to form stars and galaxies in the later Universe, masses were channelled along density gradients to self-collapsing centres. Other than gravitational attraction, cosmic expansion compressed some regions to the point where they also underwent collapse. Eventually, these highly dense centres became singularities, and primordial black holes were formed.

These ancient entities are called primordial blackholes

In the 1970s, Stephen Hawking and Bernard Carr took interest to reinforce and expand upon this model. They constrained the masses of primordial blackholes, with the famous discovery of Hawking Radiation. At the even horizon of a black hole – the size-defining boundary below which no light escapes – gravitational forces are powerful enough to emit thermal radiation. This leak of energy is slow enough that it can be countered if the black hole eats up enough other cosmic microwaves and materials; otherwise, a definitive lifetime would be set upon the black hole as it gradually ‘evaporates’ to void.

The speed at which black holes evaporate are very much dependent on their masses. Thus, considering the age of our Universe, Hawking Radiation rules out primordial black holes below roughly 1000kg. At the 1000kg threshold, primordial blackholes that underwent runaway evaporation – as the rate of radiation acceleratingly increases with decreasing mass – should be observed to burst with the force of a hydrogen bomb at present time. Searches for these spectacles have remain futile.

Nevertheless, there have been many attempts to find, or at least place limits on the numbers and masses of, primordial blackholes. A notable example is NASA’s Fermi Gamma-ray Space Telescope. It contains a module that monitors the luminosities of gamma-ray bursts, during gravitational lensing from (primordial) blackholes. It had set a mass range on primordial black holes, which, though disputed and removed for re-evaluation, inspired more varied and accurate measurements. Further making use of black holes’ lensing effect, white dwarfs, neutron stars and supernovae have become subjects of investigations. LIGO in the US probed deeply into data of gravitational waves from the 2016 black hole merger, revealing their common primordial nature.

Primordial black holes could nevertheless chip in as a runner-up

A major reason for such passion and dedication is because primordial black holes seem to be a perfect candidate for dark matter. Primordial black holes are categorised as massive compact halo objects (MACHOs), a class of astronomical and highly unobservable bodies that constitute the dark matter content in stellar haloes. Primordial black holes check every box as a MACHO: they are nearly as old as the Universe itself; they are highly stable – as long as accretion balances evaporative Hawking Radiation; they move at velocities much slower than light speed and barely collide with other astronomical bodies. Theoretically, as long as there are enough primordial black holes, the dark matter enigma can be resolved.

However, the science community has reached a consensus, albeit still debatable, that the theoretical overabundance of primordial black holes is impossible, or at least unrealistic. Dark matter comprises about 27% of matter in the Universe; and under the current set mass limits of primordial black holes, they could only contribute so many tenths or hundredths percentages to that 27%.

Despite many salvage endeavours, primordial black holes are no longer the sole solution to the dark matter enigma, yet this doesn’t rule them out as a viable solution. Contrary to the underabundance of primordial black holes is an overabundance of plausible candidates for dark matter. From MACHOs, to weakly interacting massive particles (WIMPs), to neutrinos, none of the candidates can on their own fill in all of the blank space dark matter has left. Perhaps there is a dominant species of dark matter, but primordial black holes could nevertheless chip in as a runner-up. This may after all ruin the Keplerian aesthetics to achieve harmonious perfection and unity in cosmic principles, but the search for solutions to enigmas is an unorthodox but empirical perceptiveness.

Image: Aman Pal via Unsplash

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