Primordial Black Holes

Illustration of Black Hole

Black holes are one of the most cryptic objects in the Universe. Even after 100 years of its theoretical prediction by Einstein’s General Theory of Relativity, scant knowledge about these gravitationally massive objects. Black holes can be of many types Stellar, Intermediate, Supermassive, Miniature and Primordial Black holes. All the theoretical predictions confirmed observationally to date are related to Stellar Black holes. So what precisely are Primordial Black holes? How were they formed? What are the Observational constraints on them? And why are they considered significant for the advancement of Modern Cosmology? 

In 1971, Stephen Hawking propounded that when the age of the Universe was less than a second, excessively dense regions in the Universe underwent gravitational collapse and directly formed black holes. These Black holes were known as Primordial Black holes (PBHs). Like Stellar Black holes, they do not originate from the death of a star but are birthed from the direct collapse of radiation (photons, neutrinos, leptons, quarks, etc.) filling the Universe at that epoch. The formation of PBHs might have taken place when the relative amplitude of the density perturbations of a particular over-dense region exceeded a threshold value (~unity) which satisfied the Jeans criterion for the gravitational collapse and started to contract by its own gravity against the radiation pressure at around a time when its size becomes equal to the Hubble horizon and forthrightly grows into a PBH within the free-fall time. Contrasting to the Stellar Black Holes, which are heavier than the mass of the Sun, PBH mass could be distinctly smaller. The mass of the resultant PBH is candidly proportional to the time when the PBH formed. Depending on the length scale of the over-dense region, the PBH mass could be as low as 1 gram. For instance, ten solar mass PBHs emerged when the age of the Universe is merely about a millisecond. So, PBHs are hypothetical (as even after 50 years of observational efforts after their theoretical prediction, we still haven’t detected them) Black holes that might have formed shortly after the Big Bang of the Universe.

According to the Hawking radiation hypothesis, PBHs of mass lighter than 1015 grams evaporates due to the present age of the Universe, and PBHs heavier than 1015 grams do not vaporize. Although, there are still some Observational constraints on these non-evaporating PBHs:

• Dark Matter contender: PBHs are speculated as a contender for dark matter as they move only at non-relativistic speed and interact only gravitationally (similar to cold dark matter).

• Gravitational Lensing: It can constrict detection of PBHs as they act as lenses and magnify the luminosity of background objects such as stars.

• Dynamical constraints: If there are bountiful PBHs with certain mass then, they can dynamically affect and eradicate any known Astrophysical system due to their gravitational interactions; as existence of PBHs is incompatible with these systems.

• Accretion constraints: In the early Universe, radiation emanating from accretion gas onto PBHs led to the distortion of the spectrum in the Cosmic Microwave Background (CMB). In the present Universe also PBHs produce strong radiation due to accretion of gas but non-observation of this radiation makes it onerous to detect.

• Indirect constraints: According to the most popular scenario for PBH formation is direct gravitational collapse of the rare high-σ peaks of primordial density perturbations. Although regions aside from where PBHs are formed are not in-homogeneous enough to produce PBHs but, they’re still in-homogeneous enough to induce effects that yield the observable signals. Those effects are not sourced by the PBHs but by the density perturbations that seed the PBHs, and hence the resultant constraints on the PBHs are indirect.

• The first such effect is the emission of incoherent Gravitational Waves; which are filling the present Universe, from the acoustic oscillations of the primordial density perturbations.
•  The second indirect constraint can be received from generation of the CMB spectral distortion of the primordial density perturbations.

Despite these constraints if we are able to detect PBHs then it’ll not only be a technological milestone but will also be epoch-making evolution of Modern Cosmology in many ways. Firstly, presently, there are many inflation models. If PBHs are spotted in the future, then inflation models will be constrained to those that form PBHs. Contrarily, if existence of PBHs is recanted by observations, then the inflation models envisioning presence of PBHs shall be obviated. Secondly, as PBHs behave as cold dark matter (as they too interact only gravitationally) their detection can perhaps abolish necessity to introduce a new physics. Thirdly, it is important for advancement of Gravitational-Wave Astronomy. Fourthly, detection of PBHs can impart Astrophysical explanations about origins of Supermassive Black Holes.
Hence, detection of more Black Hole binaries in the future will aid in gaining more knowledge about PBHs and early Universe. And in this path of detection of PBHs many technological, scientific advancements are on its way.

“PBHs are Dark but the Future of PBH research is Bright.” ~ Teruaki Suyama

            Author - Priyal Bordia,Public Representative Head, National Space Society (USA) — Mumbai

References:
[1] Primordial Black Holes - Teruyaki Suyama
[2] M.Sasaki, T.Suyama, T.Tanaka, S.Yokoyama, Class.Quant.Grav.35, 063001, (2018).
[3] Illustration of Black Hole