Primordial Black Holes: The Invisible Vestiges of the Young Universe

The Primordial Universe

In the primordial Universe (just after the Big Bang), the density of energy and matter was enormous. In some places, they could "collapse in on themselves". These local fluctuations could further concentrate this matter/energy. When this density crossed a critical threshold, gravity took over: the region immediately collapsed to form a primordial black hole.

In other words, primordial black holes (PBH) could have formed in the very first fractions of a second of the Universe, during extreme density fluctuations. Unlike stellar black holes resulting from the collapse of massive stars, these originate from the initial conditions of the cosmos. If a region reached such a local overdensity that its Schwarzschild radius \((R_s = 2GM/c^2)\) became comparable to its causal horizon, a black hole formed immediately.

Masses and Characteristic Scales of Primordial Black Holes

The possible masses of primordial black holes cover an immense spectrum, ranging from the subatomic scale \((10^{-5}\,\text{g})\) to several thousand solar masses. The mass is determined by the moment of formation: a black hole formed \(10^{-23}\) seconds after the Big Bang would have a mass comparable to that of a mountain, while a PBH formed one second later could reach \(10^5 M_{\odot}\).

Primordial Black Holes: A New Lead to Unravel the Mysteries of Dark Matter?

We still do not know what dark matter is, but we know it represents about 85% of the matter in the Universe. Primordial black holes (PBH) are a hypothesis to explain part (or all) of this dark matter, as they would be very numerous, invisible (except by their gravity), and formed very early in the cosmos. They could be a form of dark matter, but only within a very narrow mass range, as observations exclude most other scenarios.

Modern research (gravitational microlensing, gamma radiation from Hawking evaporation, anisotropies in the cosmic microwave background) allows us to narrow down the mass range of PBHs that could constitute a significant fraction of dark matter. Low-mass PBHs (\(<10^{15}\,\text{g}\)) should have completely evaporated, while intermediate-mass PBHs are strongly constrained by the lack of gravitational lensing events.

Indirect Signatures of Primordial Black Holes

• Gravitational microlensing: Gravitational microlensing is used to detect if many compact objects (such as PBHs) pass in front of stars, but since we do not observe enough of them, this limits the possible contribution of PBHs of certain masses to dark matter.
• Gamma radiation: The expected gamma radiation from the evaporation of small PBHs via the Hawking mechanism has not been detected, which excludes low-mass PBHs as dark matter.
• Effects on the cosmic microwave background (CMB): Anisotropies in the CMB: too many massive PBHs would disrupt the evolution of the early Universe, which we do not observe.

PBHs are considered hypothetical and not widely accepted, and their research relies on indirect signatures.

Current State of Knowledge

To date, primordial black holes (PBH) remain mainly theoretical objects. Although their existence is physically and cosmologically possible, there is still no direct evidence, and their role in dark matter is highly debated. Models predict various masses and abundances, but the scientific community does not agree on their actual importance.

Observational Constraints

Sources: Sasaki et al., 2016; Niikura et al., 2019.