At the origins of time: the birth of primordial black holes

The primordial Universe

Density fluctuations at the heart of the nascent cosmos

In the very first fractions of a second after the Big Bang, the Universe experienced a state of extreme density of energy and matter. Within this primordial plasma, local density variations could occur: in some areas, matter and energy naturally tended to accumulate and concentrate.

An immediate gravitational collapse

When a region reached a critical overdensity threshold, gravity took over from expansion, causing a spontaneous collapse. More precisely, if the Schwarzschild radius of the area in question \((R_s = 2GM/c^2)\) became comparable to its causal horizon, the matter would instantly compact to give birth to a black hole.

A distinct origin from stellar black holes

This is how primordial black holes (PBHs) were born. Unlike classical stellar black holes, which result from the death and collapse of massive stars, PBHs are direct relics of the initial conditions and the first convulsions of our Universe.

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, or even several million, solar masses in the most massive scenarios. The mass is determined by the time 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 avenue for unlocking the mysteries of dark matter?

An ideal candidate for the Universe's invisible mass

Although dark matter constitutes about 85% of the total matter in the Universe, its exact nature remains one of the greatest enigmas in modern physics. Primordial black holes (PBHs) represent an attractive hypothesis to explain all or part of this missing component: formed in the first instants of the cosmos, they would be extremely numerous and interact with their environment only through gravity.

An extremely narrow mass window

However, PBHs could only constitute dark matter within a very narrow mass range. Modern observational methods — such as the search for gravitational microlensing, detection of evaporation gamma rays, study of cosmic microwave background (CMB) anisotropies or, more recently, the hunt for binary mergers by gravitational waves — have progressively eliminated most other scenarios.

Indeed, very low-mass PBHs (\(< 10^{15}\,\text{g}\)) should have already entirely evaporated via Hawking radiation, while the absence of observed lensing events imposes very strong constraints on intermediate-mass PBHs.

Indirect signatures and observational constraints

• Gravitational microlensing: This technique aims to detect the passage of compact objects in front of distant stars. The lack of observed events severely limits the fraction of dark matter that PBHs could represent in these mass ranges.
• Gamma radiation: According to the Hawking evaporation mechanism, light PBHs should emit a continuous flux of gamma rays. The absence of such a residual signal allows us to exclude low-mass PBHs as a major component of dark matter.
• Imprint on the cosmic microwave background (CMB): The excessive presence of massive PBHs would have profoundly disturbed the thermodynamic evolution of the primordial cosmos. Precision measurements of the CMB show no trace of this, so this population is also excluded.
• Gravitational waves: The search for binary mergers of black holes of "sub-solar" mass (less than that of the Sun) constitutes an almost indisputable signature, because classical stellar evolution cannot produce such objects. See the dedicated chapter below.

A still theoretical scenario

In short, the existence of primordial black holes remains to this day hypothetical and subject to debate. Their hunt continues to rely on the detection of these indirect signatures and the progressive restriction of cosmological models.

Observational constraints on primordial black holes

From the size of a mountain to that of true cosmic giants, primordial black holes come in a wide range of masses.

As the cosmic horizon expands as the Universe grows, the epoch of formation determines the maximum mass of a primordial black hole. Recent observational data make it possible to map these mass scales and isolate the rare windows that are still open. The following table summarizes their detection methods and current constraints from observations.

On the trail of gravitational waves: the hunt for "sub-solar" black holes

An almost indisputable signature

Classical stellar evolution imposes a lower mass limit on black holes: below about one solar mass, the gravitational collapse of a star cannot produce either a black hole or even a stable neutron star (degenerate matter resists collapse, this is the Chandrasekhar limit). A compact object merging with a mass below that of the Sun, detected by gravitational waves, would therefore constitute one of the most direct proofs of the existence of primordial black holes, since no known astrophysical mechanism can produce them via this route.

Systematic searches, so far unsuccessful

Since their first observation campaigns, the LIGO, Virgo and KAGRA (LVK) collaborations have been conducting dedicated searches for "sub-solar" mass binaries in their data. These successive analyses, covering observation runs O1 to O4, have so far not identified any statistically significant candidate. These negative results are not without value: they make it possible to set increasingly strict upper limits on the merger rate of such objects, and therefore on the fraction of dark matter they could represent, in the planetary and sub-solar mass ranges.

An intriguing candidate, but unconfirmed

On November 12, 2025, the LVK collaboration reported a binary merger candidate named S251112cm, with no identified electromagnetic counterpart, whose chirp mass is estimated between 0.1 and 0.87 solar masses, with a probability greater than 99% that the lighter component is sub-solar.

This is a single candidate of still modest statistical significance, not a confirmed discovery. Its sky localization remains broad (over 1,600 square degrees), no electromagnetic counterpart has been identified, and an alternative astrophysical origin (e.g., exotic stellar) cannot be ruled out at this stage. This event nevertheless illustrates the potential of future observation campaigns, especially with the increased sensitivity expected from the Einstein Telescope, Cosmic Explorer, and LISA, to settle the question.

From primordial black holes to the seeds of supermassive black holes: the JWST enigma

A problem of too rapid growth

Since its commissioning, the James Webb Space Telescope (JWST) has revealed an unexpected population of very massive galaxies at high redshifts (\(z \gtrsim 7\)), some already hosting supermassive black holes of several million to several hundred million solar masses when the Universe was less than a billion years old. These objects, along with the compact and reddish population nicknamed "Little Red Dots" (LRDs), exhibit abnormally high black hole-to-host stellar mass ratios. Classical accretion growth, limited by the Eddington limit from ordinary stellar seeds, struggles to explain such rapid assembly.

Massive PBHs as ready-made seeds

Several teams are exploring the hypothesis that primordial black holes, formed with masses of the order of \(10^3\) to \(10^7\ M_{\odot}\) in the primordial Universe, could have served as directly available seeds for the growth of the supermassive black holes observed by JWST, without requiring a prolonged super-Eddington accretion phase. High-resolution cosmological simulations have thus attempted to reproduce the extreme properties of galaxies like A2744-QSO1, UHZ1 or GHZ9 from a primordial black hole seed coupled with star formation and radiative feedback.

One avenue among others, still debated

This hypothesis remains in competition with other non-primordial scenarios (direct collapse of pristine gas, hierarchical mergers of dense stellar clusters, prolonged super-Eddington accretion) and none of these explanations is currently favored by consensus. It nevertheless illustrates how JWST observations have revived interest in massive PBHs, in a mass range largely distinct from that considered for dark matter.