Astronomy
Primordial Black Holes as Dark Matter Candidates
Quick fact
If all dark matter were made of primordial black holes, then every cubic light-year of space would contain a black hole about the mass of a small asteroid (roughly 10^18 kg). Such light PBHs would be hard to detect because their gravitational lensing effects are tiny and they would have just survived evaporation by now.
Why this is interesting
Dark matter makes up about 85% of all matter in the universe, yet we have never seen it directly. What if some of that dark matter is made of black holes that formed in the very first moments after the Big Bang?
Read the full explanation
Understanding Primordial Black Holes as Dark Matter Candidates
To understand primordial black holes (PBHs), imagine the early universe as a boiling soup of particles. Tiny quantum fluctuations caused some regions to be slightly denser than others. In a region dense enough, gravity could overwhelm the outward pressure, causing the matter to collapse into a black hole. Unlike black holes that form from massive stars, PBHs didn't need a star to start with; they were born directly from the stresses of the Big Bang. These black holes could have any mass, from very light (a mountain) to very heavy (thousands of Suns). The key is that they are not made of normal matter like stars, but they still have gravity. Dark matter is known to exert gravity but doesn't emit light, which is exactly what a black hole does (though matter falling into it can glow, isolated PBHs would be dark). So, PBHs are a natural candidate: they are invisible and have the right gravitational effects. But to be a good candidate, their mass must be in a 'window' that avoids being ruled out by observations. For example, very light PBHs would have evaporated due to Hawking radiation by now, while heavier ones might have been detected by their lensing effects on starlight or by the gravitational waves they produce when they merge.
A deeper explanation
The formation of PBHs requires density fluctuations of order 1 (a region 10-50% denser than average) during the radiation-dominated era. These could come from inflation, but standard inflation produces fluctuations of about 10^-5, so PBH formation needs special conditions, such as a peak in the power spectrum or non-Gaussian tails. Once formed, their evolution depends on their mass. PBHs below about 10^15 grams would have completely evaporated by now via Hawking radiation, emitting gamma rays. Observations of the extragalactic gamma-ray background limit such light PBHs. For heavier PBHs, their gravitational influence is what matters. If they make up dark matter, they would affect the cosmic microwave background (by accreting gas) and the large-scale structure. They would also cause gravitational lensing of stars and quasars; microlensing surveys like OGLE and MACHO have constrained PBHs in the 10^-9 to 1000 solar mass range. In 2016, LIGO detected gravitational waves from merging black holes of about 30 solar masses, which was surprisingly heavy for stellar black holes. This sparked speculation that these could be PBHs, since PBHs could naturally form in binary systems. However, constraints from CMB and X-ray observations limit the fraction of dark matter in PBHs across most mass ranges, leaving a few allowed windows (e.g., around 10^-13 solar masses or the asteroid-mass range). Overall, PBHs are a compelling but currently disfavored candidate for being all of dark matter; they remain a possible component.