Our January 2018 newsletter discussed the possibility that what cosmologists have called “dark matter” for the last 87 years may actually be, in whole or in part, normal matter trapped inside black holes.

The operational definition of “dark matter” is matter that only interacts gravitationally with normal matter and radiation. There are seven separate lines of evidence for dark matter, including two confirming its existence in the early universe. This evidence shows that, as early as 100 seconds after the Big Bang, 84% of all the matter in our universe was “dark”.

Mainstream theories say black holes formed much later, and hence cannot account for this early dark matter. These theories say black holes only form from dying stars and that the first stars formed about 200 million years after the Big Bang.

However, if some black holes — primordial black holes (PBHs) — did form very early, say within the first microsecond, normal matter trapped within them would be invisible, isolated, and would interact only gravitationally with everything else, thus qualifying as “dark matter”.

Bernard Carr of University of London and Florian Kühnel of LMU Munich recently published a study of plausible origins of PBHs and what percentage they might be of all dark matter.

The authors say PBHs most likely formed during phase transitions during the first few seconds after the Big Bang. A phase transition occurs when the average particle energy — what we call temperature — passes through some critical value, and the state of matter changes abruptly. Water freezing at 32ºF is an example.

When our universe began, it was extremely hot. Each particle moved at nearly the speed of light, and its kinetic energy was far greater than its mass energy. Particles collided frequently, producing all types of particle/antiparticle pairs.

When the universe cooled down to 1000 trillion Kelvin, colliding particles no longer had enough energy to produce the most massive particles: Higgs, W and Z bosons, and Top quarks. These particles became non-relativistic (moving much slower than light), and started decaying. This was the first phase transition.

Relativistic particles produce very high pressures; their pressure-to-energy ratio w = 1/3, is a much greater ratio than slower particles. While the number of relativistic particle types changes during a phase transition, w and pressure briefly drop substantially, making it easier for matter to collapse, forming black holes.

An even greater phase transition occurred when the temperature dropped to 2 trillion K, and quarks combined forming protons and neutrons. Additional phase transitions occurred at lower temperatures, as ever-less-massive particles — mesons, muons, and electrons — become non-relativistic.

This cascade of phase transitions created a series of bursts of PBHs, with the PBHs formed in each burst being more massive than in preceding bursts, due to ever-expanding interaction horizons.

The result is shown here. The horizontal axis is PBH mass in units of our Sun’s mass. The vertical axis plots the possible PBH percentage of total dark matter. The green, blue, and red curves represent three different values of a parameter in the authors’ theory.

The light gray curves denote areas of the graph excluded by observations from: M for microlensing, A for accretion, W for wide binaries, X for x-rays, and E for star clusters in Eridanus. Microlensing is the focusing of light by an object much less massive than a galaxy — see my January 2020 newsletter for additional information.

All exclusion zones in the above graph involve numerous assumptions that are strongly debated. Indeed, the authors show different exclusion patterns in different graphs in their paper.

The graph above shows a small bump near a PBH mass of 30 Msun, matching the LIGO gravitational wave observations of unexpectedly many mergers of such massive black holes. The large peak at 1 Msun could account for all dark matter. Yet, the authors conservatively claim only that PBHs may account for much less.

To support the possible existence of PBH, the authors highlight now-enigmatic observations that PBHs might explain, including microlensing from:

1) An unexpectedly large number of sub-stellar masses near our galaxy’s center.

2) Invisible bodies outside galaxies.

3) Invisible objects with masses from 2 to 5 Msun near our galaxy’s center. Stars are unlikely to produce black holes in this mass range.

So, does all this finally settle the dark matter issue?

Not hardly! Conventional dark matter theories spawned a gargantuan industry, with hundreds of major experiments, thousands of careers, and many billions of dollars. This Establishment — fully invested in WIMPs, MACHOs, axions, and other exotica — will fight for its life.

Best Regards,

Robert

July 2020

Note: Previous newsletters can be found on my website.