For the last 85 years, astrophysicists have proposed that most of the matter in our universe is invisible; we call it dark matter. Measurements show normal visible matter — normal atoms comprised of protons, neutrons, and electrons — is only 16% of all matter. The other 84% is some mysterious form of matter that interacts gravitationally, but not with light nor with electric charges.
Until now, it seemed black holes could not account for this invisible mass. Black holes are usually separated into two categories: supermassive and stellar-mass. The former reside at the centers of galaxies and are millions to billions of times more massive than our Sun. As great as that is, it is less than 1% of a galaxy’s total mass. There just aren’t enough supermassives to account for dark matter.
Stellar-mass black holes — 3 to a few dozen times our Sun’s mass — are produced when mammoth, dying stars collapse under their own gravity. Since the first stars formed about 200 million years after the Big Bang, cosmologists thought stellar-mass black holes could not be the dark matter observed at earlier epochs.
But now there’s a new clue: LIGO’s detection of gravity waves (see my prior newsletters) show there are many more mid-sized black holes than previously imagined. The black holes generating LIGO’s gravity waves average 24 times our Sun’s mass. Some eminent cosmologists now wonder whether dark matter could simply be normal matter trapped within black holes.
Let's examine the evidence for dark matter that includes three observations that reveal the composition of our universe very early on, and four that reveal its more recent composition.
The earliest evidence is the cosmic abundance of deuterium, which reveals the density of unbound nucleons — protons and neutrons — in the first 100 seconds after the Big Bang. The nucleus of deuterium has one proton and one neutron. If the early density of unbound nucleons was high, deuterium would be rapidly converted into helium. At lower densities, less deuterium would be converted, leaving more deuterium to be observed today. After 100 seconds, the expansion of the universe decreased all densities so much that deuterium conversion in open space stopped.
The log-log chart below, prepared by NASA, plots the abundance of deuterium vertically versus the early density of unbound nucleons horizontally.
The curved blue line is the prediction of the Big Bang theory. The observed deuterium abundance (green line) and the observed density of unbound nucleons (red line) intersect on the curve. Deuterium abundance is therefore consistent with the density of normal visible matter.
Conversely, the early density of all matter — normal and dark — (black line) corresponds to 40 times less deuterium (orange line) than is observed. Thus dark matter cannot be made of unbound protons and neutrons. BUT, normal matter trapped around or within black holes is not unbound, and would not affect the observed deuterium abundance.
The cosmic microwave background (CMB) radiation reveals the composition of the universe through its first 376,000 years. It is usually said that the CMB proves 84% of all matter is not normal protons, neutrons, and electrons. BUT again, what it really proves is that 84% did not freely interact with light; it too could be normal matter trapped within black holes.
Supercomputer simulations of cosmic evolution also show that the total mass required to pull stars together, forming galaxies and clusters, is 6 times more than is visible. BUT, matter within black holes is not visible.
Additionally, the current amount of all matter must be 6 times more than is visible, as shown by these three observations:
(1) Galaxies orbit clusters too fast.
(2) Stars orbit galaxies too fast.
(3) Galaxies bend light too much.
Finally, the Bullet Cluster collision shown below (see my July 2012 newsletter) proves dark matter (invisible but highlighted in blue) does not interact with other matter, neither normal nor dark — the blue regions are undisrupted after passing through stars, plasma, and other dark matter. BUT, if dark matter consists of myriad mid-sized black holes, each would be much smaller than a star. Just as almost all stars escaped direct collisions, so would all the black holes.
What all this means is: if an immense number of small and mid-sized black holes formed in the early universe, before the first stars, they might have made enough normal matter invisible to account for all the so-called dark matter. If so, we need to understand how these black holes could form without stars.
Perhaps dark matter is simply normal matter in an abnormal form. As LIGO observes more gravity waves, we will certainly learn more.
January 30, 2018
Note: Previous newsletters can be found on my website