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A Fourth Layer of Epicycles on the Standard Cosmology

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From Science Daily, reporting on a paper in Physical Review Letters, “Inflatable Dark Matter“:

Standard cosmology — that is, the Big Bang Theory with its early period of exponential growth known as inflation — is the prevailing scientific model for our universe, in which the entirety of space and time ballooned out from a very hot, very dense point into a homogeneous and ever-expanding vastness. This theory accounts for many of the physical phenomena we observe. But what if that’s not all there was to it?

A new theory from physicists at the U.S. Department of Energy’s Brookhaven National Laboratory, Fermi National Accelerator Laboratory, and Stony Brook University, … suggests a shorter secondary inflationary period that could account for the amount of dark matter estimated to exist throughout the cosmos.

“In general, a fundamental theory of nature can explain certain phenomena, but it may not always end up giving you the right amount of dark matter,” said Hooman Davoudiasl, group leader in the High-Energy Theory Group at Brookhaven National Laboratory and an author on the paper. “If you come up with too little dark matter, you can suggest another source, but having too much is a problem.”

Some theories that elegantly explain perplexing oddities in physics…cannot be fully accepted because they predict more dark matter than empirical observations can support.

This new theory solves that problem. Davoudiasl and his colleagues add a step to the commonly accepted events at the inception of space and time.

What this paper represents is a fourth layer of epicycles on the standard cosmology.

As you may recall, the Big Bang expelled all the matter in the universe with a certain amount of violence or explosive velocity. Call that the “size of the Bang.” On the other hand, the universe has a certain number of galaxies dotted over it, and a certain amount of matter that we can see “in the visible universe.” To be precise, and since we don’t know where the edge of the universe is, we only know the density of material. Thus this second Big Bang quantity is the “size of the Density.”

The great mystery of the Big Bang is why the size of the Bang matched the size of the Density to 60 decimal points, 1:10^60. In concrete terms, this means the matter of the all galaxies and stars and nebulae is adjusted to within one sand grain to the size of the explosion. In cosmology jargon, this great mystery is known as the “Flatness Problem.”

So the first of what will be several epicycles was proposed to make this problem go away: inflation.

The second problem was that the glowing stars and nebulae only accounted for 10 percent of the matter density, so astronomers call the cold, non-glowing material they can’t see “dark matter.” Despite sounding mysterious, it is an all-too-common problem that astronomers can’t see dust, asteroids in other star systems, comets outside the orbit of Mars, etc., simply because they don’t emit any light. So we do know a lot about dark matter — it is cold, very black in color, smaller than a Jupiter, bigger than a pea, etc.

However, for historical and programmatic reasons, astronomers have been told that the search for dark matter would be far better served if it were mysterious, exotic, and employed particle physicists. Hence the second great epicycle — the dark sector of exotic physics particles where dark matter lives.

Astronomers pointed out that if dark matter were interacting gravitationally, which it must to solve the first problem, then it should self-clump into the center of the galaxies where it manifestly is not seen (using gravitational lensing, etc.). After casting about for “other interactions” that could prevent clumping, the astrophysicists added the third great epicycle — dark interactions to stop embarrassing clumpiness. (This is not “dark energy,” though I’m sure there are imaginative proposals to link the two.)

However, when dark matter and dark interactions are viewed from a particle-physics perspective, it interferes with the early part of the Big Bang, when the universe was so hot and dense that atoms were being built out of protons and neutrons. This dark sector changed the careful balance created by the boutique Big Bang nucleosynthesis models that made the dark sector seem necessary in the first place! The solution, yes you guessed it, was to add the fourth epicycle — a separate dark-sector inflation.

If you ask just how long this charade can continue, Copernicus used about 27 epicycles when he published his theory some 14 centuries after Ptolemy. On the other hand, since the half-life of an epicycle solution is decreasing rapidly, we may reach 27 epicycles by the end of the decade.

Image credit: Brookhaven National Laboratory.