Physical Keys to Cosmology

Only in the latter half of the 20th Century was enough physical evidence accumulated to make reasonable models of the formation process of the universe. The present standard model, the "big bang" model, was developed around three major pieces of experimental evidence:

As with most models of nature, it has seen successive refinements and has presented significant difficulties which fuel further investigation.

One of the fascinating aspects of cosmological modeling is that it reveals a number of balances of parameters which must be maintained quite precisely for the universe as we know it to exist. Some of these balances are explored in an open-ended section on the "windows of creation".

Cosmology

The standard cosmological model is the "big bang", and while the evidence supporting that model is enormous, it is not without problems. Trefil in The Moment of Creation does a nice job of pointing out those problems.

The Antimatter Problem

Why such a predominance of matter over antimatter in the universe? From Trefil, pg 38. "after the beginning of the particle era. there is no known process which can change the net particle number of the universe" " ..by the time the universe is a millisecond old, the balance between matter and antimatter is fixed forever."

A basic part of the Standard Model for matter in the universe is the idea of pair-production. It is demonstrated by electron-positron pair production. A common type of interaction between high energy X-rays or gamma-rays and ordinary atoms converts most of the energy of the photon into an electron and its antiparticle, the positron. The masses of the particles follow the Einstein relationship E=mc². The mass produced has an exactly equal number of electrons and positrons, so if all the processes of mass production were such pair production processes, there would be exactly the same amount of matter and antimatter in the universe.

Clearly there is some asymmetry in the way nature treats matter and antimatter. One promising line of investigation is that of CP symmetry violations in the decay of particles by the weak interaction. The main expermental evidence comes from the decay of neutral kaons, which shows a small violation of CP symmetry. In the decay of the kaons to electrons, we have a clear distinction between matter and antimatter, and this could be at least one of the keys to the predominance of matter over antimatter in the universe.

A new discovery at the Large Hadron Collider is a 0.8% difference in the decay rate of the D-meson and its antiparticle, which could be another contribution to the solution of the antimatter problem.

The Galaxy Formation Problem

Random nonuniformities in the expanding universe are not sufficient to allow the formation of galaxies. In the presence of the rapid expansion, the gravitational attraction is too slow for galaxies to form with any reasonable model of turbulence created by the expansion itself. "..the question of how the large-scale structure of the universe could have come into being has been a major unsolved problem in cosmology" Trefil p43 "we are forced to look to the period before 1 millisecond to explain the existence of galaxies."

The Horizon Problem

The microwave background radiation from opposite directions in the sky is characterized by the same temperature within 0.01%, but the regions of space from which they were emitted at 500,000 years were more than light transit time apart and could not have "communicated" with each other to establish the apparent thermal equilibrium - they were beyond each other's "horizon".

This situation is also referred to as the "isotropy problem", since the background radiation reaching us from all directions in space is so nearly isotropic. One way of expressing the problem is to say that the temperature of parts of space in opposite directions from us is almost exactly the same, but how could they be in thermal equilibrium with each other if they cannot communicate with each other? If you considered the ultimate lookback time as 14 billion years (14 thousand million ) as obtained from a Hubble constant of 71 km/s per megaparsec as suggested by WMAP , then these remote parts of the universe are 28 billion light years apart, so why do they have exactly the same temperature?

Being twice the age of the universe apart is enough to make the point about the horizon problem, but as Schramm points out, if you look at this problem from earlier perspectives it is even more severe. At the time the photons were actually emitted, they would have been 100 times the age of the universe apart, or 100 times causally disconnected.

This problem is one of the lines of thought which led to the inflationary hypothesis put forth by Alan Guth in the early 1980's. The answer to the horizon problem from the inflationary point of view is that there was a period of incredibly rapid inflation very early in the big bang process which increased the size of the universe by 10²⁰ or 10³⁰, and that the present observable universe is "inside" that expansion. The radiation we see is isotropic because all that space "inflated" from a tiny volume and had essentially identical initial conditions. This is a way to explain why parts of the universe so distant that they could never have communicated with each other look the same.

The Flatness Problem

Observations indicate that the amount of matter in the universe is surely greater than one-tenth and surely less than ten times the critical amount needed to stop the expansion. It is either barely open or barely closed, or "very nearly flat". There is a good analogy here - a ball thrown up from the earth slows down. With the same velocity from a small asteroid, it might never stop (Trefil pp46-47). Early in this theoretical toss from the asteroid, it might appear that you have thrown it with just the right velocity to go on forever, slowing toward zero velocity at infinite time and distance. But as time progressed, it would become more and more evident if you had missed the velocity window even a small amount. If after 20 billion years of travel, it still appeared that you had thrown it with the right velocity, then that original throw was precise indeed.

Any departures from "flatness" should become exaggerated with time, and at this stage of the universe, tiny irregularities should have been much amplified. If the density of the present universe appears to be very close to the critical density, then it must have been even closer to "flat" in earlier epochs. Alan Guth credits a lecture by Robert Dicke as one influence which put him on the "inflationary" path; Dicke pointed out that the flatness of todays universe would require that the universe be flat to one part in 10¹⁴ at one second after the big bang. Kaufmann suggests that right after the big bang, the density must have been equal to the critical density to 50 decimal places!

In the early 1980's, Alan Guth proposed that there was a brief period of extremely rapid expansion following the Planck time of 10^-43 seconds. This "inflationary model" was a way of dealing with both the flatness problem and the horizon problem. If the universe inflated by 20 to 30 orders of magnitude, then the properties of an extremely tiny volume which could have been considered to be intimately connected were spread over the whole of the known universe today, contributing both extreme flatness and the extremely isotropic nature of the cosmic background radiation.