12.1 Cosmology and particle physics  (Page 7/20)

Now let us imagine starting at TOE and moving forward in time to see what type of universe is created from various events along the way. As temperatures and average energies decrease with expansion, the universe reaches the stage where average particle separations are large enough to see differences between the strong and electroweak forces (at about ${\text{10}}^{-\text{35}}\text{s}$ ). After this time, the forces become distinct in almost all interactions—they are no longer unified or symmetric. This transition from GUT to electroweak is an example of spontaneous symmetry breaking    , in which conditions spontaneously evolved to a point where the forces were no longer unified, breaking that symmetry. This is analogous to a phase transition in the universe, and a clever proposal by American physicist Alan Guth in the early 1980s ties it to the smoothness of the CMBR. Guth proposed that spontaneous symmetry breaking (like a phase transition during cooling of normal matter) released an immense amount of energy that caused the universe to expand extremely rapidly for the brief time from ${\text{10}}^{-\text{35}}\text{s}$ to about ${\text{10}}^{-\text{32}}\text{s}$ . This expansion may have been by an incredible factor of ${\text{10}}^{\text{50}}$ or more in the size of the universe and is thus called the inflationary scenario    . One result of this inflation is that it would stretch the wrinkles in the universe nearly flat, leaving an extremely smooth CMBR. While speculative, there is as yet no other plausible explanation for the smoothness of the CMBR. Unless the CMBR is not really cosmic but local in origin, the distances between regions of similar temperatures are too great for any coordination to have caused them, since any coordination mechanism must travel at the speed of light. Again, particle physics and cosmology are intimately entwined. There is little hope that we may be able to test the inflationary scenario directly, since it occurs at energies near ${\text{10}}^{\text{14}}\text{GeV}$ , vastly greater than the limits of modern accelerators. But the idea is so attractive that it is incorporated into most cosmological theories.

Characteristics of the present universe may help us determine the validity of this intriguing idea. Additionally, the recent indications that the universe’s expansion rate may be increasing (see Dark Matter and Closure ) could even imply that we are in another inflationary epoch.

It is important to note that, if conditions such as those found in the early universe could be created in the laboratory, we would see the unification of forces directly today. The forces have not changed in time, but the average energy and separation of particles in the universe have. As discussed in The Four Basic Forces , the four basic forces in nature are distinct under most circumstances found today. The early universe and its remnants provide evidence from times when they were unified under most circumstances.

Section summary

• Cosmology is the study of the character and evolution of the universe.
• The two most important features of the universe are the cosmological red shifts of its galaxies being proportional to distance and its cosmic microwave background (CMBR). Both support the notion that there was a gigantic explosion, known as the Big Bang that created the universe.
• Galaxies farther away than our local group have, on an average, a recessional velocity given by
  $v=H_{0}d\text{,}$

  where $d$ is the distance to the galaxy and $H_0$ is the Hubble constant, taken to have the average value $H_0=20km/s\cdot \text{Mly}\text{.}$

• Explanations of the large-scale characteristics of the universe are intimately tied to particle physics.
• The dominance of matter over antimatter and the smoothness of the CMBR are two characteristics that are tied to particle physics.
• The epochs of the universe are known back to very shortly after the Big Bang, based on known laws of physics.
• The earliest epochs are tied to the unification of forces, with the electroweak epoch being partially understood, the GUT epoch being speculative, and the TOE epoch being highly speculative since it involves an unknown single superforce.
• The transition from GUT to electroweak is called spontaneous symmetry breaking. It released energy that caused the inflationary scenario, which in turn explains the smoothness of the CMBR.