Genesis: The origin of the universe

National Forum, Winter 1996 by Wersinger, J-M

No direct observational evidence exists for any of the events occurring before the radiation barrier. Cosmologists thus rely on known high-energy particle and nuclear physics to reconstruct the history of the universe before the 100,000-year mark. The details of the particle physics occurring before the radiation barrier are beyond the scope of this essay. The interested reader is encouraged to read the classic work The First Three Minutes, a Modern View of the Origin of the Universe by Steven Weinberg, published by BasicBooks in 1977, with an updated edition in 1988. In the hot conditions before the radiation barrier time, important nuclear reactions occurred. Calculations based on the Big Bang model and known nuclear reactions predicted a ratio of about 75 percent Hydrogen, 24 percent Helium-4, and definite small percentages of Helium-3, Deuterium, and Lithium to emerge from the early universe. These ratios have been verified experimentally and provide the third important test for the Big Bang model, after the evidence for the expansion and for the CBR.

It thus appears that the Big Bang is a very successful model for the history of the universe that imposed itself on a reluctant scientific community. However, as mentioned in the introduction, this model brings forth a number of puzzling paradoxes that could either be conveniently avoided by invoking special initial conditions or be resolved by searching for an exegetic mechanism.

The first challenge is to explain why the initial push of the explosion was so exquisitely well fine-tuned. If the push of the explosion had been ever so slightly stronger than it actually was, matter would have dispersed so fast that it would have been impossible for gravity to create the structure we now observe: the billions of galaxies populating the universe, themselves made of hundreds of billions of stars. Instead, the universe would now have the appearance of a very tenuous, structureless soup of atoms. Had the push been ever so slightly less than it was, gravity would have stopped the expansion long ago, and the universe would have collapsed back onto itself to disappear the way it appeared, into nothing. The borderline case between these two situations is very unlikely and very unstable, yet it is the situation we are in: our universe has lasted for billions of years and contains billions of galaxies.

The second challenge arises from a law of Relativity stating that no signal can be transmitted faster than the speed of light. Consider our place in the universe. From our vantage point there is a universe around us that we can see because the light emitted by its galaxies has had the time to reach us since their appearance. This is the observable universe. It is limited by a horizon that is constantly expanding, as more and more of the universe becomes visible with time. Because the speed of light is the cosmic speed limit, no signal coming from regions beyond the present horizon has had the time to reach us. At the time of the radiation barrier, many regions of the universe we see now were not visible from our location and also were invisible from each other, thus not in causal contact with our neck of the woods or in contact with each other. Physics tells us that for temperature to become uniform in an object, the various parts of the object have to exchange heat. Heat cannot flow faster than the speed of light. Why then does the CBR indicate such a uniform temperature over regions that were too distant to have had the time to exchange heat? This is the horizon problem. The standard Big Bang can only assume that the universe started in a very uniform state of temperature.