A Hot Big Bang

The first embellishment is to fill the Universe with a bath of thermal photons at some very early time just at or after the Big Bang (“thermal photons” is just a fancy way of saying “heat radiation”). Such a thermal bath cools down as the Universe expands (just as a jet of air spurting out of a tire feels cool), but the remarkable thing is that the bath stays perfectly thermal at a single temperature. In the jargon of physics, the spectral-energy curve of the radiation retains its classic single-temperature shape but drifts downward in frequency toward longer wavelengths as the Universe expands.

The hot Big Bang model predicts that we should see these thermal photons today, but much reduced in temperature by the expansion. Of course, we do see this radiation - it was detected as microwaves in 1964 by Penzias and Wilsonand is known as the cosmic microwave background (CMB), or “primeval fireball radiation.” Since this radiation is a gas of photons filling the whole Universe, it should not be found to emanate from individual stars or galaxies but should instead cover the whole sky evenly, as it does. If our eyes were sensitive to CMB photons, the sky would glow faintly but uniformly like the daytime sky. The CMB spectrum has a perfect thermal shape corresponding to a single temperature of exactly 2.728 degrees above absolute zero.

The thermal spectrum of the CMB is extremely telling. Many people have tried but failed to produce this radiation in sources other than the hot BB, for example, by stars, QSOs, or cool clouds heated by stars. The problem is that such objects inevitably have a broad range of temperatures, unlike the CMB radiation, which has a single temperature to better than one part in 10,000. The hot BB model is uniquely able to explain the CMB because of its high early density, which allowed heat to flow rapidly so as to even out even tiny temperature differences. No other satisfactory explanation for the CMB radiation has ever been found.

Let us now run the hot BB model backwards from today and observe how the Universe heats up. For every factor of 10 contraction in the size of the Universe, the temperature also increases by a factor of 10. Particles move faster, and photons become more energetic - the result is that at very early times the whole Universe turns into a colossal particle accelerator, the envy of every particle physicist on Earth.

Alan Guth and Neil Turok in subsequent chapters will discuss events very early in the Universe that are at or beyond the limits of present physical laws. Well after that - at a time of about 100 seconds when the temperature had fallen to 10⁹ K and the density was that of water - a key series of particle reactions occurred. Protons and neutrons fused together to make deuterium (proton + neutron), He (2 protons + 2 neutrons), and other light elements. These are the same fusion reactions that drive stars and the hydrogen bomb, and they are exceedingly well understood.The net result is that roughly 25% of the original protons and neutrons in the Universe were converted into helium plus small amounts of deuterium, lithium, etc. The predicted amounts closely match the observed abundance of these elements. Twenty-five percent turns out to be a lot of He, and the only other known source of He - nuclear burning in stars - falls short of producing enough of this element by a factor of 10 or so. This elegant argument was used by George Gamow, Ralph Alpher, and collaborators in the 1940's to predict the hot Big Bang and forecast the CMB radiation some 16 years prior to its discovery! In short, the CMB radiation and the high helium content of the Universe fit together like lock and key.

It is hard to overstate the impact of these two mutually reinforcing discoveries on the subsequent progress of cosmology. While the temperature and density of the Universe at nucleosynthesis are not extreme, the theory does entail the extrapolation of known physical laws over 10 billion years all the way back to only t = 100 seconds! The masses of the neutron and proton, the decay time of the neutron, the properties of small particles called “neutrinos,” the nuclear reaction rates, and the value of the gravitational constant G must all be identical to what they are now. The fact that the basic properties of the Universe were the same then as now inspired the notion that the Universe is inherently simple and understandable. The success of primordial nucleosynthesis emboldened cosmologists to take the even more daring leaps to the earlier times and much higher temperatures that you will hear about this afternoon.

Contributed by: Dr. Sandra Faber