The Way of Science

UNIT 4

Cosmology and Relativity

V. The Big Bang Theory

Let's start with the very simple version of "old" Big Bang, and then bring it up to date with "inflation" and the new supernovae data.

What can we say with reasonable certainty about the Evolution of our Universe? If the present estimate of the expansion rate is used to "run the film backwards," then we can make the following statements.

First, the Universe was very much smaller in the past, and therefore very much hotter and denser. The best estimate, from many lines of analysis, gives us a starting time of about 12-13 billion years ago. Second, in these early hot stages, matter could not exist: atoms would disintegrate, and at very high temperatures and density, only the ultimate elementary particles, "quarks", could exist. The four forces were joined into a single unified force, and separated as cooling occurred. At the very earliest time which modern theoretical physics can say anything, 10- 42 second, all spacetime, all matter/energy was packed into a space considerably smaller than an atom. What about before that time? We will return to that fascinating topic later, and discuss singularities, "bubble universes," etc. Take a look at Appendix 4 for a more extensive summary of these events. Here's a really brief summary of the major times: 12-13 billion years ago: Big Bang. 10- 36 second after the Big Bang: inflation. 10-5 second after the Big Bang: formation of protons and neutrons from quarks. 3 minutes after the Big Bang: synthesis of atomic nuclei. 300,000 years after the Big Bang: first atoms form, and the release of what we now see as Cosmic Microwave Background (CMB) Radiation occurs. One billion years after the Big Bang: first stars, galaxies, quasars appear. 10 billion years after the Big Bang: all galaxies have formed; stars continue to form.

All this sounds quite bizarre, but this model is testable (falsifiable) in several independent empirical ways. We will look at three of these ways, as follows: (1) Big Bang model predicts a particular distribution of chemical elements in the Universe. (2) Big Bang model also predicts/requires the presence of 3 K CMB. (3) If we look back in time, we should see a very different Universe: denser, with different kinds of celestial objects. We can, of course, look back; the finite speed of light demands that the more distant objects are seen earlier in time.

Let's take a look at each of these in turn.

  1. Distribution of elements. If Big Bang model is basically correct, then the Universe should consist of mostly hydrogen and helium in a specific ratio of abundance. So it is. (Consult your text for more details.)

  2. The Cosmic Microwave Background radiation. Be sure to read your text on this part. The CMB is particularly important material (hint, hint), in that its analysis continues to provide data for refining the original Big Bang model. Back in the nineteen-fifties, three physicists proposed that there should be characteristic "left-over" radiation from the formation of atoms after the Big Bang. This radiation, they calculated, should be in the microwave portion of the spectrum, and be coming essentially from everywhere ("isotropic"). It would be sourceless, since it is trapped in expanding and cooling spacetime. The cooling would give it a characteristic distribution of different microwave wavelength ("3 K blackbody spectrum"). The CMB is thus frequently referred to as 3 K isotropic microwave radiation (good mouthful). The original paper from the 'fifties disappeared for years into the mass of research published in physics, and was unfamiliar to many scientists. Many years later, the radiation was discovered fortuitously (and fortunately, for Penzias and Wilson; a Nobel Prize was their reward for this serendipitous event!). The CMB has no other explanation (at least at the present time) except as a Big Bang event, and it stands as the most convincing evidence yet. It eliminated other models of the Universe's expansion, particularly Gold's "steady-state" version, which required constant small creations of spacetime and matter/energy, but could not produce CMB. Be very sure that you understand how the CMB originated. It was not generated at the time of the Big Bang, but about 300,000 years later. Before that time, the temperature and density were too high to allow young atomic nuclei and electrons to stick together. Spacetime, then, was filled with particles which blocked free transmission of EM radiation. In other words, the Universe was "opaque." At about 300,000 years, cooling was sufficient to allow element formation, and the Universe became "transparent." The radiation left untrapped was now free to run in all directions without being absorbed, bounced, etc. - and thus we had the isotropic CMB.

If the Universe developed as described, then we should see some very odd beasts "way out there," and way back in time. Indeed there are such objects, and one of them - quasars - is worth examining in some detail. In doing so, black holes and the Einsteinian view of gravity can be logically introduced, and elaborated on later.

"Quasi-stellar objects" (quasars) do not occur up close, and in fact have the greatest red shifts of any celestial objects. From this fact on shift, draw some conclusions about motion, direction of motion and speed. Quasars are estimated to be about the size of our solar system. Put this fact together with what you know of their red shift, and draw a conclusion about energy production by quasars.

These are certainly odd objects. At the moment, most astronomers think that they are massive black holes located at the hearts of young and active galaxies, and/or black holes stimulated into growth by the colliding galaxies of the dense early Universe. A knowledge of black holes is now necessary; read on, and also consult your text. We are embarking on Einsteinian General Relativity, and a new way of looking at the force called gravity. This view inevitably leads one to a consideration of the large-scale shape of spacetime (is your common sense insulted yet?), and then - inevitably again - to the ultimate fate of the Universe.

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