by alecto » May 23rd, 2008, 12:39 pm
The Big Bang is not so much a theory as it is a consequence of the General Theory of Relativity and certain observations, such as the distance-dependent speed of recession of galaxies and the cosmic background radiation. These conspire to tell us that the volume of the universe once seemed much smaller than it is now. (The "seemed" exchanges with "was" in some explanations.) In any case, matter was much more dense. Explanations of the cosmic background radiation require an outrush from nuclear-density. In other words, all of space was filled with matter as dense as that in an atomic nucleus. A "pure" General Relativistic model would say that the matter and space originated from a point. More recent "inflationary" and "periodic" models say that the universe, for an unknown time, existed in this really dense state without expanding (much) but that this state became unstable and "exploded" into the present state. ("became unstable" is physics jargon with a very specific meaning, not a replacement for "I don't know.")
In any case, several things have to be remembered.
Space expanded. This means that the distance between geometric points (a la Euclid) increased over time. Most of the matter actually stayed relatively still, meaning that, for example, the Milky Way Galaxy is still relatively close to the point at which the matter it is made out of was located when the big bang occurred. By relatively close I mean within a few tens of millions of light years, which is small compared to the scale of the observed universe. The relative motions of galaxies, according to modern big bang descriptions, are due to quantum mechanical fluctuations in the momentum of the matter that rushed out in the big bang - essentially a quantum equivalent of turbulence - which is now modified by the gravitation of nearby galaxies.
No one claims to know all of the answers. Subatomic physics is in a state of flux right now as people try to iron out the details in a class of theories called "string theories" (which are technically hypotheses).
Language is bad bad bad! If someone says "compressed gas" this is because better language does not really exist. Yes, "compressed" literally means "having once been in a less compressed state but now compressed together" but only some descriptions of the big bang actually imply that (in which case the compressing cause is the contraction of a previous large phase for space, a "big crunch".) It might be better to say instead of "compressed gas" the long phrase "gas which was in a state identical to that of having been compressed without implying that it was actually compressed" but this is tedious, and would have to be used for nearly every simple phrase or term (like "expanded", "moved", or "separated from").
Good scientists (like good theologians) admit that they don't know a lot of things.
No version of string theory or supergravity works outside of the laws of thermodynamics. There are no violations of conservation laws or statistical mechanics. Scientists are aware that conservation laws are tricky (e.g. the First Law of Thermodynamics, which states that energy is conserved in the exchange of heat and motion). We know for instance that one thing being called matter while another is called energy are entirely the result of point of view. If you "weigh" the earth and calculate its mass, part of that mass will be the energy of everything going on in/on the planet. Its mass is given by the equation E= mc^2. Since we've been "warned" by the mass/energy thing we are allowed to work with hypotheses that might allow things such as exchange of charge with mass. But statistical laws like the Second Law of Thermodynamics are based on pure mathematics. We do not allow violations of such principles. If one appears to be violated, there is an error somewhere, though if a non-physicist sees one this is almost always due to errors of transmission or interpretation of information.
Errors of transmission or interpretation of information are usually not due to dishonesty.
Science news in the United States is generally terrible, so terrible that if errors of equal magnitude and consequence were made in other areas of news (such as continually mixing up "Iraq" and "Iran", reporting that Alaska had more Democratic Party delegates than New York, calling president Bush "vice chairman Bush" etc.) the writers would be fired.
Popular accounts of physics are almost always in error somewhere, in a way that the writers could not be fired for. We live in a relatively mathematically illiterate society AND the actual theories are almost entirely mathematical. It's also very difficult mathematics. One of the complaints about String Theory is that the math is so obtuse that very few people really understand it, so we don't have a lot of people working in the field who can understand it and check each other. But just because the press (including even magazines like Scientific American) leave out details, this does not mean that physicists are not aware of them, or that the information communicated to the public is so flawed as to be useless. Newton's laws of motion and of gravitation are in error, but in most cases they still give us useful information. When the folks plan the thruster firings to put spacecraft in orbit around other planets, they use Einstein's laws to do the detail work, but all of the planning and conceptualization is done using procedures adapted from Newton.
Sentio ergo est.