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Re: [Phys-l] Big Bang density



David Craig wrote:
Astronomers claim that at this time, the universe had infinite density (zero volume).


Only astronomers who don't really know a whole lot about big-bang physics would say this with a straight face. (I know, I know, you do see statements to this effect in the popular media fairly regularly, even by people who ought to know better. Maybe it's just easier than the truth.) But the, astronomers aren't really the ones doing physics in that domain to begin with.

The REASON people would say this is that, in classical general relativity, no physically reasonable equation of state can support itself against ultimate gravitational collapse in, say, a black hole singularity, or the initial "big bang" singularity.

However, for a variety of reasons, we KNOW that classical general relativity cannot be the whole story. It is, in a fundamental way, inconsistent with quantum theory. Since quantum theory and relativity are the two pillars of our modern conception of the physical universe, we KNOW that our present understanding is incomplete outside of a reasonably well understood domain of applicability.

The immediately obvious possibilities are (i) GR requires revision at certain length/energy scales, i.e. we need a "quantum theory of gravity", if you will; (ii) quantum theory requires revision; (iii) both.

Most attacks on the problem -- string theory, loop quantum gravity, causal dynamical triangulations, causal sets, to name the bigger players -- focus on (i) since, historically, this has worked well, and we have a general idea of how to go about it. (ii) is hard to do without mucking things up in the domain where we already know the theory applies, and is, I suppose, considered more "out of the mainstream". (Less so, now, though.) Some have hoped the ultimate solution to the problem will REQUIRE (iii), since we also know we don't properly understand what quantum theory is trying to tell us about the world irrespective of gravity. This may be overly ambitious. Smolin's "Three Roads to Quantum Gravity" does a reasonably even-handed job of looking at some current approaches (by no means all.)

At any rate, most of the approaches to a quantum theory of gravity which currently seem viable do resolve these infinite densities in some fashion. (In fact, many consider this to be a litmus test of whether or not a theory is viable to begin with.)

In short, I wouldn't spend a lot of time fretting about infinite densities. (Heck, I'd say that there are a number of things we KNOW to be true about the physical world that bother me a whole lot more than that!)

Bottom line: In spite of what you might see scientists who enjoy the limelight a little too much saying on TV, what we KNOW -- for sure -- about the universe (very) close to the classical big bang singularity is that we DON'T KNOW -- for sure -- what the physical world is really like on those scales.

David Craig


<http://web.lemoyne.edu/~craigda/>



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That seemed like a reasonable take. My take is more extreme - that the pointillist
Big Bang theory for astrophysics represents something of the same level of theoretical development as Biology's spontaneous generation of (macro-scale, complex structured) life as portrayed in the 18th Century at least.

Brian W
(notice the difference, - perhaps even superficial similarity - between the discredited Spontaneous Generation theory, and the immensely longer time scale that one attributes to the persuasive theory of Evolution by Natural Selection.