Shake

How dense? Some theories say infinitely dense. There are those who believe that it all started from a singularity (like a black hole), which exploded. How and why are other questions. I'm currently reading Hunting Down the Universe, which is an attempt to refute Big Bang cosmology over a steady-state universe. I'm not far enough into it to provide more help here, though.

Shadowy Man

I don't think this is really what's thought anymore. Not at least if you believe in inflationary theory.

I just got from amazon.com the book The Inflationary Universe by Alan Guth. When I'm done reading it I hope to be able to respond more accurately to questions concerning inflation.

Mr. Sammi

Shadowy Man,

Does this mean the Universe is still falling?

* * *
Personally I don't buy the microscopic universe where everything is allowed to tend towards uniformity! (I know I have no evidence). Perhaps a different thought may help this. On relative scales, a small scale universe is needed to account for the projection of uniform choice. This small universe could be quite massive with similar qualities distributed over itz entirety which did tend to be proactive with uniform choice. Finding the correct combinations or a topology of choice may prove the hazard.


Sammi Na Boodie ()

Tim Thompson

Stephen T-B: So how dense? Does this mean a concentration of matter in a small area, or a spread of dense matter in a vast one? Or is the concept of "matter" occupying any sort of "space" in the first moments of the Big Bang erroneous?

The answer to an cosmological question is "model dependent", by which I mean that the details of the answer depend on how you choose to model the cosmology. In any more-or-less standard Big Bang cosmology, the concept of "matter" occupying "space" is deceptive, because there is no such thing as "matter" in the early stages of the post Bang expansion. Replace the word "matter" with the word "energy" and you are in business.

A literal interpretation of general relativity requires that the universe begin with a singularity, which means that the entire universe exists in a single geometric point (in the literal, mathematical sense), under physical conditions that cannot be described or explained by any theory, but are essentially equivalent to zero volume and infinite density (both, once again, in a literal mathematical sense). In a strictly relativistic cosmology, the universe begins in an undefinable state.

However, it has been recognized since the 1930's, or maybe even earlier, that general relativity is an "incomplete" theory of the universe, since it does not incorporate any aspect of quantum mechanics. So the current efforts in cosmological research center around the desire to marry general relativity to quantum mechanics, which is now an effort at the frontier of cosmological research.

There are a lot of ways to do this; see, for instance, Steve Carlip's webpage for a lot of examples and links. there are "classical" forms of quantum gravity, but these days I think that loop quantum gravity and string theory are the leading contenders (at least they are the most popular theories).

It turns out that general relativity can be derived from string theory, which makes it definitive that string theory is a quantum theory of gravity, but it remains to be shown that it is the correct quantum theory of gravity. One of the principle features of string theory is that it uses not the 4 dimensions of space-time that standard general relativity uses, but 10 or 11 dimensions (10 for "normal" string theory, 11 for their proposed unifying M-theory). These extra dimensions give mathematicians a lot of room to play around in, and opens up a lot of possibilities like pre Big Bang cosmology and the Cyclic Universe.

It seems to have escaped the popular attention that the current state of quantum cosmology allows for serious & realistic speculation about what happened before the Big Bang, something which cannot be done in a classical, general relativistic cosmology. It may well be that what we call the "singularity" or the "Big Bang" is just a kind of optical illusion, fostered by our position as observers inside the universe, and limited to a paltry 4 "observable" dimensions.

Now for a perhaps more palatable answer to the original question, from our point of view, the quantum aspects of cosmology are important only during the very early stages, the "Bang" and a very short part of the post-Bang evolution of the universe. Once the inflationary stage is over, it's pretty much all general relativity. So we would see the infant universe as a sea of "pure energy" occupying space-time. As the universe expands & cools, the energy "condenses", much as steam will condense into liquid water as it expands & cools. So "matter" is formed when the energy has cooled enough to "condense" into matter (there may be more than one stage of "condensation"). After that, the interaction between matter & energy, which depends on the input physics, dominates the detailed evolution of the universe in the general expanding models. After about 300,000 years or so, the universe has cooled enough for free protons to grab free electrons, form neutral hydrogen, and make the universe transparent to the energy which eventually becomes the cosmic microwave background that we observe today.

Hope that helps.

Shadowy Man

So, in this viewpoint, what exactly is the "Bang"? My understanding (as weak as it is at the moment - though we just had a lunch talk today on it) of inflationary theory is that there was a 1st order phase transition from the false vacuum, which is what is "inflating", into the true vacuum.

Is there any beginning of the false vacuum?

Tim Thompson

Shadowy Man: So, in this viewpoint, what exactly is the "Bang"?

It depends on the model. The one you cite (a 1st order phase transition from the false vacuum) is one possibility. Steinhardt collides two 5-dimensional "branes" (short for "membranes") and that collision is the "bang". In string theory you can create a "bang" by making use of a concept calle the Hagedorn temperature, which is the maximum possible temperature allowed in string thermodynamics. At the Hagedorn temperature, string states swap from open to closed (or vice-versa, it escapes me at the moment which way it goes). That reversal of string winding states causes "distance" to change its meaning, and an apparently contracting universe will start to expand, and vice versa.

So, how one chooses to model the "bang" will ultimately depend on whose theory wins the day.

Shadowy Man

Ok.. thanks. Most of that string theory and brane stuff is way over my head, so I tend to just listen and nod.

eh

Welcome back to the forums, Tim.