Talk:Big Bang

I removed the following text between "dominant" and "theory". All current physical theories are incomplete, no?

  , though incomplete, 

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Old Talk

In 1927, the Belgian priest [Georges Lemaître] was the first to propose that the universe began with the explosion of a primeval atom. His proposal came after observing the redshift in distant nebulae by astronomers to a model of the universe based on relativity.

What does this mean?

Also, the claim that the universe was initially microscopic is suspect as discussed on talk:Redshift. If the universe is infinite, than it was infinite already at the very first moment. --AxelBoldt

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I've made a few corrections - but this is far from complete.

According to this theory, the universe emerged spontaneously between 10 and 20 billion years ago from a gravitational singularity, at which time started and all distances in the universe were zero.

• "emerged" implies existence of a previous space - it's wrong

• "from a ..." is wrong - the singularity is part of the universe

• "time started" assumes two concepts of time, since you can't conjugate a verb in time (e.g. past) if you're talking about time itself.

• "all distances...were zero" - if you think of the singularity as a single point, then it's meaningless to talk about distances - a distance only makes sense between two distinct points

A common way to think about this theory is to think of global time and space separately - the theory assumes Weyl's postulate, which states that this is possible. Thinking in this way, space itself has only existed for about 13-17 billion years. Thinking backwards in time towards the "beginning", this "beginning" can be thought of as a gravitational singularity.

The model includes all of space-time, so the question "What was there before the Big Bang?" is meaningless in terms of the standard model.

Because of this, the distance between distant galaxies increases faster than the speed of light. This is possible because special relativity only states that matter and information cannot travel through space faster than the speed of light. It doesn't limit how fast space itself can stretch.

• "It doesn't limit how fast space itself can stretch." The stretching of space is not a velocity. "fast" means a change in quantity X with respect to time, usually X=distance. It only has a meaning once you know what definitions of distance and time you are talking about.

Because of this, the change in comoving distance between distant galaxies divided by cosmological time[?] can be greater than the speed of light. This a theoretical concept and not an observational one. For example, galaxies whose light will not reach the Earth for tens of billions of years can be said to be moving away faster than the speed of light according to this definition. This does not violate the laws of special relativity, which is a local theory, which states, among other things, that matter and information cannot travel through space faster than the speed of light, but does not deal with global space-time concepts.

If an observational definition of distance to distant galaxies, e.g. the distance integrated along the path of a photon from a distant galaxy to the observer, using the locally valid distances at each point of the path, is used instead, then the change in this distance divided by cosmological time[?] cannot be greater than the speed of light.

A strict empiricist might say that using the former definition only relates to galaxies which do not exist, in the sense in which chocolates hidden in an unopened box do not exist to the observer, but further discussion of this should go to one of the philosophy pages. --boud

Good points. A couple of comments:

• The description of the singularity as "all distances being zero" seems to me to be correct. Even if it is a point, then all distances are zero: you don't need two different points to measure distance. I think it gives the beginner a good picture of what the singularity is like. Also, a point is often thought of as something infinitely or microscopically small, which doesn't fit well with the (most likely) spatially infinitely large universe right after the big bang.

• I would move Weyl's postulate out of the first paragraph, which should only outline the highlights of the theory, and explain it later. Most people intuitively separate space and time anyway, so it's no biggie that the theory does the same, except for people who constantly think about spacetime manifolds.

• Right now, the first paragraph contains too many "think"s and "thought"s. We should try to find more definite language.

• Regarding the different distance definitions: We probably need a page explaining the various defintions of distance in an expanding universe.

AxelBoldt 03:15 Oct 5, 2002 (UTC)

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Who first coined the term? I'd heard he (I know it was a he) who was mocking the idea...--dgd

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Who put the bit aboout the K-correction? Nowadays it means something slightly different, but I don't know about the history of the term to know if the reference in the text is right.--AN 00:59 Oct 25, 2002 (UTC)

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Regarding the different distance...

i've started with comoving distance

Who first coined...

The reference to Hoyle is there. And yes, that Hoyle is a he, but there's a younger F. Hoyle - Fiona Hoyle - doing observational cosmology research - who's not.

Who put the bit...

Guilty as charged. i've put in wirtz. He published in a journal whose abbreviation is identical with your nick: "AN".

You'd have to read the AN article (in German) to check - i don't have convenient access - so i don't remember whether his K-correction was a magnitude (logarithm of luminosity) correction or the redshift (shift in wavelength). But i agree the definition has probably evolved, though AFAIK this is where the term comes from.

--boud

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