Why is an infinitely small point required for the big bang?

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All explanations of the big bang that I have ever seen, refer to all the matter in the universe coming into existence at an infinitely small point, following which there was a great expansion. Wouldn't it be more likely that the big bang happened in perhaps a spherical volume?

If one estalished an 'upper limit' (why not, everything else seems to have limits or quantum values), to the possible density of matter and/or energy, then all the 'substance' in the universe could have been created in a 'region' rather than at an inifintely small point. If one assumes the diameter of the universe when the cosmic microwave background radiation was emitted as the diameter of the 'big bang' (380,000 light years?) as the upper limit of matter/energy density possible, and the cosmic microwave radiation as the 'smoke evidence' of the diameter as the proof of it's size when it was created, then a whole lot of problems get solved don't they?
 

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  • #2
Chronos
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I personally like the planck density as the description of a singularity. This does not, however, adequately explain the BB. A good guess would be the BB arose from a quantum fluctuation. This is permissible under quantum physics if the net energy of the universe is exactly zero. The evidence favoring this conclusion is not all bad.
 
  • #3
Chalnoth
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All explanations of the big bang that I have ever seen, refer to all the matter in the universe coming into existence at an infinitely small point, following which there was a great expansion. Wouldn't it be more likely that the big bang happened in perhaps a spherical volume?
The infinitely-small point is an artifact in the theory. Basically, if you have an expanding universe that obeys General Relativity and contains any amount of matter or radiation, extrapolating back in time far enough and everything was necessarily at a singularity.

This isn't a statement that there was actually a singularity: nobody seriously believes that. Rather, it's a statement that something is wrong with this model. So, what is done in practice is we take the big bang theory as an accurate description of the universe only after a certain point: before that point, some other theory has to take over. Theories for the actual beginning are, at this point, pretty speculative as we have no experimental way (yet) to distinguish most of them.
 
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jambaugh
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[...] Rather, it's a statement that something is wrong with this model. [...]
Good explanation. I'd add that it's no so much something wrong as something incomplete. Everyone acknowledges that the model must break down at some stage between now and the initial point of singularity. This is one reason (one of the main reasons) we seek a unification of General Relativity and Quantum Theory.
 
  • #5
Chalnoth
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Good explanation. I'd add that it's no so much something wrong as something incomplete. Everyone acknowledges that the model must break down at some stage between now and the initial point of singularity. This is one reason (one of the main reasons) we seek a unification of General Relativity and Quantum Theory.
Haha, well, that depends upon what you mean :) Obviously General Relativity must be giving us the wrong answer at some level, because it produces nonsensical predictions (singularities). The difficulty is that all of our experiments to date have yet to find any deviation from GR, so it is definitely a very good approximation to reality.
 
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Nobody i guess cared to answer the question whether or not the big bang could have been a region of space rather than a single point. Along this line however, I have another problem. I have worked in the area of compression of data. At some point in compression one reaches a point where there is a random state produced in the data and no further compression can occur without data loss. That the minimum state to represent n^x things is n^x unique identities. If one believes that all the energy and particles in the universe (physics is supposed to be reversible without data loss) came from one infinitely small point, then at that one single point (extreme compression) it would be neccessary to be able to encode all the information that exists today in this universe at that one point. Is there any way to explain this?
 
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  • #7
Chalnoth
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Nobody i guess cared to answer the question whether or not the big bang could have been a region of space rather than a single point.
I thought I did. Basically, there are two points to make here:
1. If you assume General Relativity is completely and utterly accurate, then there was necessarily a singularity. This is one reason why most don't think General Relativity can be completely and utterly accurate.
2. Most tend to think that, indeed, our universe started as some finite-sized region, a region that came about due to some specific physical process (e.g. a quantum vacuum fluctuation).

Along this line however, I have another problem. I have worked in the area of compression of data. At some point in compression one reaches a point where there is a random state produced in the data and no further compression can occur without data loss. That the minimum state to represent n^x things is n^x unique identities. If one believes that all the energy and particles in the universe (physics is supposed to be reversible without data loss) came from one infinitely small point, then at that one single point (extreme compression) it would be neccessary to be able to encode all the information that exists today in this universe at that one point. Is there any way to explain this?
Well, this is one more reason to believe that the infinitely-dense point is unphysical. As long as physics is unitary, then indeed this conclusion is accurate. If physics is not unitary, then there can well be more information than there was to begin with.
 
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Thank you for that clarification.

So if there is belief that there was a region of space that things 'banged' from, is there any opinion or evidence of what the diameter of that region might be?
 
  • #9
marcus
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So if there is belief that there was a region of space that things 'banged' from, is there any opinion or evidence of what the diameter of that region might be?
When you say things like "opinion" and "might be" you leave the door open for hypothesizing and conjecturing.

There is a rather prominent research group at Penn State that runs computer models of the early universe using a quantized version of Einstein gravity---which does not develop a singularity.

With them the minimum size (maximally dense) stage depends on what mass you think the U has.

The model tends to bounce when it reaches a maximum density of 41% of the socalled Planck density.

As I recall quantum effects begin to take over and make contraction slow down when you reach about 2% of Planck density. Gravity becomes repellent at that point, by the time density reaches 41% the U actually turns around, contraction stops and it starts expanding. The bounce causes a period of "inflation" very rapid expansion.

There are solvable equation models that show the same behavior as the computer sims.

We don't know the total mass of the U, but if you postulate some mass then you can calculate the size at the moment of bounce using the figure for the maximum density.

You can find out the value of Planck density in standard metric units (kilograms per cubic meter) if you want just by googling "planck units".

Naturally if you know the mass of something, and you know the density in kg per m3 it is simple arithmetic to find the volume-----you just divide the mass by the density.

This line of research involves untested conjecture. It belongs a branch of cosmology which has taken off recently. "Quantum cosmology" or QC. Here is a list of over 200 QC papers that have appeared in the last 2 years or so (2009 and later). Look at the titles to get an idea of what they are about.
Let me know if you can't get the list, the server is sometimes slow.

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E+2008&FORMAT=WWW&SEQUENCE=citecount%28d%29 [Broken]
 
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  • #10
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By opinion I meant scientific opinion:

A "scientific opinion" is any opinion formed via the scientific method, and so is necessarily evidence-backed. A scientific opinion which represents the formally-agreed consensus of a scientific body or establishment, often takes the form of a published position paper citing the research producing the scientific evidence upon which the opinion is based. "The scientific opinion" (or scientific consensus) can be compared to "the public opinion" and generally refers to the collection of the opinions of many different scientific organizations and entities and individual scientists in the relevant field.

I guess what I am getting at here is to test if there is any scientific support for an idea I had when I saw a recent 'refined picture' of the CMB.

http://news.nationalgeographic.com/...-background-radiation-big-bang-science-space/

Looking at it, I wondered if instead of a previous universes, as suggested in the article, if what instead I was looking at was some kind of gravitational lensing producing duplicate images of the big bang itself that occured around the time of the CMB.
 
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  • #11
Chalnoth
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Thank you for that clarification.

So if there is belief that there was a region of space that things 'banged' from, is there any opinion or evidence of what the diameter of that region might be?
Well, these things are highly model-dependent, so there isn't a whole lot of constraint there. Typically, however, models set a lower bound that is much smaller than the size of a proton. That is to say, the model requires that the original patch that started inflation be at least a certain size (that is much smaller than a proton), but the patch could be much larger.
 
  • #12
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Can a quark plasma with that much mass and energy exist in a space that small or is it believed that it was a different form of mass or energy at this point?

Are primordial black holes considered a possibility in the standard model(s)?
 
  • #13
Chalnoth
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Can a quark plasma with that much mass and energy exist in a space that small or is it believed that it was a different form of mass or energy at this point?

Are primordial black holes considered a possibility in the standard model(s)?
There was no plasma, quark or otherwise, during the inflationary era. During that era, our universe was essentially empty of nearly everything but the inflaton field. That, in fact, is a precondition for inflation to occur. The quark-gluon plasma wasn't produced until the inflaton field decayed after the end of inflation.
 

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