Simple description of a Singularity

[tit4tat]

I am writing a paper and need to know if the following statement provides a simple yet accurate description of a Singularity. If not, please submit suggestions for improvement. (thank you in advance for any help):

In scientific theory a Singularity is a zone of infinite density that is unimaginably small and inconceivably hot. This Singularity (the beginning of everything we know) originated in a black void and started expanding. As it expanded and cooled over the next 13 billion years, matter formed and created the universe as we know it today.

 

[phyzguy]

Well, OK, I'll take a stab at a critique, although I'm probably opening a can of worms. Two things:

(1) It's clear that the early universe was very hot, and very dense, and much smaller than it is today. But there is no theory that describes a state of infinite density, and no clear understanding of how it arrived in this initial state.

(2) I think it is inaccurate to say that the early universe "originated in a black void". The entire universe was hot and dense, and expanded from this initial state. There is no evidence that there is anything "outside" of the universe that it "expanded into". This is a common misconception.

 

[Calimero]

Well, we don't know if singularity can exist in nature. It may be flaw in theory only. You should also drop 'black void' out of description.
Generaly, when speaking about birth of universe, it is usual to say that it evolved from very hot and very dense state, not from singularity.

 

[Ich]

Try http://www.aei.mpg.de/einsteinOnline/en/spotlights/big_bangs/index.html" .
It's important to know that the Big Bang scenario works backwards, from our time and what we observe we extrapolate to the past. Extrapolating far enough, we encounter a state where curvature, density, and temperature are so large that we can't trust our physical models. These untrusted models say that these values go to infinity. That's called the singularity. A singularity is a math error, not a physical entity.
And, as already said, drop "black void".

 

[tit4tat]

Phyz: "there is no theory that describes a state of infinite density"

Okay, question... at the following site regarding Einstein's theory of general relativity they state the following "Since density is defined as mass divided by volume, the density was infinite.".
( http://bit.ly/aGl6eR ) Is this not contradictory to what you're saying?

Please forgive me if I sound argumentative, I'm just trying to understand.

 

[Ich]

At the beginning of the paragraph:
"If we simply follow the predictions of Einstein's theory..."

 

[tit4tat]

Thanks Ich.

 

[phyzguy]

I think Ich said it better than I did. He said, "we encounter a state where curvature, density, and temperature are so large that we can't trust our physical models. These untrusted models say that these values go to infinity. That's called the singularity."

Maybe I should have said, "there is no theory that we trust describes a state of infinite density".

 

[George Jones]

I am not sure that there is a completely accepted technical definition of "singularity".

Roughly, a spacetime is singular if there is a timelike curve having bounded acceleration that ends after a finite amount of proper time. Singular spacetimes have "edges".

What does this mean? "timelike curve" means a path that a person (say, Alice) or particle could traverse in a rocket. "bounded acceleration" means that the rocket always has zero or finite acceleration. "ends after a finite amount of proper time" means that after a finite of time elapses on Alice's watch, Alice falls "off of spacetime" and "into the singulairty."

By the Penrose-Hawking singularity theorems, any "reasonable" classical spacetime must be singular. Very roughly, in any "reasonable" classical spacetime, gravity is so stong that the fabric of spacetime gets ripped, thus creating an "edge".

 

[CmWhitten]

There is no evidence that there is anything "outside" of the universe that it "expanded into". This is a common misconception.

I'm not sure I understand this fully. I think that you might be hinting at a contradiction to the expanding universe theory; but if not, I really don't understand how a universe that is expanding can have 'nothing outside' of it to expand into. Forgive me if I may seem rude, but I only wish to understand this better.

Correct me if I'm wrong, but the universe is either finite or infinite. If there is nothing beyond the universe, then it must be infinite. but if it is infinite, then universal expansion would be impossible because the infinite universe would exist forever. There is nothing further for it to 'exist into'. (just like the mathematical concept [infinity + 1 =] is logically flawed and cannot produce any determined value)
However if the universe IS expanding, (using the same logical reasoning as before) then it must be finite for something to exist 'beyond' the universe for it to expand into. If not, then the universe wouldn't expand because it would simply already be there...

I would be interested and appreciated if you could help to enlighten me on the subject!
:)

 

[phyzguy]

However if the universe IS expanding, (using the same logical reasoning as before) then it must be finite for something to exist 'beyond' the universe for it to expand into. If not, then the universe wouldn't expand because it would simply already be there...
:)

I think our language and concepts of logic and geometry, which are developed in a small piece of the universe here on Earth which is static and flat, simply don't apply when talking about the universe as a whole. So your logical statements just don't follow. For example your statement above that "if the universe is expanding, then it must be finite..." How do you know this? Why couldn't there be a universe which is infinite in extent, but in which everything in this universe is getting further apart with time? Basically the "scale factor" of the universe is growing with time - that's what we mean when we say the universe is expanding. But this tells us nothing about the extent of the universe. I know it is difficult to imagine, but just because it is difficult to imagine doesn't mean that this isn't the way our universe is. We simply don't know the answers to most of these questions about the ultimate nature of the universe. The best we can do is continue to make measurements and build mathematical models to try and explain what we see. We already understand much more than we did even 20 years ago, so we are making rapid progress.

 

[tit4tat]

Originally Posted by phyzguy: "I think our language and concepts of logic and geometry, which are developed in a small piece of the universe here on Earth which is static and flat, simply don't apply when talking about the universe as a whole... I know it is difficult to imagine, but just because it is difficult to imagine doesn't mean that this isn't the way our universe is. We simply don't know the answers to most of these questions about the ultimate nature of the universe"

So, are you saying that what physicists possesses is a kind of 'metalogic' regarding the universal et al. And that the hypothosese currently being shared in physics circles are simply conjecture - since they are not based on empirical evidence?

 

[phyzguy]

So, are you saying that what physicists possesses is a kind of 'metalogic' regarding the universal et al. And that the hypothosese currently being shared in physics circles are simply conjecture - since they are not based on empirical evidence?

To which specific hypotheses are you referring? Many things you read are simply conjecture, as you say, but some are concrete quantitative hypotheses. We have to discuss them one-by-one.