
What Is the Big Bang Theory? - tdurden
http://phys.org/news/2015-12-big-theory.html
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laotzu
>Working backwards from the current state of the Universe, scientists have
theorized that it must have originated at a single point of infinite density
and finite time that began to expand

Logically, it doesn't make sense to me how something could be described as
infinite (density) and finite (time) at the same time.

Furthermore, the article states:

>However, the earliest times of the Universe – lasting from approximately
10-43 to 10-11 seconds after the Big Bang – are the subject of extensive
speculation. Given that the laws of physics as we know them could not have
existed at this time, it is difficult to fathom how the Universe could have
been governed. What's more, experiments that can create the kinds of energies
involved have not yet been conducted.

How can they even speculate that time existed at that point? If time is
relative to motion, and the universe was a singularity, then time could not be
said to exist because there is no relative motion if all that exists is a
singularity.

~~~
hellofunk
These kinds of infinity concepts are throughout math and physics. For example,
the Universe is "infinite" yet it is also expanding. There is very little that
is logical about infinity itself and it has been the source of much philosophy
throughout time. It took a couple thousand years just for mathematicians to
theoretically prove that some infinities are larger than others, for instance.

~~~
lutusp
> There is very little that is logical about infinity itself ...

On the contrary, infinity is a very logical notion -- much more logical than
many other ideas we accept. For example, what value must "m" have for this
statement to be true:

sum(n:1 to m inclusive, 2^-n) = 1 (i.e. 1/2 + 1/4 + 1/8 ...)

What value must "m" have, for this statement to be true:

lim(n -> m), (1+1/n)^n = e (base of natural logarithms)

For both these examples, for the described outcomes, m must equal infinity.

Also, you say, "For example, the Universe is 'infinite' yet it is also
expanding", but you also say, "It took a couple thousand years just for
mathematicians to theoretically prove that some infinities are larger than
others ..." If one infinity can be said to be larger than another, then this
admits the possibility of any single infinity expanding.

Imagine a cube of space, in isolation, initially measuring one meter in any of
its dimensions. If measurements reveal that this value is increasing over
time, one may conclude that the cube is expanding. Now imagine that the cube
is embedded in a space consisting of an infinity of such cubes in three
spatial dimensions, all increasing in size in the same way. Does the fact that
the cube is part of an infinity of such cubes prevent it from expanding? If
this were true, it would be an easy test of the existence of an infinite
spatial manifold -- but the idea of an expanding infinity is logically
consistent.

~~~
drdeca
I think that saying that m "equals infinity" is probably a bit of a
simplification.

One can say that some values are infinite, such as omega in the ordinal
numbers and the surreal numbers, but neither of those values are what is meant
in the examples you gave, which are both limits.

If you plugged in omega (from the surreals) for those expressions instead of
taking the limit, I think you would get a value which is off by around epsilon
(i.e. 1/omega). I'm not sure of that but I think that's how it tends to work
out.

In the surreals one can indeed have a continuously increasing infinite value.
But I'm not sure that that is a proper way to describe the "total volume of
the universe"? Maybe it is, but idk.

~~~
lutusp
> I think that saying that m "equals infinity" is probably a bit of a
> simplification.

Yes, it is, I agree. For the limit example, one might instead say, "As m
approaches infinity, the result approaches 1." By definition, limits approach
their target values, they never equal them -- otherwise we would accept
division by zero and similar things.

The other reason I agree is because infinity isn't a number. I'm guilty of
compressing the lexical description of a limit to a ... ahem ... finite size.

> ... what is meant in the examples you gave, which are both limits.

One of my examples was an infinite sum, not a limit per se, but the same
argument can be made, i.e. that infinity is addressed without being literally
achieved.

~~~
drdeca
I believe that infinite sums are usually defined in terms of limits, which is
why I said that both examples were limits.

> By definition, limits approach their target values, they never equal them --
> otherwise we would accept division by zero and similar things.

I think I would say that a convergent sequence (most of the time that one
cares) does not ever reach the target value (though, eventually constant
sequences do, but those aren't interesting), but that the limit of the
convergent sequence is equal to / is the target value. As opposed to saying
that "the limit approaches". The limit isn't the thing that changes with
another thing, but the limit of some changing?

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lordnacho
Maybe someone can enlighten me on this:

Is it that space and time start at the big bang, or is it that all the
mass/energy was in a little point and has since expanded?

When we talk about the size of the universe, do we mean that's the limits of
where stuff is, or do we mean that's the limit of where stuff can be?

~~~
Oletros
Space and time started with the BB

~~~
AnimalMuppet
Well... per our current theories, yes. But experimental confirmation is pretty
scarce, so making dogmatic statements may not be justified.

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quizotic
Question re inflation and dark energy: current claims are that big-bang
started with a very short period of inflation. Haven't heard/understood much
on why inflations started or stopped. Now we posit dark energy, that causes
accelerated expansion of the universe, with acceleration greater at greater
distances and therefore earlier historical times.

So... is it possible that inflation and dark energy are related? When we see
distant/ancient galaxies accelerating away from us, is there any chance we're
looking at the ramping down of the original inflation of the big bang?

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plurinshael
I prefer the objectively superior moniker, "Horrendous Space Kablooie".

