# Intro to Big Bang and Infinity Concepts - Comments

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PeterDonis
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2020 Award
I thought this was pretty basic stuff.

The fact that there is an observable universe, that it is not the entire universe, and that coordinate velocities in a curved spacetime can be greater than ##c##, are all pretty basic stuff, yes.

None of that implies the things that you were saying.

Hi @Arman777

I like your presentation, but I feel it would be improved by mentioning the following point.

Here is a quote.
On the surface of the sphere, we could move in some direction and we may find ourselves to the point that we are started.​
In an actual finite universe, which would typically be expanding or contracting, one might have to travel faster than the speed of light in the expanding case in order for the mover to arrive at the same spacial point. It may also be useful to mention choosing the point of interest as fixed in co-moving coordinates.

Here is a suggestion.
On the surface of a sphere which is expanding in the same manner as our universe, if we started at a fixed point in co-moving coordinates and traveled at some faster than light speed in any direction along a great circle of the sphere, we could eventually find ourselves back at the point where we started.​

Regards,
Buzz
"On the surface of a sphere which is expanding in the same manner as our universe . . . . . . "

It seems to me that you are adopting a flatlander analogy where the flatlander is living on the surface of the sphere, and has no comprehension of up or down. Thus you are postulating (quite correctly) travelling (I am English) on great circles. As your sphere's surface expands (= expansion of flatlander universe) the flatlander is unaware of the increasing radius of the sphere, which increase, however, is observable to an observer of (n + 1) dimensions (where the flatlander is n dimensional - I am avoiding time for the moment).
This means, of course, that (n + 1) can travel along a chord between two points on the surface. It also means that (n + 1) can appreciate expansion of his universe (I am purposely using small 'u's here) in a dimension beyond the comprehension of n (n being the flatlander). To 'n' it might appear that (n + 1) had travelled faster than light, or even instantaneously.

I appreciate that this is only an analogy, and proves nothing, but it helps me to understand comprehension of dimensions from different dimensional points of view. It is capable of further development.

Catastrophe (aka Cat)

There are classes of plausible theories in which time extends infinitely far into the past. One of my favorites is

https://en.wikipedia.org/wiki/Eternal_inflation
The eternal inflating universe model is future eternal, but not past eternal. Vilenkin and Guth proved that generically an inflating spacetime is geodesical incomplete to the past timeline, which means that any two particles in an inflating space must have been arbitrarily close together in the past. So, strictly speaking, eternal inflation is not eternal to the past.

However, since eternal inflation it is future eternal, the point in time in which eternal inflation must have started, can be an arbitrariry amount of time in the past, without bound. Likely inflation already started in earlier times, before our universe was born.

Further, there is no restriction for eternal inflation to have started only once, so theoretically, eternal inflation could have been going on before our time line or branch of inflation that created our universe, has started, but that would be a completely different space time, not causally connected to our spacetime, and about which nothing can be known.

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There really are three phases in cosmology as we understand it
1) t=0 the Big Bang Singularity where we don't know WHAT was going on
2) t = one Planck Time to something like t= 10E-32 seconds --- the Inflationary Period (hypothetical but likely)
3) t = the end of the Inflationary Period and onward --- the time of the Big Bang Theory

This kind of dividing cosmic history in three phases always make me wonder why - when assuming inflation - we still put the big bang singularity (purely hypothethical and only derived from GR without QM, which then only shows that GR is incomplete and one needs a full theory combining GR and QM to say something meaningfull about this) in front of this, while the inflationary period might have a totally different history (the history of the cosmos before inflation started) and is largely unknowable, as also is probably eternal into the past (there is no a priori reason to assume time itself or the universe as a whole had a finite history - with what could the universe have begun?).

Inflationary period: it depends on which model of inflation one talks here, but in many inflation models inflation is future eternal, and thus it can not simply be assumed that our universe happened to "condense out" from the inflationary space soon after inflation started, as it could as well have lasted an arbitrary amount of time after inflation started before our universe came to be.

PeterDonis
Mentor
2020 Award
why - when assuming inflation - we still put the big bang singularity (purely hypothethical and only derived from GR without QM, which then only shows that GR is incomplete and one needs a full theory combining GR and QM to say something meaningfull about this) in front of this
Who is this "we" who does this? AFAIK this is not what is done in actual textbooks and peer-reviewed papers. Pop science presentations are another matter, but that's why we advise people here on PF not to try to learn science from pop science presentations.

Who is this "we" who does this? AFAIK this is not what is done in actual textbooks and peer-reviewed papers. Pop science presentations are another matter, but that's why we advise people here on PF not to try to learn science from pop science presentations.
It does happen in scientific lectures too.

PeterDonis
Mentor
2020 Award
It does happen in scientific lectures too.
Can you give an example?