# Distinction between observable and unobservable universe

1. Dec 30, 2015

### Chiclayo guy

I know that when we talk about ‘the universe’ we’re normally referring to the observable universe. It is my understanding that the universe is 'one thing’, the only distinction being that part is visible and part is not, yet occasionally when the subjects of size and inflation are discussed comments are worded such that I have the impression that the observable universe is being treated as a separate entity existing in but independent of the total. Just so I am clear, when we talk about inflation and the initial size of the universe are we saying those concepts apply to the total universe (whatever that means) or just the observable?

2. Dec 30, 2015

### DuckAmuck

I'm not sure what you mean exactly, but yes inflation deals with the actual size of the universe. It doesn't include anything *outside* the universe.

3. Dec 30, 2015

### bcrowell

Staff Emeritus
Not true.

Phrases like "initial size of the universe" are typical in popularizations. Physicists would not typically use a phrase like that.

4. Dec 30, 2015

### Chiclayo guy

How discouraging…I thought my question was crystal clear. Let me take another run at it.

The universe is possibly infinite. Did/does inflation/expansion apply to the entire (I don’t know how else to phrase it) infinite universe?

What phrases do physicists use to describe the initial size of the universe? I’ve seen terms such as smaller than an electron, golf ball and grapefruit used on this very forum, but I’ve also seen that the BB happened “everywhere”. Does that mean everywhere within the grapefruit? It’s difficult for a layman, at least for me to make sense of that.

5. Dec 30, 2015

### DuckAmuck

Not likely, since there is a finite rate of expansion. It's very big, but probably not infinite.

The big bang happened everywhere, because it's not the material in the universe expanding, it's space itself. It wasn't just a dense ball of matter floating in large empty space.
The actual available space in the universe was once the size of a golf ball. This space has since expanded, and so you can say the big bang happened everywhere, since it's technically true.

6. Dec 30, 2015

### bcrowell

Staff Emeritus
Yes. (Inflation is irrelevant here. We don't even know for sure that inflation happened.)

Typically they would say that the cosmological scale factor had increased by a factor of x from a certain time in the past up to the present.

You can't talk about the initial size for two reasons: (1) the universe may be infinite, and (2) the cosmological scale factor was initially zero.

7. Dec 30, 2015

### DuckAmuck

I think OP might be confusing inflation with expansion.

8. Dec 30, 2015

### Orodruin

Staff Emeritus
This is irrelevant. The universe can perfectly well be infinite with a finite rate of expansion. In those situations, the universe was always infinite in extent.

9. Dec 30, 2015

### alw34

In that context, 'the [whole] universe' is everything.....there is nothing 'outside' the whole. You can't look back inside.

Inflation applies to the total/whole universe;
'initial size' has little if any meaning:

If you look at the illustration here,

https://en.wikipedia.org/wiki/Big_Bang#Overview

it starts from a single 'point' in the illustration. What popularizations may discuss is that one geometric point of origin, but it is from every such 'point', finite or infinite in number, that inflation takes place and from which the seeds of our universe appear. The origin of our universe appears to be a point time rather than a point in space.

10. Dec 30, 2015

### bcrowell

Staff Emeritus
The standard formulation in general relativity is not to describe singularities as points or sets of points. In particular, GR doesn't give any straightforward answers to questions about the size of a singularity, or even how many dimensions it has. Size and dimensionality are things that are defined for point-sets.

What we can say is that the cosmological scale factor approaches zero as time approaches the time of the big bang.

11. Dec 30, 2015

### DuckAmuck

Sure, but if the universe were infinite with a finite rate of expansion, then this would imply infinite size from the beginning, or a jump from finite to infinite at some point.
If it began with finite size and has always had a finite expansion rate, it must be finite.

12. Dec 30, 2015

### bcrowell

Staff Emeritus
GR predicts that if the universe is spatially finite now, then it has always been spatially finite. If it's spatially infinite now, then it's always been so.

Before going too far in this dialog between you and Orodruin on this point, are you sure you're both using the same definition of "rate of expansion?" Are we all talking about $\dot{a}/a$?