Distinction between observable and unobservable universe

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    Observable Universe
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Discussion Overview

The discussion revolves around the distinction between the observable universe and the total universe, particularly in the context of inflation and expansion. Participants explore the implications of these concepts on the nature of the universe, addressing both theoretical and conceptual aspects.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that the universe is typically referred to as the observable universe, while others challenge this notion, suggesting that the universe is a singular entity encompassing both observable and unobservable parts.
  • There is a discussion about whether inflation applies to the entire universe or just the observable part, with some arguing that inflation pertains to the total universe, while others express uncertainty about the implications of this.
  • Participants mention that phrases like "initial size of the universe" are often used in popularizations but are not typically employed by physicists, leading to confusion about the universe's size and expansion.
  • Some argue that the universe may be infinite, while others contend that it is likely finite due to the finite rate of expansion, though the possibility of an infinite universe with a finite expansion rate is also discussed.
  • There is a clarification that the Big Bang occurred everywhere, not just at a single point, and that the cosmological scale factor is a more appropriate way to discuss the universe's expansion over time.
  • Participants debate the implications of general relativity (GR) on the size and dimensionality of singularities, noting that GR does not provide straightforward answers regarding these concepts.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of the universe, inflation, and the implications of expansion. There is no consensus on whether the universe is finite or infinite, and the discussion remains unresolved on several key points.

Contextual Notes

Limitations include varying definitions of terms such as "rate of expansion" and the implications of inflation on the universe's size. There is also ambiguity surrounding the use of popular phrases versus technical terminology in physics.

Chiclayo guy
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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?
 
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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.
 
Chiclayo guy said:
I know that when we talk about ‘the universe’ we’re normally referring to the observable universe.
Not true.

Chiclayo guy said:
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?
Phrases like "initial size of the universe" are typical in popularizations. Physicists would not typically use a phrase like that.
 
bcrowell said:
Not true.Phrases like "initial size of the universe" are typical in popularizations. Physicists would not typically use a phrase like that.

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.
 
Chiclayo guy said:
universe is possibly infinite
Not likely, since there is a finite rate of expansion. It's very big, but probably not infinite.

Chiclayo guy said:
BB happened “everywhere”
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.
 
Chiclayo guy said:
The universe is possibly infinite. Did/does inflation/expansion apply to the entire (I don’t know how else to phrase it) infinite universe?
Yes. (Inflation is irrelevant here. We don't even know for sure that inflation happened.)

Chiclayo guy said:
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.
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.
 
I think OP might be confusing inflation with expansion.
 
DuckAmuck said:
Not likely, since there is a finite rate of expansion. It's very big, but probably not infinite.
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.
 
Chiclayo guy said:
It is my understanding that the universe is 'one thing’,

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

Chiclayo guy said:
when we talk about inflation and the initial size of the universe are we saying those concepts apply to the total universe.

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

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

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
alw34 said:
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.

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
Orodruin said:
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.

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
DuckAmuck said:
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.

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##?
 

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