Infinite Universe Or Not?

[modest]

“I sense something, a presence I have not felt since...”

 

The partypooper is back!

 

We meet again... ;)

 

All kidding aside, it's good to see ya CC.

 

I was thinking more in terms of a steady state-like model where this problem of 'infinite yet bounded' (or finite without bounds) does not arise.

 

But, many cosmologists (i.e. Hawking in A Brief History of Time and indeed Einstein) found finite yet unbound to be an elegant solution rather than a problem. Of course, observation is tending toward rejecting such elegance :winter_brr:

 

 

One problem I have with 'infinite' universe, is the relentless descriptions of how large the universe was during its early stages. I have read everything from softball size, to galactic in size after inflation.

 

If the universe was not infinite when it was the size of a softbal, why would it become such later.

 

The size of the universe being like a softball is meant only to refer to the visible universe. Calculations that determine density and energy of a given area of space—such as "when the universe was the size of a softball it had a temperature of ___ Kelvin", always refer to the amount of matter and space in our visible universe.

 

In this respect, the size of the universe being like a softball at one point and being infinite today is comparing two different things. The first refers to the space in our past light cone while the second refers to everything we can imagine existing beyond that light cone.

 

In the current and most popular Lambda-CDM model of our universe, space is infinite and unbound, and it has always been so (for as long as time has existed). Going back in time up to t=0 or "the singularity", the model still has the universe being infinite and unbound. The singularity refers more to the density of the universe than the size.

 

We can imagine today there are an infinite number of points making a straight line where each point is 5 kilometers apart. According to the LCDM model's currently agreed upon scale factor, when the universe was 379,000 years old (when it emitted CMBR) those points would have been 3.9 meters apart. Going back further they would be closer and closer until we can imagine an infinite number of points making a straight line where all the points are infinity close together at T=0.

 

In other words, according to standard cosmology today, as soon as there is *any* value to time (T>0), the universe is infinite in spatial extent.

 

~modest

[Pluto]

G'day Modest

 

You said

 

We can imagine today there are an infinite number of points making a straight line where each point is 5 kilometers apart. According to the LCDM model's currently agreed upon scale factor, when the universe was 379,000 years old (when it emitted CMBR) those points would have been 3.9 meters apart. Going back further they would be closer and closer until we can imagine an infinite number of points making a straight line where all the points are infinity close together at T=0.

 

Mate this to me is not very scientific.

 

How can you look at this logic?

 

It means that space itself is expanding.

[coldcreation]

Hello modest, good morning Pluto and you other kosmoficionados.

 

Good thread litespeed.

 

 

Yes, it's good to be back. I actually wasn't very far; skiing in the Spanish Pyrenees. I leave again tomorrow, albeit for another destination. I'll see if I can tap onto some ones WiFi to continue this and other interesting debates.

 

 

I was thinking more in terms of a steady state-like model where this problem of 'infinite yet bounded' (or finite without bounds) does not arise.

 

But, many cosmologists (i.e. Hawking in A Brief History of Time and indeed Einstein) found finite yet unbound to be an elegant solution rather than a problem. Of course, observation is tending toward rejecting such elegance ;)

 

~modest

 

True.

 

At the time, Albert Einstein’s static universe had a constant positive curvature and was spatially closed, temporally infinite (or “finite yet unbounded”), the scale factor or radius remaining constant with time. ‘Infinite,’ here refers to the continuous curvature of spherical space. Note that had time not been considered infinite the universe would have possessed a central symmetry. Both Einstein and de Sitter realized a four-dimensional sphere had the bizarre feature of allowing the coiling of time around itself, leading to the hypothetical prospect of an observer experiencing his own past. And it is precisely this paradox inherent in the physical meaning of the spherical model that prompted Willem de Sitter’s hyperboloid solution, avoiding the time paradox by leaving open the time direction. (See de Sitter Space).

 

 

 

 

Don’t fight the chill.

 

 

 

 

 

CC

[modest]

G'day Modest...

 

Mate this to me is not very scientific.

 

How can you look at this logic?

 

It means that space itself is expanding.

 

Yes, according to standard cosmology everything used to be closer together. If we consider space to be the distance between stuff then, yes, it is expanding. What do you mean unscientific and illogical?

 

Good thread litespeed.

 

Agreed :phones:

 

I actually wasn't very far; skiing in the Spanish Pyrenees.

 

I'm sure my invite was lost in the mail :confused:

 

Both Einstein and de Sitter realized a four-dimensional sphere had the bizarre feature of allowing the coiling of time around itself, leading to the hypothetical prospect of an observer experiencing his own past. And it is precisely this paradox inherent in the physical meaning of the spherical model that prompted Willem de Sitter’s hyperboloid solution, avoiding the time paradox by leaving open the time direction. (See de Sitter Space).

 

I think a closed universe allows you only to see your own past rather than experiencing it (which would require faster than light travel). Seeing your past is no more paradoxical than a video recorder. I'm not positive, I'd have to find a source...

 

~modest

[Pluto]

G'day modest

 

Scientifically prove to me how space can expand.

[Michaelangelica]

From what I have read, the universe is NOT infinite.

It has a fast expanding 'edge'

 

The intriguing question for me is

"What is on the other side of that edge?"

[Pluto]

G'day Michaelangelica

 

How can the universe be finite? Think about it for a sec.

[Tormod]

Scientifically prove to me how space can expand.

 

Hey Pluto

 

Not sure what you mean. Space as the "fabric" it's made of, or space meaning the entire universe?

 

It has been scientifically proved that space does expand, and that the cosmic expansion has increased over at least the last two billion years. This has been discussed many times here at Hypography IIRC.

 

This work was attributed to (among others) Paul Davies and his team.

 

Paul Davies: Home

 

Read more on expansion at Wiki:

Metric expansion of space - Wikipedia, the free encyclopedia

[C1ay]

From what I have read, the universe is NOT infinite.

It has a fast expanding 'edge'

 

The intriguing question for me is

"What is on the other side of that edge?"

 

Precisely. If it's expanding then it begs the question of "What is it expanding into?".

[Michaelangelica]

G'day Michaelangelica

 

How can the universe be finite? Think about it for a sec.

Are you being pedantic?

 

If you ascribe to the Big Bang theory of the evolution of the KNOWN universe it seems to have an "end" where we seem to be getting lots of static on our radios from.

 

If you want to include what is on the other side of that "edge/end" as the "Universe" I don't see any point to the question.

 

"At first there was nothing;

then it exploded."

Terry Prattchet

[C1ay]

That's where the definition of "universe" becomes the problem. If "The" Universe includes everything inside the universe as we know it and anything else beyond it then the question is impossible to anwser. If it refers only to the space and matter within the envelope of the known universe it seems obviously finite. I personally tend to refer to the known universe as the local universe which is only a small part of what I believe is a larger infinite universe that lies beyond our abilities of observation.

[modest]

G'day modest

 

Scientifically prove to me how space can expand.

 

Sure, just hold your hands out in front of you and spread them apart. The space between them just expanded. So, obviously, it *can* happen. The question is if it's happening on a universal scale—best scientific evidence says yes. Yes it is.

 

~modest

[Michaelangelica]

here is a guy with all the answers

NEW PHYSICS INSPIRED BY EINSTEIN'S FAMOUS THOUGHT EXPERIMENTS

He says the universe can't expand

 

An add for it came with my "Hypography advice of post" of this thread.!

[C1ay]

He says the universe can't expand

 

I wonder what his definition of universe is?

[Pluto]

G'day from the land of ozzzzzz

 

What does the expanding?

 

We have space without any form of matter or energy. Does this expand?

 

Modest that does not prove that space expands. We are talking about spacetime expansion. I think.

 

 

As for the universe being infinite, If you define ALL as being the universe than ALL is infinite. If you define the universe as only the observable universe than it becomes finite. Than you have a change in definition of the known universe to a UNITUNIVERSE or part of a multi-universe units.

 

As for the BBT I do not ascribe to it. Just because the theory has a standard status does not mean its reality. Far from it, its one theory on very weak foundations.

[Pluto]

G'day from the land of ozzz

 

Being a book worm I come across papers

 

This is in line with the discussion, it does not mean that I agree with it.

 

[0812.0414] Odds of observing the multiverse

Odds of observing the multiverse

 

Authors: Alex Dahlen

(Submitted on 2 Dec 2008)

 

Abstract: Eternal inflation predicts our observable universe lies within a bubble (or pocket universe) embedded in a volume of inflating space. The interior of the bubble undergoes inflation and standard cosmology, while the bubble walls expand outward and collide with other neighboring bubbles. The collisions provide either an opportunity to make a direct observation of the multiverse or, if they produce unacceptable anisotropy, a threat to inflationary theory. Here we show that the probability of an observer in our bubble detecting the effects of collisions has an absolute upper bound set by the odds of being in the part of our bubble that lies in the forward light-cone of a collision; in the case of collisions with bubbles of identical vacua, this bound is roughly one half, and it is enhanced or reduced in the case of collisions with bubbles of larger or smaller cosmological constant, respectively. In some cases, even if we do live in the causally connected region, the effects of a collision are eradicated by the round of inflation that occurs in our bubble. Still, especially for an energy landscape that produces bubbles with different types of vacua, the odds of observing the multiverse generically saturate the bound from causality.

[modest]

We have space without any form of matter or energy. Does this expand?

 

There would be no way to tell.

 

Modest that does not prove that space expands. We are talking about spacetime expansion. I think.

 

You're talking about the metric expansion of space, I'm talking about distance and we both should probably both be talking about whether the universe is infinite or not. :weather_snowing:

 

~modest