Is the Universe infinite?

**Strange** · August 19, 2017

4 minutes ago, StringJunky said:

If the universe is infinite it has always been in existence.

And the same may be true if it is finite.

StringJunky · August 19, 2017

3 minutes ago, Strange said:

And the same may be true if it is finite.

Yes, but a finite universe is not constrained to be infinitely old.

Edited August 19, 2017 by StringJunky

Tub · August 19, 2017

2 hours ago, Dan B. said:

How could it be that an infinite Universe had been set in only 13.8 BY?

Hello,Dan. The distance of around 13.8 billion light-years is the current limit of the observable size of the Universe, relative to us here on Earth. It can possibly be seen as a sort of event-horizon,and an horizon is just the limit of our sight. Beyond this horizon, also called the Hubble limit, there could be countless stars, galaxies and other astronomical bodies that are so far away that their light hasn't had time to reach us yet - or may never reach us. Hubble's Law also permits sufficiently distant space to expand faster than the speed of light so, again, light from those far, far distant galaxies will never reach us here on Earth. Of course this doesn't mean that the Universe is definitely infinite....but it still could be.

https://www.nasa.gov/feature/goddard/2016/hubble-team-breaks-cosmic-distance-record

Dan B. · August 19, 2017

55 minutes ago, Strange said:

There is no evidence that the universe was "created". It may be infinitely old. We don't (currently) know.

The early hot dense universe was finite if the universe is finite. It was infinite if the universe is infinite.

If the Universe is infinite old than its age must be infinite.

If there is no evidence that the Universe was "created", than:

1. Why we can't just assume that the Universe was there forever.

2. Why do we need to use the idea of early infinite hot dense Universe?

3. Why do we still need to use BBT as an explanation for Infinite Universe?

Edited August 19, 2017 by Dan B.

**Strange** · August 19, 2017

3 minutes ago, Dan B. said:

1. Why we can't just assume that the Universe was there forever.

Because science doesn't assume things. It looks for evidence.

4 minutes ago, Dan B. said:

2. Why do we need to use the idea of early infinite hot dense Universe?

Because we have good evidence it was hot and dense (but not infinitely so).

5 minutes ago, Dan B. said:

3. Why do we still need to use BBT as an explanation for Infinite Universe?

Because it is still the best explanation for all the evidence.

MigL · August 19, 2017

Finite but unbounded means a 'circular' universe such that if you go off in one direction, eventually you come back from the opposite direction.
the curvature would be intrinsic, ie it doesn't curve in another dimension. Hence there is no 'edge' and it doesn't expand INTO anything.

The reason we have problems with 'edges', Delta is because they are strange ( not you strange !) as you noted for the finite, bounded universe.

How do you treat the 'edge' of all there is ?

The observable universe has always been there. Even shortly after the Big Bang event, light only had limited time to get to a certain distance. So information and causality only reached so far, and that information and causality gives rise to the isotropy of the observable universe.
The amount of 'stuff' in the observable universe has been decreasing since then as more and more 'stuff' ( galactic clusters ) move out of the observable universe with increasing time.
The volume of information exchange and causality ( why is the universe isotropic and homogenous, why is it at equal temperature to 1 part in 1000, how did this information get exchanged with a finite light speed and finite time ?) was greatest at T=0, and that established al the parameters of our universe.

beecee · August 19, 2017

20 hours ago, Strange said:

I really recommend reading up on Cantor's work on infinity (if you haven't already). It is fascinating (and eye opening) stuff.

I may just give that a go. In the meantime do you (or anyone else) accept that as per the "nothing" definition, we just do not yet have a good handle of what infinity means and entails.

MigL · August 19, 2017

You should.

I would say we have a good definition, mathematically speaking.
It is just impossible for our minds to grasp or visualize the concept of infinity.

Just like 4dimensional space-time ( or even higher dimensions ), it is easily described mathematically, but impossible to describe using common, every day analogies.

( my apologies, in the previous post it should be temperature to 1 part in 10000; what happened to post numbering ? )

Edited August 19, 2017 by MigL

beecee · August 19, 2017

6 hours ago, Dan B. said:

Few questions:

1. Why do we assume now that the Universe could be infinite?

The results from the WMAP show the universe/spacetime to be very nearly flat, at least within very small error bars. A flat universe denotes an infinite universe. Of course it is also possible that an even more accurate methodology may see that flatness as simply the small part of a much larger curvature.

https://map.gsfc.nasa.gov/universe/uni_shape.html

6 hours ago, Dan B. said:

2. Why an infinite Universe has no impact on the BBT?

The BB applies to the observable universe and the evolution of spacetime as we know it.

6 hours ago, Dan B. said:

3. How could it be that an infinite Universe had been set in only 13.8 BY?

The 13.83 billion years is the age of the observable universe. The BB again was the evolution of space and time ( as we know them) from a hotter, denser state, which we now see at a 2.7K temperature.

http://www.astro.ucla.edu/~wright/infpoint.html

zapatos · August 20, 2017

6 hours ago, MigL said:

Finite but unbounded means a 'circular' universe such that if you go off in one direction, eventually you come back from the opposite direction.
the curvature would be intrinsic, ie it doesn't curve in another dimension. Hence there is no 'edge' and it doesn't expand INTO anything.

The reason we have problems with 'edges', Delta is because they are strange ( not you strange !) as you noted for the finite, bounded universe.

How do you treat the 'edge' of all there is ?

The observable universe has always been there. Even shortly after the Big Bang event, light only had limited time to get to a certain distance. So information and causality only reached so far, and that information and causality gives rise to the isotropy of the observable universe.
The amount of 'stuff' in the observable universe has been decreasing since then as more and more 'stuff' ( galactic clusters ) move out of the observable universe with increasing time.
The volume of information exchange and causality ( why is the universe isotropic and homogenous, why is it at equal temperature to 1 part in 1000, how did this information get exchanged with a finite light speed and finite time ?) was greatest at T=0, and that established al the parameters of our universe.

Thanks Migl. Those explanations were very helpful.

**Strange** · August 20, 2017

16 hours ago, MigL said:

I would say we have a good definition, mathematically speaking.

And that mathematical definition works pretty well for real stuff as well. For example, in an infinite universe no matter how far you go, you can always take one more step.

Airbrush · August 20, 2017

This is from the Nasa web site posted above:

"If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent."

Does everyone agree with this? That a "flat" universe must be infinite in size?

Does "universe" mean our big bang only, or does it include other possible big bangs that are further away? Does anyone consider that our big bang MAY be finite in size and just one of many, maybe an infinite number of other big bangs in the infinite multiverse?

Edited August 20, 2017 by Airbrush

**Strange** · August 20, 2017

2 minutes ago, Airbrush said:

Does everyone agree with this? That a "flat" universe must be infinite in size?

I think it depends on the topology. For example, the surface of a torus is flat but of finite size.

3 minutes ago, Airbrush said:

Does anyone consider that our big bang MAY be finite in size and just one of many, maybe an infinite number of other big bangs in the infinite multiverse?

From the little I know, multiverse models work with both finite and infinite universes.

Mordred · August 20, 2017

47 minutes ago, Airbrush said:

This is from the Nasa web site posted above:

"If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent."

Does everyone agree with this? That a "flat" universe must be infinite in size?

Does "universe" mean our big bang only, or does it include other possible big bangs that are further away? Does anyone consider that our big bang MAY be finite in size and just one of many, maybe an infinite number of other big bangs in the infinite multiverse?

That is an older definition for flat universe, prior to discovery of the cosmological constant. A flat universe can still be finite or infinite.

Wiki mentions this the details is in its reference 2

" For example, a universe with positive curvature is necessarily finite.^[2] Although it is usually assumed in the literature that a flat or negatively curved universe is infinite, this need not be the case if the topology is not the trivial one.^[2]"

^{https://en.m.wikipedia.org/wiki/Shape_of_the_universe}

Edited August 20, 2017 by Mordred

beecee · August 20, 2017

23 hours ago, MigL said:

I would say we have a good definition, mathematically speaking.
It is just impossible for our minds to grasp or visualize the concept of infinity.

Which was what I was trying to say, albeit rather poorly.

6 hours ago, Airbrush said:

This is from the Nasa web site posted above:

"If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent."

Does everyone agree with this? That a "flat" universe must be infinite in size?

6 hours ago, Strange said:

I think it depends on the topology. For example, the surface of a torus is flat but of finite size.

From the little I know, multiverse models work with both finite and infinite universes.

6 hours ago, Mordred said:

That is an older definition for flat universe, prior to discovery of the cosmological constant. A flat universe can still be finite or infinite.

Wiki mentions this the details is in its reference 2

" For example, a universe with positive curvature is necessarily finite.^[2] Although it is usually assumed in the literature that a flat or negatively curved universe is infinite, this need not be the case if the topology is not the trivial one.^[2]"

^{https://en.m.wikipedia.org/wiki/Shape_of_the_universe}

My apologies Airbrush, as Mordred has said, that is an older definition that has not been updated. What Strange and Mordred say is correct...It depends on non trivial topology such as torus shape.

Age of the universe - Wikipedia

https://en.wikipedia.org/wiki/Age_of_the_universe

"In physical cosmology, the age of the universe is the time elapsed since the Big Bang. ... the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time."

I assume that this extrapolation was based on the observable Universe.

However, if the Universe is infinite, than it's age must be infinite (or close to infinite).

As we set the extrapolating backwards in time (For infinite Universe) - It is still there.

It is there 20 BY ago, It is there 100 BY ago and even 1 Trillion years ago.

Hence, if the Universe is there (at any age), than we can't claim that 13.8 BY ago the time of the Universe was Zero.

How could the Big bang start without setting the time to Zero?

How could it start while the Infinite Universe is already there - full of mass?

How a Big Bang which took place 13.8 Billion years ago could affect an infinite Universe?

Edited August 21, 2017 by Dan B.

beecee · August 22, 2017

3 hours ago, Dan B. said:

Thanks

Do appreciate.

Sorry - What is the meaning of the observable Universe age in an Infinite Universe?

The age of the Universe had set as follow:

Age of the universe - Wikipedia

https://en.wikipedia.org/wiki/Age_of_the_universe

"In physical cosmology, the age of the universe is the time elapsed since the Big Bang. ... the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time."

I assume that this extrapolation was based on the observable Universe.

However, if the Universe is infinite, than it's age must be infinite (or close to infinite).

As we set the extrapolating backwards in time (For infinite Universe) - It is still there.

It is there 20 BY ago, It is there 100 BY ago and even 1 Trillion years ago.

Hence, if the Universe is there (at any age), than we can't claim that 13.8 BY ago the time of the Universe was Zero.

How could the Big bang start without setting the time to Zero?

How could it start while the Infinite Universe is already there - full of mass?

How a Big Bang which took place 13.8 Billion years ago could affect an infinite Universe?

You've asked some difficult questions, which I am unable to explain any better then the link I gave at......http://www.astro.ucla.edu/~wright/infpoint.html Perhaps some confusion again is our understanding of "infinite" and that certainly applies to your's truly! ....Does it really mean something with no ending and no beginning, or is it simply something whose boundaries are unknown and can never be known?

Quote

How could the Big bang start without setting the time to Zero?

The BB is not a theory of the creation of the universe: It is a theory of how the universe/spacetime evolved from t+10-43 seconds, from a hotter, denser state. It is also a theory pertaining to the observable universe.

Quote

How could it start while the Infinite Universe is already there - full of mass?

Again the BB describes the observable universe or that part of the universe contained by the speed of light and the time it has taken to reach us since the BB. I also believe your question re the infinite universe is explained in the link.

Quote

How a Big Bang which took place 13.8 Billion years ago could affect an infinite Universe?

It doesn't. It describes the evolution of spacetime from 10-43 seconds post t, to what we are able to observe today. If the Universe is infinite, it would always have had to be infinite, and going back in time would just shorten the distances between any two points, up to around t+10-43 seconds. Also the singularity from whence the BB arose, does not necessarily invoke a dimensionless point in spacetime...It is simply where quantities such as density and temperatures have merged. I hope that makes some sense.

Other then the links I have given, I'm not able to explain it any better, as a lay person. Perhaps one of our more professional members are able to elaborate on what I have said and what the links you yourself have given and the two of mine.

Edited August 22, 2017 by beecee

**Strange** · August 22, 2017

3 hours ago, Dan B. said:

Sorry - What is the meaning of the observable Universe age in an Infinite Universe?

As beech says, that is the age since the earliest time our models can take use back to. If you just extrapolate back using GR you can go back to time = 0 when the universe had zero size (a singularity). But no one thinks this represents anything meaningful.

This is based on what we can see; i.e. our observable universe. But the working assumption (the cosmological principle) is that the universe is pretty much the same everywhere. So an observer half way to the edge of our observable universe would have their own observable universe, and they would see it as having the same age as ours. As far as we know, that will be true wherever you are in the universe.

Quote

However, if the Universe is infinite, than it's age must be infinite (or close to infinite).

No. For the reasons above.

Quote

As we set the extrapolating backwards in time (For infinite Universe) - It is still there.

It is there 20 BY ago, It is there 100 BY ago and even 1 Trillion years ago.

It was still "there" in the sense it still existed (probably). But all we know is that from 13.8 billion years ago it has been expanding and cooling. We don't know what it was like before that. Some models suggest the universe was in that "pre expansion" state for an infinite time. Others have a universe that collapsed and then expanded again. Yet other say ...

We just don't know.

Quote

How could the Big bang start without setting the time to Zero?

You can extrapolate back to zero but, as noted, it is physically meaningless (with our current theories - a theory of quantum gravity might tell us more).

Quote

How a Big Bang which took place 13.8 Billion years ago could affect an infinite Universe?

Because the entire universe (as far as we know) is expanding and cooling.

It may be better to think of the Big Bang as the process that is still going on, rather than an event (for which we have zero evidence and that may never have happened).

Airbrush · August 22, 2017

When ancients looked out at the ocean for the first time they probably thought the ocean was infinite until they discovered it was not. When people looked out they thought the Milky Way was the entire universe, until they discovered it was not. The universe seems like higher and higher levels of organization. Stars are mostly within galaxies, galaxies form clusters and clusters form superclusters. Could the universe just be the next level of organization? After universes, you can have clusters of universes and superclusters of universes, within a multiverse. Then clusters of multiverses and so on and so forth?

My main question is why does the big bang always gets represented as a roughly spherical shape expanding outward? Why not starting out like a jagged, amorphous lightning strike, also expanding outward?

Edited August 22, 2017 by Airbrush

Dan B. · August 22, 2017

I have asked: "However, if the Universe is infinite, than it's age must be infinite (or close to infinite)."

Your aswer was:

16 hours ago, Strange said:

No. For the reasons above.

It isn't clear for me.

Why we can't just assume that an infinite age is needed to set an infinity Universe?

What is the expected age of the current Infinite Universe?

I assume that a time frame of 13.8 B years isn't long enough for that process?

Edited August 22, 2017 by Dan B.

**Strange** · August 22, 2017

Just now, Dan B. said:

I have asked: "However, if the Universe is infinite, than it's age must be infinite (or close to infinite)."

Your aswer was:

It isn't clear for me.

Why we can't just assume that an infinite age is needed to set an infinity Universe?

Based on our understanding, what is the expected age of the current Infinite Universe?

I assume that a time frame of 13.8 B years isn't long enough for that process?

The age of the universe is thought to be 13.8 billion years. Whether it is finite or infinite.

I have tried to explain why that is but because I don't understand why you think it must be more than that, it is hard to give a better explanation.

Why assume an infinite age when we have evidence for the age being 13.8 billion years. (Note that this "age"is the time since the earliest period our physics models can take us back to.)

Airbrush · August 22, 2017

Hi Strange, can you please address my questions above? Thanks.

**Strange** · August 22, 2017

3 hours ago, Airbrush said:

My main question is why does the big bang always gets represented as a roughly spherical shape expanding outward? Why not starting out like a jagged, amorphous lightning strike, also expanding outward?

1. That image can only represent a part of the universe. Typically, the observable universe, which is spherical.

2. The distribution of matter, and hence the rate of expansion, is homogenous and isotropic - the same in all directions. A spherical image is a good way of representing this.

3. Simplicity.

As for the questions about the multiverse, it is not something I know much about (lack of interest) so can't really answer.

beecee · August 22, 2017

3 hours ago, Airbrush said:

When ancients looked out at the ocean for the first time they probably thought the ocean was infinite until they discovered it was not. When people looked out they thought the Milky Way was the entire universe, until they discovered it was not. The universe seems like higher and higher levels of organization. Stars are mostly within galaxies, galaxies form clusters and clusters form superclusters. Could the universe just be the next level of organization? After universes, you can have clusters of universes and superclusters of universes, within a multiverse. Then clusters of multiverses and so on and so forth?

My main question is why does the big bang always gets represented as a roughly spherical shape expanding outward? Why not starting out like a jagged, amorphous lightning strike, also expanding outward?

Again, the BB applies to the observable universe, and the observable universe is then logically a spherical shell encompassing all that we are able to see from our vantage point on Earth, and is governed by the speed of light that has had the time to reach us since the evolution/expansion of the universe/spacetime, Likewise if we could magically transport ourselves to a galaxy near the edge of our observable universe, any prospective beings in that galaxy would also have their own spherical observational universe that would encompass us in one direction.

The expansion of the universe/spacetime is of course equal in all directions.

Nouveau · August 22, 2017

The way in which we experience reality indicates that infinity is actually real to us but equally demonstrates its almost total irrelevance given that we reside completely within its confines that can never be exceeded. For the grandeur of scale of an observable universe now thought to encompass 2 trillion galaxies each themselves containing 2 to 4 hundred billion stars, it's not unrealistic to expect speculation as to whether it may indeed be infinite beyond the furthermost reaches we able to see. Certainly, we can easily imagine infinity as being something really big, but a far easier way to understand infinity is within our purview using only mathematics, Imagine if you will the length of the room you are sitting in, divide by 2 and see how many times you can repeat it. The answer, is an infinite number of times with your original length becoming infinitely small.

What this tells us is that we exist somewhere along an infinite scale, we don't start at the smallest and we aren't the biggest we are inside a scale of infinity that can never be exceeded, it also indicates the possibility that if we are inside one scale of infinity we may well be contained within many others.

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