The Universe - infinite or not ?

[Lost in Space]

If the universe is spatially infinite, then yes.


The universe doesn't have a boundary. The observable universe has a boundary. The laws of physics don't break down at the boundary of the observable universe. The boundary of the observable universe is not a place with special physical properties. It's simply the set of all points from which light has just barely had time to reach our own planet since the Big Bang. Tomorrow, that boundary will be about 3 light-days farther from us than it is today, so a certain volume of space will have become newly available to us for observation.

BTW, we have a new entry on this topic in the cosmology forum's sticky FAQ thread.

Pardon my confusion, but I've been given to understand that the Big Bang is a boundary where the laws of physics break down? What about event horizons of black holes? And isn't the present an ever moving and growing boundary as well as we cannot view future events, only events in the past?

 

[Lost in Space]

Since the Big Bang took place a finite time ago, the Universe would have had to expand at an infinite rate to reach an infinite size. Unless it was already infinite at the time of the Big Bang.

Is it finite, or is it just us measuring a portion of time relative to our own existence? Can time be divided into infinite pieces? In other words is there a state in which the 'finite' time we perceive since the Big Bang can be said to be 'infinite'?

 

[Cosmo Novice]

If the universe is spatially infinite, then yes.

Ok. Can you please clarify.

I would suggest that even if U was spatially finite then it would probably be large enough for their to be photon emitting objects so far away that the expansion and scale factor of the intervening space would result in the photon never reaching us.

 

[Shenstar]

I find an infinite universe impossible to comprehend.

 

[thekushal276]

I can't agree with you more, Mentat. I would say, maybe, that the universe is on the verge of infinity?[?]

i don't think that universe is infinite. logic behind this is: as we know that infinite is not a real number. that means infinite is not reality. it is our imagination. the thing which cannot be counted we refer it as INFINITE. something imaginary, not real or uncountable. but that do not means that the universe is infinite. the thing is that we are not able enough to explore it and define the size of it...

 

[bcrowell]

I would suggest that even if U was spatially finite then it would probably be large enough for their to be photon emitting objects so far away that the expansion and scale factor of the intervening space would result in the photon never reaching us.

I didn't quite get that right. For a closed universe with zero cosmological constant, you get a recollapse, so nothing is ever permanently hidden from any observer. But we know that the cosmological constant isn't zero, and recollapse is ruled out.

Pardon my confusion, but I've been given to understand that the Big Bang is a boundary where the laws of physics break down? What about event horizons of black holes? And isn't the present an ever moving and growing boundary as well as we cannot view future events, only events in the past?

These are all different cases. The Big Bang is a physical singularity (not just a coordinate singularity). The event horizons of black holes are not physical singularities. The boundary between past and present isn't a uniquely defined thing in relativity. I thought you were talking about the boundary of the observable universe, which is still another thing.

 

[bcrowell]

i don't think that universe is infinite. logic behind this is: as we know that infinite is not a real number. that means infinite is not reality. it is our imagination. the thing which cannot be counted we refer it as INFINITE. something imaginary, not real or uncountable. but that do not means that the universe is infinite. the thing is that we are not able enough to explore it and define the size of it...

Please read the FAQ entry "Is the universe finite, or is it infinite?" in the thread titled "Frequently Asked Questions (FAQ) about Cosmology" at the top of the cosmology forum.

 

[Lost in Space]

These are all different cases. The Big Bang is a physical singularity (not just a coordinate singularity). The event horizons of black holes are not physical singularities. The boundary between past and present isn't a uniquely defined thing in relativity. I thought you were talking about the boundary of the observable universe, which is still another thing.

But aren't black holes physical singularities bounded by event horizons? And they're part of the observable universe are they not?

 

[bcrowell]

But aren't black holes physical singularities bounded by event horizons? And they're part of the observable universe are they not?

The physical (non-coordinate) singularity is at the black hole's center, not at its event horizon.

 

[Lost in Space]

The physical (non-coordinate) singularity is at the black hole's center, not at its event horizon.

Yes, I understand this much but surely the boundary of the black hole is its event horizon which separates us from it? Although we cannot observe the singularity at the black hole's centre, we are still aware that it's there, so isn't the event horizon in this case a boundary within the observable universe?

 

[narrator]

I find an infinite universe impossible to comprehend.

I find it difficult but not impossible. We live our lives by many paradigms and when confronted with one that's different we think "contradiction" or even "contravention".

 

[bcrowell]

Yes, I understand this much but surely the boundary of the black hole is its event horizon which separates us from it? Although we cannot observe the singularity at the black hole's centre, we are still aware that it's there, so isn't the event horizon in this case a boundary within the observable universe?

The boundary between the US and Canada is also a boundary within the observable universe. In your earlier post, you refer to "a boundary where the laws of physics break down:"

Pardon my confusion, but I've been given to understand that the Big Bang is a boundary where the laws of physics break down? What about event horizons of black holes? And isn't the present an ever moving and growing boundary as well as we cannot view future events, only events in the past?

When people refer to the laws of physics breaking down in GR, they usually mean singularities. You've been lumping together a lot of different things.

 

[DavidMcC]

I find it difficult but not impossible. We live our lives by many paradigms and when confronted with one that's different we think "contradiction" or even "contravention".

That doesn't make an infinite big bang a physical possibility, though. The only way the universe could be infinite is if it never had a beginning, ie that its beginning was an infinite time ago. Not plausible.

 

[Cosmo Novice]

That doesn't make an infinite big bang a physical possibility, though. The only way the universe could be infinite is if it never had a beginning, ie that its beginning was an infinite time ago. Not plausible.

I don't think this is necessarily true, as I understand it something does not require infinite age to be spatially infinite. If the U is infinite now, then essentially it was infinite at the moment of the BB, so you could say if the U is open and spatially flat it is temporally finite but spatially infinite.

 

[DavidMcC]

I don't think this is necessarily true, as I understand it something does not require infinite age to be spatially infinite. If the U is infinite now, then essentially it was infinite at the moment of the BB, so you could say if the U is open and spatially flat it is temporally finite but spatially infinite.

You may have to re-invent physics for that, Cosmo!

 

[narrator]

You may have to re-invent physics for that, Cosmo!

That depends on your understanding of physics. Even of those of us who understand everyday physics, very few understand the physics of black holes (for example).

As a simplistic example, under the right conditions, a fog appears everywhere with no starting point. Sure, the analogy breaks down if you get into the nitty gritty, but to me, it's a very rough analog of how the universe formed - one difference being that the "right conditions" were not localized.

The other aspect of this is that these theories have come from actual "science", not from untested fantasy.

 

[DavidMcC]

I see a fog descending on this thread, narrator!

 

[Neandethal00]

I don't think this is necessarily true, as I understand it something does not require infinite age to be spatially infinite. If the U is infinite now, then essentially it was infinite at the moment of the BB, so you could say if the U is open and spatially flat it is temporally finite but spatially infinite.

I have a growing feeling some of you are mixing up observable universe (OU) and container of the OU. OU is the region which holds galaxies , us. There may even be objects in our OU which are beyond our observation limits at this time. If the U is really expanding, it is expanding into another region, which I call container. We have absolutely ZERO knowledge about this container, which may or may not be infinite, we can only speculate about this container, our speculation would probably be light years away from 'reality'. But OU can not be infinite to humans.

My personal opinion is our 'sensory systems' are playing a big tricks on us.

 

[bcrowell]

I don't think this is necessarily true, as I understand it something does not require infinite age to be spatially infinite. If the U is infinite now, then essentially it was infinite at the moment of the BB, so you could say if the U is open and spatially flat it is temporally finite but spatially infinite.

You may have to re-invent physics for that, Cosmo!

Cosmo Novice is correct. Standard spatially flat models of the universe are spatially infinite but have only existed for a finite time.

If the U is really expanding, it is expanding into another region

This is incorrect. Here is a good explanation: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

 

[Ken G]

My own feeling is we immediately err whenever we choose to use language like "the universe is...", regardless of how we finish the sentence. It simply isn't scientific. Instead, we must ask "we gain the following benefits by modeling the universe as A, and these other benefits by modeling it as B." This language stresses two important things about science:
1) our "knowledge" is constantly changing, and
2) we have a purpose for favoring the models we favor.
So in terms of the size of the universe, we would say that we observe no spatial curvature so the model with the fewest added assumptions is one that is spatially infinite. This in no way means that the universe is infinite, nor does it make any difference if we are incredulous about it being infinite (the history of science is wall-to-wall suspension of incredulity), it just means an infinite model keeps us from having to say anything else about our model that would be completely arbitrary. It's not even a question that the universe "either is or isn't infinite", because terms like "infinite" apply to models, not to the universe itself. If we can't measure a size, even in principle, then the universe doesn't have one, neither finite nor infinite-- it is the models that are one or the other. The sole scientific statement we can make is that we have never been able to detect any curvature that would suggest a size to the universe, the rest is just model-favoring.

 

[Neandethal00]

Cosmo Novice is correct. Standard spatially flat models of the universe are spatially infinite but have only existed for a finite time.


This is incorrect. Here is a good explanation: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

From your link:

"Everything that we measure is within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about".

I'm sorry, but this type of statements just turn me off from main stream science.
There is a group of 'know all' scientists who have the audacity to claim 'we already know what we wanted to know'.

There is nothing wrong in saying "We DO NOT know, yet".

 

[Ken G]

I think you are missing a crucial element of what he said-- he did not say we know it isn't expanding into anything, he said "this is not a profitable thing to think about." That's quite an important element of science, noticing what models are profitable to think about, and what ones are just idle speculation. In science, the proof is in the pudding-- models are good not because they are right, they are good because they actually guide our observations and allow us to make successful predictions. Too many people want science to be some kind of "oracle of truth", and they get mad when their pet theories are not getting attention. It isn't because the pet theories are wrong, it is simply because they bear no fruit. You just have to kind of deal with it, you want science to be something that it isn't, and then you blame the scientists.

 

[apeiron]

As a simplistic example, under the right conditions, a fog appears everywhere with no starting point. Sure, the analogy breaks down if you get into the nitty gritty, but to me, it's a very rough analog of how the universe formed - one difference being that the "right conditions" were not localized.

This is definitely an alternative way of viewing the issue. As a phase transition. Time and space as something definite would have been born out of something far less definite. You could call it a fog, a vagueness, a pre-geometry, a perfect symmetry.

What would be "infinite" or unlimited in the fog is degrees of freedom. So there just is no issue about the size of the space that the universe emerged from, or the one it is growing into. The beginnings are defined by their lack of such dimensional organisation, and the universe by it being a state of organised, or globally constrained, dimensionality.

 

[DavidMcC]

Cosmo Novice is correct. Standard spatially flat models of the universe are spatially infinite but have only existed for a finite time.


This is incorrect. Here is a good explanation: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

You (and UCLA) are making the same mistake as George (IMO), in equating black holes to the classical theory of them, which does not allow for an LQG BH, in which it is a mass quantum effect (just like superfluids and supeconductors, etc).
Also, flatness cannot be measured to zero error, so a large, but finite, curvature is necessarily a possibility, no matter how accurate the measurement.

 

[narrator]

You (and UCLA) are making the same mistake as George (IMO), in equating black holes to the classical theory of them, which does not allow for an LQG BH, in which it is a mass quantum effect (just like superfluids and supeconductors, etc).
Also, flatness cannot be measured to zero error, so a large, but finite, curvature is necessarily a possibility, no matter how accurate the measurement.

The uncertainty principle is well established in scientific method.

 

[bcrowell]

Also, flatness cannot be measured to zero error, so a large, but finite, curvature is necessarily a possibility, no matter how accurate the measurement.

Cosmological solutions with negative spatial curvature are also spatially infinite but have existed for a finite time. You simply have an issue about cosmology that you don't understand properly, as in this quote:

That doesn't make an infinite big bang a physical possibility, though. The only way the universe could be infinite is if it never had a beginning, ie that its beginning was an infinite time ago. Not plausible.

Go to the library and pull a freshman gen ed astronomy text off the shelf. Read the chapter on cosmology. This is basic stuff that you've simply gotten wrong.

You (and UCLA) are making the same mistake as George (IMO), in equating black holes to the classical theory of them, which does not allow for an LQG BH, in which it is a mass quantum effect (just like superfluids and supeconductors, etc).

The text you quoted wasn't about black holes.

It sounds like you have some ideas about black holes and cosmology that are nonstandard. Since they're nonstandard, you can't expect other people to telepathically figure out what they are when you just make vague references to "cosmic BHs" ( https://www.physicsforums.com/showpost.php?p=3347236&postcount=73 ).

If you want to discuss your ideas about black holes and cosmology, and explain why Ned Wright doesn't know what he's talking about, I suggest you start a separate thread on that in the Independent Research forum. Lay out your ideas coherently so that they can be discussed. But please do yourself a favor and step back and try to more realistically evaluate your own knowledge. There is an extreme mismatch between the elementary mistakes you're making and your assessment of your own expertise as being superior to that of professional cosmologists like Ned Wright.

 

[Neandethal00]

This is definitely an alternative way of viewing the issue. As a phase transition. Time and space as something definite would have been born out of something far less definite. You could call it a fog, a vagueness, a pre-geometry, a perfect symmetry.

I can live with this foggy phase transition analogy.

Unfortunately, in no branch of science 'common sense reality' is replaced by 'mathematical reality' as much as it did in astronomy/astro-physics/astro-anything.

 

[Cosmo Novice]

You may have to re-invent physics for that, Cosmo!

Open and spatially euclidean flat cosmological models with 0 curvature are spatially infinite and temporally finite. This models assumes that the BB began geometrically at all points in space/time and does assume spatial infinity. This is well within the laws of physics and if space was determined to have 0 curvature then this would be the current model.

 

[dougal217]

The quadrupole and octupole modes of WMAP seem to have an interesting orientation along the ecliptic plane... A bounded, finite Universe with a center of gravity would account for a number of things, including the Pioneer anomaly, and wouldn't require either a hot BB, Inflation (which is in big trouble), but most of all, doesn't need a Cosmological Principle, which is unprovable

 

[Ken G]

But would it instead require the tooth fairy? That's not really facetious-- the problem with forming entirely new theories to fix some of the bugs in the old one is invariably that the bugs get replaced by gorillas.

 

[zahero_2007]

Infinity is an ill defined concept so the universe is not infinite

 

[DavidMcC]

Zahero, infinity is well defined, but it is a pure mathematical concept, and not useful in physics, except as an approximation (since 1/infinity = 0).
The mathematical singularity of a classical black hole is a good example.

 

[Tanelorn]

I understand that all finite universe possibilities have space time wrapping around around. Which is the most correct space or space time wrapping around?

 

[bcrowell]

Infinity is an ill defined concept so the universe is not infinite

No, this is incorrect. We have a FAQ on this: https://www.physicsforums.com/showthread.php?t=507003

I understand that all finite universe possibilities have space time wrapping around around. Which is the most correct space or space time wrapping around?

Only space.

 

[Primtall]

The last part wasn't exactly true. -

... The set of positive integers is infinite. The set of positive integers divisible by 2 is also infinite, and half the size. The set of positive integers divisible by any known prime number is infinite , and expanding (everytime a new prime number is discovered).