Why do some people believe the universe is infinite?

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Why do some people believe the universe is infinite?

Postby scratch123 » Thu Nov 08, 2012 7:50 pm UTC

The more I learn about physics the less likely that the universe seems like it could be infinite. First of all of the universe started from a single point and expanded. I have never heard of any infinite object being able to expand. Then there is the idea of the continuum which implies particles and space can be divided an infinite number of times. This is proving less and less likely to be true thanks to quantum mechanics and loop quantum gravity. There is also the idea of renormalization which basically gets rid of infinites. By getting rid of infinites certain quantum field theories were made more complete.

Also since physics is moving away from the concept of infinity I think math should follow this trend as well. I think that the concept of infinity could be replaced by the concepts of repetition and variables since infinity is basically an unknown number of repetitions of something.

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Re: Why do some people believe the universe is infinite?

Postby brenok » Thu Nov 08, 2012 8:52 pm UTC

scratch123 wrote:I think that the concept of infinity could be replaced by the concepts of repetition and variables since infinity is basically an unknown number of repetitions of something.


How, exactly, would Math benefit from this?

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Re: Why do some people believe the universe is infinite?

Postby dudiobugtron » Thu Nov 08, 2012 9:31 pm UTC

scratch123 wrote:First of all of the universe started from a single point and expanded. I have never heard of any infinite object being able to expand.

I've never heard of an infinite object that isn't able to expand. Do you have an example of one?

Then there is the idea of the continuum which implies particles and space can be divided an infinite number of times. This is proving less and less likely to be true thanks to quantum mechanics and loop quantum gravity. There is also the idea of renormalization which basically gets rid of infinites. By getting rid of infinites certain quantum field theories were made more complete.

I think a pretty fundamental physics idea is that there are parts of the universe that we can never, and will never, be able to know anything about at all.
I'm not sure it's OK to just say "the known universe is finite therefore the entire universe is finite".

Also since physics is moving away from the concept of infinity I think math should follow this trend as well. I think that the concept of infinity could be replaced by the concepts of repetition and variables since infinity is basically an unknown number of repetitions of something.

Maths has been moving away from infinity for a long time now. In fact, it can't move away from it fast enough! ;)
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Re: Why do some people believe the universe is infinite?

Postby starslayer » Thu Nov 08, 2012 9:33 pm UTC

scratch123 wrote:First of all of the universe started from a single point and expanded.
It did no such thing if it is in fact infinite. The Big Bang happened everywhere, all at once. If the universe is homogenous and isotropic everywhere so that the FLRW metric holds, the universe can be closed (constant positive curvature), flat (zero curvature), or open (constant negative curvature). If it is flat or open (the two most likely possibilities, given the current results from WMAP, Ia SNe, and baryon acoustic oscillations), then the universe is infinite and has always been so. Besides which, our current theories say nothing about t=0; they all break down before you get there. Therefore, we can't make any definite statement about the very beginning of the universe, but we can be on fairly solid ground pretty close to the very beginning.

I have never heard of any infinite object being able to expand.
This is a failure of imagination on your part, not a fault of infinite objects. Infinite objects can very easily expand and contract.

Then there is the idea of the continuum which implies particles and space can be divided an infinite number of times. This is proving less and less likely to be true thanks to quantum mechanics and loop quantum gravity. There is also the idea of renormalization which basically gets rid of infinites. By getting rid of infinites certain quantum field theories were made more complete.
I fail to see what relevance these two things have to do with the size of the universe.

Also since physics is moving away from the concept of infinity I think math should follow this trend as well. I think that the concept of infinity could be replaced by the concepts of repetition and variables since infinity is basically an unknown number of repetitions of something.
What brenok said.

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Re: Why do some people believe the universe is infinite?

Postby gmalivuk » Thu Nov 08, 2012 10:25 pm UTC

scratch123 wrote:Also since physics is moving away from the concept of infinity I think math should follow this trend as well. I think that the concept of infinity could be replaced by the concepts of repetition and variables since infinity is basically an unknown number of repetitions of something.
Tell you what:

First you come up with a powerful and internally consistent finite replacement for mathematics as we now do it. Then post in here again suggesting that we move away from the concept of infinity.

Not the other way around.
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Re: Why do some people believe the universe is infinite?

Postby Tchebu » Fri Nov 09, 2012 1:40 am UTC

Most areas of math are already formulated in a way where "infinity" isn't an actual object but rather shorthand for "just keep going". So you're not really suggesting anything groundbreakingly new with your suggestion of "repetition". For example the natural numbers are already defined in terms of a "successor" function repeatedly acting on an abstract object that we call "zero".
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Re: Why do some people believe the universe is infinite?

Postby letterX » Fri Nov 09, 2012 2:40 am UTC

starslayer wrote:
scratch123 wrote:First of all of the universe started from a single point and expanded.
It did no such thing if it is in fact infinite. The Big Bang happened everywhere, all at once. If the universe is homogenous and isotropic everywhere so that the FLRW metric holds, the universe can be closed (constant positive curvature), flat (zero curvature), or open (constant negative curvature). If it is flat or open (the two most likely possibilities, given the current results from WMAP, Ia SNe, and baryon acoustic oscillations), then the universe is infinite and has always been so. Besides which, our current theories say nothing about t=0; they all break down before you get there. Therefore, we can't make any definite statement about the very beginning of the universe, but we can be on fairly solid ground pretty close to the very beginning.

This is getting off topic a bit, but it seems to me like there isn't any immediate contradiction with the universe being infinite, and having been infinite at all times t > 0, but being a single point at t = 0.

Approximate the universe as R^3, and model expansion by letting a point x be at t*x for every t in [0, 1]. Obviously, this isn't a good model for what we observe the universal expansion to be, since it isn't isotropic (everything is expanding away from the origin, not every point expanding away from every other point equally) but it does show that there are continuous maps where the universe is infinite at all t > 0, but a single point at t = 0. Am I missing something, or is this a pretty good handwavey model for singularity. It does have the advantage that even though the universe was initially a single point, the big bang nevertheless still happened everywhere (since at t = 0, everything is the same point).

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Re: Why do some people believe the universe is infinite?

Postby starslayer » Fri Nov 09, 2012 3:03 pm UTC

letterX, I'm not following your reasoning; could you be more explicit?

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Re: Why do some people believe the universe is infinite?

Postby letterX » Fri Nov 09, 2012 3:36 pm UTC

The main point I guess I was making is the following: Even assuming that the universe evolves continuously and that it is infinite in size at all times t > 0, it doesn't necessarily have to be infinite at time t = 0.

For example, consider the function H(x, t) where x is a position in R^3 (or just 1D space R, or whatever vector space you want) and t is time ranging in [0, 1] and let H(x, t) = t*x. So, if x is the position of a given point at time t = 1, then as we vary t from 0 to 1, t*x moves in a straight line from the origin to x.

Note that, for any fixed t > 0, the range of H(x, t) is all of R^3 (in the analogy to expansion, the universe is infinite in size at all t > 0) however at t = 0, the range of H(x, t) is just {0}, since 0*x is 0 for all x.

I'm not trying to actually explain expansion in this way, so this is clearly nonsense for many reasons (like, for instance, I'm embedding the universe in an "ambient" R^3, which goes against many things observed about GR) but just making the point that "is infinite in size" isn't a closed property of spaces.

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Re: Why do some people believe the universe is infinite?

Postby Aiwendil » Fri Nov 09, 2012 5:34 pm UTC

scratch123 wrote:First of all of the universe started from a single point and expanded.


We don't know that. All that we know is that about 13.75 billion years ago, the universe was extremely hot and dense, and it's been expanding and cooling ever since. I don't think I'd trust any description of what the universe was like back beyond the Planck Era until we have an adequate quantum theory of gravity.

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Re: Why do some people believe the universe is infinite?

Postby eSOANEM » Sun Nov 11, 2012 3:35 pm UTC

scratch123 wrote:First of all of the universe started from a single point


Nope.

To elaborate. This is highly unlikely. For one thing, it would imply a privileged location which could put momentum conservation in trouble.
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Re: Why do some people believe the universe is infinite?

Postby Diadem » Sun Nov 11, 2012 5:12 pm UTC

letterX wrote:The main point I guess I was making is the following: Even assuming that the universe evolves continuously and that it is infinite in size at all times t > 0, it doesn't necessarily have to be infinite at time t = 0.

For example, consider the function H(x, t) where x is a position in R^3 (or just 1D space R, or whatever vector space you want) and t is time ranging in [0, 1] and let H(x, t) = t*x. So, if x is the position of a given point at time t = 1, then as we vary t from 0 to 1, t*x moves in a straight line from the origin to x.

Note that, for any fixed t > 0, the range of H(x, t) is all of R^3 (in the analogy to expansion, the universe is infinite in size at all t > 0) however at t = 0, the range of H(x, t) is just {0}, since 0*x is 0 for all x.

I'm not trying to actually explain expansion in this way, so this is clearly nonsense for many reasons (like, for instance, I'm embedding the universe in an "ambient" R^3, which goes against many things observed about GR) but just making the point that "is infinite in size" isn't a closed property of spaces.

While this is true, mathematically, it runs into trouble as a physical explanation. Your rate of expansion would have to be infinite. That requires infinite energy density. And not just infinite in the sense of 'a lot' but truly, mathematically, infinite. That's not very physical. And where did that expansion go? We're currently not expanding infinitely fast. So the expansion slowed down? From Infinity?

Either the universe is finite and has always been finite, or it's infinite and has always been infinite. Anything else is physically extremely problematic.
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Re: Why do some people believe the universe is infinite?

Postby FancyHat » Sun Nov 11, 2012 6:18 pm UTC

Diadem wrote:And where did that expansion go? We're currently not expanding infinitely fast. So the expansion slowed down? From Infinity?

I don't think you understood the mathematics. There's no slowing down of the expansion in that bit of mathematics. A particle travelling with the expansion, so that it's position is t*x at time t, will always be moving at a velocity of x, right from the very start. There's no slowing down at all.

Either the universe is finite and has always been finite, or it's infinite and has always been infinite. Anything else is physically extremely problematic.

We already know that we don't have any physics with which to describe the universe at the absolute start of time, or for times very shortly after.

What the bit of mathematics did was to illustrate that it is concievable that an infinitely large universe could start off with zero size at the very beginning of time. Yes, it does instantaneously go from zero size to infinite size, but that's not a problem. There would be no part of that universe that would ever move at infinite speed since there is no infinitely large real number.

If you disagree, pick a finite x so that a particle at position x at time t=1 would have had to move at infinite speed at some point.
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Re: Why do some people believe the universe is infinite?

Postby doogly » Mon Nov 12, 2012 1:18 am UTC

Going from a finite universe to an infinite universe is is an infinite expansion, which is not possible under current physics, and afterwards requires you to slow down from infinite expansion to some finite rate of expansion, which is an infinite deceleration, which is also not possible. Please listen to Diadem.

In general, no evolution under GR is going to give you the topology changing necessary to go finite-->infinite.
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Re: Why do some people believe the universe is infinite?

Postby FancyHat » Mon Nov 12, 2012 3:54 am UTC

doogly wrote:Going from a finite universe to an infinite universe is is an infinite expansion, which is not possible under current physics, and afterwards requires you to slow down from infinite expansion to some finite rate of expansion, which is an infinite deceleration, which is also not possible. Please listen to Diadem.

I don't think you've understood letterX's point, either.

Draw two axes of a graph, a horizontal x axis and a vertical t axis. Consider all straight lines of the form x=kt for all real k. All such lines pass through the origin, having x=0 at t=0. For any t>0, there is no x which doesn't have a line passing through it at (x,t). If you draw a horizontal line at that particular t, every point on that line, (x,t), will also be a point on the line x=kt where k=x/t. The only exception to this is at t=0, where the only point at which the lines pass through is (0,0).

At no point do any of the straight lines have to curve, have corners, change gradient, accelerate, decelerate, or anything else. There is no change in the rate of expansion at any time.

Of course, like letterX said, this isn't a realistic model of the expansion of the universe, it's just to make the point that there's no real problem with the idea of going from zero size at t=0 to infinite size for all t>0.

Another illustration would be the future light cone, and everything within it, from a single point in Minkowskian spacetime at time t=0, which I shall call the origin. For simplicity, we can have just one single dimension of space. Consider a non-zero time-like separation s. If you plot a curve of all the points separated from the origin by s after t=0, you'll have an endless curve that stretches infinitely far in both directions, with the future light cone as asymptotes. If you imagine placing lots of little measuring rods along that curve to measure it's length, you'll find it's infinite. If you apply a Lorentz boost, you'll end up with an identical graph (with all the points shifted along it), no matter how big that Lorentz boost is. No matter how small that time-like separation s is, as long as it's non-zero, space will be infinitely big at that separation after the origin. But for a zero time-like separation from the origin, you'll only have the future light cone itself, all points on which have zero separation from the origin.
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Re: Why do some people believe the universe is infinite?

Postby gmalivuk » Mon Nov 12, 2012 4:00 am UTC

Yes, we get that you can say that happens mathematically. The point is there's no theoretical basis for saying it might have actually happened that way. So when physicists say an infinite universe was always infinite, they aren't claiming that it's mathematically impossible to have a set that's measure 0 at t=0 but infinite in extent for all t>0.
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Re: Why do some people believe the universe is infinite?

Postby FancyHat » Mon Nov 12, 2012 4:51 am UTC

gmalivuk wrote:Yes, we get that you can say that happens mathematically. The point is there's no theoretical basis for saying it might have actually happened that way. So when physicists say an infinite universe was always infinite, they aren't claiming that it's mathematically impossible to have a set that's measure 0 at t=0 but infinite in extent for all t>0.

I was addressing doogly's claim that "Going from a finite universe to an infinite universe is is an infinite expansion ... and afterwards requires you to slow down from infinite expansion to some finite rate of expansion, which is an infinite deceleration" (emphasis mine). letterX's bit of maths didn't involve any such deceleration. doogly's claim was simply incorrect. It appeared clear that doogly didn't get letterX's mathematical point. Similarly with Diadem's question, "So the expansion slowed down? From Infinity?"

I think what both Diadem and doogly were thinking was that since letterX illustration involved an instantaneous transition from zero size to infinite size, the expansion, in some sense, must have been infinite, infinitely rapid, instantaneously going from zero size to infinite. But then, after that instant, for all times t>0, the expansion was finite, in that anything a finite distance away would be moving away at a finite speed. In that different sense, the expansion was finite, not infinite. But the size of such a space is not the same thing as the velocities within it. There was simply no deceleration in letterX's universe. doogly's claim that there would have to be "infinite deceleration" was simply incorrect.
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Re: Why do some people believe the universe is infinite?

Postby doogly » Mon Nov 12, 2012 5:08 am UTC

You are bringing lines to a discussion about general relativity. I'm pointing at the moon and you're looking at my finger.

If you want to make a distinction between the "size of the space" and "the velocities in it," I invite you to choose a metric with the property described and determine its curvature.
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Re: Why do some people believe the universe is infinite?

Postby PM 2Ring » Mon Nov 12, 2012 5:49 am UTC

doogly wrote:You are bringing lines to a discussion about general relativity. I'm pointing at the moon and you're looking at my finger.

If you want to make a distinction between the "size of the space" and "the velocities in it," I invite you to choose a metric with the property described and determine its curvature.


Sure. But letterX clearly stated that his model doesn't apply to our universe and that it's inconsistent with what GR says. All letterX was doing was coming up with a simple toy "universe" that had zero size at t=0 and infinite size at t>0. It's a purely mathematical construction, with no attempt to model a physical universe.

Still, when I first read that post, I was tempted to reply: I see what you're saying, but don't say that!, it'll only confuse people. :)

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Re: Why do some people believe the universe is infinite?

Postby FancyHat » Mon Nov 12, 2012 6:02 am UTC

doogly wrote:You are bringing lines to a discussion about general relativity. I'm pointing at the moon and you're looking at my finger.

letterX's illustration didn't involve any deceleration, infinite or otherwise. Moons and fingers don't change that.

letterX's point was that the universe being infinite at all times t>0 doesn't itself have to mean that the universe was infinite at time t=0. This was to do with scratch123's remark that "the universe started from a single point and expanded". starslayer disagreed with that remark, saying the universe "did no such thing if it is in fact infinite." letterX was pointing out that an infinite universe at times t>0 didn't automatically rule out the possibility of a finite, zero-sized universe at time t=0.

For all I know, General Relativity may well rule out such a possibility, though I thought GR was known not to apply at time t=0. But since little is known about the Planck Epoch anyway, it seems unsafe to assert, so certainly, that a subsequently infinite universe can't possibly have started off finitely or zero sized.

If you want to make a distinction between the "size of the space" and "the velocities in it," I invite you to choose a metric with the property described and determine its curvature.

Wasn't my Minkowskian illustration sufficient?
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Re: Why do some people believe the universe is infinite?

Postby Tchebu » Mon Nov 12, 2012 6:49 am UTC

If you want to make a distinction between the "size of the space" and "the velocities in it," I invite you to choose a metric with the property described and determine its curvature.


[imath]ds2= -dt2+t2(dX2+sinh2X dΩ2)[/imath]

This is a metric where the spacial slice clearly has zero size at t=0 and infinite size at all other times. This metric describes Minkowski space.
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Re: Why do some people believe the universe is infinite?

Postby letterX » Mon Nov 12, 2012 8:29 am UTC

Well rats, I seem to have made a mess of things. Anyways, I mostly brought it up because it seems like the obvious thing to ask when you hear about (for instance) homotopies of R^3 to a single point. But apparently doing this is unphysical, which I didn't know, not being a physicist. Actually, whether such a thing fit at all with GR was secretly my question in making my post...

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Re: Why do some people believe the universe is infinite?

Postby doogly » Mon Nov 12, 2012 2:33 pm UTC

Tchebu wrote:
If you want to make a distinction between the "size of the space" and "the velocities in it," I invite you to choose a metric with the property described and determine its curvature.


[imath]ds2= -dt2+t2(dX2+sinh2X dΩ2)[/imath]

This is a metric where the spacial slice clearly has zero size at t=0 and infinite size at all other times. This metric describes Minkowski space.


But if this is going to be a cosmological model and not just a perverse coordinate choice, it requires a Hubble parameter of 1/t, and an energy density for your bulk fluid of 1/t^2. You only get cosmic evolution of this sort with infinities. (mea culpa that I shouted out curvature components though.)
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Re: Why do some people believe the universe is infinite?

Postby Tchebu » Mon Nov 12, 2012 8:42 pm UTC

Fair enough, but that's a statement about the matter content of the universe and its stress tensor and in particular shouldn't depend on the choice of spacial slices. My only point was that the size of spacial slices doesn't necessarily tell us about any violence in the physics or lack thereof.
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Re: Why do some people believe the universe is infinite?

Postby doogly » Mon Nov 12, 2012 8:51 pm UTC

Yes. A better response to the entire thread might have been at the beginning to say "you are focusing on the size of spacial slices which isn't actually all that relevant for cosmological history."
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Re: Why do some people believe the universe is infinite?

Postby Badhands » Fri Feb 08, 2013 3:35 pm UTC

I have always thought of it as a finite amount of mass and energy expanding throughout an infinite space. Therefore I view the universe as infinite space with a finite amount of content. That is the simplest explanation I've come up with that (as far as I know) doesn't conflict with any of the complex maths above. Is that explanation flawed in any way? What are the arguments against that view?

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Re: Why do some people believe the universe is infinite?

Postby el matematico » Fri Feb 08, 2013 11:32 pm UTC

Your view contradicts the cosmological principle that states that the universe is roughly the same everywhere. In your model all energy is inside a limited space, and there are regions of infinite size with nothing in them (though I guess that wouldn't take into account dark energy and dark mass).
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Re: Why do some people believe the universe is infinite?

Postby eSOANEM » Sat Feb 09, 2013 12:28 am UTC

Badhands wrote:I have always thought of it as a finite amount of mass and energy expanding throughout an infinite space. Therefore I view the universe as infinite space with a finite amount of content. That is the simplest explanation I've come up with that (as far as I know) doesn't conflict with any of the complex maths above. Is that explanation flawed in any way? What are the arguments against that view?


It conflicts entirely with current cosmology.

Not only does it violate cosmological homogeneity (and isotropy), it also fails to explain why we see newer and newer matter further and further away (at least not without placing us at the centre of all the mass in the universe (which must be spherical) and happens to be the same size as predicted by proper theories of universal expansion). At best it requires far more fine tuning in order to produce equivalent predictions (of short-term phenomena at least) and fine-tuning is bad.

The important thing is that the universe is not expanding into anything at all. It is simply that the gaps between things are getting bigger.
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Re: Why do some people believe the universe is infinite?

Postby stianhat » Mon Feb 11, 2013 12:13 pm UTC

Well, just to chip in a short question.

It is perfectly fine in mathematics to have an object that has a finite volume but an infinite surface. Which is being discussed here, volume of the early universe, or its surface? (Or unit vectors of the same)

What my biggest boggle is in all this is: Given the universe has an infinite size (as in volume) but a finite mass and energy, this necessarily means that the energy and mass density of the universe should be 0. as in zip, zilch, nothing at all. But - we observe a microwave background radiation which makes the temperature of the universe >0K. Now, I have always assumed this to mean that everywhere else in the universe you should observe the same background radiation, but I might be wrong in that. But assuming that is so, we have an infinite sized universe with a finite amount of energy having a nonzero temperature. :shock:

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Re: Why do some people believe the universe is infinite?

Postby Tass » Mon Feb 11, 2013 2:02 pm UTC

stianhat wrote:...but a finite mass and energy...


What makes you think that?

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Re: Why do some people believe the universe is infinite?

Postby stianhat » Mon Feb 11, 2013 2:52 pm UTC

Tass wrote:What makes you think that?


http://map.gsfc.nasa.gov/universe/uni_matter.html

"WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). This is equivalent to a mass density of 9.9 x 10-30 g/cm3, which is equivalent to only 5.9 protons per cubic meter"

Again, finite density. If the size universe is infinite, how can it have a finite density. I can understand it if the shape of the universe is such that its surface is infinite but the volume is finite, but infinite volum and finite density? hmm.

But then again, I am a chemisist, I am not supposed to understand it

Edit; infinite size and finite density could work, if infinite mass, but still I swear I have seen somewhere someone describing the universe as finite mass. On the other hand, that might have been the observeable universe. I retract my stupidity, thanks for the patience.

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Re: Why do some people believe the universe is infinite?

Postby Diadem » Mon Feb 11, 2013 3:39 pm UTC

stianhat wrote:It is perfectly fine in mathematics to have an object that has a finite volume but an infinite surface. Which is being discussed here, volume of the early universe, or its surface? (Or unit vectors of the same)

Volume. The universe has no surface.

stianhat wrote:Again, finite density. If the size universe is infinite, how can it have a finite density. I can understand it if the shape of the universe is such that its surface is infinite but the volume is finite, but infinite volum and finite density? hmm.

Yes. Take a chessboard. It has 64 squares, 32 black, 32 white. What is the density of black squares? Well, 50% of course. Now imagine an chessboard of infinite size. The density of black squares is still 50%, so finite. But the total number of black squares is infinite.

It's perfectly possible to have an infinite total mass and a finite mass density, given that the universe is infinite.

Edit; infinite size and finite density could work, if infinite mass, but still I swear I have seen somewhere someone describing the universe as finite mass. On the other hand, that might have been the observeable universe. I retract my stupidity, thanks for the patience.

Ah, I understand the confusion.

Scientists like being concise. They will often leave out context when they assume the listener knows it. When astronomers talk about the properties of the universe, they are generally talking about the visible (or observable) universe. Since the universe has a finite age, and light as a finite speed, the visible universe is finite. So it's total mass is also finite.

In the same vein, they will often talk about the 'mass of the universe' when they mean the energy density. Just because 'mass' is a shorter word, so it's easier to use. Physicists do that all the time. For example "Lagrangian" and "Lagrangian Density" are entirely different things. But physicists like randomly taking one of those two terms and randomly applying it to one of those two concepts, and then act like it's your problem if you get confused.
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Re: Why do some people believe the universe is infinite?

Postby lorb » Mon Feb 11, 2013 4:42 pm UTC

scratch123 wrote:I think that the concept of infinity could be replaced by the concepts of repetition and variables since infinity is basically an unknown number of repetitions of something.


Besides all the trouble you have to go through formalising your claim in a mathematically sound way, "an unknown number of repetitions" is always countable many and there is another kind of infinity that you won't be able to represent this way: uncountable infinity
If you start talking about doing away with infinity, knowing the different types of infinity is the very least you will have to know about. (hint: there is more than 2)
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Re: Why do some people believe the universe is infinite?

Postby stianhat » Tue Feb 12, 2013 12:37 pm UTC

Diadem wrote:Volume. The universe has no surface.


Yeah. well. umm. what? Now, I understand the concept of infinite size must mean its surface is hard to describe, but it needs to have one... doesn't it? At the big bang, everything was contained in a small crunched up piece of space and since that it has expanded - volumetrically. Does this not necessitate having a surface to expand? I realize this ends up in the pointless question of what is outside the surface of the universe, which is as pointless to discuss as what happened before the big bang - but does it not at least need to have an inner surface?

Diadem wrote:Scientists like being concise. They will often leave out context when they assume the listener knows it. When astronomers talk about the properties of the universe, they are generally talking about the visible (or observable) universe. Since the universe has a finite age, and light as a finite speed, the visible universe is finite. So it's total mass is also finite.

In the same vein, they will often talk about the 'mass of the universe' when they mean the energy density. Just because 'mass' is a shorter word, so it's easier to use. Physicists do that all the time. For example "Lagrangian" and "Lagrangian Density" are entirely different things. But physicists like randomly taking one of those two terms and randomly applying it to one of those two concepts, and then act like it's your problem if you get confused.


Hah, yes. Quite correct. I often do that in my speciality when I think about it. Chemistry can be rough too. I find that the chief difference between chemistry and physic majors / PhDs (in my experience) is not that there are any difference to how theoretical they apply themselves to a topic, merely how. Chemisists tend to need to understand it or somehow visualize it, at least to themselves, while I have countless times seen physics people rant about not understanding something, then someone shows them an equation and they go "ah. huh. excellent".

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Re: Why do some people believe the universe is infinite?

Postby firechicago » Tue Feb 12, 2013 1:42 pm UTC

stianhat wrote:Yeah. well. umm. what? Now, I understand the concept of infinite size must mean its surface is hard to describe, but it needs to have one... doesn't it? At the big bang, everything was contained in a small crunched up piece of space and since that it has expanded - volumetrically. Does this not necessitate having a surface to expand? I realize this ends up in the pointless question of what is outside the surface of the universe, which is as pointless to discuss as what happened before the big bang - but does it not at least need to have an inner surface?

"Piece of space" implies that the Big Bang happened at a specific place, or at least it happened in some places and not others. If that were so, the cosmic microwave background wouldn't be perfectly uniform, as we observe it to be.

Instead, we are pretty certain that the Big Bang occurred everywhere all at once.

The standard metaphor for the subsequent expansion is to imagine two ants on a balloon. If you blow up the balloon the surface expands and the ants move further apart, but that doesn't imply that the surface of the balloon has a one dimensional edge somewhere where it's expanding into. Similarly the three dimensional universe expands without having a two dimensional surface where it expands into.

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Re: Why do some people believe the universe is infinite?

Postby stianhat » Tue Feb 12, 2013 2:09 pm UTC

firechicago wrote:"Piece of space" implies that the Big Bang happened at a specific place, or at least it happened in some places and not others. If that were so, the cosmic microwave background wouldn't be perfectly uniform, as we observe it to be.

Instead, we are pretty certain that the Big Bang occurred everywhere all at once.


Yes, this was why I referred to it as "piece of space" to avoid confusing it with a point in space - seeing as a point needs a location, or else it is not a point. And I do understand that everything we see around us, the entire universe was on the inside of this crunched up space. This I understand. The confusion and uncertainity follows below:

The whole idea of big bang rests on that it was the driving force for the expansion during the early days of the universe, no? Such an expansion tends to need a surface to expand or else the cooling of the universe was not adiabatic. (In which case heat was transferred from the universe to some fantasy "environment" which would make Lord Kelvin rotate in his grave) - This kinda upsets me, because "stretching" the fabric of the universe within the same surface / unchanged volume does not remove energy from the system unless you add a lot of significantly cooler "nothing". In which case mass was added to the system. Or negative energy - should something like it exist.

I am not arguing against yall, just wrestling with my own lack of understanding.

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Re: Why do some people believe the universe is infinite?

Postby doogly » Tue Feb 12, 2013 2:26 pm UTC

Thermodynamics in curved space is a high reward but high demand study.
But there's nothing like a surface or boundary, it's the whole thing that is expanding. There are acoustic horizons and the like though.
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Re: Why do some people believe the universe is infinite?

Postby mathmannix » Wed Feb 13, 2013 9:39 pm UTC

Personally, I think the universe is infinite, as in there is an infinite expanse of nothingness after the farthest-away particles. The smallest volume (sphere or whatever) enclosing all matter in the universe is (probably) finite, but there's infinite space beyond, with nothing it.
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Re: Why do some people believe the universe is infinite?

Postby Diadem » Wed Feb 13, 2013 9:44 pm UTC

mathmannix wrote:Personally, I think the universe is infinite, as in there is an infinite expanse of nothingness after the farthest-away particles. The smallest volume (sphere or whatever) enclosing all matter in the universe is (probably) finite, but there's infinite space beyond, with nothing it.

No, no, no, no, no, no, NO.

Why do people keep doing that? It has been explained why this is wrong several times in this very thread, and dozens of times in other threads. Why would you post the above? What's the point?
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Re: Why do some people believe the universe is infinite?

Postby mathmannix » Wed Feb 13, 2013 10:19 pm UTC

The point is that I disagree. Why does the universe have to be the same everywhere? And wouldn't the expansion of the matter in the universe be consistent with the idea that the energy is all in one finite portion, but is expanding naturally as particles and energy bounce around?
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