The Universe - infinite or not ?

[binbots]

There is room for our infinite universe with other infinite universe. There is infinite room in infinity. I think the question is how is our universe infinite. Are we inside a larger universe etc etc? Is our own universe apart of infinite space? Or are there infinite parellel universes. Either way it is all infinite. It is the same on the small scale. No matter how small we go we will always ask: well what is that made of? The answer to the questions of the universe is that there will always be more questions forever......

 

[Chronos]

infinity loses meaning nestled amongst a multiverse of infinities, imo - aside from the fact the concept of infinity is unmeaningful to begin with. Everything in nature has relational meaning.

 

[Neandethal00]

Infinity does not exist.
Let me phrase it the right way,
to humans infinity can not exist for any physical reality, because of our sensory systems.

 

[bcrowell]

FAQ: Is the universe finite, or is it infinite?

Standard cosmological models come in two flavors, open and closed. The open type has negative spatial curvature and infinite spatial volume. The closed one has positive curvature and finite spatial volume; spatially, it is the three-dimensional analog of a sphere. Since both types are mathematically self-consistent solutions to the Einstein field equations, the finiteness or infiniteness of the universe is something that cannot be determined by solely logic but only by observation.

Current observations of the cosmic microwave background's anisotropy show that our universe is very nearly spatially flat (on the cosmological scale). If it is exactly flat, then it is a special case lying between the more general open and closed cases. The flat case has infinite volume. However, the range of uncertainty in the curvature is wide enough to be consistent with either positive or negative curvature, so right now the finiteness or infiniteness of the universe is an open question.

Sometimes people use the word "universe" when they really mean "observable universe." The observable universe is finite in volume because light has only had a finite time to travel since the Big Bang.

 

[Chronos]

Good summary, bcrowell. An FAQ would be useful with a decent index.

 

[ViewsofMars]

The European Space Agency (ESA) Science and Technology: Glossary

Universe
Everything that exists. The size of the observable Universe is determined by the distance light has traveled since the Universe was formed in the Big Bang, 12 - 15 billion years ago.

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31489&fbodylongid=950

I think the ESA's glossary is quite informative.

 

[Mutsi]

Well, for all practical purposes, it may have the potential to continue expanding infinitely. However, it will never reach infinity (if it is finite now), and so cannot really be "on the verge of infinity".

Isnt it so that Dark Matter used to be stronger then Dark Energy ? And that the universe was contracting. How is it possible to state that the universe will keep expending perhaps the strenght of DM and DE cycle.

 

[Cosmo Novice]

This question may be the final unanswerable - obviously the OU is finite and measurable, anything outside of our OU can never be quantified. Assuming the Universe is spacially flat and homogeneous then cosmological models dictate an infinite size along its x,y,z axis.

However does time factor into this? As the second law of thermodynamics dictates the arrow of time moves only forwards then when the universe approaches the end of its life - as I understand it once matter and energy becomes separated and diffuse to the point the universe is in final heat death? Is this plausable given infinite energy and matter states?

If something expands inifinitely then while it is not infinite at any given moment it will expand to an an infinite size over infinite time but is this relevant as cosmo models indicate the final end of the universe will be a heat death?

 

[Chronos]

Olber's paradox refutes the infinitely old, spacious and star filled universe idea. The CMB refutes the old wives tale of infinite age all on its own.

 

[bcrowell]

This question may be the final unanswerable - obviously the OU is finite and measurable, anything outside of our OU can never be quantified.

Anything outside of our observable universe can certainly be quantified. Just wait a while, and it will be inside our observable universe.

Assuming the Universe is spacially flat and homogeneous then cosmological models dictate an infinite size along its x,y,z axis.

We can measure the universe's spatial curvature, so why assume it?

 

[Chronos]

How can anything outside the observable universe ever be quantified? I strongly disagree. Perhaps the source of our disagreement resides in the definition of what constitutes 'observable'.

 

[ViewsofMars]

Mutsi brought up Dark Matter. I'd like to present the lastest about it. April 14, 2011 from the Weizmann Institute of Science:

An International team of scientists in the XENON collaboration, including several from the Weizmann Institute, announced on Thursday the results of their search for the elusive component of our universe known as dark matter. This search was conducted with greater sensitivity than ever before. After one hundred days of data collection in the XENON100 experiment, carried out deep underground at the Gran Sasso National Laboratory of the INFN, in Italy, they found no evidence for the existence of Weakly Interacting Massive Particles – or WIMPs – the leading candidates for the mysterious dark matter. The three candidate events they observed were consistent with two they expected to see from background radiation. These new results reveal the highest sensitivity reported as yet by any dark matter experiment, while placing the strongest constraints on new physics models for particles of dark matter. Weizmann Institute professors Eilam Gross, Ehud Duchovni and Amos Breskin, and research student Ofer Vitells, made significant contributions to the findings by introducing a new statistical method that both increases the search sensitivity and enables new discovery.

Any direct observation of WIMP activity would link the largest observed structures in the Universe with the world of subatomic particle physics. While such detection cannot be claimed as yet, the level of sensitivity achieved by the XENON100 experiment could be high enough to allow an actual detection in the near future. What sets XENON100 apart from competing experiments is its significantly lower background radiation, which is 100 times lower, greatly reducing the potential obscuring of any dark matter signal. The XENON100 detector, which uses 62 kg of liquid xenon as its WIMP target, and which measures tiny charges and light signals produced by predicted rare collisions between WIMPs and xenon atoms, continues its search for WIMPs. New data from the 2011 run, as well as the plan to build a much larger experiment in the coming years, promise an exciting decade in the search for the solution to one of nature's most fundamental mysteries.

Cosmological observations consistently point to a picture of our universe in which the ordinary matter we know makes up only 17% of all matter; the rest – 83% – is in an as yet unobserved form – so-called dark matter. This complies with predictions of the smallest scales; necessary extensions of the Standard Model of particle physics suggest that exotic new particles exist, and these are perfect dark matter candidates. Weakly Interacting Massive Particles (WIMPs) are thus implied in both cosmology and particle physics. An additional hint for their existence lies in the fact that the calculated abundance of such particles arising from the Big Bang matches the required amount of dark matter. The search for WIMPs is thus well-founded; a direct detection of such particles would provide the central missing piece needed to confirm this new picture of our Universe.

Please read on. . .

http://wis-wander.weizmann.ac.il/experiment-narrows-dark-matter-range

 

[Cosmo Novice]

Anything outside of our observable universe can certainly be quantified. Just wait a while, and it will be inside our observable universe.


We can measure the universe's spatial curvature, so why assume it?

Well objects that come inside our OU will eventually be outside our OU once their recession >C. So while our OU may be growing now, at some point our OU will begin to shrink. as galaxies at the edge of our OU begin to recede >C.

Also while we can measure spatial curvature the degree of error still lends to pos,neg or flat curvature.

How can anything outside the observable universe ever be quantified? I strongly disagree. Perhaps the source of our disagreement resides in the definition of what constitutes 'observable'.

Do cosmo models not dictate an isotropic and homogenous U with only local variance - which does specifically quantify the unobservable by measuring the unobservable affect on the OU? So effectively although cosmo models are saying 'its just more of the same' then they can still safely make this quantifying assumption?

 

[narrator]

Infinity question: If we headed directly into space traveling at many times the speed of light (ignoring for a moment that you can't travel that fast), maintaining exactly the same course for the whole trip, would or could we eventually find ourselves heading back to Earth?

 

[Cosmo Novice]

Infinity question: If we headed directly into space traveling at many times the speed of light (ignoring for a moment that you can't travel that fast), maintaining exactly the same course for the whole trip, would or could we eventually find ourselves heading back to Earth?

This would depend entirely on the topology of U. If Euclidean and infinite then not as you would just travel indefintely.

If U was an n-sphere then yes.

I am sure someone can elaborate further but as this is off topic you may be better starting a fresh thread.

 

[narrator]

I am sure someone can elaborate further but as this is off topic you may be better starting a fresh thread.

Will do, thanks ;)

 

[George Jones]

Anything outside of our observable universe can certainly be quantified. Just wait a while, and it will be inside our observable universe.

If dark energy really is like a cosmological constant, then this isn't true for ours universe.

Well objects that come inside our OU will eventually be outside our OU once their recession >C. So while our OU may be growing now, at some point our OU will begin to shrink. as galaxies at the edge of our OU begin to recede >C.

I think that you confused the Hubble (sphere) radius with the cosmological event horizon.

For a flat universe that exponentially expands for all time, the Hubble radius is the cosmological event horizon, but (as in all universes) we never see anything cross the horizon, so we never see anything on the Hubble sphere.

In our universe, the Hubble sphere and the cosmological event horizon don't correspond, even in the distant future. If we can see galaxy A now, it will never disappear. At some future time, A will be "receding" with a speed greater than the speed of light, but, even after this time, we will see A with (exponentially) increasing redshift, and with increasing faintness. In principle, we will never lose sight of A. In fact, some stuff that we see now (for example, the CMB from the (near) the surface of last scattering) was outside the Hubble sphere when the light we now see started its journey.

 

[Cosmo Novice]

I think that you confused the Hubble (sphere) radius with the cosmological event horizon.

What is a cosmological event horizon? What I meant was that our OU will eventually (billions of years) consist of less galaxies as once a distant galaxy receeds>C and all light emiited prior to a recession>C reaches us then we will no longer see said galaxy. Over billions of years will this not be true of all galaxies, or will clusters/superclusters stay clumped?

Is this incorrect? A little more explanation would be nice.

Thanks

 

[Chronos]

The number of galaxies will not effectively change, merely the distance between them will increase and the CMB temperature will decrease. This is known as the 'heat death' of the universe.

 

[George Jones]

What is a cosmological event horizon?

Consider the following two disjoint subsets of spacetime:

1) those events that we have seen, or that we will see;

2) those events that we will never see.

The cosmological event horizon is the boundary between these two subsets of spacetime.

What I meant was that our OU will eventually (billions of years) consist of less galaxies as once a distant galaxy receeds>C and all light emiited prior to a recession>C reaches us then we will no longer see said galaxy.

As I said above this isn't true. It is true that recession speeds of galaxies that we now see will eventually exceed c, but it is not true that we loose sight of a galaxy once its recession speed exceeds c. If we see a galaxy now, then we will (in principle) always see the galaxy, even when its recession speed exceeds c. It might seem that moving to a recession speed of c represents a transition from subset 1) to subset 2), but this isn't the case.

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

 

[Khashishi]

BB doesn't say if the universe is finite or infinite. The expansion of the universe is formulated as a scaling of distances, not an increase in the size of the universe. You can have a scaling of distances in a finite or infinite universe. That said, an infinite universe can't have positive curvature (and be homogeneous, isotropic). Maybe dark energy is the universe constraining itself to not become positively curved, pushing back against the pull of gravity.

 

[Neandethal00]

Consider the following two disjoint subsets of spacetime:

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

Probably not correct. I argued with a few people in another forum whether photons have inertia or not. Eventually I realized it creates more problems, specially in experimental results with light, if we assume moving frames have no effect on photons.

Which means photons of galaxies receding with FTL speed may be traveling with the galaxies with FTL speed but photons speed inside the galaxy would remain the same c.

Btw, my logical mind says galaxies are not moving at FTL speed.

 

[George Jones]

Probably not correct.

This result can be derived from the stuff in the thread

https://www.physicsforums.com/showthread.php?t=491078.

I argued with a few people in another forum whether photons have inertia or not. Eventually I realized it creates more problems, specially in experimental results with light, if we assume moving frames have no effect on photons.

Which means photons of galaxies receding with FTL speed may be traveling with the galaxies with FTL speed but photons speed inside the galaxy would remain the same c.

Btw, my logical mind says galaxies are not moving at FTL speed.

 

[Cosmo Novice]

As I said above this isn't true. It is true that recession speeds of galaxies that we now see will eventually exceed c, but it is not true that we loose sight of a galaxy once its recession speed exceeds c. If we see a galaxy now, then we will (in principle) always see the galaxy, even when its recession speed exceeds c. It might seem that moving to a recession speed of c represents a transition from subset 1) to subset 2), but this isn't the case.

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

How will it "turn around" Can you please clarify this point.

I am in galaxy A looking at Galaxy B. At some point t in the future Galaxy B recession speed exceeds c. So surely at t the last photon ever released prior to galaxy B crossing the threshold into expansion>c is released. Once this photon gets to us would this not be the last photon we would ever see from Galaxy B?

In understand light will get redshifted, but assumed this requires expansion<c otherwise it would not be measurable.

If you can clarify this for me I would greatly appreciate this.

 

[Lost in Space]

If something is finite it can be quantified. Einstein suggested that the universe was finite but had an infinite boundary. And if something is finite the question is, is it confined to or contained in a bigger state of finity (and so on ad infinitum), or is infinity its ultimate container?

 

[George Jones]

I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55.

How will it "turn around" Can you please clarify this point.

Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:

O                                B        A        C
*                                *        *        **                    *     *     *
O                    B     A     C

The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, $t_e$. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, $t$. Notice that space has "expanded" between times $t_e$ and $t$.

Suppose that at time $t_e$: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time $t$, galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because A's recession speed at time $t_e$ is greater than c, the pulse fired towards us has actually moved away from us between times $t_e$ and $t$.

Now, suppose that the distance from us to galaxy B at time $t$ is the same as the distance to galaxy C at time $t_e$. Even though the distances are the same, the recession speed of B at time $t$ is less than than the recession speed of C at time $t_e$ because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times $t_e$ and $t$.

Since A's recession speed at time $t_e$ is greater than c, and galaxy C is farther than A, galaxy C's recession speed at time $t_e$ also is greater than c. If, however, the Hubble constant decreases enough between times $t_e$ and $t$, then B's recession speed at time $t$ can be less than c. If this is the case, then at time $t$ (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times $t_e$ and $t$.

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time $t_e$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time $t$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

 

[Astralos]

The universe is mathematically approaching the concept of infinity, but is, and will never be, itself infinite. Nothing quantitative in the known universe can be infinite.

 

[Cosmo Novice]

I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55.


Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:

O                                B        A        C
*                                *        *        *


*                    *     *     *
O                    B     A     C

The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, $t_e$. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, $t$. Notice that space has "expanded" between times $t_e$ and $t$.

Suppose that at time $t_e$: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time $t$, galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because B's recession speed at time $t_e$ is greater than c, the pulse fired towards us has actually moved away from us between times $t_e$ and $t$.

Now, suppose that the distance from us to galaxy B at time $t$ is the same as the distance to galaxy C at time $t_e$. Even though the distances are the same, the recession speed of B at time $t$ is less than than the recession speed of C at time $t_e$ because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times $t_e$ and $t$.

Since B's recession speed at time $t_e$ is greater than c, galaxy C's recession speed at time $t_e$ also is greater than c. If, however, the Hubble constant decreases enough between times $t_e$ and $t$, then B's recession speed at time $t$ can be less than c. If this is the case, then at time $t$ (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times $t_e$ and $t$.

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time $t_e$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time $t$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

Thankyou for taking the time to explain. This does make complete sense except one thing:

This assumes that for galaxies whose recession>c for their photons to reach us then there must be a decrease in the Hubble constant. I thought the Hubble constant was the rate of acceleration of expansion and as such would always increase? I understand the constant referes to spatially constant (any given point in space will be the same constant as any other place at the same time) but am unclear whether this is increasing/decreasing.

I am a complete novice so appreciate the simpligied explanation you gave.

Thanks

 

[George Jones]

This assumes that for galaxies whose recession>c for their photons to reach us then there must be a decrease in the Hubble constant. I thought the Hubble constant was the rate of acceleration of expansion and as such would always increase? I understand the constant referes to spatially constant (any given point in space will be the same constant as any other place at the same time) but am unclear whether this is increasing/decreasing.

The definition of the Hubble constant $H$ is
$$H = \frac{\mbox{rate at which scale increases}}{\mbox{scale of the universe}}.$$
The universe expands with time, so the scale of the universe increases with time. Accelerated expansion means that the rate at which the scale increases itself increases, i.e., the rate tomorrow at which the scale increases is greater than rate today at which the scale increases. If, over a given period of time, the increase in the scale of the universe is proportionately greater than the increase in the rate at which the scale increases, then the Hubble constant decreases with time (since the denominator increases faster than the numerator. Observations indicate that this true now, and that this will remain true in the future.

I might later post a specific example.

 

[Cosmo Novice]

The definition of the Hubble constant $H$ is
$$H = \frac{\mbox{rate at which scale increases}}{\mbox{scale of the universe}}.$$
The universe expands with time, so the scale of the universe increases with time. Accelerated expansion means that the rate at which the scale increases itself increases, i.e., the rate tomorrow at which the scale increases is greater than rate today at which the scale increases. If, over a given period of time, the increase in the scale of the universe is proportionately greater than the increase in the rate at which the scale increases, then the Hubble constant decreases with time (since the denominator increases faster than the numerator. Observations indicate that this true now, and that this will remain true in the future.

I might later post a specific example.

Thankyou for the explanation. Although I still find this very counter-intuitive, although I can see the logic behind galaxies whose recession>c photons still reaching us. I do not think I require a specfic example in this case but thankyou.

Ok so while I know see the logic in galaxies with recession>c light reaching us - giving certain circusmtance. Am I safe in assuming that beyond the OU current cosmological models indicate galaxies so far away and receeding so much >c that their light will never reach us?

I guess the core question I am posing is: Beyond our OU, is there a cutoff point, in terms off recession speeds>c where we will no longer receive photons from galaxies further out than this cutoff point?

 

[Lost in Space]

Aren't scales just another example of the strong anthropic principle?

 

[Chronos]

Only to the extent necessary for the universe to be sufficiently large and ancient to permit our existence at this point in its history. Our efforts to measure scale factors is motivated by curiosity about the origins and destiny of the universe, not anthropic principles.

 

[Lost in Space]

Only to the extent necessary for the universe to be sufficiently large and ancient to permit our existence at this point in its history. Our efforts to measure scale factors is motivated by curiosity about the origins and destiny of the universe, not anthropic principles.

But if the laws of physics break down at the boundaries, aren't these scale factors merely reduced to a human perspective and, given that, are they any closer to describing reality or are they merely a reflection on what we consider important in relation to ourselves?

 

[Fortnum]

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.

 

[bcrowell]

I guess the core question I am posing is: Beyond our OU, is there a cutoff point, in terms off recession speeds>c where we will no longer receive photons from galaxies further out than this cutoff point?

If the universe is spatially infinite, then yes.

But if the laws of physics break down at the boundaries, aren't these scale factors merely reduced to a human perspective and, given that, are they any closer to describing reality or are they merely a reflection on what we consider important in relation to ourselves?

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.