## Need Help Understanding Expanding Universe

Hello,

I am hoping someone can help me to understand that the universe is expanding and other questions that brings up. If it is expanding, does that mean that there is an edge or a place that the universe has not reached yet but will reach at some point? What would be in the space that it has not reached yet? Are we able to see the entirety of the universe in all directions? Meaning that we can know everything that is out there or are there places that we will never be able to see?
I realize that my questions are probably a little childlike. I already feel like a kindergartner amongst Ph.D's so please try not to be to hard on me. I am just trying to understand.

Thanks,
Courtney

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 Recognitions: Gold Member Science Advisor The usual model universe that cosmologists work with is the simplest possible that makes a good fit with the data. For simplicity, it has no outside. Space is all of space. So it is a mistake to try to picture it from the outside. You can't get outside and see it expand because there is no outside. There is no edge or boundary--that would add a lot of needless unwanted complication to the model. So think about how the creatures inside something that is expanding experience the expansion process from the inside. For them a uniform pattern of expansion is not like ordinary motion, because nobody gets anywhere by it. Everybody just gets farther apart from everybody, without moving (in the ordinary sense.) That's how they experience expansion. It helps some people to watch this simple computer simulation: http://www.astro.ucla.edu/~wright/Balloon2.html It is expansion of a reduced dimension world. All existence, all creatures are in the infinitely thin surface of the balloon. Imagine that there is only this surface, no inside of the balloon, no outside of the balloon. their experience is all within the infinitely thin surface. Watch carefully. You will see galaxies sitting still and wiggles of light moving among the galaxies. each galaxy sees the other galaxies getting farther and farther away, but nobody moves, no galaxy gets anywhere. Each galaxy stays at the same latitude longitude position in their world. That's just an analogy, a toy model. Real space is 3D, not an infinitely thin curved 2D surface. Real space may be infinite volume, or (if it somehow curves around and rejoins itself) finite volume. That's hard to imagine but it is mathematically possible.
 THANK YOU!! I really do understand exactly what you mean. One more question, if you don't mind. Are we able to see the entire universe? I guess I am asking if all of space has been mapped out. Are there still things to be discovered?

Recognitions:
Gold Member

## Need Help Understanding Expanding Universe

Courtney we *definitely* do not see the whole thing!
According to the best most recent estimates I've seen, we don't even come close.
I'm not an cosmology expert, just a retired mathematician who loves cosmology. I try to follow other people's research.
If you (or anybody) wants some numbers about how much farther it extends (at least) beyond the most distant matter we can see, then just ask.

 Recognitions: Gold Member Science Advisor I agree with marcus, with one caveat - we can 'see' the entire observable universe. The cosmic microwave background, currently at z ~ 1100, will forever be the limit of the observable universe in EM wavelengths.
 One modification to the balloon analogy that I like is to picture the radial dimension of the sphere as time. The BB happens at T=0, the center of the balloon. inside the balloon is the past, outside the balloon is the future. We see the 3D surface at T=now as space. This helps explain why the universe expands, it is simply the nature of spheres that the surface area gets larger if the radius gets larger. Similarly it is simply the nature of hyperspheres that the surface volume is larger now, with a radius of 13.7 billion years then it was with a radius of 5 billion years. Please note that while this may be a convenient way to construct a mental image it cannot be an accurate model of the cosmos. Once you start crunching the numbers with it you find that it predicts a universe that is either much smaller or much older then we know the universe to be.

 Quote by courtney1111 Hello, I am hoping someone can help me to understand that the universe is expanding and other questions that brings up. If it is expanding, does that mean that there is an edge or a place that the universe has not reached yet but will reach at some point? What would be in the space that it has not reached yet? Are we able to see the entirety of the universe in all directions? Meaning that we can know everything that is out there or are there places that we will never be able to see? I realize that my questions are probably a little childlike. I already feel like a kindergartner amongst Ph.D's so please try not to be to hard on me. I am just trying to understand. Thanks, Courtney
If the cosmos is finite then it must contain a finite amount of space, mass and energy. Each unit has a defined location, physical domain and configuration. Even if the cosmos is expanding, for any time=T simple summation should be able to determine where some mythical cosmic wall exists.

Theorists have come up with an interesting hypothetical twist in which space curves in upon itself in such a way that for every given point 'A' there is a point 'B' within a finite distance at which motion in any direction will not increase the distance between the two; in fact, if a traveler who could instantaneously traverse a sufficient distance encountered such a point, he would begin to return to his point of origin. The concept describes a "finite but unbounded" Universe and implies a cosmos enveloped in a spherical spacecage. There is no evidence, no principle of logic, science or mathematics and no law of nature that implies the existence of any point, however distant, at which progress becomes regress.

There are two ways things in nature increase in size: inflation (adding more material) and expansion (increasing volume and decreasing density). The first implies the cosmos is somehow conjuring up new materials and/or new locations to occupy (magical thinking). There is no evidence for the second.

If there ever was a Big Bang (and the jury is certainly still out), the only logical version would be that within our local neighborhood (45 billion light years or so with its theoretical expansion factor) of that infinitely populated expanse we call the cosmos, an immensely large volume of mass somehow collapsed into a hyper-critical black hole which then regurgitated. I could almost (but not quite) lend credence to that scenario, but it certainly didn't create the Universe. At most it was a colossal rearrangement of elements which already existed.

 Recognitions: Gold Member Here is a balloon model discussion from these forums: http://www.phinds.com/balloonanalogy/ A 'next step' analogy: Wikipedia has a visually enhanced model: http://en.wikipedia.org/wiki/Metric_...on_of_Universe

 Quote by Naty1 Here is a balloon model discussion from these forums: http://www.phinds.com/balloonanalogy/ A 'next step' analogy: Wikipedia has a visually enhanced model: http://en.wikipedia.org/wiki/Metric_...on_of_Universe
Unless Big Bang is a local event, Universal expansion would imply a finite cosmos. Expansion - in whatever form you wish to cast it - means increase and only that which is measurable or limited can be increased. If the cosmos is, indeed, infinite, then there is no farthest and there is no 'all'...there is always more.

 Quote by Farahday There are two ways things in nature increase in size: inflation (adding more material) and expansion (increasing volume and decreasing density). The first implies the cosmos is somehow conjuring up new materials and/or new locations to occupy (magical thinking). There is no evidence for the second.
There is actually a great deal of evidence for the second. I suggest you google cosmological redshift and supernova surveys.

 Quote by Farahday If there ever was a Big Bang (and the jury is certainly still out), the only logical version would be that within our local neighborhood (45 billion light years or so with its theoretical expansion factor) of that infinitely populated expanse we call the cosmos, an immensely large volume of mass somehow collapsed into a hyper-critical black hole which then regurgitated. I could almost (but not quite) lend credence to that scenario, but it certainly didn't create the Universe. At most it was a colossal rearrangement of elements which already existed.
This theory was postulated some years ago, but does not seem to fit the data we now have. Particularly the observation that cosmological expansion is accelerating. If this theory were correct the expansion would be slowing down due to gravity.

 Quote by Farahday Unless Big Bang is a local event, Universal expansion would imply a finite cosmos. Expansion - in whatever form you wish to cast it - means increase and only that which is measurable or limited can be increased. If the cosmos is, indeed, infinite, then there is no farthest and there is no 'all'...there is always more.
This is incorrect. Expansion does not imply finiteness. Expansion is a verifiable fact. The jury is still out on finiteness, though in my opinion infinite is more likely because only an infinite and expanding universe can have zero total energy. I find the zero energy universe idea appealing because it can spontaneously exist without violating conservation of energy.

 Quote by Farahday Unless Big Bang is a local event, Universal expansion would imply a finite cosmos. Expansion - in whatever form you wish to cast it - means increase and only that which is measurable or limited can be increased. If the cosmos is, indeed, infinite, then there is no farthest and there is no 'all'...there is always more.
\begin{rant}
Let's break this down.

 Unless Big Bang is a local event, Universal expansion would imply a finite cosmos.
Exactly what I plan to disprove.

 Expansion - in whatever form you wish to cast it - means increase
Correct,

 and only that which is measurable or limited can be increased.
And that's where we're going wrong. There's no reason to think this. Why should something being not limited mean it can't increase? At no two points in an infinite Universe is the distance between them infinite, and we can easily increase a real number. Similarly, the distance between any two points is measurable, as it is not infinite.

Anyway, even something being infinite doesn't mean it isn't measurable or it can't be increased. For instance, there's a fairly basic proof that the number of real numbers is "more" than the number of real integers.
\end{rant}

 Quote by mrspeedybob There is actually a great deal of evidence for the second. I suggest you google cosmological redshift and supernova surveys.
Google is not as good as SLAC, CERN, FERMI and other more credible sources. Been there, done that.

 This theory was postulated some years ago, but does not seem to fit the data we now have. Particularly the observation that cosmological expansion is accelerating. If this theory were correct the expansion would be slowing down due to gravity.
Not if the density of mass to space were higher OUTSIDE our sphere of observation.

 Quote by Whovian \begin{rant} And that's where we're going wrong. There's no reason to think this. Why should something being not limited mean it can't increase?
In order to detect in increase one must have a measurement. If condition A is larger than B then there is an increase. Any value you assign to A or B is a defined (finite/limited) quantity.
 At no two points in an infinite Universe is the distance between them infinite, and we can easily increase a real number. Similarly, the distance between any two points is measurable, as it is not infinite. Anyway, even something being infinite doesn't mean it isn't measurable or it can't be increased.
Yes it does. Infinity is the LACK of a quantifiable definition.
 For instance, there's a fairly basic proof that the number of real numbers is "more" than the number of real integers. \end{rant}
Duhhhh... The set of integers is a subset of the set of real numbers.

Recognitions:
Gold Member
 There are two ways things in nature increase in size: inflation (adding more material) and expansion (increasing volume and decreasing density). The first implies the cosmos is somehow conjuring up new materials and/or new locations to occupy (magical thinking). There is no evidence for the second.
Neither of these is the standard cosmology modeled by the FLRW universe. For example, inflationary expansion retained a constant energy density. Nature is more complex than either of those statements implies.

 Unless Big Bang is a local event, Universal expansion would imply a finite cosmos. Expansion - in whatever form you wish to cast it - means increase and only that which is measurable or limited can be increased. ....
Again, this conflicts with standard cosmology. We observe universal expansion, the FLRW model depicts it, and no one knows if the universeis finite or infinite. That statement is
pure speculation without supporting theoretical or observational evidence.

 Quote by Farahday In order to detect in increase one must have a measurement. If condition A is larger than B then there is an increase. Any value you assign to A or B is a defined (finite/limited) quantity.
"Infinite" quantities can also be defined. And, again, assuming the Universe is infinite, even though there is no limit to the distance between two points, it is always going to be finite.

 Yes it does. Infinity is the LACK of a quantifiable definition.
Quite incorrect. http://en.wikipedia.org/wiki/Infinity#Set_theory

 Duhhhh... The set of integers is a subset of the set of real numbers.
Oh. I had a completely different proof in mind.

 Tags expanding universe, visible space