Can we ask such a question about expansion of universe?

[aleemudasir]

How far is it correct to ask, "What is universe expanding into?". How to solve this paradox?

 

[craigi]

How far is it correct to ask, "What is universe expanding into?". How to solve this paradox?

It's an expansion of space itself. There is no paradox.

If you want to know more, there's plenty to read on this forum. A similar question gets asked very regularly and very detailed answers are given, so I'd recommend reading some of the related threads at the bottom of this page.

 

[bapowell]

It's fine to ask the question, and the answer is that the universe isn't expanding into anything. If it's infinite, then the question doesn't really apply because there is no "into" for the universe to expand into. If finite, then the universe is expected to be at least unbounded -- like a sphere (as you walk around the surface of a sphere, you encounter no boundaries). Here, the common analogy is that of an inflating balloon, where the universe corresponds to the balloon surface (this is a lower-dimensional analogy). Inevitably, someone asks about the inside of the balloon, and for that matter, the outside of the balloon. This is where the analogy falters because there is no inside or outside of the balloon! Mathematically, the balloon does not need an ambient space in which to exist (specifically, what this means is that the surface of the sphere -- as a geometric object -- does not need to be imbedded in a higher-dimensional space in order to be a sphere). So, while we always picture the 2-sphere as existing in 3D space, it needn't be so! A 2-sphere is well-defined in 2-dimensional space. If we apply this reasoning to the universe, we come to realize that there needn't be any space "outside" the universe for it to expand into -- the universe, even if finite, is all there is.

 

[Mordred]

It's an expansion of space itself. There is no paradox.

If you want to know more, there's plenty to read on this forum. A similar question gets asked very regularly and very detailed answers are given, so I'd recommend reading some of the related threads at the bottom of this page.

How do you show the related discussions at the bottom of page if you don't mind my asking?

 

[Mordred]

This link is a good reading of the balloon analogy, also google metric expansion.
http://www.phinds.com/balloonanalogy/

this thread also has some related details, more in the form of another related question. "How is space created" As you will see from this thread expansion is simply a geometric increase of volume, that is simply filled with the contents of the universe. Its a lengthy thread as its often a difficult concept for some people to grasp.

https://www.physicsforums.com/showthread.php?t=748102

 

[phinds]

How do you show the related discussions at the bottom of page if you don't mind my asking?

Just look at the bottom of the page this thread is on. Are you running from a mobile app? Maybe it doesn't show up for those.

There should be a box titled "related discussions" with links to several threads.

 

[Mordred]

Yeah I see the box, does it automatically pull up related links via the title? from what I can tell it does.

 

[phinds]

Yeah I see the box, does it automatically pull up related links via the title? from what I can tell it does.

The links each have the title of a thread and are links to the thread. Pretty easy to just click on one and see.

 

[Mordred]

I think you missed the question lol, I was asking how the site generates the related links. Seems to be based on keywords in the title posted ie from a new topics title. Not too worried about it though just a curiosity.

 

[phinds]

I think you missed the question lol, I was asking how the site generates the related links. Seems to be based on keywords in the title posted ie from a new topics title. Not too worried about it though just a curiosity.

Yeah, I WAS wondering how you missed something so simple. Your question's a good one. I'm pretty sure it's just a keyword search on titles but I'd have to poke around and look at examples to be sure. Just the impression I've gotten.

 

[iDimension]

How far is it correct to ask, "What is universe expanding into?". How to solve this paradox?

If the universe is all that exists, then the universe doesn't need something to expand into because the universe is everything. Therefor the universe can get as large as it wants, as fast as it wants without any problems.

Also just a quick question if someone could clarify for me. Do we class the universe as an object?

 

[bapowell]

Define object.

 

[iDimension]

Define object.

Something which physically exists. For example love isn't an object, gravity is not an object. They exist but they have no materialistic foundation.

 

[bapowell]

Gravity has a materialistic foundation. Yes, the universe physically exists.

 

[iDimension]

Gravity has a materialistic foundation. Yes, the universe physically exists.

How? Gravity is just the product of curved space. Gravity has no actual properties of its own does it? It doesn't weigh anything, it has no colour, it has no dimensions and it doesn't move.

What physical properties does gravity hold?

 

[bapowell]

You seem to be working with a rather arbitrary definition of "physical" and "material" and some vaguely defined terms. What does it mean for gravity to have "no dimension"? Does an electron have "dimension"? Gamma radiation has no "color", but it can disrupt DNA replication and cause cancer. Gravity does "weigh" something -- gravity gravitates.

By your definition, is light physical? Material?

 

[iDimension]

You seem to be working with a rather arbitrary definition of "physical" and "material" and some vaguely defined terms. What does it mean for gravity to have "no dimension"? Does an electron have "dimension"? Gamma radiation has no "color", but it can disrupt DNA replication and cause cancer. Gravity does "weigh" something -- gravity gravitates.

By your definition, is light physical? Material?

Light is a physical, material object because it has at least one or more properties that are bound to it, wavelength or spin for example. It's like saying that momentum is an object, momentum is just the product of something else and doesn't have any properties of its own... or does it?

I don't want to say the wrong thing because I admit I don't know much at all about physics so I'm not saying you're wrong by any means, I just can't see how gravity could be classed as an object.

 

[bapowell]

Gravity has energy. It can serve as a source of gravitation. As a field, gravity imparts a force that can be measured empirically. Gravitational waves, which are excitations of the gravitational field, have spin and energy.

In these respects, gravity is not any different from electrostatics. Do you see a difference?

 

[iDimension]

Gravity has energy. It can serve as a source of gravitation. As a field, gravity imparts a force that can be measured empirically. Gravitational waves, which are excitations of the gravitational field, have spin and energy.

In these respects, gravity is not any different from electrostatics. Do you see a difference?

Not now you've told me that. I kinda makes sense although I did think that gravitational waves were still just the product of matter curving space but I guess I just don't understand it well enough at this time.

 

[AgentSmith]

How far is it correct to ask, "What is universe expanding into?". How to solve this paradox?

We assume that beyond any given space is more space. That's how it has been for almost all of our history. After all, how can empty space just end? But empty space is not empty. Its not nothing. We cannot visualize nothing, so we think empty space is nothing. But modern physics tells us differently, as others have noted.

 

[azureorb]

So, while we always picture the 2-sphere as existing in 3D space, it needn't be so! A 2-sphere is well-defined in 2-dimensional space. If we apply this reasoning to the universe, we come to realize that there needn't be any space "outside" the universe for it to expand into -- the universe, even if finite, is all there is.

Okay, I get that. The universe is like the surface of the balloon being blown up. Where it's expansion is more within itself -- not making empty space at all edges "roll out" to create "more room", right? Hence, most galaxies grow further apart over time if they're not too close to each other via gravity being stronger.

But if space were to be infinite -- how could there be expansion of space itself? If space were infinite wouldn't it just be greater expansion between objects? But of course, that type of expansion is still creating greater volume overall. So I would think that space itself wouldn't be infinite...

... unless one means infinite from the practicality/possibility standpoint, as in even if you went the speed of light, you would keep going and going and going, because it's expanding so fast it would from your perspective be infinite as you'd be traveling infinitely before you'd get to any part to "wrap around".

Where am I going wrong here? :)

 

[phinds]

But if space were to be infinite -- how could there be expansion of space itself?

You misunderstand infinity. Google "Hilbert's Hotel".

 

[bapowell]

But if space were to be infinite -- how could there be expansion of space itself? If space were infinite wouldn't it just be greater expansion between objects? But of course, that type of expansion is still creating greater volume overall. So I would think that space itself wouldn't be infinite...

By expansion of space what is meant is increase in distance between points.

 

[azureorb]

You misunderstand infinity. Google "Hilbert's Hotel".

No, I get it now (even without googling). I brought up that reference of if space is finite in another thread (how could it not have a center/edges, without wrapping around)... and also if it IS infinite, how can that be from a seemingly finite explanation of the big bang (x amount of energy/mass, Y amount of seconds this happens, it inflates like this and that, etc)?

 

[Mordred]

I've answered your similar questions in that other thread.

https://www.physicsforums.com/showpost.php?p=4732216&postcount=123