Question about the Big Bang

[Soul Kitchen]

Did it create space as well as matter?

I remember some people implying that and it was difficult to imagine there being no space at some point. The concept of limits to space bothered me as a kid, trying to imagine what happens when you reach the end of space. Would it be an abrupt wall, or would you begin to slow as you approached it and find yourself unable to go further?

It is also said that the Universe, after expanding for a while, will begin to contract. Is this the result of space shrinking and expanding, or rather a sort of momentum from the initial bang pushing things outward and later the collective forces of gravity causing things to be pulled back?

It makes more sense to me for space to have always been there, and for it to have been filled by the Big Bang. Infinite space.

I have a feeling that the infinite space answer is the correct one and that it's silly to imagine otherwise, but I just want to know for sure.

[Wookietim]

Did it create space as well as matter?

I remember some people implying that and it was difficult to imagine there being no space at some point. The concept of limits to space bothered me as a kid, trying to imagine what happens when you reach the end of space. Would it be an abrupt wall, or would you begin to slow as you approached it and find yourself unable to go further?

It is also said that the Universe, after expanding for a while, will begin to contract. Is this the result of space shrinking and expanding, or rather a sort of momentum from the initial bang pushing things outward and later the collective forces of gravity causing things to be pulled back?

It makes more sense to me for space to have always been there, and for it to have been filled by the Big Bang. Infinite space.

I have a feeling that the infinite space answer is the correct one and that it's silly to imagine otherwise, but I just want to know for sure.

There is no "Edge" of space - after all, if something were to breach the edges of the known universe somehow, all that happens is that they end up continuing in the same direction. Remember - something from the universe is still part of the universe and if the expansion of the universe creates space, then all you do if you push the boundary is expand the universe... if you see what I mean.

[Soul Kitchen]

There is no "Edge" of space - after all, if something were to breach the edges of the known universe somehow, all that happens is that they end up continuing in the same direction. Remember - something from the universe is still part of the universe and if the expansion of the universe creates space, then all you do if you push the boundary is expand the universe... if you see what I mean.

So space, the capacity for things to inhabit, is infinite?

[Wookietim]

So space, the capacity for things to inhabit, is infinite?

Well, if you define space as "the area that things inhabit", then you can't reach the end of it since by going there, things inhabit that space... Which means that the very act of going there creates the space that that part of the universe inhabits... In other words, things don't expand into pre-existing space - the act of expanding creates the space to inhabit.

It would be impossible to take a spaceship and "Get ahead" of the expansion of the universe - the spaceship is part of the universe so wherever it would go would be part of the universe and it's expansion...

[Rlyeh]

The Universe includes space-time, the big bang is the expansion of space as well as matter.

[Soul Kitchen]

Well, if you define space as "the area that things inhabit", then you can't reach the end of it since by going there, things inhabit that space... Which means that the very act of going there creates the space that that part of the universe inhabits... In other words, things don't expand into pre-existing space - the act of expanding creates the space to inhabit.

It would be impossible to take a spaceship and "Get ahead" of the expansion of the universe - the spaceship is part of the universe so wherever it would go would be part of the universe and it's expansion...

That seems like defining the area that things inhabit rather than the capacity.

Space is still there if nothing occupies it, no?

It doesn't become space only when occupied, does it?

The potential, the capacity to be occupied, is always there.

[Wookietim]

That seems like defining the area that things inhabit rather than the capacity.

Space is still there if nothing occupies it, no?

It doesn't become space only when occupied, does it?

The potential, the capacity to be occupied, is always there.

No, "Space" is not there if the universe is not there. The universe expands - it's not expanding into anything - it creates it's one area to expand into.

[StarMountainKid]

The universe isn't expanding into pre-existing space or into anything. Space itself was created in the Big Bang and is a property of the universe.

Current cosmological theory says that the the expansion of the universe is accelerating due to the effect of dark energy and will continue expanding forever.

[Soul Kitchen]

Okay... What do you guys define as space? It sounds like we have different definitions.

Are you saying that it's not space until something occupies it?

It makes a lot more sense for an infinite area to be there before being occupied by objects of the universe.

It seems that people are giving space physical properties. What is space?

It's just space, an area with the potential to be occupied. How can that be created, and how can it expand?

How can it have limits?

It's not a tangible substance.

[Wookietim]

Okay... What do you guys define as space? It sounds like we have different definitions.

Are you saying that it's not space until something occupies it?

It makes a lot more sense for an infinite area to be there before being occupied by objects of the universe.

It seems that people are giving space physical properties. What is space?

It's just space, an area with the potential to be occupied. How can that be created, and how can it expand?

How can it have limits?

It's not a tangible substance.

Again - there is no "Area" for the universe to expand into. The universe makes it's own "Area" (Or "Space") as it expands.

[Soul Kitchen]

Again - there is no "Area" for the universe to expand into. The universe makes it's own "Area" (Or "Space") as it expands.

This is what I don't understand.

The universe makes it's own space as it expands...

So space, in a sense, is a sort of tangible thing that can expand and contract.

How is space made? What is it?

There is no potential area to be occupied outside of what is occupied?

I don't define it as an area that things inhabit, I define it as an empty area that has the potential to be inhabited.

So what makes the empty area between a couple of stars different than the area beyond the furthest piece of matter in the universe?

Is there no space beyond that piece of matter? The piece of matter can't move any further?

Or would it 'create space' by doing so?

If it moves forward, I think that it is just moving into an area that has always had the potential to be occupied.

[StarMountainKid]

So space, in a sense, is a sort of tangible thing that can expand and contract.

According Einstein's General Theory of Relativity, and the fact of the expansion of the universe, yes.

[sepulchrave]

I have a Master's degree in physics, so I might be able to help clarify things...

This is what I don't understand.

The universe makes it's own space as it expands...

So space, in a sense, is a sort of tangible thing that can expand and contract.

How is space made? What is it?

Technically speaking space is just several degrees of freedom.

What defines a pebble (for example)? We might say things like: mass, mineral composition, age, temperature, pressure, volume, location.

How many numerical values does it take to define these things? Mass is just a number, temperature is just a number, age is just a number, location is at least three numbers (we live in at least 3D space, so for example altitude, latitude, and longitude are required), volume might be a few numbers depending on how irregular the shape is, same with mineral composition, etc.

Each one of these values is a ``degree of freedom'', if we changed that value we would be talking about a different pebble.

``Space'' is set of possible values for the three (or more) degrees of freedom require to define the location of an object.

The ``expansion'' or ``contraction'' of space is just defined relative to other parameters of the object. For example, we might say that in the early moments of the Big Bang space expanded dramatically resulting in the quark-gluon plasma condensing into the fundamental particles. Alternatively we could say that space was constant but the coupling parameters of the fundamental forces and the relative ``charges'' (i.e. electric charge, colour charge, mass, etc.) changed.

Because the coupling parameters of forces and the charges of fundamental particles are related to our definition of ``energy'', and because we would like ``energy'' to be constant (for some pretty good reasons), we say that space is ``expanding''. We could also define space as the constant and say that every thing else was changing, which is totally permissible and functionally no different, but is a little more cumbersome to work with.

(Think of it this way: We can define directions like ``east'' and ``west'' as absolute, and directions like ``left'' and ``right'' as relative to the direction you happen to be facing, or we can define ``east'' and ``west'' as directions relative to you, and ``left'' and ``right'' as absolute directions. This works in a self-consistent way, but requires you to believe that you stay fixed while the Earth spins around beneath you, rather than the Earth being fixed and you just turning around.)

There is no potential area to be occupied outside of what is occupied?

I don't define it as an area that things inhabit, I define it as an empty area that has the potential to be inhabited.

I agree with your definition.

So what makes the empty area between a couple of stars different than the area beyond the furthest piece of matter in the universe?

Is there no space beyond that piece of matter? The piece of matter can't move any further?

Or would it 'create space' by doing so?

If it moves forward, I think that it is just moving into an area that has always had the potential to be occupied.

There are only two possibilities for each dimension in space: either they are infinite in scope or they are cyclic. Either way, if the matter in this Universe really is contained within a finite volume (that is less than the periodicity of a cyclic dimension), then there is no difference between the area outside that volume and the area between the Earth and the Moon, other than curvature.

However, there are also only two possibilities for the expansion of the Universe: Either the space in between every object everywhere is expanding, or we are truly at the centre of the Universe.

The latter possibility is only possible if the matter in the Universe occupies a finite, non-cyclic volume.

There are only four combined possibilities:

1. Space is infinite, matter occupies a limited region of this space but is expanding outwards, the mass and energy of the Universe is finite, and Earth occupies the centre of this region.
   
2. Space is cyclic, matter occupies a limited region of this space but is expanding outwards, the mass and energy of the Universe is finite, and the Earth occupies the centre of this region.
   
3. Space is infinite, matter is also infinite but has a finite and large-scale homogeneous density, the mass and the energy of the Universe is infinite, space itself is expanding everywhere, and the Earth occupies a random position in the Universe.
   
4. Space is cyclic, matter is spread throughout space with large-scale homogeneity, the mass and the energy of the Universe is finite, space itself is expanding everywhere, and the Earth occupies a random position in the Universe.
   

The first two possibilities are philosophically untenable (after all the Earth isn't the centre of the Solar system and the Solar system isn't the centre of the Milky Way galaxy, and the Milky Way galaxy is not very large compared to many other galaxies).

Of the second two possibilities it seems like #3 (infinite universe) is the one most supported by experimental data.

We are used to thinking that if one thing expands it will ``push'' something else out of the way. For example, if I have two glasses of water side-by-side and I magically expand one glass in all directions, it will push the other glass away. However that is because space is an external parameter for these glasses (there is one set of space values, each object occupies a distinct subset therein). There are also internal parameters, however. If I have two glasses of water side-by-side and I start adding salt to one of them, I won't affect the other glass. Salinity is an internal parameter: it is possible to have two glasses of 90% water and 10% salt, but it is not possible to have two glasses in exactly the same location at exactly the same time.

When we talk about the Universe, we are usually talking about something where every observable parameter is internal. Just like salt-content of the water is something you can only measure by looking inside the glass, we would say that space, time, mass, energy, etc. are things that can only be measured by looking inside the Universe.

[Rlyeh]

It seems that people are giving space physical properties. What is space?

The spatial dimensions, and time.

It's just space, an area with the potential to be occupied. How can that be created, and how can it expand?

How can it have limits?

It's not a tangible substance.

Gravity has been understood to manipulate space-time as if it was a "substance" like.

[ShadowSot]

One thing important to remember, at the big bang, that's where the familiar concepts of time and space began, inside what we call the universe.

Outside of the universe, concepts like time and space don't exist.

Best way I can think of it is a balloon expanding into a vacuum.