Why I think space is expanding

[5hot6un]

A thought experiment:

If occupied space is "curved", then empty space is "flat". Expansion is the process of equilibrium. At the far edge of gravitation influence the "dark energy" that pushes galaxies apart is space trying to flatten out.

 

[weeman]

Would your theory have to rule out quantum jitters?

Even Einstein once thought that empty space (where there was not matter, gravity, etc.) was perfectly flat. Quantum mechanics has shown us that this is not necessarily true. Because on the smallest of scales (far beyond any technology we have), seemingly flat space is actually very chaotic.

 

[jakeyboi]

Hmmm good idea, although space isnt really flat. The fabric of space has been illustrated in 2d (flat) where as we all know space is 4d. This is so people can visualise the effects of gravity easier. The real fabric of space is alot more complicated.

 

[5hot6un]

I understand that space isn't actually flat. That's why I used quote marks.

The point is that space is altered is the presence of matter. The influence of matter on space decreases as distance increases.

Gravity is a force that distorts space. Space therefore must have some resistance to this distortion. What then is the nature of the resistance? Could it be that this resistive nature of space is also a force? The elusive dark energy that is driving expansion?

After all, there is an infinite amount of empty space beyond the edge of the material universe. So does it not stand to reason that all the matter in the universe is disturbing a finite region of space? Thus the most entropic state of the universe is one where all matter is infinitely distributed?

 

[ramparts]

Well, your question is based on a slight misunderstanding - gravity isn't a force that distorts spacetime, it's a force caused by the distortion of spacetime. When spacetime curves, what we see as gravity is what results.

 

[5hot6un]

Well, your question is based on a slight misunderstanding - gravity isn't a force that distorts spacetime, it's a force caused by the distortion of spacetime. When spacetime curves, what we see as gravity is what results.

Yes... spacetime is distorted by matter. Therefore spacetime is UN-distorted in the absence of matter. What "force" does spacetime possess that creates the at rest state of undistorted?

 

[amshak]

Hmmm , there are many reasons for the expanding Space . Some evidence show its by the big Bang , Since we cannot travel to the past , its unexplained .

 

[SteveCNC]

One thing that bothers me about the acceleration of expansion is that that farther we look (distance/time) we see things moving faster away than closer objects . So what is it moving faster anyway , is it us (our galaxy) or them (the observed galaxy) ? If it's the observed galaxy , are we not looking way back in time so what we see is light emited over a billion years ago , so doesn't that mean that it was moving that fast a long time ago ?

 

[ramparts]

If you ask them, we're moving. If you ask us, they're moving. As far as the universe is concerned, neither of us is :lol: Everything is really getting pulled along by the motion of space.

Now, since there's no intrinsic motion, the light emitted from these galaxies isn't redshifted right away (recall that redshift is how we know that a galaxy is moving away). As the light from the galaxy travels through space, the expansion of space actually stretches the light out, causing it to appear redshifted. The farther away a galaxy is, the more time the light has had to travel to us and therefore the more it'll be redshifted, so that we can see faraway galaxies receding at higher speeds than nearby ones. We infer that this is how fast the galaxy is moving away from us because the redshift directly encodes how much space has expanded between there and here.

 

[Mordred]

Gravity is a force that distorts space. Space therefore must have some resistance to this distortion. What then is the nature of the resistance? Could it be that this resistive nature of space is also a force

As all areas of space has some matter regardless of how small or large. These in turn have the other 3 elemental forces influencing them. The weak nuclear, strong nuclear and electromagnetic. If you think of the interactions it would form a lattice of interactions. The distortions could readily be the distortions of matter upon these interactions. Or the resistance matter has on space. Space being the collection of matter added with the combined forces and energy in the area much like a large body of fluid. Gravity as the defined effect of that disruption.

 

[SpeedFreek]

One thing that bothers me about the acceleration of expansion is that that farther we look (distance/time) we see things moving faster away than closer objects . So what is it moving faster anyway , is it us (our galaxy) or them (the observed galaxy) ? If it's the observed galaxy , are we not looking way back in time so what we see is light emited over a billion years ago , so doesn't that mean that it was moving that fast a long time ago ?

It can get confusing, can't it? Especially as there are 2 different concepts involved, concerning the "speed" at which distant galaxies are, or were, receding. As we look back past 5 or 6 billion years ago we are looking at the universe when it was still decelerating, so the further we look, the faster a galaxy was apparently receding.

But it is not the acceleration of the expansion that causes apparent recession speed to increase with distance, it is the expansion itself, whether it accelerates or decelerates. At any given time, the whole universe is thought to be expanding at the same rate, but that rate changes, over time.

So at any given time, the further away an object is, the faster it would be receding, as the expansion means all distances scale up by the same factor. If 1 billion light-years becomes 2, then 10 billion light-years becomes 20! That is the principle around which we model the expansion of the universe, built using the slice of the history of the universe we can see.

With distant galaxies, their redshift tells us how much the universe has expanded since the light we are seeing was originally emitted.

If we work out how far away those galaxies were when they emitted that light (by apparent size, dimness, or proximity to something we already know the distance of, for instance), we can work out how fast they were apparently receding at that time, assuming everything we see began in the same place.

If we multiply that distance by the amount the universe has expanded since (using their redshift), we can work out how far away they would be today, and thus how fast they would have had to apparently recede to get there.

I say "apparently" recede, as they are not actually moving much relative to the space around them, it is the space between them and us that apparently expands! This has always been happening all over the universe , meaning that the further away something is, the faster it is being "moved away" by the expansion of the universe.

So when we extrapolate how the universe would be today (13.7 billion years old, 46 billion light years in radius), the further away (in distance) we "look", the faster something is receding. And when we look at the slice of the history of the universe we can actually see, the whole range of expansion rates is played out before us, with the closer objects seen as they were whilst the universe has been accelerating, and more distant objects seen as they were when the universe was decelerating, so the further we look, the faster something was receding.

Or something like that..

 

[5hot6un]

I had hoped to provoke a conversation that just will not emerge.

So let's try another angle.

So when we extrapolate how the universe would be today (13.7 billion years old, 46 billion light years in radius),

As Speedfreek stated, the universe has a finite radius. Beyond that radius is what?

Perhaps the universe is a bubble within an infinite empty void and expansion is the dissipation of matter and energy into the infinite void. Simple. Nature abhors a vacuum

My original point expands on this idea to suggest that the infinite empty void could also be thought of as flat space and the universe is a "disturbance" (sorry, I cannot find a better term) within this flat space. Expansion then is the process of evenly distributing this "disturbance" we call the universe.

I'm not trying to state this as fact. I really just want to provoke a discussion.

 

[SpeedFreek]

As Speedfreek stated, the universe has a finite radius. Beyond that radius is what?

Sorry, I should have been more specific. The age of the universe is thought to be 13.7 billion years but the overall size of the universe is unknown, and probably un-knowable. The part we can know of, the part we have received light from, is thought to be 46 billion light-years in radius, and we assume there is more. It would be a strange coincidence if we just happened to be in the exact centre of the universe and the universe just happened to be the correct age for light from its edge to just be reaching us.

Perhaps the universe is a bubble within an infinite empty void and expansion is the dissipation of matter and energy into the infinite void. Simple. Nature abhors a vacuum

My original point expands on this idea to suggest that the infinite empty void could also be thought of as flat space and the universe is a "disturbance" (sorry, I cannot find a better term) within this flat space. Expansion then is the process of evenly distributing this "disturbance" we call the universe.

I'm not trying to state this as fact. I really just want to provoke a discussion.

The current model for the universe doesn't include an infinite empty void that the universe expands into, it is the whole thing that expands, and just as a distant galaxy is receding from us, so we are receding from that galaxy at the same rate.

But I do like the way you describe expansion as the process of the universe trying to evenly distribute its contents!

Of course, we should be able to use scientists to find the mechanism that the universe is using to do this, and in fact they are trying!

 

[5hot6un]

So how has the infinite void been ruled out?

 

[ramparts]

Well, it's hard (sorry, impossible) to rule out anything beyond the observable universe, but in science we like to adopt the simplest models which fit the data, and the Universe expanding into an infinite void is just an unnecessary layer of complication.

 

[SpeedFreek]

Boundary conditions.

For something to expand evenly, the conditions in one direction have to be the same as in any other. With an "edge" of the universe and infinite void beyond, we suddenly find very different conditions in a certain direction, and this makes it very difficult to explain how the expansion can look as if it is even. Presumably, any effect of the boundary conditions would propagate inwards, meaning you would not necessarily have to be near the edge to notice that there might be an edge.

 

[captdude]

Just to throw in a "but what if"..............but what if the "edge" of the void is expanding as well.

The flat world, round world, solar system, galaxy and universe just keeps getting bigger and more complicated the longer we study it. Does anyone really think the stuff of existence has finished expanding before our ever increasing measurements of its parameters?

 

[csmyth3025]

...I say "apparently" recede, as they are not actually moving much relative to the space around them, it is the space between them and us that apparently expands! This has always been happening all over the universe , meaning that the further away something is, the faster it is being "moved away" by the expansion of the universe...

This is a concept that I keep wrestling with. Here we are, moving along at a stately pace relative to everything around us in our local patch of space which, for simplicity, we'll call the Virgo supercluster. Somewhere halfway across the universe there's another group of forum posters moving along at a stately pace relative to everything in their local patch of space (supercluster name unknown). They think we are moving away from them and we think they are moving away from us.

So far everything is hunky-dory. Then someone says:"Well, they're apparently moving away from us at nearly the speed of light, but they're not really moving that fast because in their own local space they're just drifting along at a few hundred km/sec." So my question is:"Are they really moving away from us at nearly the speed of light or not?" If they are, how much energy does it take to accelerate a whole supercluster of galaxies to nearly the speed of light (them or us - take your pick).

Chris

 

[SpeedFreek]

So far everything is hunky-dory. Then someone says:"Well, they're apparently moving away from us at nearly the speed of light, but they're not really moving that fast because in their own local space they're just drifting along at a few hundred km/sec." So my question is:"Are they really moving away from us at nearly the speed of light or not?" If they are, how much energy does it take to accelerate a whole supercluster of galaxies to nearly the speed of light (them or us - take your pick).

Chris

"Are they really moving away from us at nearly the speed of light or not?"

Really moving away at nearly the speed of light? No.

Apparently moving away faster than the speed of light? Yes.

Consider the balloon analogy, and cover the balloon with ants. Ants have a top speed they can walk across the surface of the balloon. But if you inflate the balloon, and consider ants on opposite sides of it, then the distance between those ants (around the circumference) can increase far quicker than the ants can physically walk away from each other. Each ant is moving around normally in its local region of the balloon, but distant regions of the balloon are receding from that ant far faster than any ant can move.

So the energy you are asking about is the energy required to inflate the balloon, not the energy required to move the ants across its surface. The same is true with the universe.

Cosmic inflation (as per Alan Guth) is what is currently thought to have put the "bang" into the big bang theory, and is what is currently thought to have been the initial cause of the apparently superluminal recession rates of distant galaxies.

For those distant galaxies to have achieved their current distance (a galaxy with a redshift of z=7 has a comoving distance of nearly 30 billion light-years) in only 13.7 billion years, the "fabric of space" has to have increased in size faster than light can travel across it. The "background metric", which defines cosmological distance, is increasing over time.

 

[csmyth3025]

So do we view the universe as being a googleplex of little sugar cube sized pieces of space that are each expanding a tiny bit over time, or that there are little sugar cubes of space popping up here and there between the sugar cubes of space that are already there? - or does it make any difference which way you look at it?

Chris

 

[emperor_of_localgroup]

Consider the balloon analogy, and cover the balloon with ants. Ants have a top speed they can walk across the surface of the balloon. But if you inflate the balloon, and consider ants on opposite sides of it, then the distance between those ants (around the circumference) can increase far quicker than the ants can physically walk away from each other. Each ant is moving around normally in its local region of the balloon, but distant regions of the balloon are receding from that ant far faster than any ant can move. Speedfreek.

With due respect Speed, does anyone else gets feeling how much do we have to twist a scene (balloon and ants) to conform with a mathematical model and then declare a victory for the model.

At first we take the motion of the ants, that doesn't fit the model.
Ok, let's expand the platform the ants are on, now it fits the model.
Wait, there is another problem, if we expand the entire platform, solar systems will disintegrate, galaxies will disintegrate.
Ok, keep those portions of space under gravity (even though there are light years of space between stars), let other parts of space (intergalactic) expand.
Good, now it fits the model perfectly.
Am I the only one who feels uneasy with this explanation?

Or we can simply say motions are not the only cause of redshifts.

 

[SpeedFreek]

The motion of the ants can represent the local peculiar motion of galaxies if you like, or you can have them walk in straight lines and represent photons.
If we expand the entire platform, the ants do not disintegrate.

This a toy model to aid the understanding of basic concepts. If you want to know the real model, I suggest you study General Relativity and in particular the FLRW solution to Einstein's field equations.
There is no explanation that is both accurate and simple.

 

[SpeedFreek]

So do we view the universe as being a googleplex of little sugar cube sized pieces of space that are each expanding a tiny bit over time, or that there are little sugar cubes of space popping up here and there between the sugar cubes of space that are already there? - or does it make any difference which way you look at it?

Chris

I don't think it makes any difference which way you look at it. You can even, if you like, consider that the universe is not expanding, but everything in it is shrinking! General Relativity allows you the freedom to "coordinatize" the picture in many different ways.

 

[darkmatter4brains]

Or we can simply say motions are not the only cause of redshifts.

Yes, that's a common misconception actually. The redshifts from distant galaxies are not Doppler redshifts (i.e. caused by relative velocities). Rather, they are cosmological redshifts caused by a changing metric (i.e. spacetime expanding)

In general relativity you cannot set up a global reference frame, so ideas of relative motion between distant galaxies doesn't have any real meaning. Even to say a galaxy is receding away from us, is not technically correct.

A classic example would be take two galaxies, at rest with respect to each other, in a static non-expanding Universe. Have one galaxy emit some light towards the other. No relative motions, so no Doppler redshift. Later on, after the light is emitted and while the light is still in transit, suddenly start expanding the spacetime in between the galaxies. Now that light will have a cosmological redshift due solely to the expansion of spacetime, which is related to a changing metric.

Interestingly enough, this ties over to energy conservation. Since you cannot create a global frame there is no real basis to claim there is a total energy that remains constant. Another way to look at is that conservation laws arise from symmetries. For energy it is symmetry in time that brings about its conservation. And, in the language of General Relativity, there is no time-like Killing vector to establish this symmetry for total energy in an expanding Universe. If the Universe was static, it wouldn't be an issue, but in an expanding one, things are more subtle.

 

[emperor_of_localgroup]

darkmatter4brains » Mon Jul 26, 2010 10:43 pm

The redshifts from distant galaxies are not Doppler redshifts (i.e. caused by relative velocities). Rather, they are cosmological redshifts caused by a changing metric (i.e. spacetime expanding)

Any one can correct me, I'm very familiar with derivation doppler redshift but not with of cosmological redshift. But I think the basic idea of both shifts are identical.
Change in wavelength due to change in separation between a source and a target. This separation may be caused by velocity of inertia (or whatever) or expansion of media (you call it space metric) between a source and a target. The latter is also a motion.