Why I think space is expanding

[QED]

Last I read, the curvature of space was predicted mathematically to have a curve close to zero.

 

[SpeedFreek]

Last I read, the curvature of space was predicted mathematically to have a curve close to zero.

I think you are referring to the "global" curvature of the universe, rather than any local curvature of spacetime.

 

[SpeedFreek]

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.

The latter is not a motion in the same sense, although we can derive apparent recession velocities for it, if we like, but that is simply a coordinate choice we make.

 

[darkmatter4brains]

yeah, I can provide another example from a text that is similar to mine above but will hopefully make the difference a bit clearer

Take two light sources and place them 1 million light years apart - we'll call them source A and B.

For the first part, the lights are at rest with respect to each other and in flat space. Have source A turn on for 1 sec and then shut off. After the emission of light, have the light sources speed away, within flat space, until they are much further apart and then put them at rest again. afterwards, when source B finally receives source A's light it will have ZERO redshift. This is because the light was emitted while the source and receiver were at rest with respect to each other.

For the second part, similar setup with two light sources at rest with respect to each other in flat space. Source A emits the light for 1 sec, then shuts off. While light pulse is in transit, we expand the space in between the light sources, which has the effect that they end up farther away from each other again, and then stop. However, this time when source B receives source A's light, it will be redshifted, due to the expansion of space, or a changing metric

If you look at the formula for doppler shifts it references relative velocities of the source and receiver. If you look at a simple version of a cosmological redshift formula it references energies and scale factors related to the changing metric (E1/E2)=(a2/a1), with no reference to velocities. As it should be, since relative velocity has no real meaning on cosmological scales.

Another example of redshift that is due only to gravitational effects and not to doppler effects, is the simple redshift light experiences from leaving a gravitational well. There source and receiver can be at rest with respect to each other, but you will still have a redshift.

So, they're similar in some ways, but there are subtle differences between them that have to be made.

 

[QED]

Last I read, the curvature of space was predicted mathematically to have a curve close to zero.

I think you are referring to the "global" curvature of the universe, rather than any local curvature of spacetime.

Well, it reads as "space" [without time] for a reason. The entire mass of the curvature of space determines the curvature of spacetime (GTR). The universe is more than likely has both slightly positive curve and slightly flat, since the equations in Hubble's constant call for a positive, but a flat universe allows for its eternal expansion and a curved universe allows for spacetime to exist. Some people believe that the universe will contract (Big Crunch), but I think that's phooey. The universe as it is now is only a few degrees Kelvin (almost three degrees above zero) and it is still expanding, as we can see the red shifting of galaxies still. As we know in the third law of thermodynamics, nothing can reach absolute zero. We can get pretty damn close, but no dice. The "fabric" of space is still pulling these things along at an accelerated rate.

 

[SpeedFreek]

Well, it reads as "space" [without time] for a reason. The entire mass of the curvature of space determines the curvature of spacetime (GTR).

Yes, of course.

Some people believe that the universe will contract (Big Crunch), but I think that's phooey. The universe as it is now is only a few degrees Kelvin (almost three degrees above zero) and it is still expanding, as we can see the red shifting of galaxies still. As we know in the third law of thermodynamics, nothing can reach absolute zero. We can get pretty damn close, but no dice. The "fabric" of space is still pulling these things along at an accelerated rate.

During the past decade the "Big Crunch" has fallen by the wayside since the discovery that the rate of expansion is currently accelerating, possibly due to a cosmological constant or quintessence. There is more likely to be a "Big Rip" than a "Big Crunch"! (but the big rip is considered an unlikely scenario)

One point though. If the universe had expanded in the past but had now stopped expanding, I think we would still see cosmological redshift for distant galaxies, as there is still a difference in the scale factor of the universe now, when compared to the past. Or to put it another way, the light we see was "stretched" during the time the universe was expanding.

 

[darkmatter4brains]

One point though. If the universe had expanded in the past but had now stopped expanding, I think we would still see cosmological redshift for distant galaxies, as there is still a difference in the scale factor of the universe now, when compared to the past. Or to put it another way, the light we see was "stretched" during the time the universe was expanding.

seems like you could potentially pin down when spacetime would have stopped expanding, though. For example, if it stopped expanding 5 billion years ago (just making a number up) then if we look out in space no further than 5 billion light years away, we should not see cosmological red shifts, since the metric was not changing during that time.

In addition, the redshifts we would see from galaxies further than 5 billion light years, should not match the predicted redshifts based on a scale factor, or metric, that has been changing for the entire age of the Universe.

I don't think we see anything like this though ... so probably safe to assume it's still going ...

 

[MeteorWayne]

In fact, all the latest data (and there will be more better data soon) has indicated that the expansion is continuing to accelerate.

 

[darkmatter4brains]

In fact, all the latest data (and there will be more better data soon) has indicated that the expansion is continuing to accelerate.

Wayne, is that a specific mission that will deliver that data? I'd like to look more into it, if so.

You probably watched the Morgan Freeman show tonight too, but I thought that dark energy camera they talked about sounded cool - would chart the expansion of the universe to amazing precision (compared to what we have now anyhow) I'm guessing we're quite a ways off before that data comes in though.

 

[MeteorWayne]

I believe that is one of the targets of Planck. Hubble continues to collect relevant data as well as ground based scopes.

 

[darkmatter4brains]

great, thanks! I've been excited about some other results that are supposed to come out of that one too: like m-theory cosmology vs Inflation. I'm real curious to see what they find while looking for the polarization of the CMB too. Like you said, shouldn't be too much longer.

 

[QED]

In my humble opinion (allow me to chime in again), the universe is globally flat and spacetime is curved. Kind of like a flat surface with peaks and dips on a 4D + plane. The extra dimensions, if they exist, are called for by the math (M theory - Witten). Who knows exactly where they fit.

 

[halpmaine]

Hello.

Could the universe or better the matter comprising it represent an elastic collision? Thanks.

 

[QED]

Hello.

Could the universe or better the matter comprising it represent an elastic collision? Thanks.

Do you mean the Big Crunch reoccurring? Can you be more specific?

 

[halpmaine]

yes...that is, if energy is still conserved (i.e. under all conditions) then would not this imply infinite crunches & bangs??

 

[ramparts]

You've gotta be more specific, man - why would conservation of energy imply infinite crunches and bangs?

 

[csmyth3025]

Hello.

Could the universe or better the matter comprising it represent an elastic collision? Thanks.

by halpmaine » Tue Aug 10, 2010 9:42 pm

yes...that is, if energy is still conserved (i.e. under all conditions) then would not this imply infinite crunches & bangs??

Four questions:

If one constructs a hypothetical frictionless air hockey board of infinite dimensions and pushes two hypothetical perfectly elastic hockey pucks directly towards each other, when they bang together would their collision be considered an "elastic collision"?

If so, is there any reason why these two hockey pucks would ever come back together (assuming the air hockey board is globally flat - not the surface of a sphere)?

If the air hockey board turns out to have global positive curvature (spherical), wouldn't the pucks eventually bang into each other just as they had done originally?

If the Big Bang is interpreted as an expansion of space rather than an expansion of things moving through space, does the question of whether space is globally flat or whether it has global positive curvature have any bearing on whether things will eventually "come back together"?

Chris

 

[QED]

I believe it depends on how much matter is construed throughout the edges of the universe, since gravity pulls matter along.

 

[ramparts]

Chris, to your last question (the first three seem to just be leading into it), if things are moving away from each other in a positively curved space, they'll come back to each other, but if they're moving away from each other because the space itself is expanding, they won't. Why? Well, you should know that, the balloon analogy works great. Two points on a non-expanding balloon moving away from each other will eventually come back. Two points stationary on the balloon when the balloon itself is expanding obviously never will. The key point is that in the latter case, even though the surface is positively curved, the radius of curvature is expanding. If the expansion is a motion through space, then the radius of curvature would (presumably) remain constant, ignoring gravitational effects. There's also the matter that when it's the space that's expanding, you have the points remaining in the same comoving positions within the space so they'd never meet each other anyway.

 

[csmyth3025]

Thanks Ramparts,

I was hoping that I finaly grasped the concept of things (galaxies, super clusters, etc.) in an expanding volume of space. I think that I now understand the difference between things moving through space relative to distant objects (proper motion) and things moving with space relative to distant objects as a result of the general expansion of space.

As I understand it, a distant galaxy may, indeed, have a proper motion relative to earth - but in order to determne that proper motion you first have to subtract out the effect of expansion. I believe the term for the adjustment is - as you pointed out - co-moving co-ordinates.

The question of why and by what mechanism space is expanding is, of course, the subject of a lot of discussion elsewhere.

Chris

 

[halpmaine]

Ok, in reading prior posts it seems that there is now pretty fair evidence that points to an accelerating expansion of space, do I have that right?
However, if this were not the case... and given that energy has no origin or terminus - then the big bang really couldn't represent the actual start of the universe, right?
Rather, since matter is a form of energy (i.e. all matter can be broken down to energy) and if space or even matter alone eventually stopped expanding and then began contracting - then we'd be looking at continuous bounces, if you will; the cycle of bangs and crunches would continue...Thus my initial ? pertaining to the conservation of energy and bounces, collisions, etc., etc..
Sounds like this model appears less likely given the accelerating expansion of space, etc., yes?
Thanks ramparts/All for clarifying...

 

[ramparts]

Chris - Sounds like you've got it!

Ok, in reading prior posts it seems that there is now pretty fair evidence that points to an accelerating expansion of space, do I have that right?

Definitely.

However, if this were not the case... and given that energy has no origin or terminus - then the big bang really couldn't represent the actual start of the universe, right?
Rather, since matter is a form of energy (i.e. all matter can be broken down to energy) and if space or even matter alone eventually stopped expanding and then began contracting - then we'd be looking at continuous bounces, if you will; the cycle of bangs and crunches would continue...Thus my initial ? pertaining to the conservation of energy and bounces, collisions, etc., etc..
Sounds like this model appears less likely given the accelerating expansion of space, etc., yes?
Thanks ramparts/All for clarifying...

If there were no "dark energy" causing the expansion to accelerate then yes, eventually the gravity of the matter in the Universe would turn around the expansion and lead to a crunch. Whether this would lead to another bang is unclear. But it does seem like a bit of a moot point by now

 

[csmyth3025]

As I understand it, before dark energy the astronomers were searching for evidence of the effect of gravity on the expansion of the universe. They were hoping to find data by which they could determine how much gravity was slowing down cosmological expansion. At that time (the mid-1990's) the big debate was whether there was enough matter to slow down the expansion and, subsequently, cause cosmological contraction (the Big Crunch) - or whether there wasn't enough matter in the universe to do this and the universe would just keep on expanding forever (but more slowly as time passed).

In 1998 two separate research teams - the Supernova Cosmology Project and the High-z Supernova Search Team - composed of researchers and facilities all over the world - were trying to make this determination. To their collective surprise they found something that no one expected to find. Their data suggested that, starting about 6 billion years ago, the expansion of the universe has been speeding up, not slowing down!

I can imagine these researchers being torn between thinking that someone would almost certainly find flaws in their research (an embarrasment that would put their years of work in the same category as cold fusion) and thinking - to steal a quote from Matt Damon in the movie "Good Will Hunting" - How do you like them apples!

Chris

 

[halpmaine]

It's ironic that you noted the mid-90's cosmology research. That was the last time (I know, I know...I'm dating things here) I put into 'text' my more-than-periodic thoughts about this cool stuff. Then - I had emailed a buddy of mine in the Physics world about crunches, bangs and collisions. As I recall, given the lack of strong info on dark matter, doppler shifts/accelerations of space and the like at that time, my friend did not discount the relevance of the crunch-bang-crunch hypothesis. Looks like I took too long in hooking up with Space.com!

Good stuff, thanks for the edu, Chris. Ok, I think my daughter's waking up. "I gotta go see about a girl..."

 

[csmyth3025]

....Ok, I think my daughter's waking up. "I gotta go see about a girl..."

I see you couldn't resist the temptation to steal a line from Matt Damon that he stole from Robin Williams

Chris