PDA

View Full Version : Newbie question - Universe expansion

2012-Apr-06, 10:17 AM
Hi all,

There are two related questions that I've asked myself lately without having found the answers:

1 - If I got it right objects that are furhter away from Earth appear to be moving away from us faster than objects that are closer to us (very basic version).
But when we loook to objects that are further away from Earth we are also looking into objects from a more distant past, so putting things another way this would mean the expansion rate of the univers was greater in the past then today!

2 - How can we see light from a Galaxy 13 Billion light years away from us (so I suppose we are looking into something that happened 13 Billion years ago) without one of the following:
2.1 - At some point we where moving faster than the speed of light (relative to the Galaxy in question) in an expanding Universe
2.2 - Considering the Universe not to be expanding (or at least not the way we believe it has been)
2.3 - ???

As said in the title, newbie, so the questions may seem stupid, but I would still thank you if you could answer.

Regards

speedfreek
2012-Apr-06, 10:41 AM
1 - The reason we think that the further away a galaxy is the faster it recedes is due to the redshift of the light we detect from that galaxy. The redshifting of that light is caused by the expansion of the universe during the time the light was travelling towards us. So, although the light was emitted in the distant past, it is what has happened to that light (the redshifting of it) in between the time it was emitted and the time we detect it, that tells us the universe has expanded in that time.

We aren't really measuring recession velocity at the time the light was emitted, what we are measuring is the amount the universe has expanded since the light was emitted - which tells us how far that galaxy will have receded by now.

The concept that the further away something is the faster it recedes comes from the expansion of the universe in general - in any scenario where the distance between things is scaling up over time, across the universe, the further away something is the faster it will apparently recede.

2 - The universe expands such that distances across the universe can increase faster than light! This is not a violation of Special Relativity, as nothing is moving through the universe faster than light in this scenario, it is more a case of the universe expanding the distances between objects that can all consider themselves to be at rest.

2012-Apr-06, 12:10 PM
We aren't really measuring recession velocity at the time the light was emitted, what we are measuring is the amount the universe has expanded since the light was emitted - which tells us how far that galaxy will have receded by now.

So by measuring the redshift from the most distant Galaxy observed sofar (UDFj-39546284 at around 13.2 Billion light years) we are measuring the expansion of the Universe for that amount of time.

2 - The universe expands such that distances across the universe can increase faster than light! This is not a violation of Special Relativity, as nothing is moving through the universe faster than light in this scenario, it is more a case of the universe expanding the distances between objects that can all consider themselves to be at rest.

That point is clear, but it would mean that at some point it was "expanding" faster than what it is now, otherwhise we would not be able to see that far away (unless...)!

antoniseb
2012-Apr-06, 01:05 PM
The questions you are asking are probably best answered with diagrams or even video. It is difficult to say something accurate in a few words that conveys what is really happening that doesn't also risk saying something misleading.

I think one idea that might help is to imagine that space expands by inserting tiny bits of new space into space, so that every year there's perhaps a few hundred feet that pops up between us and Pluto. By the time you're looking at things several billion miles away, the rate that space is being added increases distances between us and it at about the speed of light.

The downside to this idea is that it makes you think that "space" is something tangeable. For other parts of the science you are better off not thinking of space as being made of something.

Hopefully within that framework you can now imagine how a photon might have left an early galaxy, and traverse space coming to us, and slowly stretching as it comes.

Cougar
2012-Apr-06, 08:06 PM
I think one idea that might help is to imagine that space expands by inserting tiny bits of new space into space, so that every year there's perhaps a few hundred feet that pops up between us and Pluto. By the time you're looking at things several billion miles away, the rate that space is being added increases distances between us and it at about the speed of light.

I like the analogy, but I'd say it's not so much the "rate that space is being added," as the sheer amount of space that is being added between here and distant objects.

The downside to this idea is that it makes you think that "space" is something tangeable. For other parts of the science you are better off not thinking of space as being made of something.

I'm not so sure about that. There are at least some parts where the concept comes into play. "Perfect fluid" in google scholar yields about 25,000 papers. Then there's the Higgs ocean... And the fact that "the space formerly known as empty" is currently said to make up something like 73% of the mass-energy of the Universe.... How can it not be tangible?

Jeff Root
2012-Apr-06, 08:27 PM
I think one idea that might help is to imagine that space
expands by inserting tiny bits of new space into space, so
that every year there's perhaps a few hundred feet that
pops up between us and Pluto.
That is *if* the expansion happens throughout all space.
We only see any evidence of the expansion between very
widely-separated clusters of galaxies. It may or may not
be happening inside of galaxies.

Even if the space between us and Pluto *is* expanding,
Pluto's orbit doesn't change, because the Sun's gravity
holds onto Pluto. If the expansion were to start and stop
suddenly, it might be possible to detect such small-scale
differences between when it is operating and when it isn't.
But since it is operating constantly it can't be detected.

By the time you're looking at things several billion miles
away, the rate that space is being added increases
distances between us and it at about the speed of light.
I started writing a reply to the OP earlier, but gave up
when trying to think how to address the question of light
travelling between widely-separated galaxies in a Universe
which is expanding at a constant, unchanging rate.

Here's an animation I made depicting uniform expansion
at an unchanging rate:

http://www.freemars.org/jeff2/expand5c.htm

The farther apart two dots are, the faster they are moving
away from each other. The view from any dot looks the
same as from any other dot. If dots represent galaxies,
light from one galaxy could travel at constant speed and
reach any other galaxy eventually, even though galaxies
which are very far apart are moving away from each other
at greater than the speed of light. How this is possible
is described by the analogy of an ant walking on a rope
which is stretching along its whole length at a constant
rate. If the ant starts at one point on the rope and heads
for another, distant point, at first the distant point will
be getting farther and farther away. But eventually the
ant will be far enough along the rope that the distance
to the goal begins to decrease.

Edit: Thinking about it again, that doesn't sound right.
I don't see that the ant can ever get closer to a point on
the rope that is initially moving away from it faster than
it walks. The reason I didn't post earlier. Hmmm...

The downside to this idea is that it makes you think that
"space" is something tangeable. For other parts of the
science you are better off not thinking of space as being
I haven't been able to reconcile the two views. It is sorta
like the particle/wave nature of matter.

Hopefully within that framework you can now imagine how
a photon might have left an early galaxy, and traverse space
coming to us, and slowly stretching as it comes.
The idea of light stretching as it travels is the hardest
part of this. It is easily described in the ant analogy by
having the ant get longer and longer as the rope under
its feet stretches. But why should light stretch like that?
The Doppler component of the redshift is more direct: The
light is emitted from a source and detected by a distant
receiver. The speed difference between the source and
receiver generates the Doppler redshift. But apparently
cosmic redshift is more than that, and is accounted for
by the stretching of the space the light travels through.

-- Jeff, in Minneapolis

antoniseb
2012-Apr-06, 08:29 PM
... How can it not be tangible?
I emphasize that you should regard it as an analogy and not the physical model because it is too easy to try to extend the analogy where it doesn't work. In the analogy objects with mass consume space, and space is being added everywhere, faster than it can be consumed... but space is massless, even IF it is an ocean of Higgs particles (at 130 proton-masses each).

But really, we haven't worked it out. This analogy works for a lot of simple ways of viewing things, but is can mislead on important parts of the big picture. So I like it. It helps me visualize things. It's wrong to assert that it is right.

Selfsim
2012-Apr-07, 04:37 AM
So, I'm not sure whether this answers Jeff Root's question or not, but here's a 'stretch objective' counterintuitive concept to consider...

The Hubble "Constant" is the parameter used to describe the rate of expansion. It increases with decreasing cosmological time. As has been already stated, the older, further away galaxies, are receding at a higher velocity than the closer in, younger galaxies and so, the older galaxies have a greater Hubble "Constant" than the younger ones.

However, galaxies closer to us are accelerating away from us at a faster rate, than the older ones further away. And yet, the recession velocities of the older, further away galaxies, will always be greater than the younger, closer ones ! So, how can this be ?

This may seem very counterintuitive, because if the closer in, younger galaxies are accelerating away faster, it would seem that they would end up receding at a faster velocity than the older ones. So how come this is not so?

The answer is that the scale of the universe also increases with time. Early on, the scale was much smaller, (the early universe would have been measured in tiny units). The scale has always been increasing, so a measured, fast velocity early on, will always be faster than subsequent velocities .. and it will continue to increase. This is what is meant by the phrase: "the metric expansion of space" .. ie: 'the metric', (or scale), also expands over time.

Cheers

caveman1917
2012-Apr-07, 11:58 PM
So by measuring the redshift from the most distant Galaxy observed sofar (UDFj-39546284 at around 13.2 Billion light years) we are measuring the expansion of the Universe for that amount of time.

Yes. A perspective that might help is to think about what "redshift" actually means. It means that the wavelength of light has increased. Wavelength is quite simply the distance between one crest of a wave and the next. Suppose that the wavelength of some light that was emitted at some time is 1 meter. During the time this wave travels towards us, the universe expands, and so does the wave. Suppose at the time this wave arrives at us its wavelength has become 10 meters, so we say it has redshifted by a factor of 10, which directly translates to the universe having expanded by a factor of 10 between the time the light was emitted and when it was received. The universe expanding means that distances between "things" get larger over time, and so does the distance between consecutive crests of a lightwave, which we measure as redshift.

That point is clear, but it would mean that at some point it was "expanding" faster than what it is now, otherwhise we would not be able to see that far away (unless...)!

I can see the problem you have with that, unfortunately the solution is not all that easy. We can see stuff that has been receding at a greater speed than the speed of light because, as Selfsim says, the hubble constant has been decreasing over time. This however doesn't mean that the expansion has been slowing down (well, it has been slowing down for a period in our history, but that's not the reason), a constant hubble constant means an exponential expansion. Anything less than that will get you a decreasing hubble constant, even though the expansion may be accelerating, just as long as it isn't exactly exponential.

Consider what the hubble constant actually means. Suppose you draw an imaginary sphere around us at a constant distance. What the hubble constant tells you is how fast a galaxy recedes through that sphere. So you don't track the galaxy itself, you just look at a fixed sphere and see how fast stuff passes that sphere. A decreasing hubble constant then means that over time stuff will pass slower at this constant sphere. But we can do something better with this, when we know the hubble constant, we can find the sphere around us through which objects recede from us at the speed of light, this is called the hubble sphere. This is the important bit to understand: because the hubble constant is decreasing over time, the hubble sphere is growing over time. The place where galaxies recede at the speed of light is getting further away over time.

Now suppose we have at some time a galaxy that is exactly at our hubble sphere emit a photon towards us. Since the galaxy is receding at the speed of light, and the photon is coming towards us at the speed of light, you would think that the photon would stay stationary from our point of view. However because the hubble sphere increases over time, if we wait a bit that photon that has stayed stationary will now be within the "new" hubble sphere. It is still "trying to come towards us" at the speed of light, but the galaxy where it is now passing (a different one than the one that emitted it) is moving away from us slower than the speed of light, so the photon is beginning to win the struggle and starts coming closer to us, and will eventually reach us. Depending on how fast the hubble constant is decreasing, this also works out for a photon that started at some distance beyond the hubble sphere, just as long as the hubble sphere is increasing faster than the photon is "losing ground" so that the hubble sphere will overtake it. That's basically why we can see galaxies that are, and always have been, receding faster than the speed of light.

caveman1917
2012-Apr-08, 12:13 AM
In the analogy objects with mass consume space, and space is being added everywhere, faster than it can be consumed

Interesting analogy, you're basically stitching together proper coordinates of an FLRW spacetime and GP coordinates of a schwarzschild spacetime. That should indeed do the trick quite well as far as analogies go.

Selfsim
2012-Apr-08, 03:29 AM
Just a couple of other aside points worthwhile noting ...

i) The effects discussed in this thread are a function of the Cosmological model adopted (in this case, the 'Standard Cosmological Model' ... ie: accelerating expansion, etc). These models start out with specified physical 'assumptions'. For example some typical 'assumptions' might be: whether the Universe expands, contracts, oscillates, rotates or remains static.

ii) These models can also be written as solutions for the General Relativity, (GR), field equations, plugged into them .. and if they satisfy those equations, then GR can then provide the gravitational theory for them. Doing this also serves as a check to ensure that the physical assumptions are valid in a universe described by GR principles. This kind of 'reverse engineering' approach to solving the equations is, to the best of my knowledge, the only way anyone has been able to 'solve' the field equations so far, also. This step is also completely separate from the main topic of the thread, I might add ... (I thought I'd just throw it in as an interesting 'aside').

iii) In discussing redshift, cosmologists choose to define the frequency of light shifting, rather than a wavelength 'stretching' interpretation, for a very good reason. Frequency has the units of reciprocal time. Cosmological redshift is a result of time dilation, (described in caveman's last paragraph in post #9), as derived from the R-W metric, rather than the wavelength 'stretching'. The 'stretching' of wavelength implies an erroneous interpretation of loss of lightwave energy, which then fatally leads to the 'tired light' hypothesis, (which has been disproven).

iv) Further to the GR points made in (ii) above, GR also "applies" to non expanding cosmological models, where time dilation doesn't exist.
I should point out again that 'Metric expansion', (cosmological redshift), is a property of the cosmological model not GR. The only form of redshift which is derived by GR/SR, is gravitational redshift - which is a completely separate topic, (and cause), from cosmological redshift.

I hope the above helps to avert a couple of common misinterpretations, which I have unfortunately encountered elsewhere in the past, when discussing this topic.
:)
Regards

caveman1917
2012-Apr-12, 12:44 AM
iii) In discussing redshift, cosmologists choose to define the frequency of light shifting, rather than a wavelength 'stretching' interpretation, for a very good reason. Frequency has the units of reciprocal time. Cosmological redshift is a result of time dilation, (described in caveman's last paragraph in post #9), as derived from the R-W metric, rather than the wavelength 'stretching'. The 'stretching' of wavelength implies an erroneous interpretation of loss of lightwave energy, which then fatally leads to the 'tired light' hypothesis, (which has been disproven).

Why are you making this distinction? We are in vacuo so it doesn't really matter whether we consider frequency or wavelength since \lambda f = c. Also the change in frequency and time dilation in the FLRW metric are derived through the "stretching" of the wavelength, not the other way around. If nothing else it's at least one of those few simple analogies that are in fact equivalent to the actual math, "things expand and so do consecutive crests of a lightwave".

Selfsim
2012-Apr-12, 07:42 AM
Why are you making this distinction?I have, elsewhere, in the past, encountered pseudoscientific 'enthusiasts', determined to refute comsological redshift, by glorifying Zwicky's 1929 Tired Light hypothesis.
Happy to accept that my wording could have been better on this occasion.

Cheers

2012-Apr-12, 09:08 AM
First of all thank you for your replies.

I believe to have integrated most of the concepts, and also been reading some other interesting articles.

Meanwhile there is a question that raised and still not been able to reply adequately.

The Universe "expansion" or "stretching" does not affect the Solar system, nor a Galaxy for example, as the forces inside those systems are more important than space stretching itself.
Going "back" to the Big Bang, or just after, that same space expansion was able to "tear apart" a system far more dense than any Galaxy today and spread it to for the Universe we see today!
I'm surely missing a bit there, but haven't found it yet, and if someone could enlighten me, I would appreciate!

Regards

Cougar
2012-Apr-12, 01:29 PM
The 'stretching' of wavelength implies an erroneous interpretation of loss of lightwave energy, which then fatally leads to the 'tired light' hypothesis, (which has been disproven).

You better tell Brian Greene that. In his 2011 book, The Hidden Reality, he decidedly uses the stretching wavelength analogy at p.137:

"It's as if light waves are threads stitched through a piece of spandex. Just as stretching the spandex stretches the stitching, so expanding the spatial fabric stretches the light waves."

On this topic, Greene's index also refers to endnote 5 at p.336, where I expected a more scientific explanation. But no. Not only did he stick with the idea that photon wavelengths stretch as they travel through long expanses of expanding space, he also casually notes that this causes a corresponding decrease in the photon wavelength's energy:

"...the stretching of space causes the wavelength of photons to stretch too, and, correspondingly, their energy to decrease..."

Of course, tired light has been discussed and debunked on this board numerous times. Apparently Greene does not think that such energy loss due to the expansion of space implies a tired light explanation. As I recall, any other mechanism that could cause such energy loss would also cause some scattering of the photons. But such scattering is not observed, since we consistently see very distant objects in remarkably sharp focus. That is why tired light fails. In fact, Greene goes on to say in note 5:

"But the stretching of photons - the diminishment of their energy - has another effect: It accentuates the dimming of a distant source."

Greene points out that to properly determine the distance to a distant source, this extra dimming must be taken into account, along with the usual inverse-square dilution of photon number density due to distance.

Now, I'll be the first to admit that The Hidden Reality is dumbed-down for a general audience. It is not a textbook on General Relativity. But if photon stretching during travel is a flawed analogy, I would expect Greene to clear this up in the more scientifically accurate endnotes. He did not.

caveman1917
2012-Apr-12, 08:51 PM
The Universe "expansion" or "stretching" does not affect the Solar system, nor a Galaxy for example, as the forces inside those systems are more important than space stretching itself.
Going "back" to the Big Bang, or just after, that same space expansion was able to "tear apart" a system far more dense than any Galaxy today and spread it to for the Universe we see today!
I'm surely missing a bit there, but haven't found it yet, and if someone could enlighten me, I would appreciate!

The problem is in saying that the solar system doesn't expand because of the forces inside. This is strictly speaking incorrect. The concept of expansion simply doesn't apply in a non-homogeneous background. Talking about expansion is only meaningful when you have constant curvature. To visualize this, think of a sphere that some time later has become a bigger sphere. Then you can talk about the sphere having "expanded". If however you have gravitational gradients (the mass in a system not being homogeneous) you will in general have some weird shaped geometry that some time later becomes a different weird shape, and then you can't meaningfully talk about whether it "expanded".

So rather than say that the solar system doesn't expand because the forces inside "win" against the expansion, better would be to say that the entire concept of expansion simply doesn't apply to such systems, only to the universe at large (which you can take to be homogeneous).

caveman1917
2012-Apr-12, 08:54 PM
I have, elsewhere, in the past, encountered pseudoscientific 'enthusiasts', determined to refute comsological redshift, by glorifying Zwicky's 1929 Tired Light hypothesis.
Happy to accept that my wording could have been better on this occasion.

Cheers

Well, light does lose energy by the stretching of the wavelength over time. The difference is what cause is attached to that phenomenom. The current model says that the cause of this is that the wavelengths are stretched by the expansion of the universe, where the tired light model says that the wavelength is stretched because the light loses energy to some undetected medium permeating space. It just seemed a bit odd that you seemed to be arguing against the light losing energy by its wavelength stretching, since that is the same in both models, they merely differ in which cause they assign to that.

caveman1917
2012-Apr-12, 08:58 PM
Now, I'll be the first to admit that The Hidden Reality is dumbed-down for a general audience. It is not a textbook on General Relativity. But if photon stretching during travel is a flawed analogy, I would expect Greene to clear this up in the more scientifically accurate endnotes. He did not.

It's not a flawed analogy, it's one of the few that are in fact accurate wordings of the actual math.

Selfsim
2012-Apr-12, 10:47 PM
Well, light does lose energy by the stretching of the wavelength over time. The difference is what cause is attached to that phenomenom. The current model says that the cause of this is that the wavelengths are stretched by the expansion of the universe, where the tired light model says that the wavelength is stretched because the light loses energy to some undetected medium permeating space. It just seemed a bit odd that you seemed to be arguing against the light losing energy by its wavelength stretching, since that is the same in both models, they merely differ in which cause they assign to that.Agreed !

Apologies to all for creating the confusion. (I was also unaware of the BAUT history on this topic, as well).

My statements about energy/wavelength changes, due to 'stretching', withdrawn.
I was intending to address the causal interpretation, although I didn't make this clear.
.. A bad day at the office !

I think my reaction was caused by the 'stretching' term .. admittedly an over-reaction in this particular thread.

Regards

Selfsim
2012-Apr-12, 11:04 PM
You better tell Brian Greene that. … Brian's cool ...
Thanks for the reference, and post, there Cougar.
From memory, in 'Fabric of the Cosmos', I think he goes into more detail and explains further (in a historical context) ..
Cheers

Cougar
2012-Apr-12, 11:22 PM
From memory, in 'Fabric of the Cosmos', I think he goes into more detail and explains further (in a historical context)

I think Fabric was much better than Hidden Reality. I also like Greene, but I'm pretty disappointed with Hidden Reality. [ETA: At times, it's almost like reading Michio Kaku, as in: "Let me tell you the very weirdest stuff out there today..."] It's probably my imagination, but I wonder if he came out with that book similar to why a currently popular actor accepts a lot of film offers - he may not be so popular after some years and won't be getting those offers... Sorry - rank speculation.

In some sense, I'm disappointed with the messenger, since Greene is describing a wide variety of multiverse scenarios proposed by various other scientists. Still, he's promoting the concept, and I remain thoroughly unconvinced. Especially when he throws in Everett's many worlds. [begins muttering incoherently]

caveman1917
2012-Apr-12, 11:34 PM
Greene is describing a wide variety of multiverse scenarios proposed by various other scientists. Still, he's promoting the concept, and I remain thoroughly unconvinced. Especially when he throws in Everett's many worlds.

Don't worry, there's bound to be an infinite number of you's in other universes who are convinced :p

Cougar
2012-Apr-13, 12:30 AM
Don't worry, there's bound to be an infinite number of you's in other universes who are convinced :p

I think they'd all be convinced that that is extremely funny. :lol:

Jeff Root
2012-Apr-13, 01:06 AM
caveman,

It could be that the expansion occurs on small scales,
such as within the Solar System. It also could be that
it doesn't occur on small scales. If it does, in principle
it could be measured. But as I suggested above, it
would pretty much have to shut off and turn back on
again so that we could measure the difference between
conditions with the expansion turned on and conditions
with the expansion turned off. And even then the
difference would likely be unmeasureably small.

-- Jeff, in Minneapolis

Gomar
2012-Apr-13, 01:47 AM
Hi all,

1 - If I got it right objects that are furhter away from Earth appear to be moving away from us faster than objects that are closer to us (very basic version).
Regards

If a plane left the airport at 1pm flying 500mph to Paris, and then another plane takes off at 3pm flying 750mph, eventhough
it left later, it will eventually catch up and overtake the 1st plane. Same with Earth, and galaxies. Some are infront, and
some behind our point of reference. Thus, if our galaxy is flying 1/4 SOL, while Andromeda is at 1/2 SOL, we'll never catch
up with it. But we do see its light.

Jeff Root
2012-Apr-13, 02:43 PM
Gomar,

I don't understand your point. It sounds like it might not be relevant.
Maybe it is just that the examples you used are inappropriate.
Earth, the Milky Way, and Andromeda are all local to each other
compared to the distances involved in cosmic expansion, and none
of them is moving away from the others.

-- Jeff, in Minneapolis

caveman1917
2012-Apr-14, 01:05 AM
It could be that the expansion occurs on small scales,
such as within the Solar System. It also could be that
it doesn't occur on small scales. If it does, in principle
it could be measured. But as I suggested above, it
would pretty much have to shut off and turn back on
again so that we could measure the difference between
conditions with the expansion turned on and conditions
with the expansion turned off. And even then the
difference would likely be unmeasureably small.

-- Jeff, in Minneapolis

Well it's certainly possible that it actually might be expanding on that scale, but it's a bit stretching things to claim that it actually does but is countered by other forces (not that i'm saying you did, just a general remark), the way the current model works it doesn't apply on that scale (or rather that degree of inhomogeneity). In any case, even if it did, we wouldn't be able to measure expansion per se, only the acceleration of it.

Jeff Root
2012-Apr-14, 08:07 AM
It could be that the expansion occurs on small scales,
such as within the Solar System. It also could be that
it doesn't occur on small scales. If it does, in principle
it could be measured. But as I suggested above, it
would pretty much have to shut off and turn back on
again so that we could measure the difference between
conditions with the expansion turned on and conditions
with the expansion turned off. And even then the
difference would likely be unmeasureably small.
Well it's certainly possible that it actually might be expanding
on that scale, but it's a bit stretching things to claim that it
actually does but is countered by other forces (not that i'm
saying you did, just a general remark), the way the current
model works it doesn't apply on that scale (or rather that
degree of inhomogeneity).
This is about the eighth or tenth time that the question
has been asked on BAUT that I've noticed, whether the
expansion affects small scales. When I have added my
comments, I have usually been the only one to suggest
that the effect might not apply to small scales. All the
regular question-answerers as knowledgeable or more
knowledgeable about relativity and cosmology than I am
have always said that objects smaller than galaxy clusters
are held together by gravity or electric and nuclear forces.
This thread is the very first time anyone other than I
suggested that it just doesn't apply on small scales.

My reasons for suggesting it are:

1) No small-scale expansion has been observed,
2) The cause of the expansion is unknown, and
3) My pet idea suggests a cause that would only
apply on very large scales.

(1) doesn't mean much since the expansion is too feeble to
detect on small scales in any case, and I don't take (3) very
seriously because it is such an obvious idea that somebody
more capable than I must have considered it already. So
reason (2) predominates.

Some instances of the question being raised involved the
possible "Big Rip" in which everything eventually flies apart.
This is obviously based on the notion that the expansion
does apply on all scales, and the last time I saw it mentioned
on BAUT -- less than a year ago, I think -- it was put forward
as a likely scenario, with no dissenters.

-- Jeff, in Minneapolis

Cougar
2012-Apr-14, 01:53 PM