The truth behind the first cheesy special effects
February 23, 2005 5:57 PM   Subscribe

This addresses the distinction between expansion occurring within space and the expansion of space itself, a frequent stumbling block. It's an interesting article, Gyan, and thanks for posting it.
posted by Songdog at 6:08 PM on February 23, 2005

"Even astronomers frequently get it wrong".

And so we're sure they got all this right. Right? Or is that another common misconception?
posted by spock at 6:15 PM on February 23, 2005

And so we're sure they got all this right. Right? Or is that another common misconception?

"This" is experimentally verifiable. Or does this boil down to a question of agreeing on experimental results?
posted by AlexReynolds at 6:22 PM on February 23, 2005

Astronomy is the least experimentally verifiable of sciences.
posted by spock at 6:34 PM on February 23, 2005

Observationally, then.
posted by mr_roboto at 6:41 PM on February 23, 2005

Astronomy is the least experimentally verifiable of sciences.

They make theories and verify them against the data they collect, same as every other science. What are you basing your opinion on, I'm curious?
posted by AlexReynolds at 6:47 PM on February 23, 2005

This balloon analogy should not be stretched too far.

Priceless..
posted by c13 at 6:51 PM on February 23, 2005

That was an excellent read, thanks for posting it. I have had several misconceptions corrected in a nice polite fashion, and learned a load of new stuff. If only everything went that smoothly.
posted by Gamecat at 6:53 PM on February 23, 2005

They make theories and verify them against the data they collect, same as every other science. What are you basing your opinion on, I'm curious?

I think he means that you can't perform a controlled experiment. Of course, this is not unique to Astronomy. It holds true for any of the "historical" sciences: geology, paleontology, zoology and botany, planetary science, oceanography, climatology, etc. I don't see where he's getting the "least" from. It seems pretty arbitrary.
posted by mr_roboto at 7:08 PM on February 23, 2005

It was totally worth reading for the aphid bit alone.
posted by Vulpyne at 7:08 PM on February 23, 2005

I'm a big astronomy buff (amateur only) and have a fair collection of books. I can't pull a particular quote out for you at the moment. I'm not downplaying science or anything. It is just that (most) astronomy (or perhaps I should more correctly say cosmology - which is what the Big Bang is all about) is not all about proving things by experimentation. I think it was one of Sagan's books that made the point. It is natural to think that what we know now is correct - but that is also what they thought not that many decades ago when the Steady State theory still held sway.
posted by spock at 7:13 PM on February 23, 2005

zoology and botany

As an experimental botanist/ecologist, I'm not entirely sure why you consider these historical sciences in which you can't perform a controlled experiment. Perhaps you mean "taxonomy"?
posted by Jimbob at 7:23 PM on February 23, 2005

Yeah; I meant the non-experimental zoology and botany. Probably a better term would have been "evolutionary biology". Even this would be a generalization, I suppose.
posted by mr_roboto at 7:48 PM on February 23, 2005

The part that bugs me is the assertion that all of space is expanding, but individual galaxies are not. I can imagine that the radius of an atom (say, of hydrogen) is an electromagnetically determined constant, and that as space stretches, the atom stretches a little and then recollapses to its preferred size. I can imagine the same line of reasoning working on the scale of the Earth.

But then I get confused once we get to the scale of a single galaxy. Why isn't a galaxy expanding as space stretches? If not, then it seems that some parts of space (those between galaxies, apparently) are stretching faster than others. Why is that?

Or should I just decide that galaxies are sufficiently ill-understood at this point (e.g. that we need to posit dark matter to explain how they manage to rotate so fast without breaking up) that the effects I'm complaining about would be negligible anyway as compared to our current level of misunderstanding?
posted by Aknaton at 9:57 PM on February 23, 2005

The article explained that pretty well. You are confusing things the movement or expansion of the objects that reside within space with the expansion of space itself (the distances between those objects).

Think of yourself on a moving walkway (like in an airport). You are standing still on it, as is the person 10 feet ahead of you. The distance between the two of you doesn't change. But if the walk way was made of elastic and was stretching as time went on, both you and the other guy are still standing in one place - but you are getting farther away from each other. (simplified, but helpful?)
posted by spock at 10:05 PM on February 23, 2005

But isn't space the distance between things? So while the electron (yourself) and the person 10 feet ahead (the nucleus) don't change size, you do move further apart.

So why does the space between galaxies -- or galaxy clusters, whatever -- expand, but not the space between parts of an atom?

Maybe it's not space that stretches, but time.
posted by five fresh fish at 11:05 PM on February 23, 2005

fff, the article explains this. The distances between parts of a coherent object are governed by the forces that cause those parts to cohere -- that cause it to be an object. Gravity holds a galaxy together and it's much stronger than the expansion force. An expanding universe exerts a bit of a tug on these objects and will result in them being minutely larger than they might otherwise have been, but they stabilize at that (very slightly) larger size and do not continue to expand -- the universe expands around them and while they "feel" the tug they resist it because of their binding force. If the expansion were to stop, these objects would contract slightly, once, and achieve a new equilibrium and remain stable at the slightly smaller size.
posted by George_Spiggott at 11:49 PM on February 23, 2005

So, objects are affected, but the effect is negligible. Initially, I got the impression that the reason was fundamental.
posted by Gyan at 11:52 PM on February 23, 2005

That's not quite what the article is saying, George_Spiggot. As I read it, it seem to say that with constant expansion matter and clumps of matter bound together by any force will not "expand". They try to explain how an accelerating expansion causes the slight outward pressure effect you're describing.

Now, I may have misread it. But the assertion I think they are making can't be true because all the galaxies are certainly bound together by gravity (no matter how loosely), and so if any "binding" forces negate expansion of clumps of matter, then the galaxies wouldn't be rushing apart! So I've obviously misread it, but I can't figure out what the problem is.

I would have given George_Spiggot's explanation.

A key in this, somehow, is that a constant expansion isn't an accerlation, while an accelerating expansion is. As an acceleration, we move into general relativity. We get something that can be considered the equivalent of a "force".

I've been involved in an email discussion with two friends about this article tonight, and this portion is the only thing that I didn't know or have trouble with.

I'm not terribly happy with how they've debunked the misunderstandings they've debunked—in this email conversation I've been involved in the authors seem to have created new confusion as much as they've eliminated old confusion.

Really, the key point they're making can be easily summarized (even if this isn't helpful for many people): the redshifting we observe because of relative motion is because of special relativity. But the redshifting that we observe due to the expansion of space is general relativity. The redshifting of distant galaxies can easily be confused with redshifting of relative motion; and, when it is, some wrong conclusions follow.

They try to explain, or allude to, black body radiation (as a way of inadequately explaining distinctive spectra and how we can see that they are shifted) and they use "temperature" in that context, which I think will likely mislead most readers. They complicate this further by comparing the expanding universe to an expanding cloud of gas that's under pressure (to explain cooling). It seems to me that they're inviting exactly the same sort of confusion they're debunking. Specifically, a gas temperature is not radiation temperature and a material pressure wave is not a radiation wave. The analogy is very convenient, but it invites misconception.
posted by Ethereal Bligh at 12:22 AM on February 24, 2005

Is the universe expanding or is light slowing down?
posted by ZippityBuddha at 3:25 AM on February 24, 2005

And is there any difference between the two?
posted by ZippityBuddha at 3:29 AM on February 24, 2005

It's expanding. Yes.
posted by Ethereal Bligh at 3:33 AM on February 24, 2005

How can we tell?

The speed of light is the constraint on our movement around space. If the speed of light changes, the 'distance' between two objects changes. Its no further away, it just takes longer (or shorter) to get there.
posted by ZippityBuddha at 3:48 AM on February 24, 2005

c is a measurement of both velocity and time. Assuming that the speed of light did change (which according to all current theory it can't), the physical distance between object wouldn't change but our perception of that distance might (compared to what we perceive now with our universal constant of c). But since c is fundamental to the way we see our universe and the means by which we exist, such comparisons are ultimately meaningless.

This stuff makes my head hurt.
posted by NeonSurge at 4:16 AM on February 24, 2005

Perhaps another crude illustration: Imagine a bucket of soapy water. Pour it all at once on a parking lot. Soap bubbles form that are held together by their internal structure - some cluster together, others become individual bubbles*. The puddle of water spreads out over a wider and wider area, carrying the bubbles farther apart from one another. But the fact that the medium (water=space) is getting larger (expanding) does not mean that the structure of the bubbles is expanding. The forces at work on the local level are negligable - certainly not enough to overcome the forces holding the bubble together internally.

*Interestingly, if you were able to stand back and map the distribution of galaxies in the universe, their shape would be like soap bubbles (they would appear to form on what would be the skin of clusters of bubbles) - or so they currently think.

Crappy illustration?
posted by spock at 6:06 AM on February 24, 2005

that is also what they thought not that many decades ago when the Steady State theory still held sway.

Not true. The Steady State theory never "held sway" -- it was clung to by Fred Hoyle and a few acolytes, and everybody else derided them. In the '60s, when contrary evidence became overwhelming, Hoyle gave up and renounced his theory.

This is a fantastic article, and I thank you for posting it, Gyan. I started it thinking smugly "Yeah, yeah, expansion like a balloon, galaxies themselves aren't expanding, I know this stuff." Then I hit things like:

Observers living in the Andromeda galaxy and beyond have their own observable universes that are different from but overlap with ours. Andromedans can see galaxies we cannot, simply by virtue of being slightly closer to them, and vice versa. Their observable universe also used to be the size of a grapefruit. Thus, we can conceive of the early universe as a pile of overlapping grapefruits that stretches infinitely in all directions. Correspondingly, the idea that the big bang was "small" is misleading. The totality of space could be infinite. Shrink an infinite space by an arbitrary amount, and it is still infinite.

and

In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us. The galaxy they came from, though, may continue to recede superluminally. Thus, we can observe light from galaxies that have always been and will always be receding faster than the speed of light.

and I began realizing this stuff is hard.

EB, George_Spiggot: I think you've both slightly misread the article. Rather than give my own summary, I'll quote the relevant paragraphs, bolding the crucial conditions:

For example, if gravity got stronger, your spinal cord would compress until the electrons in your vertebrae reached a new equilibrium slightly closer together. You would be a shorter person, but you would not continue to shrink. In the same way, if we lived in a universe dominated by the attractive force of gravity, as most cosmologists thought until a few years ago, the expansion would decelerate, putting a gentle squeeze on bodies in the universe, making them reach a smaller equilibrium size. Having done so, they would not keep shrinking.

In fact, in our universe the expansion is accelerating, and that exerts a gentle outward force on bodies. Consequently, bound objects are slightly larger than they would be in a nonaccelerating universe, because the equilibrium among forces is reached at a slightly larger size. At Earth's surface, the outward acceleration away from the planet's center equals a tiny fraction (10^–30) of the normal inward gravitational acceleration. If this acceleration is constant, it does not make Earth expand; rather the planet simply settles into a static equilibrium size slightly larger than the size it would have attained.

This reasoning changes if acceleration is not constant, as some cosmologists have speculated. If the acceleration itself increased, it could eventually grow strong enough to tear apart all structures, leading to a "big rip." But this rip would occur not because of expansion or acceleration per se but because of an accelerating acceleration.

Note "the expansion would decelerate," and not (as George has it) "if the expansion were to stop."

It's depressing but predictable that a MeFi post explaining subtle, exciting ideas of modern cosmology would be followed by standard-issue MeFi snarks about "but do they really know anything, ma-a-a-an?" Ah well.
posted by languagehat at 6:47 AM on February 24, 2005

EB - i've not read the article, but maybe the following helps clarify the issues you raise slightly (disclaimer - although i was once an astronomer, cosmology was never my strong point).

if you observe two points to be a fixed distance apart, for some period of time, even though space is expanding at a uniform rate, then they will stay a fixed distance apart. what else would you expect them to do? from the basic symmetry of the situation, they have to remain as they were. nothing has changed - space continues to expand at the same rate as when you were watching them. how would "nature" "know" that they should start to drift apart if they were previously stationary?

the above argument doesn't change if the rate of expansion varies because then symmetry is broken. one time is not like another - things have changed (the rate of expansion has changed), so a simple appeal to symmetry doesn't help you.

note that there's no physics in the above argument, really. it's just symmetry. well, there is physics because, in the end, physics comes down to symmetry. but the argument is stronger because it is so fundamental, not weaker because it doesn't obviously contain arguments about "forces" etc.

ok, that was the first thing. second was whether or not things move apart or stick together when the rate of expansion changes. the answer is that "it depends". if the expansion is very small, or the force attracting things is large then things will stay together. if the expansion is large, or the mutual attraction weak, they will drift apart.

hence the different behaviour of stars and galaxies.

inside a galaxy, stars whizz round in a local "gravitational well". they're all close together (relatively) and so forces are strong enough to keep them there (gravitational force is inversely proportional to distance^2). separate galaxies, however, tend to be much further apart and so, typically, expand away from each other.

there are exceptions - clusters of galaxies, or small groups, interacting pairs etc - where there are sufficient galaxies in a small enough space for this not to be true.

and if you imagine the history of the universe, with expansion initially being strong and slowly declining, you can see how with time, as expansion decreases, collections of matter on different scales transition. at some point, as expansion slows, gravity becomes strong enough to overcome it, and things start to collapse. how this happens (small things first? large things?) depends on various complicated details (what is the distribution of fluctuations in density?) and is the kind of thing that people were very worried about until recently, because no-one could get the numbers to add up. if you simulate things, you didn't get stars forming soon enough, or galaxies were too big, or didn't form, or whatever - i don't know the details. what currently saves the day is the addition of an extra expansion factor that appears to make everything work out ("dark energy"), but no-one currently has much of an idea what that "really" is....
posted by andrew cooke at 6:50 AM on February 24, 2005

well, steady state was accepted for a time. it was wrong. what we have now might be wrong. probably, certainly is. the important point isn't that this means science has somehow failed, but that we're getting better. steady state was a good first approximation - nice and simple, fit the data. when we got more data it had to go. in some sense it was a dead end, so we back-track a bit and look for a better explanation. we're making our way through a maze. sometimes we have to go back a bit and then edge forwards again. the point is that we slowly(?), on average, move outwards, even if we sometimes have a tactical retreat. science is flexible. criticism makes it stronger, but it's never "perfect". it just keeps getting better (in a pretty well defined way - we can reproduce the results of previous generations and explain the stuff they couldn't).
posted by andrew cooke at 6:57 AM on February 24, 2005

Of course, the real question, brought to us by jhilton™ in this thread, is how to create The Next Big Bang™.
posted by casu marzu at 8:01 AM on February 24, 2005

But the assertion I think they are making can't be true because all the galaxies are certainly bound together by gravity (no matter how loosely), and so if any "binding" forces negate expansion of clumps of matter, then the galaxies wouldn't be rushing apart!

This is another common misunderstanding. If you read the article carefully, especially languagehat's excerpts above, what is being stated is that the universe is isotropic.

At the largest scales, it makes no sense to say that objects within the Universe are gravitationally bound. If our Galaxy for example is being "pulled" in all directions, then it is being pulled in no direction. As a first approximation this is true even if you use Newtonian Gravity (although its easier to see if you use Gauss's Law and the common example of how the G field behaves in a hollow sphere)

Still, if you apply energy conservation laws, the gravitational potential (which does not cancel out) and which is negative, implies that everything *must* have a velocity with respect to everything else. This is the simplest way to see the argument that the Universe must be expanding (or contracting) and that Steady state (with nothing like matter creation) is an unstable state.

Also, to reiterate, a constant expansion produces no forces. Force by definition, is acceleration (remember F=ma?) An accelerating expansion would produce an isotropic outward force on everything. Its trying to stretch everything apart.

Another thing worth repeating is that whenever Cosmology talks about the Universe, it always means "the obervable Universe." It could be that the "actual" Universe could be infinite in all directions. the problem with making any inferences about what is "outside" is that, according to modern physical theory, it cannot affect us in any way. (That's not strictly true since we may in the future be affected by galaxies that were affected by galaxies we will never see but thats another topic)

Still, theres lots more interesting stuff here including Inflation and how we explain the general uniformity, the formation of the CMB etc. etc.
posted by vacapinta at 8:56 AM on February 24, 2005

whenever Cosmology talks about the Universe, it always means "the observable Universe"

That is a good point. We are continually learning to "see" the universe in a "new light" (pun sorta intended). First we simply had visible light. Then we broke that light into spectrum (astromonical spectroscopy). Since that time, the ability to "see" beyond the visible spectrum has given us additional information (infrared, x-ray, ultraviolet, neutrino, etc.) Some of them have required putting instrumentation outside our atmosphere to evade its filtering effects.
posted by spock at 9:56 AM on February 24, 2005

whenever Cosmology talks about the Universe, it always means "the observable Universe"

is it that simple? doesn't the cmb show power fluctuations in areas of the universe which are outside our cone doodah? in which case we can say something about parts of the universe we cannot currently observe, right? this is the kind of thing that makes my head hurt...
posted by andrew cooke at 10:10 AM on February 24, 2005

I'm confused about universe vs. galaxy - the author speaks about our universe vs. the Andromedan's universe - wha? Are not the Milky Way and Andromenda galaxies in the same universe? We can't see any other universes yet, can we?
posted by mouthnoize at 11:07 AM on February 24, 2005

is it that simple? doesn't the cmb show power fluctuations in areas of the universe which are outside our cone doodah?

Well, no, not really that simple. If you really want to get into it there are actually several different horizons. The CMB defines one horizon which is different than the event horizon or whats called the particle horizon. The reason has to do with the specifics of how cosmic expansion has taken place. There are galaxies for example that expanded away faster than our light cone and so they are no longer visible but they once were (remember: things can expand away faster than light and it doesnt violate relativity)

One other thing that they would have done well to clear up: When we look at distant galaxies we are also looking backwards in time. People understand this. But also keep in mind that "back" in time the Universe was also smaller. This means that the exact shape of our light-sphere is not really a sphere but another topological entity altogether, more of a pear-shape.

An example might help: If we can actually see far back enough to see young galaxies then a galaxy we see when we look North up at the sky and a galaxy we see when we look South may actually be/have been very close to each other. Space itself gets compressed as we look into the past.

Anyways, this article might help. And here's some pictures which may help get your head around it.
posted by vacapinta at 11:11 AM on February 24, 2005

bleagh. ok, i've just read the article. the last section's equivalence of expansion with a force is a bad move, imho. i can see why that's confusing people. it's simpler, in my experience, to not think of expansion as a force at all, but simply to imagine space getting bigger between things - "growing" in every gap. then you need forces to overcome that and keep things together (if the expansion accelerates). but it's not "a force" because if it dominated your motion (everything flying apart - imagine two massless points expanding with the universe) you'd actually feel no acceleration. calling something a force when it doesn't cause acceleration in your local rest frame is a bit sneaky (as you say, F=ma).

imagine flicking a switch that turned off electromagnetism and gravity in an expanding universe. if the expansion increases, atoms will drift apart. but no-one is experiencing what i, at least, whould call a force.

hoping that makes sense...
posted by andrew cooke at 12:17 PM on February 24, 2005

Great article. Thanks, Gyan.
posted by homunculus at 12:57 PM on February 24, 2005

What I'm having a problem with is the assertion that galaxies aren't "expanding"; or, put more precisely, there is no redshift differential between stars nearest to us in another galaxy and the stars farthest from us in that same galaxy. They're not saying that the difference is undetectable or insignificant; they're saying it's not happening at all. I just don't see how this can be the case where galaxies are, in contrast, redshifted with regard to each other in this way. Yes, the inverse square law; yes, galaxies are far, farther apart than stars in a galaxy are (but not proportionally, I don't think). But one good reason why astronomers don't like people to call gravity a "force" is because it's not bounded, I didn't think. Under GR, gravitational "attraction" between any two given objects, no matter how distant, may be incredibly small, but it still exists, doesn't it?
posted by Ethereal Bligh at 1:09 PM on February 24, 2005

galaxies aren't expanding because they stars are being pulled together by gravity. imagine two weights sitting on a rubber sheet, fastened together by a spring. now start stretching the (infinitely stretchable) rubber sheet at a constant rate. at first the two weights will be dragged apart by the sheet, but that will stretch the spring. as they are dragged further the spring will pull stronger until, at some moment, the pull of the string is equal to the dragging of the sheet, at which point the weights will start to slide on the sheet and remain at a fixed distance apart, even though the sheet underneath continues to stretch.

that equilibrium situation describes stars in galaxies. the combined mass of the galaxy forms a deep potential well. the expanding universe "drags" stars out of the well, to the point where they are displaced slightly from what you would otherwise expect, so that the increased gravitational pull counteracts the expansion. so there is no redshift.

i hope that helps. i wouldn't put money on it being a very correct explanation because, as i said, i'm not that good at this (and it raises further problems that i'll keep quiet about), but i think the general idea is more-or-less consistent with what the article is saying (you can safely assume that the article author knows much more than me! ;o)
posted by andrew cooke at 5:56 AM on February 25, 2005

Ah, the word "equilibrium" made it click for me.
posted by Ethereal Bligh at 10:06 AM on February 25, 2005