Why "expanding space"?

Peter Watkins

When the "faster with distance" view was first discovered the natural assumption was that this movement was ballistic. As far as I am aware, this remained the case for most of the rest of the 20th century. Nowadays this movement is said to be due to the expansion of space, (which, as an aside, begs the question, "where is the need for dark energy"?). So what aspect of the the original "expansion from a single point of origin, via the energy imparted by the big bang," cannot be explained without the need for either/or expanding space and dark energy?

Dmitry67

Dark energy is needed to explain why expansion is accelerating, while it should be slowed down by gravity

Chronos

What makes you think the universe arose from a 'single point of origin'? It was not a 'point' in the manner you suggest.

Peter Watkins

Logic. If a missile passes by and heads away from your right, then even if you didn't see it coming, it would seem logical that if you could reverse time, that it would come back and head away from your left. Similarly, it would seem logical that the galaxies that Hubble saw moving away from us and each other would come together at some point if time, and hence their movement, were reversed. As for the apparent acceleration, the "raisin bread" analogy can explain that.
And as Thaddeus says in post# 64 of "all on one page", if the big bang were an everywhere at once event, wouldn't galaxies be headed every which-way?

marcus

You are wildly misinformed, Peter. Ballistic was never the view of the scientific mainstream.

There were a small number on the fringe who tried to promote the "ballistic" interpretation. One name that comes to mind is Eddie Milne in 1935, a kind of contrarian or renegade. His ballistic view was not taken seriously by the professional majority.

The ballistic view gives a bad mathematical fit, for many reasons, one being that so much of the distance expansion we see goes on at rates far exceeding the speed of light. Verbally it may sound good to you but quantitatively the explosion picture works only in a vague kinda sorta way.

From it's beginning in 1915, General Relativity has been understood as a theory of dynamic geometry---of distances that change in other words.

As early as 1923 it was shown that GR applied to cosmology called for either a pattern of increasing distances, or the reverse: a pattern of shrinking distances.

No ballistics, Peter. That is only what you get in popularized accounts for the general public--and the journalists who put things in oversimplified terms like that do the public a disservice.

GR is our theory of gravity and its predictions have been shown to be exquisitely accurate. However it says you have no right to expect distances to remain constant unless locked to some physical system like a piece of metal, or rock, or a planetary orbits. Unless you have a model of gravity which can rival the precision of GR, and which does not have expanding largescale distance as corollary, you have to accept dynamic geometry, as scientists of our grandparents' and great-grandparents' generation already did back in the 1920s.

============

Btw I only respond to where you misrepresent the scientific consensus all thru the 20th century. We shouldn't mislead people about what the astronomers back then thought.

But if you personally want to believe in an explosion model, that's fine.

marcus

In case anyone wants to respond, here is Thaddeus post #64 from the other thread:

I don't think we've heard from Thaddeus for a while.

Peter Watkins

Hello Marcus. I'm sure you already know this, but Hubble went to his grave not believing Einstein's notions on space curvature etc.. Re Chronos on #2; the einstein-online. info that you yourself regularly promote, states that a reversal of time would see a gathering of all matter at a single point. With regard to a "bounce", it will not happen. This little "altered state" universe is a one off, single use, throwaway universe.

marcus

No indeed! I didn't already know this! You have taught me an interesting bit of history, if true. I wish you had a link to an online source for that Hubble story.

I'd rather not have to scroll thru pages of stuff searching so if you can give me a link and say how many paragraphs down the page I'd like to see where you think Einstein Online says everything in the universe was collected into one dense blob. They may have actually said everything in the currently observable portion of the universe. Or they may have qualified it in some other way. I'd need to see what they actually said, in context.

Ich

Hi marcus,

I tend to disagree. Since Old Smuggler set me on the right track (you witnessed), I learned to distrust the "Davis/Lineweaver-approach".
The Milne Model is just one coordinate transformation away from an empty expanding universe, and so are superluminal speeds from subluminal. Expanding space means increasing distances - nothing else -, and there is always a region of space, or a local coordinate system, where it is sensible to speak of increasing distances as velocity. It's just a matter of coordinates.
Of course, our universe is not empty, and the Milne model is an artificial setup like the ether, but it presents a different point of view - at least locally - that helps to demystify some coordinate-dependent statements that tend to mislead struggling students like me. One of these is the notion of motion through space as opposed to motion of space. Motion through space is not a well defined concept, it contradicts the principle of relativity, and in fact makes sense only in an (admittedly somehow "preferred") coordinate basis.
It's my point of view that we can break down "cosmological mechanics" to the familiar concepts of ballistic motion and gravitational influence on a local scale, and that GR allows the extrapolation that this picture is valid at every point in space, mostly because it gets rid of the concept of gravitational (proper) acceleration.
Sorry, this post seems to be somewhat incoherent, but as promised I will try to explain my thoughts in the "Balloon Analogy" thread.

russ_watters

Everything in that explanation is fine, but it doesn't necessarily imply what you claimed it does. What you are missing is just that since every point in the unverse was at that single point of origin, after the big bang, every point remains a usable origin for the expansion. The rasin bread analogy is just an analogy and is not meant to show an edge or center. You are misusing it.

Your problem here is your understanding of the geometry....in addition to a simple refusal to consider that your view could be wrong: You keep asking the same questions over and over. You aren't going to get different answers.

marcus

Thanks for the link, Ich. I'll go back and have a look at your conversation with Old Smuggler.

For me, what anchors me whenever we have a controversy about this at PF is the CMB.
The bath of radiation from the evenly dispersed matter in the early universe provides a criterion of rest.

If some galaxy really were moving at 0.999 c then its people would be roasting from a Doppler hotspot in the CMB sky.

Coordinate transformations that depart from universal rest seem contrived and unintuitive to me.

That and the fact that the Hubble law and Friedman model use my kind of coordinates and my kind of distance.

I'll have to see what Old Smuggler could have said to so Milnerize you. I know you're smart so there must be something persuasive in it.

Ich

No, it's not that bad. I'm fully aware of the advantages of a global coordinate system, and I'd never say that "private space"-coordinates are the only ones. Nor do I think of the Milne model as more than a useful toy model.
But I believe that the description in comoving coordinates somehow got reified among many cosmologists and especially in public outreach, giving rise to some misconceptions and inappropriate mysticism.
Now, I start sounding like a crank. I better show what I mean in a separate post.

marcus

No! you are certainly not sounding like a crank.
The best paper I know in line with what you say is Bunn and Hogg (2008).
I will get the link.
They show that cosmo redshift can (and they think for mathematical rigor's sake ought to) be considered as the cumulative limit of a very large number of small Dopplershift steps.
I think there is no question about this, and that the paper is quite solid and well-intentioned.

They see the main danger that people will re-ify space and think of it as a rubber sheet and believe that the wavelengths are actually stretched by being in this physical material which is being stretched. So they defend against what they see as the main danger.

What I see as the worst obstacle to understanding that people come here to PF with is something different. Newcomers think of the cosmo redshift naively as a one-time (or perhaps two-time) Dopplershift involving one or two definite velocities----and they picture a cosmology as an explosion. In my opinion this misconception is more primitive and deep-rooted than the rubbersheet reification (imaginary materialization) and more of an obstacle.

In fact the excellent Bunn Hogg paper shows that cosmo redshift is not any simple kind of Doppler. It can be analyzed mathematically as an infinite series of small "epsilon" Dopplers. The effect depends on the whole expansion history while the light is in transit.

And if one should make a mental picture of that long series of small Dopplers, it comes to much the same as the stretching rubber, except that there is no rubber.

So everybody has their own different pedagogical problems, and needs different tracts, pamphlets, and sermons
Bunn Hogg is a sermon preached from the other side, but it is a good one which actually supports the conventional view as well, that the way to treat redshift is simply as

1 + z = a(now)/a(then)

the ratio of the Friedmann model scale-factor.

http://arxiv.org/abs/0808.1081
The kinematic origin of the cosmological redshift
Emory F. Bunn, David W. Hogg
14 pages; Am. J. Phys.
(Submitted on 7 Aug 2008)
"A common belief among cosmologists is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must instead be viewed in terms of the stretching of space. We argue that the most natural interpretation of the redshift is in fact as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts..."

I haven't followed all the discussion about this topic. It's possible you have already seen this paper, but if not I think you would like it.

tickle_monste

If the universe is expanding, what is it expanding relative to? Couldn't one say there's an equally accurate perspective from which one would see something other than an expanding universe? If the universe had a boundary that increased in size over time, THEN one could say that the universe is actually expanding. But if the universe had a boundary that was expanding, it would be expanding relative to its own center, and if you were to be floating right along next to the boundary as it expanded, you would be able to look at the center and see everything getting smaller. As I understand it, the universe has no boundary, and no center, so this hypothetical scenario doesn't work, and all motion can still stay relative. But I just don't see how the universe could be called "expanding" without having an expanding boundary and a center relative to which it can be called "expanding". As an aside, the way I see it, cooling is generally linked to contraction in size. A cool galaxy should seemingly occupy much less space than it would were all its energy just "free".

Wallace

The problem with this whole discussion is that both in the public domain (such as these forums) and the scienitific literature, much of the noise is made by the least informed. People can convince each other of all kinds of non-sense but unfortunately it doesn't make it any more right.

Bunn and Hogg seems like a reasonable paper, I think anyone who reads and understands that should be satisified and no longer feel the need to endlessly debate this uninteresting topic. The problem is that every few years someone comes along and re-mangles the ideas all over again (see for instance several papers by Ambramowicz et al in recent years) that requires someone else coming along to set the story straight again.

The only thing I would tell anyone about the expansion of space is that it is a metaphor. Not physics. If you like, you can debate the usefulness of the metaphor, but the problem that comes up over and over again is that people start debating and motion over the surface of balloons and through rising bread and think they are discussing physics. They are not. It is simply not possible to assess the truth or otherwise of the Big Bang by thinking and talking about expanding space. Unfortunately this is what Peter Watkins and others are trying to do. It simply won't work because the expansion of space is a flawed metaphor, as all metaphors must be at some level. You can break the metaphor very easily but that does not mean you have broken the physics. You need to understand the physics to do that, and to that there is no shortcut.

The science behind modern cosmology makes no reference to the ideas that are most frequently discussed in forums such as these as if they were fundamental pillars. I'm not sure what the solution is, but it's a sorry state of affairs.

marcus

I am glad you think so, Wallace. I spotted it when it first came out and thought it was excellent. That is, mathematically. The interpretive part where they preach a little sermon subsequent to their mathematics----well if you find the controversy over different people's spins tiresome, I would heartily agree!

Peter Watkins

Marcus, einstein-online.info cosmology/spotlight on relativity/tale of two bangs, para.3.