There seems to be something in the air these days that is making people speak out against the idea that space is expanding. For evidence, check out these recent papers:
The kinematic origin of the cosmological redshift
Emory F. Bunn, David W. HoggA diatribe on expanding space
J.A. PeacockExpanding Space: the Root of all Evil?
Matthew J. Francis, Luke A. Barnes, J. Berian James, Geraint F. Lewis
Admittedly, my first sentence is unfair. The correct thing way to paraphrase the underlying argument here is to say that “space is expanding” is not the right way to think about certain observable properties of particles in general-relativistic cosmologies. These aren’t crackpots arguing against the Big Bang; these are real scientists attacking the Does the Earth move around the Sun? problem. I.e., they are asking whether these are the right words to be attaching to certain indisputable features of a particular theory.
Respectable scientific theories are phrased as formal systems, usually in terms of equations. But most of us don’t think in equations, we think in words and/or pictures. This is true not only for non-specialists interested in science, but for scientists themselves; we’re not happy to just write down the equations, we want sensible ways to think about them. Inevitably, we “translate” the equations into natural-language words. But these translations aren’t the original theory; they are more like an analogy. And analogies tend to break under pressure.
So the respectable cosmologists above are calling into question the invocation of expanding space in certain situations. Bunn and Hogg want to argue against a favorite cosmological talking point, that the cosmological redshift is not an old-fashioned Doppler shift, but a novel feature of general relativity due to the expansion of space. Peacock argues against the notion of expanding space more generally, admitting that while it is occasionally well-defined, it often can be exchanged for ordinary Newtonian kinematics by an appropriate choice of coordinates.
They each have a point. And there are equally valid points for the other side. But it’s not anything to get worked up about. These are not arguments about the theory — everyone agrees on what GR predicts for observables in cosmology. These are only arguments about an analogy, i.e. the translation into English words. For example, the motivation of B&H is to do away with confusions in students caused by the “rubber sheet” analogy for expanding space. Taken too seriously, thinking of space as an expanding rubber sheet convinces students that the galaxy should be expanding, or that Brooklyn should be expanding — and that’s not a prediction of GR, it’s just wrong. In fact, they argue, it is perfectly possible to think of the cosmological redshift as a Doppler shift, and that’s what we should do.
Well, maybe. On the other hand, there is another pernicious mistake that people tend to make: the tendency, quite understandable in Newtonian mechanics, to talk about the relative speed between two far-away objects. Subtracting vectors at distinct points, if you like. In general relativity, you just can’t do that. And realizing that you just can’t do that helps avoid confusions along the lines of “Don’t sufficiently distant galaxies travel faster than light?” And reifying a distinction between the Doppler shift and the cosmological redshift is a good first step toward appreciating that you can’t compare the velocities of two objects that are far away from each other.
The point is, arguments about analogies (and, by extension, the proper words in which to translate some well-accepted scientific phenomenon) are not “right” or “wrong.” The analogies are simply “useful” or “useless,” “helpful” or “misleading.” And which of these categories they fall into may depend on the context. Personally, I think “expanding space” is an extremely useful concept. My universe will keep expanding.
Hi Geraint,
> “Sigh – this is a coordinate dependent quantity and so there is no single “correct” answer.”
So much sighing and groaning… I feel like the foolish Kung Fu Grasshopper at the knee of the great master.
I have agreed that there are an infinite number of coordinate systems which yield an infinite number of different perspectives. BUT, I think there is at least one coordinate system in which the relationships I proposed might be correct.
What’s important here is not the *absolute* values of distance, speed and acceleration, it’s the relationship of relative values as a function of time and density as perceived in the distinct frames of the emitter and observer. Surely one or both of those frames has some special logical relevance to the issue of observed redshift.
Geraint, your papers refer frequently to superluminal recession velocities, which you say are inevitable and open ended at large distances in both the FLRW and conformally Minkowski metrics. You also apply Gauss’ Law in your radar ranging paper in the same manner I do, and you specifically apply it to round-trip velocities in your two selected coordinate systems. You also specifically state that redshift is a result of a discrete transformation at the observer’s frame rather than an accumulation of infintesimal redshifts en route. The terminology I’m using and the points I’m trying to make are similar to those in your papers, although the particular redshift mechanism is original.
Why not address my specific questions and ideas rather than reciting broad mantras: “There are no absolutes my son … everything is relative… ”
Jon
Jon Corthell,
“I agree that photons don’t accelerate LOCALLY. But at cosmological distances, their travel speed relative to both the observer and emitter MUST change. Otherwise, how could distant galaxies have been receding at superluminal velocities at the time of emission, yet the photons approach us at exactly c today?”
You grasp the math better than me, but I think you have got trapped in the same “logical confusion” as me. It’s probably “over bold” for an amateur like to tell you how to think, but after debating this topic over the last weeks, the logic finally make sense:
* We cannot visually see anything that has moved toward us at superluminal speed, including redshift or photons.
* We can calculate, using comoving distance and cosmological time, that the objects that emitted light towards us 700 million years after the big bang, *NOW* should be accelerating at superluminal speeds, at physical distances further than 13,7 light years. BUT please observe – we cannot physically see these objects *NOW*.
Take a good look at this picture of the embedded Lambda-CDM geometry
The brown line on the diagram is the worldline of the Earth (or, at earlier times, of the matter which condensed to form the Earth). The yellow line is the worldline of the most distant known quasar. The red line is the path of a light beam emitted by the quasar about 13 billion years ago and reaching the Earth in the present day. The orange line shows the present-day distance between the quasar and the Earth, about 28 billion light years.
Here is very basic and helping information from good old Wikipedia – Understanding the expansion of space
Speedy
Correction: “…further than 13,7 billion light years.”
NOTE: THIS MATERIAL COULD BE COPYRIGHTED!
(I honestly don’t know. Since this is from The Caltech Years, Sean can probably decide if it’s ok?)
Anybody want to listen to Richard Feynman for 9 hours explaining the laws of nature? Guessed so!
I’ve found very rare Richard Feynman’s lectures on Physics for all physicists and amateurs!
The following are a collection of rare lectures by the man himself (MP3 Sound):
V1 Ch07 Theory Of Gravitation – 51:42 (20.4 MB)
V1 Ch08 Motion – 53:46 (23.3 MB)
V1 Ch09 Newton’s Laws of Dynamics – 54:42 (23.9 MB)
V1 Ch10 Conservation of Momentum – 53:44 (23.4 MB)
V1 Ch11 Vectors – 52:41 (23.2 MB)
V1 Ch12 Characteristics of Force – 59:36 (26.1 MB)
V1 Ch13 Work and Potential Energy 1 – 55:45 (24.4 MB)
V1 Ch14 Work and Potential Energy 2 – 52:15 (22.8 MB)
V1 Ch15 Special Theory of Relativity – 50:00 ( (25.6 MB)
V1 Ch16 Relativistic Energy And Momentum – 54:57 (24.7 MB)
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RapidShare – Feynman Lectures 7-9.rar (66.2 MB)
RapidShare – Feynman Lectures 10-12.rar (71.3 MB)
RapidShare – Feynman Lectures 13-15.rar (71.6 MB)
RapidShare – Feynman Lectures 16.rar (24.3 MB)
Each of the first three RAR files has three lectures in them, and the last one has lecture 16 alone.
Enjoy!
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