Expanding Space: the Root of all Evil?

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Discussion Overview

The discussion revolves around the concept of expanding space in cosmology, particularly in relation to its implications and the clarity of its communication. Participants explore various analogies and interpretations of the expanding universe, as well as the pedagogical aspects of explaining this concept to different audiences.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested
  • Meta-discussion

Main Points Raised

  • Some participants highlight that the concept of expanding space is often poorly articulated and can lead to misconceptions, as noted in the referenced paper.
  • One participant proposes an analogy comparing the expanding universe to a gas expanding in a vacuum, suggesting that galaxies behave like gas molecules held together by gravity.
  • Another participant counters this analogy, asserting that a Newtonian understanding of expansion is insufficient and that General Relativity provides a more accurate framework where space itself expands over time.
  • There is a discussion about the importance of using precise language, such as emphasizing that distances expand rather than space itself, to avoid confusion.
  • Some participants express enjoyment of the paper and its pedagogical approach, while others question whether there is anything controversial in the discussion.
  • References are made to previous threads that discuss similar topics, indicating ongoing interest and debate in the community regarding the nature of space and expansion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best way to describe the expansion of space, with multiple competing views and analogies presented. The discussion remains unresolved regarding the implications of these differing interpretations.

Contextual Notes

Some limitations in understanding arise from the dependence on definitions of space and time, as well as the varying interpretations of cosmological models. The discussion reflects a range of perspectives without settling on a definitive explanation.

Who May Find This Useful

This discussion may be of interest to those studying cosmology, educators looking for effective ways to communicate complex concepts, and individuals curious about the philosophical implications of space and time in physics.

marcus
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thoughtful paper on how to convey an idea with least confusion
http://arxiv.org/abs/0707.0380
Expanding Space: the Root of all Evil?
Matthew J. Francis, Luke A. Barnes, J. Berian James, Geraint F. Lewis
8 pages, accepted for publication in PASA
(Submitted on 3 Jul 2007)

"While it remains the staple of virtually all cosmological teaching, the concept of expanding space in explaining the increasing separation of galaxies has recently come under fire as a dangerous idea whose application leads to the development of confusion and the establishment of misconceptions. In this paper, we develop a notion of expanding space that is completely valid as a framework for the description of the evolution of the universe and whose application allows an intuitive understanding of the influence of universal expansion. We also demonstrate how arguments against the concept in general have failed thus far, as they imbue expanding space with physical properties not consistent with the expectations of general relativity."
 
Space news on Phys.org
3 Conclusion
Despite (and perhaps in part because of) its ubiquity, the concept of expanding space has often been articulated poorly and formulated in contradictory ways. That addressing this issue is important must be placed beyond doubt, as the phrase ‘expansion of space’ is in such a wide use—from technical papers, through to textbooks and material intended for school students or the general public—that it is no exaggeration to label it the most prominent feature of Big Bang cosmologies. In this paper, we have shown how a consistent description of cosmological dynamics emerges from the idea that the expansion of space is neither more nor less than the increase over time of the distance between observers at rest with respect to the cosmic fluid.

As stated here.

Garth
 
Interesting paper! When I think of an expanding universe analogy I think of a gas expanding in a vacuum. The molecules of the gas behave as the galaxies in space. They are held together with molecular forces while galaxies are held together by gravity. Does this make sense?

I am not a physicist, just an old retired Chemical Engineer with an interest in physics. Thanks for your thoughts.
 
PRDan4th said:
Interesting paper! When I think of an expanding universe analogy I think of a gas expanding in a vacuum. The molecules of the gas behave as the galaxies in space. They are held together with molecular forces while galaxies are held together by gravity. Do

Not really, that would be a Newtonian understanding of the expansion of the universe, i.e. the galaxies expand into an already existing empty space. A General Relativity understanding would be that in one cosmological solution to the 'shape' of space-time given by the Robertson-Walker (RW) metric is that it is the space itself that expands with passing time.

This has to be understood physically as the distance between co-moving observers, as measured by rigid (steel) rulers, increases with time .

Garth
 
I really enjoyed reading this paper. The tethered galaxy example is right on, and if you'd like your physical intuition to work for you in cosmology this is a must read!
 
i can imagine time without space but not space without time...

...so is space what's left after time has moved on ?
 
Waiting for Wallace to reply...
 
matt.o said:
Waiting for Wallace to reply...

Reply to what? I have forgotten if Wallace has distinctive views on this subject.
I didn't detect anything controversial about the paper----it was more pedagogical or studying how best to communicate to students and the public.
Trying to help, rather than support some kind of debate.

did you see something potentially controversial, Matt. O.?

Wait. Are you one of the authors of the paper?
 
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  • #10
George Jones said:
Maybe matt.o is waiting for Wallace to reply because Wallace often has interesting and insightful things to say, or because Wallace started a thread titled "https://www.physicsforums.com/showthread.php?t=162727"".

Right! Now I remember that "Does Space Expand?" thread. And someone has just bumped it into view.

Did we come to some peaceable consensus in that thread? something I hope like:

"lets not talk so much about space expanding, let's say DISTANCES expand, especially distances that are out in the middle of nowhere."
 
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  • #11
ooo I'm famous! Thought I'd better say something so as to not disappoint!

Actually over https://www.physicsforums.com/showthread.php?t=176380" there is another of the many expanding space discussions, with some interesting comments.

I think matt.o was waiting for a reply as he has inside information that I'm one of the authors of this paper. There goes my anonymity, I'll have to stop flaming people now!
 
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  • #12
marcus said:
Right! Now I remember that "Does Space Expand?" thread. And someone has just bumped it into view.

Did we come to some peaceable consensus in that thread? something I hope like:

"lets not talk so much about space expanding, let's say DISTANCES expand, especially distances that are out in the middle of nowhere."

that someone would be me and if anyone would be so kind as to feedback on my analogy it would be much appreciated...

...or would that be opening up a can of wormholes called 'units of space' ?
 
  • #13
Wallace said:
ooo I'm famous! Thought I'd better say something so as to not disappoint!

Actually over https://www.physicsforums.com/showthread.php?t=176380" there is another of the many expanding space discussions, with some interesting comments.

I think matt.o was waiting for a reply as he has inside information that I'm one of the authors of this paper. There goes my anonymity, I'll have to stop flaming people now!

Haha. Sorry if you wanted to keep anonymity! :biggrin:
 
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