I've written an article 'On the Relativity of Redshifts: Does Space Really "Expand"?' which has appeared in Australian Physics (2016, 53(3), 95-100). For some reason, the arXiv has put the article on hold, so you can download it here.

I like it :)

I like it :)

Haven't read it yet, but will do so soon. Surprised that the references contain neither Harrison's ApJ paper nor Bunn and Hogg.

ReplyDeleteMore later.

And do let me know why the paper was put on hold, if you ever find out, and let me know when it shows up on arXiv. Personal email would be best (and post here as well if you want).

It's a semi-popular level article, not a paper. The goal was not to provide a complete review of the literature, but an overview. Hence, not that many fires to my own work :)

DeleteIn this corner, we have Peacock, Rees, and Weinberg. And in this corner, we have Harrison, certainly no slouch. Here and in his textbook, Harrison defends the expanding-space view.

ReplyDeleteBunn and Hogg have written a paper which takes the opposite point of view, and explains Narlikar's work in a more accessible way.

For me, the idea of an empty universe which is expanding is "proof" that space expands. Yes, it can be transformed away, but I still think that it is a useful picture. In particular, the entire concept of horizons, as well as questions like "how can this galaxy be receding at more than the speed of light", are easier to understand in this picture.

At the end of the day, does it matter? You mention that developing an intuition is important, and I agree. However, different intuitions work for different people. Some want to see an equation, some want to make a plot, some want a series of figures. Some think mathematically, some not; some visually, some not. What really matters is whether one gets the right answer. Like polar and cartesian coordinates, it is not so much a question of which is right as of which is more convenient (in a particular situation).

Yes, abusing the expanding-space picture can lead to wrong results, but so can abusing a model which does not include expanding space. I'm sure that all interesting results can be derived in the expanding-space picture as well.

An analogy is perhaps the membrane paradigm with respect to black holes. It's a different picture, but can be useful. Or different interpretations of quantum mechanics. As long as the calculations turn out correctly, does it matter?

Some talk about expanding space as a "thing" and that, simply, is wrong. It's a picture for some, and that is all.

DeleteConsider a universe with a positive cosmological constant, lambda, > 1, and zero matter density. This is the Lanczos model, an empty bounce model. At least in the expanding-space view, it has a definite volume, measured in pints if you like, which changes with time. There is no matter in it. The energy density due to the cosmological constant does not change with time.

ReplyDeleteHow can one describe this without using the expanding-space view?

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