Another paper helping to clarify expansion-congrats, authors :-)

[marcus]

another paper helping to clarify expansion--congrats, authors :-)

http://arxiv.org/abs/0707.2106

Coordinate Confusion in Conformal Cosmology
Geraint F. Lewis, Matthew J. Francis, Luke A. Barnes, J. Berian James
5 pages, accepted for publication in MNRAS Letters
(Submitted on 13 Jul 2007)

"A straight-forward interpretation of standard Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies is that objects move apart due to the expansion of space, and that sufficiently distant galaxies must be receding at velocities exceeding the speed of light. Recently, however, it has been suggested that a simple transformation into conformal coordinates can remove superluminal recession velocities, and hence the concept of the expansion of space should be abandoned. This work demonstrates that such conformal transformations do not eliminate superluminal recession velocities for open or flat matter-only FRLW cosmologies, and all possesses superluminal expansion. Hence, the attack on the concept of the expansion of space based on this is poorly founded. This work concludes by emphasizing that the expansion of space is perfectly valid in the general relativistic framework, however, asking the question of whether space really expands is a futile exercise."

 

[oldman]

http://arxiv.org/abs/0707.2106

This paper (together with "Expanding Space: the Root of all Evil?") that Marcus has kindly drawn attention to is, I believe, an interesting reflection of the conservative cosmological prejudice that finds it convenient to always use the familiar word "expansion" to describe what is going down in this ancient universe of ours.

Both papers that Marcus mentions conclude that despite Steven Weinberg's pungent question quoted in the latter paper:

... how can nothing expand?...space does not expand... (cosmologists) should know better")

it is nevertheless safe to continue with this confusing but convenient practice, although, as the authors explain, it has led to many difficulties among both professional cosmologists and neophytes.

In this forum,too, there is strong support for the convenience of talking about the "expansion" of the universe and, more particularly, about the "expansion" of space. Over a year ago Space Tiger summed it up neatly in the thread: How do you interpret the Robertson-Walker metric as follows:

Attributing (redshift) to the expansion of space would not be wrong, only crude and colloquial. Most laymen don't appreciate it when you respond to their questions with tensor equations or metrics, so we need more "user-friendly" terms. If you're not comfortable with those terms, then it's your prerogative to learn the mathematics the words are meant to describe

Is it not time (again) to consider whether exclusive use of the word "expansion" to describe change in the universe is fully justified, or whether this convenience is indeed only "crude and colloquial", and perhaps conceals subtelties.

The redshift, taken in the context of general relativity, tells us only that the changes we observe in the universe can be consistently modeled by allowing the ratio of the space and the time coefficients of the metric to change isotropically and uniformly. But in fact the redshift does not tell us which of these dimension is to blame for the change in this ratio. For historical astronomical reasons early cosmologists fixed on change in the space dimensions, via the "scale factor" of the FRW metric, and described change as "expansion". And indeed this choice has adequately explained how the universe evolved from extreme primordial conditions.

But is that the end of the story? We understand physics in the time of extreme primordial conditions only poorly, and many physicist are now happy to consider the possibility that we live in a multidimensional universe where only three space dimensions are "unfolded".

From this multidimensional perspective it is perhaps time to consider the possibility that the changes we observe began and possibly continue today with an unfolding of dimensions rather than the semantically confusing "expansion" of "nothing".

 

[wolram]

A ball rolling down a hill needs little explanation.
A ball moving because it is kicked needs little explanation.
Two balls moving apart from each other needs explanation, an explanation that can be understood in an (every day) way, cosmology seems to be only for (a group) of academics these days, with few tests to prove the theories.

 

[marcus]

distances increase, dammit

 

[Hell_SD]

yeah i mean what's so hard to understand ?

...the balls aren't moving further apart, the distance between them is increasing

huh ?