Talk:Deceleration parameter

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[edit] Hubble factor business

This business of the Hubble factor still decreasing needs some explanation. What I came to realize is that a, as the scale factor, determines whether there is an acceleration or not. Beynod that, the Hubble factor will decrease even with q=0 since as the universe gets bigger the galaxies moving away at a given rate get farther away. Hence the decrease. That the Hubble factor is still decreasing indicates that this is not a rapid expansion. In any case, some explanation of this is needed in the article. --EMS | Talk 04:04, 12 August 2006 (UTC)

Okay, I've rewritten q as:

$q = \frac{4 \pi G (\rho + \frac{3p}{c^2}) - \Lambda}{3H^2}$

Which shows independence of the sign of q from the sign of H. --Michael C. Price ^talk 09:34, 12 August 2006 (UTC)

All that you have done is to confuse things even more. For a reader,

What is the Hubble parameter?
How can it be decreasing when the universe's expansion is accelerating?
What is the significance of its decreasing while the expansion is accelerating?

These issues need to be answered with a well thought-out write-up, not by throwing another equation at the reader. I took that business out of the general relativity article because it was more confusing than informative given the issues that it raised. I can accept it here, but only if the issues are dealt with. --EMS | Talk 18:12, 12 August 2006 (UTC)

Your first point completely baffles me. Have you looked the article itself, where the Hubble parameter is defined and wikilinked? What more of an explanation are you after? --Michael C. Price ^talk 18:21, 12 August 2006 (UTC)

Regarding the second point, that can only be explained by presenting the equations, which show that the sign of q and the sign of $\dot{H}$ are independent.

Regarding the third point, I have no idea of its significance.

If you find my explanation confused them please expand it. But statements in the literature that the cosmic expansion is accelerating do beg the question of how this can be compatible with H still decreasing. This is what I have tried to address in this article.--Michael C. Price ^talk 18:45, 12 August 2006 (UTC)

*Sigh*. $\dot{a}$ is the exapnsion factor of the universe, not the Hubble parameter. The Hubble parameter relates the redshift of galaxies to the galaxy's distance from the Earth, and therefore is a measure of the recessional velocity of the distant galaxies. The Hubble parameter is decreasing because as a galaxy gets further away (and given that its recessional velocity/redshift is not changing) the parameter needs to decrease to account for the increased distance. This is a low acceleration in that its tendency to increase the Hubble parameter is not outweighed by the normal tendency of the Hubble parameter to decrease.

Please correct the article. --EMS | Talk 19:34, 12 August 2006 (UTC)

*Sigh* Thank you for your patronising response, O Wise One. Please insert your explanation of why cosmologists define

$-q \equiv \frac{\ddot{a} a }{\dot{a}^2}$

instead of

$-q \equiv \dot{a}$

into the article, since I am clearly too stupid to do it myself. And thanks for the explanation that

$\dot{a} \ne \frac{\dot{a}}{a} \equiv H$

I would have never figured that out either. --Michael C. Price ^talk 20:30, 12 August 2006 (UTC)

Perhaps you mean to type $-q \equiv \ddot{a}$ above. $\dot{a}$ is just the rate change in the scale factor after all. Also the article itself says that

$H \equiv \frac{\dot{a}}{a}$ is the Hubble parameter, the rate of the local expansion of the universe.

A look at

q

reveals that it includes

H - 1

, which means that it permits comparison across all times, and also $\ddot{a} / \dot{a}$ , which permits comparison aginst different expansion factors. [Or be careful what you ask for: You just may get it! :-) ]

I will take a crack at strightenning this article out in a day or two. I want to take my time with this since you have not done a bad job with it overall, and since cosmology is not my forte. --EMS | Talk 02:15, 13 August 2006 (UTC)

I had meant to say $\dot{a}$ , although, yes $\ddot{a}$ would have been better ;-). Neither will do, though, because we are after a dimensionless measure of acceleration.

I've added some stuff about

Ω

, see what you think. --Michael C. Price ^talk 10:52, 13 August 2006 (UTC)

I've tossed out the equations, and reduced this back to being a tame stub. If you really think that $\dot{H}$ and $\ddot{a}$ (or

- q

) having different signs is so important, then you can bring it back, but if you do so please explain it in a way that a non-technical reader can understand. Wikipedia is not a research journal. --EMS | Talk 16:06, 13 August 2006 (UTC)

That was not OR, that was straight from my GR college notes (and other wiki-articles). I don't understand this obsession people have about removing information. --Michael C. Price ^talk 20:27, 13 August 2006 (UTC)

Restored the equations, which any non-technical reader can ignore if they so choose. This way we give them the option. Why I should have to explain this I really don't know. --Michael C. Price ^talk 20:59, 13 August 2006 (UTC)

I know this stuff is not OR. My complaint is that it leads the reader nowhere. You leave a reader with more questions that answers. That is why this article is better off without the Friedman equations business. Leave that for the cosmology pages, where it belongs. --EMS | Talk 00:03, 14 August 2006 (UTC)

Unanswered questions should be dealt with by answering them, not deleting them.

This is a cosmology page.

q is not dealt with in detail anywhere else. --Michael C. Price ^talk 00:21, 14 August 2006 (UTC)

Then explain why you are presenting those equations. Explain how the terms interrelate. Explain this "cosmological fluid" that you refer to. Explain the dots over the variables! (I use that dot notation regularly myself, but for the average reader who will come to this page, it is most likely gibberish.) I can use my skills to validate the technical contents of this article, and that appears OK to me. Hovever, when I mentally "coast" and look to see now much I can glean from this article with a minimum of effort, I come up empty. I find that this is a better article without the Friedman equation and the derivation of q, as I can then focus on a narrower set of information and take that in easily. Sometimes, the less said, the better. --EMS | Talk 01:51, 14 August 2006 (UTC)

I gave rewriting the page a shot to make it a little clearer. Essentially, once you have defined q – which isn't used that much any more, I think – the page has served its function, and anything else is icing. It will never be an extensive article, like dark energy. –Joke 03:05, 14 August 2006 (UTC)

I think that you did a good job of tying up all the loose ends that were here. Thank you very much. --EMS | Talk 04:09, 14 August 2006 (UTC)

Nice overall look, but ignores

q = - 1

during inflation. The non-flat case needs restoring and, for consistency with the Friedman equations,

Λ

needs including. --Michael C. Price ^talk 06:59, 14 August 2006 (UTC)

I thought these were unnecessary, but I'm also happy with the additions you've made. My one comment is that, assuming a scale invariant spectrum of primordial curvature perturbations, the cosmic microwave background measures the curvature directly – the chain of reasoning is more direct than CMB -> inflation -> flatness. This comes directly from the angular scale of the first peak which is determined by the redshift of the surface of last scattering, which is easy to calculate, and the geometry of the space the photons have travelled through since recombination. –Joke 22:49, 17 August 2006 (UTC)

[edit] More explanation required for the redirect to stand

For editors coming from the redirect at accelerating universe, there is no clue in this article as to why they have been redirected here. This version of accelerating universe at least contained some explanation of what observations were driving the theory. This article, in contrast, is poorly written and even talks about "recent measurements of dark energy". Dark energy hasn't been measured at all. It's dark, after all. What have been measured are supernovae and the microwave background. This article requires some serious attention and correction in order to make it useful for readers who have come here because they have wanted to know about an accelerating universe. Uncle G 16:11, 15 November 2006 (UTC)

Dark energy has been measured -- although dark its existence and basic characteristics can be inferred, otherwise the concept would not exist. As for the issue of readability, why don't we restore the older version of the article as "accelerating universe" (i.e. copy it into the redirect) and keep the current article at this page (i.e. "Deceleration parameter")? Does that sound reasonable? --Michael C. Price ^talk 16:34, 15 November 2006 (UTC)
- Inferring the existence of something is not the same as measuring it. If you think that dark energy has been measured, please point to a report of that measurement, to counter the many things that one can cite that state that its existence is currently hypothetical. Uncle G 00:57, 16 November 2006 (UTC)
  - Read dark energy: 74% of the energy density of the universe is dark energy. That's a measurement. What about the redirect proposal I made? Does anyone object? --Michael C. Price ^talk 10:03, 16 November 2006 (UTC)
    - Wrong. I suggest that you read it yourself, first, and read the above sources that I cited. It's not a measurement. It's a hypothesis, an attempt at an explanation for what have actually been measured, which are supernovae and the microwave background, as I stated above. Please work from sources. Uncle G 12:41, 16 November 2006 (UTC)