Cosmological constant agrees with Dark Energy ?

[Royston]

Just read this article from sciencedaily, thought some of you may be interested...

 

http://www.sciencedaily.com/releases/2007/11/071127142128.htm

 

"The magic value is -1 exactly," Krisciunas said. "If the number turns out to be precisely -1, then this dark energy is a relatively simple thing -- it is Einstein's cosmological constant." The team won't have the final results until later next year, but right now, the measurement is coming in at -1 plus or minus 10 percent error, Suntzeff said, so the initial data seems to point to Einstein being correct.

 

Just incase anybody is a little confused by the article, a small excerpt from wiki explaining how the CC will agree with this data, and not a static model for the universe...

 

It is now thought that adding the cosmological constant to Einstein's equations does not lead to a static universe at equilibrium because the equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a universe which contracts slightly will continue contracting.

 

http://en.wikipedia.org/wiki/Cosmological_constant

[Martin]

Just read this article from sciencedaily, thought some of you may be interested...

 

http://www.sciencedaily.com/releases/2007/11/071127142128.htm

 

thanks for flagging this. it is part of a steady ongoing effort since 1998 of narrowing down the estimate of a parameter called w the dark energy equation of state.

 

the dominant assumption has always been that w = -1 exactly, and this is assumed in the mainstream cosmology model called LCDM or LambdaCDM.

LCDM is the automatic model of choice. the reason is it fits the data best of any of the models they've tried.

 

But there have been a number of studies aimed at seeing if maybe w is not exactly -1, but is some other number close to that.

 

In that case dark energy would not behave exactly as if it comes from Lambda cosmological constant-----then we would NOT use the LCDM model any more.

 

So there have been dozens of papers trying to pin down w more and more closely, by observing more and more supernovae (at higher redshifts too, so going back further in time).

 

this ESSENCE study is part of that series of efforts, and what they are quoted as saying is not especially surprising but it is reassuring. They apparently are finding that w equals - 1 WITHIN TEN PERCENT

 

so it would be roughly between - 0.9 and -1.1, if the quote is right. that is good news! Everybody can keep on using LCDM.

 

=======================

Wow! It turns out I know one of the people doing the study! Had several semesters courses with him.

Also Tamara Davis, who got her PhD from Lineweaver (and co-authored that SciAm article with him)

A lot of important names. Major league study.

Here is what they said in January 2007.

 

http://arxiv.org/abs/astro-ph/0701041

Observational Constraints on the Nature of the Dark Energy: First Cosmological Results from the ESSENCE Supernova Survey

W. M. Wood-Vasey, G. Miknaitis, C. W. Stubbs, S. Jha, A. G. Riess, P. M. Garnavich, R. P. Kirshner, C. Aguilera, A. C. Becker, J. W. Blackman, S. Blondin, P. Challis, A. Clocchiatti, A. Conley, R. Covarrubias, T. M. Davis, A. V. Filippenko, R. J. Foley, A. Garg, M. Hicken, K. Krisciunas, B. Leibundgut, W. Li, T. Matheson, A. Miceli, G. Narayan, G. Pignata, J. L. Prieto, A. Rest, M. E. Salvo, B. P. Schmidt, R. C. Smith, J. Sollerman, J. Spyromilio, J. L. Tonry, N. B. Suntzeff, A. Zenteno

(Submitted on 2 Jan 2007)

 

"We present constraints on the dark energy equation-of-state parameter, w=P/(rho c^2), using 60 Type Ia supernovae (SNe Ia) from the ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat Universe. By including constraints on (Omega_M, w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w=-1.05^{+0.13}_{-0.12} (stat; 1 sigma) +- 0.11 (sys) and Omega_M=0.274^{+0.033}_{-0.020} (stat; 1 sigma) with a best-fit chi^2/DoF of 0.96. These results are consistent with those reported by the SuperNova Legacy Survey in a similar program measuring supernova distances and redshifts. We evaluate sources of systematic error that afflict supernova observations and present Monte Carlo simulations that explore these effects. Currently, the largest systematic currently with the potential to affect our measurements is the treatment of extinction due to dust in the supernova host galaxies. Combining our set of ESSENCE SNe Ia with the SuperNova Legacy Survey SNe Ia, we obtain a joint constraint of w=-1.07^{+0.09}_{-0.09} (stat; 1 sigma) +- 0.12 (sys), Omega_M=0.267^{+0.028}_{-0.018} (stat; 1 sigma) with a best-fit chi^2/DoF of 0.91. The current SN Ia data are fully consistent with a cosmological constant. "

 

I guess is not quite as good as it sounded in ScienceDaily, but still good. the error bar could be

[-1.05 - 0.12, -1.05 + 0.13] which is [- 1.17, - 0.92]

 

and I'm not including their estimate of SYSTEMATIC error because I don't understand it. this is just their primary errorbar from their supernova data.

 

the reason that it is CONSISTENT with the LambdaCDM model is because the value -1 is in the range [-1.17, - 0.92]

and even sort of mid-range in there, so it is a comfortable fit.

 

But of course it is still not conclusive that the observed accelerated expansion is behaving as if caused by a constant Lambda.

They will still go on testing more and more and narrowing it down.

[Royston]

Thanks Martin

 

But of course it is still not conclusive that the observed accelerated expansion is behaving as if caused by a constant Lambda.

They will still go on testing more and more and narrowing it down.

 

I took the article as 'looking more encouraging', and wasn't sure if they were exaggerating slightly (it's not the best article), however if the results will mean a consensus next year, I'll keep my eyes peeled for any updates et.c If the Lambda CDM model is scrapped, or retained, it's obviously significant either way.

 

Without running through the maths, I can't really appreciate the affect of the w parameter on other variables, vacuum and energy density et.c Perhaps somebody will be willing to explain this ? I do remember if the CC doesn't agree, then the alternative is quintessence, though I'd have to do some thorough reading to appreciate how that operates compared to a fixed constant. Anyhooo, I'll try to remember to post more on this later, if I come across or can think of anything relevant.