The new Hubble rate estimate of 74 means a new critical density.

  • #1
marcus
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Main Question or Discussion Point

Around 1998 we got the estimate of 71 from Wendy Freedman's Hubble Space Telescope team. That was called the "key project" of the HST program and was one of the main reasons for HST. So much depends on it that it is important to keep trying to improve the accuracy.
That 71 has stood for 10 years.

Now it looks like Adam Riess' team has a more accurate figure of 74, based on HST observations since Freedman 1998.

74 point something, with some confidence interval or error range. Like +/- 3. So we will have a little turbulence around this for a while.

Ned Wright has put Riess et al's figure in his News of the Universe page on the web.
http://www.astro.ucla.edu/~wright/cosmolog.htm
It behoves us to consider the possibility that 74 might be righter than 71.

Let's recalculate the critical density. It will come out in nanopascal, which is the same as nanojoules per cubic meter.

I put this into the google window and press return
(74 km/s per megaparsec)^2*3*c^2/(8 pi G)
and it tells me 0.92 nanopascal.

That is the average density around us----0.92 nanojoules per cubic meter.

It has to be that, because we can see that it is flat or nearly flat.

So if the Riess figure is accepted, this is what you take 73 percent of, and 27 percent of, and 4 percent of, etc. Talking about the makeup of the universe.

Wright's news item sounds like Riess figure will be accepted, and he's where he should know. He says:

More Accurate Hubble Constant

07 May 2009 - Riess et al. report a new value for the Hubble constant of Ho = 74.2 +/- 3.6 km/sec/Mpc based on Cepheid measurements in galaxies that have hosted Type Ia supernovae, including the nuclear ring maser galaxy NGC 4258 which has a very precise distance determined by geometric means.
 
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  • #2
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Thanks for the update. A consequent result for the equation-of-state I find even more interesting: w= -1.12 +/- 0.12.

Substantiates the suspicion that the observations favor phantom energy over a cosmological constant.
 
  • #3
nicksauce
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What were the errors bars on the old 71?
 
  • #4
turbo
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It should be noted that Sandage et al derived a much smaller Hubble constant, and that even quite recently there have been higher estimates than those of Riess. An example:

http://arxiv.org/abs/0812.1288

[Disclaimer] I am a co-author with Russell and another researcher on a paper on an unrelated subject and am not neutral on the subject of the Hubble constant, having been on the sidelines and briefed regularly during the research and development of this work. Astronomy is necessarily an observational science, and my preference is to let observation and interpretation drive theory, not the other way around. As Michael Disney has explained, cosmology relies on observations of more and more extreme objects, faint objects, and objects from which the spectra (given the low flux-density) are difficult to establish. [/disclaimer] We have to keep an open mind about the redshift-distance relation.
 
  • #5
Chronos
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No recent study has ruled out a dead flat universe within published error bars. This does not necessarily mean the universe is spatially infinite, it could still be finite but unbounded. I'm unsure what observation might resolve that issue. We live in interesting times. The Planck or Herschel instruments may yield some 'light' on the matter. I agree the redshift distance relationship is still open to discussion, but, it still appears to have a compelling edge over any competing theory. Perhaps there are exceptions, but, they appear to be few and far between. I believe any such exceptions can be accomodated without tuning modern theory on its head. I have never dismissed observational evidence, merely its interpretation.
 
  • #6
Chalnoth
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Thanks for the update. A consequent result for the equation-of-state I find even more interesting: w= -1.12 +/- 0.12.

Substantiates the suspicion that the observations favor phantom energy over a cosmological constant.
Er, no. That's just a misunderstanding of statistics. The number after the +/- is the expected deviation from the true value. That means we can't seriously take a value different from -1 at that level: we expect a deviation, just from pure chance, of approximately that magnitude. And since the probability is 50/50 that it would be above or below -1, well, we really can't take it as seriously supporting w < -1.
 
  • #7
Ich
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And since the probability is 50/50 that it would be above or below -1, well, we really can't take it as seriously supporting w < -1.
It means w<-1 with 84% probability.
 
  • #8
Chalnoth
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It means w<-1 with 84% probability.
That sort of statement can't be taken seriously, however, as we expect a result about that far away from the true value, with a 50/50 chance of being high or low. It makes no sense to give a value any significance whatsoever at the level of one standard deviation away, because that is the expected result.
 
  • #9
marcus
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Assuming the Riess 74 is accepted, they will presumably have to adjust the matter fraction from its current estimate of 0.27.

Here is a guess as to the new Omegam.

The Komatsu WMAP report, in Table 1, gave a figure of 0.1358 +/- 0.0036 for
Omegamh2.
So we just have to divide 0.1358 by the square of 0.74
0.248, call it 0.25.

To a rough approximation the space she is flat, so let's put OmegaLambda = 1 - Omegam = 0.75.

So you know how Wright calculator has default parameters of 0.27, 0.73, 71.
Now I would guess the new default would become 0.25, 0.75, 74.

If that happens it could give us new estimates of the particle horizon, age, distance to last scatter and so forth. No reason to jump the gun, but I'm curious about the size of the putative changes.

I'll put the old CMB redshift z = 1090 into Wright calculator, with these three adjusted parameters, and see.

Well the age of expansion is now 13.4 billion years, which is a little different.
The particle horizon is now pretty much right on 46 billion lightyears.
The distance to last scatter is now 45.2 instead of 45.5 (as it was with the old parameters).

It is not awfully different, on the whole.

Here is Komatsu et al, if you want to check Table 1:
http://arxiv.org/abs/0803.0547v2
Here is Riess et al:
http://arxiv.org/abs/0905.0695
 
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  • #10
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So the Big Rip, right? In Superstring Theory quintessence can be just a brane changing its metric. How quintessence can be explained in the Loop Gravity?
 
  • #11
marcus
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So the Big Rip, right?
Wrong. We have insufficient grounds to assume w < -1, or to anticipate a Rip. Have a look at the discussion preceding your post. Chalnoth has been making this point:


... A consequent result for the equation-of-state I find even more interesting: w= -1.12 +/- 0.12.

Substantiates the suspicion that the observations favor phantom energy over a cosmological constant.
Er, no. That's just a misunderstanding of statistics. The number after the +/- is the expected deviation from the true value. That means we can't seriously take a value different from -1 at that level: we expect a deviation, just from pure chance, of approximately that magnitude. And since the probability is 50/50 that it would be above or below -1, well, we really can't take it as seriously supporting w < -1.
It means w<-1 with 84% probability.
That sort of statement can't be taken seriously, however, as we expect a result about that far away from the true value, with a 50/50 chance of being high or low. It makes no sense to give a value any significance whatsoever at the level of one standard deviation away, because that is the expected result.
Rip is certainly possible, Dima. It has not been ruled out. But it is too early to call, at this point. We live in interesting times :biggrin:
 

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