Change in some basic parameters may be under way

[Martin]

The Hubble rate figure of 71 (km/s per megaparsec) has been the standard one to use for over 10 years now, since 1998. One of the main reasons for the HST was the socalled "Hubble Key Project" of determining it. And that team reported in 1998. There was a range of uncertainty though.

 

Now it looks like, using the HST during the years since 1998, they have narrowed the uncertainty to about 4 percent and have a new estimate of 74.

 

It will take some time for this to be accepted, and for changes to be made. Like the default parameters in Ned Wright's calculator are still 0.27, 0.73, 71.

I would guess that eventually the default numbers will change to 0.25, 0.75, and 74.

 

That leaves us in a slight dilemma. Should we continue using the old numbers or should we change over to what we guess the new ones will be?

 

Well, what difference does it make, if any?

 

Percentage-wise not much. The CMB redshift stays 1090. Redshifts in general don't change. The age of expansion becomes 13.4 billion years instead of 13.7 or 13.8 or whatever it was.

 

The distance to last scatter becomes 45.2 billion lightyears, which is about what we were saying before. The particle horizon---the radius of the observable---is 46 billion lightyears, which is just what we have been calling it!

 

So since we have been speaking in approximate terms all along, and the percentage changes are not very big, I'm kind of tempted to shift over to the new numbers. Don't want to jump the gun, but if this kind of thing only happens every 10 years or so, why not enjoy it?

 

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In case anyone is curious here are technical details.

 

The Komatsu WMAP report, in Table 1, gave a figure of 0.1358 +/- 0.0036 for Omega_mh².

Cosmologists have a conventional symbol h to show dependence on the Hubble parameter. It is just the numerical value of H divided by 100, so with the new Hubble rate of 74, this h will be 0.74

 

To get the matter fraction Omega_m we just have to divide 0.1358 by the square of 0.74

That gives 0.248, call it 0.25.

 

To a rough approximation, space she is flat, so let's put Omega_Lambda = 1 - Omega_m = 0.75.

 

Sources:

Riess et al give the new value of 74:

http://arxiv.org/abs/0905.0695

Here is Komatsu et al, if you want to check Table 1 for Omega_m:

http://arxiv.org/abs/0803.0547v2

The figure of 0.1358 is an average determination three datasets: WMAP + BAO + SN, that means the microwave background plus galaxy counts (ripples in the overall distribution of matter) plus supernovae. They tend to prefer that to basing the determination of numbers on just one approach. Table 1 gives the separate estimates and then it gives the average in the column labeled WMAP + BAO + SN

Edited May 15, 2009 by Martin

[Airbrush]

Interesting news, thanks for that. According to wiki one gigaparsec is about 3.262 Billion light-years so one megaparsec is about 3.262 Million light years, or about 60% further from us than the Andromeda galaxy. I grew up using light years. Megaparsecs is not a familiar unit of measure to me and perhaps some others.

 

Space is expanding at the rate of 74 km/sec every megaparsec. If expansion is accelerating, how long until the expansion rate is 75 km/second?

 

Correct me if I am wrong, but using simple math we can calculated how far away this expansion adds up to light speed. That would be at 13.2 Billion light years away where the expansion adds up to light speed (300,000/74 km/sec = 4,054 megaparsecs = 13.216 Billion LY away), which coincidentally is near the horizon of the furthest visible galaxies or quasars. How can we detect the CMB radiation if it is expanding away from us at far beyond light speed?

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Merged post follows:

Consecutive posts merged

To answer my own question, we can now detect the CMB radiation because it left that region so long ago, when it was much closer to us, so that it had enough time to arrive here.

Edited May 18, 2009 by Airbrush

[Arch2008]

As a consequence of this refinement to the Hubble constant, the derivation of what dark energy might be improves as well:

 

http://www.nasa.gov/mission_pages/hubble/science/hstimg_ngc3021.html