Einstein's Cosmological Constant

[Finger Painter]

recently i heard a prominent english physicist using 70 as his value for ecc. what value do you like and where did you get it from?

 

[HallsofIvy]

70 what?? In other words, what units?

 

[marcus]

Originally posted by Finger Painter
recently i heard a prominent english physicist using 70 as his value for ecc. what value do you like and where did you get it from?

70 percent is a common way of expressing the generally agreed-on size of it

there is a widely understood energy density called
the critical density (written "rho_crit")

and the ecc. can be viewed as the density of dark energy or vacuum energy

so it can be expressed as a percentage of rho-crit

and the closest fit to the data is currently 73 percent or 0.73

but many people just say 0.7 or 70 percent because at this
point approximate magnitude is what matters so it sounds too
finicky and overprecise otherwise

 

[marcus]

Originally posted by Finger Painter
what value do you like and where did you get it from?

I personally don't "like" any particular value, I just report what
figure they are using. It is a parameter that is measurable and the best determinations of it come from an instrument called WMAP that is orbiting the sun in a somewhat larger orbit than the earth's. I must say I like the way the ecc. is measured!

Where do you get the 0.73 (or the approximate 0.7) from?
there are a ton of sources for that, many of them online

Michael Turner "Making sense of the new cosmology"
http://arxiv.org/astro-ph/0202008

Charles Lineweaver "Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179

Ned Wright's online cosmology tutorial

There is a report by Bennett et al of WMAP results that gives
the latest rundown on measurements of a bunch of cosmological
parameters including this one. If you want URLs ask, this is just a sampling

 

[Finger Painter]

thanks for your replies. I've also heard 80 as a value, but it does seem a bit arbitrary. until it is proven, i guess maybe that's all we can expect. will it be difficult to get a definitive answer?

 

[marcus]

Originally posted by Finger Painter
thanks for your replies. I've also heard 80 as a value, but it does seem a bit arbitrary. until it is proven, i guess maybe that's all we can expect. will it be difficult to get a definitive answer?

For a look at over a dozen parameters, including this one, currently measured values with error bars showing uncertainty see Table 1 on page 32 of Charles Lineweaver "Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179

That is as of May 2003. As far as the cosm. const goes you can see from the table that the error bar currently looks like this

Ω_Λ = 0.73 ± 0.04

That means that experts responsible for publishing the data are betting their professional reputations on future improved observations by their colleagues never getting out of the bracket
0.69 - 0.77

I would guess that future measurements will likely
cluster about 0.73 but there is a margin of error and it is currently plus/minus 0.04

Frankly I don't entirely understand where the certitude comes from but they are the pros and not me and I respect the work to the extent I'm able to judge it, so I accept the data

The WMAP project is pretty amazing and that is where the data in Table one comes from---a recent WMAP report. Have a look and form your own opinion