View Full Version : Value of Hubbles constant
2006-Nov-27, 11:50 AM
Is there a generally accepted value of Hubbles constant nowadays,
Apparently the age of the universe is quoted as 13.7 billion years, which corresponds to about 71.4km/s/Mpc
Is this accepted nowadays or could it be lower?
2006-Nov-27, 01:08 PM
Tim Thompson made an interesting post recently on this topic, but essentially I think it can be paraphrased as this. The local Hubble constant is about 72km/sec/MPc, but since the universe is accelerating, this number was somewhat lower a few billion years ago (hence the age is higher). Tim estimates the actual age to be around 14.2 billion years. I don't know how to factor in the idea that the Hubble constant must have been a little higher in the very early universe because of more gravitational deceleration.
In any case the Hubble 'constant' has varied a few percent.
2006-Nov-27, 01:30 PM
Tim estimates the actual age to be around 14.2 billion years. I don't know how to factor in the idea that the Hubble constant must have been a little [B]higher[/] in the very early universe because of more gravitational deceleration.
In any case the Hubble 'constant' has varied a few percent.Did you mean smaller commensurate with slower expansion? How would we determine the value of the Hubble variable prior to what we believe to be BB+ 300,000 years? It could have been much less than unity giving a rather large (not necessarily infinite) age for the universe.
2006-Nov-27, 01:54 PM
1. Did you mean smaller commensurate with slower expansion? 2. How would we determine the value of the Hubble variable prior to what we believe to be BB+ 300,000 years?
I added numbers above for clarity.
1. Yes. A smaller Hubble constant means slower expansion. As the expansion accelerates, the 'constant' gets bigger.
2. I'm not sure that that there is an obvious way to measure it. If someone comes up with an idea for this, I'll be interested in knowing. Otherwise, we go with extrapolating what we do know, and applying it to the current model of how we got here.
2006-Nov-27, 10:36 PM
See the Hubble constant webpage (http://cfa-www.harvard.edu/~huchra/hubble/) by John Huchra. It's as good an explanation as you will find on the web, and incudes data files (http://cfa-www.harvard.edu/~huchra/hubble/hubble.plot.dat) listing all published values of the Hubble constant, through 2004. Things have not changed much, somewhere in the 72 km/sec/Mpc, give or take a few. Most everybody who publishes anything that is based on an assumed H0 uses something like that.
2006-Nov-28, 03:34 AM
Further, H0 has now been determined using two methods that are quite independent of the distance ladder (http://www.bautforum.com/showpost.php?p=868651&postcount=139) - lensed quasars and the SZE (Sunyaev-Zel'dovich effect).
Comfortingly, the values determined by these independent methods are consistent with 72 (though the error bars are rather larger than those from the Key HST Project).
2006-Nov-28, 11:22 AM
Powered by vBulletin® Version 4.2.0 Copyright © 2013 vBulletin Solutions, Inc. All rights reserved.