PDA

View Full Version : Redshift Question



peter eldergill
2010-Jul-17, 01:18 AM
What is the minimum distance out that astronomers can start to detect redshift and hubble expansion? Are we talking on the order of a million lightyears? 100 million?

Just curious

Pete

RussT
2010-Jul-17, 01:42 AM
I think the more pertinent question would be...

What is the maximum range that can be used to correctly determine blue shifted entities coming toward us ;)

Ken G
2010-Jul-17, 02:24 AM
From this graph, http://en.wikipedia.org/wiki/File:Hubble_constant.JPG, or this one, http://outreach.atnf.csiro.au/education/senior/cosmicengine/images/cosmoimg/hublaw1.gif, you can see that it starts to appear, in a statistical way, pretty clearly by about 2-3 Mpc. Converting that to light years, given 3.3 LY in a pc, it says that you can indeed begin to see the expansion by 5 million light years, and by 10 million LY there is considerable confidence. You can't get the slope very accurately though, for that you need to go to something like 30 million LY, and the farther the better.

peter eldergill
2010-Jul-17, 04:10 AM
Thanks Ken, that's exactly the answer I was looking for

I do understand that I might have been able to spend a couple of hours of google looking for that but I've always found it more fun and interesting to interact with the folks here

Pete

Edit: Are the data points galaxies? There's actually not that many data points on those graphs. Is there a reason for that? Keep in mind I've never really thought about this concept before

Ken G
2010-Jul-17, 06:30 AM
Edit: Are the data points galaxies? There's actually not that many data points on those graphs. Is there a reason for that? Keep in mind I've never really thought about this concept before
Yes, those data points are galaxies. They probably needed to have Cepheids in them to get the distances, and keep in mind that 5 million LY really isn't that far, so there aren't all that many galaxies in that volume in the first place. But maybe some have been excluded because it's hard to get their distance, or there are other problems with them, I'm not sure how complete it is.

RussT
2010-Jul-17, 06:56 AM
Yes, those data points are galaxies. They probably needed to have Cepheids in them to get the distances, and keep in mind that 5 million LY really isn't that far, so there aren't all that many galaxies in that volume in the first place. But maybe some have been excluded because it's hard to get their distance, or there are other problems with them, I'm not sure how complete it is.

Yes...5 million LY really isn't that far...it's only twice the distance of Andromeda at ~2.5 million LY's...I guess it was a good idea that I limited my insistance on "Local Neighborhood" not to exceed the Local Group in our "Distance: Now VS Then" Thread ;)