How do we allow for gravitational red-shift at source when estimating distance?

  • 4 Replies
  • 1567 Views

0 Members and 1 Guest are viewing this topic.

*

Offline MikeS

  • Neilep Level Member
  • ******
  • 1044
  • The Devils Advocate
    • View Profile
Imagine two galaxies far, far away but the same distance from us, one large with lots of mass, the other smaller with little mass.  Both galaxies have about the same brightness.

Light leaving both galaxies will be red shifted at source.  The larger galaxy being more red shifted than the smaller.
To us this would appear that the larger galaxy is further away than the smaller but we know this to be untrue.
How do we recognise and reconcile the difference in red-shift and is it significant enough to cause difficulty in judging distance?
« Last Edit: 30/01/2012 16:22:00 by MikeS »

*

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
In general gravitational red shift at the galaxy level is far too small to introduce any significant errors to the expansion red shift.  Even very close to the intense gravity of a white dwarf star gravitational red shift only amounts to the equivalent of a few tens of km/sec velocity.  The gravitational fields of galaxies are many orders of magnitude less than this.  OK they have lots of mass but they are also very sparse so the gravity is very weak.  Also the random velocities of galaxies in clusters are hundreds of km/sec velocity.
Learn, create, test and tell
evolution rules in all things
God says so!

*

Offline MikeS

  • Neilep Level Member
  • ******
  • 1044
  • The Devils Advocate
    • View Profile
Soul Surfer
Thanks, that appears to answer the question.

*

Offline imatfaal

  • Neilep Level Member
  • ******
  • 2787
  • rouge moderator
    • View Profile
I was hoping SoulSurfer would answer this one  - I could not think of a simple (or otherwise) answer, probably cos I was neglecting the most basic principle of checking the amount of the effect.
Thereís no sense in being precise when you donít even know what youíre talking about.  John Von Neumann

At the surface, we may appear as intellects, helpful people, friendly staff or protectors of the interwebs. Deep down inside, we're all trolls. CaptainPanic @ sf.n

*

Offline syhprum

  • Neilep Level Member
  • ******
  • 3897
    • View Profile
I prepared an answer similar to that posted by Soul Surfer but it got lost in "Cyber Space" some how.
syhprum