# The Naked Scientists Forum

### Author Topic: How do we allow for gravitational red-shift at source when estimating distance?  (Read 1528 times)

#### MikeS

• Neilep Level Member
• Posts: 1044
##### How do we allow for gravitational red-shift at source when estimating distance?
« on: 30/01/2012 09:11:52 »
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 »

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### Re: How do we allow for gravitational red-shift at source when estimating distance?
« Reply #1 on: 30/01/2012 23:56:13 »
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.

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Re: How do we allow for gravitational red-shift at source when estimating distance?
« Reply #2 on: 31/01/2012 08:45:37 »
Soul Surfer
Thanks, that appears to answer the question.

#### imatfaal

• Neilep Level Member
• Posts: 2787
• rouge moderator
##### Re: How do we allow for gravitational red-shift at source when estimating distance?
« Reply #3 on: 31/01/2012 09:49:07 »
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.

#### syhprum

• Neilep Level Member
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##### Re: How do we allow for gravitational red-shift at source when estimating distance?
« Reply #4 on: 31/01/2012 10:05:25 »
I prepared an answer similar to that posted by Soul Surfer but it got lost in "Cyber Space" some how.

#### The Naked Scientists Forum

##### Re: How do we allow for gravitational red-shift at source when estimating distance?
« Reply #4 on: 31/01/2012 10:05:25 »