Tolman surface brightness test -

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Discussion Overview

The discussion revolves around the Tolman surface brightness test, specifically addressing the implications of surface brightness in an expanding universe and the relationship between surface brightness, flux, and redshift. Participants are exploring theoretical aspects and clarifying concepts related to cosmological observations.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Richard Tolman's derivation suggests that in an expanding universe, the surface brightness of identical objects decreases by a factor of (1+z)^4.
  • One participant questions whether surface brightness is equivalent to flux (W/m^2).
  • There is confusion regarding the relationship between the decrease in received flux, which is understood to decrease with the distance squared, and the (1+z)^4 factor, leading to speculation about its relevance at smaller distances.
  • Another participant notes a discrepancy in sources, with some stating that surface brightness decreases by (1+z)^2 instead of (1+z)^4, prompting a request for clarification on which is correct.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the correct factor for the decrease in surface brightness, with conflicting information about whether it is (1+z)^2 or (1+z)^4. The discussion remains unresolved on this point.

Contextual Notes

The discussion highlights potential limitations in understanding the relationship between surface brightness, flux, and redshift, as well as the implications of distance in cosmological observations.

_Andreas
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Tolman surface brightness test -- need help!

Richard Tolman derived that in an expanding universe with any arbitrary geometry, the surface brightness of a set of identical objects will decrease by (1+z)^4.

I have two questions:

1) The surface brightness is the same as the flux (W/m^2), right?
2) My understanding so far has been that the received flux decreases with the distance squared, so reading that it will decrease by a factor of (1+z)^4 is a bit confusing. Is it perhaps a factor that is ignored when the distance from Earth is relatively small (because z is so small)?

Need help urgently!
 
Last edited:
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Oh, and another question. Some sources say the SB will decrease by (1+z)^2 instead of (1+z)^4 in an expanding universe. Which is correct?
 
I've received answers to the first two questions already, so no need for anyone to bother with them. The question in my second post is still unanswered, though.
 
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