Help: Explaining Expanding/Accelerating Universe Without Redshift Fallacy

[Egregious]

Preface: I am not a physicist, but I do tend to accept the theories of an expanding and accelerating universe. I am not inquiring to dispute these ideas, but it recently occurred to me that there is an obvious fallacy in the arguments for these ideas as presented in the popular media. I wish to navigate this logical flaw.

Fallacious template: If A, then B. B; therefore A.

From the popular media:

(1) If (A) [light sources] are moving away, then (B) there is red-shift.
(2) (B); therefore (A)

(1) is well established, but I also understand the following to be false:

(A) is the only thing that causes (B)

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This fallacy first appeared for me while I was considering the idea of an accelerating universe. An accelerating universe is consistent with a further red-shift of light sources that are farther away. I wondered, though why there could not be a feature of space that would also cause red-shift in a way that is proportional to the distance to the light source. In other words, (asking myself) why is the accelerating universe theory the only/best explanation for this observation?

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If someone would kindly fill in the missing pieces for a lay-enthusiast to logically establish that for light sources at cosmological distances (A) is true, your efforts would be greatly appreciated!

Many thanks,

~Egr

 

[Drakkith]

If A is KNOWN to cause B, and nothing else is known to cause B, then if we see B we assume A caused it until we have reason to believe otherwise. (Note that while redshift can happen due to gravity, it would NOT be the same, so I don't list it as a cause of B since we know what that would look like)There have actually been a few theories on "tired light", which is exactly what you were talking about with light losing energy and redshifting from another source other than relative motion. However all of these have either failed to be observed or would be far more complicated and make more assumptions than an expanding universe.

Note that we don't claim that expansion is the ONLY explanation for redshift of this sort, only that it is currently the most likely explanation by quite a wide margin.

 

[Chalnoth]

However all of these have either failed to be observed or would be far more complicated and make more assumptions than an expanding universe.

More accurately, they haven't fit with the observations we have made.

Either way, it's incredibly difficult to come up with a model that correlates redshift and distance that is substantially different from an expanding universe.

 

[Chronos]

At cosmological distances [100+ Mpc] the doppler and gravitational contributions to redshift are overwhelmed by the cosmological redshift and generally ignored. It becomes obvious by the time you detect objects with a redshift of 1+ neither proper motion or gravity can possibly account for such values. Various ideas like 'intrinsic' redshift and 'tired' light have been proposed over the years, but, have no theoretical basis and have been largely dismissed by mainstream scientists.

 

[Whovian]

I wondered, though why there could not be a feature of space that would also cause red-shift in a way that is proportional to the distance to the light source.

There is. It's called expansion.

 

[Chronos]

Just to clarify, there is no simple way to differentiate one kind of redshift from another. Gravitational and doppler redshifts are spectroscopically identical to cosmological redshift. They cannot be told apart without knowing something about the emitting source. If you happen to know the source is either a black hole or neutron star, you can approximate the gravitational redshift contribution. If it is a galaxy you can approximate its kinematical redshift by examining redshift of opposing arms [assuming your view is not parallel to its rotational axis].