Why are physicists not more concerned that there are too many explanations for redshift in the universe?

Question

There are speculative explanations for red shift such as the tired light theory, but I am not referring to those. There are three mainstream explanations

1. Red shift due the expansion of the universe giving rise to a Doppler effect.

2. Cosmological red shift. The red shift is due to the stretching of light as the universe expands. Numerically, this seems to explain all of red shift, leaving no room for the other explanations.

3. Gravitational time dilation. Time runs slower as the force of gravity, or the gravitational field, gets stronger. Slower time equates to a reduced frequency, which is a red shift. There is definitely gravitational red shift at the local scale but I think there is at the cosmological scale. As the universe gets younger, the density increases, as will the force of gravity, or at least the gravitational field.

It is interesting that Doppler red shift can be termed temporary red shift. When the velocity stops the red shift stops. On the other hand, gravitational red shift due to the expansion of the universe could be termed permanent red shift. If the expansion stops, the red shift will remain. I think cosmological red shift must be permanent red shift.

How do all of these different red shifts relate to each other?

Answer 1

One of the difficult concepts for people starting in general relativity is that we can and often do use radically different coordinates to describe the same physical system. It often appears that we are using different physical theories when really it is the same theory (GR) just in different coordinates.

In this case, the equations that every student learns for the expanding universe are written in a coordinate system called comoving coordinates. In these coordinates everything in the universe is (approximately) stationary. Since everything is stationary the increasing distances between objects, and the red shift, is interpreted as the universe expanding.

However it is also possible to use coordinates in which the universe is not expanding. This isn't often done since the maths gets messy, but it can be done and in this case we get a universe that is not expanding but in which the velocities of objects increase with distance from the observer. In that case the red shift is indeed described by the Doppler equation.

So your question is slightly off target. There are not different mainstream explanations for red shift. Instead there are different coordinate systems we can use to describe the same physics. Physicists are not concerned that different coordinate systems exist. Rather the reverse because different coordinate systems can be very useful for doing calculations in different ways.