How cosmologists know if the observed redshift of galaxies is due to expansion and not intrinsic to the galaxy?

Question

Is there a way to experimentally test if the cosmological red shift observed is due to the expansion or intrinsic to the galaxy? If anyone knows how to do this or how to extract from the data please share this information here.

Answer 1

There're a few ways to do it. If we assume that the observed redshift is not due to expansion, then something else must be causing it. What could that something else be?

• Maybe the light is losing energy because it is climbing out of a potential well (aka gravitational redshift). This is something that can be checked, because the equations governing gravitational redshift are well-known. The effect turns out to be too small to account for the observed redshift. See this 2011 paper, for which the gravitational redshift of galaxy clusters turns out to be a few km/s, not nearly enough to match cosmological redshift (which is many orders of magnitude larger).
• Maybe the light is losing energy because it is traveling through (non-expanding) space. This is the tired light hypothesis, which is generally regarded as falsified.
• Maybe space is static but everything else is receding from us. This makes the redshift a pure doppler redshift. Problem with this is, how why are those galaxies receding from us? Did we kick them? If so, how? See also Copernican principle.
• Maybe stars in faraway galaxies are just redder. But how would you create a star that is redder? Starlight is well-understood (see main sequence), and you can't get stars that are that much redder from the same building blocks as stars around us (i.e. hydrogen/helium), unless you postulate that the laws of physics themselves are different far away from us, which has its own set of problems. Another issue with this is that you need to be able to explain how your modified stars are still able to go supernova (necessary because we can observe supernovae in faraway galaxies).

So: it's not that people don't try, it's just that we cannot find another explanation that matches the data.

Answer 2

The intrinsic gravitational redshift (which would be too small to explain high redshift galaxies in any case - also, why would redshift depend on distance?) would be proportional to $M/R$.

Galaxies often come in pairs, groups or clusters. Yet the galaxies within the pair, group or cluster share very similar redshifts despite often having very different masses and radii.

Answer 3

Yes, there is a proposal claiming that the universe isn't expanding, and red shift is due to something "intrinsic to the galaxy". Specifically, in Wetterich's "Universe without expansion" scenario, it's the masses of fundamental particles which are changing and causing the redshift.

Wetterich's growing mass interpretation may be plausible, but it comes with a big problem: the red-shifted photon frequency emitted from a given atom is related to the reduced mass of the electron $$ \mu= \frac{M m_e}{M + m_e} = m_e\frac{1}{1 + \frac{m_e}{M}}, $$ where $m_e$ is the electron mass and $M$ is the mass of the nucleus.

In order to be consistent with the observed uniform redshift for all atoms, one is expecting to hold ratio $m_e/M$ constant. Giving that $m_e$ is coming from Higgs mechanism and $M$ is coming mostly from QCD, it's a tall order.

See details here.

Answer 4

One way is to look at the 21 cm hydrogen line, which will be redshifted.

Cepheid variable may help, as their periods and luminosity are related. The luminosity is mostly $1/r^2$, but there should be a $T/z$ (or $z-1$, ask an expert) temperature shift, and the period should be Doppler shifted too, in which case there is unique velocity and distance determined by a single object...though I don't know if it's done this way. Ask Astronomy.