Astronomer Michael J. Disney describes Big Bang cosmology this way:
Big Bang cosmology is not a single theory; rather, it is five separate theories constructed on top of one another. The ground floor is a theory, historically but not fundamentally rooted in general relativity, to explain the redshifts—this is Expansion, which happily also accounts for the cosmic background radiation.
https://www.americanscientist.org/article/modern-cosmology-science-or-folktale
Redshifting has been assumed to be an evidence of the expansion of the universe. But is it?
Back in the 1930s, Cal Tech professor Richard Tolman proposed a test for determining if Redshifting and the expansion of the universe are linked.
Tolman calculated that the surface brightness (the apparent brightness per unit area) of receding galaxies should fall off in a particularly dramatic way with redshift—indeed, so dramatically that those of us building the first cameras for the Hubble Space Telescope in the 1980s were told by cosmologists not to worry about distant galaxies, because we simply wouldn’t see them. Imagine our surprise therefore when every deep Hubble image turned out to have hundreds of apparently distant galaxies scattered all over it (as seen in the first image in this piece).
Tolman couldn’t be wrong: his physics is as sound of a bell. So, if the universe really is expanding, which seems likely, why do all the galaxies, irrespective of their redshift, appear to have the same identical surface- brightness? High redshift galaxies are no less than 4000 times brighter than they ought to be. There’s no escaping from that. 4000 times! (https://mjdisney.org/2022/07/at-the-beginning-of-time-what-the-james-webb-telescope-might-see/astronomy/)
Physicist Eric Lerner agrees that the surface brightness data support the non-expanding universe hypothesis:
…we find that when the effect of telescope resolution is taken into account, the r–z relationships for disc and elliptical galaxies are identical. Both are excellently fit by SEU [static Euclidian Universe] predictions.
https://academic.oup.com/mnras/article/477/3/3185/4951333?login=false
What does this mean? The theory of General Relativity predicts an expanding universe, but redshifting data does not confirm expansion. If the universe is not expanding, the ground floor of the Big Bang theory has just been destroyed!!
A second implication: When astronomers measure the size of the universe, they use several techniques of the “cosmic distance ladder” to estimate distances from the Earth. The fourth and final step of the “cosmic distance ladder” estimates distances based on redshifts and the implied doppler effect. That fourth step is no longer valid. Surface brightness observations prove that redshifts are not analogous to the doppler effect.
A third implication has to do with the concept of “time since creation.” If the universe is not expanding, then the idea that measuring redshifts lets us look back in time is invalid.