2203312 said:
If the Universe stopped expansion 10 million years ago and started to collapse
Whould it their be such a thing as cosmological blueshift?Would the scalefactor-redshift be R=1/(z - 1) or R=1/(1+z)?
I think since universe is collapsing then spectra will be blueshifted!
The scale factor is still relevant but the formula is changed to 1/1(z-1)!
If the universe had stopped expanding only fairly recently, like say a billion years ago, and had started to contract, then some galaxies would have a NEGATIVE z.
It would continue to be true that 1+z equals the ratio by which distances have changed while the light is in transit.
Some light would have z = 0 for example if the light was emitted 2 billion years ago and for the first billion year that it was in transit there was expansion and then for the next billion years there was contraction---then there would be no net change in the wavelength.
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You don't need any new formula. You can always assume that 1+z is the ratio of distances, and also the ratio of wavelengths.
If the universe had been contracting for several billion years, so there was enough time for some distances to contract by a factor of 10, then there would be some galaxies with z = -0.9
That means the ratio of wavelengths is 1+z = 0.1
The wavelengths are ten times shorter----in other words bluer, more UV
The way that z is defined it cannot take on the value -1 or any value more negative than -1. The ratio of wavelengths is always a positive number 1+z>0 and that means that z>-1.
