# Are most galaxies expanding from us as previously calculated?

In this question: Can a photon that is emitted from a denser part of the universe to a less dense part appear redshifted? The answers there were yes, yes and yes primarily. With our space less dense than most, is it possible that our own region of galaxy or universe space could be collapsing much faster, becoming denser faster more than metric expansion as a whole, to a point where it makes the rest of the universe appear to be accelerated expanding and red shifted?

Is it possable that matter in the universe is just expanding and collapsing imperceptibly in our existence on Earth?

• Hi Jen, I just stuck in a few commas and hopefully made the question stand out. The title is your own business , but it might not be as clear as you want it to be. How about "Are nearby galaxies expanding from us slower than previously calculated"? Anyway, best of luck with it
– user108787
Dec 12 '16 at 22:52
• @CountTo10 have you read my profile? Feel free to improve and increase the quality of anything I post.
– Muze
Dec 12 '16 at 23:53

It is not hard to see that the redshift due to the mass of a galaxy is not that significant. A galaxy has around $10^{11}$ stars with a mass $\sim~10^{41}kg$. We think of the photons as leaving a region at the outer reaches of a galaxy $\sim~10^5$ light years or $10^{21}$m. We now evaluate $2GM/rc^2$, with $G~=~6.7\times 10^{-11}Nm^2/kg^2$. This gives an order of magnitude estimate that $$\frac{2GM}{rc^2}~\simeq~10^{-9}.$$ The Schwarzschild factor $1~-~2GM/rc^2$ gives the redshift as $$\frac{\lambda_\infty}{\lambda}~=~\frac{\lim_{r\rightarrow\infty}(1~-~2GM/rc^2)}{1~-~2GM/rc^2}$$ $$\simeq~1~-~2GM/rc^2~\simeq~1~+~10^{-9}.$$ Here $r~\rightarrow~\infty$ means the radius from the mass to the observer very far away.
• Essentially that will be even less than what I calculated. The average density of matter, as I recall without looking it up, is $10^{-29}g/cm^3$. Galaxies and galaxy clusters are rather small anisotropic or inhomogeneous variations on this. The redshift factor is very slight. Dec 13 '16 at 0:28