I'm a bit fixated on the idea of redshift being proof of cosmic expansion, and I can't wrap my head around the idea that space is supposedly expanding everywhere, all at once.
If space itself is expanding everywhere, why is it that we don't see stars in galaxies, or even our own Milky Way, visibly drifting apart over time. Instead, the stars within galaxies (and even galaxy clusters, for that matter) appear to remain stable in structure, giving the impression that cosmic expansion seems to occur only between galaxies.
How does this align with the theory that space is expanding uniformly throughout the universe?
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Here's an update on what I believe I understand so far, and why it's a problem for me:
- Astronomers observed that the light from many galaxies appeared to be redshifted, meaning its wavelength was stretched, shifting it toward the red part of the spectrum.
- By measuring the redshift of light from galaxies at various distances, astronomers noticed that galaxies farther away from us tend to have higher redshift values. This led to the observation that the redshift is correlated with distance.
- To explain this correlation, Edwin Hubble and others hypothesized that the redshift was due to the expansion of space itself. According to this hypothesis, as space expands, it stretches the light traveling through it, increasing its wavelength causing redshift.
- Assuming that their cosmic expansion hypothesis was correct, they interpreted the redshift of a galaxy as an indicator of its velocity. The farther away a galaxy was, the more redshifted its light, which was taken to mean that distant galaxies are moving away from us faster.
- Because they already assumed they were right about redshift, Hubble created a law that velocity was equal to Hubble's constant multiplied by the distance. Basically, he made math that fit his assumption, so of course it works out mathematically...
- The assumption that redshift is due to galaxies moving away was then used to argue that the universe is expanding, with galaxies moving away from each other over time. The idea of cosmological redshift became a circular reference to support the idea of cosmic expansion.
- Arguments that try to explain why cosmic expanse does not occur where gravity exists include: a) Gravitational binding holding the galaxies together. - If this were the case, then why do immense galaxies "move" at the same rate smaller ones do? Shouldn't their increased gravitational attraction to nearby galaxies hold them in proximity more so than smaller galaxies against one another? b) Cosmic expansion only affects unbound systems. - The critique above works against this theory as well. c) The scale of the expansion is too small to measure locally. d) Local velocities are greater than the effect of cosmic expansion. e) Einstein Field Equation only applies to large scale, and not local spacetime. - So... somehow using different math makes physics change?
