What Actually Expands In An Expanding Universe?

What people said

Recommends Hartmut Braun · 2020-10 · Physics Stack Exchange · on “Redshift of distant galaxies: why not a doppler effect?”

This happened to be published just yesterday! Essentially it says that Dopplershift, gravitational redshift and redshift from expansion are the same thing. [this video]

Recommends Himanshu · 2021-12 · Physics Stack Exchange · on “Do gravitational redshift depends on reference frames?”

It does, If you know the expression for shift $$\frac{\nu_R}{\nu_E}=\frac{p_0(B)}{p_0(A)}\left[\frac{g_{00}(A)}{g_{00}(B)}\right]^{1/2}$$ See General Relativity: An introduction to Physicists [LINK] Appendix 9A for the proof. [LINK] It's clear from here that different observers have different $p_0$ , and therefore have a different shifts. It might be helpful to watch this [this video] veritesium video.

Recommends RC_23 · 2022-05 · Physics Stack Exchange · on “Is redshift an accurate method for measuring distances of distant galaxies and quasars?”

This may not be detailed enough for a real answer, but I wanted to point out that the cosmological redshift is not caused by the original speed of the emitting galaxy, it is caused by what happens to the emitted light between being emitted and observed (or maybe more accurately, what happens to the Earth in that time). There's a great video on this [this video]

Recommends mmesser314 · 2023-10 · Physics Stack Exchange · on “Why is the cosmic background 4K? Where are the other photons?”

You seem to have a few misconceptions about photons and the Big Bang. Here are a couple videos that might help. Misconceptions About the Universe [LINK] , What Actually Expands In An Expanding Universe? [this video]

Recommends mmesser314 · 2023-11 · Physics Stack Exchange · on “Is there a Doppler effect on the surface of an expanding balloon?”

Suppose the mechanical properties of the balloon were not affected by being stretched. That is, suppose you took a balloon and stretched it $10$ %. The speed of waves is the same as an unstretched balloon. The frequency and wavelength would be the same in both. Travel time would increase $10$ % because the distance traveled increased that much. Now suppose you emit a wave as the balloon stretches. Let us call the stretching rate $\alpha$ . In a time interval $\Delta t$ , a length $l$ grows by $\Delta l$ . $$\alpha = \frac{\Delta l}{l \Delta t}$$ If the source and receiver are separated by a distance d, their separation is increasing at velocity $\dot d =\alpha d$ . So $$d = d_0 e^{\alpha t}$

Recommends mmesser314 · 2023-12 · Physics Stack Exchange · on “Does the accelerated expansion of the universe have any effects in the orbital precession of galaxies? And in their eccentricity?”

Galaxies don't expand with the expansion of the universe. See this Veritasium video. What Actually Expands In An Expanding Universe? [this video]

Recommends mmesser314 · 2025-02 · Physics Stack Exchange · on “Experiment to test for shrinking matter?”

Controlgroup makes the point that the Hubble constant is too small to measure the way you are thinking (+1). But suppose it was bigger. Suppose the Earth and moon shrank 1% in a year. If we lined up a bunch of rulers, we would indeed think distance to the moon was 1% bigger. But suppose we measured by measuring the time it takes a pulse of light to travel to the moon, bounce off retroreflectors, and return. We would get the same distance. You might also find this Veritasium video interesting. What Actually Expands In An Expanding Universe? [this video]

Recommends mmesser314 · 2026-01 · Physics Stack Exchange · on “Is the acceleration of cosmological expansion just for the cosmos as a whole or is it also more localized?”

Gravitationally bound objects do not expand. See this Veritasium video - What Actually Expands In An Expanding Universe? [this video]