The Doppler equation used for sound calculations cannot be used in this situation. This is because galaxies are receding moving away at such high speeds that relativistic effects need to be considered in calculations. The following equation is used to calculate redshift:.
I would say that at this level letting go of a rigid space is a good idea and using language like "expanding space makes it look like the galaxies are retreating ever faster and it causes a doppler effect-like redshift" is OK. Add a comment. Active Oldest Votes. Improve this answer.
FMunley FMunley 51 2 2 bronze badges. Bunn's treatment is sensible but arises from a sophisticated understanding of what "velocity" is involved. However it is not observationally different from a Doppler shift. Sean E. Lake Sean E. Lake 19k 2 2 gold badges 31 31 silver badges 70 70 bronze badges.
For example: Flat spacetime is static in two different ways the obvious way and Rindler coordinates , and the SR and gravitational redshift formulas agree in each case. De Sitter space is static and is FLRW in at least two ways, and the gravitational and cosmological redshift formulas agree.
The comoving velocity turns out to be the SR rapidity times c , so you could also use a version of the formula that takes rapidities "natively. Why the light is stretching - there are two equivalent ways to think about this.
In the sources rest frame: the light source emits light at the rest wavelength and as the light propagates through space it's physical properties wavelength expand with space. In the observers rest frame: the source is moving away from us, so when it emits the light is immediately Doppler shifted red-wards, and then proceeds through space at this wavelength. Hope this helps. Judge Judge 1 1 gold badge 6 6 silver badges 8 8 bronze badges. Marco Ocram Marco Ocram Hartmut Braun Hartmut Braun 4 4 silver badges 9 9 bronze badges.
Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Featured on Meta. Now live: A fully responsive profile. Linked 8. Related 4. The diagram shows part of the emission spectrum of light from a distant galaxy. Astronomers see red-shift in virtually all galaxies. It is a result of the space between the Earth and the galaxies expanding. This expansion stretches out the light waves during their journey to us, shifting them towards the red end of the spectrum.
For example:. When Hubble plotted the redshift vs. Hubble concluded that the fainter and smaller the galaxy, the more distant it is, and the faster it is moving away from us, or that the recessional velocity of a galaxy is proportional to its distance from us:. The line goes through the origin 0,0 because that represents our home position zero distance and we are not moving away from ourselves zero speed.
To determine a galaxy's distance, we must rely on indirect methods.
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