The discussion revolves around the implications of the expanding universe on the formation of galaxies, the nature of redshift, and the mechanisms behind these phenomena. Participants explore theoretical aspects, observational evidence, and the relationship between cosmic expansion and gravitational interactions.
Participants express multiple competing views regarding the mechanisms of redshift and the implications of cosmic expansion on galaxy formation. The discussion remains unresolved with differing interpretations of the evidence and theoretical implications.
Some statements rely on specific definitions of cosmological terms and concepts, which may not be universally agreed upon. The discussion includes assumptions about gravitational interactions and the nature of redshift that are not fully elaborated.
The expansion is an overall, average effect. However, different parts of the universe early-on were slightly more dense than other parts. The parts that were dense enough had enough self-gravity to stop expanding and collapse inward, even while the overall expansion continued.Brook said:If space is expanding then how did the planets and stars come together to form galaxies?
Why are the galaxies themselves not expanding and the solar system not expanding?
Both, sort of.Brook said:And the red shift seen in the distant galaxies...is this due to the expansion or to the doppler effect?
Brook said:so how does expansion create a red shift? what is the mechanism...
Brook said:If space is expanding then how did the planets and stars come together to form galaxies?
Why are the galaxies themselves not expanding and the solar system not expanding?
Draw a curved line on a sheet of rubber, extend it, and you will see an increased wavelength. As the speed of light is constant, an increased wavelength corresponds to a reduced frequency.Brook said:so how does expansion create a red shift? what is the mechanism...
phinds said:If one were to say that the entire red shift of distance galaxies was due to the Doppler effect, that would be equivalent to saying that the universe is not expanding.
Brook said:can you explain this further. I know that the doppler effect can show which direction things are orbiting when viewed from earth.
Why can't the doppler effect explain the red shift seen for distant galaxies? why would this mean that the universe is not expanding?
phinds said:Reread post #4
Rather the inferred velocities don't make much sense.Brook said:So let me see if I get this...
If the doppler effect was used to interpret the red shifts then it wouldn't make sense because the red shifts calculated would be too large to be explained by the velocity of the galaxies?
Chalnoth said:Rather the inferred velocities don't make much sense.
It's the other way around. Too small. These objects measured to be too far away to be explained by their inferred velocities by assuming the redshift is due to the Doppler shift. If we instead infer their velocities based upon their distances, the further away galaxies are and always have been receding at faster than the speed of light.Brook said:right, so the velocities needed to explain the red shifts would be too great...were these assumed velocities approaching light speed?
Brook said:So let me see if I get this...
If the doppler effect was used to interpret the red shifts then it wouldn't make sense because the red shifts calculated would be too large to be explained by the velocity of the galaxies?
So instead of the red shift being due to the velocity of the galaxy it is due to the expansion of space itself and of the wavelength of the photon?
am i starting to get it right?
Brook said:hmmm...
so where does the energy go when the photon wavelength expands?
and is this red shift the only evidence that we have of an expanding universe?
If space is expanding then how did the planets and stars come together to form galaxies?
That's not what that means. The era of matter domination was the era where matter (normal matter + dark matter) made up most of the energy density in the universe.Naty1 said:A point not mentioned so far is that in the distant past, the universe was 'matter dominated'...that is, stuff was close enough together for gravity to be relatively strong between matter.
It depends how we want to define the Doppler effect as to whether cosmological redshift is a Doppler effect. If we define it as something that exists only when the observer and observed are in the same Lorentz frame then by definition the redshift of distant galaxies cannot be Doppler because any frame incorporating both us and them has too much curvature to be Lorentz.phinds said:With the Doppler effect, what matters is the speed of the receding object at the time light is emitted. This alone determines the red shift. With metric expansion, however, the distance between the photon and us keeps getting bigger as the photon travels towards us, and this is what causes the red shift (which as Chalnoth pointed out, is for distant galaxies almost entirely due to this effect, NOT to the Doppler effect).
andrewkirk said:It depends how we want to define the Doppler effect as to whether cosmological redshift is a Doppler effect. If we define it as something that exists only when the observer and observed are in the same Lorentz frame then by definition the redshift of distant galaxies cannot be Doppler because any frame incorporating both us and them has too much curvature to be Lorentz.
However, if we define Doppler as a spectral shift arising from relative motion, or that can be described by the sqrt((1+V)/(1-V)) formula then all spectral shifts - local Doppler, gravitational and cosmological - can be regarded as Doppler.