Hi,
Related to last night's presentation...
I was unaware of the general relativistic effect on
photons... I knew Hubble initially assumed doppler
shifts. Somehow I missed any mention of this other
effect. I know people were arguing about "tired photons"
at one point and I think that was rejected.
Guess the books I read about this when I was young were
already out of date? :-)
Anyway, our lecturer last night did a great job.
I definitely learned something!
This is from https://en.wikipedia.org/wiki/Redshift:
Expansion of space
Main article: Metric expansion of space
<https://en.wikipedia.org/wiki/Metric_expansion_of_space>
In the early part of the twentieth century, Slipher, Hubble and others
made the first measurements of the redshifts and blueshifts of galaxies
beyond the Milky Way <https://en.wikipedia.org/wiki/Milky_Way>. They
initially interpreted these redshifts and blueshifts as due solely to
the Doppler effect, but later Hubble discovered a rough correlation
between the increasing redshifts and the increasing distance of
galaxies. Theorists almost immediately realized that these observations
could be explained by a different mechanism for producing redshifts.
Hubble's law <https://en.wikipedia.org/wiki/Hubble%27s_law> of the
correlation between redshifts and distances is required by models of
cosmology derived from general relativity that have a metric expansion
of space <https://en.wikipedia.org/wiki/Metric_expansion_of_space>.[18]
<https://en.wikipedia.org/wiki/Redshift#cite_note-Eddington-18> As a
result, photons propagating through the expanding space are stretched,
creating the cosmological redshift
<https://en.wikipedia.org/wiki/Cosmological_redshift>.
There is a distinction between a redshift in cosmological context as
compared to that witnessed when nearby objects exhibit a local
<https://en.wikipedia.org/wiki/Local_reference_frame> Doppler-effect
redshift. Rather than cosmological redshifts being a consequence of
relative velocities, the photons instead increase in wavelength and
redshift because of a feature of the spacetime
<https://en.wikipedia.org/wiki/Spacetime_topology> through which they
are traveling that causes space to expand
<https://en.wikipedia.org/wiki/Metric_expansion_of_space>.[27]
<https://en.wikipedia.org/wiki/Redshift#cite_note-27> Due to the
expansion increasing as distances increase, the distance between two
remote galaxies can increase at more than 3×108 m/s, but this does not
imply that the galaxies move faster than the speed of light at their
present location (which is forbidden by Lorentz covariance
<https://en.wikipedia.org/wiki/Lorentz_covariance>).
Thanks,
Howard
