l**n
发帖数: 67 | 1 Cosmological redshift(Hubble redshift)
Now let go back to cosmology and for the time being forget about
the gravitational redshift since it's small anyway. In principle
the cosmological redshift can be described similarly, but we
must stress that the cosmological redshift(z) is deeply rooted
in the fact that we live in a (accelarating?) expanding,(curved?),
universe. In order to describe this z, we need to have a model
of universe. It's mathmatical description is model dependent!
comoving coordi | i*******n
发帖数: 166 | 2
why \sqrt R(t)? I think it is R(t).
BTW, in last chapter, you said that there are two kinds of redshift:
gravitational and cosmological. I think you should add another: Doppler
redshift.
【在 l**n 的大作中提到】
: Cosmological redshift(Hubble redshift)
: Now let go back to cosmology and for the time being forget about
: the gravitational redshift since it's small anyway. In principle
: the cosmological redshift can be described similarly, but we
: must stress that the cosmological redshift(z) is deeply rooted
: in the fact that we live in a (accelarating?) expanding,(curved?),
: universe. In order to describe this z, we need to have a model
: of universe. It's mathmatical description is model dependent!
: comoving coordi
| f*******d
发帖数: 339 | 3 I think you meant Copernicus principle. Also, such reasoning
for introducing comoving coordinate is unnecessary and incorrect.
Even if we do not use comoving coordinate, the expansion of
Universe does not violate Copernicus principle: all these galaxies end
up in infinite far away (if the Universe is forever expanding)!
Indeed, according to general relativity, we could describe the space time
in any well-posed coordinate system. Introducing comoving coordinates
is merely a convinience.
The disti | l**n
发帖数: 67 | 4
end
time
coordinates
Thank you very much for your comment. I guess we all agree that
whatever
physics should be independent of the coordinates we are using. It's
just
a matter of convinience to use specific coordinate in specific
questions.
I think(guess) the reason why comoving coordinate is used here is
because
the universe is expanding. It's perfectly valid to use euclidean frame
also
but with more trouble in maths. To me a euclidean frame should be used
to describe the static universe in wh
【在 f*******d 的大作中提到】
: I think you meant Copernicus principle. Also, such reasoning
: for introducing comoving coordinate is unnecessary and incorrect.
: Even if we do not use comoving coordinate, the expansion of
: Universe does not violate Copernicus principle: all these galaxies end
: up in infinite far away (if the Universe is forever expanding)!
: Indeed, according to general relativity, we could describe the space time
: in any well-posed coordinate system. Introducing comoving coordinates
: is merely a convinience.
: The disti
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