post for sci.astro.research

Ted Bunn wrote:


Personally, I think that Phillip tends to oversell the notion that you
can never (not even at low redshifts) think of the cosmological
redshift as a Doppler shift. At low redshifts (distances much smaller
than the horizon or the curvature scale), spacetime is Minkowski to an
excellent approximation, and in that approximation galaxies are flying
away from us. By all means go ahead and think of the redshifts of
those nearby galaxies as ordinary Doppler shifts if you like.

The attractiveness of the unified approach to spectral shifts is
that this approach clearly shows how spectral shifts are related
to the geometry of space-time.

In particular, spectral shifts due to curved space-time geometry
should never be thought of as ordinary Doppler shifts in flat space-time.
That is, it is not meaningful to approximate curved space-time with
flat space-time and at the same time keeping spectral shifts due
to curved space-time; such a scheme would be inconsistent.

In particular this is true for a weak gravitational field, and also for
the Friedmann models.

Ted Bunn wrote:


Personally, I think that Phillip tends to oversell the notion that you
can never (not even at low redshifts) think of the cosmological
redshift as a Doppler shift. At low redshifts (distances much smaller
than the horizon or the curvature scale), spacetime is Minkowski to an
excellent approximation, and in that approximation galaxies are flying
away from us. By all means go ahead and think of the redshifts of
those nearby galaxies as ordinary Doppler shifts if you like.

The attractiveness of the unified approach to spectral shifts is
that this approach clearly shows how spectral shifts are related
to the geometry of space-time.

In particular, spectral shifts due to curved space-time geometry
should never be thought of as ordinary Doppler shifts in flat space-time.
That is, it is not meaningful to approximate curved space-time with
flat space-time and at the same time keeping spectral shifts due
to curved space-time; such a scheme would be inconsistent.

In particular this is true for a weak gravitational field, and also for
the Friedmann models.

In article , Ted Bunn
writes:

On the other hand, let me urge everyone not to think that this
justifies saying that the cosmological redshift "really is" a Doppler
shift rather than a gravitational redshift.

Right, but cosmological redshifts are different than conventional
gravitational redshifts. Of course, in practice objects have all three,
but the typical object at a cosmological distance has a cosmological
redshift much greater than (and independent of) any Doppler or
gravitational redshift.

In article , Ted Bunn
writes:

If you use this recipe to calculate the cosmological redshift of a
distant galaxy, the v that you get by this procedure does not
correspond to the recession speed of the galaxy in any of the usual
senses. For instance, you might reasonably define the present
recession speed of a galaxy to be dr/dt, where t is cosmic time and r
is the distance to the galaxy measured at constant t. (This is the
thing people most often mean when they talk about the recession speed
of a cosmological object.) That quantity dr/dt is not the same as the
v that goes into the above formula. That (I think) is what Phillip
Helbig meant by the comment that you can't use the
special-relativistic formula to calculate cosmological redshifts, and
he's quite right about that.

Yes, this is the important point. If someone mentions the normal
Doppler formula (relativistic version or not) in the context of
cosmological redshifts, it's just wrong.

Personally, I think that Phillip tends to oversell the notion that you
can never (not even at low redshifts) think of the cosmological
redshift as a Doppler shift.

You CAN, I just say that you SHOULDN'T. :-)

I think the disadvantages of overselling it are far outweighed by the
advantages of avoiding the confusion created by mentioning Doppler
shifts in the context of cosmological redshifts.

In article , Ted Bunn
writes:

On the other hand, let me urge everyone not to think that this
justifies saying that the cosmological redshift "really is" a Doppler
shift rather than a gravitational redshift.

Right, but cosmological redshifts are different than conventional
gravitational redshifts. Of course, in practice objects have all three,
but the typical object at a cosmological distance has a cosmological
redshift much greater than (and independent of) any Doppler or
gravitational redshift.

In article , Ted Bunn
writes:

If you use this recipe to calculate the cosmological redshift of a
distant galaxy, the v that you get by this procedure does not
correspond to the recession speed of the galaxy in any of the usual
senses. For instance, you might reasonably define the present
recession speed of a galaxy to be dr/dt, where t is cosmic time and r
is the distance to the galaxy measured at constant t. (This is the
thing people most often mean when they talk about the recession speed
of a cosmological object.) That quantity dr/dt is not the same as the
v that goes into the above formula. That (I think) is what Phillip
Helbig meant by the comment that you can't use the
special-relativistic formula to calculate cosmological redshifts, and
he's quite right about that.

Yes, this is the important point. If someone mentions the normal
Doppler formula (relativistic version or not) in the context of
cosmological redshifts, it's just wrong.

Personally, I think that Phillip tends to oversell the notion that you
can never (not even at low redshifts) think of the cosmological
redshift as a Doppler shift.

You CAN, I just say that you SHOULDN'T. :-)

I think the disadvantages of overselling it are far outweighed by the
advantages of avoiding the confusion created by mentioning Doppler
shifts in the context of cosmological redshifts.

In article ,
writes:

Personally, I think that Phillip tends to oversell the notion that you
can never (not even at low redshifts) think of the cosmological
redshift as a Doppler shift.

You CAN, I just say that you SHOULDN'T. :-)

I think the disadvantages of overselling it are far outweighed by the
advantages of avoiding the confusion created by mentioning Doppler
shifts in the context of cosmological redshifts.

Let me ask you this. Suppose you lived in an open FRW Universe with
Omega = 10^(-50). You observe a (very low-mass!) galaxy at a redshift
z = 0.1. Would you say it's wrong to call that redshift a Doppler
shift? Remember that in this spacetime, the geometry is Minkowski,
with errors of only one part in 10^50 all the way out to the horizon
scale!

In particular, the geometry of this spacetime is much closer
to Minkowski than the geometry of spacetime near Earth's surface,
yet the cops use radar guns to measure speeds (interpreting frequency
shifts as Doppler shifts) all the time.

In this case, I would say it's OK. In our univese, if the redshift is
small enough, it's OK. Still, I don't see what advantage it brings,
since at larger redshift in our universe it is not correct. My
experience has been that it is more trouble than it is worth, in
practice. (I've even seen folks try to explain the cosmological
redshift to a popular audience and mention the Doppler effect---but they
had to explain the Doppler effect first! That's certainly going
overboard. IF one already knows what the Doppler effect is, then it
might be OK to mention it, but one should point out that it is only
correct in the limiting cases.)

This might be a good time to direct readers of this thread to
astro-ph/0310808.

In article ,
writes:

Personally, I think that Phillip tends to oversell the notion that you
can never (not even at low redshifts) think of the cosmological
redshift as a Doppler shift.

You CAN, I just say that you SHOULDN'T. :-)

I think the disadvantages of overselling it are far outweighed by the
advantages of avoiding the confusion created by mentioning Doppler
shifts in the context of cosmological redshifts.

Let me ask you this. Suppose you lived in an open FRW Universe with
Omega = 10^(-50). You observe a (very low-mass!) galaxy at a redshift
z = 0.1. Would you say it's wrong to call that redshift a Doppler
shift? Remember that in this spacetime, the geometry is Minkowski,
with errors of only one part in 10^50 all the way out to the horizon
scale!

In particular, the geometry of this spacetime is much closer
to Minkowski than the geometry of spacetime near Earth's surface,
yet the cops use radar guns to measure speeds (interpreting frequency
shifts as Doppler shifts) all the time.

In this case, I would say it's OK. In our univese, if the redshift is
small enough, it's OK. Still, I don't see what advantage it brings,
since at larger redshift in our universe it is not correct. My
experience has been that it is more trouble than it is worth, in
practice. (I've even seen folks try to explain the cosmological
redshift to a popular audience and mention the Doppler effect---but they
had to explain the Doppler effect first! That's certainly going
overboard. IF one already knows what the Doppler effect is, then it
might be OK to mention it, but one should point out that it is only
correct in the limiting cases.)

This might be a good time to direct readers of this thread to
astro-ph/0310808.