Re: Cosmic Background Radiation

Thanks George.
I guess I was lazy to work this out. Bringing together Relativity and
the concept of the local 'prefered frame of reference' of the CMBR
somehow didn't compute for a while there.
Now, I think I have it. It would seem to me that the important
concepthere is that an AVERAGE acceleration in any given direction
directly away from Earth, that leaves the craft at a velocity (relative
to Earth) of (distance x H ), would ensure that our theoretical
astronaut observer would see the CMBR as (almost) uniform in all
directions. To land on the planet that was the subject of the original
thought experiment, the astronaut would have to have the exact average
acceleration in that vector that leaves them at
(distance x H ) velocity relative to Earth when they land.
Thanks for the nudge.
JMetolius


George Dishman wrote:
"jmetolius" <jmetolius@xxxxxxxxx> wrote in message
news:1148761125.216761.251840@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
It is generally accepted that the uniformity of the cosmic background
radiation provides a ready made test of whether the observer is
actually moving with the cosmic flow of space.
In our case, the background radiation shows a slight blue shift in one
direction that is explained by our 600km/sec relative motion withn that
cosmic flow...(our Earth's motion around the Sun, the Sun's motion
around the galactic center and the entire Milky Way has velocity, in
excess of the cosmic expansion, in the direction of the constellation
Hydra).

Now, could someone please help me understand the following thought
experiment......
If we imagine a planet in a particular far away galaxy, where this
galaxy has a considerable relative recession to ourselves, and where
this relative velocity is almost entirely attributable to the cosmic
expansion ( let's say 20,000 km/sec). The inhabitants of this planet
will, of course, see themselves as almost 'at rest' relative to the
cosmic background radiation. They are almost entirely just moving with
the cosmic flow.
If we then imagine an intrepid astronaut from Earth accelerating off in
the direction of this galaxy, eventually catching it up and landing on
the planet in question. That astronaut now has a 20,000 km/sec
relative motion with their mother Earth's background radiation, and
therefore must see that background radiation as considerably blue
shifted in their direction of travel.
The astronaut may then have tea with an alien on that planet, who views
their backgrond radiation as (almost) not shfted at all. How can this
be? If the astronaut mates with the alien, how will their son see the
cosmic background shifted?

Obviously, I am missing something.....Please help. Thanks

What you are missing is that the material which produced
the CMBR is itself expanding. The radiation seen on the
distant planet was produced in a different location to
the radiation seen by the observer from Earth so it has
a different mean motion. Suppose we currently see CMBR
which was produced 13 billion years ago. In one sense
(ignoring the change of scale in between) you could say
that radiation was produced 13 billion light years away
in all directions, hence in a sphere round us. If I just
represent a line passing through Earth 'E' and your
distant planet 'P', it intersects the material that
produced the radiation we see at W and X like this:

--W----E----X--P-------

Similarly the radiation seen at the same time on P was
produced by plasma at locations Y and Z:

--W----E--Y-X--P----Z--

Now that whole line was expanding at the time the
radiation was produced so W and X were separating from
E to give the red shift we see, and Y and Z were
separating from P. If astronomers could have viewed it
when the redshift was just a factor of 2 (z=1) then the
line would be like this:

----W---------E-----Y---X-----P---------Z----

Now here's the mistake, you said:

If we then imagine an intrepid astronaut from Earth accelerating off in
the direction of this galaxy, eventually catching it up and landing on
the planet in question. That astronaut now has a 20,000 km/sec
relative motion with their mother Earth's background radiation, and
therefore must see that background radiation ...
^^^^

By the time he gets to the planet, he isn't seeing "that"
background radiation, meaning the radiation from the same
source material that he saw from Earth, he is seeing
radiation produced at Y and Z, both of which were
apparently moving from left to right when it was emitted,
and if P seems to be moving at 20000 km/s relative to
Earth, the average motion of Y and Z was also 20000 km/s
in Earth coordinates.

HTH
George

.

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