Re: Cosmic Background Radiation
- From: "George Dishman" <george@xxxxxxxxxxxxxxxxx>
- Date: Sun, 28 May 2006 21:14:10 +0100
"jmetolius" <jmetolius@xxxxxxxxx> wrote in message
news:1148841195.990997.108570@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.
That's actually reasonable because there isn't a
global preferred frame of reference defined by
the CMBR in the sense of special relativity, it
is only local. If you scatter space with lots of
little probes each of which can use thrusters to
arrange its speed so that it sees no anisotropy,
the grid pattern formed by them would itself be
expanding.
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.
You have to be careful because any real motion gets
you to your destination at a later time than when
you set out so you have to account for the ongoing
expansion during the journey. What matters of course
is that your astronaut should get to the distant
planet in a reasonable time and then match his speed
to that of the planet. If the astronaut measures the
CMBR while moving past it at high speed, obviously he
will get a different value for the anisotropy.
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.
Better to have much higher acceleration, get there
fast and then "fire the retros" to stop. The key is to
match velocity. You are right about averages though
since there is a redshift observed in all directions,
the proper motion is defined as the deviation from
uniformity hence it is the dipole moment of the
deviation from the average.
Thanks for the nudge.
Glad to help
best regards
George
.
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