Cosmic Background Radiation

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
J.Metolius

Short answer - once located on the surface of the planet, the astronaut
makes the same observation as the native inhabitants and that is there
will be a slight blueshift attributed to the general motion of that
planet relative to the background, and a redshift in the opposite
direction.

Your thought experiment fails because you are attempting to put the
motion of the astronaut during flight into the observation made by the
astronaut once on the planet. IF the astronaut could travel at such
high speeds, during that motion there would be a larger blueshift in the
CMB in the direction of motion and larger redshift in the opposite
direction than we measure here. That motion is unimportant once the
astronaut reaches the surface of the distant planet. On the surface,
measurements would yield the same type of results as here.

jmetolius wrote:
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
J.Metolius

Thank you Scott. Would you mind if I ask a few further questions? While
I'm sure you are correct, could you (or anyone else) offer a more
detailed explanation of why ' that motion is unimportant once the
astronaut reaches the surface of the planet' . I'm still missing
something here. At what point does the astronaut stop experiencing the
CMB being blue shifted in their direction of travel? I understand that,
to catch up with the receding planet, the theoretical astronaut must
accelerate to a greater speed (relative to Earth), and then eventually
to decelerate before landing, but the deceleration will still leave
them traveling at 20,000 km/sec relative to earth after they have
landed, so why wouldn't they be experiencing a blue shifted CMB?
Thanks again, JMetolius

Scott Miller wrote:
Short answer - once located on the surface of the planet, the astronaut
makes the same observation as the native inhabitants and that is there
will be a slight blueshift attributed to the general motion of that
planet relative to the background, and a redshift in the opposite
direction.

Your thought experiment fails because you are attempting to put the
motion of the astronaut during flight into the observation made by the
astronaut once on the planet. IF the astronaut could travel at such
high speeds, during that motion there would be a larger blueshift in the
CMB in the direction of motion and larger redshift in the opposite
direction than we measure here. That motion is unimportant once the
astronaut reaches the surface of the distant planet. On the surface,
measurements would yield the same type of results as here.

jmetolius wrote:
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
J.Metolius

Dear jmetolius:

"jmetolius" wrote in message
oups.com...
Thank you Scott. Would you mind if I ask a few
further questions? While I'm sure you are correct,
could you (or anyone else) offer a more detailed
explanation of why ' that motion is unimportant
once the astronaut reaches the surface of the
planet' .

Are you asking why the red/blue shift with respect to the CMB is
unimportant?

I'm still missing something here. At what point
does the astronaut stop experiencing the
CMB being blue shifted in their direction of travel?

When they are at rest with respect to it.

I understand that, to catch up with the
receding planet, the theoretical astronaut
must accelerate to a greater speed (relative
to Earth), and then eventually to decelerate
before landing, but the deceleration will still
leave them traveling at 20,000 km/sec
relative to earth after they have landed, so
why wouldn't they be experiencing a blue
shifted CMB?

Lets get a minor detail handled. The Earth is moving wrt the
CMBR (and the average population of observable objects) with a
speed of only about 300 km/sec. So "20,000" is a little high for
your proposed planet to be at rest wrt the CMBR.

They will see the CMBR at the same temperature in all directions
when they are at rest wrt to it. So even orbitting a star or a
galactic center could be a problem.

And now, why do you think it would be important whether or not
this planet is at rest?

David A. Smith

David...
Please read the initial thought experiment, then I would be pleased if
you can offer an explanation.
The 20,000km/sec is just a number. It can be any velocity you like:
make it 2,000km/sec if you like. It is just a thought experiment to get
a handle on the physics involved.


N:dlzc D:aol T:com (dlzc) wrote:
Dear jmetolius:

"jmetolius" wrote in message
oups.com...
Thank you Scott. Would you mind if I ask a few
further questions? While I'm sure you are correct,
could you (or anyone else) offer a more detailed
explanation of why ' that motion is unimportant
once the astronaut reaches the surface of the
planet' .

Are you asking why the red/blue shift with respect to the CMB is
unimportant?

I'm still missing something here. At what point
does the astronaut stop experiencing the
CMB being blue shifted in their direction of travel?

When they are at rest with respect to it.

I understand that, to catch up with the
receding planet, the theoretical astronaut
must accelerate to a greater speed (relative
to Earth), and then eventually to decelerate
before landing, but the deceleration will still
leave them traveling at 20,000 km/sec
relative to earth after they have landed, so
why wouldn't they be experiencing a blue
shifted CMB?

Lets get a minor detail handled. The Earth is moving wrt the
CMBR (and the average population of observable objects) with a
speed of only about 300 km/sec. So "20,000" is a little high for
your proposed planet to be at rest wrt the CMBR.

They will see the CMBR at the same temperature in all directions
when they are at rest wrt to it. So even orbitting a star or a
galactic center could be a problem.

And now, why do you think it would be important whether or not
this planet is at rest?

David A. Smith

Dear jmetolius:

"jmetolius" wrote in message
oups.com...
N:dlzc D:aol T:com (dlzc) wrote:
Dear jmetolius:

"jmetolius" wrote in message
oups.com...
Thank you Scott. Would you mind if I ask a few
further questions? While I'm sure you are correct,
could you (or anyone else) offer a more detailed
explanation of why ' that motion is unimportant
once the astronaut reaches the surface of the
planet' .

Are you asking why the red/blue shift with respect to the CMB
is
unimportant?

I'm still missing something here. At what point
does the astronaut stop experiencing the
CMB being blue shifted in their direction of travel?

When they are at rest with respect to it.

I understand that, to catch up with the
receding planet, the theoretical astronaut
must accelerate to a greater speed (relative
to Earth), and then eventually to decelerate
before landing, but the deceleration will still
leave them traveling at 20,000 km/sec
relative to earth after they have landed, so
why wouldn't they be experiencing a blue
shifted CMB?

Lets get a minor detail handled. The Earth is
moving wrt the CMBR (and the average
population of observable objects) with a
speed of only about 300 km/sec. So "20,000"
is a little high for your proposed planet to be
at rest wrt the CMBR.

They will see the CMBR at the same
temperature in all directions when they are
at rest wrt to it. So even orbitting a star or a
galactic center could be a problem.

And now, why do you think it would be
important whether or not this planet is at
rest?

Please read the initial thought experiment,

I read it. I read it before I replied.

then I would be pleased if
you can offer an explanation.

You didn't answer my question.

The 20,000km/sec is just a number. It
can be any velocity you like: make it
2,000km/sec if you like.

I "like" the number 300 km/sec, since that could be at rest with
the CMBR.

It is just a thought experiment to get
a handle on the physics involved.

The physics is Doppler shift. And if someone were at rest with
respect to the CMBR, they *might* age a tiny bit faster than an
Earthman. Other than that, the physics *there* is the same as we
see here.

David A. Smith

"jmetolius" wrote in message
oups.com...
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

Dear jmetolius:

"jmetolius" wrote in message
oups.com...
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 concept here 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.

No more than the CMBR seems "(almost) uniform in all directions"
now. The variance in the temperature of the CMBR is very slight
now. 300 km/sec *in one particluar direction* will make that
difference disappear. Any other choice of direction, or
different speed, doesn't. The CMBR is expanding away from every
point in every frame. It is just that some frames see the
current "expansion speed" as different values in different
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.

It isn't where the other planet is. It does matter in which
direction and what speed it is moving wrt the Earth. It doesn't
matter what the acceleration profile is, as long as you don't
kill your astronaut and yet still end up on your planet with zero
relative velocity to the planet (assuming you have infinite
fuel).

David A. Smith

"jmetolius" wrote in message
oups.com...
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

Dear George,

Yes, the DISTANCE in the term (distance x H ) velocity (away from
Earth), is the distance of the planet from Earth when the astronaut
arrives.

Any observer traveling (distance x H ) velocity (away from Earth) will
always experience a uniform CMBR.

David, I suggest you read George's contribution.

Thanks again George.

JMetolius

George Dishman wrote:
"jmetolius" wrote in message
oups.com...
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