CMB dipole anisotropies- we are moving towards cmb?

• ChrisVer
In summary, the conversation revolves around the concept of the CMB rest frame and its relationship to absolute rest in the universe. The speakers discuss the idea of moving towards or away from the CMB rest frame and the implications of this movement. They also touch on the concept of absolute rest and whether it exists in the universe. Ultimately, they come to the conclusion that the CMB rest frame is a useful reference point, but it does not necessarily represent absolute rest in the universe. There is also mention of some theoretical considerations related to this topic.

ChrisVer

Gold Member
Don't kill me, but I have a problem understanding and answering a question of what it means to be moving towards/away from CMB rest frame.

The CMB is a sphere around earth, and corresponds to the last scattering surface of the Universe. If we are moving with respect to it, doesn't that mean we are moving towards the BB? Not only I wasn't able to answer this question, but it also became problematic to me...

Where do you read "moving towards/away from CMB rest frame"? We are moving in this frame, but there is no meaningful way to call this "away/towards" because all directions are the same.

mfb said:
Where do you read "moving towards/away from CMB rest frame"? We are moving in this frame, but there is no meaningful way to call this "away/towards" because all directions are the same.

I don't know... but the blueshift/redshift (higher/lower temp) should be connected with how you are moving away from/towards it...

Think of CMB rest frame as the only frame in which the CMB looks isotropic. If you'll have any movement in this frame, you'll see anisotropies. It doesn't matter which direction you go, you'll see blue-shift in the direction you're moving towards and red-shift in the opposite one. You can't be moving 'towards BB', regardless of whether it means anything, if any direction is fine.

TEFLing and ChrisVer
ChrisVer said:
Don't kill me, but I have a problem understanding and answering a question of what it means to be moving towards/away from CMB rest frame.

The CMB is a sphere around earth, and corresponds to the last scattering surface of the Universe. If we are moving with respect to it, doesn't that mean we are moving towards the BB? Not only I wasn't able to answer this question, but it also became problematic to me...
We're moving through the CMB. The CMB is a gas of photons, and we're experiencing a bit of a headwind as we travel through it.

wabbit and TEFLing
The interesting fact related to this question is the existence of the frame of absolute rest in our universe (which is in rest in CMB) in opposition to commonly presented statement, that all frames are equivalent and that there is no absolute rest and absolute movement. In every place in the Universe we can define and point out the frame of absolute rest.
However it is truth, that these frames are moving away from each other. So two guys being in absolute rest are moving (quite fast if they are far away) against each other.

fizykus said:
The interesting fact related to this question is the existence of the frame of absolute rest in our universe (which is in rest in CMB) in opposition to commonly presented statement, that all frames are equivalent and that there is no absolute rest and absolute movement. In every place in the Universe we can define and point out the frame of absolute rest.
However it is truth, that these frames are moving away from each other. So two guys being in absolute rest are moving (quite fast if they are far away) against each other.
CMB-rest frame is most emphatically not the absolute rest frame. It's a useful frame to choose in the same way as a frame in which Earth, Sun or Milky Way is at rest are. Neither of those is preferred in the relativistic sense (laws of physics are the same in all of these, light speed is measured at c etc.).

TEFLing
fizykus said:
that all frames are equivalent
In terms of physics, they are. The laws of physics are the same in all of them.

The CMB rest frame is special for the CMB in the same way the rest frame of the sun is very interesting for the solar system.

TEFLing
ChrisVer said:
Don't kill me, but I have a problem understanding and answering a question of what it means to be moving towards/away from CMB rest frame.

The CMB is a sphere around earth, and corresponds to the last scattering surface of the Universe. If we are moving with respect to it, doesn't that mean we are moving towards the BB? Not only I wasn't able to answer this question, but it also became problematic to me...
fizykus said:
The interesting fact related to this question is the existence of the frame of absolute rest in our universe (which is in rest in CMB) in opposition to commonly presented statement, that all frames are equivalent and that there is no absolute rest and absolute movement. In every place in the Universe we can define and point out the frame of absolute rest.
However it is truth, that these frames are moving away from each other. So two guys being in absolute rest are moving (quite fast if they are far away) against each other.

Answers seems so clear, and questions so simple. However if you think a little you would understand the problem and doubts of author of the question.
I can show that we have right to say that we are moving "away". Each reference frame has its origin. If we choose the reference frame which is in the rest with CMB with it origin right in the place we are in this particular moment we pose our question, we clearly are moving away from the origin of this frame with the speed of about 280? km/s. Its kind of joke obvious for everyone who answers this question, but for me most important is the possibility of defining frame of absolute rest.

The CMB rest frame is special for the CMB in the same way the rest frame of the sun is very interesting for the solar system.[/QUOTE]

I do not agree. CMB - expanding photon gas is in rest vs expanding Universe. So frame in rest in CMB is in rest in the Universe and defines absolute rest in Universe.

fizykus said:
I do not agree. CMB - expanding photon gas is in rest vs expanding Universe. So frame in rest in CMB is in rest in the Universe and defines absolute rest in Universe.
That's still not what is meant by absolute.

Granted, some theorists have considered the possibility of making the expansion rest frame into an absolute rest frame, but so far such investigations have been fruitless.

I have attached a (rough, hand-drawn) spacetime diagram that uses conformal time. O1 is an observer for whom the dipole is zero. O2 is an observer who moves spatially in O1's frame. N1 and N2 are CMB "photons" that propagate in opposite spatial directions in O1's frame. In O1's frame, observer O2 moves in the same spatial direction as N2, and in the opposite spatial direction to N1. The dashed line is to indicate that CMB photons come from all directions, i.e., a lighcone. In O1's frame, O2 picks out two special spatial directions for CMB photon's on this cone.

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