CMB dipole anisotropies- we are moving towards cmb?

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Discussion Overview

The discussion revolves around the concept of moving towards or away from the Cosmic Microwave Background (CMB) rest frame, exploring the implications of such movement in relation to the universe's expansion and the nature of absolute rest frames. Participants engage in a technical examination of the CMB's properties and its significance in cosmology, including the effects of motion on observed anisotropies.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants express confusion about the meaning of moving towards or away from the CMB rest frame, questioning if such movement implies a motion towards the Big Bang.
  • Others argue that the CMB rest frame is isotropic, and any movement within this frame results in observable anisotropies, with blue-shifts in the direction of motion and red-shifts in the opposite direction.
  • A participant suggests that the CMB can be viewed as a gas of photons, and that the Earth is moving through it, experiencing a headwind.
  • Some participants introduce the idea of an absolute rest frame defined by the CMB, contrasting it with the notion that all frames are equivalent in physics.
  • Others challenge the concept of an absolute rest frame, stating that while the CMB rest frame is useful, it does not constitute an absolute frame in the relativistic sense.
  • One participant mentions that some theorists have explored the idea of an expansion rest frame as an absolute frame, but these investigations have not yielded conclusive results.
  • A participant shares a spacetime diagram to illustrate the relationship between observers in different frames and the propagation of CMB photons.

Areas of Agreement / Disagreement

Participants express a range of views, with no consensus reached on the interpretation of movement relative to the CMB rest frame or the existence of an absolute rest frame. Disagreements persist regarding the implications of these concepts in cosmology.

Contextual Notes

Some discussions highlight the limitations of definitions and assumptions regarding rest frames and motion, as well as the complexities involved in understanding the CMB's role in the universe's structure and expansion.

ChrisVer
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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...
 
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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.
 
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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.
 
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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.).
 
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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.
 
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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.
 
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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.
 
  • #11
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.
 
  • #12
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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