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What is CMB rest?
The Cosmic Microwave Background is remarkably uniform -- the temperature of the light is the same from all directions in the sky to within about one thousandth of one percent!
That is, if you first adjust for the effects of solar system/orbital motion.
Solar system motion (including motion within the solar system of whatever instrument is mapping temperature) causes a Doppler hot-spot to appear in the direction of motion, warmer by over a hundredth of one percent -- ten times more than the variation seen otherwise. There is a cold-spot in the opposite direction. The temperature data is adjusted to get rid of the effect of this "Doppler dipole" effect of the instrument's own motion relative to the ancient light. Our maps of the CMB temperature represent the microwave sky as it would appear to an observer at rest relative to the CMB, for whom, in other words, there is no Doppler dipole due to his or her individual motion.
The idea of an observer being at CMB rest is identical to the idea of being at rest relative to the "Hubble flow," i.e., relative to the average motion of nearby galaxies. All galaxies are approximately at CMB rest, so that galaxies A and B can both be "at rest" by the CMB definition, and yet the distance between them is increasing. One way of verbally describing this is that the space between the galaxies is expanding.
The concept of CMB rest comes in handy in many ways. For example, when we say that the universe is 13.8 billion years old, we're referring to the time measured on a hypothetical clock that is in a state of CMB rest.The following forum members have contributed to this FAQ:
bcrowell
George Jones
jim mcnamara
marcus
PAllen
tiny-tim
vela
The Cosmic Microwave Background is remarkably uniform -- the temperature of the light is the same from all directions in the sky to within about one thousandth of one percent!
That is, if you first adjust for the effects of solar system/orbital motion.
Solar system motion (including motion within the solar system of whatever instrument is mapping temperature) causes a Doppler hot-spot to appear in the direction of motion, warmer by over a hundredth of one percent -- ten times more than the variation seen otherwise. There is a cold-spot in the opposite direction. The temperature data is adjusted to get rid of the effect of this "Doppler dipole" effect of the instrument's own motion relative to the ancient light. Our maps of the CMB temperature represent the microwave sky as it would appear to an observer at rest relative to the CMB, for whom, in other words, there is no Doppler dipole due to his or her individual motion.
The idea of an observer being at CMB rest is identical to the idea of being at rest relative to the "Hubble flow," i.e., relative to the average motion of nearby galaxies. All galaxies are approximately at CMB rest, so that galaxies A and B can both be "at rest" by the CMB definition, and yet the distance between them is increasing. One way of verbally describing this is that the space between the galaxies is expanding.
The concept of CMB rest comes in handy in many ways. For example, when we say that the universe is 13.8 billion years old, we're referring to the time measured on a hypothetical clock that is in a state of CMB rest.The following forum members have contributed to this FAQ:
bcrowell
George Jones
jim mcnamara
marcus
PAllen
tiny-tim
vela