Celestial objects’ speed through space is measured relative to other things. Often that speed is based on the reference of the cosmic microwave background radiation. Cosmic microwave background radiation is photons, that are remaining from the first light that was able to travel freely through the universe. Light is traveling at C (and experiences no time). Eli5 How is that light/photon cloud a standard for a motionless frame of reference when its photons are moving at the speed of light?
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You’ll hear that the CMB has a “temperature” of about 2.7 Kelvin. Roughly what that means is the CMB photons vary in frequency, but the most common frequency is typically what you’d expect from the heat radiation emitted from an object that cold: about 160.2 GHz
If we look in different directions in space, it seems to be about 2.7 Kelvin in all directions. In all directions, the main frequency is about 160.2 GHz. But if we hopped on a rocket ship, and sped up to half the speed of light towards (say) Andromeda, then the CMB from that direction would be blue-shifted, and the CMB from the opposite directions would be red-shifted.
* In front of the ship, the CMB would have a peak frequency of 273 Ghz, which corresponds to a temperature of 4.6 Kelvin
* Behind the ship, the CMB would have a peak frequency of 91 Ghz, which corresponds to a temperature of 1.55 Kelvin.
So people on the ship would notice that the CMB isn’t uniform, but has a general trend across the sky. They could use this to calculate “we are travelling at half the speed of light, relative to the CMB”.
If we do that calculation on earth, it turns out we’re traveling at about 368 km/s relative to the CMB, about 0.1% of the speed of light.
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