Oct 24, 2026 · 16 min · 13 segments
Viewers like you help make PBS (Thank you 😃) . Support your local PBS Member Station here: https://to.pbs.org/DonateSPACE Uploaded 02-08-17 Why does the universe…
Matt O'DowdHost
Galilean relativity tells us that there is no preferred frame of reference to define absolute stillness.

And yet, there is one frame of reference that in some ways is a little more preferred, or at least special, than all of the others.

It's the frame of reference of the cosmic microwave background radiation, the CMB.

In all directions, it appears to be the same temperature, around 2.7 Kelvin, and hence, the same microwave wavelength.


This motion is due to the sun's orbit around the Milky Way and the Milky Way falling to the great attractor.

Its wavelength's a little stretched out behind and a little more compacted ahead.


In that reference frame, if you added together the peculiar velocities of all galaxies in the universe, you'd expect them to cancel out.

After all, those galaxies formed from the hot hydrogen plasma that produced the CMB.

So the peculiar motion of galaxies can be defined as motion relative to the cosmic microwave background.

The CMB can be used to define this notion of stillness, but it can also be used to measure deviations from that stillness.

On the larger scales of the cosmos, there's controversial evidence of a slight drift in one preferred direction.

Galilean relativity tells us that there is no preferred frame of reference to define absolute stillness.

And yet, there is one frame of reference that in some ways is a little more preferred, or at least special, than all of the others.

It's the frame of reference of the cosmic microwave background radiation, the CMB.

In all directions, it appears to be the same temperature, around 2.7 Kelvin, and hence, the same microwave wavelength.


This motion is due to the sun's orbit around the Milky Way and the Milky Way falling to the great attractor.

Its wavelength's a little stretched out behind and a little more compacted ahead.


In that reference frame, if you added together the peculiar velocities of all galaxies in the universe, you'd expect them to cancel out.

After all, those galaxies formed from the hot hydrogen plasma that produced the CMB.

So the peculiar motion of galaxies can be defined as motion relative to the cosmic microwave background.

The CMB can be used to define this notion of stillness, but it can also be used to measure deviations from that stillness.

On the larger scales of the cosmos, there's controversial evidence of a slight drift in one preferred direction.
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