The universe could be rotating
Einstein’s theory of general relativity is, so far, our only validated theory of the universe as a whole. Without it we would have difficulty explaining where the universe came from and where it is going.
One of the earliest predictions of the theory was the universe’s expansion. The universe expands everywhere all at once like the surface of a balloon being blown up. We can see it expanding by looking at distant galaxies. Based on what we know about the composition of those galaxies, we have a good idea of what wavelengths of light they should be producing. But what we actually see is that those wavelengths are extended. Thus, waves in the visible spectrum that the human eye can see are shifted towards the red.
This effect, called the Doppler effect, is familiar from the sound of a police or ambulance siren as it passes. When it is approaching, the sound is a higher pitch as the wavelengths are compressed. As it moves away the sound is lower pitched because the wavelengths are elongated.
The expansion of the universe means that it is “curved” in the time dimension because the space dimensions are all getting bigger while the time dimension is (as far as we can measure with clocks) not expanding. Therefore, when we measure how the universe’s expansion changes over time, we are measuring the universe’s time curvature.
