Cosmology Takes a Turn

NASA/ESA

Everything in the Universe spins. Galaxies, planets, stars, and black holes all rotate, even if just a bit. It comes from the fact that the clouds of scattered gas and dust of the cosmos are never perfectly symmetrical. But the Universe as a whole does not rotate. Some objects spin one way, some another, but add them all up, and the total rotation is zero. At least that’s what we’ve thought. But a new study suggests that the Universe does rotate, and this rotation solves the big mystery of cosmology known as the Hubble tension.¹

The idea of a rotating universe isn’t new. Even Einstein contemplated the idea. Most famously, Kurt Gödel devised a rotating model universe that is a solution to Einstein’s field equations. The Gödel metric, as it’s known, has some really strange properties. While standard cosmology metrics such as the one used in our standard model have space plus one time dimension, Gödel’s model has two time-like dimensions. Distant galaxies would have a strange rotational bias to them, and light would spontaneously polarize. Even more bizarre is that the universe would be filled with time loops. Known as closed timelike curves (CTCs), they would allow you to meet yourself in the past simply by traveling through space.

Gödel’s model is an interesting exploration of the theoretical limits of relativity, but it doesn’t resemble the real Universe in any significant way. Astronomers have looked for things like galactic spin bias and asymmetries in the cosmic microwave background that would suggest cosmic rotation, and they all support a non-rotating universe to the limits of observation.

But our observational limits have gotten better over time, and they seem to have painted us into a theoretical quandary. We know the Universe began in a hot, dense state we call the Big Bang, and since that time, about 13.8 billion years ago, the cosmos has expanded. It continues to expand at an ever-increasing rate, thanks to dark energy. But what that increasing rate is, exactly, is a bit of a mystery. Observations of fluctuations in the cosmic microwave background find that the Universe expands at about 67 km/s per Mpc. However, observations of supernovae and Cepheid variables in distant galaxies find the Universe expanding at a faster rate of 73 km/s per Mpc. According to the standard model, these values should agree. The fact that they don’t is known as the Hubble tension.

This mystery stands at the heart of modern cosmology, so it isn’t surprising that all sorts of wild solutions have been proposed. Modified gravity, quintessence, exotic supersymmetry particles—you name it. So why not reconsider the rotating universe model?

Szigeti, et al

This new model doesn’t use the Gödel metric, but instead presumes the Universe is filled with a uniform rotating fluid. Non-rotating versions of this fluid have been proposed to explain dark energy, but the addition of rotation means that spacetime is given a twist, similar to the frame-dragging effect we’ve observed for rotating masses such as Earth. As the authors show, the rotating fluid would bias our observation of cosmic expansion with increasing distance. In other words, the more distant our measure of the Hubble parameter, the smaller it would appear.

Taking the nearby observations of cosmic expansion as the most accurate, the team assumes the 73 km/s per Mpc value is correct and then asks how fast the fluid would have to rotate to match the 67 km/s per Mpc observations of the cosmic microwave background. Matching the data, they obtained a rotation rate of ω = 3.5 radians per billion years. On a basic level, cosmic rotation can solve the Hubble tension problem. Interestingly, the rotation rate the authors found is just under the limit that would create closed timelike curves. The model rotates as fast as it possibly can before all temporal logic is tossed out the window.

While it’s a fascinating result, there are plenty of caveats. To begin with, the model is a tweak model. The authors assume it’s true to prove it works. There’s nothing inherently wrong with tweak models, but tweak models are weak models, as the saying goes. Just because it can solve the problem doesn’t mean it’s the solution. Additionally, the introduction of a rotating fluid will affect things such as galactic evolution and galaxy clustering. The authors are aware of this and note that their next goal is to determine observable tests of their model. Time (and space) will tell if the model works in the end.