r/cosmology • u/AutoModerator • 3d ago

Basic cosmology questions weekly thread

Ask your cosmology related questions in this thread.

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u/Hsotuhsa9 3d ago

is rotating universe true? if yes, how does it explain expansion of the uni?

another quest- what's simplest way a student can calculate the hubble const. using the data on web?

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u/mick645 3d ago

Exact solutions to Einstein’s equations exist that theoretically allow for rotating models of the universe, such as Gödel’s solution. However, observational evidence suggests otherwise: if there is any large‐scale rotation, it is extremely small. The standard cosmological model, ΛCDM (based on the FLRW metric), does not incorporate rotations. Although recent observations released by DESI indicate that the ΛCDM model may not be perfect - hinting at subtle changes in the expansion history (suggestions that the accelerated expansion rate might be evolving), but a rotating universe is unlikely to be the explanation.

Gödel’s model is interesting in its own right and technically permits time travel via closed timelike curves. A good read of that with some nice diagrams is: http://mekurt.blogspot.com/2011/03/how-godel-broke-taboo-of-time-travel.html?m=1

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u/Hsotuhsa9 2d ago

thank you! i'll read the blog, seems interesting.

for the first paragraph, can you cite sources or maybe some good lectures of yt to understand them profoundly. I've read about the DESI though very recently, I'm stunned by it's operations tbh.

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u/mick645 2d ago edited 2d ago

Of course! Firstly, Gödel’s paper can be found at:

A couple of papers that put constraints the Universe’s rotation - by assessing how isotropic the Universe is - are:

Since Bianchi models do not assume isotropy from the outset, they provide an good framework for constraining any anisotropic effects, including cosmic rotation. Hence, by comparing the predictions of these models with the rather uniform CMB data, one can place tight upper limits on how much rotation (or other anisotropic expansion) might be present.

Edit: I found a good yt video on this: