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comment by Devac
Devac  ·  24 days ago  ·  link  ·    ·  parent  ·  post: Cosmology Has Some Big Problems

I really like this article. It's refreshing in that it doesn't yammer on about MOND Saving the Day™ while remaining healthily sceptical and informative.

Recalling one of the multi-week problems we were assigned during the General Relativity II, you can reasonably easily extend GR to derive a model of the universe without dark matter by 'simply' assuming that the flow of time measured by an observer has to be indistinguishable from a locally flat universe. When you check for (rotational) velocity dispersion function in such a model, it will be consistent with observations, provided you consider motion with respect to the CMB. However, as a consequence, it will turn out that the age of our universe is a couple of billion years older (16 or 17 IIRC, it depends on energy-mass density), which – to my knowledge – isn't easy to reconcile with other observations.

There's not much point to those ramblings, however, problems like those make me very happy I went for condensed matter instead of cosmology. :P




mk  ·  24 days ago  ·  link  ·  

Do you remember what some of those difficult to reconcile observations were?

Devac  ·  23 days ago  ·  link  ·  

Oof, I'd have to find those notebooks to be certain, but, logically, initial inflation would have to be a lot slower and happen later in the universe's evolution to accommodate this model. Presumably, the observable universe would be smaller than what we can observe, but that doesn't need to be the case, just something that I find likely to be one.

What I do remember is that we did a cluster dynamics simulation[1] starting with standard initial conditions (matter starts randomly positioned on a sphere, mass distribution of 'chunks' is logarithmic and something about the distribution of initial velocities that I can't recall but will probably add in an edit) and the end result lacked those bubbles surrounded by supercluster filaments you get with dark matter (which, in fact, do exist).

That said, I'll try to find those notes, if only because I don't like to second-guess my memory.

[1] - Reasonably smaller scale than the ones you can do on a proper supercomputer. IIRC, we were constrained to 128 GB memory and only got allocated 50 hours on uni grid.

kleinbl00  ·  23 days ago  ·  link  ·  

Max Tegmark's Our Mathematical Universe delves into measurements of the age of the universe in a highly readable way. The problem isn't so much that the observations are "difficult to reconcile" it's that independent observational methods have gained agreement to about three orders of magnitude since 2012.