Only Dark Matter (And Not Modified Gravity) Can Explain The Universe
“Modified gravity cannot successfully predict the large-scale structure of the Universe the way that a Universe full of dark matter can. Period. And until it can, it’s not worth paying any mind to as a serious competitor. You cannot ignore physical cosmology in your attempts to decipher the cosmos, and the predictions of large-scale structure, the microwave background, the light elements, and the bending of starlight are some of the most basic and important predictions that come out of physical cosmology. MOND does have a big victory over dark matter: it explains the rotation curves of galaxies better than dark matter ever has, including all the way up to the present day. But it is not yet a physical theory, and it is not consistent with the full suite of observations we have at our disposal. Until that day comes, dark matter will deservedly be the leading theory of what makes up the mass in our Universe.”
You’ve heard of the big controversy: between dark matter explaining the missing mass of the Universe on one hand, and on the possibility of modifying gravity on the other. If you’re not a physical cosmologist yourself, how do you know which camp is right? Should you just go with whatever answer sounds better to you, or fits your gut instinct better? Of course not! Instead, you need to look at the full suite of data, and you need to look in the regime where the predictions are the most robust and the easiest to discern from one another. Where is that? On the largest scales, at the earliest times, and in general in the linear regime of structure formation. There are four observations that I highlight here, and remarkably, dark matter can explain all four with ease. Modified gravity? It can’t get you even two of them with the same modification, not unless you also include dark matter.