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Lambda CDM assumes that the initial density distribution is a Gaussian random field with a spectral index close to 1. There is only one free parameter setting t

Lambda CDM assumes that the initial density distribution is a Gaussian random field with a spectral index close to 1. There is only one free parameter setting the distribution of matter: the deviation of the spectral index from 1. The detailed distribution of matter that we view in the present-day universe is a consequence of gravity (and other known physical laws) acting on that initial, random (but following a very simple statistical distribution) density distribution.

throwawaymaths 4y ago
This is not true. There is not enough explanatory power in LCDM to explain the distributions of dark matter we apply to each specific galaxy, therefore there must be a proliferation of parameters.
- DiogenesKynikos 4y ago
  I recommend you read about Gaussian random fields in cosmology. Cosmology treats the initial conditions as random, but with a distribution that is describable by only a few parameters. You're confusing the exact realization of the random field with model parameters.
  - AnimalMuppet 4y ago
    Replying late, because I was on vacation, but just in case anyone reads this: There is a sense in which what you are saying is true. And yet, when people say "The dark matter distribution in the Bullet Cluster is <some non-trivial distribution>, they are not deriving that from the "few parameters" that describe the cosmological distribution. They're deriving it from what they see of the local situation in the Bullet Cluster. And then they say "dark matter explains what we see in the Bullet Cluster". That's what people are referring to when they say "a tunable parameter at each point in space". Without that, you have something that explains cosmology, but not the Bullet Cluster. With that, you have something that explains both cosmology and the Bullet Cluster (and a number of other odd things out there), but you no longer have "only a few parameters".

throwawaymaths 4y ago

This is not true. There is not enough explanatory power in LCDM to explain the distributions of dark matter we apply to each specific galaxy, therefore there must be a proliferation of parameters.

DiogenesKynikos 4y ago
I recommend you read about Gaussian random fields in cosmology. Cosmology treats the initial conditions as random, but with a distribution that is describable by only a few parameters. You're confusing the exact realization of the random field with model parameters.
- AnimalMuppet 4y ago
  Replying late, because I was on vacation, but just in case anyone reads this: There is a sense in which what you are saying is true. And yet, when people say "The dark matter distribution in the Bullet Cluster is <some non-trivial distribution>, they are not deriving that from the "few parameters" that describe the cosmological distribution. They're deriving it from what they see of the local situation in the Bullet Cluster. And then they say "dark matter explains what we see in the Bullet Cluster". That's what people are referring to when they say "a tunable parameter at each point in space". Without that, you have something that explains cosmology, but not the Bullet Cluster. With that, you have something that explains both cosmology and the Bullet Cluster (and a number of other odd things out there), but you no longer have "only a few parameters".