In the Dark

There’s a nice little paper – a summary of a talk – by Eric Linder on the arXiv here. The abstract is:

Reports of “cosmology in crisis” are in vogue, but as Mark Twain said, “the report of my death was an exaggeration”. We explore what we might actually mean by the standard cosmological model, how tensions – or their apparent resolutions – might arise from too narrow a view, and why looking at the big picture is so essential. This is based on the seminar “All Cosmology, All the Time”.

You can find a PDF here.

The paper discusses not only the question “what is the standard cosmological model?” in a fairly general way but also the more basic question “what is cosmology?” I’d think you’d be surprised how different would be the answers to that question from different cosmologists!

Anyway, I usually say when I give talks that the following are the six main ingredients of the standard model:

1. General Relativity
2. The Cosmological Principle
3. Cold Dark Matter
4. Cosmological Constant
5. Primordial (nearly) Gaussian adiabatic fluctuations
6. Inflation in the very early Universe

There are other ingredients of course, such as baryons and neutrinos but I don’t include them in a model because I feel one should distinguish at some level between the ingredients of a model and the  ingredients of the actual Universe. What I mean by that is that we know baryons exist (though we may not know their cosmic abundance precisely) but we don’t know for sure whether Cold Dark Matter exists.

Note that (1) isn’t really a model because it has no free parameters: it’s only when you add (2) that it comes a FLRW model with e.g. the curvature as a free parameter, and some assumed form of the energy-momentum tensor. The other ingredients have one or more free parameters, e.g. the density of CDM (3) and the value of Λ (4); these can be made more flexible by including, e.g., a dark energy term with equation of state w. Ingredient (5) needs the user to specify an initial power spectrum, which is at least two parameters (amplitude and slope), which may or may not be motivated by (6).

Anyway, the following items in the above list are – to a greater or lesser extent – open to question:

1. General Relativity
2. The Cosmological Principle
3. Cold Dark Matter
4. Cosmological Constant
5. Primordial (nearly) Gaussian adiabatic fluctuations
6. Inflation in the very early Universe

We’d be unwise to question only, e.g., 3 or 4 while ignoring the possibility that we may be wrong about the others!