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1. WHAT IS THE STANDARD MODEL OF COSMOLOGY?

The currently best-fitting picture for describing the statistics of the Universe on large scales, the standard model of cosmology (or SMC), is often known as ΛCDM, since it's a model in which the matter is mostly cold and dark (i.e. effectively collisionless and with no electromagnetic interactions, CDM), with the bulk of the energy density of the Universe behaving like vacuum energy (i.e. like the cosmological constant of general relativity, Λ). But things are even more specific than that, with the values of only about half-a-dozen free parameters being enough to make a Universe that looks statistically just like the one we live in – and several of those parameters are now known to a really impressive level of precision. So the “SMC” is now quite precisely prescribed.

It is an astonishing achievement of modern cosmology that we have come to have such a successful model, especially when one considers that there is no a posteriori reason to expect things to be this simple. In physics we are driven to accept a model for several reasons – certainly that it fits the data, but also because of some less well defined notion of æsthetics. The simple group theoretical underpinnings of the standard model of particle physics and the elegance of the field equations of general relativity are obvious examples of this. Sure, they fit lots and lots of experimental data, but they're also really nice! But for cosmology, no one would claim that the SMC is beautiful, or even that it has to be correct because all alternatives are uglier. Certainly the SMC has some degree of simplicity (since it doesn't need many free parameters), but why do those parameters have the values that they do (see Sects. 2 and 3)? And why aren't there lots of other parameters required (see Sect. 4)? Despite the fact that nothing in the basic cosmological picture has changed since the early-to-mid-1990s (see Sect. 5), most cosmologists are expecting something else to be just around the corner. After all, surely the SMC can't be all there is?

In these notes I'd like to bring some attention to the idea that we should be skeptical [1] here, since we're dealing with very large themes. A model that purports to describe the whole of the observable Universe should be met with a decent dose of incredulity! It's important that we retain a healthy level of skepticism when discussing any such claims. But at the same time we should also remember to be skeptical about counter-claims (Sects. 6 and 7) that haven't passed the same level of scrutiny. And we should keep in mind criteria that define what skepticism is (Sect. 8), so that we can isolate the successes of the SMC, while remembering that parts of modern cosmology's lore remain quite speculative (Sect. 9).

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