It is clear that the standard model of cosmology is now well established. So well established in fact that a great deal of effort in modern cosmology is directed towards trying to find extensions to the model. For example, searching for evidence that the dark energy is evolving or that more than two parameters are needed to characterise the perturbations. Such searches for “physics beyond the standard model” makes one think of similar endeavours to find evidence to extend the Standard Model of particle physics. When this sort of thing comes up, it has been traditional for cosmologists to claim that the SMC is relatively young and still in an exploratory stage – so it's nothing like the chasing of 3 σ effects that appears to have motivated much experimental particle physics for decades. But in fact the SMC is actually quite long in the tooth itself by now!
So how old is the SMC? Certainly if one goes back a dozen years to a previous overview by this same author [12], one finds little that has changed. The model is much more precisely determined of course, but all the ingredients are already in place. Indeed, one can go back earlier, e.g. to the paper “What Have We Already Learned from the Cosmic Microwave Background?” (also known as “What Has the CMB Ever Done For Us?”) written in 1998 [13], and find that the basic picture is just the same. In fact many expositions of the history of cosmology state that the model became established with the detection of cosmic acceleration in 1998. Of course that was an important part of the story, but I think it's clear that what we now call ΛCDM was already the best-fitting model when the supernova data came in and confirmed it – to the extent that even the most skeptical cosmologists had to take Λ seriously. Many papers had already pointed out before 1998 that a collection of results pointed to a flat Λ model being the best way of extending what had previously been called “standard CDM” (or sCDM), i.e. a CDM-dominated model with Ωm = 1 and an initial power spectrum of exactly the Harrison-Zeldovich-Peebles form (n = 1). Among these results were: the need for more power on large scales (to match galaxy clustering data); the fact that most measurements of the density parameter tended to give Ω ≲ 0.3; that few measurements of H0 gave values in the ≃ 50 km s−1 Mpc−1 range that were needed to make the age of the Universe old enough for the stars within it; that the amplitude of density perturbations from the COBE satellite's CMB anisotropies pointed towards adiabatic perturbations in a Λ-dominated (rather than open) model; and that indications from smaller-scale CMB anisotropies were suggesting an acoustic scale consistent with flat geometry [14, 15].
Several of these arguments were compiled in two essays in 1995, one by Ostriker & Steinhardt [16] and the other by Krauss & Turner [17]. Not everyone was convinced of course, and some nostalgic theorists still tried to cling to the Einstein-de Sitter elegance of sCDM [18, 19]. However, the writing was on the wall, and of all the flavours to add to sCDM, it was apparent by the mid-90s that ΛCDM gave the best fit (even if you didn't necessarily like it!). Indeed it is possible to find earlier papers pointing to this model being preferred by a combination of data – and here the 1990 Nature paper by Efstathiou, Maddox & Sutherland [20] is a particular standout. That's not to say that there weren't papers proposing quite different models at the same time, but just that the currently understood SMC was already there in the early 90s, with reasons to believe that it provided the “best-buy” cosmology.
What this means is that the SMC is older than most people appreciate – something like a quarter of century old, making it more than half the age of the SM of particle physics! As an indication of just how long ago that was, in the early 90s we were using dial-up modems to connect with the internet, the main browser was (the pre-Netscape) Mosaic and the world's first text message was being sent!