9. BEYOND THE SMC

We'd all like to understand where the whole Universe comes from, or explain away the dark matter and dark energy. Speculation is certainly good, but believing your speculation (before it has passed any tests) is bad science! The correct approach should be to investigate the consequences of your idea and try to determine if there are definitive predictions that can be confronted with data.

I feel that there's a kind of malady that infects some cosmologists, where pretty much any outlandish and unorthodox idea is considered at the same level as the conventional picture – rather than giving it a higher degree of skepticism, like all extraordinary claims deserve. Perhaps part of the blame here is that modern physics in general, and the SMC in particular, contain some fairly bizarre-sounding concepts. We teach students about quantum mechanics and black holes, that we can build a model for the whole observable Universe, that there are hypothetical particles that dominate all matter and that a negative-pressure fluid is driving the cosmic acceleration. So perhaps students start to think that any hare-brained scheme is equally worth pursuing?

I can't shake the feeling that a good dose of skepticism would help keep things in perspective.

An example of this is inflation (see [31] and [32] for discussions). It is undoubtedly an appealing idea, and there is a great deal of circumstantial evidence to support it – so I think it's entirely reasonable to be a fan of inflationary cosmology. But since inflation is really a framework rather than a model, we can't assert that any of the observations actually prove that inflation is correct [33]. It seems reasonable to assume that whatever picture turns out to describe the early Universe, and generates the perturbations, it will contain some of the features of the current inflationary paradigm. But I don't think we can proclaim that we know that it will include all the ingredients – not until we have some more direct evidence.

However, one of the problems with assessing the merits of inflation is that there isn't a good alternative. Sure, there might be some ideas suggested as counter-proposals, but they tend to seem much more ad hoc, or create more problems than they solve, or have predictions that are less well developed. And the same issue applies more broadly across other “alternative” theories. The SMC has been developed over decades and the calculations are relatively straightforward (involving Gaussian perturbations, linear theory, well-understood physics, etc.) – but there's no reason to expect the same to be true for some unconventional new idea. So if an alternative is proposed, then it's not trivial to determine whether it can match the precision tests of the SMC. We just have to be a little patient until the calculations can be done accurately enough.

Despite the need to be open to alternatives, when there are clear predictions, it's still important to be skeptical if they just don't fit the data. As an example, we call the dominant form of energy in the Universe “dark energy”, as though its properties were mysterious and unknown – and a huge amount of effort is going into measuring its equation of state (w as a function of redshift) with increasing precision. But the reality is that all measurements so far are consistent with this component being simply vacuum energy with w = −1. I've heard people say that it's much more likely to be a model with w≠−1, since w = −1 has zero probability! But really, there's no sensible model that gives a definite prediction other than pure vacuum, and so we're left with the notion that there's just a universal constant, Λ, that gives a small (but non-zero) energy density to empty space.

Another example is dark matter. It's obvious that an alternative explanation for galaxy rotation curves might be that we can modify our theory of gravity. And there have been several suggestions along those lines (see e.g. [34]). However, the evidence for dark matter comes from a lot more than rotation curves of galaxies, e.g. the depth of cluster potential wells and measurements of gravitational lensing. But in fact the most robust evidence for dark matter comes from the CMB anisotropies – there is no model for fitting the power spectra that doesn't include a lot more CDM than baryonic matter. Here we have a choice between abandoning GR (or even Newtonian gravity) or just imagining that there's a component of matter that's not very shiny! Even without guidance from data, it seems fairly clear that the parsimonious explanation is to have a particle that's like a heavier version of the neutrino. But the skeptic should come down more heavily on the side of CDM when comparing with clustering, lensing and (particularly) CMB data.

A related issue is the evaluation of some of the small-scale puzzles associated with galaxies. It has become common to propose models ascribing these to some property of the dark matter (just strong enough to detect, without messing up the SMC predictions entirely). However, galaxy formation is a complicated business [35], involving non-linear complexity, hydrodynamics, feedback processes, etc. Since we know that we don't fully understand baryonic physics, we should be skeptical of assertions that some new property of dark matter has been discovered because of indications coming from non-linear scales.

Despite the examples given here, the SMC is in no sense a complete model, and there will surely be several additions eventually. Table VI lists some potential questions relating to physics beyond the SMC. Will one of these lead to the next breakthrough? Right now the path to progress isn't at all clear. Maybe it will turn out to be something else entirely, something unexpected and outlandish – but only if the evidence strongly supports that.