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2.3. Particle Physics

Let me briefly discuss some particle physics implications. For a more complete discussion see the recent review by Kamionkowski & Kosowsky [38].

The best-fitting models for the CMB, in conjunction with other cosmological constraints, seem to imply that there may be a positive cosmological constant, or other form of matter which behaves in a similar way. Since `extraordinary claims require extraordinary evidence' I think it is premature to say that the energy density of the vacuum has now been measured. But since OmegaLambda appeq 0.6 really is the best fit at the moment, it is worth exploring this more fully. The implications for particle physics models are obviously profound.

The CMB has little to say about the neutrino, unless its mass is high enough that free-streaming of the dark matter particles is important (which is only the case for mnu gtapprox few eV, see [39] for discussion). But the CMB certainly requires (again much more strongly when other constraints are taken into account) that most of the matter content is in some cold dark, non-baryonic form - and some new particle is the favoured candidate. Whether it is the axion, the lightest supersymmetric particle, or something else, remains to be seen.

There are many other constraints on particle physics which are beginning to be discussed. The basic idea is that the Universe couldn't have been too crazy at z ~ 1000, otherwise the microwave sky would appear very different. Already there have been limits placed on: strong primordial magnetic fields; large domains of anti-matter; large lepton asymmetry; particle decays; and early phase transitions. At the moment the limits are not too severe on things that anyone believed in the first place, but this will certainly change as the data improve.

I'm not sure how much this belongs in the particle physics section, but there are various models in which the large-scale structure of the Universe is non-trivial, either in terms of global rotation or topology. If these are too extreme they lead to detectable patterns on the CMB sky [40]. In the simplest models with strange topology, the scale has to be so close to the Hubble scale as to be hardly worth considering [41], although in models with hyperbolic geometry things are much less clear [42]. But of course, open models are not currently in vogue. In any case the conclusion is that

The large-scale structure of spacetime appears to be simple.

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