4. TEST IT!

Laboratory experiments have explored the energy range up to about 100 GeV, and quantum gravity must become important around the Planck energy ~ 10¹⁹ GeV: where will new physics appear in this vast energy range? There are reasons to think that the origin of particle masses will be found at some energy 10³ GeV. If they are indeed due to some elementary scalar Higgs field, this will provide a prototype for the inflaton. If the Higgs is accompanied by supersymmetry, this may provide the cold dark matter that fills the Universe. Some circumstantial evidence for supersymmetry may be provided by the anomalous magnetic moment of the muon and by measurements of the electroweak mixing angle , in the framework of grand unified theories. These would operate at energy scales ~ 10¹⁶ GeV, far beyond the direct reach of accelerators. The first hints in favour of such theories have already been provided by experiments on astrophysical (solar and atmospheric) neutrinos, and cosmology may provide the best probes of grand unified theories, e.g., via inflation and/or super-heavy relic particles, which might be responsible for the ultra-high-energy cosmic rays. Cosmology may also be providing our first information about quantum gravity, in the form of the vacuum energy.

The LHC will extend the direct exploration of the energy frontier up to ~ 10³ GeV. However, as these examples indicate, the unparallelled energies attained in the early Universe and in some astrophysical sources may provide the most direct tests of many ideas for physics beyond the Standard Model. The continuing dialogue between the two may tell us the origins of dark matter and dark energy.