Connecting cosmology to fundamental physics

Cosmology has now a standard model: with a handful of parameters the LCDM model fits remarkably well observations of the Universe from 380000 years after the big bang to the present day (13,7 billion years after the big bang). The model's parameters are measured with exquisite precision, however there is still much we do not understand and the open questions in cosmology have deep implication for fundamental physics. In the LCDM model about 23% of the matter that makes up the Universe is non-baryonic, it has only be detected indirectly via its gravitational effects. More surprisingly 73% of the Universe is dark energy: a form of vacuum energy that manifests its effects only on scales comparable to the horizon and that became the dominant component only relatively recently in the Universe's history. The next logical step is then to test this model and look for deviations from it. The approach taken so far (and probably also in the near future) is to deviate from the base-LCDM model in one direction (i.e. one parameter) at the time. More complex deviations are also possible, but, unless there are compelling motivations for that, the resulting model would be more baroque.

For example in the LCDM model dark energy is taken to be a cosmological constant. But then one is faced with the cosmological constant problem: "why is it that small?" or, in other words, "why is that the dark energy became important only recently?" This is the "coincidence" or "why now?" problem. Alternatively dark energy could be associated to the energy of an extremely slowly rolling scalar field. Such a dynamical solution would alleviate the "why now?" problem. Dark energy, wether it is a cosmological constant or a slowly rolling scalar field is one of the (if not "the") biggest puzzle in physics today. On-going and planned large-scale structure surveys have the goal of shedding light on the nature of dark energy by measuring the expansion history and the growth of structure. More on this on the large-scale structure section .

In the standard LCDM model there are three masseless neutrinos. Neutrino oscillations however indicate that neutrinos have a small but non-zero mass. For the precision of current cosmological data the assumption of masseless neutrinos is satisfactory, however massive neutrinos do leave their imprint on the cosmological observables (through small changes in the matter-radiation equality and on structure formation via the shape of the matter power spectrum). It is expected that forthcoming large-scale structure surveys will be able to see these effects even for the smallest masses allowed by what it is currently known from neutrino oscillations. Cosmological data are key to determine the absolute neutrino mass scale offering therefore complementary information to particle physics experiments. Cosmology cal also offer a unique oportunity to detect, albeit indirectly, the cosmic neutrino background. Moreover, in principle, there may be enough signal in the sky to determine the neutrino mass hierarchy. However, when dealing with observations of cosmological large-scale structures, especially galaxy surveys, it is important to keep in mind that one is looking at highly non-linear objects where complicated astrophysical processes are at play: control of systematic errors is of paramount importance. While forecasted statistical errors in principle allow neutrino properties to be measured from cosmological surveys, it is yet to be demonstrated that systematic errors can be kept below the statistical ones: it will be an exciting and challenging task.

In the standard cosmological analyses it is also assumed that photons are conserved, that the Universe is transparent and so supernovae-inferred luminosity distance can be combined with other geometrical measurements to infer constraints on cosmological parameters. One can however not assume necessarily that the Universe is perfectly transparent: by combining distance measures that are insensitive to photon number conservation and measurements that are sensitive to it, constraints n transparency can be obtained. Even more interestingly, violation of photon number conservation could be due to Axion-photon mixing, chameleons, or the existence of mini-charged particles. Interesting constraints of such beyond the standard model physics can be obtained from present data. Forthcoming data also offer extremely promising results.

Other deviations from the standard LCDM model involve the nature of initial conditions, see that section.

For ways of testing the physics of the early Universe see the CMB section.