Cosmology and theories of dark matter and energy

Since October 1st, 2015 Dr. Alex Vikman from LMU (Munich), recipient of Purkyně Fellowship, has joined our group. His research focuses on cosmology and the puzzles of dark matter and dark energy.

In the past two decades cosmology experienced a golden age. During these years we learned several fundamental striking facts about the universe where we live in.

First it was observed that the universe expansion is currently accelerated by some unknown substance called Dark Energy (DE). The latter approximately builds 70% of the energy budget of our universe. Observations only allow for negligibly weak interactions of this substance with light and other standard model (SM) fields. So far Dark Energy reveals itself only through gravity. Even more astonishing is that in accordance with the observations the pressure of DE is negative with the magnitude approximately equal to the DE energy density. Thus DE has the equation of state close to that of vacuum in quantum field theories (QFT). In turn, the gravitational effects of the quantum vacuum are indistinguishable from those cased by the cosmological constant (CC) introduced by Einstein at the dawn of the theory of General Relativity. General Relativity does not predict the value of CC, while our current estimations of the vacuum energy from QFT are many orders of magnitude larger than the observational results. The origin of Dark Energy and the theory behind the proper value of the cosmological constant remains one of the biggest puzzles of theoretical physics.

Further, it turned out that out of the remaining 30% of the energy budget of our universe only 5% correspond to the ordinary SM luminous matter whereas the rest is still dark and unknown. The good news here is that for cosmological purposes these 25% behave like dust. This dark dust is called Dark Matter (DM). Despite of all recent efforts, no interaction of DM with the SM fields has been detected so far. Thus, similarly to DE, the only manifestation of DM is purely gravitational. However, contrary to DE, there are tremendous amount of particle physics models beyond the SM which can be candidates to explain the DM phenomenon. The origin of DM is another fundamental problem of theoretical physics and so far the only observational fact which necessarily requires physics beyond the Standard Model.

Finally, during recent years we observed that the spatial sections of the universe have Euclidean geometry with a very high precision. We have also learned that the cosmological density perturbations have a slightly red-tilted spectrum. This means that the amplitude of these fluctuations away from homogeneity and isotropy (large scale structure of the universe) is slightly increasing towards larger scales. These two observational facts confirm main predictions of inflation and of the quantum origin of the cosmological perturbations. This is a fantastic success of the theoretical cosmology. However, the exact particle physics model behind inflation and the quantum creation of the large scale structure is still unknown. Usually such models require new fundamental physics beyond the SM.

These main problems of cosmology is an active area of current research of the cosmology group. For the most recent publications of the members of the cosmology group see here.