Dark Matter and Dark Energy

Despite the generally accepted success of the Cold Dark Matter cosmology on large scales the model still inhibits a number of serious deviations from observations on galactic scales. Moreover, none of the putative dark matter particle candidates have yet been detected. Modified Newtonian dynamics (MOND) is a modification to Newton's second law of motion capable of explaining most of the observations without the need for dark matter. Another possibility to bring theory predictions into better agreement with observations is to fine-tune the mass of the dark matter particle and to "heat" it up resulting in Warm Dark Matter, respectively.

Cold Dark Matter vs. Warm Dark Matter

We show visual impression of dark matter halos in cosmological simulations with Cold Dark Matter (left) and Warm Dark Matter (right). These halos are selected at environments which could represent the Milky Way, the Andromeda nebula M31 and M33. We just recently found that the fewer subhalos in Warm Dark Matter tend to have higher orbital velocities.

Dark Energy

New developments in theoretical physics suggest that the physical vacuum has gravitational effects that can be measured with deep and wide angle galaxy surveys. We investigated the precision with which such experiments can measure the parameters of a dynamical vacuum or dark energy. Baryonic oscillations in the power spectrum of the spatial distribution of galaxies and galaxy clusters can be used as standard ruler in the deep universe. In measuring their apparent wavelength we can reconstruct the evolution history of the Universe, and we can derive the properties of the basic cosmic constituents.

Baryonic Acoustic Oscillations

At the Hobby-Eberly-telescope in Texas we plan to observe about one million galaxies for the study of Dark Energy (american-german HETDEX-project) with VIRUS - Visible Integral-field Replicable Unit Spectrograph - between redshifts z=2 and z=4. Test observations with a prototype built in Potsdam were successful. With the collaboration on BOSS (Baryon Oscillation Spectroscopic Survey) we will measure the galaxy distribution to z=0.8 and quasars between z=2 and z=2.5 over a quarter of the sky. The planned space mission eROSITA (extended Roentgen Survey with an Imaging Teleskope Array) will idendify some thousands of galaxy clusters up to z=1.5. Using very large simulation boxes of about 4 billion light years we simulate the galaxy distribution at these redshifts and derive baryonic oscillations in the model galaxy distribution. We compare the expected accuracy of dark energy surveys using Supernovae surveys, anisotropy measurements of the cosmic microwave background with the forthcoming Planck satellite, and of the baryon oscillations. The evolution parameter of the dynamical dark energy is significantly restricted if we combine the analysis of the spatial distribution along the line of sight and on the plane of the sky. According to our simulations, an effective constant equation of state of dark energy is measured with 6 % accuracy.

The baryon oscillations become significant as ratio of the measure and the smoothed power spectrum and a reconstruction of the true density field that almost reproduces the input theoretical power spectrum. Estimated confidence level for parameters of a dynamical dark energy from Supernovae measurments (the blue range), further adding expected constraints from the Planck-satellite (green), and both combined with baryonic acoustic oscillations (red).

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