Jacob Brandbyge

Solar-like Oscillations in the G2 Subgiant β Hydri from Dual-Site Observations

We have observed oscillations in the nearby G2 subgiant starHyi using high-precision velocity observations obtained over more than a week with the HARPS and UCLES spectrographs. The oscillation frequencies show a regularcombstructure,asexpectedforsolar-likeoscillations,butwithseverall ¼ 1modesbeingstronglyaffectedby avoided crossings. These data, combined with those we obtained five years earlier, allow us to identify 28 oscillation modes.Byscalingthelarge-frequencyseparationfromtheSun,wemeasurethemeandensityofHyitoanaccuracy of 0.6%. The amplitudes of the oscillations are about 2.5 times solar and the mode lifetime is 2.3 days. A detailed comparison of the mixed l ¼ 1 modes with theoretical models should allow a precise estimate of the age of the star.

The effect of thermal neutrino motion on the non-linear cosmological matter power spectrum

We have performed detailed studies of non-linear structure formation in cosmological models with light neutrinos. For the first time the effect of neutrino thermal velocities has been included in a consistent way, and the effect on the matter power spectrum is found to be significant. The effect is large enough to be measured in future, high precision surveys. Additionally, we provide a simple but accurate analytic expression for the suppression of fluctuation power due to massive neutrinos. Finally, we describe a simple and fast method for including the effect of massive neutrinos in large-scale N-body simulations which is accurate at the 1% level for P m� . 0.15eV.

Grid based linear neutrino perturbations in cosmological N-body simulations

We present a novel, fast and precise method for including the effect of light neutrinos in cosmological N-body simulations. The effect of the neutrino component is included by using the linear theory neutrino perturbations in the calculation of the gravitational potential in the N-body simulation. By comparing this new method with the full non-linear evolution first presented in [1], where the neutrino component was treated as particles, we find that the new method calculates the matter power spectrum with an accuracy better than 1% for ?m???0.5?eV at z = 0. This error scales approximately as (?m?)2, making the new linear neutrino method extremely accurate for a total neutrino mass in the range 0.05?0.3?eV. At z = 1 the error is below 0.3% for ?m???0.5?eV and becomes negligible at higher redshifts. This new method is computationally much more efficient than representing the neutrino component by N-body particles.

Neutrinos in non-linear structure formation — the effect on halo properties

We use N-body simulations to find the effect of neutrino masses on halo properties, and investigate how the density profiles of both the neutrino and the dark matter components change as a function of the neutrino mass. We compare our neutrino density profiles with results from the N-one-body method and find good agreement. We also show and explain why the Tremaine-Gunn bound for the neutrinos is not saturated. Finally, using N-body simulations we study how the halo mass function changes as a function of the neutrino mass and compare our results with the Sheth-Tormen semi-analytic formulae. Our results are important for surveys which aim at probing cosmological parameters using clusters, as well as future experiments aiming at measuring the cosmic neutrino background directly.

Resolving cosmic neutrino structure: a hybrid neutrino N-body scheme

We present the first simulation capable of resolving the structure of neutrino clustering on Mpc scales. The method combines grid- and particle-based methods and achieves very good accuracy on both small and large scales, while keeping CPU consumption under control. Such simulations are not only ideal for calculating the non-linear matter power spectrum but also particularly relevant for studies of how neutrinos cluster in galaxy- or cluster-sized halos. We perform the largest neutrino N-body simulation to date, effectively containing 10 different neutrino hot dark matter components with different thermal properties.

Angular Signatures of Annihilating Dark Matter in the Cosmic Gamma-Ray Background

The extragalactic cosmic gamma-ray background (CGB) is an interesting channel to look for signatures of dark matter annihilation. In particular, besides the imprint in the energy spectrum, peculiar anisotropy patterns are expected compared to the case of a pure astrophysical origin of the CGB. We take into account the uncertainties in the dark matter clustering properties on subgalactic scales, deriving two possible anisotropy scenarios. A clear dark matter angular signature is achieved when the annihilation signal receives only a moderate contribution from subgalactic clumps and/or cuspy haloes. Experimentally, if galactic foreground systematics are efficiently kept under control, the angular differences are detectable with the forthcoming GLAST observatory, provided that the annihilation signal contributes to the CGB for a fraction

Cosmological

\ensuremath{\gtrsim}10\char21{}20%

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. If, instead, subgalactic structures have a more prominent role, the astrophysical and dark matter anisotropies become degenerate, correspondingly diluting the dark matter signature. As complementary observables we also introduce the cross correlation between surveys of galaxies and the CGB and the cross correlation between different energy bands of the CGB, and we find that they provide a further sensitive tool to detect the dark matter angular signatures.

-body simulations including radiation perturbations

Cosmological

The Cosmic Neutrino Background Anisotropy - Linear Theory

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