Ole Eggers Bjaelde

Neutrino dark energy—revisiting the stability issue

A coupling between a light scalar field and neutrinos has been widely discussed as a mechanism for linking (time varying) neutrino masses and the present energy density and equation of state of dark energy. However, it has been pointed out that the viability of this scenario in the non-relativistic neutrino regime is threatened by the strong growth of hydrodynamic perturbations associated with a negative adiabatic sound speed squared. In this paper we revisit the stability issue in the framework of linear perturbation theory in a model independent way. The criterion for the stability of a model is translated into a constraint on the scalar–neutrino coupling, which depends on the ratio of the energy densities in neutrinos and cold dark matter. We illustrate our results by providing meaningful examples for both stable and unstable models.

Dark energy properties from large future galaxy surveys

We perform a detailed forecast on how well a Euclid-like survey will be able to constrain dark energy and neutrino parameters from a combination of its cosmic shear power spectrum, galaxy power spectrum, and cluster mass function measurements. We find that the combination of these three probes vastly improves the surveys potential to measure the time evolution of dark energy. In terms of a dark energy figure-of-merit defined as (sigma(w_0) sigma(w_a))^-1, we find a value of 454 for Euclid-like data combined with Planck-like measurements of the cosmic microwave background (CMB) anisotropies in a fiducial LambdaCDM cosmology, a number that is quite conservative compared with existing estimates because of our choice of model parameter space and analysis method, but still represents a factor of 3 to 8 improvement over using either CMB+galaxy clustering+cosmic shear data, or CMB+cluster mass function alone. We consider also the surveys potential to measure dark energy perturbations in models wherein the dark energy is parameterised as a fluid with a nonstandard non-adiabatic sound speed, and find that in an optimistic scenario in which w_0 deviates by as much as is currently observationally allowed from -1, models with c_s^2 = 10^-6 and c_s^2 = 1 can be distinguished at more than 2sigma significance. Under the same optimistic assumptions, if the Jeans mass associated with dark energy clustering falls within the cluster mass range observed by the survey, then the order of magnitude of the dark energy sound speed can potentially be pinned down. Finally, we find that the sum of neutrino masses can be measured with a 1sigma precision of 0.01eV, even in complex cosmological models in which the dark energy equation of state varies with time. (abridged)

Spherical collapse of dark energy with an arbitrary sound speed

We consider a generic type of dark energy fluid, characterised by a constant equation of state parameter w and sound speed cs, and investigate the impact of dark energy clustering on cosmic structure formation using the spherical collapse model. Along the way, we also discuss in detail the evolution of dark energy perturbations in the linear regime. We find that the introduction of a finite sound speed into the picture necessarily induces a scale-dependence in the dark energy clustering, which in turn affects the dynamics of the spherical collapse in a scale-dependent way. As with other, more conventional fluids, we can define a Jeans scale for the dark energy clustering, and hence a Jeans mass MJ for the dark matter which feels the effect of dark energy clustering via gravitational interactions. For bound objects (halos) with masses M MJ, the effect of dark energy clustering is maximal. For those with M MJ, the dark energy component is effectively homogeneous, and its role in the formation of these structures is reduced to its effects on the Hubble expansion rate. To compute quantitatively the virial density and the linearly extrapolated threshold density, we use a quasi-linear approach which is expected to be valid up to around the Jeans mass. We find an interesting dependence of these quantities on the halo mass M, given some w and cs. The dependence is the strongest for masses lying in the vicinity of M ~ MJ. Observing this M-dependence will be a tell-tale sign that dark energy is dynamic, and a great leap towards pinning down its clustering properties.

Are cosmological neutrinos free-streaming?

Precision data from cosmology suggest neutrinos stream freely and hence interact very weakly around the epoch of recombination. We study this issue in a simple framework where neutrinos recouple instantaneously and stop streaming freely at a redshift z{sub i}. The latest cosmological data imply z{sub i} < or approx. 1500, the exact constraint depending somewhat on the assumed prior on z{sub i}. This bound can be translated into a bound on the coupling strength between neutrinos and majoronlike particles.

Origin of ΔNeff as a result of an interaction between dark radiation and dark matter

Results from the Wilkinson Microwave Anisotropy Probe (WMAP), Atacama Cosmology Telescope (ACT) and recently from the South Pole Telescope (SPT) have indicated the possible existence of an extra radiation component in addition to the well known three neutrino species predicted by the Standard Model of particle physics. In this paper, we explore the possibility of the apparent extra dark radiation being linked directly to the physics of cold dark matter (CDM). In particular, we consider a generic scenario where dark radiation, as a result of an interaction, is produced directly by a fraction of the dark matter density effectively decaying into dark radiation. At an early epoch when the dark matter density is negligible, as an obvious consequence, the density of dark radiation is also very small. As the Universe approaches matter radiation equality, the dark matter density starts to dominate thereby increasing the content of dark radiation and changing the expansion rate of the Universe. As this increase in dark radiation content happens naturally after Big Bang Nucleosynthesis (BBN), it can relax the possible tension with lower values of radiation degrees of freedom measured from light element abundances compared to that of the CMB. We numerically confront this scenario with WMAP+ACT and WMAP+SPT data and derive an upper limit on the allowed fraction of dark matter decaying into dark radiation.

Cosmology when living near the Great Attractor

If we live in the vicinity of the hypothesized Great Attractor, the age of the universe as inferred from the local expansion rate can be off by 3 per cent. We study the effect that living inside or near a massive overdensity has on cosmological parameters induced from observations of supernovae, the Hubble parameter and the cosmic microwave background. We compare the results to those for an observer in a perfectly homogeneous Λ cold dark matter universe. We find that for instance the inferred value for the global Hubble parameter changes by around three per cent if we happen to live inside a massive overdensity such as the hypothesized Great Attractor. Taking into account the effect of such structures on our perception of the universe makes cosmology perhaps less precise, but more accurate.

Dark matter decaying into a Fermi sea of neutrinos

We study the possible decay of a coherently oscillating scalar field, interpreted as dark matter, into light fermions. Specifically, we consider a scalar field with sub-eV mass decaying into a Fermi sea of neutrinos. We recognize the similarity between our scenario and inflationary preheating where a coherently oscillating scalar field decays into standard model particles. Like the case of fermionic preheating, we find that Pauli blocking controls the dark matter decay into the neutrino sea. The radius of the Fermi sphere depends on the expansion of the universe leading to a time varying equation of state of dark matter. This makes the scenario very rich and we show that the decay rate might be different at different cosmological epochs. We categorize this in two interesting regimes and then study the cosmological perturbations to find the impact on structure formation. We find that the decay may help in alleviating some of the standard problems related to cold dark matter.

Probing the spin of the central black hole in the Galactic Centre with secondary images

This paper explores the possibility of determining the spin of the supermassive black hole (SMBH) in Sgr A*, by using secondary images of stars orbiting the SMBH. The photons propagate close to the SMBH and their trajectories probe the space time in a region where the spin of the SMBH is important. We find the appearance of spikes in the secondary image, which depends on the angular momentum and spin axis of the SMBH and study the specific case of the star S2 in detail. The spikes has a magnitude of

Mass Varying Neutrinos With More Than One Species Of Neutrinos

\sim 29

Confronting the sound speed of dark energy with future cluster surveys

in the K-band and the required angular resolution is of order 15-20