Jostein R. Kristiansen

Constraining dark energy anisotropic stress

We investigate the possibility of using cosmological observations to probe and constrain an imperfect dark energy fluid. We consider a general parametrization of the dark energy component accounting for an equation of state, speed of sound and viscosity. We use present and future data from the cosmic microwave background (CMB) radiation, large-scale structures and Type Ia supernovae. We find that both the speed of sound and viscosity parameters are difficult to nail down with the present cosmological data. Also, we argue that it will be hard to improve the constraints significantly with future CMB data sets. The implication is that a perfect fluid description might ultimately turn out to be a phenomenologically sufficient description of all the observational consequences of dark energy. The fundamental lesson is, however, that even then one cannot exclude, by appealing to observational evidence alone, the possibility of imperfectness in dark energy.

Prospects for polarized foreground removal

In this report we discuss the impact of polarized foregrounds on a future CMBPol satellite mission. We review our current knowledge of Galactic polarized emission at microwave frequencies, including synchrotron and thermal dust emission. We use existing data and our understanding of the physical behavior of the sources of foreground emission to generate sky templates, and start to assess how well primordial gravitational wave signals can be separated from foreground contaminants for a CMBPol mission. At the estimated foreground minimum of ∼100 GHz, the polarized foregrounds are expected to be lower than a primordial polarization signal with tensor‐to‐scalar ratio r = 0.01, in a small patch (∼1%) of the sky known to have low Galactic emission. Over 75% of the sky we expect the foreground amplitude to exceed the primordial signal by about a factor of eight at the foreground minimum and on scales of two degrees. Only on the largest scales does the polarized foreground amplitude exceed the primordial signal by a larger factor of about 20. The prospects for detecting an r = 0.01 signal including degree‐scale measurements appear promising, with 5σ_r∼0.003 forecast from multiple methods. A mission that observes a range of scales offers better prospects from the foregrounds perspective than one targeting only the lowest few multipoles. We begin to explore how optimizing the composition of frequency channels in the focal plane can maximize our ability to perform component separation, with a range of typically 40 ≲ ν ≲ 300 GHz preferred for ten channels. Foreground cleaning methods are already in place to tackle a CMBPol mission data set, and further investigation of the optimization and detectability of the primordial signal will be useful for mission design.

Do WMAP data favor neutrino mass and a coupling between Cold Dark Matter and Dark Energy

We fit WMAP5 and related data by allowing for a CDM-DE coupling and non-zero neutrino masses, simultaneously. We find a significant correlation between these parameters, so that simultaneous higher coupling and {nu}-masses are allowed. Furthermore, models with a significant coupling and {nu}-mass are statistically favoured in respect to a cosmology with no coupling and negligible neutrino mass (our best fits are: C{approx}1/2 m{sub p}, m{sub {nu}{approx}0}.12 eV per flavor). We use a standard Monte Carlo Markov Chain approach, by assuming DE to be a scalar field self-interacting through Ratra-Peebles or SUGRA potentials.

Coupling between cold dark matter and dark energy from neutrino mass experiments

Abstract We consider cosmological models with dynamical dark energy (dDE) coupled to cold dark matter (CDM), while simultaneously allowing neutrinos to be massive. Using a MCMC approach, we compare these models with a wide range of cosmological data sets. We find a strong correlation between this coupling strength and the neutrino mass. This correlation persists when BAO data are included in the analysis. We add then priors on ν mass from particle experiments. The claimed detection of ν mass from the Heidelberg–Moscow neutrinoless double- β decay experiment would imply a 7– 8 σ detection of CDM–DE coupling. Similarly, the detection of ν mass from coming KATRIN tritium β decay experiment will imply a safe detection of a coupling in the dark sector. Previous attempts to accommodate cosmic phenomenology with such possible ν mass data made recourse to a w - 1 eoS. We compare such an option with the coupling option and find that the latter allows a drastic improvement.

Using the cluster mass function from weak lensing to constrain neutrino masses

We discuss the variation of cosmological upper bounds on M{sub {nu}}, the sum of the neutrino masses, with the choice of data sets included in the analysis, pointing out a few oddities not easily seen when all data sets are combined. For example, the effect of applying different priors varies significantly depending on whether we use the power spectrum from the 2dFGRS or SDSS galaxy survey. A conservative neutrino mass limit of M{sub {nu}}<1.43 eV (95% C.L.) is obtained by combining the WMAP 3 yr data with the cluster mass function measured by weak gravitational lensing. This limit has the virtue of not making any assumptions about the bias of luminous matter with respect to the dark matter, and is in this sense (and this sense only) bias-free.

Dynamical Dark Energy model parameters with or without massive neutrinos

We use WMAP5 and other cosmological data to constrain model parameters in quintessence cosmologies, focusing also on their shift when we allow for non-vanishing neutrino masses. The Ratra-Peebles (RP) and SUGRA potentials are used here, as examples of slowly or fastly varying state parameter w(a). Both potentials depend on an energy scale Λ. Here we confirm the results of previous analysis with WMAP3 data on the upper limits on Λ, which turn out to be rather small (down to ~ 10−9 in RP cosmologies and ~ 10−5 for SUGRA). Our constraints on Λ are not heavily affected by the inclusion of neutrino mass as a free parameter. On the contrary, when the neutrino mass degree of freedom is opened, significant shifts in the best-fit values of other parameters occur.

How fast could Usain Bolt have run? A dynamical study

Since that very memorable day at the Beijing 2008 Olympics, a big question on every sports commentator’s mind has been “What would the 100 meter dash world record have been, had Usain Bolt not celebrated at the end of his race?” Glen Mills, Bolt’s coach suggested at a recent press conference that the time could have been 9.52 seconds or better. We revisit this question by measuring Bolt’s position as a function of time using footage of the run, and then extrapolate into the last two seconds based on two different assumptions. First, we conservatively assume that Bolt could have maintained Richard Thompson’s, the runner-up, acceleration during the end of the race. Second, based on the race development prior to the celebration, we assume that he could also have kept an acceleration of 0.5 m/s higher than Thompson. In these two cases, we find that the new world record would have been 9.61± 0.04 and 9.55± 0.04 seconds, respectively, where the uncertainties denote 95% statistical errors. Subject headings: popular science — image analysis — Beijing 2008Since that memorable day at the Beijing 2008 Olympics, a big question has been, “What would the 100m dash world record have been had Usain Bolt not celebrated at the end of his race?” Bolt’s coach suggested that the time could have been 9.52s or better. We consider this question by measuring Bolt’s position as a function of time using footage of the run, and then extrapolate the last 2s with two different assumptions. First, we conservatively assume that Bolt could have maintained the runner-up’s acceleration during the end of the race. Second, based on the race development prior to the celebration, we assume that Bolt could have kept an acceleration of 0.5m∕s2 greater than the runner-up. We find that the new world record in these two cases would have been 9.61±0.04 and 9.55±0.04s, respectively, where the uncertainties denote 95% statistical errors.

Reactor sterile neutrinos, dark energy, and the age of the universe

There are indications that the neutrino oscillation data from reactor experiments and the LSND and MiniBooNE experiments show a preference for two sterile neutrino species, both with masses in the eV region. We show that this result has a significant impact on some important cosmological parameters. Specifically, we use a combination of CMB, LSS, and SN1A data and show that the existence of two light, sterile neutrinos would rule out the cosmological constant as dark energy at a 95% confidence level and lower the expansion age of the universe to 12.58 ± 0.26 Gyr.

Constraining primordial magnetic fields with CMB polarization experiments

We calculate the effect that a primordial homogeneous magnetic field, B{sub 0}, will have on the different CMB power spectra due to Faraday rotation. Concentrating on the TB, EB, and BB correlations, we forecast the ability for future CMB polarization experiments to constrain B{sub 0}. Our results depend on how well the foregrounds can be subtracted from the CMB maps, but we find a predicted error between {sigma}{sub B{sub 0}}=4x10{sup -11} G (for the QUIET experiment with foregrounds perfectly subtracted) and 3x10{sup -10} G (with the Clover experiment with no foreground subtraction). These constraints are two orders of magnitude better than the present limits on B{sub 0}.

Revised WMAP constraints on neutrino masses and other extensions of the minimal {lambda}CDM model

Recently, two issues concerning the three-year Wilkinson Microwave Anisotropy Probe (WMAP) likelihood code were pointed out. On large angular scales (l ~300), over-subtraction of unresolved point sources produced a small power deficit. For a minimal six-parameter cosmological model, these two effects conspired to decrease the value of ns by ~0.7sigma. In this paper, we study the change in preferred parameter ranges for extended cosmological models, including running of ns, massive neutrinos, curvature, and the equation of state for dark energy. We also include large-scale structure and supernova data in our analysis. We find that the parameter ranges for alphas, Omegak and w are not much altered by the modified analysis. For massive neutrinos the upper limit on the sum of the neutrino masses decreases from Mnu<1.90 eV to Mnu<1.57 eV when using the modified WMAP code and WMAP data only. We also find that the shift of ns to higher values is quite robust to these extensions of the minimal cosmological model.