Loris P. L. Colombo

The Sky Polarization Observatory

Abstract The Sky Polarization Observatory (SPOrt) is an ASI-funded experiment specifically designed to measure the sky polarization at 22, 32 and 90 GHz, which was selected in 1997 by ESA to be flown on the International Space Station. Starting in 2006 and for at least 18 months, it will be taking direct and simultaneous measurements of the Stokes parameters Q and U at 660 sky pixels, with FWHM=7°. Due to development efforts over the past few years, the design specifications have been significantly improved with respect to the first proposal. Here we present an up-to-date description of the instrument, which now warrants a pixel sensitivity of 1.7 μK for the polarization of the cosmic background radiation, assuming two years of observations. We discuss SPOrt scientific goals in the light of WMAP results, in particular in connection with the emerging double-reionization cosmological scenario.

Dark matter and dark energy from a single scalar field and cosmic microwave background data

Axions are likely to be the dark matter (DM) that cosmological data require. They arise in the Peccei-Quinn solution of the strong-CP problem. In a previous work we showed that their model has a simple and natural generalization that also yields dark energy (DE), in fair proportions, without tuning any parameter: DM and DE arise from a single scalar field and are weakly coupled in the present era. In this paper we extend the analysis of this dual-axion cosmology and fit it to WMAP data, by using a Markov chain technique. We find that ΛCDM, dynamical DE with a SUGRA potential, DE with a SUGRA potential and a constant DE-DM coupling, and the dual-axion model with a SUGRA potential fit data with a similar accuracy. The best-fit parameters are, however, fairly different, although consistency is mostly recovered at the 2 σ level. A peculiarity of the dual-axion model with a SUGRA potential is to cause more stringent constraints on most parameters and to favor high values of the Hubble parameter.

Constraints on quintessence using recent cosmological data

Recent data, including the three-year WMAP data, the full 2dF galaxy power spectrum and the first-year data of the Supernova Legacy Survey, are used to constrain model parameters in quintessence cosmologies. In particular, we discuss the inverse power-law (RP) and SUGRA potentials and compare parameter constraints with those for ΛCDM. Both potentials fit current observations with a goodness of fit comparable or better than ΛCDM. The constraints on the energy scale ΛDE appearing in both potential expressions are however different. For RP, only energy scales around the cosmological constant limit are allowed, making the allowed models quite similar to ΛCDM. For SUGRA, ΛDE values approximately up to the electroweak energy scale are still allowed, while other parameter intervals are slightly but significantly displaced. In particular, a value of the primeval spectral index ns = 1 is still allowed at the 95% c.l., and this can have an impact on constraints on possible inflationary potentials.

Constraining the reionization history with large angle cosmic microwave background polarization

The first-year WMAP (Wilkinson Microwave Anisotropy Probe) data release showed that the reionization optical depth to cosmic microwave background (CMB) photons is greater than previously thought. This follows from unexpectedly high values of the CTEl spectrum, for l up to ~30, presumably allowing a measurement of the E-mode polarization spectrum CEl sooner than expected. This paper aims to test the capability of large-angle polarization experiments to explore the history of the cosmological reionization, considering also the impact of cosmic variance. In particular, we discuss how well the ionized fraction xe and the reionization redshift zr can be separately measured, at various levels of instrumental noise.

Cosmic opacity to CMB photons and polarization measurements

Anisotropy data analysis leaves a significant degeneracy between primeval spectral index (ns) and cosmic opacity to CMB photons (τ). Low-l polarization measures, in principle, can remove it. We perform a likelihood analysis to see how cosmic variance possibly affects such a problem. We find that, for a sufficiently low noise level (σPpix) and if τ is not negligibly low, the degeneracy is greatly reduced, while the residual impact of cosmic variance on ns and τ determinations is under control. On the contrary, if σPpix is too high, cosmic variance effects appear to be magnified. We apply general results to specific experiments and find that, if favorable conditions occur, it is possible that a 2-σ detection of a lower limit on τ is provided by the SPOrt experiment. Furthermore, if the PLANCK experiment will measure polarization with the expected precision, the error on low-l harmonics is adequate to determine τ, without significant magnification of the cosmic variance. This however indicates that high sensitivity might be more important than high resolution in τ determinations. We also outline that a determination of τ is critical to perform detailed analyses on the nature of dark energy and/or on the presence of primeval gravitational waves.

Nature of Dark Energy and Polarization Measurements

Abstract High sensitivity polarization measures, on wide angular scales, together with data on anisotropy, can be used to fix DE parameters. In this paper, first of all, we aim to determine the sensitivity needed to provide significant limits. Our analysis puts in evidence that there is a class of DE models that polarization measures can possibly exclude soon. This class includes models with DE due to a Ratra–Peebles (RP) potential. Using a likelihood analysis, we show that it is possible to distinguish RP models from ΛCDM and other dynamical DE models, already with the sensitivity of experiments like SPOrt or WMAP, thanks to their negative TE correlation at low-l, when the optical depth τ is sufficiently large. On the contrary, fixing the energy scale Λ for RP potentials or distinguishing between ΛCDM and other DE potentials requires a much lower pixel noise, that no planned polarization experiment will achieve. While reviewing this paper after the referee report, the first-year WMAP data were released. WMAP finds large positive anisotropy–polarization correlations at low l; this apparently excludes DE models with RP potentials.

Scaling Laws and Luminosity Segregation

We debate how the scaling properties of the angular correlation function w(θ) depend on luminosity segregation. Under the approximation that there is no deviation from Euclidean geometry and no evolution, we find that the scaling with catalog depth (D*) is the same both for a luminosity (L) independent clustering length (r0) and for a generic dependence of r0 on L. Recent angular data, however, extend to depths where the above approximation is unsuitable and the simple scaling w ∝ D should be modified. We find that such modifications depend on the shape of the L-dependence of r0 and are indeed different depending on whether luminosity segregation is or is not considered. In particular, we find that a luminosity segregation as observed at z = 0 causes effects of the same order as varying the rate of clustering evolution. For the sake of example, we apply our expressions to available angular galaxy data in the B- and R-bands and show that significant constraints on the evolution of clustering can already be found with public data.

The Dark Side and its Nature

Although the cosmic concordance cosmology is quite successfull in fitting data, fine tuning and coincidence problems apparently weaken it. We review several possibilities to ease its problems, by considering various kinds of dynamical Dark Energy and possibly its coupling to Dark Matter, trying to set observational limits on Dark Energy state equation and coupling.