http://diposit.ub.edu/dspace/bitstream/2445/119262/1/670717.pdf

Beyond ΛCDM: Problems, solutions, and the road ahead

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, $\Lambda$CDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of $\Lambda$CDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.

Beyond $\Lambda$CDM: Problems, solutions, and the road ahead

Establishing homogeneity of the universe in the shadow of dark energy

We revisit the question of hemispherical power asymmetry in the WMAP and Planck temperature sky maps by measuring the local variance over the sky and on disks of various sizes. For the 2013 Planck sky map we find that none of the 1000 available isotropic Planck "Full Focal Plane" simulations have a larger variance asymmetry than that estimated from the data, suggesting the presence of an anisotropic signature formally significant at least at the $3.3\sigma$ level. For the WMAP 9-year data we find that 5 out of 1000 simulations have a larger asymmetry. The preferred direction for the asymmetry from the Planck data is $(l,b)=(212^\circ,-13^\circ)$, in good agreement with previous reports of the same hemispherical power asymmetry.

Power asymmetry in WMAP and Planck temperature sky maps as measured by a local variance estimator

Cosmology seeks to characterise our Universe in terms of models based on well-understood and tested physics. Today we know our Universe with a precision that once would have been unthinkable. This book develops the entire mathematical, physical and statistical framework within which this has been achieved. It tells the story of how we arrive at our profound conclusions, starting from the early twentieth century and following developments up to the latest data analysis of big astronomical datasets. It provides an enlightening description of the mathematical, physical and statistical basis for understanding and interpreting the results of key space- and ground-based data. Subjects covered include general relativity, cosmological models, the inhomogeneous Universe, physics of the cosmic background radiation, and methods and results of data analysis. Extensive online supplementary notes, exercises, teaching materials, and exercises in Python make this the perfect companion for researchers, teachers and students in physics, mathematics, and astrophysics.

Precision Cosmology: The First Half Million Years

Averaging in general relativity is a complicated operation, due to the general covariance of the theory and the nonlinearity of Einstein's equations. The latter of these ensures that smoothing spacetime over cosmological scales does not yield the same result as solving Einstein's equations with a smooth matter distribution, and that the smooth models we fit to observations need not be simply related to the actual geometry of spacetime. One specific consequence of this is a decoupling of the geometrical spatial curvature term in the metric from the dynamical spatial curvature in the Friedmann equation. Here we investigate the consequences of this decoupling by fitting to a combination of Hubble Space Telescope (HST), CMB, type Ia supernovae (SNIa), and baryon acoustic oscillation (BAO) data sets. We find that only the geometrical spatial curvature is tightly constrained and that our ability to constrain dark energy dynamics will be severely impaired until we gain a thorough understanding of the averaging problem in cosmology.

Observational constraints on the averaged universe

We study the coherent temperature and polarization patterns produced in homogeneous but anisotropic cosmological models We show results for all Bianchi types with a Friedman-Robertson-Walker limit (ie Types I, V, VII0, VIIh and IX) to illustrate the range of possible behaviour We discuss the role of spatial curvature, shear and rotation in the geodesic equations for each model and establish some basic results concerning the symmetries of the patterns produced We also give examples of the time-evolution of these patterns in terms of the Stokes parameters I, Q and U

/pdf/temperature-and-polarization-patterns-in-anisotropic-2wakfx5mzv.pdf

Temperature and Polarization Patterns in Anisotropic Cosmologies

We consider the issue of characterizing the coherent large-scale patterns from cosmic microwave background (CMB) temperature maps in globally anisotropic cosmologies. The methods we investigate are reasonably general; the particular models we test them on are the homogeneous but anisotropic relativistic cosmologies described by the Bianchi classification. Although the temperature variations produced in these models are not stochastic, they give rise to a ‘non-Gaussian’ distribution of temperature fluctuations over the sky that is a partial diagnostic of the model. We explore two methods for quantifying non-Gaussian and/or non-stationary fluctuation fields in order to see how they respond to the Bianchi models. We first investigate the behaviour of phase correlations between the spherical harmonic modes of the maps. Then we examine the behaviour of the multipole vectors of the temperature distribution which, though defined in harmonic space, can indicate the presence of a preferred direction in real space, i.e. on the 2-sphere. These methods give extremely clear signals of the presence of anisotropy when applied to the models we discuss, suggesting that they have some promise as diagnostics of the presence of global asymmetry in the Universe.

/pdf/statistical-characterization-of-cosmic-microwave-background-4hspeftdwj.pdf

Statistical characterization of cosmic microwave background temperature patterns in anisotropic cosmologies

We consider the issue of characterizing the coherent large-scale patterns from CMB temperature maps in globally anisotropic cosmologies. The methods we investigate are reasonably general, the particular models we test them on are the homogeneous but anisotropic relativistic cosmologies described by the Bianchi classification. Although the temperature variations produced in these models are not stochastic they give rise to a “non–Gaussian” distribution of temperature fluctuations over the sky that is a partial diagnostic of the model. We then explore two methods for quantifying non– Gaussian and/or non-stationary fluctuation fields in order to see how they respond to the Bianchi models. We first investigate the behaviour of phase correlations between the spherical harmonic modes of the maps. Then we examine the behaviour of the multipole vectors of the temperature distribution which, though defined in harmonic space, can indicate the presence of a preferred direction in real space, i.e. on the 2-sphere. These methods give extremely clear signals of the presence of anisotropy when applied to the models we discuss, suggesting that they have some promise as diagnostics of the presence of global asymmetry in the Universe.

/pdf/statistical-characterization-of-temperature-patterns-in-4fr8xaloc5.pdf

Statistical characterization of temperature patterns in anisotropic cosmologies

We calculate the temperature and polarization patterns generated in anisotropic cosmological models drawn from the Bianchi classification. We show that localized features in the temperature pattern, perhaps similar to the cold spot observed in the Wilkinson Microwave Anisotropy Probe (WMAP) data, can be generated in models with negative spatial curvature, i.e. Bianchi types V and VIIh. Both these models also generate coherent polarization patterns. In Bianchi VIIh, however, rotation of the polarization angle as light propagates along geodesics can convert E modes into B modes but in Bianchi V this is not necessarily the case. It is therefore possible, at least in principle, to generate localized temperature features without violating existing observational constraints on the odd-parity component of the cosmic microwave background polarization.

Rockhee Sung

Papers

Observational constraints on the averaged universe

Temperature and Polarization Patterns in Anisotropic Cosmologies

Statistical characterization of cosmic microwave background temperature patterns in anisotropic cosmologies

Statistical characterization of temperature patterns in anisotropic cosmologies

Polarized spots in anisotropic open universes