Ebook: New Horizons for Observational Cosmology

This volume contains the lectures and talks given at the International School of Physics “Enrico Fermi” entitled New Horizons for Observational Cosmology, which was held in the town of Varenna, by the lake Como, from July 1 to July 6, 2013.

This School came at a very unique and exciting time in cosmology. Our understanding of the universe has been revolutionized by observations of the cosmic microwave background, the large-scale structure of the universe, and distant supernovae. These studies have conclusively shown that we are living in a strange universe: 96% of the present-day energy density of the universe is dominated by the so-called dark matter and dark energy. However, we do not know what dark matter and dark energy actually are. The data also suggest that it is likely that the universe underwent a rapid accelerating expansion phase in the very early universe called the inflationary phase. However, we still do not know how inflation happened. Now, we are about to have another revolution in cosmology because, during the next couple of years, we expect to see a further qualitative jump in our knowledge of the universe. The Planck satellite collaboration, mostly funded by ESA, in particular, just published the cosmological results from its first year of survey. These results, while confirming again in a spectacular way the expectations of the standard cosmological scenario, are also hinting to the presence of some tensions or deviations from the standard scenario that need further investigations.

The future Planck data release expected around the end of 2014 will provide an essentially complete view of temperature anisotropy of the cosmic microwave background, as well as full-sky maps of polarised emission at many frequencies. These data may resolve some of the current tensions, and also provide important information on the polarised Galactic foregrounds. A host of ground-based experiments measuring polarisation of the cosmic microwave background (e.g., ACTpol, SPTpol, Polarbear, BICEP2/Keck Array) are also in the process of reporting their results soon. The next-generation galaxy surveys (BOSS, DES, HSC, HETDEX) will begin to yield data. These new data will undoubtedly address fundamental questions about the universe: what is the nature of dark energy and dark matter? What powered the Big Bang? Did inflation occur? If it did, how did it occur? What is the mass of neutrinos? When and how were the first stars and galaxies formed?

Maybe some answers to these exciting questions will come from the future work of the excellent students that attended our School.

A. Cooray, E. Komatsu, A. Melchiorri and L. Lamagna

After recombination the cosmic gas was left in a cold and neutral state. However, as the first stars and black holes formed within early galactic systems, their UV and X-ray radiation induced a gradual phase transition of the intergalactic gas into the warm and ionized state we currently observe. This process is known as cosmic reionization. Understanding how the energy deposition connected with galaxy and star formation shaped the properties of the intergalactic gas is one of the primary goals of present-day cosmology. In addition, reionization back reacts on galaxy evolution, determining many of the properties of the high-redshift galaxy population that represent the current frontier of our discovery of the cosmos. In these two lectures we provide a pedagogical overview of cosmic reionization and intergalactic medium and of some of the open questions in these fields.

Luminous tracers have been used extensively to map the large-scale matter distribution in the Universe. Similarly the dynamics of stars or galaxies can be used to estimate masses of galaxies and clusters of galaxies. However, assumptions need to be made about the dynamical state or how well galaxies trace the underlying dark matter distribution. The gravitational tidal field affects the paths of photons, leading to observable effects. This phenomenon, gravitational lensing, has become an important tool in cosmology because it probes the mass distribution directly. In these lecture notes we introduce the main relevant quantities and terminology, but the subsequent discussion is mostly limited to weak gravitational lensing, the small coherent distortion of the shapes of distant galaxies by intervening structures. We focus on some of the issues in measuring accurate shapes and review the various applications of weak gravitational lensing, as well as some recent results.

Galaxy surveys are enjoying a renaissance thanks to the advent of multi-object spectrographs on ground-based telescopes. The last 15 years have seen the fruits of this experimental advance, including the 2-degree Field Galaxy Redshift Survey (2dFGRS; Colless M. et al., arXiv:0306581 (2003)) and the Sloan Digital Sky Survey (SDSS; York D.E. et al., Astron. J., 120 (2000) 1579). Most recently, the Baryon Oscillation Spectroscopic Survey (BOSS; Dawson K.S. et al., Astron. J., 145 (2013) 10), part of the SDSS-III project Eisenstein D.J. et al., Astron. J., 142 (2011) 72, has provided the largest volume of the low-redshift Universe ever surveyed with a galaxy density useful for high-precision cosmology. This set of lecture notes looks at some of the physical processes that underpin these measurements, the evolution of measurements themselves, and looks ahead to the next 15 years and the advent of surveys such as the enhanced Baryon Oscillation Spectroscopic Survey (eBOSS), the Dark Energy Spectroscopic Instrument (DESI) and the ESA Euclid satellite mission.

Galaxy formation is at the forefront of observation and theory in cosmology. An improved understanding is essential for improving our knowledge both of the cosmological parameters, of the contents of the universe, and of our origins. In these lectures intended for graduate students, galaxy formation theory is reviewed and confronted with recent observational issues. In lecture 1, the following topics are presented: star formation considerations, including IMF, star formation efficiency and star formation rate, the origin of the galaxy luminosity function, and feedback in dwarf galaxies. In lecture 2, we describe formation of disks and massive spheroids, including the growth of supermassive black holes, negative feedback in spheroids, the AGN-star formation connection, star formation rates at high redshift and the baryon fraction in galaxies.

Cosmology has come a long way from being based on a small number of observations to being a data-driven precision science. We discuss the questions “What is observable?”, “What in the Universe is knowable?” and “What are the fundamental limits to cosmological knowledge?”. We then describe the methodology for investigation: theoretical hypotheses are used to model, predict and anticipate results; data is used to infer theory. We illustrate with concrete examples of principled analysis approaches from the study of cosmic microwave background anisotropies and surveys of large-scale structure, culminating in a summary of the highest-precision probe to date of the physical origin of cosmic structures: the Planck 2013 constraints on primordial non-Gaussianity.

Recently, the new Planck Cosmic Microwave Background (CMB) measurements have greatly improved our knowledge of the Universe. Nevertheless the dark component of the radiation content of the Universe (coined dark radiation) remains an unsolved case and its existence is strongly dependent on the external data sets included in the analysis and on the assumed cosmological model. For instance, the discrepancy between the Planck results and the Hubble Space Telescope ones in the matter of the value of the Hubble constant can be reduced at the expenses of the standard ΛCDM model which must be enlarged including extra relativistic degrees of freedom, i.e. dark radiation. This dark-radiation case is extremely important because it can represent the cosmological counterpart to the sterile neutrinos hinted at by the short-baseline experiments. Here we update the cosmological constraints on sterile neutrinos using Planck CMB data, the Hubble Space Telescope measurements and the matter power spectrum extracted from the Data Release 9 (DR9) of the CMASS sample of galaxies. Furthermore, we assume an extended cosmological scenario with a varying lensing amplitude. If the new Planck data are analyzed in the framework of this cosmological model, we find an evidence of additional massive species above the 2σ c.l.

We present new constraints on possible features in the primordial inflationary density perturbations power spectrum in the light of the recent Cosmic Microwave Background Anisotropies measurements from the Planck satellite. We found that the Planck data hints at the presence of features in two different ranges of angular scales, corresponding to multipoles 10<ℓ<60 and 150<ℓ<300, with a decrease in the best-fit χ² value with respect to the featureless “vanilla” ΛCDM model of Δχ²~9 in both cases.

A major challenge in the implementation of optical elements in the millimeter band is to minimize the radiation loss caused by reflection. The widely used method is based on the realization of an Anti-Reflection Coating (ARC), which exploits the behaviour of interference of the radiation generated by the deposition of one or more layers of polymeric material on the optical element, so as to obtain a unique surface having a refractive index conveniently selected. The major limitations that affect the ARC reside in the complexity of manufacturing, with consequent increase of costs, and possible bonding process issues after several cooling cycles; an essential feature to achieve measurements at these wavelengths. Here we report about an alternative and innovative technique based on the realization of mechanical Anti-Reflection Structures (ARS), possibly a simpler and more economic manufacture, based on the mechanical processing of the surface of a dielectric material. The antireflective behaviour is linked to the geometrical texture of the surface. The simulations of these ARS provide us with a detailed analysis of the optical properties for different geometrical realizations. The results will be compared with those of the ARC and further experimentally validated. Particular attention is put on to the analysis of the possible presence of spurious polarization effects, because this type of technology could find employment in the realization of cold optical elements for telescopes devoted to Cosmic Microwave Background (CMB) observations.

OLIMPO is a balloon-borne telescope designed to observe the sky in the mm and sub-mm bands, with an unprecedented combination of angular resolution, frequency coverage, and sensitivity. The experiment uses a 2.6 m reflective telescope with four diffraction-limited bolometric detector arrays; it is equipped with a custom attitude control system, and is designed for long-duration polar flights. OLIMPO features original spectroscopic capabilities, including a plug-in differential Fourier transform spectrometer (DFTS). This is an imaging spectrometer with very high throughput, wide spectral coverage, medium to high spectral resolution and excellent rejection of common-mode signals. The main objective of the experiment is to measure the Sunyaev-Zel'dovich (SZ) effect on selected clusters of galaxies. For this reason we explore four frequency bands centered at 145, 210, 345 and 480 GHz, matching the negative, zero and positive regions of the SZ spectrum. Simulations show that measuring the spectrum of the SZ within the four bands, in addition to making wide-band photometry in the same bands degeneracy in cluster parameters, can be avoided. The experiment is being prepared for a launch in 2014, devoted to the observation of ~50 clusters per flight, and the measurement of spatial-spectral Cosmic Microwave Background (CMB) anisotropy in a deep blind survey area of ~100 square degrees.

Recent Planck data, combined with previous CMB data and Hubble constant measurements from the HST, provide a constraint on the effective number of relativistic degrees of freedom N_eff=3.62^+0.50_−0.48 at 95% CL, allowing to place limits on models containing relativistic species at the decoupling epoch. We present the bounds on sterile neutrino models combining Planck data with galaxy clustering information, and we review as well the bounds on extended dark sectors with additional light species.

Measurements of the mass function of galaxy clusters provide tight constraints on various cosmological parameters. However, in order to fully exploit this powerful probe it is necessary to develop accurate models for the scaling relations between the total cluster mass and its observable properties. In this note I describe a semi-analytic approach to modeling the hierarchical evolution of galaxy clusters, which allows to estimate mass-observable relations and their scatter. This model is computationally efficient, and will be particularly useful in the analysis of current and upcoming cluster surveys.

We present new constraints on the rest-frame sound speed, c_eff², and the viscosity parameter, c_vis², of the Cosmic Neutrino Background from the recent measurements of the Cosmic Microwave Background anisotropies provided by the Planck satellite. While broadly consistent with the expectations of c_eff²=c_vis²=1/3 in the standard scenario, the Planck dataset hints for a higher value of the viscosity parameter, with c_vis²=0.60±0.18 at 68% c.l., and a lower value of the sound speed, with c_eff²=0.304±0.013 at 68% c.l. We find a correlation between the neutrino parameters and the lensing amplitude of the temperature power spectrum A_L. When the latter parameter is allowed to vary, we find a better consistency with the standard model with c_vis²=0.51±0.22, c_eff²=0.311±0.019 and A_L=1.08±0.18 at 68% c.l. This result indicates that the anomalous large value of A_L measured by Planck could be connected to non-standard neutrino properties.

We present here bounds on neutrino masses from the combination of recent Planck Cosmic Microwave Background measurements and galaxy clustering information from the Baryon Oscillation Spectroscopic Survey (BOSS), part of the Sloan Digital Sky Survey-III. We explore the full shape of either the photometric angular clustering (Data Release 8) and the 3D spectroscopic clustering (Data Release 9) power spectrum as well as the associated BAO signature in different cosmological scenarios.

The European Space Agency (ESA) Planck satellite has been studying microwave and submillimetre sky with unprecedented sensitivity and high angular resolution since August 2009. The High-Frequency Instrument (HFI) on Planck has observed simultaneously in six bands in the range from 100 GHz to 857 GHz. The inclusion of non-CMB bands allowed for removal of foreground sources from the data. This paper is concerned with the modelling of the specialized multi-mode feedhorns used in the highest-frequency channel centred on 857 GHz. Multi-mode systems have the advantage of increasing the throughput, and thus sensitivity, of the detection assembly when diffraction-limited resolution is not required. The horns were configured in a back-to-back setup which transmits the signal through filters to a detector horn. The modelling of the broadband beam patterns on the sky requires careful analysis of the complex propagation properties of the horns. This presentation describes the approach to modelling the highest frequency, 857 GHz, channel and discusses how the electromagnetic modelling of the horns predicts the beam patterns on the sky and the spill over at the telescope mirrors.

A modification of the action of general relativity leaves an imprint on the cosmic microwave background (CMB) anisotropies due to a different pattern for the growth of the cosmic structures below a certain length-scale. We re-examine the upper limits on the length-scale parameter B₀ of f(R) models using the recent data from the Planck satellite experiment. We also investigate the combined constraints obtained when including the Hubble Space Telescope H₀ measurement and the baryon acoustic oscillations measurements from the SDSS, WiggleZ and BOSS surveys.

Upcoming low-medium redshift observations will greatly improve our possibility to determine the nature and properties of Dark Energy. In this work we investigate the constraining power of several future surveys on parameters describing the redshift evolution of Dark Energy's equation of state. In particular we focus on Dark Energy models where the expansion is driven by a scalar field also investigating the possible coupling of this scalar field with electromagnetism, which could lead to time variation of the fine structure constant α, in order to understand how this coupling could affect Dark Energy constraints.

In this brief paper we present the first cosmological constraints on the fine-structure constant based on Planck measurements of the cosmic microwave background (CMB) temperature and lensing-potential power spectra. Planck data can fix the value of the fine-structure constant with extreme good precision at the epoch of recombination at redshift z≈1100 with respect to the previuos experiments.

We present an estimator based upon the combination of two powerful statistical tools that appears to be sensitive enough to detect tiny deviations from Gaussianity in CMB maps: the Minkowski Functionals, widely used to study non-Gaussian signals, and Neural Networks, designed to identify patterns in a data set. We test our estimator by analyzing simulated CMB maps contaminated with different amounts of local primordial non-Gaussianity, quantified by the dimensionless parameter f_NL. Applying it to sets of simulated maps we find >~98% of chance of positive detection, even for small intensity local non-Gaussianity like f_NL=38±18, the current limit set by the Planck satellite for large angular scales.

Future redshift-drift measurements will be crucial to probe the so-called “redshift desert”, thus providing a new tool for cosmological studies. In this paper we quantify the ability of a future measurement of the redshift-drift signal by a CODEX-like spectrograph to constrain a phenomenological parametrization of dynamical dark energy, specifically by obtaining constraints on w₀ and w_a. We also demonstrate that if used alongside CMB data, the redshift-drift measurements will be able to break degeneracies between expansion parameters, thus greatly improving cosmological constraints.

Modern surveys allow us to access to high-quality large-scale structure measurements. In this framework, cosmic voids appear as a new potential probe of Cosmology. We discuss the use of cosmic voids as standard spheres and their capacity to constrain new physics, dark energy and cosmological models. We introduce the Alcock-Paczynski test and illustrate its use with voids. We discuss the main difficulties in treating with cosmic voids: redshift-space distortions and sparsity of data. We present a method to reconstruct the spherical density profiles of void stacks in real space, without redshift-space distortions. We show its application to a toy model and a full dark matter simulation. Finally we present a first application of the algorithm to reconstruct real cosmic void stacks density profiles in real space from the Sloan Digital Sky Survey.

We combine measurements of Cosmic Microwave Background anisotropies, Supernovae luminosity distances and Baryonic Acoustic Oscillations to derive constraints on the dark energy equation of state w in the redshift range 0<z<2, using a principal components approach. We find no significant deviations from the expectation of a cosmological constant. However, combining those datasets with expansion rate measurements, we find a slight preference for w<−1 at low redshift, thus highlighting how these probes favour a non-constant w. Nevertheless the cosmological constant is still in agreement with these observations, while we find that some classes of alternative models may be in tension with the inferred w(z) behaviour.

We present new constraints on interacting dark energy from the recent measurements of the Cosmic Microwave Background Anisotropies from the Planck satellite experiment. We show that an interacting dark-energy model is compatible with the Planck measurements, deriving a weak bound on the dark matter-dark energy coupling parameter ξ=−0.49^+0.19_−0.31 at 68% c.l. Moreover if Planck data are fitted to an interacting dark energy scenario, the constraint on the Hubble constant is relaxed to H₀=72.1^+3.2_−2.3 km/s/Mpc, solving the tension with the Hubble Space Telescope measurement. We find that a combined Planck+HST analysis provides significant evidence for coupled dark energy finding a non-zero value for the coupling parameter ξ, with −0.90<ξ<−0.22 at 95% c.l.

The Dark Energy Survey has just started the task of mapping 5000 square degrees of the southern sky using DECam, a 570 mega-pixel camera set at the 4-meter Blanco telescope in CTIO, Chile. This will provide the collaboration with photometric information of almost 300 milion galaxies, to study dark energy using several probes. A “Science Verification” (SV) period of observations provided science-quality images for about 150 square degrees at the nominal depth of the survey. In this study we use N-body simulations (DES-MICE) of DES-SV size to combine weak gravitational lensing with galaxy clustering and extract the galaxy bias together with the cross-correlation coefficient. In particular, we use galaxy clustering to measure the bias as a function of scale and, with this in hand, make usage of galaxy-galaxy lensing to extract the cross-correlation coefficient. Additionally, we employ cosmic shear to determine the amplitude of matter fluctuations, σ₈. We achieve precisions of order 5–7% in all three parameters: σ₈, b and r, assuming them to be independent of scale. We conclude that the measurement is competitive, and therefore will be applied to DES-SV data as soon as the final data processing becomes available.