Martin Kunz - Geneva University, Dept. Theor. Phys. and African Inst. Math. Sci., Cape Town

Martin Kunz
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Name
Martin Kunz
Affiliation
Geneva University, Dept. Theor. Phys. and African Inst. Math. Sci., Cape Town
City
Cape Town
Country
South Africa

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Cosmology and Nongalactic Astrophysics (49)
 
General Relativity and Quantum Cosmology (32)
 
High Energy Physics - Theory (16)
 
High Energy Physics - Phenomenology (10)
 
Instrumentation and Methods for Astrophysics (3)
 
Statistics - Applications (1)
 
Physics - Data Analysis; Statistics and Probability (1)
 
Physics - Computational Physics (1)
 
Physics - Biological Physics (1)
 
Physics - Materials Science (1)

Publications Authored By Martin Kunz

We investigate cosmic string networks in the Abelian Higgs model using data from a campaign of large-scale numerical simulations on lattices of up to $4096^3$ grid points. We observe scaling or self-similarity of the networks over a wide range of scales, and estimate the asymptotic values of the mean string separation in horizon length units $\dot{\xi}$ and of the mean square string velocity $\bar v^2$ in the continuum and large time limits. The scaling occurs because the strings lose energy into classical radiation of the scalar and gauge fields of the Abelian Higgs model. Read More

We give an analytical understanding of how subsamples-based internal covariance estimators lead to biased estimates of the covariance due to underestimating the super-sample covariance (SSC). This includes jackknife and bootstrap as estimators for the full survey area, and subsampling as an estimator of the covariance of subsamples. The limitations of the jackknife covariance have been previously presented in the literature, basically because it is effectively a rescaling of the covariance of the subsample area. Read More

Modified Gravity theories generally affect the Poisson equation and the gravitational slip (effective anisotropic stress) in an observable way, that can be parameterized by two generic functions ($\eta$ and $\mu$) of time and space. We bin the time dependence of these functions in redshift and present forecasts on each bin for future surveys like Euclid. We consider both Galaxy Clustering and Weak Lensing surveys, showing the impact of the non-linear regime, treated with two different semi-analytical approximations. Read More

2016Dec
Authors: CORE Collaboration, Fabio Finelli, Martin Bucher, Ana Achúcarro, Mario Ballardini, Nicola Bartolo, Daniel Baumann, Sébastien Clesse, Josquin Errard, Will Handley, Mark Hindmarsh, Kimmo Kiiveri, Martin Kunz, Anthony Lasenby, Michele Liguori, Daniela Paoletti, Christophe Ringeval, Jussi Väliviita, Bartjan van Tent, Vincent Vennin, Rupert Allison, Frederico Arroja, Marc Ashdown, A. J. Banday, Ranajoy Banerji, James G. Bartlett, Soumen Basak, Jochem Baselmans, Paolo de Bernardis, Marco Bersanelli, Anna Bonaldi, Julian Borril, François R. Bouchet, François Boulanger, Thejs Brinckmann, Carlo Burigana, Alessandro Buzzelli, Zhen-Yi Cai, Martino Calvo, Carla Sofia Carvalho, Gabriella Castellano, Anthony Challinor, Jens Chluba, Ivan Colantoni, Martin Crook, Giuseppe D'Alessandro, Guido D'Amico, Jacques Delabrouille, Vincent Desjacques, Gianfranco De Zotti, Jose Maria Diego, Eleonora Di Valentino, Stephen Feeney, James R. Fergusson, Raul Fernandez-Cobos, Simone Ferraro, Francesco Forastieri, Silvia Galli, Juan García-Bellido, Giancarlo de Gasperis, Ricardo T. Génova-Santos, Martina Gerbino, Joaquin González-Nuevo, Sebastian Grandis, Josh Greenslade, Steffen Hagstotz, Shaul Hanany, Dhiraj K. Hazra, Carlos Hernández-Monteagudo, Carlos Hervias-Caimapo, Matthew Hills, Eric Hivon, Bin Hu, Ted Kisner, Thomas Kitching, Ely D. Kovetz, Hannu Kurki-Suonio, Luca Lamagna, Massimiliano Lattanzi, Julien Lesgourgues, Antony Lewis, Valtteri Lindholm, Joanes Lizarraga, Marcos López-Caniego, Gemma Luzzi, Bruno Maffei, Nazzareno Mandolesi, Enrique Martínez-González, Carlos J. A. P. Martins, Silvia Masi, Darragh McCarthy, Sabino Matarrese, Alessandro Melchiorri, Jean-Baptiste Melin, Diego Molinari, Alessandro Monfardini, Paolo Natoli, Mattia Negrello, Alessio Notari, Filippo Oppizzi, Alessandro Paiella, Enrico Pajer, Guillaume Patanchon, Subodh P. Patil, Michael Piat, Giampaolo Pisano, Linda Polastri, Gianluca Polenta, Agnieszka Pollo, Vivian Poulin, Miguel Quartin, Andrea Ravenni, Mathieu Remazeilles, Alessandro Renzi, Diederik Roest, Matthieu Roman, Jose Alberto Rubiño-Martin, Laura Salvati, Alexei A. Starobinsky, Andrea Tartari, Gianmassimo Tasinato, Maurizio Tomasi, Jesús Torrado, Neil Trappe, Tiziana Trombetti, Carole Tucker, Marco Tucci, Jon Urrestilla, Rien van de Weygaert, Patricio Vielva, Nicola Vittorio, Karl Young

We forecast the scientific capabilities to improve our understanding of cosmic inflation of CORE, a proposed CMB space satellite submitted in response to the ESA fifth call for a medium-size mission opportunity. The CORE satellite will map the CMB anisotropies in temperature and polarization in 19 frequency channels spanning the range 60-600 GHz. CORE will have an aggregate noise sensitivity of $1. Read More

2016Dec
Affiliations: 1U. Geneva, Dept. Theor. Phys. and ASCR, Inst. Phys., Prague, 2Lisbon U., 3U. Geneva, Dept. Theor. Phys., 4U. Heidelberg, ITP, 5U. Geneva, Dept. Theor. Phys. and African Inst. Math. Sci., Cape Town

In many generalized models of gravity, perfect fluids in cosmology give rise to gravitational slip. Simultaneously, in very broad classes of such models, the propagation of gravitational waves is altered. We investigate the extent to which there is a one-to-one relationship between these two properties in three classes of models with one extra degree of freedom: scalar (Horndeski and beyond), vector (Einstein-Aether) and tensor (bimetric). Read More

2016Nov
Authors: Eleonora Di Valentino, Thejs Brinckmann, Martina Gerbino, Vivian Poulin, François R. Bouchet, Julien Lesgourgues, Alessandro Melchiorri, Jens Chluba, Sebastien Clesse, Jacques Delabrouille, Cora Dvorkin, Francesco Forastieri, Silvia Galli, Deanna C. Hooper, Massimiliano Lattanzi, Carlos J. A. P. Martins, Laura Salvati, Giovanni Cabass, Andrea Caputo, Elena Giusarma, Eric Hivon, Paolo Natoli, Luca Pagano, Simone Paradiso, Jose Alberto Rubino-Martin, Ana Achucarro, Peter Ade, Rupert Allison, Frederico Arroja, Marc Ashdown, Mario Ballardini, A. J. Banday, Ranajoy Banerji, Nicola Bartolo, James G. Bartlett, Soumen Basak, Jochem Baselmans, Daniel Baumann, Paolo de Bernardis, Marco Bersanelli, Anna Bonaldi, Matteo Bonato, Julian Borrill, François Boulanger, Martin Bucher, Carlo Burigana, Alessandro Buzzelli, Zhen-Yi Cai, Martino Calvo, Carla Sofia Carvalho, Gabriella Castellano, Anthony Challinor, Ivan Charles, Ivan Colantoni, Alessandro Coppolecchia, Martin Crook, Giuseppe D'Alessandro, Marco De Petris, Gianfranco De Zotti, Josè Maria Diego, Josquin Errard, Stephen Feeney, Raul Fernandez-Cobos, Simone Ferraro, Fabio Finelli, Giancarlo de Gasperis, Ricardo T. Génova-Santos, Joaquin González-Nuevo, Sebastian Grandis, Josh Greenslade, Steffen Hagstotz, Shaul Hanany, Will Handley, Dhiraj K. Hazra, Carlos Hernández-Monteagudo, Carlos Hervias-Caimapo, Matthew Hills, Kimmo Kiiveri, Ted Kisner, Thomas Kitching, Martin Kunz, Hannu Kurki-Suonio, Luca Lamagna, Anthony Lasenby, Antony Lewis, Michele Liguori, Valtteri Lindholm, Marcos Lopez-Caniego, Gemma Luzzi, Bruno Maffei, Sylvain Martin, Enrique Martinez-Gonzalez, Silvia Masi, Darragh McCarthy, Jean-Baptiste Melin, Joseph J. Mohr, Diego Molinari, Alessandro Monfardini, Mattia Negrello, Alessio Notari, Alessandro Paiella, Daniela Paoletti, Guillaume Patanchon, Francesco Piacentini, Michael Piat, Giampaolo Pisano, Linda Polastri, Gianluca Polenta, Agnieszka Pollo, Miguel Quartin, Mathieu Remazeilles, Matthieu Roman, Christophe Ringeval, Andrea Tartari, Maurizio Tomasi, Denis Tramonte, Neil Trappe, Tiziana Trombetti, Carole Tucker, Jussi Väliviita, Rien van de Weygaert, Bartjan Van Tent, Vincent Vennin, Gérard Vermeulen, Patricio Vielva, Nicola Vittorio, Karl Young, Mario Zannoni, for the CORE collaboration

We forecast the main cosmological parameter constraints achievable with the CORE space mission which is dedicated to mapping the polarisation of the Cosmic Microwave Background (CMB). CORE was recently submitted in response to ESA's fifth call for medium-sized mission proposals (M5). Here we report the results from our pre-submission study of the impact of various instrumental options, in particular the telescope size and sensitivity level, and review the great, transformative potential of the mission as proposed. Read More

We re-analyse recent Cepheid data to estimate the Hubble parameter $H_0$ by using Bayesian hyper-parameters (HPs). We consider the two data sets from Riess et al 2011 and 2016 (labelled R11 and R16, with R11 containing less than half the data of R16) and include the available anchor distances (megamaser system NGC4258, detached eclipsing binary distances to LMC and M31, and MW Cepheids with parallaxes), use a weak metallicity prior and no period cut for Cepheids. We find that part of the R11 data is down-weighted by the HPs but that R16 is mostly consistent with expectations for a Gaussian distribution, meaning that there is no need to down-weight the R16 data set. Read More

We present cosmic microwave background (CMB) power spectra from recent numerical simulations of cosmic strings in the Abelian Higgs model and compare them to CMB power spectra measured by Planck. We obtain revised constraints on the cosmic string tension parameter $G\mu$. For example, in the $\Lambda$CDM model with the addition of strings and no primordial tensor perturbations, we find $G\mu < 2. Read More

The angular power spectrum of the cosmic infrared background (CIB) is a sensitive probe of the local primordial bispectrum. CIB measurements are integrated over a large volume so that the scale dependent bias from the primordial non-Gaussianity leaves a strong signal in the CIB power spectrum. Although galactic dust dominates over the non-Gaussian CIB signal, it is possible to mitigate the dust contamination with enough frequency channels, especially if high frequencies such as the Planck 857 GHz channel are available. Read More

Euclid is a European Space Agency medium class mission selected for launch in 2020 within the Cosmic Vision 2015 2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Read More

We present a new N-body code, gevolution, for the evolution of large scale structure in the Universe. Our code is based on a weak field expansion of General Relativity and calculates all six metric degrees of freedom in Poisson gauge. N-body particles are evolved by solving the geodesic equation which we write in terms of a canonical momentum such that it remains valid also for relativistic particles. Read More

We use large-scale cosmological observations to place constraints on the dark-matter pressure, sound speed and viscosity, and infer a limit on the mass of warm-dark-matter particles. Measurements of the cosmic microwave background (CMB) anisotropies constrain the equation of state and sound speed of the dark matter at last scattering at the per mille level. Since the redshifting of collisionless particles universally implies that these quantities scale like $a^{-2}$ absent shell crossing, we infer that today $w_{\rm (DM)}< 10^{-10. Read More

We demonstrate the importance of including the lensing contribution in galaxy clustering analyses with large galaxy redshift surveys. It is well known that radial cross-correlations between different redshift bins of galaxy surveys are dominated by lensing. But we show here that also neglecting lensing in the autocorrelations within one bin severely biases cosmological parameter estimation with redshift surveys. Read More

We present a comprehensive and updated comparison with cosmological observations of two non-local modifications of gravity previously introduced by our group, the so called RR and RT models. We implement the background evolution and the cosmological perturbations of the models in a modified Boltzmann code, using CLASS. We then test the non-local models against the {\em Planck} 2015 TT, TE, EE and Cosmic Microwave Background (CMB) lensing data, isotropic and anisotropic Baryonic Acoustic Oscillations (BAO) data, JLA supernovae, $H_0$ measurements and growth rate data, and we perform Bayesian parameter estimation. Read More

We report on the energy-momentum correlators obtained with recent numerical simulations of the Abelian Higgs model, essential for the computation of cosmic microwave background and matter perturbations of cosmic strings. Due to significant improvements both in raw computing power and in our parallel simulation framework, the dynamical range of the simulations has increased four-fold both in space and time, and for the first time we are able to simulate strings with a constant physical width in both the radiation and matter eras. The new simulations improve the accuracy of the measurements of the correlation functions at the horizon scale and confirm the shape around the peak. Read More

Radio interferometers suffer from the problem of missing information in their data, due to the gaps between the antennas. This results in artifacts, such as bright rings around sources, in the images obtained. Multiple deconvolution algorithms have been proposed to solve this problem and produce cleaner radio images. Read More

Numerical simulations are a versatile tool providing insight into the complicated process of structure formation in cosmology. This process is mainly governed by gravity, which is the dominant force on large scales. To date, a century after the formulation of general relativity, numerical codes for structure formation still employ Newton's law of gravitation. Read More

2015Jul
Affiliations: 1U. Geneva, Dept. Theor. Phys. and African Inst. Math. Sci., Cape Town, 2U. Geneva, Dept. Theor. Phys., 3U. Geneva, Dept. Theor. Phys.

The latest Planck results reconfirm the existence of a slight but chronic tension between the best-fit Cosmic Microwave Background (CMB) and low-redshift observables: power seems to be consistently lacking in the late universe across a range of observables (e.g.~weak lensing, cluster counts). Read More

New telescopes like the Square Kilometre Array (SKA) will push into a new sensitivity regime and expose systematics, such as direction-dependent effects, that could previously be ignored. Current methods for handling such systematics rely on alternating best estimates of instrumental calibration and models of the underlying sky, which can lead to inadequate uncertainty estimates and biased results because any correlations between parameters are ignored. These deconvolution algorithms produce a single image that is assumed to be a true representation of the sky, when in fact it is just one realization of an infinite ensemble of images compatible with the noise in the data. Read More

Continuum and HI surveys with the Square Kilometre Array (SKA) will allow us to probe some of the most fundamental assumptions of modern cosmology, including the Cosmological Principle. SKA all-sky surveys will map an enormous slice of space-time and reveal cosmology at superhorizon scales and redshifts of order unity. We illustrate the potential of these surveys and discuss the prospects to measure the cosmic radio dipole at high fidelity. Read More

In this work we discuss the possibility of cosmic defects being responsible for the B-mode signal measured by the BICEP2 collaboration. We also allow for the presence of other cosmological sources of B-modes such as inflationary gravitational waves and polarized dust foregrounds, which might contribute to or dominate the signal. On the one hand, we find that defects alone give a poor fit to the data points. Read More

We study the cosmological predictions of two recently proposed non-local modifications of General Relativity. Both models have the same number of parameters as $\Lambda$CDM, with a mass parameter $m$ replacing the cosmological constant. We implement the cosmological perturbations of the non-local models into a modification of the CLASS Boltzmann code, and we make a full comparison to CMB, BAO and supernova data. Read More

We analyse the possible contribution of topological defects to cosmic microwave anisotropies, both temperature and polarisation. We allow for the presence of both inflationary scalars and tensors, and of polarised dust foregrounds that may contribute to or dominate the B-mode polarisation signal. We confirm and quantify our previous statements that topological defects on their own are a poor fit to the B-mode signal. Read More

We present a framework for general relativistic N-body simulations in the regime of weak gravitational fields. In this approach, Einstein's equations are expanded in terms of metric perturbations about a Friedmann-Lema\^itre background, which are assumed to remain small. The metric perturbations themselves are only kept to linear order, but we keep their first spatial derivatives to second order and treat their second spatial derivatives as well as sources of stress-energy fully non-perturbatively. Read More

The large-scale homogeneity and isotropy of the universe is generally thought to imply a well defined background cosmological model. It may not. Smoothing over structure adds in an extra contribution, transferring power from small scales up to large. Read More

From the galaxy power spectrum in redshift space, we derive semi-analytical results on the generic degeneracy of galaxy clustering measurements. Defining the observables $\bar{A}= Gb\sigma_8$ and $\bar{R} = Gf\sigma_8$, (being $G$ the growth function, $b$ the bias, $f$ the growth rate, and $\sigma_8$ the amplitude of the power spectrum), we perform a Fisher matrix formalism to forecast the expected precision of these quantities for a Euclid-like survey. Among the results we found that galaxy surveys have generically a slightly negative correlation between $\bar{A}$ and $\bar{R}$, and they can always measure $\bar{R}$ about 3. Read More

2014Jun
Affiliations: 1Nottingham U., 2Geneva U., Dept. Theor. Phys. and African Inst. Math. Sci., Cape Town, 3U. Heidelberg, ITP, 4Geneva U., Dept. Theor. Phys. and African Inst. Math. Sci., Cape Town

We make precise the heretofore ambiguous statement that anisotropic stress is a sign of a modification of gravity. We show that in cosmological solutions of very general classes of models extending gravity --- all scalar-tensor theories (Horndeski), Einstein-Aether models and bimetric massive gravity --- a direct correspondence exists between perfect fluids apparently carrying anisotropic stress and a modification in the propagation of gravitational waves. Since the anisotropic stress can be measured in a model-independent manner, a comparison of the behavior of gravitational waves from cosmological sources with large-scale-structure formation could in principle lead to new constraints on the theory of gravity. Read More

We show that self-ordering scalar fields (SOSF), i.e. non-topological cosmic defects arising after a global phase transition, cannot explain the B-mode signal recently announced by BICEP2. Read More

We study the cosmological consequences of a recently proposed nonlocal modification of general relativity, obtained by adding a term $m^2R\,\Box^{-2}R$ to the Einstein-Hilbert action. The model has the same number of parameters as $\Lambda$CDM, with $m$ replacing $\Omega_{\Lambda}$, and is very predictive. At the background level, after fixing $m$ so as to reproduce the observed value of $\Omega_M$, we get a pure prediction for the equation of state of dark energy as a function of redshift, $w_{\rm DE}(z)$, with $w_{\rm DE}(0)$ in the range $[-1. Read More

We show that the B-mode polarization signal detected at low multipoles by BICEP2 cannot be entirely due to topological defects. This would be incompatible with the high-multipole B-mode polarization data and also with existing temperature anisotropy data. Adding cosmic strings to a model with tensors, we find that B-modes on their own provide a comparable limit on the defects to that already coming from Planck satellite temperature data. Read More

We study a dark energy model with non-zero anisotropic stress, either linked to the dark energy density or to the dark matter density. We compute approximate solutions that allow to characterise the behaviour of the dark energy model and to assess the stability of the perturbations. We also determine the current limits on such an anisotropic stress from the cosmic microwave background data by the Planck satellite, and derive the corresponding constraints on the modified growth parameters like the growth index, the effective Newton's constant and the gravitational slip. Read More

Distance measurements are usually thought to probe the background metric of the universe, but in reality the presence of perturbations will lead to deviations from the result expected in an exactly homogeneous and isotropic universe. At least in principle the presence of perturbations could even explain the observed distance-redshift relation without the need for dark energy. In this paper we re-investigate a toy model where perturbations are plane symmetric, and for which exact solutions are known in the fluid limit. Read More

The effective anisotropic stress or gravitational slip $\eta=-\Phi/\Psi$ is a key variable in the characterisation of the physical origin of the dark energy, as it allows to test for a non-minimal coupling of the dark sector to gravity in the Jordan frame. It is however important to use a fully model-independent approach when measuring $\eta$ to avoid introducing a theoretical bias into the results. In this paper we forecast the precision with which future large surveys can determine $\eta$ in a way that only relies on directly observable quantities. Read More

We determine the full C_l spectra and correlation functions of the temperature and polarization anisotropies in the CMB, generated by a source modeled by the large N limit of spontaneously broken global O(N)-theories. We point out a problem in the standard approach of treating the radiation-matter transition by interpolating the eigenvectors of the unequal-time correlators of the source energy-momentum tensor. This affects the CMB predictions from all type of cosmic defects. Read More

2013Oct
Authors: PRISM Collaboration, Philippe André, Carlo Baccigalupi, Anthony Banday, Domingos Barbosa, Belen Barreiro, James Bartlett, Nicola Bartolo, Elia Battistelli, Richard Battye, George Bendo, Alain Benoît, Jean-Philippe Bernard, Marco Bersanelli, Matthieu Béthermin, Pawel Bielewicz, Anna Bonaldi, François Bouchet, François Boulanger, Jan Brand, Martin Bucher, Carlo Burigana, Zhen-Yi Cai, Philippe Camus, Francisco Casas, Viviana Casasola, Guillaume Castex, Anthony Challinor, Jens Chluba, Gayoung Chon, Sergio Colafrancesco, Barbara Comis, Francesco Cuttaia, Giuseppe D'Alessandro, Antonio Da Silva, Richard Davis, Miguel de Avillez, Paolo de Bernardis, Marco de Petris, Adriano de Rosa, Gianfranco de Zotti, Jacques Delabrouille, François-Xavier Désert, Clive Dickinson, Jose Maria Diego, Joanna Dunkley, Torsten Enßlin, Josquin Errard, Edith Falgarone, Pedro Ferreira, Katia Ferrière, Fabio Finelli, Andrew Fletcher, Pablo Fosalba, Gary Fuller, Silvia Galli, Ken Ganga, Juan García-Bellido, Adnan Ghribi, Martin Giard, Yannick Giraud-Héraud, Joaquin Gonzalez-Nuevo, Keith Grainge, Alessandro Gruppuso, Alex Hall, Jean-Christophe Hamilton, Marijke Haverkorn, Carlos Hernandez-Monte\-agudo, Diego Herranz, Mark Jackson, Andrew Jaffe, Rishi Khatri, Martin Kunz, Luca Lamagna, Massimiliano Lattanzi, Paddy Leahy, Julien Lesgourgues, Michele Liguori, Elisabetta Liuzzo, Marcos Lopez-Caniego, Juan Macias-Perez, Bruno Maffei, Davide Maino, Anna Mangilli, Enrique Martinez-Gonzalez, Carlos Martins, Silvia Masi, Marcella Massardi, Sabino Matarrese, Alessandro Melchiorri, Jean-Baptiste Melin, Aniello Mennella, Arturo Mignano, Marc-Antoine Miville-Deschênes, Alessandro Monfardini, Anthony Murphy, Pavel Naselsky, Federico Nati, Paolo Natoli, Mattia Negrello, Fabio Noviello, Créidhe O'Sullivan, Francesco Paci, Luca Pagano, Rosita Paladino, Nathalie Palanque-Delabrouille, Daniela Paoletti, Hiranya Peiris, Francesca Perrotta, Francesco Piacentini, Michel Piat, Lucio Piccirillo, Giampaolo Pisano, Gianluca Polenta, Agnieszka Pollo, Nicolas Ponthieu, Mathieu Remazeilles, Sara Ricciardi, Matthieu Roman, Cyrille Rosset, Jose-Alberto Rubino-Martin, Maria Salatino, Alessandro Schillaci, Paul Shellard, Joseph Silk, Alexei Starobinsky, Radek Stompor, Rashid Sunyaev, Andrea Tartari, Luca Terenzi, Luigi Toffolatti, Maurizio Tomasi, Neil Trappe, Matthieu Tristram, Tiziana Trombetti, Marco Tucci, Rien Van de Weijgaert, Bartjan Van Tent, Licia Verde, Patricio Vielva, Ben Wandelt, Robert Watson, Stafford Withington

PRISM (Polarized Radiation Imaging and Spectroscopy Mission) was proposed to ESA in May 2013 as a large-class mission for investigating within the framework of the ESA Cosmic Vision program a set of important scientific questions that require high resolution, high sensitivity, full-sky observations of the sky emission at wavelengths ranging from millimeter-wave to the far-infrared. PRISM's main objective is to explore the distant universe, probing cosmic history from very early times until now as well as the structures, distribution of matter, and velocity flows throughout our Hubble volume. PRISM will survey the full sky in a large number of frequency bands in both intensity and polarization and will measure the absolute spectrum of sky emission more than three orders of magnitude better than COBE FIRAS. Read More

We present the spectroscopy from 5254 galaxies that hosted supernovae (SNe) or other transient events in the Sloan Digital Sky Survey II (SDSS-II). Obtained during SDSS-I, SDSS-II, and the Baryon Oscillation Spectroscopic Survey (BOSS), this sample represents the largest systematic, unbiased, magnitude limited spectroscopic survey of supernova (SN) host galaxies. Using the host galaxy redshifts, we test the impact of photometric SN classification based on SDSS imaging data with and without using spectroscopic redshifts of the host galaxies. Read More

We develop a formalism for General Relativistic N-body simulations in the weak field regime, suitable for cosmological applications. The problem is kept tractable by retaining the metric perturbations to first order, the first derivatives to second order and second derivatives to all orders, thus taking into account the most important nonlinear effects of Einstein gravity. It is also expected that any significant "backreaction" should appear at this order. Read More

Recent work has demonstrated that it is important to constrain the dynamics of cosmological perturbations, in addition to the evolution of the background, if we want to distinguish among different models of the dark sector. Especially the anisotropic stress of the (possibly effective) dark energy fluid has been shown to be an important discriminator between modified gravity and dark energy models. In this paper we use approximate analytical solutions of the perturbation equations in the presence of viscosity to study how the anisotropic stress affects the weak lensing and galaxy power spectrum. Read More

2013Apr
Affiliations: 1U. Campinas and U. Heidelberg, ITP, 2U. Heidelberg, ITP, 3Nottingham U., 4U. Heidelberg, ITP, 5U. Geneva and African Inst. Math. Sci., Cape Town

We consider the consequences of having no prior knowledge of the true dark energy model for the interpretation of cosmological observations. The magnitude of redshift-space distortions and weak-lensing shear is determined by the metric on the geodesics of which galaxies and light propagate. We show that, given precise enough observations, we can use these data to completely reconstruct the metric on our past lightcone and therefore to measure the scale- and time-dependence of the anisotropic stress and the evolution of the gravitational potentials in a model-independent manner. Read More

For future surveys, spectroscopic follow-up for all supernovae will be extremely difficult. However, one can use light curve fitters, to obtain the probability that an object is a Type Ia. One may consider applying a probability cut to the data, but we show that the resulting non-Ia contamination can lead to biases in the estimation of cosmological parameters. Read More

We argue that there is an intrinsic noise on measurements of the equation of state parameter $w=p/\rho$ from large-scale structure around us. The presence of the large-scale structure leads to an ambiguity in the definition of the background universe and thus there is a maximal precision with which we can determine the equation of state of dark energy. To study the uncertainty due to local structure, we model density perturbations stemming from a standard inflationary power spectrum by means of the exact Lema\^{i}tre-Tolman-Bondi solution of Einstein's equation, and show that the usual distribution of matter inhomogeneities in a $\Lambda$CDM cosmology causes a variation of $w$ -- as inferred from distance measures -- of several percent. Read More

Nacre is a layered, iridescent lining found inside many mollusk shells, with a unique brick-and-mortar periodic structure at the sub-micron scale, and remarkable resistance to fracture. Despite extensive studies, it remains unclear how nacre forms. Here we present 20-nm, 2{\deg}-resolution Polarization-dependent Imaging Contrast (PIC) images of shells from 15 mollusk shell species, mapping nacre tablets and their orientation patterns, showing where new crystal orientations appear and how they propagate across organic sheets as nacre grows. Read More

We present the cosmological analysis of 752 photometrically-classified Type Ia Supernovae (SNe Ia) obtained from the full Sloan Digital Sky Survey II (SDSS-II) Supernova (SN) Survey, supplemented with host-galaxy spectroscopy from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). Our photometric-classification method is based on the SN typing technique of Sako et al. (2011), aided by host galaxy redshifts (0. Read More

We introduce Bayesian Estimation Applied to Multiple Species (BEAMS), an algorithm designed to deal with parameter estimation when using contaminated data. We present the algorithm and demonstrate how it works with the help of a Gaussian simulation. We then apply it to supernova data from the Sloan Digital Sky Survey (SDSS), showing how the resulting confidence contours of the cosmological parameters shrink significantly. Read More

2012Oct
Affiliations: 1ITP, U. Heidelberg, 2Geneva U., 3ITP, U. Heidelberg and Campinas State U., 4Nottingham U., 5ITP, U. Heidelberg

The aim of this paper is to answer the following two questions: (1) Given cosmological observations of the expansion history and linear perturbations in a range of redshifts and scales as precise as is required, which of the properties of dark energy could actually be reconstructed without imposing any parameterization? (2) Are these observables sufficient to rule out not just a particular dark energy model, but the entire general class of viable models comprising a single scalar field? This paper bears both good and bad news. On one hand, we find that the goal of reconstructing dark energy models is fundamentally limited by the unobservability of the present values of the matter density Omega_m0, the perturbation normalization sigma_8 as well as the present matter power spectrum. On the other, we find that, under certain conditions, cosmological observations can nonetheless rule out the entire class of the most general single scalar-field models, i. Read More

2012Aug

We present a new prescription for analysing cosmological perturbations in a more-general class of scalar-field dark-energy models where the energy-momentum tensor has an imperfect-fluid form. This class includes Brans-Dicke models, f(R) gravity, theories with kinetic gravity braiding and generalised galileons. We employ the intuitive language of fluids, allowing us to explicitly maintain a dependence on physical and potentially measurable properties. Read More

Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Read More

New supernova surveys such as the Dark Energy Survey, Pan-STARRS and the LSST will produce an unprecedented number of photometric supernova candidates, most with no spectroscopic data. Avoiding biases in cosmological parameters due to the resulting inevitable contamination from non-Ia supernovae can be achieved with the BEAMS formalism, allowing for fully photometric supernova cosmology studies. Here we extend BEAMS to deal with the case in which the supernovae are correlated by systematic uncertainties. Read More

In this mini-review we discuss first why we should investigate cosmological models beyond LCDM. We then show how to describe dark energy or modified gravity models in a fluid language with the help of one background and two perturbation quantities. We review a range of dark energy models and study how they fit into the phenomenological framework, including generalizations like phantom crossing, sound speeds different from c and non-zero anisotropic stress, and how these effective quantities are linked to the underlying physical models. Read More

We propose a fully nonlinear framework to construct consistency relations for testing generic cosmological scenarios using the evolution of large scale structure. It is based on the covariant approach in combination with a frame that is purely given by the metric, the normal frame. As an example, we apply this framework to the LambdaCDM model, by extending the usual first order conditions on the metric potentials to second order, where the two potentials start to differ from each other. Read More