scholarly journals Features and new physical scales in primordial observables: Theory and observation

2015 ◽  
Vol 24 (10) ◽  
pp. 1530023 ◽  
Author(s):  
Jens Chluba ◽  
Jan Hamann ◽  
Subodh P. Patil
Keyword(s):  
Correlation Functions ◽  
Degrees Of Freedom ◽  
Large Scale ◽  
Initial Conditions ◽  
Linear Response Theory ◽  
Microwave Background ◽  
Scale Invariant ◽  
Cosmological Observations ◽  
Background Models ◽  
Characteristic Scales

All cosmological observations to date are consistent with adiabatic, Gaussian and nearly scale invariant initial conditions. These findings provide strong evidence for a particular symmetry breaking pattern in the very early universe (with a close to vanishing order parameter, ϵ), widely accepted as conforming to the predictions of the simplest realizations of the inflationary paradigm. However, given that our observations are only privy to perturbations, in inferring something about the background that gave rise to them, it should be clear that many different underlying constructions project onto the same set of cosmological observables. Features in the primordial correlation functions, if present, would offer a unique and discriminating window onto the parent theory in which the mechanism that generated the initial conditions is embedded. In certain contexts, simple linear response theory allows us to infer new characteristic scales from the presence of features that can break the aforementioned degeneracies among different background models, and in some cases can even offer a limited spectroscopy of the heavier degrees of freedom that couple to the inflaton. In this review, we offer a pedagogical survey of the diverse, theoretically well-grounded mechanisms which can imprint features into primordial correlation functions in addition to reviewing the techniques one can employ to probe observations. These observations include cosmic microwave background (CMB) anisotropies and spectral distortions as well as the matter two and three point functions as inferred from large-scale structure (LSS) and potentially, 21 cm surveys.

scholarly journals Measuring Gravity at Cosmological Scales

Universe ◽  
2020 ◽  
Vol 6 (2) ◽  
pp. 20 ◽  
Author(s):  
Luca Amendola ◽  
Dario Bettoni ◽  
Ana Marta Pinho ◽  
Santiago Casas
Keyword(s):  
Degrees Of Freedom ◽  
Large Scale ◽  
Early Years ◽  
Microwave Background ◽  
Four Dimensions ◽  
Tensor Theory ◽  
Cosmological Observations ◽  
Local Gravity ◽  
Model Independent ◽  
Models Of Gravity

This review is a pedagogical introduction to models of gravity and how they are constrained through cosmological observations. We focus on the Horndeski scalar-tensor theory and on the quantities that can be measured with a minimum of assumptions. Alternatives or extensions of general relativity have been proposed ever since its early years. Because of the Lovelock theorem, modifying gravity in four dimensions typically means adding new degrees of freedom. The simplest way is to include a scalar field coupled to the curvature tensor terms. The most general way of doing so without incurring in the Ostrogradski instability is the Horndeski Lagrangian and its extensions. Testing gravity means therefore, in its simplest term, testing the Horndeski Lagrangian. Since local gravity experiments can always be evaded by assuming some screening mechanism or that baryons are decoupled, or even that the effects of modified gravity are visible only at early times, we need to test gravity with cosmological observations in the late universe (large-scale structure) and in the early universe (cosmic microwave background). In this work, we review the basic tools to test gravity at cosmological scales, focusing on model-independent measurements.

scholarly journals A gravitational puzzle

2011 ◽  
Vol 369 (1957) ◽  
pp. 4998-5002
Author(s):  
Robert R. Caldwell
Keyword(s):  
Gravitational Lensing ◽  
Large Scale ◽  
Current Data ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Microwave Background ◽  
Scale Free ◽  
Cosmological Observations ◽  
Stress Energy ◽  
The Relationship

The challenge to understand the physical origin of the cosmic acceleration is framed as a problem of gravitation. Specifically, does the relationship between stress–energy and space–time curvature differ on large scales from the predictions of general relativity. In this article, we describe efforts to model and test a generalized relationship between the matter and the metric using cosmological observations. Late-time tracers of large-scale structure, including the cosmic microwave background, weak gravitational lensing, and clustering are shown to provide good tests of the proposed solution. Current data are very close to proving a critical test, leaving only a small window in parameter space in the case that the generalized relationship is scale free above galactic scales.

scholarly journals The Weird Side of the Universe: Preferred Axis

2017 ◽  
Vol 45 ◽  
pp. 1760009 ◽  
Author(s):  
Wen Zhao ◽  
Larissa Santos
Keyword(s):  
Cosmic Microwave Background ◽  
Large Scale ◽  
Directional Statistics ◽  
Temperature Anisotropy ◽  
Microwave Background ◽  
The Past ◽  
Cosmological Observations ◽  
Definition Of ◽  
The Universe

In both WMAP and Planck observations on the temperature anisotropy of cosmic microwave background (CMB) radiation a number of large-scale anomalies were discovered in the past years, including the CMB parity asymmetry in the low multipoles. By defining a directional statistics, we find that the CMB parity asymmetry is directional dependent, and the preferred axis is stable, which means that it is independent of the chosen CMB map, the definition of the statistic, or the CMB masks. Meanwhile, we find that this preferred axis strongly aligns with those of the CMB quadrupole, octopole, as well as those of other large-scale observations. In addition, all of them aligns with the CMB kinematic dipole, which hints to the non-cosmological origin of these directional anomalies in cosmological observations.

ANALYTICAL EXPRESSIONS FOR STABLE AND UNSTABLE MANIFOLDS IN HIGHER DEGREE OF FREEDOM HAMILTONIAN SYSTEMS

1996 ◽  
Vol 06 (11) ◽  
pp. 1997-2013 ◽  
Author(s):  
HARRY DANKOWICZ
Keyword(s):  
Hamiltonian Systems ◽  
Degrees Of Freedom ◽  
Large Scale ◽  
Initial Conditions ◽  
Saddle Points ◽  
Melnikov Method ◽  
Type Systems ◽  
Stable And Unstable Manifolds ◽  
Unstable Manifolds ◽  
Analytical Expressions

Perturbations of completely integrable Hamiltonian systems with three or more degrees of freedom are studied. In particular, the unperturbed systems are assumed to be separable into a product of simple oscillator-type systems and a system containing homo- or heteroclinic connections consisting of stable and unstable manifolds of saddle points. Under a perturbation, the manifolds persist but separate and may no longer intersect. In this paper we show how, with proper choices for initial conditions, one may solve the variational equations to obtain analytical expressions for orbits on the perturbed manifolds in the form of expansions in the small parameter characterizing the perturbation. The derivation also shows how the distance between the manifolds can be uniquely defined, and thus provides an alternative to the traditional higher dimensional Melnikov method. It is finally argued that the approximate knowledge of the shape and position of the perturbed manifolds could be utilized for the study of large-scale phase-space motions, such as those associated with Arnold diffusion. The theory is further illuminated in two example problems.

scholarly journals Scaling and similarity of a stream-power incision and linear diffusion landscape evolution model

2018 ◽  
Vol 6 (3) ◽  
pp. 779-808 ◽  
Author(s):  
Nikos Theodoratos ◽  
Hansjörg Seybold ◽  
James W. Kirchner
Keyword(s):  
Steady State ◽  
Peclet Number ◽  
Degrees Of Freedom ◽  
Landscape Evolution ◽  
Initial Conditions ◽  
Evolution Model ◽  
Model Parameters ◽  
Stream Power ◽  
Linear Diffusion ◽  
Characteristic Scales

Abstract. The scaling and similarity of fluvial landscapes can reveal fundamental aspects of the physics driving their evolution. Here, we perform a dimensional analysis of the governing equation of a widely used landscape evolution model (LEM) that combines stream-power incision and linear diffusion laws. Our analysis assumes that length and height are conceptually distinct dimensions and uses characteristic scales that depend only on the model parameters (incision coefficient, diffusion coefficient, and uplift rate) rather than on the size of the domain or of landscape features. We use previously defined characteristic scales of length, height, and time, but, for the first time, we combine all three in a single analysis. Using these characteristic scales, we non-dimensionalize the LEM such that it includes only dimensionless variables and no parameters. This significantly simplifies the LEM by removing all parameter-related degrees of freedom. The only remaining degrees of freedom are in the boundary and initial conditions. Thus, for any given set of dimensionless boundary and initial conditions, all simulations, regardless of parameters, are just rescaled copies of each other, both in steady state and throughout their evolution. Therefore, the entire model parameter space can be explored by temporally and spatially rescaling a single simulation. This is orders of magnitude faster than performing multiple simulations to span multidimensional parameter spaces. The characteristic scales of length, height and time are geomorphologically interpretable; they define relationships between topography and the relative strengths of landscape-forming processes. The characteristic height scale specifies how drainage areas and slopes must be related to curvatures for a landscape to be in steady state and leads to methods for defining valleys, estimating model parameters, and testing whether real topography follows the LEM. The characteristic length scale is roughly equal to the scale of the transition from diffusion-dominated to advection-dominated propagation of topographic perturbations (e.g., knickpoints). We introduce a modified definition of the landscape Péclet number, which quantifies the relative influence of advective versus diffusive propagation of perturbations. Our Péclet number definition can account for the scaling of basin length with basin area, which depends on topographic convergence versus divergence.

scholarly journals Primordial Non-Gaussianity and Bispectrum Measurements in the Cosmic Microwave Background and Large-Scale Structure

2010 ◽  
Vol 2010 ◽  
pp. 1-64 ◽  
Author(s):  
Michele Liguori ◽  
Emiliano Sefusatti ◽  
James R. Fergusson ◽  
E. P. S. Shellard
Keyword(s):  
Cosmic Microwave Background ◽  
Large Scale ◽  
Initial Conditions ◽  
Temperature Fluctuations ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Future Prospects ◽  
Microwave Background ◽  
Galaxy Distribution ◽  
Non Gaussian

The most direct probe of non-Gaussian initial conditions has come from bispectrum measurements of temperature fluctuations in the Cosmic Microwave Background and of the matter and galaxy distribution at large scales. Such bispectrum estimators are expected to continue to provide the best constraints on the non-Gaussian parameters in future observations. We review and compare the theoretical and observational problems, current results, and future prospects for the detection of a nonvanishing primordial component in the bispectrum of the Cosmic Microwave Background and large-scale structure, and the relation to specific predictions from different inflationary models.

scholarly journals Neutrino Mass from Cosmology

2012 ◽  
Vol 2012 ◽  
pp. 1-34 ◽  
Author(s):  
Julien Lesgourgues ◽  
Sergio Pastor
Keyword(s):  
Beta Decay ◽  
Large Scale ◽  
Neutrinoless Double Beta Decay ◽  
Double Beta Decay ◽  
Neutrino Masses ◽  
Microwave Background ◽  
Complementary Information ◽  
Cosmological Observations ◽  
Cosmological Data ◽  
The Universe

Neutrinos can play an important role in the evolution of the universe, modifying some of the cosmological observables. In this contribution we summarize the main aspects of cosmological relic neutrinos, and we describe how the precision of present cosmological data can be used to learn about neutrino properties, in particular their mass, providing complementary information to beta decay and neutrinoless double-beta decay experiments. We show how the analysis of current cosmological observations, such as the anisotropies of the cosmic microwave background or the distribution of large-scale structure, provides an upper bound on the sum of neutrino masses of order 1 eV or less, with very good perspectives from future cosmological measurements which are expected to be sensitive to neutrino masses well into the sub-eV range.

scholarly journals Cosmic Dark Radiation and Neutrinos

2013 ◽  
Vol 2013 ◽  
pp. 1-14 ◽  
Author(s):  
Maria Archidiacono ◽  
Elena Giusarma ◽  
Steen Hannestad ◽  
Olga Mena
Keyword(s):  
Degrees Of Freedom ◽  
Hubble Constant ◽  
Point Of View ◽  
Data Sets ◽  
Beyond The Standard Model ◽  
Microwave Background ◽  
Dark Radiation ◽  
Cosmological Observations ◽  
Local Measurements ◽  
The Moment

New measurements of the cosmic microwave background (CMB) by the Planck mission have greatly increased our knowledge about the universe. Dark radiation, a weakly interacting component of radiation, is one of the important ingredients in our cosmological model which is testable by Planck and other observational probes. At the moment, the possible existence of dark radiation is an unsolved question. For instance, the discrepancy between the value of the Hubble constant,H0, inferred from the Planck data and local measurements ofH0can to some extent be alleviated by enlarging the minimalΛCDM model to include additional relativistic degrees of freedom. From a fundamental physics point of view, dark radiation is no less interesting. Indeed, it could well be one of the most accessible windows to physics beyond the standard model, for example, sterile neutrinos. Here, we review the most recent cosmological results including a complete investigation of the dark radiation sector in order to provide an overview of models that are still compatible with new cosmological observations. Furthermore, we update the cosmological constraints on neutrino physics and dark radiation properties focusing on tensions between data sets and degeneracies among parameters that can degrade our information or mimic the existence of extra species.

NON-SPHERICAL VOIDS: THE BEST ALTERNATIVE TO DARK ENERGY?

2011 ◽  
Vol 20 (supp01) ◽  
pp. 94-101
Author(s):  
ROBERTO A SUSSMAN
Keyword(s):  
Dark Energy ◽  
Degrees Of Freedom ◽  
Large Scale ◽  
Observation Point ◽  
Coarse Grained ◽  
Spherical Void ◽  
Cosmological Observations ◽  
Definition Of ◽  
Almost All ◽  
Spherical Voids

If our cosmic location lies within a large-scale under–dense region or "void", then current cosmological observations can be explained without resorting to a cosmological constant or to an exotic and elusive source like "dark energy". If we further assume this void region to be spherical (as almost all current void models do), then fitting observational data severely constrains our position to be very near the void center, which is a very special and unlikely observation point. We argue in this article that existing spherical void models must be regarded as gross approximations that arise by smoothing out more realistic non–spherical configurations that may fit observations without the limitations imposed by spherical symmetry. In particular, the class of quasi–spherical Szekeres models provides sufficient degrees of freedom to describe the evolution of non–spherical inhomogeneities, including a configuration consisting of several elongated supercluster–like overdense filaments with large underdense regions between them. We summarize a recently published example of such configuration, showing that it yields a reasonable coarse-grained description of realistic observed structures. While the density distribution is not spherically symmetric, its proper volume average yields a spherical density void profile of 250 Mpc that roughly agrees with observations. Also, once we consider our location to lie within a non-spherical void, the definition of a "center" location becomes more nuanced, and thus the constraints placed by the fitting of observations on our position with respect to this location become less restrictive.

scholarly journals Scaling and similarity of a stream-power incision and linear diffusion landscape evolution model

2018 ◽  
Author(s):  
Nikos Theodoratos ◽  
Hansjörg Seybold ◽  
James W. Kirchner
Keyword(s):  
Steady State ◽  
Degrees Of Freedom ◽  
Characteristic Length ◽  
Initial Conditions ◽  
Model Parameters ◽  
Length Scale ◽  
Characteristic Length Scale ◽  
Stream Power ◽  
Linear Diffusion ◽  
Characteristic Scales

Abstract. Scaling and similarity of fluvial landscapes can reveal fundamental aspects of the physics driving their evolution. Here we perform dimensional analysis on a widely used landscape evolution model (LEM) that combines stream-power incision and linear diffusion laws. Our analysis assumes that length and height are conceptually distinct dimensions, and uses characteristic scales that depend only on the model parameters (incision coefficient, diffusion coefficient, and uplift rate) rather than on the size of the domain or of landscape features. We use a previously defined characteristic length scale, but introduce new characteristic height and time scales. We use these characteristic scales to non-dimensionalize the LEM, such that it includes only dimensionless variables and no parameters. This significantly simplifies the LEM by removing all parameter-related degrees of freedom. The only remaining degrees of freedom are in the boundary and initial conditions. Thus, for any given set of dimensionless boundary and initial conditions, all simulations, regardless of parameters, are just re-scaled copies of each other, both in steady state and throughout their evolution. Therefore, the entire model parameter space can be explored by temporally and spatially re-scaling a single simulation. This is orders of magnitude faster than performing multiple simulations to span multi-dimensional parameter spaces. The characteristic length, height, and time scales are geomorphologically interpretable; they define relationships between topography and the relative strengths of landscape-forming processes. The characteristic height scale specifies how drainage areas and slopes must be related to curvatures for a landscape to be in steady state, and leads to methods for defining valleys, estimating model parameters, and testing whether real topography follows the LEM. The characteristic length scale is roughly equal to the scale of the transition from diffusion-dominated to advection-dominated propagation of topographic perturbations (e.g., knickpoints). We introduce a modified definition of the landscape Péclet number, which quantifies the relative influence of advective versus diffusive propagation of perturbations. Our Péclet number definition can account for the scaling of basin length with basin area, which depends on topographic convergence versus divergence.

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