arXiv subjects

Eric V. Linder

Publications and source records attributed to Eric V. Linder.

At least 145 records · Page 8Linked to original sources

arXiv2011

From Asymptotic Safety to Dark Energy

We consider renormalization group flow applied to the cosmological dynamical equations. A consistency condition arising from energy-momentum conservation links the flow parameters to the cosmological evolution, restricting possible behaviors. Three classes of cosmological fixed points for dark energy plus a barotropic fluid are found: a dark energy dominated universe, which can be either accelerating or decelerating depending on the RG flow parameters, a barotropic dominated universe where dark energy fades away, and solutions where the gravitational and potential couplings cease to flow. If the IR fixed point coincides with the asymptotically safe UV fixed point then the dark energy pressure vanishes in the first class, while (only) in the de Sitter limit of the third class the RG cutoff scale becomes the Hubble scale.

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arXiv2011

Limits on Dark Radiation, Early Dark Energy, and Relativistic Degrees of Freedom

Recent cosmological data analyses hint at the presence of an extra relativistic energy component in the early universe. This component is often parametrized as an excess of the effective neutrino number N_{eff} over the standard value of 3.046. The excess relativistic energy could be an indication for an extra (sterile) neutrino, but early dark energy and barotropic dark energy also contribute to the relativistic degrees of freedom. We examine the capabilities of current and future data to constrain and discriminate between these explanations, and to detect the early dark energy density associated with them. We found that while early dark energy does not alter the current constraints on N_{eff}, a dark radiation component, such as that provided by barotropic dark energy models, can substantially change current constraints on N_{eff}, bringing its value back to agreement with the theoretical prediction. Both dark energy models also have implications for the primordial mass fraction of Helium Y_p and the scalar perturbation index n_s. The ongoing Planck satellite mission will be able to further discriminate between sterile neutrinos and early dark energy.

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arXiv2011

Model Independent Tests of Cosmic Gravity

Gravitation governs the expansion and fate of the universe, and the growth of large scale structure within it, but has not been tested in detail on these cosmic scales. The observed acceleration of the expansion may provide signs of gravitational laws beyond general relativity. Since the form of any such extension is not clear, from either theory or data, we adopt a model independent approach to parametrising deviations to the Einstein framework. We explore the phase space dynamics of two key post-GR functions and derive a classification scheme and an absolute criterion on accuracy necessary for distinguishing classes of gravity models. Future surveys will be able to constrain the post-GR functions' amplitudes and forms to the required precision, and hence reveal new aspects of gravitation.

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arXiv2011

Weak Lensing Science, Surveys, and Systematics

Weak gravitational lensing is one of the key probes of the cosmological model, dark energy, and dark matter, providing insight into both the cosmic expansion history and large scale structure growth history. Taking into account a broad spectrum of physics affecting growth - dynamical dark energy, extended gravity, neutrino masses, and spatial curvature - we analyze the cosmological constraints. Similarly we consider the effects of a range of systematic uncertainties, in shear measurement, photometric redshifts, and the nonlinear power spectrum, on cosmological parameter extraction. We also investigate, and provide fitting formulas for, the influence of survey parameters such as redshift depth, galaxy number densities, and sky area. Finally, we examine the robustness of results for different fiducial cosmologies.

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arXiv2011

Correlated Supernova Systematics and Ground Based Surveys

Supernova distances provide a direct probe of cosmic acceleration, constraining dark energy. This leverage increases with survey redshift depth at a rate bounded by the systematic uncertainties. We investigate the impact of a wavelength-dependent, global correlation model of systematics in comparison to the standard local-redshift correlation model. This can arise from subclass uncertainties as features in the supernova spectrum redshift out of the observer photometric filters or spectral range. We explore the impact of such a systematic on ground-based supernova surveys such as Dark Energy Survey and LSST, finding distinctive implications. Extending the wavelength sensitivity to 1.05 microns through "extreme red" CCDs can improve the dark energy figure of merit by up to a factor 2.

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arXiv2010

Induced Gravity and the Attractor Dynamics of Dark Energy/Dark Matter

Attractor solutions that give dynamical reasons for dark energy to act like the cosmological constant, or behavior close to it, are interesting possibilities to explain cosmic acceleration. Coupling the scalar field to matter or to gravity enlarges the dynamical behavior; we consider both couplings together, which can ameliorate some problems for each individually. Such theories have also been proposed in a Higgs-like fashion to induce gravity and unify dark energy and dark matter origins. We explore restrictions on such theories due to their dynamical behavior compared to observations of the cosmic expansion. Quartic potentials in particular have viable stability properties and asymptotically approach general relativity.

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arXiv2010

Confronting General Relativity with Further Cosmological Data

Deviations from general relativity in order to explain cosmic acceleration generically have both time and scale dependent signatures in cosmological data. We extend our previous work by investigating model independent gravitational deviations in bins of redshift and length scale, by incorporating further cosmological probes such as temperature-galaxy and galaxy-galaxy cross-correlations, and by examining correlations between deviations. Markov Chain Monte Carlo likelihood analysis of the model independent parameters fitting current data indicates that at low redshift general relativity deviates from the best fit at the 99% confidence level. We trace this to two different properties of the CFHTLS weak lensing data set and demonstrate that COSMOS weak lensing data does not show such deviation. Upcoming galaxy survey data will greatly improve the ability to test time and scale dependent extensions to gravity and we calculate the constraints that the BigBOSS galaxy redshift survey could enable.

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arXiv2010

Future CMB Constraints on Early, Cold, or Stressed Dark Energy

We investigate future constraints on early dark energy (EDE) achievable by the Planck and CMBPol experiments, including cosmic microwave background (CMB) lensing. For the dark energy, we include the possibility of clustering through a sound speed c_s^2 <1 (cold dark energy) and anisotropic stresses parameterized with a viscosity parameter c_vis^2. We discuss the degeneracies between cosmological parameters and EDE parameters. In particular we show that the presence of anisotropic stresses in EDE models can substantially undermine the determination of the EDE sound speed parameter c_s^2. The constraints on EDE primordial energy density are however unaffected. We also calculate the future CMB constraints on neutrino masses and find that they are weakened by a factor of 2 when allowing for the presence of EDE, and highly biased if it is incorrectly ignored.

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arXiv2010

Frontiers of Dark Energy

Cosmologists are just beginning to probe the properties of the cosmic vacuum and its role in reversing the attractive pull of gravity to cause an acceleration in the expansion of the cosmos. The cause of this acceleration is given the generic name of dark energy, whether it is due to a true vacuum, a false, temporary vacuum, or a new relation between the vacuum and the force of gravity. Despite the common name, the distinction between these origins is of utmost interest and physicists are actively engaged in finding ways to use cosmological observations to distinguish which is the true, new physics. Here we will discuss how to relate the theoretical ideas to the experimental constraints, how to understand the influences of dark energy on the expansion and structure in the universe, and what frontiers of new physics are being illuminated by current and near-term data.

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arXiv2010

Dark Before Light: Testing the Cosmic Expansion History through the Cosmic Microwave Background

The cosmic expansion history proceeds in broad terms from a radiation dominated epoch to matter domination to an accelerated, dark energy dominated epoch. We investigate whether intermittent periods of acceleration are possible in the early universe -- between Big Bang nucleosynthesis (BBN) and recombination and beyond. We establish that the standard picture is remarkably robust: observations of anisotropies in the cosmic microwave background exclude any extra period of accelerated expansion between 1 \leq z \lesssim 10^5 (corresponding to 5\times10^{-4}\ {\rm eV} \leq T \lesssim 25\ {\rm eV}).

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arXiv2010

Einstein's Other Gravity and the Acceleration of the Universe

Spacetime curvature plays the primary role in general relativity but Einstein later considered a theory where torsion was the central quantity. Just as the Einstein-Hilbert action in the Ricci curvature scalar R can be generalized to f(R) gravity, we consider extensions of teleparallel, or torsion scalar T, gravity to f(T) theories. The field equations are naturally second order, avoiding pathologies, and can give rise to cosmic acceleration with unique features.

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arXiv2010

Uniqueness of Current Cosmic Acceleration

One of the strongest arguments against the cosmological constant as an explanation of the current epoch of accelerated cosmic expansion is the existence of an earlier, dynamical acceleration, i.e. inflation. We examine the likelihood that acceleration is an occasional phenomenon, putting stringent limits on the length of any accelerating epoch between recombination and the recent acceleration; such an epoch must last less than 0.05 e-fold (at z>2) or the matter power spectrum is modified by more than 20%.

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arXiv2010

Testing General Relativity with Current Cosmological Data

Deviations from general relativity, such as could be responsible for the cosmic acceleration, would influence the growth of large scale structure and the deflection of light by that structure. We clarify the relations between several different model independent approaches to deviations from general relativity appearing in the literature, devising a translation table. We examine current constraints on such deviations, using weak gravitational lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background radiation data of WMAP5, and supernova distance data of Union2. Markov Chain Monte Carlo likelihood analysis of the parameters over various redshift ranges yields consistency with general relativity at the 95% confidence level.

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arXiv2010

Constraining Models of Dark Energy

One of the great endeavors of the past decade has been the evaluation of different observational techniques for measuring dark energy properties and of theoretical techniques for constraining models of cosmic acceleration given cosmological data. This chapter reviews a few of the key developments, promises, and cautions for revealing dark energy. We also present a few new calculations, on direct detection of acceleration through redshift drift, the minimum uncertainty in the equation of state, and testing gravity.

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arXiv2010

Old Dark Energy

Dark energy dynamics in the recent universe is influenced by its evolution through the long, matter dominated expansion history. A particular dynamical property, the flow variable, remains constant in several classes of scalar field models as long as matter dominates; the dark energy is only free to diverge in behavior at recent times. This gives natural initial conditions for Monte Carlo studies of dark energy dynamics. We propose a parametrization for the later evolution that covers a wide range of possible behaviors, is tractable in making predictions, and can be constrained by observations. We compare the approach to directly parametrizing the potential, which does not take into account the maturity of the dark energy dynamics.

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arXiv2010

Measuring the Speed of Dark: Detecting Dark Energy Perturbations

The nature of dark energy can be probed not only through its equation of state, but also through its microphysics, characterized by the sound speed of perturbations to the dark energy density and pressure. As the sound speed drops below the speed of light, dark energy inhomogeneities increase, affecting both CMB and matter power spectra. We show that current data can put no significant constraints on the value of the sound speed when dark energy is purely a recent phenomenon, but can begin to show more interesting results for early dark energy models. For example, the best fit model for current data has a slight preference for dynamics (w(a)\ne-1), degrees of freedom distinct from quintessence (c_s\ne1), and early presence of dark energy (Omega_ de(a<<1)\ne0). Future data may open a new window on dark energy by measuring its spatial as well as time variation.

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arXiv2010

Generating and Analyzing Constrained Dark Energy Equations of State and Systematics Functions

Some functions entering cosmological analysis, such as the dark energy equation of state or systematic uncertainties, are unknown functions of redshift. To include them without assuming a particular form we derive an efficient method for generating realizations of all possible functions subject to certain bounds or physical conditions, e.g. w\in[-1,+1] as for quintessence. The method is optimal in the sense that it is both pure and complete in filling the allowed space of principal components. The technique is applied to propagation of systematic uncertainties in supernova population drift and dust corrections and calibration through to cosmology parameter estimation and bias in the magnitude-redshift Hubble diagram. We identify specific ranges of redshift and wavelength bands where the greatest improvements in supernova systematics due to population evolution and dust correction can be achieved.

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arXiv2009

Exponential Gravity

We investigate a f(R) modification of gravity that is exponential in the Ricci scalar R to explain cosmic acceleration. The steepness of this dependence provides extra freedom to satisfy solar system and other curvature regime constraints. With a parameter to alleviate the usual fine tuning of having the modification strengthen near the present, the total number of parameters is only one more than LCDM. The resulting class of solutions asymptotes to w=-1 but has no cosmological constant. We calculate the dynamics in detail, examine the effect on the matter power spectrum, and provide a simple fitting form relating the two.

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