arXiv subjects

Eric V. Linder

Publications and source records attributed to Eric V. Linder.

At least 199 records · Page 11Linked to original sources

arXiv2005

On Oscillating Dark Energy

Distance-redshift data can impose strong constraints on dark energy models even when the equation of state is oscillatory. Despite the double integral dependence of the distance on the equation of state, precision measurement of the distance-redshift relation for z=0-2 is more incisive than the linear growth factor, CMB last scattering surface distance, and the age of the universe in distinguishing oscillatory behavior from an average behavior. While oscillating models might help solve the coincidence problem (since acceleration occurs periodically), next generation observations will strongly constrain such possibilities.

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arXiv2005

Seeing Darkness: the New Cosmology

We present some useful ways to visualize the nature of dark energy and the effects of the accelerating expansion on cosmological quantities. Expansion probes such as Type Ia supernovae distances and growth probes such as weak gravitational lensing and the evolution of large scale structure provide powerful tests in complementarity. We present a ``ladder'' diagram, showing that in addition to dramatic improvements in precision, next generation probes will provide insight through an increasing ability to test assumptions of the cosmological framework, including gravity beyond general relativity.

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arXiv2005

The Influence of Evolving Dark Energy on Cosmology

Observational evidence indicating that the expansion of the universe is accelerating has surprised cosmologists in recent years. Cosmological models have sought to explain this acceleration by incorporating `dark energy', of which the traditional cosmological constant is just one possible candidate. Several cosmological models involving an evolving equation of state of the dark energy have been proposed, as well as possible energy exchange to other components, such as dark matter. This paper summarises the forms of the most prominent models and discusses their implications for cosmology and astrophysics. Finally, this paper examines the current and future observational constraints on the nature of dark energy.

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arXiv2005

Curved Space or Curved Vacuum?

While the simple picture of a spatially flat, matter plus cosmological constant universe fits current observation of the accelerated expansion, strong consideration has also been given to models with dynamical vacuum energy. We examine the tradeoff of ``curving'' the vacuum but retaining spatial flatness, vs. curving space but retaining the cosmological constant. These different breakdowns in the simple picture could readily be distinguished by combined high accuracy supernovae and cosmic microwave background distance measurements. If we allow the uneasy situation of both breakdowns, the curvature can still be measured to 1%, but at the price of degrading estimation of the equation of state time variation by 60% or more, unless additional information (such as weak lensing data or a tight matter density prior) is included.

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arXiv2005

Going Nonlinear with Dark Energy Cosmologies

We propose an efficient method for generating high accuracy (~1%) nonlinear power spectra for grids of dark energy cosmologies. Our prescription for matching matter growth automatically matches the main features of the cosmic microwave background anisotropy power spectrum, thus naturally including ``CMB priors''.

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arXiv2005

Cosmic Growth History and Expansion History

The cosmic expansion history tests the dynamics of the global evolution of the universe and its energy density contents, while the cosmic growth history tests the evolution of the inhomogeneous part of the energy density. Precision comparison of the two histories can distinguish the nature of the physics responsible for the accelerating cosmic expansion: an additional smooth component - dark energy - or a modification of the gravitational field equations. With the aid of a new fitting formula for linear perturbation growth accurate to 0.05-0.2%, we separate out the growth dependence on the expansion history and introduce a new growth index parameter γthat quantifies the gravitational modification.

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arXiv2005

The Limits of Quintessence

We present evidence that the simplest particle-physics scalar-field models of dynamical dark energy can be separated into distinct behaviors based on the acceleration or deceleration of the field as it evolves down its potential towards a zero minimum. We show that these models occupy narrow regions in the phase-plane of w and w', the dark energy equation-of-state and its time-derivative in units of the Hubble time. Restricting an energy scale of the dark energy microphysics limits how closely a scalar field can resemble a cosmological constant. These results, indicating a desired measurement resolution of order σ(w')\approx (1+w), define firm targets for observational tests of the physics of dark energy.

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arXiv2005

How many dark energy parameters?

For exploring the physics behind the accelerating universe a crucial question is how much we can learn about the dynamics through next generation cosmological experiments. For example, in defining the dark energy behavior through an effective equation of state, how many parameters can we realistically expect to tightly constrain? Through both general and specific examples (including new parametrizations and principal component analysis) we argue that the answer is 42 - no, wait, two. Cosmological parameter analyses involving a measure of the equation of state value at some epoch (e.g. w_0) and a measure of the change in equation of state (e.g. w') are therefore realistic in projecting dark energy parameter constraints. More elaborate parametrizations could have some uses (e.g. testing for bias or comparison with model features), but do not lead to accurately measured dark energy parameters.

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arXiv2005

Dealing with Dark Energy

Discoveries in the last few years have revolutionized our knowledge of the universe and our ideas of its ultimate fate. Measurements of the expansion of the universe show that it is not slowing down under normal gravity but accelerating due to an unknown, gravitationally repulsive ``dark energy''. This may be a clue to new properties of quantum physics or of gravity beyond Einstein. I present an overview of the puzzles of dark energy and the means for unraveling them through cosmological probes, on both a generally accessible and a technical level. I also highlight the strong benefits of meshing supernova distance and weak lensing methods. Next generation experiments such as the Supernova/Acceleration Probe (SNAP) satellite would measure the supernova distance-redshift relation to high accuracy and map the evolution of structure and dark matter through gravitational lensing. These observations will explore the frontiers of physics and aim to uncover what makes up the still unknown 95% of our universe.

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arXiv2004

Safety in numbers: Gravitational Lensing Degradation of the Luminosity Distance-Redshift Relation

Observation of the expansion history of the Universe allows exploration of the physical properties and energy density of the Universe's various constituents. Standardizable candles such as Type Ia supernovae remain one of the most promising and robust tools in this endeavor, by allowing for a direct measure of the luminosity distance-redshift curve, and thereby producing detailed studies of the dark energy responsible for the Universe's currently accelerating expansion. As such observations are pushed to higher redshifts, the observed flux is increasingly affected by gravitational lensing magnification due to intervening structure along the line-of-sight. We simulate and analyze the non-Gaussian probability distribution function of de/amplification due to lensing of standard candles, quantify the effect of a convolution over many independent sources (which acts to restore the intrinsic average (unlensed) luminosity due to flux conservation), and compute the additional uncertainty due to lensing on derived cosmological parameters. For example, the ``degradation factor'' due to lensing is a factor of three reduction in the effective number of usable supernovae at z=1.5 (for sources with intrinsic flux dispersion of 10%). We also derive a useful expression for the effective increased dispersion in standard candles due to lensing, as a function of redshift.

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arXiv2004

Is Dark Energy Dynamical? Prospects for an Answer

Recent data advances offer the exciting prospect of a first look at whether dark energy has a dynamical equation of state or not. While formally theories exist with a constant equation of state, they are nongeneric -- Einstein's cosmological constant is a notable exception. So limits on the time variation, w', directly tell us crucial physics. Two recent improvements in supernova data from the Hubble Space Telescope allow important steps forward in constraining the dynamics of dark energy, possessing the ability to exclude models with w'\ga 1, if the universe truly has a cosmological constant. These data bring us much closer to the ``systematics'' era, where further improvements will come predominantly from more accurate, not merely more, observations. We examine the possible gains and point out the complementary roles of space and ground based observations in the near future. To achieve the leap to precision understanding of dark energy in the next generation will require specially designed space based measurements; we estimate the confidence level of detection of dynamics (e.g. distinguishing between $w'=0$ and $w'=1$) will be ~1.8σafter the ongoing generation, improving to more than 6.5σin the dedicated space generation.

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arXiv2004

Dark Entropy: Holographic Cosmic Acceleration

In cosmic holography, the fundamental quantity is the degrees of freedom on a horizon surface rather than the material contents within the volume. That is, the horizon area and hence cosmological expansion rate H is related to the entropy. Using this as a guide, we examine the consequences of an effective dark {\it entropy} on the past and future dynamics of the universe. The results, including nonmonotonic expansion behavior, are interesting apart from the theoretical motivation.

hep-th↗

arXiv2004

Observing Dark Energy with SNAP

The nature of dark energy is of such fundamental importance -- yet such a mystery -- that a dedicated dark energy experiment should be as comprehensive and powerfully incisive as possible. The Supernova/Acceleration Probe robustly controls for a wide variety of systematic uncertainties, employing the Type Ia supernova distance method, with high signal to noise light curves and spectra over the full redshift range from z=0.1-1.7, and the weak gravitational lensing method with an accurate and stable point spread function.

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arXiv2004

Reconstructing and Deconstructing Dark Energy

The acceleration of the expansion of the universe, ascribed to a dark energy, is one of the most intriguing discoveries in science. In addition to precise, systematics controlled data, clear, robust interpretation of the observations is required to reveal the nature of dark energy. Even for the simplest question: is the data consistent with the cosmological constant? there are important subtleties in the reconstruction of the dark energy properties. We discuss the roles of analysis both in terms of the Hubble expansion rate or dark energy density ρ_{DE}(z) and in terms of the dark energy equation of state w(z), arguing that each has its carefully defined place. Fitting the density is best for learning about the density, but using it to probe the equation of state can lead to instability and bias.

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arXiv2004

Light Thoughts on Dark Energy

The physical process leading to the acceleration of the expansion of the universe is unknown. It may involve new high energy physics or extensions to gravitation. Calling this generically dark energy, we examine the consistencies and relations between these two approaches, showing that an effective equation of state function w(z) is broadly useful in describing the properties of the dark energy. A variety of cosmological observations can provide important information on the dynamics of dark energy and the future looks bright for constraining dark energy, though both the measurements and the interpretation will be challenging. We also discuss a more direct relation between the spacetime geometry and acceleration, via ``geometric dark energy'' from the Ricci scalar, and superacceleration or phantom energy where the fate of the universe may be more gentle than the Big Rip.

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arXiv2004

Probing Gravitation, Dark Energy, and Acceleration

The acceleration of the expansion of the universe arises from unknown physical processes involving either new fields in high energy physics or modifications of gravitation theory. It is crucial for our understanding to characterize the properties of the dark energy or gravity through cosmological observations and compare and distinguish between them. In fact, close consistencies exist between a dark energy equation of state function w(z) and changes to the framework of the Friedmann cosmological equations as well as direct spacetime geometry quantities involving the acceleration, such as ``geometric dark energy'' from the Ricci scalar. We investigate these interrelationships, including for the case of superacceleration or phantom energy where the fate of the universe may be more gentle than the Big Rip.

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arXiv2004

Strong Gravitational Lensing and Dark Energy Complementarity

In the search for the nature of dark energy most cosmological probes measure simple functions of the expansion rate. While powerful, these all involve roughly the same dependence on the dark energy equation of state parameters, with anticorrelation between its present value w_0 and time variation w_a. Quantities that have instead positive correlation and so a sensitivity direction largely orthogonal to, e.g., distance probes offer the hope of achieving tight constraints through complementarity. Such quantities are found in strong gravitational lensing observations of image separations and time delays. While degeneracy between cosmological parameters prevents full complementarity, strong lensing measurements to 1% accuracy can improve equation of state characterization by 15-50%. Next generation surveys should provide data on roughly 10^5 lens systems, though systematic errors will remain challenging.

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arXiv2003

Testing the Cosmological Constant as a Candidate for Dark Energy

It may be difficult to single out the best model of dark energy on the basis of the existing and planned cosmological observations, because many different models can lead to similar observational consequences. However, each particular model can be studied and either found consistent with observations or ruled out. In this paper, we concentrate on the possibility to test and rule out the simplest and by far the most popular of the models of dark energy, the theory described by general relativity with positive vacuum energy (the cosmological constant). We evaluate the conditions under which this model could be ruled out by the future observations made by the Supernova/Acceleration Probe SNAP (both for supernovae and weak lensing) and by the Planck Surveyor cosmic microwave background satellite.

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