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A class of inflation theories called $\ensuremath{\alpha}$-attractors has been investigated recently with interesting properties interpolating between quadratic potentials, the Starobinsky model, and an attractor limit. Here we examine their use for late-time cosmic acceleration. We generalize the class and demonstrate how it can interpolate between thawing and freezing dark energy, and reduce the fine-tuning of initial conditions, allowing $w\ensuremath{\approx}\ensuremath{-}1$ for a prolonged period or as a de Sitter attractor.

Strong lensing time delay cosmography has excellent complementarity with other dark energy probes, and will soon have abundant systems detected. We investigate two issues in the imaging and spectroscopic followup required to obtain the time delay distance. The first is optimization of spectroscopic resources. We develop a code to optimize the cosmological leverage under the constraint of constant spectroscopic time, and find that sculpting the lens system redshift distribution can deliver a 40% improvement in dark energy figure of merit. The second is the role of systematics, correlated between different quantities of a given system or model errors common to all systems. We show how the levels of different systematics affect the cosmological parameter estimation, and derive guidance for the fraction of double image vs quad image systems to follow as a function of differing systematics between them., 9 pages, 7 figures

Current cosmological data puts increasing pressure on models of dark energy in the freezing class, e.g. early dark energy or those with equation of state $w$ substantially different from $-1$. We investigate to what extent data will distinguish the thawing class of quintessence from a cosmological constant. Since thawing dark energy deviates from $w=-1$ only at late times, we find that deviations $1+w\lesssim0.1$ are difficult to see even with next generation measurements; however, modest redshift drift data can improve the sensitivity by a factor of two. Furthermore, technical naturalness prefers specific thawing models., Comment: 5 pages, 2 figures

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., Comment: 7 pages, 6 figures; minor changes to match version accepted to Astroparticle Physics and correct typo at bottom of page 1

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., invited overview at Dark Matter/Dark Energy 2004; 6 pages

Dark energy has a dramatic effect on the dynamics of the universe, causing the recently discovered acceleration of the expansion. The dynamics are also central to the behavior of the growth of large scale structure, offering the possibility that observations of structure formation provide a sensitive probe of the cosmology and dark energy characteristics. In particular, dark energy with a time varying equation of state can have an influence on structure formation stretching back well into the matter dominated epoch. We analyze this impact, first calculating the linear perturbation results, including those for weak gravitational lensing. These dynamical models possess definite observable differences from constant equation of state models. Then we present a large scale numerical simulation of structure formation, including the largest volume to date involving a time varying equation of state. We find the halo mass function is well described by the Jenkins et al. mass function formula. We also show how to interpret modifications of the Friedmann equation in terms of a time variable equation of state. The results presented here provide steps toward realistic computation of the effect of dark energy in cosmological probes involving large scale structure, such as cluster counts, Sunyaev-Zel'dovich effect, or weak gravitational lensing.

Adding to our previous method for dealing with gravitational modifications at redshift $z\gtrsim3$ through a single parameter, we investigate treatment of lower redshift modifications to linear growth observables. We establish subpercent accurate fits to the redshift space distortion observable $f\sigma_8(a)$ using two parameters binned in redshift, testing the results for modifications with time dependence that rises, falls, is nonmonotonic, is multipeaked, and corresponds to $f(R)$ and braneworld gravity. The residuals are then propagated to cosmological parameter biases for DESI observations, and found to cause a shift in the dark energy joint confidence contour by less than the equivalent of $\sim0.1\sigma$. The proposed 2--3 parameter modified gravity description also can reveal physical characteristics of the underlying theory., Comment: 8 pages, 6 figures

We present a weak-lensing study of SPT-CLJ2040-4451 and IDCSJ1426+3508 at z=1.48 and 1.75, respectively. The two clusters were observed in our "See Change" program, a HST survey of 12 massive high-redshift clusters aimed at high-z supernova measurements and weak-lensing estimation of accurate cluster masses. We detect weak but significant galaxy shape distortions using IR images from the WFC3, which has not yet been used for weak-lensing studies. Both clusters appear to possess relaxed morphology in projected mass distribution, and their mass centroids agree nicely with those defined by both the galaxy luminosity and X-ray emission. Using an NFW profile, for which we assume that the mass is tightly correlated with the concentration parameter, we determine the masses of SPT-CL J2040-4451 and IDCS J1426+3508 to be M_{200}=8.6_{-1.4}^{+1.7}x10^14 M_sun and 2.2_{-0.7}^{+1.1}x10^14 M_sun, respectively. The weak-lensing mass of SPT-CLJ2040-4451 shows that the cluster is clearly a rare object. Adopting the central value, the expected abundance of such a massive cluster at z>1.48 is only ~0.07 in the parent 2500 sq. deg. survey. However, it is yet premature to claim that the presence of this cluster creates a serious tension with the current LCDM paradigm unless that tension will remain in future studies after marginalizing over many sources of uncertainties such as the accuracy of the mass function and the mass-concentration relation at the high mass end. The mass of IDCSJ1426+3508 is in excellent agreement with our previous ACS-based weak-lensing result while the much higher source density from our WFC3 imaging data makes the current statistical uncertainty ~40% smaller., Comment: Accepted to ApJ

Cosmic growth of large scale structure probes the entire history of cosmic expansion and gravitational coupling. To get a clear picture of the effects of modification of gravity we consider a deviation in the coupling strength (effective Newton's constant) at different redshifts, with different durations and amplitudes. We derive, analytically and numerically, the impact on the growth rate and growth amplitude. Galaxy redshift surveys can measure a product of these through redshift space distortions and we connect the modified gravity to the observable in a way that may provide a useful parametrization of the ability of future surveys to test gravity. In particular, modifications during the matter dominated era can be treated by a single parameter, the "area" of the modification, to an accuracy of $\sim0.3\%$ in the observables. We project constraints on both early and late time gravity for the Dark Energy Spectroscopic Instrument and discuss what is needed for tightening tests of gravity to better than 5% uncertainty., Comment: 12 pages, 14 figures

We forecast dark energy constraints that could be obtained from a new large sample of Type Ia supernovae where those at high redshift are acquired with the Euclid space mission. We simulate a three-prong SN survey: a z, Comment: 21 pages. Accepted for publication in A&A

We investigate how the use of higher wavenumbers (smaller scales) in the galaxy clustering power spectrum influences cosmological constraints. We take into account uncertainties from nonlinear density fluctuations, (scale dependent) galaxy bias, and baryonic effects. Allowing for substantially model independent uncertainties through separate fit parameters in each wavenumber bin that also allow for the redshift evolution, we quantify strong gains in dark energy and neutrino mass leverage with increasing maximum wavenumber, despite marginalizing over numerous (up to 125) extra fit parameters. The leverage is due to not only an increased number of modes but, more significantly, breaking of degeneracies beyond the linear regime., 10 pages, 6 figures

The dark sector of the Universe need not be completely separable into distinct dark matter and dark energy components. We consider a model of early dark energy in which the dark energy mimics a dark matter component in both evolution and perturbations at early times. Barotropic aether dark energy scales as a fixed fraction, possibly greater than one, of the dark matter density and has vanishing sound speed at early times before undergoing a transition. This gives signatures not only in cosmic expansion but in sound speed and inhomogeneities, and in number of effective neutrino species. Model parameters describe the timing, sharpness of the transition, and the relative abundance at early times. Upon comparison with current data, we find viable regimes in which the dark energy behaves like dark matter at early times: for transitions well before recombination the dark energy to dark matter fraction can equal or exceed unity, while for transitions near recombination the ratio can only be a few percent. After the transition, dark energy goes its separate way, ultimately driving cosmic acceleration and approaching a cosmological constant in this scenario., Comment: 10 pages, 8 figures

Modified gravity theories on cosmic scales have three key deviations from general relativity. They can cause cosmic acceleration without a physical, highly negative pressure fluid, can cause a gravitational slip between the two metric potentials, and can cause gravitational waves to propagate differently, e.g. with a speed different from the speed of light. We examine whether the deviations in the metric potentials as observable through modified Poisson equations for scalar density perturbations are related to or independent from deviations in the tensor gravitational waves. We show analytically they are independent instantaneously in covariant Galileon gravity -- e.g. at some time one of them can have the general relativity value while the other deviates -- though related globally -- if one deviates over a finite period, the other at some point shows a deviation. We present expressions for the early time and late time de Sitter limits, and numerically illustrate their full evolution. This in(ter)dependence of the scalar and tensor deviations highlights complementarity between cosmic structure surveys and future gravitational wave measurements., 6 pages, 3 figures; v2 minor changes to match PRD version

We analyze the clustering of large scale structure in the Universe in a model independent method, accounting for anisotropic effects along and transverse to the line of sight. The Baryon Oscillation Spectroscopy Survey Data Release 11 provides a large sample of 690,000 galaxies, allowing determination of the Hubble expansion H, angular distance D_A, and growth rate G_T at an effective redshift of z=0.57. After careful bias and convergence studies of the effects from small scale clustering, we find that cutting transverse separations below 40 Mpc/h delivers robust results while smaller scale data leads to a bias due to unmodelled nonlinear and velocity effects. The converged results are in agreement with concordance LCDM cosmology, general relativity, and minimal neutrino mass, all within the 68% confidence level. We also present results separately for the northern and southern hemisphere sky, finding a slight tension in the growth rate -- potentially a signature of anisotropic stress, or just covariance with small scale velocities -- but within 68% CL., 9 pages, 6 figures

The most stringent bounds on the absolute neutrino mass scale come from cosmological data. These bounds are made possible because massive relic neutrinos affect the expansion history of the universe and lead to a suppression of matter clustering on scales smaller than the associated free streaming length. However, the resulting effect on cosmological perturbations is relative to the primordial power spectrum of density perturbations from inflation, so freedom in the primordial power spectrum affects neutrino mass constraints. Using measurements of the cosmic microwave background, the galaxy power spectrum and the Hubble constant, we constrain neutrino mass and number of species for a model independent primordial power spectrum. Describing the primordial power spectrum by a 20-node spline, we find that the neutrino mass upper limit is a factor three weaker than when a power law form is imposed, if only CMB data are used. The primordial power spectrum itself is constrained to better than 10 % in the wave vector range $k \approx 0.01 - 0.25$ Mpc$^{-1}$. Galaxy clustering data and a determination of the Hubble constant play a key role in reining in the effects of inflationary freedom on neutrino constraints. The inclusion of both eliminates the inflationary freedom degradation of the neutrino mass bound, giving for the sum of neutrino masses $\Sigma m_\nu < 0.18$ eV (at 95 % confidence level, Planck+BOSS+$H_0$), approximately independent of the assumed primordial power spectrum model. When allowing for a free effective number of species, $N_{eff}$, the joint constraints on $\Sigma m_\nu$ and $N_{eff}$ are loosened by a factor 1.7 when the power law form of the primordial power spectrum is abandoned in favor of the spline parametrization., Comment: 17 pages, 8 figures; v2 accepted for publication in Phys Rev D, minor changes relative to v1

We reconstruct the gravitational lensing convergence signal from Cosmic Microwave Background (CMB) polarization data taken by the POLARBEAR experiment and cross-correlate it with Cosmic Infrared Background (CIB) maps from the Herschel satellite. From the cross-spectra, we obtain evidence for gravitational lensing of the CMB polarization at a statistical significance of 4.0$\sigma$ and evidence for the presence of a lensing $B$-mode signal at a significance of 2.3$\sigma$. We demonstrate that our results are not biased by instrumental and astrophysical systematic errors by performing null-tests, checks with simulated and real data, and analytical calculations. This measurement of polarization lensing, made via the robust cross-correlation channel, not only reinforces POLARBEAR auto-correlation measurements, but also represents one of the early steps towards establishing CMB polarization lensing as a powerful new probe of cosmology and astrophysics., Comment: 6 pages, 2 figures. v2: replaced with version accepted for publication in Physical Review Letters. The companion paper (arXiv:1312.6646) describes a measurement of the polarization lensing power spectrum

Our observations of the Universe are fundamentally anisotropic, with data from galaxies separated transverse to the line of sight coming from the same epoch while that from galaxies separated parallel to the line of sight coming from different times. Moreover, galaxy velocities along the line of sight change their redshift, giving redshift space distortions. We perform a full two-dimensional anisotropy analysis of galaxy clustering data, fitting in a substantially model independent manner the angular diameter distance D_A, Hubble parameter H, and growth rate ddelta/dln a without assuming a dark energy model. The results demonstrate consistency with LCDM expansion and growth, hence also testing general relativity. We also point out the interpretation dependence of the effective redshift z_eff, and its cosmological impact for next generation surveys., Comment: 12 pages, 14 figures

POLARBEAR-2 (PB-2) is a cosmic microwave background (CMB) polarization experiment for B-mode detection. The PB-2 receiver has a large focal plane and aperture that consists of 7588 transition edge sensor (TES) bolometers at 250 mK. The receiver consists of the optical cryostat housing reimaging lenses and infrared filters, and the detector cryostat housing TES bolometers. The large focal plane places substantial requirements on the thermal design of the optical elements at the 4K, 50K, and 300K stages. Infrared filters and lenses inside the optical cryostat are made of alumina for this purpose. We measure basic properties of alumina, such as the index of refraction, loss tangent and thermal conductivity. All results meet our requirements. We also optically characterize filters and lenses made of alumina. Finally, we perform a cooling test of the entire optical cryostat. All measured temperature values satisfy our requirements. In particular, the temperature rise between the center and edge of the alumina infrared filter at 50 K is only 2:0 ± 1:4 K. Based on the measurements, we estimate the incident power to each thermal stage. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

International audience; The Simons Array is an expansion of the POLARBEAR cosmic microwave background (CMB) polarization experiment currently observing from the Atacama Desert in Northern Chile. This expansion will create an array of three 3.5m telescopes each coupled to a multichroic bolometric receiver. The Simons Array will have the sensitivity to produce a ≥ 5σ detection of inationary gravitational waves with a tensor-to-scalar ratio r ≥ 0:01, detect the known minimum 58 meV sum of the neutrino masses with 3σ confidence when combined with a next-generation baryon acoustic oscillation measurement, and make a lensing map of large-scale structure over the 80% of the sky available from its Chilean site. These goals require high sensitivity and the ability to extract the CMB signal from contaminating astrophysical foregrounds; these requirements are met by coupling the three high-throughput telescopes to novel multichroic lenslet-coupled pixels each measuring CMB photons in both linear polarization states over multiple spectral bands. We present the status of this instrument already under construction, and an analysis of its capabilities.

The time delays between point-like images in gravitational lens systems can be used to measure cosmological parameters. The number of lenses with measured time delays is growing rapidly; the upcoming \emph{Large Synoptic Survey Telescope} (LSST) will monitor $\sim10^3$ strongly lensed quasars. In an effort to assess the present capabilities of the community to accurately measure the time delays, and to provide input to dedicated monitoring campaigns and future LSST cosmology feasibility studies, we have invited the community to take part in a "Time Delay Challenge" (TDC). The challenge is organized as a set of "ladders," each containing a group of simulated datasets to be analyzed blindly by participating teams. Each rung on a ladder consists of a set of realistic mock observed lensed quasar light curves, with the rungs' datasets increasing in complexity and realism. The initial challenge described here has two ladders, TDC0 and TDC1. TDC0 has a small number of datasets, and is designed to be used as a practice set by the participating teams. The (non-mandatory) deadline for completion of TDC0 was the TDC1 launch date, December 1, 2013. The TDC1 deadline was July 1 2014. Here we give an overview of the challenge, we introduce a set of metrics that will be used to quantify the goodness-of-fit, efficiency, precision, and accuracy of the algorithms, and we present the results of TDC0. Thirteen teams participated in TDC0 using 47 different methods. Seven of those teams qualified for TDC1, which is described in the companion paper II., referee's comments incorporated, to appear in ApJ; challenge data and instructions can be accessed at http://timedelaychallenge.org

Cosmic microwave background measurements have pushed to higher resolution, lower noise, and more sky coverage. These data enable a unique test of the early Universe's expansion rate and constituents such as effective number of relativistic degrees of freedom and dark energy. Using the most recent data from Planck and WMAP9, we constrain the expansion history in a model-independent manner from today back to redshift z=10(5). The Hubble parameter is mapped to a few percent precision, limiting early dark energy and extra relativistic degrees of freedom within a model-independent approach to 2%-16% and 0.71 equivalent neutrino species, respectively (95% C.L.). Within dark radiation, barotropic ether, and Doran-Robbers models, the early dark energy constraints are 3.3%, 1.9%, and 1.2%, respectively.

Cluster abundances are oddly insensitive to canonical early dark energy. Early dark energy with sound speed equal to the speed of light cannot be distinguished from a quintessence model with the equivalent expansion history for $zl2$ but negligible early dark energy density, despite the different early growth rate. However, cold early dark energy, with a sound speed much smaller than the speed of light, can give a detectable signature. Combining cluster abundances with cosmic microwave background (CMB) power spectra can determine the early dark energy fraction to 0.3% and distinguish a true sound speed of 0.1 from 1 at 99% confidence. We project constraints on early dark energy from the Euclid cluster survey, as well as the Dark Energy Survey, using both current and projected Planck CMB data, and assess the impact of cluster mass systematics. We also quantify the importance of dark energy perturbations, and the role of sound speed during a crossing of $w=\ensuremath{-}1$.

The growth rate of large scale structure can probe whether dark matter clusters at gravitational strength or deviates from this, e.g. due to self interactions. Measurement of the growth rate through redshift space distortions in galaxy redshift surveys constrains the clustering strength, and its redshift dependence. We compare such effects on growth to those from high redshift deviations (e.g. early dark energy) or modified gravity, and give a simple, highly accurate analytic prescription. Current observations can constrain the dark matter clustering strength to F_{cl}=0.99\pm0.02 of standard, if all other parameters are held fixed, but substantial covariances exist. Future galaxy redshift surveys may constrain an evolving clustering strength to 28%, marginalizing over the other parameters, or 4% if the dark energy parameters are held fixed while fitting for dark matter growth. Tighter constraints on the nature of dark matter could be obtained by combining cosmological and astrophysical probes., 8 pages, 5 figures; v2 matches JCAP version, minor changes, clearer Fig. 4

I.H. Park, S. Ahmad, P. Barrillon, S. Brandt, C. Budtz-Jorgensen, A.J. Castro-Tirado, P. Chen, Y.J. Choi, P. Connell, S. Dagoret-Campagne, C. Eyles, B. Grossan, M.–H.A. Huang, A. Jung, S. Jeong, J.E. Kim, M.B. Kim, S.-W. Kim, Y.W. Kim, A.S. Krasnov, J. Lee, H. Lim, E.V. Linder, T.–C. Liu, N. Lund, K.W. Min, G.W. Na, J.W. Nam, M.I. Panasyuk, J. Ripa, V. Reglero, J.M. Rodrigo, G.F. Smoot, J.E. Suh, S. Svertilov, N. Vedenkin, M.–Z. Wang, I. Yashin Ewha Womans University, Seoul, Korea University of Paris-Sud 11, Orsay, France Technical University of Denmark, Copenhagen, Denmark Instituto de Astrofisica de Andalucia CSIC, Granada, Spain National Taiwan University, Taipei, Taiwan Korea Advanced Institute of Science and Technology, Daejeon, Korea University of Valencia, Valencia, Spain University of California, Berkeley, USA National United University, Miao-Li, Taiwan Yonsei University, Seoul, Korea Moscow State University, Moscow, Russia E-mail: ipark@ewha.ac.kr

Wide area cosmological surveys enable investigation of whether dark energy properties are the same in different directions on the sky. Cosmic microwave background observations strongly restrict any dynamical effects from anisotropy, in an integrated sense. For more local constraints we compute limits from simulated distance measurements for various distributions of survey fields in a Bianchi I anisotropic universe. We then consider the effects of fitting for line of sight properties where isotropic dynamics is assumed (testing the accuracy through simulations) and compare sensitivities of observational probes for anisotropies, from astrophysical systematics as well as dark energy. We also point out some interesting features of anisotropic expansion in Bianchi I cosmology., Comment: 9 pages, 5 figures; v2 corrected equations, conclusions unchanged, matches PRD version

Extended gravity origins for cosmic acceleration can solve some fine tuning issues and have useful characteristics, but generally have little to say regarding the cosmological constant problem. Fab 5 gravity can be ghost free and stable, have attractor solutions in the past and future, and possess self tuning that solves the original cosmological constant problem. Here we show however it does not possess all these qualities at the same time. We also demonstrate that the self tuning is so powerful that it not only cancels the cosmological constant but also all other energy density, and we derive the scalings of its approach to a renormalized de Sitter cosmology. While this strong cancellation is bad for the late universe, it greatly eases early universe inflation., Comment: 6 pages, 2 figures; v2 minor change to match published version

This document presents the results from the Distances subgroup of the Cosmic Frontier Community Planning Study (Snowmass 2013). We summarize the current state of the field as well as future prospects and challenges. In addition to the established probes using Type IA supernovae and baryon acoustic oscillations, we also consider prospective methods based on clusters, active galactic nuclei, gravitational wave sirens and strong lensing time delays., Comment: Report from the "Dark Energy and CMB" working group for the American Physical Society's Division of Particles and Fields long-term planning exercise ("Snowmass"). Accepted for publication in Astroparticle Physics

Strong gravitational lensing of time variable sources such as quasars and supernovae creates observable time delays between the multiple images. Time delays can provide a powerful cosmographic probe through the "time delay distance" involving the ratio of lens, source, and lens-source distances. However, lightcurves of lensed images have measurement gaps, noise, systematics such as microlensing from substructure along an image line of sight, and no a priori functional model, making robust time delay estimation challenging. Using Gaussian process techniques, we demonstrate success in accurate blind reconstruction of time delays and reduction in uncertainties for real data., Comment: 6 pages, 3 figures; v2 improved results using damped random walk kernel, matches PRD version

The effective field theory of cosmic acceleration systematizes possible contributions to the action, accounting for both dark energy and modifications of gravity. Rather than making model dependent assumptions, it includes all terms, subject to the required symmetries, with four (seven) functions of time for the coefficients. These correspond respectively to the Horndeski and general beyond Horndeski class of theories. We address the question of whether this general systematization is actually effective, i.e. useful in revealing the nature of cosmic acceleration when compared with cosmological data. The answer is no and yes: {\it there is no simple time dependence of the free functions} -- assumed forms in the literature are poor fits, but one can derive some general characteristics in early and late time limits. For example, we prove that the gravitational slip must restore to general relativity in the de Sitter limit of Horndeski theories, and why it doesn't more generally. We also clarify the relation between the tensor and scalar sectors, and its important relation to observations; in a real sense the expansion history $H(z)$ or dark energy equation of state $w(z)$ is $1/5$ or less of the functional information! In addition we discuss the de Sitter, Horndeski, and decoupling limits of the theory utilizing Goldstone techniques., Comment: 16+5 pages, 7 figures

The discovery that the expansion of the Universe is accelerating opens up new frontiers for our understanding of cosmology and particle physics. The nature of the dark energy responsible is intimately tied to the high energy theory and gravitation. Measuring the properties of the accelerating universe and studying both the dark energy and the dark matter of the Universe using supernovae and weak gravitational lensing is the primary aim of the Supernova/Acceleration Probe (SNAP). SNAP can discover and follow thousands of Type Ia supernovae at redshifts z = 0.3 − 1.7. The resulting magnitude-redshift relation can determine the cosmological and dark energy parameters with high precision: the dark energy equation of state w to 0.05 and its time variation w′ = dw dz to ±0.15. Wide area weak gravitational lensing studies will map the distribution of dark matter in the universe.

This white paper describes the LSST Dark Energy Science Collaboration (DESC), whose goal is the study of dark energy and related topics in fundamental physics with data from the Large Synoptic Survey Telescope (LSST). It provides an overview of dark energy science and describes the current and anticipated state of the field. It makes the case for the DESC by laying out a robust analytical framework for dark energy science that has been defined by its members and the comprehensive three-year work plan they have developed for implementing that framework. The analysis working groups cover five key probes of dark energy: weak lensing, large scale structure, galaxy clusters, Type Ia supernovae, and strong lensing. The computing working groups span cosmological simulations, galaxy catalogs, photon simulations and a systematic software and computational framework for LSST dark energy data analysis. The technical working groups make the connection between dark energy science and the LSST system. The working groups have close linkages, especially through the use of the photon simulations to study the impact of instrument design and survey strategy on analysis methodology and cosmological parameter estimation. The white paper describes several high priority tasks identified by each of the 16 working groups. Over the next three years these tasks will help prepare for LSST analysis, make synergistic connections with ongoing cosmological surveys and provide the dark energy community with state of the art analysis tools. Members of the community are invited to join the LSST DESC, according to the membership policies described in the white paper. Applications to sign up for associate membership may be made by submitting the Web form at http://www.slac.stanford.edu/exp/lsst/desc/signup.html with a short statement of the work they wish to pursue that is relevant to the LSST DESC.

Cosmic microwave background (CMB) polarization encodes information not only on the early universe but also dark energy, neutrino mass, and gravity in the late universe through CMB lensing. Ground-based surveys, such as ACTpol, PolarBear, and SPTpol, significantly complement cosmological constraints from the Planck satellite, strengthening the CMB dark energy figure of merit and neutrino mass constraints by factors of 3--4. This changes the dark energy probe landscape. We evaluate the state of knowledge in 2017 from ongoing experiments including dark energy surveys (supernovae, weak lensing, galaxy clustering), fitting for dynamical dark energy, neutrino mass, and a modified gravitational growth index. Adding a modest strong lensing time delay survey improves those dark energy constraints by a further 32%, and an enhanced low-redshift supernova program improves them by 26%.

Gravity directs the paths of light rays and the growth of structure. Moreover, gravity on cosmological scales does not simply point down: it accelerates the universal expansion by pulling outward, either due to a highly negative pressure dark energy or an extension of general relativity. We examine methods to test the properties of gravity through cosmological measurements. We then consider specific possibilities for a sound gravitational theory based on the Galileon shift symmetry. The evolution of the laws of gravity from the early universe to the present acceleration to the future fate -- the paths of gravity -- carries rich information on this fundamental force of physics and on the mystery of dark energy., Comment: 8 pages, 2 new figures

We examine model independent constraints on the high redshift and prerecombination expansion history from cosmic microwave background observations, using a combination of principal component analysis and other techniques. This can be translated to model independent limits on early dark energy and the number of relativistic species $N_{\rm eff}$. Models such as scaling (Doran-Robbers), dark radiation ($\Delta N_{\rm eff}$), and barotropic aether fall into distinct regions of eigenspace and can be easily distinguished from each other. Incoming CMB data will map the expansion history from $z=0$--$10^5$, achieving subpercent precision around recombination, and enable determination of the amount of early dark energy and valuable guidance to its nature., Comment: 11 pages, 8 figures

Gaussian processes provide a method for extracting cosmological information from observations without assuming a cosmological model. We carry out cosmography -- mapping the time evolution of the cosmic expansion -- in a model-independent manner using kinematic variables and a geometric probe of cosmology. Using the state of the art supernova distance data from the Union2.1 compilation, we constrain, without any assumptions about dark energy parametrization or matter density, the Hubble parameter and deceleration parameter as a function of redshift. Extraction of these relations is tested successfully against models with features on various coherence scales, subject to certain statistical cautions., Comment: 9 pages, 5 figures; v2 minor changes to match PRD version

We use Gaussian Processes to map the expansion history of the universe in a model independent manner from the Union2.1 supernovae data and then apply these reconstructed results to solve for the growth history. By comparing this to BOSS and WiggleZ large scale structure data we examine whether growth is determined wholly by expansion, with the measured gravitational growth index testing gravity without assuming a model for dark energy. A further model independent analysis looks for redshift dependent deviations of growth from the general relativity solution without assuming the growth index form. Both approaches give results consistent with general relativity., Comment: 5 pages, 3 figures; v2 matches PRD version

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.

This report is based on the BigBOSS proposal submission to NOAO in October 2010, and reflects the project status at that time with minor updates; BigBOSS is a Stage IV ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with a wide-area galaxy and quasar redshift survey over 14,000 square degrees. It has been conditionally accepted by NOAO in response to a call for major new instrumentation and a high-impact science program for the 4-m Mayall telescope at Kitt Peak. The BigBOSS instrument is a robotically-actuated, fiber-fed spectrograph capable of taking 5000 simultaneous spectra over a wavelength range from 340 nm to 1060 nm, with a resolution R = 3000-4800. Using data from imaging surveys that are already underway, spectroscopic targets are selected that trace the underlying dark matter distribution. In particular, targets include luminous red galaxies (LRGs) up to z = 1.0, extending the BOSS LRG survey in both redshift and survey area. To probe the universe out to even higher redshift, BigBOSS will target bright [OII] emission line galaxies (ELGs) up to z = 1.7. In total, 20 million galaxy redshifts are obtained to measure the BAO feature, trace the matter power spectrum at smaller scales, and detect redshift space distortions. BigBOSS will provide additional constraints on early dark energy and on the curvature of the universe by measuring the Ly-alpha forest in the spectra of over 600,000 2.2 < z < 3.5 quasars. BigBOSS galaxy BAO measurements combined with an analysis of the broadband power, including the Ly-alpha forest in BigBOSS quasar spectra, achieves a FOM of 395 with Planck plus Stage III priors. This FOM is based on conservative assumptions for the analysis of broad band power (kmax = 0.15), and could grow to over 600 if current work allows us to push the analysis to higher wave numbers (kmax = 0.3). BigBOSS will also place constraints on theories of modified gravity and inflation, and will measure the sum of neutrino masses to 0.024 eV accuracy.

We examine the dark energy and matter densities allowed by precision measurements of distances out to various redshifts, in the presence of spatial curvature and (near) arbitrary behavior of the dark energy equation of state. Degeneracies among the parameters permit a remarkably large variation in their values when using only distance measurements of the late time universe and making no assumptions about the dark energy or curvature. Going beyond distance measurements to a lower limit on the growth of structure bounds the allowed region significantly but still leaves considerable freedom to match a flat LCDM model with dark energy very different from a cosmological constant. The combination of distances with Hubble parameter, gravitational lensing or other large scale structure data is essential to determining robustly the cosmological model., Comment: 6 pages, 3 figures, minor changes, results unchanged, matched Phys. Rev. D published version

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., Comment: 8 pages, 5 figures

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., Comment: 10 pages, 7 figures

Cosmic acceleration can be achieved not only with a sufficiently flat scalar field potential but through kinetic terms coupled to gravity. These derivative couplings impose a shift symmetry on the scalar field, aiding naturalness. We write the most general purely kinetic action not exceeding mass dimension six and obeying second order field equations. The result reduces to a simple form involving a coupling of the Einstein tensor with the kinetic term and can be interpreted as adding a new term to Galileon gravity in curved spacetime. We examine the cosmological implications of the effective dark energy and classify the dynamical attractor solutions, finding a quasistable loitering phase mimicking late time acceleration by a cosmological constant., Comment: 7 pages, 3 figures

Time delays in strong gravitational lensing systems possess significant complementarity with distance measurements to determine the dark energy equation of state, as well as the matter density and Hubble constant. Time delays are most useful when observations permit detailed lens modeling and variability studies, requiring high resolution imaging, long time monitoring, and rapid cadence. We quantify the constraints possible between a sample of 150 such time delay lenses and a near term supernova program, such as might become available from an Antarctic telescope such as KDUST and the Dark Energy Survey. Adding time delay data to supernovae plus cosmic microwave background information can improve the dark energy figure of merit by almost a factor 5 and determine the matter density \Omega_m to 0.004, Hubble constant h to 0.7%, and dark energy equation of state time variation w_a to 0.26, systematics permitting., Comment: 6 pages, 3 figures; v2 added detail on potential time delay, matches PRD version

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., Comment: 6 pages, 1 figure

Cosmological and astrophysical measurements provide powerful constraints on neutrino masses complementary to those from accelerators and reactors. Here we provide a guide to these different probes, for each explaining its physical basis, underlying assumptions, current and future reach., 11 pages

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., We gratefully acknowledge a UC-MEXUS-CONACYT Visiting Fellowship for JLCC to spend a sabbatical at Berkeley. JLCC thanks the Berkeley Center for Cosmological Physics for hospitality and CONACYT for grant No. 84133-F. This work has been supported in part by the Director, Office of Science, Office of High Energy Physics, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and the World Class University grant R32-2009-000-10130-0 through the National Research Foundation, Ministry of Education, Science and Technology of Korea.

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., Comment: 13 pages; 11 figures; updated to published version

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%., 6 pages, 5 figures; v2 corrected typo in Eq. 8