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The 2nd International Conference "Exploring the Energetic Universe 2019"

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https://testres.nu.edu.kz/handle/123456789/4005

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Open Access
Accelerated Parameter Estimation with DALE X
(NURIS; Energetic Cosmos Laboratory, 2017-05-08) Daniel, Scott F.; Linder, Eric V.
We consider methods for improving the estimation of constraints on a high-dimensional parameter space with a computationally expensive likelihood function. In such cases Markov chain Monte Carlo (MCMC) can take a long time to converge and concentrates on finding the maxima rather than the often-desired confidence con-tours for accurate error estimation. We employ DALEχ(Direct Analysis of Limits via the Exterior ofχ2) for determining confidence contours by minimizing a cost function parametrized to incentivize points in parameter space which are both on the confidence limit and far from previously sampled points. We compare DALEχ to the nested sampling algorithm implemented in MultiNest on a toy likelihood function that is highly non-Gaussian and non-linear in the mapping between parameter values and χ2. We find that in high-dimensional cases DALEχfinds the same confidence limit as Multi-Nest using roughly an order of magnitude fewer evaluations of the likelihood function.DALE χ is open-source and available athttps://github.com/danielsf/Dalex.git.
Open Access
The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: Constraining modified gravity
(American Physical Society, 2016-12-02) Mueller, Eva-Maria; Percival, Will; Linder, Eric; Alam, Shadab; Zhao, Gong-Bo; Sanchez, Ariel G.; Beutler, Florian
We use baryon acoustic oscillation and redshift space distortion from the completed Baryon Oscillation Spectroscopic Survey, corresponding to data release 12 of the Sloan Digital Sky Survey, combined sample analysis in combination with the cosmic microwave background, supernova and redshift space distortion measurements from additional spectroscopic surveys to test deviations from general relativity. We present constraints on several phenomenological models of modified gravity.
Open Access
Constraining dark energy dynamics in extended parameter space
(American Physical Society, 2017-07-19) Di Valentino, Eleonora; Melchiorri, Alessandro; Linder, Eric V.; Silk, Joseph
Dynamical dark energy has been recently suggested as a promising and physical way to solve the 3 sigma tension on the value of the Hubble constant H0 between the direct measurement of Riess et al. (2016) (R16, hereafter) and the indirect constraint from cosmic microwave anisotropies obtained by the Planck satellite under the assumption of a ΛCDM model. In this paper, by parametrizing dark energy evolution using the w0−wa approach, and considering a 12 parameter extended scenario, we find that: (a) the tension on the Hubble constant can indeed be solved with dynamical dark energy, (b) a cosmological constant is ruled out at more than 95% c.l. by the Planck+R16 dataset, and (c) all of the standard quintessence and half of the “downward going” dark energy model space (characterized by an equation of state that decreases with time) is also excluded at more than 95% c.l. These results are further confirmed when cosmic shear, CMB lensing, or SN Ia luminosity distance data are also included. The best fit value of the χ2 for the Planck+R16 data set improves by Δχ2=−12.9 when moving to 12 parameters respect to standard ΛCDM. However, tension remains with the BAO dataset. A cosmological constant and small portion of the freezing quintessence models are still in agreement with the Planck+R16+BAO data set at between 68% and 95% c.l. Conversely, for Planck plus a phenomenological H0 prior, both thawing and freezing quintessence models prefer a Hubble constant of less than 70 km/s/Mpc. The general conclusions hold also when considering models with nonzero spatial curvature.
Open Access
Cosmic Curvature Tested Directly from Observations
(NURIS; Energetic Cosmos Laboratory, 2018-02-13) Denissenya, Mikhail; Linder, Eric V.; Shafieloo, Arman
Cosmic spatial curvature is a fundamental geometric quantity of the Universe. We investigate a model independent, geometric approach to measure spatial curvature directly from observations,without any derivatives of data. This employs strong lensing time delays and supernova distance measurements to measure the curvature itself, rather than just testing consistency with flatness.We define two curvature estimators, with differing error propagation characteristics, that can cross-check each other, and also show how they can be used to map the curvature in redshift slices, totest constancy of curvature as required by the Robertson-Walker metric. Simulating realizations ofredshift distributions and distance measurements of lenses and sources, we estimate uncertainties onthe curvature enabled by next generation measurements. Theresults indicate that the model independent methods, using only geometry without assuming forms for the energy density constituents,can determine the curvature at the∼6×10^−3 level.
Open Access
Cosmic Growth Signatures of Modified Gravitational Strength
(NURIS; Energetic Cosmos Laboratory, 2018-09-05) Denissenya, Mikhail; Linder, Eric V.
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 ina 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∼0.3% 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
Open Access
Dark Energy from α-attractors: Phenomenology and Observational Constraints
(NURIS; Energetic Cosmos Laboratory, 2018-03-01) Garcia-Garcia, Carlos; Linder, Eric V.; Ruiz-Lapuente, Pilar; Zumalacaregui, Miguel
The possibility of linking inflation and late cosmic accelerated expansion using the α-attractor models has received increasing attention due to their physical motivation. In the early universe, α-attractors provide an inflationary mechanism compatible with Planck satellite CMB observations and predictive for future gravitational wave CMB modes. Additionally α-attractors can be written as quintessence models with a potential that connects a power law regime with a plateau or uplifted exponential, allowing a late cosmic accelerated expansion that can mimic behavior near a cosmological constant. In this paper we study a generalized dark energy α-attractor model. We thoroughly investigate its phenomenology, including the role of all model parameters and the possibility of large-scale tachyonic instability clustering. We verify the relation that 1+w∼1/α (while the gravitational wave power r∼α) so these models predict that a signature should appear in either the primordial B-modes or in late time deviation from a cosmological constant. We constrain the model parameters with current datasets, including the cosmic microwave background (Planck 2015 compressed likelihood), baryon acoustic oscillations (BOSS DR12) and supernovae (Pantheon compressed). Our results show that expansion histories close to a cosmological constant exist in large regions of the parameter space, not requiring a fine-tuning of the parameters or initial conditions.
Open Access
Debiasing Cosmic Gravitational Wave Sirens [Article]
(NURIS; Energetic Cosmos Laboratory, 2019-05-27) Keeley, Ryan E.; Shafieloo, Arman; L'Huillier, Benjamin; Linder, Eric V.
Accurate estimation of the Hubble constant, and other cosmological parameters, from distances measured by cosmic gravitational wave sirens requires sufficient allowance for the dark energy evolution. We demonstrate how model independent statistical methods, specifically Gaussian process regression, can remove bias in the reconstruction of H(z), and can be combined model independently with supernova distances.
Open Access
Developments in Cosmic Growth and Gravitation [Article]
(NURIS; Energetic Cosmos Laboratory, 2019-04-23) Linder, Eric V.
Cosmic surveys of large scale structure have imaged hundreds of millions of galaxies and mapped the 3D positions of over a million. Surveys starting over the next few years will increase these numbers more than tenfold. Simultaneously, developments in extracting information on dark energy, dark matter, neutrinos, and gravity on cosmic scales have advanced greatly, with many important works from Asian institutions.
Open Access
The End of Cosmic Growth
(NURIS; Energetic Cosmos Laboratory, 2018-10-24) Linder, Eric V.; Polarski, David
The growth of large scale structure is a battle between gravitational attraction and cosmic acceleration. We investigate the future behavior of cosmic growth under both general relativity (GR)and modified gravity during prolonged acceleration, deriving analytic asymptotic behaviors and showing that gravity generally loses and growth ends. We also note the “why now” problem is equally striking when viewed in terms of the shut down of growth. For many models inside GR the gravitational growth indexγalso shows today as a unique time between constant behavior in the past and a higher asymptotic value in the future. Interestingly, while f(R) models depart in this respect dramatically from GR today and in the recent past, their growth indices are identical in the asymptotic future and past.
Open Access
Eternal and Evanescent Black Holes: It’s All Done With Mirrors
(NURIS; Energetic Cosmos Laboratory, 2017-11-27) Good, Michael R.R.; Linder, Eric V.
The analogy between black hole radiation and accelerating mirror radiation (the dynamical Casimir effect) is particularly strong for mirror trajectories giving rise to a constant thermal flux of particles. We present new ways to achieve such thermal plateaus, and customize their finite, semi-infinite, and eternal presence, corresponding to forming/collapsing, complete-evaporation/remnants,and eternal black holes. We find simple expressions for the energy flux in terms of the mirror rapidity as a function of proper time and null time.
Open Access
Finite Energy but Infinite Entropy Production from Moving Mirrors
(NURIS; Energetic Cosmos Laboratory, 2018-07-20) Good, Michael R.R.; Linder, Eric V.
Accelerating mirrors provide a simple conceptual laboratory for studying particle production and the relation between trajectory and particle, energy, and entropy fluxes. We focus on the relation between energy and entropy, studying some special cases with finite total energy but infinite integrated entropy (though the entropy flux may be finite at any particular moment). We present a new asymptotically static moving mirror trajectory with solvable beta Bogolyubov coefficients, total energy and fully relativistic particle count. The integrated entropy diverges despite finite global radiative particle and energy emission. Another class of models includes exponentially accelerated mirrors in proper time; one of its unexpected behaviors is finite energy emission but divergent entropy. We compare mirrors exponentially accelerated in other coordinates as well, showing their close relation and an interesting duality property.
Open Access
Gravity’s Islands: Parametrizing Horndeski Stability
(NURIS; Energetic Cosmos Laboratory, 2018-07-31) Denissenya, Mikhail; Linder, Eric V.
Cosmic acceleration may be due to modified gravity, with effective field theory or property functions describing the theory. Connection to cosmological observations through practical parametrization of these functions is difficult and also faces the issue that not all assumed time dependence or parts of parameter space give a stable theory. We investigate the relation between parametrization and stability in Horndeski gravity, showing that the results are highly dependent on the function parametrization. This can cause misinterpretations of cosmological observations, hiding and even ruling out key theoretical signatures. We discuss approaches and constraints that can be placed on the property functions and scalar sound speed to preserve some observational properties, but find that parametrizations closest to the observations, e.g. in terms of the gravitational strengths, offer more robust physical interpretations. In addition we present an example of how future observations of the B-mode polarization of the cosmic microwave background from primordial gravitational wave scan probe different aspects of gravity
Open Access
GW170817 Most Likely Made a Black Hole
(NURIS; Energetic Cosmos Laboratory, 2018-05-14) Pooley, David; Kumar, Pawan; Wheeler, J. Craig; Grossan, Bruce
There are two outstanding issues regarding the neutron-star merger event GW170817: the nature of the compact remnant and the interstellar shock. The mass of the remnant of GW170817, ∼2.7 M⊙, implies the remnant could be either a massive, rotating, neutron star, or a black hole. We report Chandra Director's Discretionary Time observations made in 2017 December and 2018 January, and we reanalyze earlier observations from 2017 August and 2017 September, in order to address these unresolved issues. We estimate the X-ray flux from a neutron star remnant and compare that to the measured X-ray flux. If we assume that the spin-down luminosity of any putative neutron star is converted to pulsar wind nebula X-ray emission in the 0.5-8 keV band with an efficiency of 10−3, for a dipole magnetic field with 3×1011 G < B < 1014 G, a rising X-ray signal would result and would be brighter than that observed by day 107, we therefore conclude that the remnant of GW170817 is most likely a black hole. Independent of any assumptions of X-ray efficiency, however, if the remnant is a rapidly-rotating, magnetized, neutron star, the total energy in the external shock should rise by a factor ∼102 (to ∼1052 erg) after a few years, therefore, Chandra observations over the next year or two that do not show substantial brightening will rule out such a remnant. The same observations can distinguish between two different models for the relativistic outflow, either an angular or radially varying structure.
Open Access
Joining bits and pieces of reionization history
(NURIS; Energetic Cosmos Laboratory, 2019-04-02) Hazra, Dhiraj Kumar; Paoletti, Daniela; Finelli, Fabio; Smoot, George F.
Cosmic Microwave Background temperature and polarization anisotropies from Planck have estimated lower value of the optical depth to reionization (τ) compared to WMAP.
Open Access
Model independent expansion history from supernovae: Cosmology versus systematics
(NURIS; Energetic Cosmos Laboratory, 2019-03-05) L'Huillier, Benjamin; Shafieloo, Arman; Linder, Eric V.; Kim, Alex G.
We examine the Pantheon supernovae distance data compilation in a model independent analysis to test the validity of cosmic history reconstructions beyond the concordance ΛCDM cosmology. Strong deviations are allowed by the data at z≳1 in the reconstructed Hubble parameter, Om diagnostic, and dark energy equation of state. We explore three interpretations: 1) possibility of the true cosmology being far from ΛCDM, 2) supernovae property evolution, and 3) survey selection effects.
Open Access
Modified Gravity Away from a Lambda CDM Background [Article]
(NURIS; Energetic Cosmos Laboratory, 2019-05-01) Brando, Guilherme; Falciano, Felipe T.; Linder, Eric V.; Velten, Hermano E. S.
Within the effective field theory approach to cosmic acceleration, the background expansion can be specified separately from the gravitational modifications. We explore the impact of modified gravity in a background different from a cosmological constant plus cold dark matter (ΛCDM) on the stability and cosmological observables, including covariance between gravity and expansion parameters. In No Slip Gravity the more general background allows more gravitational freedom, including both positive and negative Planck mass running. We examine the effects on cosmic structure growth, as well as showing that a viable positive integrated Sachs-Wolfe effect crosscorrelation easily arises from this modified gravity theory.
Open Access
No Run Gravity[Article]
(NURIS; Energetic Cosmos Laboratory, 2019-03-05) Linder, Eric V.
Considering the dark energy/gravity landscape if next generation surveys of galaxies, cosmic microwave background radiation, and gravitational waves do not find clear modification of gravity, we develop No Run Gravity as a counterexample to the conclusion that this would imply general relativity with an expansion history described by an equation of state w(z). No Run Gravity is a cubic Horndeski theory with a constant Planck mass, no gravitational slip, and no modification of gravitational waves, but a rich phenomenology beyond w(z). We calculate the evolution of gravitational strength, sound speed, and cosmic growth within the theory and project sensitivities for upcoming DESI redshift space distortion data.
Open Access
No Slip CMB
(NURIS; Energetic Cosmos Laboratory, 2018-10-29) Brush, Micah; Linder, Eric V.; Zumalacárregui, Miguel
No Slip Gravity is a simple modified gravity theory with only one free function and the interesting characteristic that -- unlike many modified gravity theories -- it suppresses growth. This allows it to fit current redshift space distortion and σ8 mass fluctuation amplitude data better than ΛCDM in general relativity, while retaining a ΛCDM background expansion and hence distances. Since it has no gravitational slip it alters equally CMB lensing and matter density growth, and in addition affects the CMB gravitational wave B-mode polarization power spectrum. We investigate and compute the effects of No Slip Gravity for CMB lensing and B-modes, and present a simple analytic approximation. Using a Monte Carlo analysis, we place constraints on the theory from current CMB data.
Open Access
No Slip Gravity
(NURIS; Energetic Cosmos Laboratory, 2018-01-04) Linder, Eric V.
A subclass of the Horndeski modified gravity theory we call No Slip Gravity has particularly interesting properties: 1) a speed of gravitational wave propagation equal to the speed of light, 2) equality between the effective gravitational coupling strengths to matter and light, G matter and G light, hence no slip between the metric potentials, yet difference from Newton's constant, and 3) suppressed growth to give better agreement with galaxy clustering observations. We explore the characteristics and implications of this theory, and project observational constraints. We also give a simple expression for the ratio of the gravitational wave standard siren distance to the photon standard candle distance, in this theory and others, and enable a direct comparison of modified gravity in structure growth and in gravitational waves, an important crosscheck.
Open Access
On the Use of Fast Radio Burst Dispersion Measures as Distance Measures[Article]
(NURIS; Energetic Cosmos Laboratory, 2019-03-19) Kumar, Pawan; Linder, Eric V.
Fast radio bursts appear to be cosmological signals whose frequency-time structure provides a dispersion measure. The dispersion measure is a convolution of the cosmic distance element and the electron density, and contains the possibility of using these events as new cosmological distance measures. We explore the challenges of extracting the distance in a robust manner, and estimate the systematics control needed for fast radio bursts to become a competitive distance probe. The methodology can also be applied to their use for mapping electron density fluctuations or helium reionization.