Gamma-ray burst distances and the timescape cosmology

Mon. Not. R. Astron. Soc.

2016

We characterize a cosmic rest frame in which the monopole variation of the spherically averaged nonlinear Hubble expansion is most uniform, under arbitrary local Lorentz boosts of the central observer. Using the COMPOSITE sample of 4534 galaxies, we identify a degenerate set of candidate minimum nonlinear variation frames, which includes the rest frame of the Local Group (LG) of galaxies, but excludes the standard Cosmic Microwave Background (CMB) frame. Candidate rest frames defined by a boost from the LG frame close to the plane of the galaxy have a statistical likelihood similar to the LG frame. This may result from a lack of constraining data in the Zone of Avoidance. We extend our analysis to the Cosmicflows-2 (CF2) sample of 8162 galaxies. While the signature of a systematic boost offset between the CMB and LG frame averages is still detected, the spherically averaged nonlinear expansion variation in all rest frames is significantly larger in the CF2 sample than would be reasonably expected. We trace this to the CF2 distances being reported without a correction for inhomogeneous distribution Malmquist bias. Systematic differences in the inclusion of the large SFI++ subsample into the COMPOSITE and CF2 catalogues are analysed. Our results highlight the importance of a careful treatment of Malmquist biases for future peculiar velocities studies, including tests of the hypothesis of [Phys. Rev. D, 88, 083529] that a significant fraction of the CMB temperature dipole may be nonkinematic in origin.

Section: Discussionmentioning

confidence: 99%

Defining the frame of minimum non-linear Hubble expansion variation

McKay

Mon. Not. R. Astron. Soc.

2016

“…The timescape model has passed a number of independent observational tests (Leith, Ng & Wiltshire 2008;Smale & Wiltshire 2011;Smale 2011;Duley, Nazer & Wiltshire 2013;Nazer & Wiltshire 2015). Its distance-redshift relation is very close to that of particular Λ cold dark matter (ΛCDM) models over small redshift ranges, but effectively interpolates (Wiltshire 2009(Wiltshire , 2014 between spatially flat ΛCDM cosmologies with different values of Ω M 0 and Ω Λ0 over larger redshift ranges.…”

Section: Inhomogeneous Alternatives To Dark Energymentioning

confidence: 95%

Apparent cosmic acceleration from Type Ia supernovae

Dam

Heinesen

Monthly Notices of the Royal Astronomical Society

2017

Parameters that quantify the acceleration of cosmic expansion are conventionally determined within the standard Friedmann-Lemaître-Robertson-Walker (FLRW) model, which fixes spatial curvature to be homogeneous. Generic averages of Einstein's equations in inhomogeneous cosmology lead to models with non-rigidly evolving average spatial curvature, and different parametrizations of apparent cosmic acceleration. The timescape cosmology is a viable example of such a model without dark energy. Using the largest available supernova data set, the JLA catalogue, we find that the timescape model fits the luminosity distance-redshift data with a likelihood that is statistically indistinguishable from the standard spatially flat Λ cold dark matter cosmology by Bayesian comparison. In the timescape case cosmic acceleration is nonzero but has a marginal amplitude, with best-fitting apparent deceleration parameter, q 0 = −0.043−0.000 . Systematic issues regarding standardization of supernova light curves are analysed. Cuts of data at the statistical homogeneity scale affect light curve parameter fits independent of cosmology. A cosmological model dependence of empirical changes to the mean colour parameter is also found. Irrespective of which model ultimately fits better, we argue that as a competitive model with a non-FLRW expansion history, the timescape model may prove a useful diagnostic tool for disentangling selection effects and astrophysical systematics from the underlying expansion history.Key words: cosmological parameters -dark energy -cosmology: observationscosmology: theory INHOMOGENEOUS ALTERNATIVES TO DARK ENERGYOne of the most important discoveries in cosmology (Riess et al. 1998;Perlmutter et al. 1999) is the observation that the luminosity distances and redshifts of type Ia supernovae (SneIa) are well matched to the expansion history of a spatial homogeneous and isotropic Friedmann-Lemaître-Robertson-Walker (FLRW) model only if the Universe began an epoch of accelerated expansion late in cosmic history. Since gravity with matter obeying the Strong Energy Condition is universally attractive, this demands a cosmological constant, Λ, or some other unknown source of spatially homogeneous dark energy with an equation of state, P = wρc 2 , that subsequent tests find to be close to the cosmological constant case, w = −1. Independently of SneIa, since the late 1990s other data sets including the cosmic microwave background (CMB) and galaxy clustering statistics, have been found to independently require late epoch cosmic acceleration in the FLRW model.

“…After a variety of observational tests [5][6][7][8] the timescape model remains a viable alternative to the standard model. On large scales its expansion history is so close to that of the standard model that differences in luminosity distances are at the level of current systematic uncertainties in type Ia supernova data [6]; which model fits better depends on the method by which supernova light curves are reduced.…”

Section: Introductionmentioning

confidence: 99%

Cosmic microwave background anisotropies in the timescape cosmology

Nazer

2015

Phys. Rev. D

We analyze the spectrum of cosmic microwave background (CMB) anisotropies in the timescape cosmology: a potentially viable alternative to homogeneous isotropic cosmologies without dark energy. We exploit the fact that the timescape cosmology is extremely close to the standard cosmology at early epochs to adapt existing numerical codes to produce CMB anisotropy spectra, and to match these as closely as possible to the timescape expansion history. A variety of matching methods are studied and compared. We perform Markov chain Monte Carlo analyses on the parameter space, and fit CMB multipoles 50 ≤ ℓ ≤ 2500 to the Planck satellite data. Parameter fits include a dressed Hubble constant, H 0 = 61.0 km sec −1 Mpc −1 (±1.3% stat) (±8% sys), and a present void volume fraction fv0 = 0.627 (±2.3% stat) (±13% sys). We find best fit likelihoods which are comparable to that of the best fit ΛCDM cosmology in the same multipole range. In contrast to earlier results, the parameter constraints afforded by this analysis no longer admit the possibility of a solution to the primordial lithium abundance anomaly. This issue is related to a strong constraint between the ratio of baryonic to nonbaryonic dark matter and the ratio of heights of the second and third acoustic peaks, which cannot be changed as long as the standard cosmology is assumed up to the surface of last scattering. These conclusions may change if backreaction terms are also included in the radiation-dominated primordial plasma. Physical Review D 91, 063519 (2015).