We consider a low redshift (z < 0.7) cosmological data set comprising megamasers, cosmic chronometers, type Ia supernovae and baryon acoustic oscillations, which we bin according to their redshift. For each bin, we read the value of H 0 by fitting directly to the flat ΛCDM model. Doing so, we find that H 0 descends with redshift, allowing one to fit a line with a nonzero slope of statistical significance 2.1σ. Our analysis rests on the use of cosmic chronometers to break a degeneracy in baryon acoustic oscillations data and it will be imperative to revisit this feature as data improves. Nevertheless, our results provide the first independent indication of the descending trend reported by the H0LiCOW Collaboration. If substantiated going forward, early Universe solutions to the Hubble tension will struggle explaining this trend.
An anisotropic measurement of the baryon acoustic oscillation (BAO) feature fixes the product of the Hubble constant and the acoustic scale H 0 r d . Therefore, regardless of the dark energy dynamics, to accommodate a higher value of H 0 one needs a lower r d and so necessarily a modification of early time cosmology. One must either reduce the age of the Universe at the drag epoch or else the speed of sound in the primordial plasma. The first can be achieved, for example, with dark radiation or very early dark energy, automatically preserving the angular size of the acoustic scale in the Cosmic Microwave Background (CMB) with no modifications to post-recombination dark energy. However it is known that the simplest such modifications fall afoul of CMB constraints at higher multipoles. As an example, we combine anisotropic BAO with geometric measurements from strong lensing time delays from H0LiCOW and megamasers from the Megamaser Cosmology Project to measure r d , with and without the local distance ladder measurement of H 0 . We find that the best fit value of r d is indeed quite insensitive to the dark energy model, and is also hardly affected by the inclusion of the local distance ladder data.
Assuming that the Universe at higher redshifts (z ∼ 4 and beyond) is consistent with ΛCDM model as constrained by the Planck measurements, we reanalyze the low redshift cosmological data to reconstruct the Hubble parameter as a function of redshift. This enables us to address the H 0 and other tensions between low z observations and high z Planck measurement from CMB. From the reconstructed H(z), we compute the energy density for the "dark energy" sector of the Universe as a function of redshift without assuming a specific model for dark energy. We find that the dark energy density has a minimum for certain redshift range and that the value of dark energy at this minimum ρ min DE is negative. This behavior can most simply be described by a negative cosmological constant plus an evolving dark energy component. We discuss possible theoretical and observational implications of such a scenario.Key words: cosmology: theory -large-scale structure of Universe -cosmology: diffuse radiation -cosmology: Dark energy INTRODUCTIONThanks to various sets of cosmological data, we can now talk about "the standard model of cosmology", the ΛCDM Universe Ade et al. (2016). It provides the simplest paradigm that fits remarkably well to most of the current cosmological observations. As the precision of the low redshift data increases, there are however emerging tensions in ΛCDM model which is otherwise consistent with high redshift CMB observations by Planck. The major tension is between the model independent measurement of H0 parameter (∼ 73 km/s/Mpc) by SH0ES collaboration Riess et al. (2016, 2018) and that by Planck assuming ΛCDM model Ade et al. (2016); Y. Akrami et al. (2018). The latest Planck-2018 data shows, H0 = 67.8 ± 0.9 km/s/Mpc Y. Akrami et al. (2018) and this is at tension over 3.5σ with the SH0ES measurement which is H0 = 73.52 ± 1.62 km/s/Mpc. A similar mild inconsistency in H0 for ΛCDM model is also observed by Strong Lensing experiments like H0LiCow using time delay measurements Bonvin et al. (2017) which measured H0 = 71.9 +2.4 −3.0 km/s/Mpc for ΛCDM Universe. Moreover the BOSS survey for baryon acoustic oscillations measurements using Lyman-α forest Delubac et al. (2015) has also measured the expansion rate of the Universe at z = 2.34. This measured expansion rate of the Universe at z = 2.34 is also at tension over 2σ with Planck result for ΛCDM Sahni et al. (2014). The important consequences of these tensions are the prediction of the dark energy density evolution with time 1 and more importantly for us, the possibility of having negative dark energy density at higher redshifts as discussed by Sahni et al. (2014); Delubac et al. (2015); Poulin et al. (2018). Similar conclusion has also been obtained recently by Wang et al. (2018) using a dark energy model independent approach. In their study, Wang et al. attributed the evolution of dark energy density as well
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