![]() Several studies have looked at this, drawing upon a range of sky surveys. Determine the age of thousands of stars at various distances, then use statistics to gauge a minimum cosmological age at different epochs, and from that calculate a minimum Hubble parameter. But you can expand upon the idea as a cosmological test. ![]() This doesn't work because the age of these stars is uncertain enough to be younger than the universe. For example, there are a few stars that appear to be older than the universe, which Big Bang skeptics often point to as disproving the Big Bang. If this age disagrees with LCDM, then LCDM must be wrong. If you know the age of a star or galaxy a billion light-years away, then you know the universe must have been at least that old a billion years ago. It's an alphabetical list of data that shows just how deep this cosmological mystery is. In a recent article uploaded to the arXiv preprint server (and later published in the journal Universe), he outlines seven reasons to suspect dark energy won't be enough to solve the problem. Perhaps tweaking dark energy will solve Hubble Tension, but Sunny Vagnozzi doesn't think so. ![]() We know so little about dark energy that there are lots of theoretical possibilities. Maybe the rate was greater in the period of early galaxies, then slowed down, hence the different observations. But perhaps dark energy is an exotic scalar field, one that would allow a variable expansion rate or even an expansion that varies slightly depending on which direction you look. In Einstein's early model, cosmic expansion is an inherent part of the structure of space and time, a cosmological constant that expands the universe at a steady rate. Much of the effort to solve this mystery has focused on better understanding the nature of dark energy. This is known as the Hubble tension problem, and it's one of the deepest mysteries of cosmology at the moment. In the past, the uncertainty of these values was large enough that they overlapped, but we've now measured them with such precision that they truly disagree. If we look at distant supernova to measure it, we get a value of around 73 km/s per megaparsec. For example, if we use fluctuations in the cosmic microwave background to calculate the parameter, we get a value of about 68 km/s per megaparsec. This can produce an additional systematic error of the order of 1-3 per cent depending on the redshift, comparable to the statistical errors the we aim at achieving with future high-precision surveys such as Euclid.A central difficulty is the fact that increasingly, our various measures of the Hubble parameter aren't lining up. Finally, we also test the impact of neglecting the presence of non-negligible velocity bias with respect to mass in the galaxy catalogues. Its effect is particularly severe for most luminous galaxies, for which systematic effects in the modelling might be more difficult to mitigate and have to be further investigated. ![]() The scale dependence of galaxy bias plays a role on recovering unbiased estimates of f when fitting quasi-non-linear scales. We find that the model of Taruya et al., the most sophisticated one considered in this analysis, provides in general the most unbiased estimates of the growth rate of structure, with systematic errors within ±4 per cent over a wide range of galaxy populations spanning luminosities between L > L* and L > 3L*. Results are compared to those obtained using the standard dispersion model, over different ranges of scales. We consider the possibility of including the linear component of galaxy bias as a free parameter and directly estimate the growth rate of structure f. We examine the most recent developments in redshift-space distortion modelling, which account for non-linearities on both small and intermediate scales produced, respectively, by randomized motions in virialized structures and non-linear coupling between the density and velocity fields. To this aim, we make use of a set of simulated catalogues at z = 0.1 and 1 with different luminosity thresholds, obtained by populating dark matter haloes from a large N-body simulation using halo occupation prescriptions. We investigate the ability of state-of-the-art redshift-space distortion models for the galaxy anisotropic two-point correlation function, ξ(r ⊥, r ∥), to recover precise and unbiased estimates of the linear growth rate of structure f, when applied to catalogues of galaxies characterized by a realistic bias relation.
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