Spyros Basilakos

The Aspen–Amsterdam void finder comparison project

Despite a history that dates back at least a quarter of a century studies of voids in the large–scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void–finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi– analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than mB = 20 is found to be smaller than about 0.8 by all our void finding algorithms.

Hubble expansion and structure formation in time varying vacuum models

We investigate the properties of the FLRW flat cosmological models in which the vacuum energy density evolves with time,

The size and shape of local voids

\ensuremath{\Lambda}(t)

Expansion History with Decaying Vacuum: A Complete Cosmological Scenario

. Using different versions of the

New Schwarzschild-like solutions in f(T) gravity through Noether symmetries

\ensuremath{\Lambda}(t)

Searching for cluster substructure using APM and ROSAT data

model, namely, quantum field vacuum, power series vacuum and power law vacuum, we find that the main cosmological functions such as the scale factor of the Universe, the Hubble expansion rate

Testing gamma-ray bursts as standard candles

H

Viable

, and the energy densities are defined analytically. Performing a joint likelihood analysis of the recent supernovae type Ia data, the cosmic microwave background shift parameter and the baryonic acoustic oscillations traced by the Sloan Digital Sky Survey galaxies, we put tight constraints on the main cosmological parameters of the

f(T)

\ensuremath{\Lambda}(t)

models are practically indistinguishable from

scenarios. Furthermore, we study the linear matter fluctuation field of the above vacuum models. We find that the patterns of the power series vacuum