Continuum and HI surveys with the Square Kilometre Array (SKA) will allow us to probe some of the most fundamental assumptions of modern cosmology, including the Cosmological Principle. SKA all-sky surveys will map an enormous slice of space-time and
reveal cosmology at superhorizon scales and redshifts of order unity. We illustrate the potential of these surveys and discuss the prospects to measure the cosmic radio dipole at high fidelity. We outline several potentially transformational tests of cosmology to be carried out by means of SKA all-sky surveys.
ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) is a Flagship Project financed by the Italian Ministry of Education, University and Research, and led by INAF, the Italian National Institute of Astrophysics. Main goals of the ASTRI pr
oject are the realization of an end-to-end prototype of a Small Size Telescope (SST) for the Cherenkov Telescope Array (CTA) in a dual-mirror configuration (SST-2M) and, subsequently, of a mini-array composed of a few SST-2M telescopes to be placed at the final CTA Southern Site. Here we present the main features of the Mini-Array Software System (MASS) that has a central role in the success of the ASTRI Project and will also serve as a prototype for the CTA software system. The MASS will provide a set of tools to prepare an observing proposal, to perform the observations specified therein (monitoring and controlling all the hardware components of each telescope), to analyze the acquired data online and to store/retrieve all the data products to/from the archive.}
We assume cold dark matter to possess a small bulk-viscous pressure which typically attenuates the growth of inhomogeneities. Explicit calculations, based on Eckarts theory of dissipative processes, reveal that for viscous cold dark matter the usual
Newtonian approximation for perturbation scales smaller than the Hubble scale is no longer valid. We advocate the use of a neo-Newtonian approach which consistently incorporates pressure effects into the fluid dynamics and correctly reproduces the general relativistic dynamics. This result is of interest for numerical simulations of nonlinear structure formation involving nonstandard dark-matter fluids. We obtain upper limits on the magnitude of the viscous pressure by requiring that relevant perturbation amplitudes should grow sufficiently to enter the nonlinear stage.
We show, using differential dynamic microscopy, that the diffusivity of non-motile cells in a three-dimensional (3D) population of motile E. coli is enhanced by an amount proportional to the active cell flux. While non-motile mutants without flagella
and mutants with paralysed flagella have quite different thermal diffusivities and therefore hydrodynamic radii, their diffusivities are enhanced to the same extent by swimmers in the regime of cell densities explored here. Integrating the advective motion of non-swimmers caused by swimmers with finite persistence-length trajectories predicts our observations to within 2%, indicating that fluid entrainment is not relevant for diffusion enhancement in 3D.
We continue our study of the physical properties of the recurrent nova T Pyx, focussing on the structure of the ejecta in the nebular stage of expansion during the 2011 outburst. The nova was observed contemporaneously with the Nordic Optical Telesco
pe (NOT), at high resolution spectroscopic resolution (R ~ 65000) on 2011 Oct. 11 and 2012 Apr. 8 (without absolute flux calibration), and with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope, at high resolution (R ~ 30000) on 2011 Oct. 10 and 2012 Mar. 28 (absolute fluxes). We use standard plasma diagnostics (e.g. [O III] and [N II] line ratios and the H$beta$ line fluxes) to constrain electron densities and temperatures. Using Monte Carlo modeling of the ejecta, we derive the structure and filling factor from comparisons to the optical and ultraviolet line profiles. The ejecta can be modeled using an axisymmetric conical -- bipolar -- geometry with a low inclination of the axis to the line of sight, i=15+/-5 degrees, compatible with published results from high angular resolution optical spectro-interferometry. The structure is similar to that observed in the other short orbital period recurrent novae during their nebular stages. We show that the electron density scales as $t^{-3}$ as expected from a ballistically ejected constant mass shell; there is no need to invoke a continuing mass outflow following the eruption. The derived mass for the ejecta with filling factor f ~ 3%, M_ej ~ 2E-6$M_sun is similar to that obtained for other recurrent nova ejecta but inconsistent with the previously reported extended optically thick epoch of the explosion. We suggest that the system underwent a common envelope phase following the explosion that produced the recombination event. Implications for the dynamics of the recurrent novae are discussed. (truncated)
We present a short (and necessarily incomplete) review of the evidence for the accelerated expansion of the Universe. The most direct probe of acceleration relies on the detailed study of supernovae (SN) of type Ia. Assuming that these are standardiz
able candles and that they fairly sample a homogeneous and isotropic Universe, the evidence for acceleration can be tested in a model- and calibration-independent way. Various light-curve fitting procedures have been proposed and tested. While several fitters give consistent results for the so-called Constitution set, they lead to inconsistent results for the recently released SDSS SN. Adopting the SALT fitter and relying on the Union set, cosmic acceleration is detected by a purely kinematic test at 7 sigma when spatial flatness is assumed and at 4 sigma without assumption on the spatial geometry. A weak point of the described method is the local set of SN (at z < 0.2), as these SN are essential to anchor the Hubble diagram. These SN are drawn from a volume much smaller than the Hubble volume and could be affected by local structure. Without the assumption of homogeneity, there is no evidence for acceleration, as the effects of acceleration are degenerate with the effects of inhomogeneities. Unless we sit in the centre of the Universe, such inhomogeneities can be constrained by SN observations by means of tests of the isotropy of the Hubble flow.
This work summarises some of the attempts to explain the phenomenon of dark energy as an effective description of complex gravitational physics and the proper interpretation of observations. Cosmological backreaction has been shown to be relevant for
observational (precision) cosmology, nevertheless no convincing explanation of dark energy by means of backreaction has been given so far.
We propose a version of chaotic inflation, in which a fundamental scale M, well below the Planck scale M_P, fixes the initial value of the effective potential. If this scale happens to be the scale of grand unified theories, there are just enough e-f
oldings of inflation. An initial epoch of fast-roll breaks scale-invariance at the largest observable scales.
The cosmological principle says that the Universe is spatially homogeneous and isotropic. It predicts, among other phenomena, the cosmic redshift of light and the Hubble law. Nevertheless, the existence of structure in the Universe violates the (exac
t) cosmological principle. A more precise formulation of the cosmological principle must allow for the formation of structure and must therefore incorporate probability distributions. In this contribution to the Memorial Volume for Wolfgang Kummer, a great teacher and mentor to me, I discuss how we could formulate a new version of the cosmological principle, how to test it, and how to possibly justify it by fundamental physics. My contribution starts with some of my memories of Wolfgang.
The present standard model of cosmology states that the known particles carry only a tiny fraction of total mass and energy of the Universe. Rather, unknown dark matter and dark energy are the dominant contributions to the cosmic energy budget. We re
view the logic that leads to the postulated dark energy and present an alternative point of view, in which the puzzle may be solved by properly taking into account the influence of cosmic structures on global observables. We illustrate the effect of averaging on the measurement of the Hubble constant.
