Wednesday Colloquium


"The Greatest Myth in the History of Astronomy"

Owen Gingerich (Harvard University)


"Wildly oscillating stars - a new physical phenomenon? Eliminating the impossible to find the improbable"

Victoria Antoci (University of Aarhus)

Here I present our photometric and spectroscopic analyses of a sample of gamma Dor and hybrid pulsators that exhibit (additional) frequency patterns inconsistent with anything we know about these type of stars. More precisely we find peaks, that show nice long ridges reminiscent of consecutive high radial order g-modes, however, their period spacings do not match the expected values for main sequence stars but rather those of compact objects. On the other hand, if we assume that the peaks are split in frequency instead of periods we find repeating patterns of multiplets reoccurring in a very narrow frequency range. This phenomenon occurs in a significant number of stars, not always at the same frequency and with the same separation, which leads us to conclude that these signals are intrinsic to the stars and may describe a new phenomenon in A and F stars on and near the main sequence.


"GRB light curve decay indices in the afterglow phase"

Roberta Del Vecchio (Astronomical Observatory, Jagiellonian University)

I will start with presentation of basic information about cosmic Gamma-ray Burst (GRB) phenomenon. Then our study of the distribution of temporal power-law decay indices, α, in the GRB afterglow phase will be described, based on a sample of 176 GRBs with known redshifts. In this analysis we discovered a convincing regular trend between α and the afterglow luminosity at the end time of the plateau phase, La. Even stronger systematic trend is visible between and the luminosity ratio computed by dividing La by the respective luminosity at the fitted Luminosity-Time correlation line (Dainotti et al. 2008). This systematic effect provides a new constraint on the GRB physical models. Finally, a proposed toy model accounting for this systematics applied to the luminosity of the analyzed GRB distribution results in a slight decrease of the scatter within the LT correlation, possibly a small step towards turning GRBs into cosmological standard candles.


"Dark energy as silent virialisation"

Boud Roukema (Astronomical Observatory, Toruń)

The Friedmann-Lemaitre-Robertson-Walker cosmological solutions of the Einstein equation assume a stress-energy tensor with positive density and zero pressure. These solutions do not allow for gravitational collapse on galaxy and galaxy cluster scales. Gravitational collapse and virialisation, together with general spatial expansion, can be modelled by either (i) assuming stable clustering in virialised domains (D) and the kinematical backreaction (Q_D) Zel'dovich approximation (ZA) for normal volume evolution (VQZA); or, equivalently, (ii) defining a negative effective gravitational pressure that locally opposes gravitational attraction, thus enabling virialisation to be represented, with a stable end state p_D = -_D/3. The silent universe hypothesis is assumed in both cases. An implementation of (i) VQZA will be presented. This shows that scalar averaging with a domain size of a typical non-linear structure scale, about 2.5 Mpc/h, yields "dark energy" as an artefact of ignoring virialisation in the stress-energy tensor.