Vacuum energy and cosmological evolution

An expanding universe is not expected to have a static vacuum energy density. The so-called cosmological constant $\Lambda$ should be an approximation, certainly a good one for a fraction of a Hubble time, but it is most likely a temporary description of a true dynamical vacuum energy variable that is evolving from the inflationary epoch to the present day. We can compare the evolving vacuum energy with a Casimir device where the parallel plates slowly move apart ("expand"). The total vacuum energy density cannot be measured, only the effect associated to the presence of the plates, and then also their increasing separation with time. In the universe there is a nonvanishing spacetime curvature $R$ as compared to Minkowskian spacetime that is changing with the expansion. The vacuum energy density must change accordingly, and we naturally expect $\delta\Lambda\sim R\sim H^2$. A class of dynamical vacuum models that trace such rate of change can be constructed. They are compatible with the current cosmological data, and conveniently extended can account for the complete cosmic evolution from the inflationary epoch till the present days. These models are very close to the $\Lambda$CDM model for the late universe, but very different from it at the early times. Traces of the inherent vacuum dynamics could be detectable in our recent past.Comment: Slightly extended discussion, typos corrected and references adde

Once Upon Four Robbers: the magic of subversion (introduction)

Once Upon Four Robbers is the first in the series of dramas that Osofisan refers to as the ‘magic boon’ plays. Written between 1976 and 1978, the play was premiered at the Arts Theatre, University of Ibadan in March 1979. The other play in the series is Esu and the Vagabond Minstrels. The central motif of the magic boon plays is borrowed or adapted from the world of folklore. A group of persons in anguish or dilemma suddenly obtains a magical power from a mysterious agent. The power is capable of changing their circumstances as long as they adhere strictly to expressed injunctions. Osofisan confesses to be fascinated by the possibilities that magic presents in dramatising societal issues that may otherwise prove difficult to discuss, ‘especially when you are criticising the government.’ The dramatist planned ten magic boon plays but only two have been written, or produced so far. Four Robbers is popular with students because of the topicality of the theme of armed robbery and the ability to be produced on a bare stage, without a cumbersome setting. The play is also adaptable; and though the prescribed setting is market place, producers have sometimes changed that setting to a bank, a beach or casino. Four Robbers is set in a market square, which is a symbolic location in Yoruba worldview. Osofisan wrote the play to contribute to the debate on public execution of armed robbers in Nigeria. The play examines the moral and legal definitions of ‘robbery’ in the wider sense, and the implications of the Armed Robbery and Firearms Decree 47 of 1970 on the public psyche

The Top-Quark Width in the Light of ZZ-Boson Physics

We discuss possible non-standard contributions to the top-quark width, particularly the virtual effects on the standard decay $t\rightarrow W^+\,b$ within the context of the MSSM. We also place a renewed emphasis on the unconventional mode $t\rightarrow H^+\,b$ in the light of recent analyses of $Z$-boson observables. It turns out that in the region of parameter space highlighted by $Z$-boson physics, the charged Higgs mode should exhibite an appreciable branching fraction as compared to the standard decay of the top quark. Remarkably enough, the corresponding quantum effects in this region are also rather large, slowly decoupling, and most likely resolvable in the next generation of experiments at Tevatron and at LHC.Comment: One new figure added, with supplementary comments. 17 pages, LaTeX, 2 figures included via psbox. A postcript version can be obtained from anonymous ftp at ftp://ftp.ifae.es/preprint/ft/uabft372.ps Talk presented at the Workshop on Physics of the Top Quark, Iowa State University, May 199