Astrophysics

We suggest here a mechanism for the seeding of the primordial density fluctuations. We point out that a process like reheating at the end of inflation will inevitably generate perturbations, even on superhorizon scales, by the local diffusion of energy. Provided that the final temperature is of order the GUT scale, the density contrast $\delta_R$ for spheres of radius $R$ will be of order $10^{-5}$ at horizon entry, consistent with the values measured by \texttt{WMAP}. If this were a purely classical process, $\delta_R^2$ would fall as $1/R^4$ beyond the horizon, and the resulting primordial density power spectrum would be $P(k) \propto k^n$ with $n=4$. However, as shown by Gabrielli et al, a quantum diffusion process can generate a power spectrum with any index in the range $0<N\LEQ $n="1$" for R^4$ $1 and $n<1$ R^{3+n}$ 1 $\propto be then will ($\delta_R^2$ observed the to close values including 4$,>1$). Thus, the two characteristic parameters that determine the appearance of present day structures could be natural consequences of this mechanism. These are in any case the minimum density variations that must have formed if the universe was rapidly heated to GUT temperatures by the decay of a `false vacuum'. There is then no \emph{a priori} necessity to postulate additional (and fine tuned) quantum fluctuations in the `false vacuum', nor a pre-inflationary period. Given also the very stringent pre-conditions required to trigger a satisfactory period of inflation, altogether it seems at least as natural to assume that the universe began in a flat and homogeneously expanding phase.

The black body nature of the first acoustic peak of the cosmic microwave background (CMB) was tested using foreground reduced WMAP 5-year data, by producing subtraction maps between pairs of cosmological bands, viz. the Q, V, and W bands, for masked sky areas that avoid the Galactic disk. The resulting maps revealed a non black body signal that has three main properties. (a) It fluctuates on the degree scale preferentially in one half of the sky, producing an extra {\it random} noise there of amplitude $\approx$ 3.5 $\mu$K, which is $\gtrsim$ 10 $\sigma$ above the pixel noise even after beam size differences between bands are taken into account. (b) The signal exhibits large scale asymmetry in the form of a dipole ($\approx$ 3 $\mu$K) in the Q-V and Q-W maps; and (c) a quadrupole ($\approx$ 1.5 $\mu$K) in the Q-V, Q-W, and V-W maps. While (b) is due most probably to cross-band calibration residuals of the CMB COBE dipole, the amplitude of (c) is well beyond systematics of the kind, and in any case no {\it a priori} quadrupole in the CMB exists to leave behind such a residual. The axes of symmetry of (a), (b), and (c) are tilted towards the same general direction of the ecliptic plane. This strongly suggests that foreground emission contaminates the CMB signatures at the 4 -- 5 % level even on the angular scale of the first acoustic peak.

Astronomers believe our Universe began in a Big Bang, and is expanding around us. Brian Schmidt will describe the life of the Universe that we live in, and how astronomers have used observations to trace our Universe's history back more than 13 Billion years. With this data a puzzling picture has been pieced together where 96% of the Cosmos is made up of two mysterious substances, Dark Matter and Dark Energy. These two mysterious forms of matter are in a battle for domination of the Universe, and Schmidt will describe experiments that are monitoring the struggle between Dark Energy and Dark Matter, trying better understand these elusive pieces of our Universe, and predict the ultimate fate of the Cosmos.

I will give account of a work in progress in which I attempt to modify the metric-manifold structure of GR in the infra-red. The proposed modification does not contain any massive parameter as it is effective at length scales comparable with the inverse (extrinsic) curvature. The guiding line for this modification is an "ultra-strong" equivalence principle, according to which even semi-classical gravitational effects (i.e. particle production) are definitely banned from a sufficiently small free-falling elevator. Some cosmological consequences of this modification will be discussed.

Cosmologists are struggling to understand why the expansion rate of our universe is now accelerating. There are two sets of explanations for this remarkable observation: dark energy fills space or general relativity fails on cosmological scales. If dark energy is the solution to the cosmic acceleration problem, then the logarithmic growth rate of structure $dlnG/dlna = \Omega^\gamma$, where $\Omega$ is the matter density independent of scale in a dark matter plus dark energy model. By combining measurements of the amplitude of redshift space, $\beta = (1/b) dlnG/dlna$ with measurements of galaxy bias, $b$, from cross-correlations with CMB lensing, redshift surveys will be able to determine the logarithmic growth rate as a function of scale and redshift. I will discuss the role of upcoming surveys in improving our ability to understand the origin of cosmic acceleration.

The currently accelerating Hubble expansion is in accord with the old heuristic prediction, from causal set theory, of a fluctuating and ever-present cosmological "constant''. More recently, a phenomenological model based on certain of the ideas behind the prediction has been devised, but it remains incomplete. I will review these developments and also mention a possible consequence for the dimensionality of spacetime.

Normalizing the Einstein-Hilbert action by the volume&nbsp;functional&nbsp;makes the theory invariant under constant&nbsp;shifts in the Lagrangian.&nbsp;The field equations then resemble unimodular gravity whose&nbsp;otherwise&nbsp;arbitrary cosmological constant is now determined as a&nbsp;Machian&nbsp;universal&nbsp;average. We first prove that an empty space-time is necessarily flat,&nbsp;and then demonstrate, by invoking the scalar field paradigm,&nbsp;that the cosmological constant is actually zero. Normalized general relativistic cosmology is then discussed at the&nbsp;mini-superspace&nbsp;level, where we confirm the generic zero cosmological constant result, and furthermore prove that if&nbsp;matter is attractive&nbsp;then the Universe cannot be closed.&nbsp;The only non-vanishing cosmological constant solutions are&nbsp;associated with static Einstein-like closed universes.