Power spectrum in effective theories for inflation

Tuesday, September 10th

Mauricio Gamonal San Martin, Penn State
Primordial power spectrum in effective theories for inflation>
PDFof the talk (12M)
Audio+Slidesof the talk (170M))

By Jorge Pullin, LSU.

The main achievement of inflationary cosmology is how it reproduces the inhomogeneities of the Universe using a relatively simple model. You put a quantum field in its simplest state -the vacuum-, evolve it through inflation (a period of exponential expansion of the Universe) and voila, the quantum state acquires non-trivial features that leave an imprint on the Universe that matches beautifully the inhomogeneities we observe in the cosmic microwave background (CMB). 

Beyond that simple story, people have proposed slightly different models for inflation to address several issues. In particular, if one considers quantum gravity further modifications are possible.While quantum gravity effects are typically too weak if inflation lasted too long, they could play a role in setting different initial conditions if a pre-inflationary epoch occurred, for example by preparing an excited vacuum state for the quantum fields. This is exciting because it means that quantum gravity effects could leave observable imprints in the observations of the CMB.

This talk presented a unified treatment of a large group of models of inflation and their potential observable imprints on the CMB, including those stemming from loop quantum gravity and spinfoams.

Localized energy of gravitational waves

Tuesday, Mar 5th 

Simone Speziale, Aix Marseille University
Localized energy of gravitational waves
PDF of the talk (11M)
Audio+Slidesof the talk (360M))
VTT (Subtitles) of the talk (100k)

By Jorge Pullin (LSU)

The idea of associating a local energy density to a gravitational field has been problematic in general relativity. In electromagnetism, a region with an electric or magnetic field contains an energy whose density (energy per unit volume at a point) is proportional to the field squared. In gravity, however, things are more complicated. This is illustrated by a thought experiment first proposed by Einstein. Consider an observer in a windowless elevator and two situations: a) the elevator is in outer space, where gravity is negligible, but accelerating at a rate of 9.8 m/s2, the acceleration of gravity on Earth, and b) it is motionless on the surface of the Earth. In both cases if the observer releases a mass, it will fall to the floor with the same acceleration. No experiment carried out in the elevator can distinguish both situations. Therefore, one could not claim that one has an energy content and the other does not.

Over the years this has led to confusion and to attempts to create local energy densities that are problematic, namely, they are ambiguous and coordinate-dependent. For many years the issue of if gravitational waves carried an energy flux was contentious, eventually being settled in the1960's. The end result is that one cannot define a local energy density but must discuss energy considerations only considering complete regions of space-time and studying them from far away. Mathematically this requires treating fields at infinity. One can define energies and fluxes in an invariant way at infinity only.

This talk discussed some of the subtleties involved and presents a framework where infinity and other regions of interest, like the horizons that surround black holes, can be treated in a unified way. This may lead to insights into what are the true physical degrees of freedom that one should consider quantizing in a theory of quantum gravity.