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Effects of structures on the Hubble diagram
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Europe/Paris
1222-RC-08 (LPNHE)
1222-RC-08
LPNHE
Description
Cosmological observations often refers to the measurement of averaged quantities over the sky and more precisely, on our past light-cone. A rigorous, in particular gauge invariant, definition of the average of scalar quantities (such as matter density, luminosity flux, distance...) on this past light-cone is not trivial and has only been developed recently in the context of the averaging problem.
I will present this definition on the light-cone and show that it can be simplified by the choice of an adapted system of coordinates. This system, so-called geodesic light-cone coordinates, turns out to be very well adapted to calculations on this cone and gives elegant expressions for observational quantities. I will show its usefulness with particularly the computation of the luminosity distance at second order in perturbations around a spatially flat FLRW model (in the Poisson gauge). Assuming then a realistic power spectrum for inhomogeneities, it is possible to estimate the effect of structures on the Hubble diagram and show that it is dominated by weak lensing. Interestingly, inhomogeneities can have strongly different effects on different observables and they can generate a large error on type Ia supernovae measurements which is comparable with their intrinsic dispersion at large redshifts.
I will present this definition on the light-cone and show that it can be simplified by the choice of an adapted system of coordinates. This system, so-called geodesic light-cone coordinates, turns out to be very well adapted to calculations on this cone and gives elegant expressions for observational quantities. I will show its usefulness with particularly the computation of the luminosity distance at second order in perturbations around a spatially flat FLRW model (in the Poisson gauge). Assuming then a realistic power spectrum for inhomogeneities, it is possible to estimate the effect of structures on the Hubble diagram and show that it is dominated by weak lensing. Interestingly, inhomogeneities can have strongly different effects on different observables and they can generate a large error on type Ia supernovae measurements which is comparable with their intrinsic dispersion at large redshifts.
