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Backreaction The effect of clumpiness in cosmology. Syksy Räsänen University of Geneva. A factor of 2. Observed distances in the late universe are a factor of 2 longer than predicted in homogeneous and isotropic models with ordinary matter and gravity. There are three possibilities:
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BackreactionThe effect of clumpinessin cosmology Syksy Räsänen University of Geneva
A factor of 2 • Observed distances in the late universe are a factor of 2 longer than predicted in homogeneous and isotropic models with ordinary matter and gravity. • There are three possibilities: 1) There is matter with negative pressure. 2) General relativity does not hold. 3) The universe is not homogeneous and isotropic.
Our clumpy universe • The early universe is exactly homogeneous and isotropic, up to linear perturbations. • At late times, the universe is only statistically homogeneous and isotropic, on scales > 100 Mpc. • The average evolution of an inhomogeneous and/or anisotropic spacetime is not the same as the evolution of the corresponding smooth spacetime. • Describing the average behaviour of a clumpy universe was termed the fitting problem by George Ellis in 1983. • Clumpiness affects the expansion rate, light propagation and their relationship.
A scanner lightly • Since most observations probe the expansion rate only via the distance scale, it could be possible to explain the data without accelerated expansion. • The effect of clumpiness on light propagation was first studied by Zel’dovich and Feynman in 1964. • Since then, numerous papers with various conclusions have appeared. To summarise(arXiv:0801.2692): It appears that the effect of clumpiness on light propagation is small for realistically sized, randomly distributed structures, assuming that the average expansion rate does not change.
Love in a void • Speculative large structures can have a significant effect on light propagation. • The old idea that we are located in a large (100 Mpc-1 Gpc) underdense region has been recently studied with the Lemaître-Tolman-Bondi (LTB) model. • It is possible to fit the SNIa, CMB and BAO data, as well as the age of the universe and Hubble parameter(arXiv:0802.1523). • However, we have to be near the center (within ∼10 Mpc) to avoid a large CMB dipole(astro-ph/0607334). • There are also constraints from spectral distortion, the kinetic SZ effect and the difference between radial and angular expansion from BAO(arXiv:0711.3459, arXiv:0807.1326, arXiv:0809.3761).
Going faster • The average expansion can differ from the FRW model, as shown by Buchert in 1999(gr-qc/9906015), even when the universe is statistically homogeneous and isotropic and structures are small. • The average expansion rate can accelerate because the fraction of volume occupied by faster expanding regions grows(astro-ph/0607626). • Acceleration has been demonstrated in the LTB model(astro-ph/0512651, gr-qc/0605120, astro-ph/0605195). • In a simple model with a realistic distribution of structures, Ht grows by 10-30% around 10 billion years(arXiv:0801.2692). • There is no fully realistic calculation yet. • The connection to light propagation needs more work.
Summary • Observations of the late universe are inconsistent with a homogeneous and isotropic model with ordinary matter and gravity. • FRW models do not include the effect of non-linear structures. • The effect on light propagation is likely to be small unless there is a speculative large structure or the expansion rate changes. • Local void models are under pressure, but not ruled out. • The effect on the expansion rate can be large. • The correct order of magnitude and timescale emerge from a simple model of structures. • Before concluding that new physics is needed, it is necessary to quantify the effect of non-linear structures.
