Backreaction The effect of clumpiness in cosmology

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BackreactionThe effect of clumpinessin
cosmology

• Syksy Räsänen
• University of Geneva

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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.

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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 gt 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.

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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 Zeldovich and Feynman in 1964.
• Since then, numerous papers with various
  conclusions have appeared.
• To summarise(arXiv0801.2692)
• 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.

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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(arXiv0802.1523).
• However, we have to be near the center (within
  10 Mpc) to avoid a large CMB dipole(astro-ph/0607
  334).
• There are also constraints from spectral
  distortion, the kinetic SZ effect and the
  difference between radial and angular expansion
  from BAO(arXiv0711.3459, arXiv0807.1326,
  arXiv0809.3761).

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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(arXiv0801.2692).
• There is no fully realistic calculation yet.
• The connection to light propagation needs more
  work.

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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.