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Does small scale structure significantly affect cosmological dynamics?

Julian Adamek1, Chris Clarkson2, Ruth Durrer1

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Cosmological backreaction, the effect of small-scale structures on large-scale universe properties, is smaller than predicted. Virialization of structure limits this effect, making it independent of initial conditions for precision cosmology.

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Area of Science:

  • Cosmology
  • Astrophysics
  • General Relativity

Background:

  • The large-scale homogeneity and isotropy of the Universe suggest a standard cosmological model.
  • However, smoothing over structures can introduce a backreaction effect, transferring power from small to large scales.
  • Second-order perturbation theory indicates this effect is small but may signal a non-convergent perturbation series.

Purpose of the Study:

  • To investigate the impact of cosmological backreaction on large-scale universe properties.
  • To determine if backreaction is dependent on initial conditions, particularly the ratio of Hubble scales.
  • To assess the significance of backreaction for precision cosmology.

Main Methods:

  • Utilized two N-body numerical simulations: a 3D Newtonian and a 1D simulation incorporating relativistic effects.
  • Varied initial conditions, specifically the amount of small-scale power, to observe its effect on backreaction.
  • Analyzed the impact of structure virialization on the backreaction phenomenon.

Main Results:

  • Contrary to perturbation theory predictions, the virialization of structure saturates the backreaction effect.
  • The saturated backreaction level is independent of the matter-radiation equality scale, implying independence from initial conditions.
  • Backreaction remains a small effect, irrespective of the initial power spectrum.

Conclusions:

  • Cosmological backreaction is a limited effect, independent of initial conditions due to structure virialization.
  • While small, backreaction can contribute at the percent level to cosmological observables.
  • This effect cannot be neglected in precision cosmology studies.