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How Isotropic is the Universe?

Daniela Saadeh1, Stephen M Feeney2, Andrew Pontzen1

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The Universe is strongly favored to be isotropic, with new analysis of cosmic microwave background data disfavoring anisotropic expansion. This cosmic microwave background (CMB) analysis provides the tightest constraints yet on deviations from cosmic isotropy.

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Area of Science:

  • Cosmology
  • Astrophysics
  • General Relativity

Background:

  • The standard model of cosmology assumes the Universe is isotropic on large scales.
  • Bianchi cosmologies offer solutions to Einstein's field equations that allow for anisotropic expansion.
  • Previous tests of Bianchi cosmologies have only considered subsets of possible anisotropic solutions.

Purpose of the Study:

  • To conduct a general test of cosmic isotropy by considering all degrees of freedom in Bianchi cosmologies.
  • To constrain anisotropic expansion using cosmic microwave background (CMB) temperature and polarization data.

Main Methods:

  • Utilized Planck satellite's cosmic microwave background (CMB) temperature and polarization data.
  • Performed a comprehensive analysis of all degrees of freedom within Bianchi cosmologies.
  • Set upper limits on various modes of anisotropic expansion, including vector and tensor modes.

Main Results:

  • Established a stringent limit on anisotropic expansion due to vorticity: (σ_{V}/H)_{0}<4.7×10^{-11} (95% C.L.).
  • Constrained the regular tensor mode of anisotropic expansion to (σ_{T,reg}/H)_{0}<1.0×10^{-6} (95% C.L.).
  • Found strong statistical evidence against anisotropic expansion, with odds of 121,000:1 against it.

Conclusions:

  • The Universe is strongly favored to be isotropic on large scales.
  • This study provides the tightest constraints to date on deviations from cosmic isotropy.
  • The comprehensive analysis of Bianchi cosmologies supports the standard cosmological model's assumption of isotropy.