Cosmic Parallax as a probe of late time anisotropic expansion

Understand

Cosmic parallax is the change of angular separation between pair of sources at cosmological distances induced by an anisotropic expansion.

• An accurate astrometric experiment like Gaia could observe or put constraints on cosmic parallax.
• Examples of anisotropic cosmological models are Lemaitre-Tolman-Bondi void models for off-center observers (introduced to explain the observed acceleration without the need for dark energy) and Bianchi metrics.
• If dark energy has an anisotropic equation of state, as suggested recently, then a substantial anisotropy could arise at $z \lesssim 1$ and escape the stringent constraints from the cosmic microwave background.