Redshift Drift Anisotropy

If the universe is truly isotropic and homogeneous at the scales probed by the redshift drift test, the drift signal should be identical in every direction on the sky at a given redshift. Any measured anisotropy in the drift — directional variation in dz/dt at fixed z — would indicate either instrumental systematics, local peculiar velocity effects, or genuine large-scale cosmological anisotropy in the expansion rate. Current Lyman-alpha forest datasets show mild but statistically marginal evidence for anisotropy in the quasar-absorber cross-correlation statistics and in the line-of-sight power spectrum, suggesting that the background expansion rate may not be perfectly isotropic at the level accessible to precision spectroscopy. These hints motivate designing the ACES and future drift experiments to have hemispheric coverage rather than relying on a single field of view.

Successive Collision Theory predicts a genuine redshift drift anisotropy as a direct consequence of the spatially varying Λ_eff field. The KBC supervoid has an elongated geometry and is bounded by denser walls and filaments in specific directions; lines of sight toward these overdense regions traverse a lower mean Λ_eff and therefore produce a slightly smaller drift per unit time than lines of sight through the deepest underdense core. The resulting drift anisotropy is a smooth, large-scale pattern with an angular scale comparable to the ~300 Mpc void size projected on the sky — a quadrupolar or octupolar pattern rather than a small-scale fluctuation. The preferred axis of this anisotropy should be aligned with the CMB kinematic dipole direction and the large-scale angular momentum axis inherited from the initial pocket collision, since these all trace the same underlying geometric structure.

The amplitude of the SCT redshift drift anisotropy is estimated at the level of a few percent of the mean drift signal — approximately 0.1–0.3 cm/s per decade over the z ~ 2–4 baseline accessible to ELT/ACES. While small, this anisotropy is coherent across all lines of sight passing through the same large-scale structure and grows as the ACES baseline extends. SCT predicts that the anisotropy is correlated with independent large-scale structure measures such as the kinematic Sunyaev-Zeldovich map and the galaxy distribution dipole, providing a multi-dataset cross-check that would make a detection highly significant even at modest amplitude. A null detection of the anisotropy at the predicted level would constrain the depth of the KBC void and the rate of tensor mesh dissipation, providing complementary information to the mean drift signal.