Time flows forward. Objects fall, entropy increases, causes precede effects. The arrow of time feels fundamental — written into the structure of the universe rather than emerging from it. But fundamental physics at the microscopic level is time-reversal symmetric: the laws of motion work equally well run backward. The arrow is emergent, and where it emerges from remains one of the deepest questions in physics.
Susskind (arXiv:2603.12434, March 2026) proposes that time-reversal symmetry in de Sitter space — the geometry that describes an expanding universe with a positive cosmological constant, which our universe approximately is — is a gauge symmetry. Gauge symmetries are redundancies in description, not physical symmetries: changing the gauge doesn't change the physics. If time reversal is a gauge symmetry, forward and backward are different descriptions of the same physics.
But we observe time asymmetry. Susskind's resolution: the gauge symmetry is spontaneously broken. Like the Higgs mechanism, which breaks the electroweak symmetry to give particles mass, something in the cosmological state breaks time-reversal symmetry to give the universe a direction.
The evidence is a closed curve in de Sitter space — a specific geometric path that, when traversed, flips forward-going clocks to backward-going clocks. This is the holonomy: a rotation acquired by parallel transport around a closed loop. The holonomy is the gauge transformation. Its existence means time reversal is available as a symmetry operation. Its spontaneous breaking means the universe has chosen a direction from among equivalent options.
The structural lesson is a reconciliation of two opposing positions. Harlow and Numasawa argued time reversal is a gauge symmetry. Witten argued it is not. Susskind's answer: it is, but you can't see it, because it's hidden by the same mechanism that hides the symmetry between the W and Z bosons. The arrow of time is not fundamental and not absent — it is fundamental symmetry rendered invisible by the state of the universe.