Horizon Problem (No Inflation)

The horizon problem asks how regions of the CMB sky separated by more than ~2 degrees — which would have been causally disconnected under standard Big Bang expansion — can have the same temperature to within one part in 100,000. In ΛCDM, inflation resolves this by positing a period of superluminal expansion that stretched a tiny causally connected region to encompass the entire observable universe. Successive Collision Theory makes inflation unnecessary through the superluminal collision mechanism itself. Two spacetime pockets approaching with relative velocity v_rel > 2c have a collision front that sweeps the full overlap volume at a rate that exceeds any internal signal speed. The thermalization of the baryon-photon plasma therefore occurs across the entire overlap volume essentially simultaneously — not sequentially from some central point outward. Every location within the 46.5 Gly radius of our observable universe reached the same temperature and density at the same moment because the collision front that thermalized it was moving superluminally, not because pre-existing causal contact established thermal equilibrium.

This resolution does not require any new physics, fields, or violations of SR. Special Relativity constrains objects that are accelerated through a single inertial frame by local forces — it does not forbid two objects from being separated at superluminal rates by the expansion of the spacetime between them, as ΛCDM itself accepts for galaxies beyond the Hubble radius. SCT extends this same accepted SR logic to the relative velocity of two independently formed spacetime pockets. The collision front propagates superluminally not because any signal travels faster than light within a local frame, but because the two pocket boundaries are closing on each other through the expansion of the parent frame that contains both. The resulting simultaneous thermalization explains CMB isotropy with the same physical mechanism — superluminal relative motion of gravitationally bound structures — that ΛCDM already accepts for cosmological recession velocities, requiring no inflaton field and no epoch of exponential expansion.