Hawking1,2 has shown that event horizons produce thermal radiation. I propose here a new cosmological model which has an early event horizon and in which the observed3 cosmic microwave background radiation is Hawking radiation. The model starts with de Sitter space—a space-time of constant curvature which is a solution to Einstein's vacuum field equations with a positive cosmological constant. Associated with an event E in de Sitter space there is a quantum barrier penetration tunnelling which leads to an open, negatively curved (k = −1) cosmology. This has an early exponential expansion phase but turns into a standard big-bang solution at late times. The geometry and quantum mechanical treatment within the future light cone of E are similar to that found in the Brout, Englert and Spindel (BES) theory4,5. This model has the following advantages: (1) It has no singularities. (2) The observed isotropy of the cosmic microwave background is explained because the different regions we observe have all been in causal contact. (3) The temperature at early epochs (T0 ∼ 1019 Ge V) is high enough to allow grand unified theories (GUTs) to produce the observed baryon excess from an initial thermal distribution through CP violations6,7. (4) T0 is correct to make the BES scenario work. (5) The early exponential expansion phase can naturally account for the observed large number n0 ∼ 1088 of particles within a volume a3 (where a is the radius of curvature) and the Guth8 flatness problem. (6) It predicts that our Universe is an open k = −1, Ω<1 cosmology consistent with the amount of mass detected in the universe so far9–11. (7) The existence of the event horizon makes it possible to create from the original de Sitter space other k = −1 universes (perhaps an infinite number) which are entirely disjoint from our own and from each other.