Gravitational waves from global phase transitions

It is shown that causality implies that any global symmetry breaking transition in the early universe should produce, at the horizon, an approximate scale invariant spectrum of gravitational waves - even in the absence of inflation. This can result in, among other effects, a quadrupole anistropy in the cosmic microwave background today. Comparing this with existing upper limits can place a rigid upper limit on the scale of symmetry breaking assoaiated with various scenarios for the formation of large scale structure. The preliminary estimates presented here suggest this limit should be less than 10¹⁷ GeV.