We present a detailed analysis of the phase transition in the standard model at finite temperature. Using an improved perturbation theory, where plasma masses are determined from a set of one-loop gap equations, we evaluate the effective potential V ef f (ϕ, T ) in nextto-leading order, i.e., including terms cubic in the gauge coupling g, the scalar self-coupling λ 1/2 and the top-quark Yukawa coupling f t . The gap equations yield a non-vanishing magnetic plasma mass for the gauge bosons, originating from the non-abelian self-interactions. We discuss in detail size and origin of higher order effects and conclude that the phase transition is weakly first-order up to Higgs masses of about 70 GeV , above which our calculation is no longer self-consistent. For larger Higgs masses even an approximation containing all g 4 contributions to V ef f is not sufficient, at least a full calculation to order g 6 is needed. These results turn out to be rather insensitive to the topquark mass in the range m t = 100 − 180 GeV . Using Langer's theory of metastability we calculate the nucleation rate of critical droplets and discuss some aspects of the cosmological electroweak phase transition.

We study the decay of the metastable symmetric phase in the standard model at finite temperature. For the SU(2)-Higgs model the two wave function correction terms Z ϕ (ϕ 2 , T ) and Z χ (ϕ 2 , T ) of Higgs and Goldstone boson fields are calculated to one-loop order. We find that the derivative expansion of the effective action is reliable for Higgs masses smaller than the W-boson mass. We propose a new procedure to evaluate the decay rate by first integrating out the vector field and the components of the scalar fields with non-zero Matsubara frequencies. The static part of the scalar field is treated in the saddle point approximation. As a by-product we obtain a formula for the decay rate of a homogeneous unstable state. The course of the cosmological electroweak phase transition is evaluated numerically for different Higgs boson masses and non-vanishing magnetic mass of the gauge boson. For Higgs masses above ∼ 60 GeV the latent heat can reheat the system to the critical temperature, which qualitatively changes the nature of the transition.

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The post-Newtonian metric of the general scalar-tensor theory with a massive scalar field is calculated. The result is used to recalculate several relativistic effects in the solar system. Experimental bounds for the massless sralar-tensor theory that have been obtained from these effects are used to get new bounds for th£' massive case. Furthermore we give a cormter-example to the conjecture that all theories with only one gravitational field obey the strong equivalence principle.

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