Primordial Black Holes With Variable Gravity

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We have studied the evolution of primordial black holes (PBHs) in a universe with a variable gravitational constant and bulk viscosity. We have found that the strength of gravity has changed appreciably in the early universe. The gravitational constant attained its greatest value at t=10⁻²³ sec after the Big Bang. PBHs formed at the GUT and electroweak epochs would have masses about 1.85× 10⁹ g and 1.8× 10⁻⁷ g respectively. Their temperatures when they explode are 6× 10⁸K and 6 K respectively. PBHs formed during nuclear epoch are hard to detect at the present time. The gravitational constant (G) is found to increase as G∝ t² in the radiation epoch. The gamma rays bursts (GRBs) may have their origin in the evaporation of the PBHs formed during GUT time.