In this book, the author, by taking into account also the contribution of the gravitational potential energy to the determination of the gravitational masses of the particles in a gravitational field (contrary to the erroneous common use), finds that for the particles with rest mass greater than zero there is always an escape velocity (less than the velocity of light in vacuum c) from any gravitational field and that the particles with rest mass equal to zero are always free to go away from any gravitational field. Consequently, there is not any event horizon, and therefore there is not any black hole. Moreover, the author also shows that the presence of an event horizon would imply the violation of the principle of conservation of energy. In particular, all this is confirmed also by the symmetry with respect to time of the Einstein’s field equation of the General Theory of Relativity and by the correct solutions of this same equation. Consequently, this book can be useful also for explaining in a linear way the possibility of the Big Bang and the possibility of a cyclic universe. Finally, the experimental results are compatible so far with this book.