Chapter 7: The Effective Energy Density

Since there exist different prescriptions for the energy–momentum (pseudo)-tensor of the relic gravitons, the purpose of this chapter is to compare them and to introduce the observables that are customarily employed for the analysis of concrete physical situations. From the practical viewpoint the problem is to find an appropriate definition of the energy density of the relic gravitons because the different prescriptions introduced through the years are not always equivalent and, in some cases, even contradictory. We could naively think that the energy–momentum tensor of the relic gravitons should always be uniquely defined but this is not the case for a number of reasons which are explored in this chapter…