Although General Relativity has passed all its classical tests conducted since Eddington’s light deflection test in 1919, it faces many issues in which it is not as quite promising. One of the biggest setbacks of General Relativity is its inability to be pertubatively quantized, making it extremely difficult to unify the theory with other fundamental interactions. From a cosmological point of view, the big bang model deduced from general relativity has many areas of vulnerability as it is based on a few assumed initial parameters such as isotropy, flat space and the current value of the cosmological constant. It would be desirable to deduce these parameters from the settings of a new alternate theory, equally linked with discussions on a black hole’s event horizon, the curvature of space-time and the current accepted age of the universe. From an experimental point of view, the Newtonian weak field limit potential has only been verified to a scale of 10^-4 m. General relativity fails to predict the the velocity of stars in the outermost edges of galaxies, as shown from data, without using large amounts of dark matter. This has come to be known as the ‘Dark Matter’ dilemma, where we assume the existence of large amounts of dark matter or we modify the Newtonian potential over cosmic distances in order to find a solution. Therefore, an alternative theory may provide the solution such that it is compatible with general relativity in certain limits, but which can also explain the equivocal sectors such as the unification of gravitation with the standard model and other limitations of general relativity. The most desirable theories are those which satisfy the two main properties.
1) The Use of Precision Tests- The predictions of an alternate theory must be consistent with all tests performed to verify General Relativity. The theory must also reveal certain solutions that correlate with the observed phenomenon.
2) It must be motivated from fundamental physics. There must be some fundamental theory from which the alternative theories are derived. This fundamental theory should solve some fundamental problem like linking gravitation to Quantum Mechanics.
The Tensor Scalar theories are not only consistent with the precision tests but are also well motivated by fundamental physics. It must now be clear that alternative theories of gravity are highly desirable. If the new theory matches with the laboratory and solar system gravitational tests, along with properties like renormaliziblity or scale invariance. These alternative theories might also provide a further dynamical insight on the cosmological evolution of the universe. Metric theories of gravity are given more priority than other alternate theories, as the metric tensor guv of the gravitational field has a direct affect on matter. The metric tensor carries the information of the gravitational field, expressing the main properties of Einstein’s theory.
