Why look for a new theory of gravity if the big bang cosmology is correct?

Occasionally we read in the popular press, especially online, that someone has come up with a new theory of gravity. Why is that even necessary if the current theory describing the evolution of the universe is so correct?

The standard ΛCDM big bang cosmology is derived from an application of certain non-biblical boundary conditions to the physics of Einstein’s general relativity theory. But when that was applied to the universe as a whole, two problems developed for the secular model. One is the need to add in dark energy (or the cosmological constant, Λ (Lambda), to Einstein’s field equations) and the other is the need for a significant amount of invisible cold dark matter (CDM).

On the scale of galaxies and even clusters of galaxies Newtonian physics is used as it is the low gravity limit of general relativity. But without the addition of dark matter the resulting theory, using the known density of visible matter in galaxies (see Fig. 1) and clusters, does not match observations. But for more than 40 years now dark matter has been sought in various lab experiments with consistently negative results. This has developed into what is called the dark matter crisis.1

galaxy-rotation-curve

Figure 1: Typical rotation curve of a spiral galaxy: Speeds (V) in km/s units as a function of distance from the centre of the galaxy (R) in 1000 light-year (ly) units. The upper curve shows the speeds of the stars in disk region determined from their visible light and the gasses beyond that determined from radio frequency emissions. The lower curve shows what standard Newtonian physics predicts should be observed. The discrepancy is made up by positing the existence of invisible dark matter. Credit: Wikipedia

Occasionally a claim is made that a theorist has some inkling of what dark matter particles might be but the crisis remains.2 Dark matter particles have been sought without success in the Galaxy using very sensitive detectors deep in underground mines,3 or with the Large Hadron Collider (LHC) over 10 years of experiments looking for the lowest mass stable particle in a theorised class of as-yet-undiscovered supersymmetric particles.4

The observational data from thousands of galaxies together with the negative outcome of all the experiments searching for Dark Matter particles indicate that either something is wrong with the physics we use or that the expected dark matter is much more elusive than supposed, or, indeed, does not, in fact, exist—which gets us back to something being wrong with the physics. Continue reading