GRSI model

The GRSI model is an attempt to explain astrophysical and cosmological observations without dark matter, dark energy or modifying the laws of gravity as they are currently established. This model is an alternative to Lambda-CDM, the standard model of cosmology.

History and description
The model was proposed in a series of articles, the first dating from 2003. The basic point is that since within General Relativity, gravitational fields couple to each other, this can effectively increase the gravitational interaction between massive objects. The additional gravitational strength then avoid the need for dark matter. This field coupling is the origin of General Relativity's non-linear behavior. It can be understood, in particle language, as gravitons interacting with each other (despite being massless) because they carry energy-momentum.

A natural implication of this model is its explanation of the accelerating expansion of the universe without resorting to dark energy. The increased binding energy within a galaxy requires, by energy conservation, a weakening of gravitational attraction outside said galaxy. This mimics the repulsion of dark energy.

The GRSI model is inspired from the Strong Nuclear Force, where a comparable phenomenon occurs. The interaction between gluons emitted by static or nearly static quarks dramatically strengthens quark-quark interaction, ultimately leading to quark confinement on the one hand (analogous to the need of stronger gravity to explain away dark matter) and the suppression of the Strong Nuclear Force outside hadrons (analogous to the repulsion of dark energy that balances gravitational attraction at large scales.) Two other parallel phenomena are the Tully-Fisher relation in galaxy dynamics that is analogous to the Regge trajectories emerging from the strong force. In both cases, the phenomenological formulas describing these observations are similar, albeit with different numerical factors.

These parallels are expected from a theoretical point of view: General Relativity and the Strong Interaction Lagrangians have the same form. The validity of the GRSI model then simply hinges on whether the coupling of the gravitational fields is large enough so that the same effects that occur in hadrons also occur in very massive systems. This coupling is effectively given by $$\sqrt{GM/L}$$, where $$G$$ is the gravitational constant, $$M$$ is the mass of the system, and $$L$$ is a characteristic length of the system. The claim of the GRSI proponents, based either on lattice calculations, a background-field model. or the coincidental phenomenologies in galactic or hadronic dynamics mentioned in the previous paragraph, is that $$\sqrt{GM/L}$$ is indeed sufficiently large for large systems such as galaxies.

List of topics studied in the Model
The main observations that appear to require dark matter and/or dark energy can be explained within this model. Namely,


 * The flat rotation curves of galaxies.  These results, however, have been challenged.
 * The Cosmic Microwave Background anisotropies.
 * The fainter luminosities of distant supernovae and their consequence on the accelerating expansion of the universe.
 * The formation of the Universe's large structures.
 * The matter power spectrum.
 * The internal dynamics of galaxy clusters, including that of the Bullet Cluster.

Additionally, the model explains observations that are currently challenging to understand within Lambda-CDM:


 * The Tully-Fisher relation.
 * The radial acceleration relation.
 * The Hubble tension.
 * The cosmic coincidence, that is the fact that at present time, the purported repulsion of dark energy nearly exactly cancels the action of gravity in the overall dynamics of the universe.

Finally, the model made a prediction that the amount of missing mass (i.e., the dark mass in dark matter approaches) in elliptical galaxies correlates with the ellipticity of the galaxies. This was tested and verified.