Lee Smolin: Galaxy rotation curves: missing matter, or missing physics?

The speaker, Lee Smolin, expresses gratitude for the opportunity to speak and acknowledges the influence of philosophy on the development of fundamental quantum gravity. He emphasizes the importance of philosophical inputs, such as the hole argument, relationalism, and diffeomorphism invariance, in shaping the field. Smolin also introduces his talk's focus on the search for new principles in quantum gravity and the need for a quantum theory that goes beyond existing models.

Smolin discusses the necessity for a quantum theory of gravity to be based on fundamental principles, like Einstein's general theory of relativity. He proposes a strategy for quantum gravity that starts with principles and leads to testable predictions. The speaker outlines seven key points for his presentation and hints at the potential for new domains of quantum gravitational phenomena.

The speaker delves into the challenges of extending the equivalence principle to quantum mechanics, especially in the presence of a cosmological constant. Smolin examines the tension between the non-locality of the quantum vacuum and the local demands of the equivalence principle, setting the stage for a new formulation of the principle in the context of quantum gravity.

Smolin explores the implications of a non-zero cosmological constant on the formulation of the equivalence principle. He suggests that the principle may need to be re-evaluated in light of the observed acceleration of the universe and proposes a new domain of quantum gravitational phenomena associated with this acceleration.

The speaker identifies a new domain of quantum gravity phenomena characterized by accelerations small compared to the universe's acceleration due to a positive cosmological constant. He proposes that this domain could be explored using the far-infrared limit of quantum gravity and suggests that galaxy rotation curves might provide a laboratory for studying this domain.

Smolin examines the rotation curves of galaxies, which show a departure from Newtonian dynamics at low accelerations, and suggests that these curves might be indicative of a new domain of quantum gravity. He discusses the observed relationship between the acceleration of stars in galaxies and the predicted acceleration based on luminosity, highlighting a critical acceleration scale that matches the universe's acceleration.

The speaker discusses the phenomenological model proposed by Milgram for the low acceleration regime, known as Modified Newtonian Dynamics (MOND). Smolin suggests that MOND might be a manifestation of the new domain of quantum gravity he previously described and emphasizes the need for a deeper understanding of this phenomenon.

Smolin hypothesizes that in the presence of a small positive cosmological constant, the equivalence of inertial and gravitational mass may be renormalized due to the thermal bath experienced by static observers in an accelerating universe. He proposes a renormalization factor that depends on the ratio of the Unruh temperature to the de Sitter temperature.

The speaker introduces the concept of scale invariance in the context of the new domain of quantum gravity phenomena. He suggests that the dynamics in this domain should be invariant under scaling transformations, leading to a characteristic relation between the observed acceleration and the Newtonian acceleration.

Smolin presents a thermodynamic perspective on the origin of inertia, modified by the presence of a cosmological constant. He proposes an entropic force that accounts for the observed phenomena in MOND and suggests that Newton's laws may emerge from a deeper quantum and relativistic context.

In the final part of his talk, Smolin discusses the extension of his hypothesis to the context of general relativity using bimetric theories. He suggests that the renormalization factor could be incorporated into the metric tensor, providing an alternative description of the observed phenomena in the language of general relativity.