Playing with Ghosts

Ghosts are particles with negative mass, i.e., they have kinetic energy unbounded from below. No such particles have been detected so far. However, ghosts often appear in theories of modified gravity and are very interesting from the point of view of cosmological applications. Until recently, it was commonly believed that the appearance of ghosts is always connected with severe instabilities. However, recent papers [1] and [2] have demonstrated that mechanical systems with ghosts can be manifestly stable, at least with a low number of degrees of freedom. This fully funded project will investigate:

1. How interactions of positive mass particles in realistic systems can mimic ghosts.
2. How to extend the results from [1,2] to the quantum domain.
3. How to extend the results from [1,2] to a large number of degrees of freedom, particularly in classical and quantum field theories.
4. What is the role of gravitational interaction in such models?

This project will involve intensive collaborations with France and Japan.

References:

[1] Cédric Deffayet, Shinji Mukohyama, Alexander Vikman, „Ghosts without Runaway Instabilities“, Phys.Rev.Lett. 128 (2022) 4, 041301 e-Print: 2108.06294
[2] Cédric Deffayet, Aaron Held, Shinji Mukohyama, Alexander Vikman, „Global and Local Stability for Ghosts Coupled to Positive Energy Degrees of Freedom“, JCAP 11 (2023) 031 • e-Print: 2305.09631