Dynamics of Screening in Modified Gravity.

Area of Science:

• Cosmology
• Astrophysics
• Theoretical Physics

Background:

• Cosmological acceleration is typically explained by a cosmological constant within General Relativity.
• Alternative gravitational theories propose scalar-tensor theories to explain cosmic acceleration without a cosmological constant.
• These theories require a 'screening mechanism' to reconcile cosmic acceleration with local gravitational tests.

Purpose of the Study:

• To investigate the stability and dynamical evolution of screened stars in a simple scalar-tensor theory with first-order derivative self-interactions.
• To determine if these screened solutions can be reliably evolved numerically.

Main Methods:

• Studied isolated, static, and spherically symmetric nonrelativistic and relativistic stars within a specific scalar-tensor theory.
• Generated screened solutions and used them as initial data for nonlinear numerical evolutions in spherical symmetry.

Main Results:

• Screened stellar solutions were found to be stable under large perturbations, provided gravitational collapse was not triggered.
• Upon triggering gravitational collapse, characteristic speeds in the scalar evolution equation diverged prematurely, before horizon formation.

Conclusions:

• The stability of screened stars in these theories is confirmed under non-collapsing conditions.
• The divergence of scalar speeds during collapse raises significant doubts about the predictive power of these effective field theories for the dynamical evolution of screened stars.