Instability of anisotropic cosmological solutions supported by vector fields

Area of Science:

• Cosmology
• Theoretical Physics
• Particle Physics

Background:

• Models with vector fields acquiring a nonvanishing vacuum expectation value along one spatial direction have been proposed for anisotropic accelerated expansion.
• These models have been explored for early time inflation and late time dark energy, potentially explaining cosmic microwave background anomalies.

Purpose of the Study:

• To investigate the stability of proposed models for anisotropic accelerated expansion.
• To determine the reliability of phenomenological results derived from these models.

Main Methods:

• Analysis of vector field models with nonvanishing vacuum expectation values.
• Investigation of linearized perturbations to identify instabilities (ghosts or unstable growth).
• Relating instabilities to the longitudinal vector polarization within the models.

Main Results:

• Proposed models for prolonged anisotropic accelerated expansion are generally plagued by instabilities.
• These instabilities, including ghosts and unstable perturbation growth, are linked to longitudinal vector polarization.
• The identified instabilities undermine the reliability of phenomenological predictions from these models.

Conclusions:

• Concrete realizations of anisotropic accelerated expansion models based on vector fields are fundamentally unstable.
• The presence of longitudinal vector polarization is a key factor contributing to these instabilities.
• Phenomenological results derived from these unstable models are unreliable for cosmological applications.