16 research outputs found
Cosmological Viability of Theories with Massive Spin-2 Fields
Theories of spin-2 fields take on a particular role in modern physics. They do not only describe the mediation of gravity, the only theory of fundamental interactions of
which no quantum field theoretical description exists, it furthermore was thought that they necessarily predict massless gauge bosons. Just recently, a consistent theory of
a massive graviton was constructed and, subsequently, generalized to a bimetric theory of two interacting spin-2 fields. This thesis studies both the viability and consequences at cosmological scales in massive gravity as well as bimetric theories. We show that all consistent models that are free of gradient and ghost instabilities behave like the cosmological standard model, LCDM. In addition, we construct a new theory of massive gravity which is stable at both classical background and quantum level, even though it suffers from the Boulware-Deser ghost
A spectre is haunting the cosmos: Quantum stability of massive gravity with ghosts
Many theories of modified gravity with higher order derivatives are usually
ignored because of serious problems that appear due to an additional ghost
degree of freedom. Most dangerously, it causes an immediate decay of the
vacuum. However, breaking Lorentz invariance can cure such abominable behavior.
By analyzing a model that describes a massive graviton together with a
remaining Boulware-Deser ghost mode we show that even ghostly theories of
modified gravity can yield models that are viable at both classical and quantum
levels and, therefore, they should not generally be ruled out. Furthermore, we
identify the most dangerous quantum scattering process that has the main impact
on the decay time and find differences to simple theories that only describe an
ordinary scalar field and a ghost. Additionally, constraints on the parameters
of the theory including some upper bounds on the Lorentz-breaking cutoff scale
are presented. In particular, for a simple theory of massive gravity we find
that a breaking of Lorentz invariance is allowed to happen even at scales above
the Planck mass. Finally, we discuss the relevance to other theories of
modified gravity.Comment: 18 pages, 3 figures, version published in JHE
Bimetric gravity is cosmologically viable
Bimetric theory describes gravitational interactions in the presence of an
extra spin-2 field. Previous work has suggested that its cosmological solutions
are generically plagued by instabilities. We show that by taking the Planck
mass for the second metric, , to be small, these instabilities can be
pushed back to unobservably early times. In this limit, the theory approaches
general relativity with an effective cosmological constant which is,
remarkably, determined by the spin-2 interaction scale. This provides a
late-time expansion history which is extremely close to CDM, but with
a technically-natural value for the cosmological constant. We find should
be no larger than the electroweak scale in order for cosmological perturbations
to be stable by big-bang nucleosynthesis. We further show that in this limit
the helicity-0 mode is no longer strongly-coupled at low energy scales.Comment: 8+2 pages, 2 tables. Version published in PLB. Minor typo corrections
from v