2 research outputs found
Ghosts in the self-accelerating universe
The self-accelerating universe realizes the accelerated expansion of the
universe at late times by large-distance modification of general relativity
without a cosmological constant. The Dvali-Gabadadze-Porrati (DGP) braneworld
model provides an explicit example of the self-accelerating universe. Recently,
the DGP model becomes very popular to study the observational consequences of
the modified gravity models as an alternative to dark energy models in GR.
However, it has been shown that the self-accelerating universe in the DGP model
contains a ghost at the linearized level. The ghost carries negative energy
densities and it leads to the instability of the spacetime. In this article, we
review the origin of the ghost in the self-accelerating universe and explore
the physical implication of the existence of the ghost.Comment: Invited topical review for Classical and Quantum Gravity, 20 pages, 4
figure
Stealth Acceleration and Modified Gravity
We show how to construct consistent braneworld models which exhibit late time
acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our
models have the standard Minkowski vacuum and accelerate only in the presence
of matter, which we dub ``stealth-acceleration''. We use an effective action
for the brane which includes an induced gravity term, and allow for an
asymmetric set-up. We study the linear stability of flat brane vacua and find
the regions of parameter space where the set-up is stable. The 4-dimensional
graviton is only quasi-localised in this set-up and as a result gravity is
modified at late times. One of the two regions is strongly coupled and the
scalar mode is eaten up by an extra symmetry that arises in this limit. Having
filtered the well-defined theories we then focus on their cosmology. When the
graviton is quasi-localised we find two main examples of acceleration. In each
case, we provide an illustrative model and compare it to LambdaCDM.Comment: 32 pages, 5 figure