# The critical tension in the Cascading DGP model

## Abstract

We study the behaviour of weak gravitational fields in the 6D Cascading DGP model using a bulk-based approach. To deal with the ambiguity in the thin limit of branes of codimension higher than one, we consider a specific regularization of the internal structure of the branes where the 5D brane can be considered thin with respect to the 4D one. We consider the solutions corresponding to pure tension sources on the 4D brane, and study perturbations at first order around these background solutions. We adopt a 4D scalar-vector-tensor decomposition, and focus on the scalar sector of perturbations. We show that, in a suitable 4D limit, the trace part of the 4D metric perturbations obeys a decoupled equation which suggests that it is a ghost for background tensions smaller than a critical tension, while it is a healthy field otherwise. We give a geometrical interpretation of the existence of the critical tension and of the reason why the relevant field is a ghost or not depending on the background tension. We however find a value of the critical tension which is different from the one already found in the literature. Differently from the results in the literature, our analysis implies that,more »

- Authors:

- Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Portsmouth, PO1 3FX (United Kingdom)

- Publication Date:

- OSTI Identifier:
- 22375869

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Journal of Cosmology and Astroparticle Physics; Journal Volume: 2014; Journal Issue: 09; Other Information: Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BRANES; GRAVITATIONAL FIELDS; MATHEMATICAL SOLUTIONS; PERTURBATION THEORY

### Citation Formats

```
Sbisà, Fulvio, and Koyama, Kazuya, E-mail: fulvio.sbisa@port.ac.uk, E-mail: kazuya.koyama@port.ac.uk.
```*The critical tension in the Cascading DGP model*. United States: N. p., 2014.
Web. doi:10.1088/1475-7516/2014/09/038.

```
Sbisà, Fulvio, & Koyama, Kazuya, E-mail: fulvio.sbisa@port.ac.uk, E-mail: kazuya.koyama@port.ac.uk.
```*The critical tension in the Cascading DGP model*. United States. doi:10.1088/1475-7516/2014/09/038.

```
Sbisà, Fulvio, and Koyama, Kazuya, E-mail: fulvio.sbisa@port.ac.uk, E-mail: kazuya.koyama@port.ac.uk. Mon .
"The critical tension in the Cascading DGP model". United States.
doi:10.1088/1475-7516/2014/09/038.
```

```
@article{osti_22375869,
```

title = {The critical tension in the Cascading DGP model},

author = {Sbisà, Fulvio and Koyama, Kazuya, E-mail: fulvio.sbisa@port.ac.uk, E-mail: kazuya.koyama@port.ac.uk},

abstractNote = {We study the behaviour of weak gravitational fields in the 6D Cascading DGP model using a bulk-based approach. To deal with the ambiguity in the thin limit of branes of codimension higher than one, we consider a specific regularization of the internal structure of the branes where the 5D brane can be considered thin with respect to the 4D one. We consider the solutions corresponding to pure tension sources on the 4D brane, and study perturbations at first order around these background solutions. We adopt a 4D scalar-vector-tensor decomposition, and focus on the scalar sector of perturbations. We show that, in a suitable 4D limit, the trace part of the 4D metric perturbations obeys a decoupled equation which suggests that it is a ghost for background tensions smaller than a critical tension, while it is a healthy field otherwise. We give a geometrical interpretation of the existence of the critical tension and of the reason why the relevant field is a ghost or not depending on the background tension. We however find a value of the critical tension which is different from the one already found in the literature. Differently from the results in the literature, our analysis implies that, choosing the background tension suitably, we can construct ghost-free models for any value of the free parameters of the theory. We suggest that the difference lies in the procedure used to evaluate the pillbox integration across the codimension-2 brane. We confirm the validity of our analysis by performing numerically the integration in a particular case where the solution inside the thick cod-2 brane is known exactly. We stress that the singular structure of the perturbation fields in the nested branes set-ups is very subtle, and that great care has to be taken when deriving the codimension-2 junction conditions.},

doi = {10.1088/1475-7516/2014/09/038},

journal = {Journal of Cosmology and Astroparticle Physics},

number = 09,

volume = 2014,

place = {United States},

year = {Mon Sep 01 00:00:00 EDT 2014},

month = {Mon Sep 01 00:00:00 EDT 2014}

}