## Project Details

### Description

Acronym | FWOTM474 |
---|---|

Status | Finished |

Effective start/end date | 1/10/08 → 30/09/14 |

### Keywords

- Physics

### Flemish discipline codes in use since 2023

- Physical sciences

- Koshelev, Alexey (Administrative Promotor)
- Sevrin, Alexandre (Co-Promotor)

The combined analysis of the data on type Ia supernovae, galaxy clusters measurements and WMAP (Wilkinson Microwave Anisotropy Probe) gives clear evidence of the accelerated expansion of the Universe. The cosmological acceleration suggests that the current Universe is dominated by the smoothly distributed, slowly varying Dark Energy (DE). Recent results of WMAP together with the data on Ia supernovae strongly support that the DE state parameter w is currently close to -1 but not necessarily above or below -1 [1]. Nowadays, w?-1 comes out as one of the main problem of the contemporarily observed Universe. An exact equation of state w=-1 corresponds to a cosmological constant. Such a constant should be roughly 120 orders of magnitude less than what one would naively expect based on Quantum Field Theory. Such a huge difference is the main objection against having a cosmological constant in the gravity action. w-1 or vice versa is forbidden under general assumptions in single local scalar field models. In addition a coincidence problem is of main concern of the present research efforts. The coincidence problem is the question why we live in the epoch when the energy densities of the three main components of the Universe (Dark Energy, Dark Matter and visible matter) are comparable. Besides a direct construction of a model describing intriguing present phenomena, there is hope that a fundamental theory explaining current observational results exists. The aim of the proposed research is to find an explanation of the present epoch accelerated Universe expansion [2] in the framework of string theory, in particular D-branes. D-branes are non-perturbative objects in string theory, which is the main candidates for a theory of Great Unification. In this regard it is proposed to use the following approaches seeking for adequate models: String Field Theory (SFT) [3], Dirac-Born-Infeld (DBI) effective action for the D-brane dynamics and brane-world scenarios as well as general time-dependent backgrounds. The first approach, SFT, so far is the only possibility to have a systematic off-shell description of the string theory. One of the main success of SFT in the past few years is Sen's description of unstable D-brane decay (known as Sen's conjecture). Full understanding of this process, which is not achieved so far, will shade light on the vacuum structure of the String Theory. In recent papers with my co-authorship and my single-author paper [4] it was shown the non-local open string tachyon coming from SFT can play the role of a scalar field governing an observed accelerating cosmic expansion. Therefore, it is important to study cosmological scenarios with open and closed string scalar excitations (tachyons, dilaton) coupled to gravity as a background. Such cosmological models may exhibit already known properties but being found in string theory they will have a much more natural motivation. Another aspect of the SFT approach is the connection of unstable brane decay with cosmology. This issue has been studied a lot using the DBI approach but there are too few attempts using String Field Theory [5]. A significant role of Vacuum SFT is foreseen in this context. Vacuum SFT is a theory in the true vacuum observed after the brane decay. Such a vacuum should not contain, according to Sen's conjecture, open string excitations and have only closed strings. DBI is an effective action while SFT is a more systematic description of strings and branes. However, it incorporates many significant properties of the brane dynamics and moreover, cosmological scenarios built upon DBI action exhibit novel interesting features. Thus, it seems to be of great importance to understand a connection between SFT and DBI descriptions of the brane dynamics. It is a very exciting question I'm going to address in my research work. It is interesting to study the above mentioned non-local models in a broader context and understand a possible role of non-local interactions. Indeed, non-locality is a general property of SFT interaction. Thus, once SFT is supposed to describe the off-shell dynamics of strings, one has to take the non-locality int account. Moreover, simple non-local scalar field models may have interesting properties from the cosmological point of view. For example, such a model may have a phantom phase without usual phantom instability and there is a possibility for crossing the line w=-1. As it was stated above, such a crossing is impossible for a single local scalar field. Studying of open-closed strings interaction in the framework of SFT [6] is another problem to be investigated. Being interesting and difficult question itself it is also very important for the brane decay processes as well as for cosmological scenarios. In fact, the graviton is a state in the spectrum of closed strings and understanding of its action and interaction with other fields from the very fundamental principals will definitely help to understand the structure of the whole theory. It is proposed to explore the known low-level actions constructed in the closed SFT from the point of view of cosmology. This may give a tool to select more realistic models. As a next step in the exploring of SFT based models it is proposed to explore not only open or closed string tachyons but also an interaction with other low-level fields. Here of most importance are vector field of the open SFT and dilaton of closed SFT. These are zero mass fields which will definitely affect the dynamics just because they are on the same mass level as graviton and cannot be integrated out as massive fields usually do. It is expected according to the preliminary calculations that a presence of the dilaton field plays a significant role in managing scales of the cosmological models to be built. It is very important because of connection with a vast amount of experimental data. Influence of vector field is not clear so far but according to recent studies it may be one of the candidate for a successful model of DE without usual phantom pathologies [7]. Brane-world building [8] is another approach and so far there are, for example, Randall-Sundrum and DGP models which are very rich phenomenologically. However, relation of brane-worlds with fundamental string theory is weak and has to be clarified. It seems that above mentioned approaches may give some information about such a relation. For instance, it is very interesting to see how brane-worlds can emerge in SFT. Despite of direct searching it is possible to implement some indirect checks. One has to study how significant properties of SFT are reflected in the brane-world models. As a first step in this direction it is proposed to see how non-local interactions affect brane-world models.

Acronym | FWOTM474 |
---|---|

Status | Finished |

Effective start/end date | 1/10/08 → 30/09/14 |

- Physics

- Physical sciences

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