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String Theory
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Lecture 3 | Topics in String Theory

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Title: D-branes and cosmic structure
Authors: Danielle Wills, Konstantinos Dimopoulos, Ivonne Zavala

We outline the embedding of the vector curvaton scenario as a promising mechanism to generate statistical anisotropy within Type IIB string theory, where the vector field on a single D-brane plays the role of the vector curvaton. We first consider a toy model in the context of open string inflation, and then begin to construct a concrete model in the context of closed string inflation.

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String Theory Helps To Explain Quantum Phases Of Matter

Newly discovered states of matter embody what Einstein called "spooky action at a distance." They defy explanation, but lately answers have come from a seemingly unrelated corner of physics: string theory
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Type-II string theories
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Title: Topological Ghosts: the Teeming of the Shrews
Authors: Nemanja Kaloper, McCullen Sandora

We consider dynamics of spacetime volume-filling form fields with "wrong sign" kinetic terms, such as in so-called Type-II^* string theories. Locally, these form fields are just additive renormalisations of the cosmological constant. They have no fluctuating degrees of freedom. However, once the fields are coupled to membranes charged under them, their configurations are unstable: by a process analogous to Schwinger pair production the field space-filling flux increases. This reduces the cosmological constant, and preserves the null energy condition, since the processes that can violate it by reducing the form flux are very suppressed. The increase of the form flux implies that as time goes on the probability for further membrane increases, in contrast to the usual case where the field approaches its vacuum value and ceases to induce further transitions. Thus, in such models spaces with tiny positive vacuum energy are ultimately unstable, but the instability may be slow and localized. In a cosmological setting, this instability can enhance black hole rate formation, by locally making the vacuum energy negative at late times, which constrains the scales controlling membrane dynamics, and may even collapse a large region of the visible universe.

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Title: Signatures of Brane Inflation
Authors: Ashok Thillaisundaram

We study slow-roll inflation on a three-brane in a five-dimensional bulk where the effects of energy loss from the brane due to graviton emission is included in a self-consistent manner. We explicitly derive the form of the energy loss term due to inflaton-to-graviton scattering and thus determine the precise dynamics of the two resulting inflationary solutions. What is also remarkable is that nonconservation of energy on the brane causes the curvature perturbation to not be conserved on superhorizon scales even for the purely adiabatic perturbations produced in single-field inflation. Thus the standard method of calculating the power spectrum of inflaton fluctuations at Hubble exit and equating it to the power spectrum at horizon reentry no longer holds. The superhorizon evolution of the perturbations must be tracked from horizon exit through to when the modes reenter the horizon for the late time power spectrum to be calculated. We develop the methodology to do this in this paper as well.

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 Do we need String Theory for Quantum Gravity? - Lance Dixon (SETI Talks)



The strong, weak, and electromagnetic interactions all have consistent, relativistic and quantum mechanical descriptions in terms of pointlike particles, but Einstein's theory of gravitation has long resisted a similar treatment, because of severe ultraviolet divergences. String theory solves these problems, but it introduces a new length scale, perhaps 16 orders of magnitude below what can be tested experimentally. Dr. Dixon will describe recent theoretical progress in showing that a particular pointlike theory of gravity, called N=8 supergravity, might also be quantum mechanically consistent. In particular, N=8 supergravity has been shown explicitly to have no ultraviolet divergences in perturbation theory through the four-loop order. Dr. Dixon will also discuss the possible implications of these results.



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Title: Generalised Hooke Law for Relativistic Membranes and p-branes
Authors: A. A. Zheltukhin

The character of elastic forces of relativistic membranes and p-branes encoded in their nonlinear equations is studied. The toroidal brane equations are reduced to the classical equations of anharmonic elastic media described by monomial potentials. Integrability of the equations is discussed and some of their exact solutions are constructed.

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The mechanism that explains why our universe was born with 3 dimensions

Superstring theory predicts a space with 9 dimensions, which poses the big puzzle of how this can be consistent with the 3-dimensional space that we live in.
A group of 3 researchers, Jun Nishimura (associate professor at KEK), Asato Tsuchiya (associate professor at Shizuoka University) and Sang-Woo Kim (project researcher at Osaka University) has succeeded in simulating the birth of the universe, using a supercomputer for calculations based on superstring theory. This showed that the universe had 9 spatial dimensions at the beginning, but only 3 of these underwent expansion at some point in time.
This work will be published soon in Physical Review Letters.

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String theory
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Part 01: String theory

Part02: String Theory



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Part 03: String Theory

Part 04: String Theory

Part 05: String Theory



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