scholarly journals Looking beyond inflationary cosmology

2006 ◽  
Vol 84 (6-7) ◽  
pp. 437-446
Author(s):  
R H Brandenberger
Keyword(s):  
String Theory ◽  
Scalar Fields ◽  
Inflationary Cosmology ◽  
Inflationary Universe ◽  
Basic Principles ◽  
String Gas Cosmology ◽  
98.80 Cq ◽  
Conceptual Problems

In spite of the phenomenological successes of the inflationary universe scenario, the current realizations of inflation making use of scalar fields lead to serious conceptual problems that are reviewed in this lecture. String theory may provide an avenue towards addressing these problems. One particular approach to combining string theory and cosmology is String Gas Cosmology. The basic principles of this approach are summarized.PACS No.: 98.80.Cq}

MULTIVERSE COSMOLOGICAL MODELS

2004 ◽  
Vol 19 (10) ◽  
pp. 727-743 ◽  
Author(s):  
P. C. W. DAVIES
Keyword(s):  
String Theory ◽  
Selection Effect ◽  
Elementary Particles ◽  
Cosmological Models ◽  
Anthropic Principle ◽  
Coupling Constants ◽  
Inflationary Cosmology ◽  
Physical Parameters ◽  
Recent Advances ◽  
The Masses

Recent advances in string theory and inflationary cosmology have led to a surge of interest in the possible existence of an ensemble of cosmic regions, or "universes", among the members of which key physical parameters, such as the masses of elementary particles and the coupling constants, might assume different values. The observed values in our cosmic region are then attributed to an observer selection effect (the so-called anthropic principle). The assemblage of universes has been dubbed "the multiverse". In this paper we review the multiverse concept and the criticisms that have been advanced against it on both scientific and philosophical grounds.

QUANTIZATION OF THE LIOUVILLE MODE AND STRING THEORY

1989 ◽  
Vol 04 (11) ◽  
pp. 1033-1041 ◽  
Author(s):  
SUMIT R. DAS ◽  
SATCHIDANANDA NAIK ◽  
SPENTA R. WADIA
Keyword(s):  
String Theory ◽  
Dimensional Space ◽  
Scalar Fields ◽  
Space Time ◽  
Two Dimensions ◽  
General Covariance ◽  
World Sheet ◽  
Lorentzian Signature ◽  
Dimensional Space Time ◽  
Background Fields

We discuss the space-time interpretation of bosonic string theories, which involve d scalar fields coupled to gravity in two dimensions, with a proper quantization of the world-sheet metric. We show that for d>25, the theory cannot describe string modes consistently coupled to each other. For d=25 this is possible; however, in this case the Liouville mode acts as an extra timelike variable and one really has a string moving in 26-dimensional space-time with a Lorentzian signature. By analyzing such a string theory in background fields, we show that the d=25 theory possesses the full 26-dimensional general covariance.

Review of the possible role of self-ordering scalar fields in production of a stochastic background of gravitational waves

2015 ◽  
Vol 24 (04) ◽  
pp. 1541005
Author(s):  
James B. Dent
Keyword(s):  
Phase Transition ◽  
Scalar Field ◽  
Gravitational Wave ◽  
Gravitational Waves ◽  
Scalar Fields ◽  
Inflationary Cosmology ◽  
Tensor Power ◽  
Stochastic Background ◽  
Gravitational Wave Background

A primordial gravitational wave background is a hallmark of inflationary cosmology. The recent announcement made by the BICEP2 collaboration of a possible measurement of B-mode polarization of the CMB on degree scales has produced an abundance of ideas and speculations on how such a signal constrains the inflationary paradigm, or possible alternative mechanisms of gravitational wave production. Here the possibility of a contribution to the gravitational wave background from the relaxation of a scalar field after a global phase transition is reviewed. The general contribution to the overall power is shown, and it is then demonstrated that if the BICEP2 result were to hold, this mechanism could at best produce a very small fraction of the measured tensor power.

scholarly journals DILATONIC GRAVITY NEAR TWO DIMENSIONS AS A STRING THEORY

1995 ◽  
Vol 10 (27) ◽  
pp. 2001-2008 ◽  
Author(s):  
E. ELIZALDE ◽  
S.D. ODINTSOV
Keyword(s):  
Fixed Point ◽  
Renormalization Group ◽  
String Theory ◽  
Sigma Model ◽  
Scalar Fields ◽  
Two Dimensions ◽  
Gravitational Coupling ◽  
Dilaton Coupling

Using the renormalization group formalism, a sigma model of a special type — in which the metric and the dilaton depend explicitly on one of the string coordinates only — is investigated near two dimensions. It is seen that dilatonic gravity coupled to N scalar fields can be expressed in this form, using a string parametrization, and that it possesses the usual uv fixed point. However, in this stringy parametrization of the theory the fixed point for the scalar-dilaton coupling turns out to be trivial, while that for the gravitational coupling remains the same as in previous studies being, in particular, nontrivial.

Supersymmetric QFT in Six Dimensions

2020 ◽  
Author(s):  
Alessandro Tomasiello
Keyword(s):  
String Theory ◽  
Scalar Fields ◽  
Field Theories ◽  
Conformal Field Theories ◽  
Tensor Fields ◽  
Conformal Field ◽  
High Energies ◽  
Six Dimensions ◽  
Full Classification

Quantum field theory (QFT) in six dimensions is more challenging than its four-dimensional counterpart: most models tend to become ill-defined at high energies. A combination of supersymmetry and string theory has yielded many QFTs that evade this problem and are low-energy effective manifestations of conformal field theories (CFTs). Besides the usual vector, spinor and scalar fields, the new ingredients are self-dual tensor fields, analogs of the electromagnetic field with an additional spacetime index, sometimes with an additional non-Abelian structure. A recent wave of interest in this field has produced several classification results, notably of models that have a holographic dual in string theory and of models that can be realized in F-theory. Several precise quantitative checks of the overall picture are now available, and give confidence that a full classification of all six-dimensional CFTs may be at hand.conformal field theories, supersymmetry, extra dimensions, holography, string theory, D-branes, F-theory

BRANE–ANTIBRANE INFLATION

2002 ◽  
Vol 11 (10) ◽  
pp. 1597-1601 ◽  
Author(s):  
C. P. BURGESS
Keyword(s):  
String Theory ◽  
Initial Conditions ◽  
Big Bang ◽  
Inflationary Cosmology ◽  
Big Bang Model ◽  
Insight Into

Inflationary cosmology has become central to our understanding of the initial conditions on whose foundations the current successes of the Hot Big Bang model rest. This is despite the well-known difficulties in finding systems whose dynamics naturally provide all of the features which successful inflation demands. Although string theory provides our best description of the physics of the relevant energy scales, it has only recently begun to shed insight into what the inflationary dynamics might be: the physics of brane-antibrane collisions. This essay is meant to summarize the difficulties which have blocked this realization until now, as well as the new insights about inflation which are now beginning to emerge.

scholarly journals RELAXATION MECHANISMS: FROM STRING MODULI TO AXIONS

1995 ◽  
Vol 10 (39) ◽  
pp. 2993-2999 ◽  
Author(s):  
C.E. VAYONAKIS
Keyword(s):  
String Theory ◽  
Vacuum Expectation ◽  
Vacuum Expectation Value ◽  
Scalar Fields ◽  
Relaxation Mechanism ◽  
Expectation Value

The relaxation mechanism of Damour-Polyakov for fixing the vacuum expectation value of certain scalar fields (moduli) in string theory could provide a convenient framework for the Peccei- Quinn relaxation mechanism and remove the narrow “axion window”.

scholarly journals A symmetric DBI action theory and its applications to inflationary cosmology

2021 ◽  
Vol 81 (12) ◽  
Author(s):  
Xiaokun Yang ◽  
Wu-Long Xu ◽  
Yong-Chang Huang
Keyword(s):  
Field Theory ◽  
String Theory ◽  
Power Spectrum ◽  
Action Theory ◽  
Density Perturbation ◽  
Inflationary Cosmology ◽  
Scalar Perturbation ◽  
Causal Mechanism ◽  
Perturbation Spectrum ◽  
The Universe

AbstractThe Dirac-Born-Infeld (DBI) field theory in string theory is important and can provide the field of the universe’s inflation. At the same time, it provides a causal mechanism for generating the original density perturbation, thereby providing the necessary density perturbation for existing the dense and sparse matter distributions of the universe. We deduce a symmetric DBI action, introduce it into inflationary cosmology to calculate various inflation parameters, further calculate the scalar perturbation spectrum and the tensor-scalar ratio, which are compared with Planck + WMAP9 + BAO data, the power spectrum predicted by the new general DBI inflation theory satisfies the CMB Experiment constraints, i.e., is consistent with the current theories and experimental observations. Consequently, the theory of this paper conforms to current experiments and is supplying the current theories, and also a new way of explaining the inflation of the universe.

scholarly journals Conformal frames in cosmology

2016 ◽  
Vol 25 (13) ◽  
pp. 1645006 ◽  
Author(s):  
Guillem Domènech ◽  
Misao Sasaki
Keyword(s):  
String Theory ◽  
Cosmic Microwave Background ◽  
Scalar Fields ◽  
The Other ◽  
Microwave Background ◽  
Classical Level ◽  
Higher Dimensional ◽  
The Universe ◽  
Matter Sector ◽  
Conformal Frames

From higher dimensional theories, e.g. string theory, one expects the presence of nonminimally coupled scalar fields. We review the notion of conformal frames in cosmology and emphasize their physical equivalence, which holds at least at a classical level. Furthermore, if there is a field, or fields, which dominates the universe, as it is often the case in cosmology, we can use such notion of frames to treat our system, matter and gravity, as two different sectors. On one hand, the gravity sector which describes the dynamics of the geometry and on the other hand, the matter sector which has such geometry as a playground. We use this interpretation to build a model where the fact that a curvaton couples to a particular frame metric could leave an imprint in the cosmic microwave background (CMB).

The Quest for a Theory of Everything

2020 ◽  
pp. 7-17
Author(s):  
Demetris Nicolaides
Keyword(s):  
String Theory ◽  
Standard Model ◽  
Basic Principles ◽  
The Standard Model ◽  
Theory Of Everything ◽  
Modern Physics ◽  
Quantum Version ◽  
Fundamental Forces

Thales’s question, What are things made of?, is still the most difficult in physics. He reasoned that despite the apparent diversity and complexity in nature, all things are made from the same stuff (water), and everything obeys a common set of unchanging basic principles (water’s transformations). Thus, nature is characterized by a certain sameness or unity between all things, however diverse they appear to be. Presently, according to the standard model of physics, everything is made from quarks and leptons. And the plethora of diverse things is partly due to their transformations. Thales’s quest for sameness is modern physics’ search for a theory of everything. It tries to unify the four fundamental forces of nature—the electromagnetic, the nuclear strong, the nuclear weak, and gravity. The challenge of this undertaking is to find a quantum version of gravity. String theory claims to have succeeded, but its hypotheses are still experimentally unverified.

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