Collisions of Cosmic F- and D-strings

Recent work suggests that fundamental and Dirichlet strings, and their (p,q) bound states, may be observed as cosmic strings. The evolution of cosmic string networks, and therefore their observational signals, depends on what happens when two strings collide. We study this in string perturbation theory for collisions between all possible pairs of strings; different cases involve sphere, disk, and annulus amplitudes. The result also depends on the details of compactification; the dependence on ratios of scales is only logarithmic, but this is still numerically important. We study a range of models and parameters, and find that in most cases these strings can be distinguished from cosmic strings that arise as gauge theory solitons.Comment: 42 pages, 7 figures; v.2: added references, expanded discussion of reconnection in field theor

A Note on the evolution of cosmic string/superstring networks

In the context of brane world scenario, cosmic superstrings can be formed in D-brane annihilation at the end of the brane inflationary era. The cosmic superstring network has a scaling solution and the characteristic scale of the network is proportional to the square root of the reconnection probability.Comment: 13 pages, 12 figures (revised version

String windings in the early universe

We study string dynamics in the early universe. Our motivation is the proposal of Brandenberger and Vafa, that string winding modes may play a key role in decompactifying three spatial dimensions. We model the universe as a homogeneous but anisotropic 9-torus filled with a gas of excited strings. We adopt initial conditions which fix the dilaton and the volume of the torus, but otherwise assume all states are equally likely. We study the evolution of the system both analytically and numerically to determine the late-time behavior. We find that, although dynamical evolution can indeed lead to three large spatial dimensions, such an outcome is not statistically favored.Comment: 26 pages, LaTeX, 4 eps figure

Reconnection of Non-Abelian Cosmic Strings

Cosmic strings in non-abelian gauge theories naturally gain a spectrum of massless, or light, excitations arising from their embedding in color and flavor space. This opens up the possibility that colliding strings miss each other in the internal space, reducing the probability of reconnection. We study the topology of the non-abelian vortex moduli space to determine the outcome of string collision. Surprisingly we find that the probability of classical reconnection in this system remains unity, with strings passing through each other only for finely tuned initial conditions. We proceed to show how this conclusion can be changed by symmetry breaking effects, or by quantum effects associated to fermionic zero modes, and present examples where the probability of reconnection in a U(N) gauge theory ranges from 1/N for low-energy collisions to one at higher energies.Comment: 25 Pages, 3 Figures. v2: comment added, reference adde

Cosmic Strings and Superstrings

Cosmic strings are predicted by many field-theory models, and may have been formed at a symmetry-breaking transition early in the history of the universe, such as that associated with grand unification. They could have important cosmological effects. Scenarios suggested by fundamental string theory or M-theory, in particular the popular idea of brane inflation, also strongly suggest the appearance of similar structures. Here we review the reasons for postulating the existence of cosmic strings or superstrings, the various possible ways in which they might be detected observationally, and the special features that might discriminate between ordinary cosmic strings and superstrings.Comment: Minor errors corrected and some references added, 34 pages, 6 figure

Brane Inflation and Cosmic String Tension in Superstring Theory

In a simple reanalysis of the KKLMMT scenario, we argue that the slow roll condition in the D3-anti-D3-brane inflationary scenario in superstring theory requires no more than a moderate tuning. The cosmic string tension is very sensitive to the conformal coupling: with less fine-tuning, the cosmic string tension (as well as the ratio of tensor to scalar perturbation mode) increases rapidly and can easily saturate the present observational bound. In a multi-throat brane inflationary scenario, this feature substantially improves the chance of detecting and measuring the properties of the cosmic strings as a window to the superstring theory and our pre-inflationary universe.Comment: Combined bounds from WMAP and SDSS Lyman alpha experiments are added for analysis, changes are added to the tabl

A Note on Noncommutative Brane Inflation

In this paper, we investigate the noncommutative KKLMMT D3/anti-D3 brane inflation scenario in detail. Incorporation of the brane inflation scenario and the noncommutative inflation scenario can nicely explain the large negative running of the spectral index as indicated by WMAP three-year data and can significantly release the fine-tuning for the parameter $\beta$. Using the WMAP three year results (blue-tilted spectral index with large negative running), we explore the parameter space and give the constraints and predictions for the inflationary parameters and cosmological observables in this scenario. We show that this scenario predicts a quite large tensor/scalar ratio and what is more, a too large cosmic string tension (assuming that the string coupling $g_s$ is in its likely range from 0.1 to 1) to be compatible with the present observational bound. A more detailed analysis reveals that this model has some inconsistencies according to the fit to WMAP three year results.Comment: 20 pages, 5 figures; accepted for publication in JCA

Cosmic F- and D-strings

Macroscopic fundamental and Dirichlet strings have several potential instabilities: breakage, tachyon decays, and confinement by axion domain walls. We investigate the conditions under which metastable strings can exist, and we find that such strings are present in many models. There are various possibilities, the most notable being a network of (p,q) strings. Cosmic strings give a potentially large window into string physics.Comment: 27 pages, 5 figures; v. 5: JHEP style, added comments in section 2.

Seeking String Theory in the Cosmos

We review the existence, formation and properties of cosmic strings in string theory, the wide variety of observational techniques that are being employed to detect them, and the constraints that current observations impose on string theory models.Comment: 25 pages; contribution for String Cosmology issue of Classical and Quantum Gravity. References added and other improvements. Matches journal versio