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. 1 1

Gravitational wave detectors are already operating at interesting sensitivity levels, and they have an upgrade path that should result in secure detections by 2014. We review the physics of gravitational waves, how they interact with detectors (bars and interferometers), and how these detectors operate. We study the most likely sources of gravitational waves and review the data analysis methods that are used to extract their signals from detector noise. Then we consider the consequences of gravitational wave detections and observations for physics, astrophysics, and cosmology.

Gravity is one of the fundamental forces of Nature, and it is the dominant force in most astronomical systems. In common with all other phenomena, gravity must obey the principles of Special Relativity. In particular, gravitational forces must not be transmitted or communicated faster than light. This means that when the gravitational field of an object changes, the changes ripple outwards through space and take a finite time to reach other objects. These ripples are called gravitational radiation or gravitational waves. 1 In Einstein’s theory of gravitation (see General Relativity and Gravitation), as in many other modern theories of gravity (see Non-general Relativity Theories of Gravity), gravitational waves travel at exactly the speed of light. Different theories make different predictions, however, about details, such as their strength and polarization. There is strong indirect observational evidence (see Binary Stars as a Probe of General Relativity, Hulse-Taylor Pulsar) that gravitational waves follow the predictions of general relativity, and instruments now under construction are expected to make the first direct detections of them in the first years of the 21st century. These instruments and plans for future instruments in space are described in the article Gravitational Radiation Detection on Earth and in Space. Detectors must look for gravitational radiation from astronomical systems, because it is not possible to generate detectable levels of radiation in the laboratory. It follows that gravitational wave detection is also a branch of observational astronomy. The most striking aspect of gravitational waves is their weakness. A comparison with the energy in light ∗ To be published in the Encyclopedia of Astronomy and Astrophysics

Gravitational wave detectors are already operating at interesting sensitivity levels, and they have an upgrade path that should result in secure detections by 2014. We review the physics of gravitational waves, how they interact with detectors (bars and interferometers), and how these detectors operate. We study the most likely sources of gravitational waves and review the data analysis methods that are used to extract their signals from detector noise. Then we consider the consequences of gravitational wave detections and observations for physics, astrophysics, and cosmology.

We construct a model of inflation in string theory after carefully taking into account moduli stabilization. The setting is a warped compactification of Type IIB string theory in the presence of D3 and anti-D3-branes. The inflaton is the position of a D3-brane in the internal space. By suitably adjusting fluxes and the location of symmetrically placed anti-D3-branes, we show that at a point of enhanced symmetry, the inflaton potential V can have a broad maximum, satisfying the condition V ′′ /V ≪ 1 in Planck units. On starting close to the top of this potential the slow-roll conditions can be met. Observational constraints impose significant restrictions. As a first pass we show that these can be satisfied and determine the important scales in the compactification to within an order of magnitude. One robust feature is that the scale of inflation is low, H = O(1010) GeV. Removing the observational constraints makes it much easier to construct a slow-roll inflationary model. Generalizations and consequences including the possibility of eternal inflation