My Research

# What is String Theory?

We live in a wonderfully complex universe, and we are curious about it by nature. Time and again we have wondered--- why are we here? Where did we and the world come from? What is the world made of? It is our privilege to live in a time when enormous progress has been made towards finding some of the answers. String theory is our most recent attempt to answer these questions.

The essential idea behind string theory is this: all of the different 'fundamental ' particles of the Standard Model are really just different manifestations of one basic object: a string. How can that be? Well, we would ordinarily picture an electron, for instance, as a point with no internal structure. A point cannot do anything but move. But, if string theory is correct, then under an extremely powerful 'microscope' we would realize that the electron is not really a point, but a tiny loop of string. A string can do something aside from moving--- it can oscillate in different ways. If it oscillates a certain way, then from a distance, unable to tell it is really a string, we see an electron. But if it oscillates some other way, well, then we call it a photon, or a quark, or a ... you get the idea. So, if string theory is correct, the whole world may be made of strings!

### The Gauge Theory/Gravity Connection

The AdS/CFT correspondence (anti-De-Sitter space/conformal field theory correspondence), sometimes called the Maldacena duality, is the conjectured equivalence between a string theory defined on one space, and a quantum field theory without gravity defined on the conformal boundary of this space, whose dimension is lower by at least one.

The name suggests that the first space is the product of anti de Sitter space (AdS) with some closed manifold like sphere, orbifold, or noncommutative space, and that the quantum field theory is a conformal field theory (CFT) - one in which size or scale really doesn’t matter. The fact that it’s a duality means that each predicted phenomenon and quantity in one theory has an analogue in the other theory, with a "dictionary" translating from one theory to the other.

This branch of string theory may lead to new insights on quantum chromodynamics, a gauge theory which is the fundamental theory of the strong nuclear force. To this end, it is hoped that a gravitational theory dual to quantum chromodynamics will eventually be found in string theory.

### Strings and Cosmology

Reading so far, there are a probably a number of questions that might occur to you: Can string theory make any cosmological predictions relevant to Big Bang physics? What happens to the extra dimensions? What can string theory tell us about quantum gravity and cosmology?

Superstring Cosmology is the name of the relatively new branch of string theory that attempts to address such questions. Superstring cosmology is enormously complicated by the presence of the additional six (or seven in the case of M theory) extra space dimensions that are required for quantum consistency of the theory. While these are difficult enough to deal with in string theory, in the time-dependent and dynamic environment of cosmology, the extra dimensions are evolving in time according to the physics of the Big Bang and whatever happened before it. The question of what keeps the extra dimensions from expanding to get as big as the three space dimensions that we observe and measure in our Universe is one of the major unsolved problems of this field called the moduli problem. Solving this and other problems will not only require a better model of the low energy dynamics of string theory but, probably a much more detailed understanding of the degrees of freedom in the full string theory but which holds the promise of nothing short of unlocking the nature of the Big Bang itself.