The hardest problem of theoretical physics is the **quantum nature of gravity** (QG).
The leading candidate theory is **string theory**. String theory attempts to give a complete unified
theory of everything, ie. all interactions for all matter. At the moment string theory faces several problems like
having an infinite number of equally acceptable vacuum states. Each vacuum represents, no more no less, a universe. This situation has lead many prominent physicists to accept the anthropic principle: the universe is as it is because we are here to observe it.
The hope of string theorists is to find a way to determine the "correct" vacuum somehow, or at least to give probabilities for the different vacua.

Experimental observations requiring new theory include dark energy and dark matter in the universe. These questions are discussed briefly eg. *here*.

People doing research on various aspects of QG (strings, branes; spin networks, etc) are too numerous to list here. Some, having an own web site, are

of Princeton IAS, String Theory (review)Edward Witten

of UC Santa Barbara, The BigJoe PolchinskiBook of String Theoryin two vols.

,String CosmologyAll about stringspeople

of Stanford, String Theory and theLeonard SusskindAnthropic Principle

of Stanford, Inflatory CosmologiesAndrei Linde

of Princeton, Cyclic CosmologiesPaul Steinhartd

of Perimeter Inst., Quantum GravityLee Smolin

of Penn State, Mathematics for new physicsAbhay Ashtekar

of Utrecht, EverythingGerard 't Hooft

of Cambridge Black Hole radiation, Wave Function of the UniverseStephen Hawking

Other important developments include *
Alain Connes*'

Links to some 'randomly' chosen *
recent papers (my blog)*.