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Martin Bojowald and Others

 

Martin Bojowald is a German-born physicist who now works at the Institute for Gravitation and the Cosmos of the Pennsylvania State University, USA. In 2005 he joined the permanent staff of the IGC, after spending several years at the Max-Planck Institute for Gravitational Physicin Golm, Germany. He works on loop quantum gravity and physical cosmology and is credited with establishing the sub-field of loop quantum cosmology.

Quantum gravity is expected to be necessary in order to understand situations in which classical general relativity breaks down. In particular in cosmology one has to deal with initial singularities, i.e., the fact that the backward evolution of a classical spacetime inevitably comes to an end after a finite amount of proper time. This presents a breakdown of the classical picture and requires an extended theory for a meaningful description. Since small length scales and high curvatures are involved, quantum effects must play a role. Not only the singularity itself but also the surrounding space time is then modified. One particular theory is loop quantum cosmology, an application of loop quantum gravity to homogeneous systems, which removes classical singularities. Its implications can be studied at different levels.  
The main effects are introduced into effective classical equations, which allow one to avoid the interpretational problems of quantum theory. They give rise to new kinds of early-universe phenomenology with applications to inflation and cyclic models. To resolve classical singularities and to understand the structure of geometry around them, the quantum description is necessary. Classical evolution is then replaced by a difference equation for a wave function, which allows an extension of quantum space time beyond classical singularities. One main question is how these homogeneous scenarios are related to full loop quantum gravity, which can be dealt with at the level of distributional symmetric states.

Finally, the new structure of space time arising in loop quantum gravity and its application to cosmology sheds light on more general issues, such as the nature of time.