Black hole collapse and bounce in effective loop quantum gravity

 Tuesday, November 24th

Edward Wilson-Ewing, University of New Brunswick

Black hole collapse and bounce in effective loop quantum gravity
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By Jorge Pullin, LSU

Stars are balls of fluid that are try to contract through their own gravitational attraction but are kept form doing so by burning nuclear fuel, which also makes them shine. When the fuel gets exhausted they start to contract. Depending on the details, the contraction can become uncontrollable, leading to an object so dense that gravity is so intense that not even light can escape from them. That is what is known as black hole. The matter continues to contract inside the black hole and eventually get highly concentrated. In classical general relativity, this leads to a "singularity", a point where density is infinite. It is expected that quantum gravity will eliminate such singularities, replacing them by a highly quantum region of high curvature.

Loop quantum gravity has led to scenarios of that nature. These investigations are pursued by restricting strongly the degrees of freedom of the problem before quantizing, this makes quantization possible. In this talk one of such proposals was considered. The particular freezing of degrees of freedom requires choosing certain coordinate systems that simplify the equations. This allows to treat the problem including the presence of matter. This in turn opens the possibility of studying how the matter collapses, forms the black hole, and then, since things never become singular, the matter explodes into a "white hole", the time reversal of a black hole. This opens new possibilities for understanding the ultimate fate of black holes and what happens to the information that falls into a black hole, is it lost or is it recovered? Further research will shed light on these issues.

Quantum gravity at the corner

 Tuesday, October 27th

Marc Geiller, ENS Lyon

Quantum gravity at the corner 
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By Jorge Pullin, LSU

Many physical theories are described in terms of more variables than needed. That includes field theories like general relativity. For an analogy consider a pendulum. We can describe it by giving the x,y coordinates of the bob, even though everything is completely characterized if one just gives the angle of the pendulum wire with respect to the vertical. When one has extra variables there may exist many sets of values of them that correspond to the same physical situation. In the pendulum x=1,y=1 and x=2,y=2 both correspond to the wire at 45 degrees. So it is said that these theories have symmetries in the sense that many mathematical configurations correspond to the same physical situation. These are mathematical, not physical symmetries. However, if one considers bounded regions of space-time those mathematical symmetries translate into physical symmetries and into conserved quantities. For instance the electric charge. To define electric charge one needs to define a region it is contained in.

More recently the concept has appeared in physics that one can describe what is happening in a region of space-time by describing what is happening at its boundary. An example of this is the so called AdS/CFT or Maldacena conjecture in string theory. This applies to a specific type of space-times called anti de Sitter (AdS) and it says that the description of gravity in the space-time is equivalent to a special type of field theory called conformal field theory (CFT)  that lives on the boundary of the space-time. This property of encoding the information of a space-time in its boundary is known as "holography" by analogy with the optical phenomenon where three dimensional images are captured on a two dimensional photograph.

This talk addressed studying bounded regions of space-time, more precisely bounded regions of a spatial slice of a space-time, where the boundary is two dimensional and is called "corner" in math. It was explored what is the most general set of symmetries that one can formulate and their implications in the corners. It was observed that certain properties of loop quantum gravity, like the quantization of the areas, arise naturally in this context. This way of viewing things opens a new approach to loop quantum gravity that may offer connections with the ideas of holography in string theory.

I benefited from discussions with Ivan Agulló while preparing this text.