Tuesday, May 5, 2020

Alleviating tensions in the cosmic microwave background using loop quantum cosmology


Tuesday, Feb. 18th.
Brajesh Gupt/Abhay Ashtekar, TACC/PSU

Title: Alleviating tensions in the CMB using LQC.
PDF of the talk (7M)
Audio+Slides of the talk (230M)
SRT (Subtitles) of the talk (85K)
By Jorge Pullin, LSU

The cosmic microwave background is radiation that reaches us from the big bang. Its wavelength (temperature) is incredibly uniform. If one looks in one direction in the sky and then another, the temperature is the same to one part in 100,000. But the tiny temperature differences between microwaves coming from different directions have been measured and they are not completely random. If one looks in one direction in the sky and then considers the ring of all possible directions a certain angle away from the original one and one averages the temperature along the ring, one does not get zero, as would be the case if the deviations were random. If one plots that deviation as a function of the angle, one obtains a curve with clear features (Credit NASA/WMAP team):

Remarkably, this curve can be rather straightforwardly predicted by the so called inflationary model. In it, the universe suffers a period of rapid expansion. If one considers a quantum field living in the universe in its simplest state (the vacuum) at the beginning of inflation and one evolves it through inflation, the field will develop correlations and those correlations are the ones observed. In the above figure the red dots are experimental points obtained by the WMAP satellite of NASA and the green curve is the prediction of inflation. The agreement is astonishing.

In spite of the agreement, there are some anomalies. If you look at the curve for large angles (left of the diagram) the points do not align as well as in the rest. Another anomaly is the lensing amplitude anomaly. It appears when one studies more complicated correlations than the one discussed before. That one was what is called the "two point" correlation function for the two directions in the sky one looks at. There are more complicated correlations involving three and four points. In the latter, the predictions of standard inflation scenario in the standard cosmological model do not square as well with observations, though the discrepancies are small.

Loop quantum gravity slightly modifies the predictions of inflation. In loop quantum gravity the Big Bang gets replaced by a "bounce" from a previous universe. In such a scenario, there is no good reason to put the quantum field in its simplest state at the outset of inflation. It would be much more naturally to either put it at the bounce or at the beginning of the previous universe. It turns out that things do not change much if one chooses one or the other of those options. The important thing is that by the time inflation starts, the field is not in a vacuum anymore and that modifies the correlations one sees in the cosmic microwave background.


This talk argued that the different correlations that loop quantum gravity predicts actually allow to solve the two anomalies we described above. Loop quantum gravity is not the only model that explains the anomalies, but compared to others, it is much cleaner in that includes essentially no free parameters to tweak and it is therefore more remarkable that it agrees with nature than other models with more freedom to tweak.

Thursday, March 12, 2020

Effect of ambiguities in loop cosmology on primordial power spectrum

Tuesday, Feb. 4th.
Parampreet Singh, LSU

Title: Effect of ambiguities in loop cosmology on primordial power spectrum
PDF of the talk (1M)
Audio+Slides of the talk (260M)
SRT (Subtitles)of the talk (60K)

By Jorge Pullin, LSU.


Cosmology is the study of the universe as a whole. You might ask, how can they study the universe, such a complicated system? The answer: very coarsely. One ignores many degrees of freedom and concentrates on a few. In its simplest incarnation, the study of cosmology is what is known as a minisuperspace approximation. One freezes all degrees of freedom except a handful. In the simplest case, one only concentrates on the universe's size. Since this is just one number, the equations for it become very simple.

However, we would like to study more features of the universe. To this aim, a technique used is known as perturbations. One assumes that the universe is simple enough that one can concentrate on its size, this will constitute the "background" on which small deviations are considered. One can then write equations for those small deviations that are simple enough to deal with. Such approach has led to spectacular predictions.

Perhaps the most striking ones ar the predictions for the anisotropies of the cosmic microwave background.  This is composed by light that arrivesat the Earth after traveling all the way from the Big Bang. Because the universe has expanded in the meantime, it has "cooled" (its wavelength havs become larger) and that is why we receive it as microwaves, which have comparatively large wavelengths compared to ordinary light. It turns out if one looks into two different directions of the sky the "temperature" (wavelength) of the microwaves that are incoming are exactly the same. They agree to one part in 100,000. The tiny disagreements however, are not random in nature, they have patterns in their structure. And those patterns have been measured with microwave satellites. And they agree remarkably well with the predictions of perturbation theory.

 Loop quantum cosmology is the application of loop quantum gravity techniques to cosmology. The resulting quantum cosmologies have been studied with tiny perturbations living in them. The results are that the predictions are almost the same of the classical theory, but with some deviations that at the moment are unobservable experimentally.

When one quantizes theories, there is not a single procedure you can follow. Different procedures lead to slightly different theories, with different predictions. This talk concentrated on how such differences in the treatment of the background solution impact the predictions on the anisotropies of the cosmic microwave background. The main conclusion is that, in spite of the ambiguities in the quantization, the resulting predictions exhibit robustness, enhancing our confidence on their physical plausibility. These predictions are perhaps the closest we are to an experimental test of quantum gravity so it is very important that they do not have significant ambiguities in them.

Monday, December 9, 2019

Effective Quantum Extended Spacetime of Polymer Schwarzschild Black Hole

Tuesday, Dec 3rd.
Fabio Mele and Johannes Muench, University of Regensburg

Title: Effective Quantum Extended Spacetime of Polymer Schwarzschild Black Hole
PDF of the talk (700k)
Audio+Slides of the talk (220M)
SRT (Subtitles) of the talk (60K)

 

By Jorge Pullin, LSU

When one crosses the horizon into a black hole the role of time and radius get interchanged. One is inevitably dragged into "the future" along the r direction towards the singularity at r=-0. The metric of the space-time, which outside the black hole depends on r, inside depends on t, it becomes time dependent as if it were a cosmological space-time. This in fact can be made into a mathematically precise correspondence. The space-time in the interior of a black hole is equivalent to a type of cosmology known as Kantowksi-Sachs. This led people to study it using loop quantum cosmology tools. In particular one finds that the singularity inside black holes gets replaced by a "bounce" into another region in space-time just like the Big Bang is replaced by a bounce in loop quantum cosmology.

This talk focused on some technical issues of these types of studies. In particular, when one studies physically observable quantities, mathematically one finds there are two of them, but only one has a physical interpretation, it is the mass of the black hole. Through a change of variables the authors were able to identify physically two observable quantities. They correspond to the mass of the black hole and the mass of the "white hole" that emerges once one bounces through the singularity. The new variables have several other advantages, in particular they simplify the dynamics quite a bit and it actually becomes exactly solvable.

Tuesday, December 3, 2019

Quasilocal holography in quantum gravity in 3 dimensions


Tuesday, Nov 5



Aldo Riello, Perimeter Institute

Title: Quasilocal holography in Quantum Gravity in 3 Dimensions
PDF of the talk (13M)
Audio+Slides of the talk (220M)
SRT (Subtitles) of the talk (60K)

By Jorge Pullin

Holography in the context of gravitational physics refers to describing a region of space-time through a theory on its boundary. The idea arises in several scenarios, in particular in string theory in the so called AdS/CFT correspondence.

In this talk the concept of holography was first studied in three dimensions. General relativity simplifies considerably in three dimensions since the Einstein equations in empty space imply that space-times are all flat. This allows to make considerable progress and in fact there exists a quantum theory of gravity in three dimensions, it is known as the Regge-Ponzano model. In the talk the relation of this model with degrees of freedom on the boundary of space-time were discussed. Both the case with and without a cosmological constant were considered. Many parallels with other results in the literature were found. The relation of the dynamics in the interior with that on the boundary and its corresponding states was outlined. Connections with other boundary treatments that were discussed in International Loop Seminars recently by Dittrich and Bonzom and by Wieland were also presented.

Thursday, October 24, 2019

Quasi local energy from loop quantum gravity boundary modes

Tuesday, Oct 22

Wolfgang Wieland, Perimeter Institute
Title: Quasi-local energy from LQG boundary modes
PDF of the talk (700K)
Audio+Slides of the talk (200M)
SRT (Subtitles) of the talk (60K)
By Jorge Pullin

In physical theories situations can be broken up into subsystems that one can follow individually characterized by a finite number of variables.

However, for nonlinear theories like general relativity one does not necessarily know how to do that. This is related to several problems in the theory like the averaging problem in cosmology. In the latter one typically concentrates on a small number of degrees of freedom of the universe, like its scale, and writes equations for them, pretending the universe is homogeneous, it does not change from one point to the next. This is a coarse approximation, but several important results can be derived from it. The matter content of the universe is not homogeneous and therefore to treat it one typically considers an average. But the average of non-linear functions of some variables is not the same as evaluating those quantities with the average values of the variables. This is known as the averaging problem in cosmology.

In this talk it was proposed to build degrees of freedom for subsystems in general relativity by constructing the degrees of freedom of the whole system starting from those of subsystems. The construction was discussed first in the context of classical general relativity. In the last part of the talk a connection with loop quantum gravity was presented. When one considers subregions of space time in loop quantum gravity the loops pierce the boundaries of the subregions and constitute field theories of "punctures" on them. These naturally embody the degrees of freedom of the subregion. A connection with a particular formulation of loop quantum gravity known as the spinor formulation was also suggested.

Tuesday, October 15, 2019

Effective dynamics from full loop quantum gravity

Tuesday, Oct 8

Muxin Han, Florida Atlantic University
Title: Effective dynamics from full loop quantum gravity
PDF of the talk (1M)
Audio+Slides of the talk (272M)
SRT (Subtitles) of the talk (98K)
By Jorge Pullin, LSU


A hot topic of research is how to derive the equations of loop quantum cosmology from loop quantum gravity. Initial investigations started by freezing most degrees of freedom and keeping the ones relevant for cosmology and proceeding to quantize them using loop quantum gravity inspired techniques. In recent years the focus has moved towards trying to derive things directly from full loop quantum gravity. In this talk a proposal along these lines is put forward. The idea is to use the path integral approach to quantization. This is an approach in which the quantum theory is built by considering all possible paths of the dynamics of the system and assigning probabilities to them. The idea is to perform the path integral using a set of states known as coherent states and study the resulting equations of motion. The technique is applied to several proposals for the evolution operator (Hamiltonian) of the theory that have been put out in the literature. The technique is suitable for numerical evolution opening a contact with numerical relativity. It may be applicable in other situations of interest like cosmological perturbations and binary black holes.

New Loop Quantum Cosmology modifications from Symplectic Structures

Tuesday, May 14th

Klaus Liegener, LSU
Title: New Loop Quantum Cosmology modifications from Symplectic Structures
PDFof the talk (2M)
Audio+Slidesof the talk (38M)
By Jorge Pullin, LSU

Loop quantum gravity is based on a new set of variables for describing general relativity that were introduced by Abhay Ashtekar. These variables have a certain amount of redundancy, known as a gauge symmetry.

Loop quantum cosmology is an approximation to loop quantum gravity that attempts to model cosmologies by following only a very limited number of degrees of freedom. A current topic of great interest is to understand how and if this approximation captures behaviors of the full theory. This talk concentrated on the role the redundancies in the variables Ashtekar introduced play in the construction of loop quantum cosmologies. It proposes to use certain variables that better behave under the presence of these redundancies and draws implications for the dynamics of the resulting theory. In particular it implies, as is common in loop quantum cosmology, that the Big Bang at the beginning of the universe is replaced by a "bounce" from a previous universe. However, the dynamics of the current model implies the bounce is asymmetrical, the universe before and after it do not look the same. This had been encountered in some previous proposals for loop quantum cosmologies, but not in all of them.