Issue of the Beginning: Initial Conditions for Cosmological Perturbations

by Abhay Ashtekar and Brajesh Gupt.


ashtekar

Abhay Ashtekar holds the Eberly Chair in Physics and the Director of the Institute for Gravitation and the Cosmos at the Pennsylvania State University. Currently, he is a Visiting Professor at the CNRS Centre de Physique Théorique at Aix-Marseille Université.

Although our universe has an interesting and intricate large-scale structure now, observations show that it was extraordinarily simple at the surface of last scattering. From a theoretical perspective, this simplicity is surprising. Is there a principle to weed out the plethora of initial conditions which would have led to a much more complicated behavior also at early times?

In the late 1970s Penrose proposed such a principle through his Weyl curvature hypothesis (WCH) [1,2]: in spite of the strong curvature singularity, Big Bang is very special in that the Weyl curvature vanishes there. This hypothesis is attractive especially because it is purely geometric and completely general; it is not tied to a specific early universe scenario such as inflation.

However, the WCH is tied to general relativity and its Big Bang where classical physics comes to an abrupt halt. It is generally believed that quantum gravity effects would intervene and resolve the big bang singularity. The question then is Continue reading

Pants on fire!

One of the authors, Ian Jubb, discussing a pair of trousers with his colleagues at Imperial College London. Ian Jubb is currently the PhD student of Fay Dowker at Imperial College London.

One of the authors, Ian Jubb, discussing a pair of trousers with his colleagues at Imperial College London. Ian Jubb is currently the PhD student of Fay Dowker in the Theoretical Physics group at Imperial College London.

by Ian Jubb and Michel Buck.


Did you know that Quantum Gravity literally sets pants on fire?

Your pants are not just a nifty garment, they are also a perfect example of a space undergoing a process known as topology change. Take a space that initially consists of two separate circles. If they were to meet and merge into a single circle, the topology of the space would have changed. The trousers allow us to visualise each stage of this process, with cross sections higher up the trouser leg corresponding to later times in the process (if we hold the trousers upside-down, we get the reverse process, corresponding to a single circle splitting into two circles). Instead of viewing this process as the space changing in time, Einstein would tell us to view the trousers in their entirety, as one whole spacetime — the trousers spacetime.

But why should we care about spaces that can ‘split’ and ’attach’ like this? It turns out that there are good reasons to believe that Continue reading

Want to crush a singularity? First make it strong and then …

by Parampreet Singh.


Parampreet Singh

Parampreet Singh with a young student who often asks him the most difficult and so far unanswerable questions on the resolution of singularities. Dr Parampreet Singh is Associate Professor at Department of Physics and Astronomy at Louisiana State University.

Einstein’s theory of classical general relativity breaks down when spacetime curvature
becomes extremely large near the singularities. To answer the fundamental questions
about the origin of our Universe or what happens at the central singularity of the black holes thus lies beyond the validity of Einstein’s theory. Our research deals with discovering the framework which guarantees resolution of singularities.

It has been long expected that quantum gravitational effects tame the classical singularities leading to insights on the above questions. A final theory of quantum gravity is not yet there but the underlying techniques can be used to understand whether quantum gravitational effects resolve cosmological and black hole singularities. Our goal is Continue reading

Setting space on fire

by Yasaman K. Yazdi and Niayesh Afshordi.


Niayesh Afshordi and Yasaman Yazdi discover that firewalls have consequences

Niayesh Afshordi and Yasaman Yazdi discover that firewalls have consequences. Yasaman K. Yazdi is a PhD candidate at the University of Waterloo and the Perimeter Institute for Theoretical Physics. Niayesh Afshordi is an associate professor at the University of Waterloo and the Perimeter Institute for Theoretical Physics.

Thought experiments highlight the edge of our understanding of our theories.  Sometimes, however, we can get so caught up in heated debates about the solution to a thought experiment, that we may forget that we are talking about physical objects, and that an actual experiment or observation may give the answer.  In this Insight we discuss a proposed solution to the black hole information puzzle, and a possible observational signal that might confirm it.

 

The black hole information puzzle and a potential solution

The black hole information loss problem is a decades old problem that highlights the tensions between some of the pillars of modern theoretical physics. It has evolved from being Continue reading

The possible emptiness of a final theory

Written by Jesper Møller Grimstrup, an independent danish theoretical physicist. He has collaborated with the mathematician Johannes Aastrup for more than a decade developing what they now call quantum holonomy theory. His present research is financed by an Indiegogo crowdfunding campaign (still open). Find more information on www.jespergrimstrup.org.


Could the laws of nature originate from a principle, that borders a triviality?

Does a final theory that cannot be explained by yet another, deeper theory, exist? What could such a theory possibly look like — and what might we learn from it?

J M Grimstrup

Jesper Møller Grimstrup

These are the million dollar questions. Will the ladder of scientific explanations that take us from biology to chemistry and down through atomic, nuclear and particle physics, end somewhere? Will we one day reach a point where it is clear that it is no longer possible to dig deeper into the fabric of reality? Will we reach the bottom?

Together with the mathematician Johannes Aastrup I have developed a new approach to this question. Our theory  we call it quantum holonomy theory is based on an elementary algebra, that essentially encodes how stuff is moved around in a three-dimensional space.

img_0192This algebra, which we call the quantum holonomy-diffeomorphism (QHD) algebra [1], is interesting for two reasons Continue reading

A Study of Time Delay from Different Time Zones

Netta Engelhardt (University of California, Santa Barbara) and Sebastian Fischetti (Imperial College) gave us an insight into their communication methods whilst collaborating for their research paper recently published in CQG.


Snetta

On a dark London evening and a sunny California day — January 19, 2016, to be precise — Netta sent Sebastian a Skype message:

Image_1

So began a new project for this dynamic duo, published recently in CQG. Unlike our previous project, this one presented a new challenge (with which researchers are all too familiar): we were separated by an eight-hour time difference. Thus began a three-way collaboration: Netta, Sebastian, and Skype (with the third member being the least cooperative).

The process began Continue reading

Why is our universe about to decay?

Dr Kin-ya Oda (left, Osaka university) and Dr Masatoshi Yamada (right, Kyoto university).

Dr Kin-ya Oda (left, Osaka university) and Dr Masatoshi Yamada (right, Kyoto university).

It has been revealed that we are living on the edge of vacuum instability by the discovery of Higgs particle at the Large Hadron Collider since 2012. The determination of Higgs mass finally provides the last-missed parameter, the Higgs self coupling, to be 0.12 in the Standard Model of particle physics after nearly half century of its foundation. This value completes the initial conditions for a set of differential equations, called renormalization group (RG) equations, which govern how particles interact at very high energy scales. It turns out that the self coupling can vanish or even become negative at the Planck scale, where the quantum gravity effects become significant. We note for later reference that the Yukawa coupling between the Higgs and top quark plays a crucial role to reduce the Higgs self coupling in its RG evolution. The Higgs potential is about to become Continue reading

Book review: Covariant Loop Quantum Gravity, an elementary introduction to quantum gravity and spinfoam theory

Rodolfo Gambini is Professor of Physics at Universidad de la República, Montevideo Uruguay

Rodolfo Gambini is Professor of Physics at Universidad de la República, Montevideo Uruguay

Review of “Covariant Loop Quantum Gravity, an elementary introduction to quantum gravity and spinfoam theory” by Carlo Rovelli and Francesca Vidotto

One of the central problems of contemporary physics is finding a theory that allows for describing the quantum behavior of the gravitational field. This book is a remarkable update on one of the most promising approaches for the treatment of this problem: loop quantum gravity. It places special emphasis on the covariant techniques, which provide with a definition of the path integral, an approach known as spin foams. It is a field that has undergone quite a bit of development in the last two decades. The book gives an overview of this area, discussing a series of results that are presented with great clarity. Both students and established researchers will benefit from the book, which provides a dependable introduction and reference material for further studies. Only a basic knowledge of general relativity, quantum mechanics and quantum field theory is assumed. The conceptual aspects and key ideas are discussed in the main body of the book and Continue reading

The importance of being Melvin

The authors, Jennie Traschen and David Kastor, enjoy the wit and humor of Oscar Wilde.

The authors, Jennie Traschen and David Kastor, enjoy the wit and humor of Oscar Wilde. The image above has been obtained from the Wikimedia website, where it is stated to have been released into the public domain. It is included within this blog post on that basis.

Like Oscar Wilde’s famous 1895 play, our recent CQG article “Melvin Magnetic Fluxtube/Cosmology Correspondence,” features an intricate interplay of dual and concealed identities. While our paper lacks the biting wit of Wilde’s dialogue, e.g.

“I do not approve of anything that tampers with natural ignorance. Ignorance is like a delicate exotic fruit; touch it and the bloom is gone. The whole theory of modern education is radically unsound. Fortunately in England, at any rate, education produces no effect whatsoever,”

our revelations regarding true identity do play out on a more vast, indeed a cosmic stage.

Melvin’s solution to the Einstein-Maxwell equations describes a static bundle of magnetic flux-lines bound together by self-gravity. Originally discovered in 1963, it has a rich and influential history. In 1964, Thorne studied the stability of what he called “Melvin’s Magnetic Universe.” Its resistance to gravitational collapse was an important clue leading to the formulation of his well-known hoop conjecture. In 1975, Ernst showed that Continue reading

The gravitational Hamiltonian, first order action, Poincaré charges and surface terms


Read the full article for free* in Classical and Quantum Gravity:
The gravitational Hamiltonian, first order action, Poincaré charges and surface terms
Alejandro Corichi and Juan D Reyes 2015 Class. Quantum Grav. 32 195024

arXiv:1505.01518
*until 18/11/15


Ever since Einstein and Hilbert were racing to complete the general theory of relativity, almost 100 years ago, having a variational principle for it was at the forefront of the theoretical efforts. An action and the variational principle accompanying it are the preferred ways to describe a physical theory. At the classical level, all the information one can possibly ask about a physical system is conveniently codified into a single scalar function S. Additionally, in covariant approaches to quantum mechanics, the action S provides, through the path integral, a fundamental link between the classical and quantum descriptions. Ideally, the Hamiltonian structure of the theory itself -the starting point for canonical quantization- may too be extracted from the same action. Continue reading