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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Thursday, April 24, 2008

Cosmology: 5 Needed Breakthroughs
-- Robert Brandenberger

[Today's guest in our ongoing feature '5 Breakthroughs' is Robert Brandenberger, Canada Research Chair and Professor of Physics of McGill University, where he taught and conducted research since 2004. Before that he was a professor at Brown University for about 18 years.

In his long career spanning about a quarter of a century (He received his PhD from Harvard University in 1983; Thesis "Topics in Quantum Field Theory and Cosmology'), Prof. Brandenberger made crucial contributions in various important subfields of cosmology (link to a list of publications).

His current research interests cover a wide spectrum of topics in cosmology and related fields and include
(A) Conceptual problems in inflationary universe cosmology, in particular, trans-Planckian problem for cosmology,
(B) Theory of cosmological perturbations, in particular, back reaction problems, evolution of perturbations in nonsingular cosmologies, and parametric amplification of fluctuations during reheating,
(C) Superstring cosmology, in particular, string gas cosmology and structure formation, mechanisms for obtaining inflation from string theory, resolution of cosmological singularities in string theory, dualities and brane gases in the early universe,
(D) Topological defects in cosmology, in particular, topological defects and Baryogenesis, topological defects and direct signatures, stabilization of embedded defects by plasma effects,
(E) Nonequilibrium processes, in particular, parametric resonance during reheating in inflationary cosmology, nonequilibrium production of topological defects,
(F) Particle-Astrophysics, in particular, constraining physics beyond the Standard Model using cosmology, new mechanisms for CP violation and Baryogenesis,
(G) Large-scale structure, in particular, use of topological statistics to analyze large-scale redshift surveys, studies of weak gravitational lensing maps using new statistics.
(H) Formation of structure in the early universe, in particular, coupling of adiabatic and entropy fluctuations in multi-field, and cosmological models.

In March, 2008 issue of 'Physics Today', Prof. Brandenberger presented an excellent account of current status of inflationary cosmology in his article 'Alternatives to cosmological inflation' (article link here).

Prof. Brandenberger is a Fellow of the American Physical Society. He was an Alfred P. Sloan Research Fellow in years 1988-1992 and received the Outstanding Junior Investigator award of Department of Energy in years 1988-1991.

It gives us great pleasure to present this list of 5 most important breakthroughs that Prof. Brandenberger would like to see in Cosmology.
-- 2Physics.com ]

Breakthrough 1:
Solution of the (old) cosmological constant problem: why is the cosmological constant not given by the cut off scale of relativistic quantum field theory?

Breakthrough 2:
Solution of the new cosmological constant problem: why is there an apparent cosmological constant which is beginning to dominate the evolution of the universe at the current cosmological epoch?

Breakthrough 3:
Resolution of the cosmological singularity: without resolving the cosmological singularity a cosmological model will always be incomplete. Standard Big Bang cosmology had to be replaced by a new early universe cosmology because of this problem. The current paradigm, scalar field-driven inflationary cosmology still suffers from this problem and is therefore incomplete.

Breakthrough 4:
Non-perturbative understanding of superstring theory: will lead to a new cosmological model of the very early universe which will either yield a convincing realization of inflationary cosmology or else to an alternative model.

Breakthrough 5:
An observational discovery of a cosmic superstring: this will cement the link between string theory and cosmology and will also lead to a new theory of the very early universe.

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At 9:51 PM, Blogger Sanjay said...

I know Prof Robert Brandenberger for many years now, having met him in 1988. So, I am ware of his outstanding contributions to various fields. It is indeed a pleasure to see his posting related to Cosmology.

His first four needed breakthroughs are related to the basic theory of gravity, Einstein's field equations. Of course, when these equations of replaced by appropriate ``equations'' not admitting any singularities of the spacetime, we will have resolved these vexing issues.

If the geometry of the space is our ``free choice'', then the first three issues raised by Prof Brandenberger will have their solution.


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