Chameleon mechanism

Tuesday 28 of October I gave a small!!! talk in physics department,in series of cosmology weekly seminars.

The general title of talk is the Accelerating universe,Theory and phenomenology and the subtitle is Chameleon scalars mechanism and modified gravity.

The general title is the field which I want to be engaged in my Ph.D.thesis.

And what about the chameleon theories?

The accelerating universe open a new paradigm in cosmology.

There are many many models trying to describe this phenomena,but generally we can divided all the models in 2 general subdivisions.

If we accept the context of general relativity, which means that we know gravity well, we will come to the realization that new physical entity needed to describe the accelerating universe,such as cosmological constant or scalar fields generally named quintessence.

The second main models come to existence from the assumption that the general relativity needed to be modified in long ranges and small curvatures.This models named Modified gravity.

But what is chameleon?

The point is that we know that general relativity works well in solar system and local tests of gravity.So if we change the action of Einstein -Hilbert to have new effects in cosmological scales also we will affect the local tests of gravity, so we need a mechanism! in order to conceal this effect in local tests and disappear it and bring it back to existence in cosmological scales. This mechanism is called Chameleon mechanism after the name of lizard!!!like animal which changes its skin color with environment.So the scalar field equivalent to modified gravity /We know about the equivalence of modified gravity with scalar field theories/

changes its behaviour with respect of the density of environment like Chameleons.

BUT

in theoretical point of view this mechanism needs more investigations to be reliable,I will study it more...

Ads/CFT

It is about a year that in our physics department in Sharif university, each Tuesday held a seminar among the three active theoretical groups of department.

1)Particle physics 2)Cosmology 3)Conformal field theory group

The idea and the motivation of seminars is nice:it is to be connected and aware of the work of each other.Last Tuesday particle group talk about really complex subject.I want to respell the ideas of it in order to have a snapshot of this.

The talk is given by Mohsen Alishahia and is about about Ads/CFT correspondence more vastly Anti de sitter space/Conformal field theory correspondence which is also called after Maldacena the Argentinian physicists MAldacena duality.What is all about?

The theory shows that the string theory in a Anti desitter space plus a sphere or orbifold is physically equivalent to a conformal field theory with out gravity defined in its boundary.

It seems complicated anyhow it is interesting because of presenting a way to solve the quantum gravity theory and also it is inspired by the holographic principle which claim that you can extract the physics of a volume from its boundary.Holographic principle is a a speculative idea about quantum gravity originally proposed by Gerard 't Hooft and improved and promoted by Leonard Susskind.

Finally the lecturer imposed the probability that maybe this correspondence first of all can be used to describe the gravitational effects on inflationary models.

Let me show up some important papers on this genius work.

Reminding you that In about five years, Maldacena's article had 3000 citations and became one of the most important conceptual breakthroughs in theoretical physics of the 1990s, providing stark new insight into both quantum gravity and QCD.

1)Maldacena, The Large N Limit of Superconformal Field Theories and Supergravity, arΧiv:hep-th/9711200.
2)Witten, Anti-de Sitter Space and Holography, arΧiv:hep-th/9802150.
3)Gubser, Klebanov and Polyakov, Gauge Theory Correlators from Non-Critical String Theory, arΧiv:hep-th/9802109.
4)Aharony et al, Large N Field Theories, String Theory and Gravity, arΧiv:hep-th/9905111.
AdS/CFT correspondence on arxiv.org.

2008 Physics Nobel Prize

Finally the 2008 Physics Nobel prize goes to Japanese.

Yoichiro Nambu from Chicago University /http://physics.uchicago.edu/research/areas/particle_t.html#Nambu /for his contribution to the finding of the mechanism of spontaneous broken symmetry in subatomic physics get 1/2 of the Nobel prize and the other half goes to Makoto Kobayashi and Toshihide Maskawa for for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature.

One another Nobel prize goes to particle physicists campaign,although their are hope to see the prize in cosmology for the discovery Accelerating universe by Supernovas but ...

Anyhow in the upcoming posts I will try to describe the work of Nobel prize winners...