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Shin'ichi Nojiri talks with and answers a few questions about this month's Fast Breaking Paper in the field of Physics.
Nojiri Article Title: Introduction to modified gravity and gravitational alternative for dark energy
Authors: Nojiri, S;Odintsov, SD
Volume: 4
Issue: 1
Page: 115-145
Year: FEB 2007
* Natl Def Acad, Dept Appl Phys, Yokosuka, Kanagawa 2398686, Japan.
* Natl Def Acad, Dept Appl Phys, Yokosuka, Kanagawa 2398686, Japan.
(addresses have been truncated)

Why do you think your paper is highly cited?

Among the remarkable observations of the Universe, where we have found that the expansion of the Universe is accelerating and that the Universe is spatially flat; implied is the existence of an unknown component in the Universe, that is, dark energy. In order to generate the accelerating expansion of the Universe, dark energy must possess a large negative pressure, which is quite unusual.

Many people believe that dark energy should be one of the most important problems undertaken in the study of physics in this century. Many researchers are actively studying this problem. Some people have proposed that Einstein's theory of general relativity could be modified to explain dark energy. The so-called ƒ(R) gravity is one of several strong candidates in such potential modifications of the Einstein gravity. In the ƒ(R) gravity, the Einstein-Hilbert action, which is linear in the scalar curvature R, is replaced with a proper function of R.

Click photo to enlarge.
Left to right: Shin'ichi Nojiri, interviewer, coauthor Sergei D. Odintsov

Another candidate is the scalar Einstein-Gauss-Bonnet gravity, where the action of a scalar field and the Gauss-Bonnet invariant coupled with the scalar field are added to the Einstein-Hilbert action. Such terms could appear as a correction to the Einstein gravity from the superstring theory. My paper coauthored with Sergei D. Odintsov, "Introduction to modified gravity," is basically a review of these articles, which includes works with collaborators, about such modified gravities. General properties of modified gravities and useful formulae are given explicitly in our paper. As many researchers are currently working on modified gravity theories, this paper could indeed prove quite useful for them, thus explaining its high citation rate.

Does it describe a new discovery, methodology, or synthesis of knowledge?

Although it has not yet been confirmed if these modified gravity theories do actually describe dark energy, we have discovered that these concepts could potentially be promising ones.

Would you summarize the significance of your paper in layman's terms?

The Sloan Digital Sky Survey results, as well as other studies, agree with the prediction for a Universe composed of 5% atoms, 25% dark matter and 70% of the unknown component known as dark energy. Dark energy is a form of energy with negative pressure. It is surely unusual and we have not previously known about such matter/energy in the entire history of physics. The one possible exception may be the cosmological constant, which is also a naïve candidate for dark energy.

Our paper might give a clue to a clarification of what dark energy could be. I also expect that the problem of dark energy could be directly related to the problem of a unified field theory of all elementary particles and their interaction.

How did you become involved in this research, and were there any problems along the way?

I am an elementary particle physicist. The most important problem in the field of particle physics is attempting to combine an understanding of all particles and their interactions in a unified way. Among these interactions, gravity has a different nature when compared with other interactions. All these interactions, other than gravity, can be described by the quantum theory.

"The most important problem in the field of particle physics is attempting to combine an understanding of all particles and their interactions in a unified way."

No one has yet succeeded in constructing a quantum theory of gravity, which could imply that Einstein's theory should be modified. The strongest candidate for quantum gravity beyond Einstein's theory could be the superstring theory. Superstring theory could unify all these elementary particles and their interactions. I thought that dark energy could be related to a real gravity beyond Einstein's gravity, which is the initial reason why I began to study dark energy.

Where do you see your research leading in the future?

I believe that the understanding of dark energy could lead to an understanding of the quantum theory of gravity. I expect that this research will finally provide a unification of all elementary particles and their interactions.

Do you foresee any social or political implications for your research?

Although my research does not have any obviously direct social or political implications, a better understanding of the Universe could lead to a better comprehension among human beings of our unique place in the Universe, where we may even learn enough about the nature of time and space to help us better understand how we should live now. The Universe is 70% dark energy and, in a sense, understanding the dark energy will deepen our understanding of the entire Universe. Being a part of this Universe, it is my scientific obligation to help others understand it better.

Shin'ichi Nojiri
Particle Cosmology Group (Q-Lab)
Department of Physics
Graduate School of Science
Nagoya University
Nagoya, Japan

Keywords: dark energy, f(r) gravity, quantum gravity, einstein's gravity, einstein-hilbert, einstein-gauss-bonnet gravity, gauss-bonnet invariant, quantum gravity, superstring theory, particle physics.


2008 : June 2008 - Fast Breaking Papers : Shin'ichi Nojiri
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