Shin'ichi Nojiri talks with
ScienceWatch.com and answers a few questions about
this month's Fast Breaking Paper in the field of
Article Title: Introduction to modified gravity and
gravitational alternative for
Journal: INT J GEOM METHODS MOD PHYS
Year: FEB 2007
* Natl Def Acad, Dept Appl Phys, Yokosuka, Kanagawa
* Natl Def Acad, Dept Appl Phys, Yokosuka, Kanagawa
(addresses have been truncated)
Why do you think your paper is highly
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.
Left to right: Shin'ichi Nojiri,
interviewer, coauthor Sergei D.
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
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
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
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
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
Do you foresee any social or political implications for
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.
Particle Cosmology Group (Q-Lab)
Department of Physics
Graduate School of Science
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