Luca Amendola talks with
ScienceWatch.com and answers a few questions about
this month's New Hot Paper in the field of
Article Title: Conditions for the cosmological
viability of f(R) dark energy models
L;Gannouji, R;Polarski, D;Tsujikawa, S
Journal: PHYS REV D
Year: no.-083504 APR 2007
* Osserv Astron Roma, INAF, Via Frascati 33, I-00040 Monte
Porzio Catone, Roma, Italy.
(addresses have been truncated)
Why do you think your paper is highly
Probably the reason is that this paper gives finally an exhaustive set of
rules for judging whether theories that propose a form of "modified
gravity" (i.e. gravity different from Einstein's Relativity theory) are
acceptable or not. This is helpful because so far many scientists, at least
since the 1970s, have proposed models of modified gravity and often it was
not easy to ascertain whether such models were capable of explaining the
current cosmological observations.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
It's a new methodology because it shows how to understand forms of modified
gravity—the so-called f(R) models—without extensive numerical
and analytical analysis, simply by looking at the form of a suitable
"characteristic function." This allows one to easily find the models that
have the potential to explain the cosmological observations and to reject
those that fail some basic criteria.
"Our paper finds a simple
way to detect some of these problems and to
classify in a general and compact way all f(R)
It also synthesizes previous knowledge: many of these f(R) models had been
already been studied with different methods, but now we have a set of
uniform criteria that can be used to classify the various cases. I think we
produced a useful roadmap to gravity.
Would you summarize the significance of your paper
in layman's terms?
Theories of gravity that go beyond Einstein's have often been proposed to
help explain puzzling features of our universe: its accelerated expansion,
the large amount of invisible matter and energy, the initial states, etc.
Many of these models are called f(R) theories since they generalize
Einstein's gravity, which describes the space-time curvature R by
introducing an arbitrary function f(R). Often however, such models,
although explaining some observations, turn out to be incompatible with
other features: for instance, they are inconsistent with the formation of
galaxies and the cluster of galaxies.
Our paper finds a simple way to detect some of these problems and to
classify in a general and compact way all f(R) models. We detail the
criteria that any f(R) model have to satisfy in order to reproduce the
current knowledge on the cosmic expansion. This shows at once that many
models are ruled out; on the other hand, it shows how to build acceptable
How did you become involved in this research, and
were there any problems along the way?
For many years I have worked on the theoretical and observational
properties of gravity and modified gravity. At some point it became clear
that it would have been very useful to have general criteria to evaluate
models of modified gravity. We knew that some f(R) models are unacceptable
and others work fine, but we did not have a simple and general way to
classify these cases and understand why they were acceptable or rejected.
So we started looking for such classification and ended up with the results
described in our paper.
The main problem was to try to be as general as possible. We did not want
to study just one or a few models: we really meant to give general
criteria. It took us almost a year to realize that indeed there was a
simple graphical solution to the problem.
Where do you see your research leading in the
Gravity is the most important force at work in our universe, and the one
about which we know the least. In the future, I think most cosmological
research will be dedicated to a better understanding of gravity, especially
at astrophysical scales, and I will continue working on this.
Do you foresee any social or political
implications for your research?
Not in direct way. However, understanding the large-scale geography and
history of our universe is a necessary step to expand human's reach in
Astronomical Observatory of Rome
Italian National Institute of Astrophysics
Keywords: dark energy, models, theories, Einstein's gravity,
modified gravity, cosmological observations, galaxies.