Science Daily — From various independent observations, cosmologists have established that ordinary matter, made of protons and neutrons, accounts for only 4% of the total energy content of the Universe. The remaining 96% is made of puzzling ingredients Dark Matter and Dark Energy. Researchers at the Laboratory Universe and Theories from the Observatory of Paris and the Belgian Fonds de la Recherche Scientifique have recently suggested the Abnormally Weighting Energy (AWE) Hypothesis to describe the dark side of the Universe as a revolutionary aspect of gravitational physics.
In the past decade, cosmology has entered an era of high precision, and in the future it may become a unique laboratory to test theories of fundamental physics, from gravitation laws to microphysics. Amongst the many questions raised by this science in turmoil, one of the most important is indisputably the one of the energy content of the Universe. Knowing what the Universe is precisely made of, and in which proportions, allows not only to determine its age but also to reconstruct the history, to predict its past and future. In fact in the attempt to solve this question cosmologists have made two of the most promising discoveries in the history of modern physics: the existence of dark matter and dark energy.
While dark matter is unavoidable to explain at the same time the
angular fluctuations of the cosmic microwave background and the
formation and the properties of galaxies, dark energy has been
originally invoked to account for the observed recent acceleration of
the cosmic expansion. The so-called concordance model of cosmology
assumes that this dark energy is in fact the cosmological constant once
introduced by Einstein himself as an attempt to incorporate Mach’s
principle within general relativity.
However, the usual
interpretation of the cosmological constant in terms of quantum vacuum
fluctuations is in disagreement with observed value by a few dozens
orders of magnitude! Furthermore, as the vacuum energy is assumed
constant everywhere at all times, it is hard to explain how it became
dominant only a few billion years ago. This would mean that we live in
a very particular, and even privileged, epoch of cosmic history… Is
this an extraordinary coincidence? Yet this anthropic consideration is
quite deceiving for scientists.
To overcome these difficulties,
the authors, Jean-Michel Alimi and André Füzfa, have proposed the AWE
Hypothesis (« Abnormally Weighting Energy ») in which the dark sector
of cosmic matter violates the equivalence principle on cosmological
scales. This principle, as well introduced by Einstein, assumes that
all kinds of energies produce and undergo the same form of gravity.
This principle is extremely well tested (to a part out of a thousand
billion) in laboratories, i.e. at local scales, in contrast what would
happen if violation of the equivalence principle would be
scale-dependant. In other words, what would happen if the equivalence
principle was rigorously verified at local scales, where dark matter
and dark energy are present in tiny amount, but is violated on
cosmological scales where dark matter and dark energy are dominant?
The
authors have precisely shown that this could naturally happen if some
particles, those of dark matter for instance, do not couple to
gravitation in the same way as ordinary matter. These particles would
therefore see gravitational fields with a gravitational strength
different from ordinary matter. The authors have answered these
questions by showing how at a given scale the gravitational strength
becomes dependent on dark matter concentration…
If the amount of
dark matter at sub-galactic scales is negligible, so is the amplitude
of this effect. This is not the case on cosmological scales where dark
matter dominates the energy content of the Universe. The team has shown
that over such cosmic distances, ordinary matter has experienced a
stronger cosmic expansion, as its own gravitational coupling strength
has been adapting to the dark matter domination. This change in the
matter gravitational coupling results in an accelerating cosmic
expansion until equilibrium is reached such that the gravitational
coupling on cosmological scales stabilizes at a value which differs
from the one measured in our Solar system.
The resulting dark
energy mechanism exhibits key features which appear very promising. (i)
First, it does not require the existence of negative pressures such as
in the case of the cosmological constant or other proposed models like
quintessence. (ii) It allows explaining naturally the cosmic
coincidence as result of the stabilization mechanism of the
gravitational constant during the matter-dominated era. (iii) It fairly
accounts for the Hubble diagram of type Ia supernovae by predicting
independently the amount of ordinary matter and dark matter as obtained
by the detailed analysis of cosmic microwave background anisotropies.
This suggests an explanation to the remarkable adequacy of the
concordance model while predicting an age of the Universe which is
compatible with existing observations. Finally, (iv) in the future this
mechanism leads to a decelerated cosmic expansion described by the
well-known Einstein-de Sitter cosmological model. Most important is the
AWE hypothesis allows reducing dark energy as a new property of
gravitation: the anomalous gravity of dark matter.
Note: This story has been adapted from a news release issued by Observatory of Paris.
Story from: http://www.sciencedaily.com/releases/2007/04/070407175959.htm
