**DARK ENERGY: TRIBUTE TO ALBERT EINSTEIN**

Fundación Corp, Group Centro Cultural. Caracas, Venezuela**April – May, 2005**

*By Claudio Mendoza (Astrophysicist)*

*“I have a deep faith that the principle of the universe will be beautiful and simple.”*

– Albert Einstein

– Albert Einstein

The year 2005 has been declared the World Year of Physics by the United Nations General Assembly in celebration of the centennial of Albert Einstein's Annus Mirabilis, denoted by a series of articles that became the foundations of modern physics and cosmology. Never, before or after, such an influential expansion of the frontiers of knowledge has occurred in such a brief interval of time as in the spring of 1905. In March of that year, an unknown Level II Technical Expert of the Patent Office in Bern, Switzerland, hardly 26 years old, proposed the concept of the light "quantum" in order to explain the basic interaction of radiation with matter, detonating an avalanche of discoveries that would end up with the quantum theory of the microscopic underworld. By the end of April, he completed his doctorate thesis proposing a theoretical method for estimating the molecular radius and Avogadro's Number (the number of molecules in a molar mass of a compound), and eleven days later he published an interpretation of Brownian motion. Both are solid evidence of the existence of atoms with which he would deduce that the behavior of the macroscopic properties of a body could be determined from the sum of the random walks of its particles, the basis of present day statistical physics. In his fourth publication in June, he presented the Theory of Special Relativity revolutionizing the interrelation of space-time which, with the inclusion of gravity some years later in a Theory of General Relativity, would establish the theoretical framework for understanding the origin, state and evolution of the universe. In September 1905, he concluded in a brief publication that "the mass of a body is a measure of its energetic content"; in other words, he discovered his famous equation E=Mc2.

Rolando Pena joins these celebrations with the exhibition Dark Energy: Tribute to Albert Einstein as a continuation of his previous enquiries on energy and matter. In this context we can remember the installations The Standard Model of Matter and Spontaneous Symmetry Breaking: God's Barrel where he established a definite alliance with the expression of scientific achievement and its popularization in a collective that is practically alien to its aesthetics. In this occasion he chooses the current controversy on the cosmological constant that Einstein considered his greatest error, and that curiously, has been recently revived to characterize the peculiarities of the Big Bang expansion, specifically those related to dark energy.

The Big Bang is regarded as the standard model of the origin and evolution of our universe. It is based, on the one hand, on Einstein's Theory of General Relativity where gravity, instead of being described by a force field, is considered a distortion of space-time; and on the other, on the Cosmological Principle that assumes that the average large-scale distribution of matter in the universe is homogeneous and isotropic. Although the equations of the General Theory naturally predict a dynamic universe, Einstein's original cosmological model was static; this was achieved by introducing an ad hoc "anti-gravitational" term (the cosmological constant) that counteracted the gravitational attraction of matter. The Russian cosmologist Alexander Friedmann and the Belgium priest Georges Lemaitre were the first to mathematically demonstrate the dynamic and expansive character of the relativistic universe, in spite of Einstein's initial reluctance, an expansion that was later corroborated with the astronomical observations of receding galaxies by Edwin Hubble in 1929. These observations suggested a definite moment in time, call it Big Bang, when the universe was actually born.

The Big Bang model is nowadays supported by further evidence that is difficult to refute such as the cosmic microwave background postulated by George Gamow in 1948 and discovered in 1965 by Arno Penzias and Robert Wilson and the primordial abundance of chemical elements, namely hydrogen, helium and lithium. However, the theory does not account for the fluctuations that have given rise to the observed structure, for instance galaxies, and the packing of all the universal content in zero volume at time zero. With these problems in mind, Alan Guth has proposed an exponential inflation at the very beginning of the expansion when the distribution of the universe was dominated by vacuum energy, the dark energy associated with the cosmological constant. When it decays and through spontaneous symmetry breaking, it generates the matter and radiation that fill the universe today; in other words, inflation literally produces the universe out of nothing.

The properties of the universe are determined by the quantity and type of matter it contains and by the dark energy. For instance, its geometry, is it open (infinite) or closed (finite)? The expansion rate, does it accelerate or decelerate? And its history, how old is it? Therefore great efforts are currently given to characterize the expansion, specifically two independent international projects directed by Saul Perlmutter and Robert Kirshner that observe distant supernovae have confirmed an accelerated expansion dominated by dark energy. Moreover, a recent space mission by NASA to measure small variations of the cosmic microwave background, the Wilkinson Microwave Anisotropy Probe, has determined with great accuracy the age of the universe at 13.7 billion years, that its geometry is neither close nor open but an intermediate state referred to as "flat" and that 70% of the universe is dark energy. This last prediction seems to be in accord with the statistics resulting from observations of gravitational lenses.

With this headache and total confusion about the dark energy, with a growing number of astronomical observations in favor and crucified with a concept previously discarded by Einstein, we enter the 21st century, the World Year of Physics and the Black Prince's multimedia installation. With some much darkness about, it is perhaps timely to think about the need of another Messiah, because Albert Einstein indeed illuminated the most distinguished scientific minds of last century and the general public with the universality of his marvelous theories. The fine arts have finally given in with this much deserved tribute.

– Claudio Mendoza, Astrophysicist* Caracas, July 2004*