Super-Nobel in Physics 2011

The first observation of a supernova is dated 1572 by Tycho Brahe, but the hystorically most important supernova's observation is the Galilei's observation in 1604:
The supernova of 1604 caused even more excitement than Tycho's because its appearance happened to coincide with a so-called Great Conjunction or close approach of Jupiter, Mars and Saturn.(1)
The Galilei's discover was revolutionary for one important reason:
Galileo's observations and those made elsewhere in Italy and in Northern Europe indicated that it was beyond the Moon, in the region where the new star of 1572 had appeared. The appearance of a new body outside the Earth-Moon system had challenged the traditional belief, embodied in Aristotle's Cosmology, that the material of planets was unalterable and that nothing new could occur in the heavens.(1)
About the new star
Galileo states that [it] was initially small but grew rapidly in size such as to appear bigger than all the stars, and all planets with the exception of Venus.(1)
We can confrount the observation with modern definitions:
Novae are the result of explosions on the surface of faint white dwarfs, caused by matter falling on their surfaces from the atmosphere of larger binary companions. A supernova is also a star that suddenly increases dramatically in brightness, then slowly dims again, eventually fading from view, but it is much brighter, about ten thousand times more than a nova.(1)
These dramatical events became soon a good tools in order to observe the expansion of the universe:
Type Ia supernovae are empirical tools whose precision and intrinsic brightness make them sensitive probes of the cosmological expansion.(5)
And observing a series of supernovae the team of Brian Schmidt (1967) and Adam Riess (1969) in 1998(3) and the team of Saul Perlmutter (1959) in 1999(4) found an important consmological observation: Universe is accelerating!
The vision of the Universe it was very simple: a quickly, great expansion from a high density quark-gluon plasma (or something else); a cooling of the Universe with an aggregation between particles with the birth of stars, planets, galaxies; a deceleration in the expansion dued by gravitational field; an unknown future with Universe in the balance between eternal expansion and gravitational collapse. The observation of Schmidt's and Perlmutter's teams changed the decelerating-scenario, substituting it with an accelerating-scenario, that is compatible with a nonzero cosmological constant (in the plot, the cosmological constant is $\Omega_\Lambda$, that is also the vacuum energy density):
We can write the expansion of the Universe using the following formula, that is a sperimentally verifiable version of Einstein's equation(2): \[\left ( \frac{\text{d} a}{\text{d} \tau} \right )^2 = 1 + \Omega_M \left ( \frac{1}{a} - 1 \right ) + \Omega_\Lambda (a^2 - 1)\] where $a$ is the expansion factor, a function of the redshift $z$(6), $\tau = H_0 t$, with $H_0$ the Hubble's constant, and $\Omega_M$ the matter density of the universe.
These is the brief story of the Nobel Prize in Physics assigned today to Schmidt, Riess and Perlmutter, a discover that has brought to the attention of cosmology the dark matter introduced by Fritz Zwicky. But that's another story!

(1) Shea, W. Galileo and the Supernova of 1604. 1604-2004: Supernovae as Cosmological Lighthouses, ASP Conference Series, Vol. 342, Proceedings of the conference held 15-19 June, 2004 in Padua, Italy.
(2) Carroll, Sean M.; Press, William H.; Turner, Edwin L. The cosmological constant. Annual review of astronomy and astrophysics. Vol. 30 (A93-25826 09-90), p. 499-542.
(3) Riess, Adam G et al. Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant. The Astronomical Journal, Volume 116, Issue 3, pp. 1009-1038. (arXiv)
(4) S. Perlmutter et al.. Measurements of $\Omega$ and $\Lambda$ from 42 High-Redshift Supernovae. Astrophysical Journal, 517, 565-586. (arXiv)
(5) Perlmutter, S. and Schmidt, B.P. Measuring Cosmology with Supernovae. Lecture Notes in Physics, 2003, Volume 598/2003, 195-217. (arXiv)
(6) $a = \frac{1}{1+z}$
Nobel Prize official resources: Press release, Useful links and further readings

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