Paul J. Steinhardt is the Albert Einstein Professor of Science at Princeton University. Director, Center for Theoretical Science   He is on the faculty of both the Department of Physics and the Department of Astrophysical Sciences.
Steinhardt is one of the architects of the "inflationary model of the universe."

This is a modification of the standard Big Bang picture, which proposes a brief period of extraordinary, superluminal expansion in the early universe to explain the homogeneity and large-scale structure of the universe.
He and collaborators constructed the first successful model and then showed how quantum fluctuations generated during inflation may seed the formation of galaxies and also produce temperature variations in the cosmic microwave background. In the last year, Steinhardt and Neil Turok (Cambridge University) proposed a radical alternativethe cyclic universe." In this scenario, the universe avoids inflation and instead, undergoes an infinite time in the past and the future, going through an endless sequence of bangs and crunches.
Steinhardt has made numerous contributions to the study of dark matter and dark energy. Steinhardt and David Spergel (Princeton) developed the idea of "strongly iself-interacting dark matter" to explain observations of subgalactic structure. Steinhardt also introduced the concept of "quintessence" to explain the recently observed acceleration of the universe. In condensed matter physics, Steinhardt and Dov Levine (Technion) predicted a new form of matter, known as a "quasicrystal." This is a solid that exhibits five-fold and other symmetries that are impossible for ordinary crystals. He pioneered studies of their physical and structural properties and is currently investigating their application to photonics. He has edited two books on the subject, The Physics of Quasicrystals and Quasicrystals: The State of the Art.
Website: feynman.princeton.edu/~steinh
Articles on KurzweilAI.net written by Paul J. Steinhardt:

universo senza fine

Sebbene la teoria del Big Bang sia la più accreditata per spiegare 1'origine dell'universo, negli ultimi trent'anni è stata costantemente rivista e integrata per rispondere alle domande sulla nascita delle galassie e sulla velocità di espansione dell'universo. Eppure, il quesito più importante, cosa scatenò il Big Bang, non ha ancora trovato una risposta. In UNIVERSO SENZA FINE
"Universo senza fine" i fisici teorici Paul J. Steinhardt e Neil Turok presentano un nuovo modello di cosmologia, affrontando il Big Bang non come un momento germinale nella storia dell'universo, ma come un passaggio nell'infinita serie di collisioni tra il nostro e un universo parallelo; Un "universo ciclico" il cui modello si avvale dei più recenti sviluppi nella ricerca sulla fisica delle particene e della teoria delle superstringhe.
lafeltrinelli.it

The Cyclic Universe   http://www.kurzweilai.net/the-cyclic-universe

http://www.kurzweilai.net/paul-j-steinhardt

http://www.princeton.edu/physics/ 

Quintessence and the Missing Energy Problem
Recent evidence suggests that the total matter density of the universe is significantly less than the critical density. The shortfall may be explained by curvature (an open universe), vacuum energy density (a cosmological constant), or quintessence (a time-evolving, spatially inhomogeneous component with negative pressure). In all three cases, a key problem is to explain the initial conditions required to have the energy density nearly coincident with the matter density today. A possible solution is ?tracker fields,? a form of quintessence with an attractor-like solution which leads to the same conditions today for a very wide range of initial conditions. Tracker field make quintessence a more viable candidate for the missing energy component and produce more distinctive predictions.          

physica.org       physicsweb.org  

.PDF   FULL TEXT   http://www.iop.org

http://it.wikipedia.org/wiki/Quintessenza

Cosmologists have proposed that a mysterious substance called quintessence can explain why our universe is accelerating. But what is it made of?
A revolution is taking place in cosmology. New ideas are usurping traditional notions about the composition of the universe, the relationship between geometry and destiny, and Einstein's greatest blunder. As numerous observations and experiments reshape the field, many cosmologists are exploring the possibility that the vast majority of the energy in the universe is in the form of a hitherto undiscovered substance called "quintessence".
Quintessence has the striking physical characteristic that it causes the expansion of the universe to speed up. Most forms of energy, such as matter or radiation, cause the expansion to slow down due to the attractive force of gravity. For quintessence, however, the gravitational force is repulsive, and this causes the expansion of the universe to accelerate.
The name has historical precedents. In philosophy, quintessence refers to the fifth element - after air, earth, fire and water - proposed by the ancient Greeks to describe a sublime, perfect substance. In literature, Quintessence is the queen of a land of speculative science in Rabelais' Gargantua.
In cosmology, quintessence is a real form of energy distinct from any normal matter or radiation, or even "dark matter". Its bulk properties - energy density, pressure and so forth - lead to novel behaviour and unusual astrophysical phenomena. So far its existence has only been inferred indirectly from a range of observations, but a number of current and planned experiments will make direct searches for this elusive form of energy.
Although cosmological quintessence bears some superficial resemblance to the historical version, there is plenty of substance in the modern invocation of this classical name.              physicsweb.org
 

paul j. steinhardt

A Quintessential Introduction to Dark Energy

http://www.feynman.princeton.edu