The Holographic Universe
  I give a critical review of the holographic hypothesis, which posits that a
universe with gravity can be described by a quantum field theory in fewer
dimensions. I first recall how the idea originated from considerations on black
hole thermodynamics and the so-called information paradox that arises when
Hawking radiation is taken into account. String Quantum Gravity tried to solve
the puzzle using the AdS/CFT correspondence, according to which a black hole in
a 5-D anti-de Sitter space is like a flat 4-D field of particles and radiation.
Although such an interesting holographic property, also called gauge/gravity
duality, has never been proved rigorously, it has impulsed a number of research
programs in fields as diverse as nuclear physics, condensed matter physics,
general relativity and cosmology. I finally discuss the pros and cons of the
holographic conjecture, and emphasizes the key role played by black holes for
understanding quantum gravity and possible dualities between distant fields of
theoretical physics.
