T HE UNIVERSE AS A HOLOGRAM Maria Ioanna Christodoulou Supervisor: Prof. Kostas Skenderis

O UR UNIVERSE IN A NUTSHELL Q UANTUM MECHANICS

is a branch of physics that describes The between the most basic building blocks of our Universe are the behaviour of maer on micro- interacons scopic scales of about 10-5 cm or less. governed by four forces: the familiar electromagnec force, the strong or nuclear According to quantum mechanics, if force that binds neutrons and protons together, the weak force which is responsible for you know exactly where a parcle is, radioacve decays, and gravity.

you have very limited knowledge of how fast it is moving. This is not a These forces can be described on the smallest of length scales using quantum limitaon of the measuring apparatus mechanics. However, the unificaon of the theory of gravity with quantum but a fundamental property of maer. mechanics, into a single theory of quantum gravity is very challenging.

B LACK HOLES HOLD THE ANSWER

B LACK HOLES are Black holes are objects that need a quantum gravity objects in space whose descripon. In the 70s Jacob Bekenstein and Stephen Hawking showed that gravitaonal field is so strong that even light the informaon that makes up the three dimensional cannot escape it. They black hole can be contained on a two dimensional contained a huge amount Illustraon: NASA/CXC/M. Weiss surface. This is analogous to how a hologram works. of maer squeezed in a very small volume. Since they involve strong gravitaonal fields and very small This observaon was one of the main movaons for the Holographic length scales, they need a quantum gravity Principle. descripon.

A HOLOGRAM is a three dimensional image T HE HOLOGRAPHIC PRINCIPLE generated using a two dimensional film. Although one perceives a three dimensional object, all In 1993, Nobel laureate Gerard ’t Hoo proposed that the physical descripon informaon is encoded in the two dimensional of a system occupying a three dimensional region and film. involving quantum gravity is equivalent to that of another Look at the photo on the le. A credit card security hologram dove system living on the two dimensional boundary and appears three dimensional when without gravity. The two descripons are referred to as the bulk viewed from the right angle, and boundary theory, respecvely. Photo: Credit card security however the hologram is actually hologram/M.I. G ERARD ’T H OOFT Christodoulou engraved on a two dimensional foil. is the recipient of the 1999 This implies that our Universe, with gravity, is equivalent to a two dimensional Nobel Prize in Physics for work universe that is governed by forces like electromagnesm and the strong and weak he did in elementary parcle nuclear forces, but not gravity. physics. In addion to parcle physics, he has made signifi- Boundary cant contribuons to the Bulk Photo: G. ‘t Hoo/From Theory without gravity in one less ‘t Hoo’s personal theory of black holes, gravity, Theory with gravity website dimension and quantum mechanics.

A N EXAMPLE OF THE HOLOGRAPHIC PRINCIPLE

An-de Sier universe In 1997 Juan Maldacena proposed a realisaon of the holographic principle.

The bulk theory describes a special, very symmetric universe called An-

parcle in the An- de Sier space. In the figure this is represented by the interior of the black hole de Sier universe sphere.

The boundary theory, corresponding in the figure to the surface of the sphere, has a special feature: it looks the same when you zoom in or out, move in any parcle in the conformal theory direcon, or rotate. A theory with this feature is called conformal.

conformal boundary The equivalence of these two theories allows sciensts like myself to probe Illustraon: Arsc representaon of the correspondence between the An-de Sier universe the physics of quantum gravity. and the boundary conformal theory/M. I. Christodoulou