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Infinity in a Second

There are mysteries we may never solve. The origin of the universe is one of them, and one that physicists, cosmologists, and philosophers have puzzled over for millennia. Yet it was only in the early 20th century that scientists discovered that they could travel back in time, at least through their equations, rewinding its history all the way back to its origin, the famous Big Bang. And although there is a long way to go before scientists reach a consensus on the exact scenario, cosmologists are increasingly convinced that it is within their grasp.
Traveling back through time, one may be surprised to learn that the greatest questions surround a very short time-period, namely the universe's first second. As Jean-Philippe Uzan, from the Astrophysics Institute in Paris (IAP),1 points out, “After the first second of the Big Bang, the temperature was close to 10 billion kelvins, and everyone is more or less in agreement about what happened (see illustration). There is no longer any doubt that the universe was expanding then and that it emerged from a hot, dense phase.”

A short history of the universe

© NASA/WMAP Science Team

Click to enlarge.



The Original Content

In fact, central to the problem is understanding what happened during this very first second, starting with the question of what the universe was made of at that moment. “And to find out what type of matter the universe contained, we need to turn to particle physics,” explains Uzan. “The problem, though, is that the current model–the standard model–is only valid up to energy densities that are rapidly exceeded as you go back in time.” For example, cosmologists believe that one millionth of a second (10-6 second) after the Big Bang, quarks joined together to form protons and neutrons. But like all the other particles of matter, quarks appeared a long time before, probably before 10-12 seconds after the Big Bang.
For that time-period, known as the Grand Unification Era, we have yet to construct a theory that describes elementary particles and their interactions.
Although the universe was already populated with particles that we know–quarks, electrons, etc.–it must also have harbored other particles that are today unknown. Physicists think that some of these particles might make up the elusive “dark matter.” Although it has never been directly observed, its gravity is thought to give the universe and galaxies their structure, and scientists believe it is far more abundant than ordinary matter. Furthermore, according to our current theoretical understanding, at this time in the distant past, there should have been as much matter as antimatter.
Scientists believe that every particle is associated with an antiparticle–absolutely identical to it, but with an opposite electrical charge. For instance, the antiparticle of the electron is the positron. Yet astonishingly, today's universe contains almost nothing but ordinary matter, and the reason for this lack of symmetry is not yet understood. Something CERN's2 giant accelerator, the Large Hadron Collider (LHC), might be able to elucidate in the coming years. “It will give us access to physics that goes beyond the standard model,” adds Uzan. This means that we should soon improve our understanding of the state of matter during the universe's primordial phase.”

How It Unravelled
When it comes to the geometry of the primordial universe, cosmologists have constructed elegant theories about the evolution of space-time, a concept first introduced by Einstein in his theory of relativity. For instance, specialists believe that approximately 10-35 seconds after the Big Bang, the universe underwent an extremely rapid phase of expansion–called “inflation”–during which its size increased by an incredible several billion orders of magnitude (see illustrations). For Thibault Damour, tenured professor at France's Institut des Hautes Études Scientifiques (IHES), “there is a broad consensus on inflation, although it remains a speculative model.” The European Space Agency's (ESA) Planck satellite, successfully launched this May, might shed some light on the exact nature of inflation, by measuring its precise effects on the properties of fossil radiation, known as the cosmic microwave background (CMB).

In the Beginning Was Naught
Turning back the clock further, we come to what scientists believe to be the last stage before the Big Bang: the “Planck wall,” which took place approximately 10-43 seconds after the Big Bang. At this stage, today's physics is powerless. As Uzan puts it, “during this period, gravity becomes quantum– and the problem is that we don't yet have a quantum theory of gravity.” There is a need for a theory that unifies general relativity, which describes how space-time changes under the effect of gravity, and quantum mechanics, the theory of the infinitesimally small. This has led to a host of hypotheses that paint a truly mind-boggling picture of the birth of the universe. Many of these theories stem from string theory. Extremely complex and still highly speculative, string theory emerged in the 1970s with the aim of uniting the two irreconcilable theories of physics: quantum mechanics and gravity. According to this theory, the fundamental building blocks of the universe are not discrete particles but tiny linear filaments called strings. The strings vibrate in a space-time that has many more than our classical four dimensions (the three dimensions of space, plus that of time).
Some researchers have constructed hypotheses that envisage a time before the Big Bang. For instance, Pierre Binetruy, from the Astroparticle physics and cosmology laboratory (APC)3 in Paris, has developed a theory where a Big Bang is created by the collision of two four-dimensional universes. “We hypothesize that there are several universes which sometimes collide,” Binetruy explains. “Admittedly, with this kind of approach we don't know about the physics at the precise moment of the Big Bang, but are able to describe in detail the state of the universe immediately before and after.” In this hypothesis, the universe therefore existed before the Big Bang, which was nothing more than one episode in a never-ending history.
Meanwhile, Damour, together with Marc Henneaux and Hermann Nicolai, has developed an approach in which space-time emerges from an earlier situation: “According to our theory, as you approach the singularity–the Big Bang– space vanishes. Nothing remains but time, from which space emerged.” As Damour himself recognizes, “all this is highly speculative, but it is nonetheless extremely precise mathematically, and ties in with sound parts of string theory.”

The Universe according to the standard model

© Particle Data Group, LBNL 2008

Click to enlarge.



The Barrier of Proof

Will we ever be able to confirm, or simply derive facts from these abstract theories? Indirectly, perhaps. If physicists manage to demonstrate the validity of string theory, if only in part, it will make some of these hypotheses more credible. “I believe that cosmology is at a turning point,” says Nathalie Deruelle, also from APC. “Admittedly, a new theoretical model comes out every day. A new understanding of our universe will surely emerge from this ferment of ideas.”
Will such a model enable us to understand the mysteries of the origin of the universe? It's hard to say. According to Étienne Klein, from the French Atomic Energy Commission (CEA),4 “to make headway, [science] needs an 'already there,' a starting point–one made up of existing principles, laws, or objects. However, the absolute origin of the universe is not part of the 'already-there,' since it corresponds to the emergence of something in the absence of any other thing. [...] That's why the question of the origin of the universe remains unanswerable.”5 Perhaps a purely philosophical question after all.

Mathieu Grousson

The Standard Model describes elementary particles and their strong, weak, and electromagnetic interactions. Gravitational force does not yet fit into the theory.

Theory of Relativity
A theory developed by Einstein to describe gravity and in which the notions of space and time are replaced by space-time. The geometry of this space-time is determined by the matter distribution and is the source of the gravitational force.

Cosmic Microwave Background (CMB) radiation
This radiation, emitted some 380,000 years after the Big Bang and still detectable today, is a precious witness to the very young universe. It therefore carries essential information about inflation.

 

Notes :

1. Institut d'astrophysique de Paris (CNRS / Université Paris-VI).
2. European Organization for Nuclear Research.
3. Astroparticule et cosmologie (CNRS / CEA / Université Paris-VII / Observatoire de Paris).
4. Commissariat à l'énergie atomique.
5. Etienne Klein, “Mystères de l'Univers,” Le Monde, November 21, 2008.

Contacts :

Jean-Philippe Uzan,
uzan@iap.fr
Thibault Damour,
damour@ihes.fr
Pierre Binetruy,
binetruy@apc.univ-paris7.fr
Nathalie Deruelle,
deruelle@apc.univ-paris7.fr


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