In this file, published in its
bulletin,
the CNRS expresses its opinion on the astrophysics and cosmology work
by Jean-Pierre Petit
March 8, 2005
Big Bang Dark Energy
THE GREAT UNKNOWN
In 1998, the earthquake struck, recounts Pierre Astier, from the Laboratory of Nuclear and High Energy Physics: the expansion of the Universe is accelerating under the influence of a mysterious
dark energy.
This energy accounts for about 70% of the cosmos. And we know nothing about its composition. Until then, within the framework of the Big Bang theory and based on Albert Einstein’s general relativity, we knew that the Universe was expanding uniformly due to an initial "explosion." Researchers’ work thus consisted in cataloging the contents of the cosmos to "weigh" it and determine whether expansion would eventually stop under gravity’s influence. But seven years ago, ideas were completely overturned. Two independent teams entered the scene: the Supernova Cosmology Project and the High z Supernova Search Team. They observed around fifty distant stellar explosions (spread between 1 and 6 billion light-years away), identified as Type Ia supernovae—natural nuclear detonations. These rare phenomena are called "standard candles" because their absolute brightness is known, allowing their distance to be determined by two different methods. However, during the 1998 observations, the most distant supernovae appeared dimmer than expected in a Universe expanding solely due to matter. Therefore, a conclusion became unavoidable: the apparent dimness is explained by the object’s greater distance. Its host galaxy lies farther away than previously thought. The Universe must therefore have expanded faster than predicted. To explain this phenomenon, a mysterious dark energy must be invented, giving a boost to expansion. Would this discovery shake the Big Bang theory? "Not at all. It renews interest and adds excitement," reassures Pierre Astier.
Indeed, since 1998, two confirmations of acceleration have further strengthened this hypothesis. And "the latest results indicate that this expansion regime has prevailed for about four to five billion years—roughly 35% of the Universe’s history," notes the physicist, who belongs to the research group created by Reynald Pain, the only French scientist involved in the American Supernova Cosmology Project. French researchers now use several telescopes, including the Canada-France-Hawaii Telescope and the European Very Large Telescope in Chile. The hunt for distant supernovae has become a major objective for these observatories. The goal? "To discover hundreds," says Pierre Astier enthusiastically. This is essential if we are to estimate how the acceleration of expansion has evolved in the past. Some physicists see in this repulsive force the signature of what they call "vacuum energy."
This reflects the incredible quantum frenzy of the cosmos at small scales. Scientists view the vacuum—"what remains when everything is removed"—as a strange medium. Absolute nothingness does not exist. Particles and antiparticles constantly appear and instantly disappear. They are associated with an irreducible energy known as the "ground level" or, more simply, "vacuum energy." According to an experiment proposed in 1948 by Dutch physicist Hendrick Casimir, vacuum energy behaves like pressure—a repulsive force. Hence the idea of identifying it with dark energy, which seems to be "blowing apart" the Universe.
For others, if dark energy evolves over time, it might correspond to an entire zoo of exotic objects constituting quintessence—or "fifth essence"—alongside the four fundamental forces (see page 28). As you can see, the fate of the entire cosmos is at stake. It has become urgent to resolve these ambiguities. With ongoing projects, the French team aims to detect 600 new supernovae within five years. Meanwhile, the Hubble Space Telescope discovered sixteen supernovae in 2003. May the best win in the quest for dark energy!
Frédéric Guérin
CONTACT
Pierre Astier, astier@in2p3.fr
DARK MATTER: THE INVISIBLE OCEAN
This is a stubborn mystery: ordinary matter accounts for less than 5% of the cosmos, and over 95% of the total remains unaccounted for. If dark energy makes up 70% of the Universe, what constitutes the rest? It all began in 1933, when American astrophysicist Fritz Zwicky examined the Coma cluster in the constellation of Virgo, located 150 million light-years away. This object consists of thousands of galaxies similar to our Milky Way. He discovered that their motion within the cluster suggested there was a hundred times more matter than what is visible. In other words, the tip of the iceberg. In the background, a clear conclusion emerged: bright stars, gas clouds, and dust filling galaxies are merely trivial debris floating on a vast, invisible, and powerful ocean that moves them around. Similarly, in the 1960s–1970s, studies of star movements at the centers of galaxies seemed to prove that these galaxies contain five to ten times more invisible matter than visible matter. To preserve appearances, scientists thus had to "invent" a different kind of matter—dark or invisible matter. For the past ten years, scientists have observed a dramatic improvement in both the quality and quantity of data.
On one hand, observations of galaxy rotation have confirmed the presence of a dark matter halo ten times more massive than the stars. On the other hand, clusters contain hot gas heated to millions of degrees. Measurements in X-rays from satellites Chandra (NASA) and XMM (Europe) point to a quantity of dark matter a hundred times greater than visible matter. Finally, another tool has recently proven effective: gravitational lenses—natural mirages predicted by relativity—provide a powerful means to probe the geometry of the Universe. By monitoring microlensing effects, scientists have determined that galactic dark matter does not consist of atoms in the classical sense, with a nucleus (protons and neutrons) and electrons. These enigmatic objects were named MACHOs (Massive Compact Halo Objects), and researchers estimate they account for less than 10% of the total matter in our galaxy. So, what makes up the remainder of the cosmos, which does not correspond to any atomic nucleus or known particle? No one knows—but ideas are beginning to emerge. Physicists already have a candidate...