Hunting Dark Matter and Dark Energy

Chandra has also weighed in on two of the profound mysteries of cosmology: the nature of dark matter and dark energy. These two components of the universe account for 95 percent of its be­havior, yet the physical basis for them is not known and is not part of standard physics. Dark matter outweighs normal matter by a factor of six and binds galaxies and clusters and stops them from flying apart, as well as causing the expanding universe to decelerate for most of the first two thirds of its existence. Dark energy dominates dark matter by a factor of three and has caused the cosmic expansion to accelerate in the most recent third of the universe’s existence.67

Dark matter and dark energy don’t interact strongly with nor­mal matter like the atoms in our bodies, so stealth and cunning must be used to ensnare them and measure them. In both cases the laboratories used are rich clusters of galaxies, consisting of thousands of galaxies moving swiftly under the action of gravity and a huge cloud of superhot gas, so energetic that it emits X – rays. In 2006, an object called the Bullet Cluster was used for a convincing demonstration that dark matter actually exists. In the Bullet Cluster, a bullet-shaped cloud of hundred-million-degree gas is produced by a high-speed collision between a large cluster and a smaller one. The hot gas was slowed by the collision, due to a drag force analogous to air resistance. By contrast, the dark mat­ter hardly interacted at all and sailed through during the collision, ending up on either side of the hot gas.68 This result would not have occurred unless weakly interacting dark matter dominated the mass of both clusters. In particular, alternate theories of grav­ity, conjured up to avoid needing dark matter, fail to explain the observations. It seems we have to live with dark matter.

Dark energy is even more ephemeral, announcing its presence (and ubiquity) only by the effect it has on the cosmic expansion. The long and hard search for independent evidence of its existence settled on clusters of galaxies. In a study that took nearly a de­cade to complete, researchers showed that rich clusters suffer from “arrested development.”69 It’s more difficult for clusters to grow when space is being stretched. By comparing the size and age of clusters with simulations of how they should grow under differ­ent conditions of cosmology, the results cement the interpretation of dark energy as a universal repulsive agent. They also rule out alterations to gravity theory and confirm that general relativity is a good description of the behavior of matter and radiation on large scales. The enigma of dark energy has not been solved, but its status as the biggest challenge in both physics and cosmology has been enhanced. Whether it is a black hole devouring a companion, massive black holes causing mayhem in the centers of galaxies, or clusters being pulled apart by the accelerating expansion of space, Chandra has provided data to illustrate the violence of the uni­verse we inhabit.

A result of these insights is a new sense of the power of gravity. Stars are powered by gravity, and gravity also governs the nature of the most compact and energetic objects we’ve ever discovered. Dark forces even govern the expansion of the universe. When we think of other worlds, we think of planets illuminated by stars, where any life that exists is beholden to the energy from the star. But in a fundamental sense, starlight is just the inefficient leakage of radiation from mass-energy conversion by fusion. The true source of the starlight is gravity—a star is a gravitational engine. Black holes and neutron stars have no light but they have intense gravity. While they seem alien and utterly different from our world, given suitable protection or adaptation living creatures might be able to live near these compact stellar husks, using gravity to power their dreams.