Dark Force

The Hubble Space Telescope has touched every area of astronomy. But some of its key contributions have profoundly shaped our view of the universe. One of the original hopes for the Hubble was to extend the Key Project, which measured the local or current ex­pansion rate, and measure the entire expansion history of the uni­verse over cosmic time. Hubble’s sensitivity and resolution allow it to observe supernovae in distant galaxies. A Type 1a supernova is a white dwarf star that detonates as a result of mass steadily siphoned onto it from a massive companion star. When the white dwarf exceeds the Chandrasekhar limit, which we encountered in the last chapter, it collapses and then explodes as a supernova. This well-regulated process means it’s a “standard bomb” with an in­trinsic brightness that doesn’t vary from one supernova to another by more than 15 percent. At peak brightness, the supernova rivals its parent galaxy in brightness (figure 11.4), so these explosions can be seen to distances of 10 billion light-years or more.39

In the mid-1990s, two groups studying supernovae saw some­thing utterly unexpected.40 In an expanding universe, the effect of normal matter and dark matter is to slow down the expansion rate. By looking back in time with more and more distant super­novae, these researchers had expected to see supernovae appear­ing slightly brighter than expected for a constant expansion rate. The reasoning is that deceleration reduces the distance between

us and a supernova relative to constant expansion, making it ap­pear brighter. Instead, they saw the opposite effect: the distant su­pernovae were dimmer than expected. The interpretation was the distance to the dying star was larger than expected. Rather than decelerating, the universe has been accelerating! Cosmic accelera­tion is a very puzzling effect, because it implies a force acting in op­position to gravity. No such force is known in physics, so the cause of the phenomenon was called “dark energy,” where that phrase is really just a placeholder for ignorance. Dark energy seems to have the character of the cosmological constant, the term that Ein­stein added to the solutions of his equations of general relativity to suppress natural expansion (and which he later called the greatest blunder of his life). It’s new and fundamental physics.

The discovery of cosmic acceleration was based on images and spectra taken with ground-based telescopes. The Hubble Space Telescope didn’t make the initial discovery. But confirming the re­sult, extending the measurements to higher redshift, and putting constraints on the nature of dark energy—all of those have been essential contributions. Hubble’s depth has been used to trace the expansion history back over two thirds of the age of the universe. The data show that acceleration reverts to deceleration more than 5 billion years ago.41 Astronomers can now apportion the two major components of the universe: dark matter and dark energy. Dark energy accounts for 68 percent, dark matter accounts for 27 percent, diffuse and hot gas in intergalactic space is about 4.5 percent, and all the stars in all of the galaxies in the observable uni­verse amount to only 0.4 percent of the cosmic “pie.” Dark forces govern the universe.

Imagine the expanding universe with a brake and an accelerator. The “driver” isn’t very competent so they press both pedals at the same time. Dark matter is the brake because gravity slows the ex­pansion. Dark energy is the accelerator. In the first two thirds of the expansion history, the dark matter dominates and the expansion slows with time. But the effect of dark matter weakens, because the density and the gravity force go down, while dark energy has a constant strength in both time and space. So the pressure on the brake eases while the accelerator is pressed the same amount, and about 5 billion years ago all galaxies started to separate at ever – increasing rates. Some cosmologists consider it an unexplained coincidence that dark energy and dark matter—two mysterious entities with fundamentally different behavior—happen to have roughly the same strength and crossed paths relatively recently in cosmic time. This is the only time in cosmic history they are close to equal strength; for most of the early history of the universe, dark matter was utterly dominant and forever into the future dark energy will dominate. This coincidence only sharpens the enigma.