Free Origins: Fourteen Billion Years of Cosmic Evolution by Donald Goldsmith, Neil deGrasse Tyson

exploit dark matter—once we figure out what the stuff is made of. Imagine invisible toys, cars that pass through one another, or super stealth airplanes. The history of obscure and obtuse discoveries in science is rich with examples of clever people who came later and who figured out how to exploit such knowledge for their own gain or for the benefit of life on Earth.

CHAPTER 5
    Let There Be More Dark
    T he cosmos, we now know, has both a light and a dark side. The light side embraces all familiar heavenly objects—the stars, which group by the billions into galaxies, as well as the planets and smaller cosmic debris that may not produce visible light but do emit other forms of electromagnetic radiation, such as infrared or radio waves.
    We have discovered that the dark side of the universe embraces the puzzling dark matter, detected only by its gravitational influence on visible matter but otherwise of completely unknown form and composition. A modest amount of this dark matter may be ordinary matter that remains invisible because it produces no detectable radiation. But, as detailed in the previous chapter, the great bulk of the dark matter must consist of non-ordinary matter, whose nature continues to elude us—except for its gravitational force on matter we can see.
    Beyond all issues concerning dark matter, the dark side of the universe has another, entirely different aspect. One that resides not in matter of any kind, but in space itself. We owe this concept, along with the amazing results that it implies, to the father of modern cosmology, none other than Albert Einstein himself.
    Ninety years ago, while the newly perfected machine guns of World War I slaughtered soldiers by the thousands a few hundred miles to the west, Albert Einstein sat in his office in Berlin, pondering the universe. As the war began, Einstein and a colleague had circulated an antiwar petition among his peers, gathering two other signatures in addition to their own. This act set him apart from his fellow scientists, most of whom had signed an appeal to aid Germany’s war effort, and ruined his colleague’s career. But Einstein’s engaging personality and scientific fame allowed him to keep the esteem of his peers. He continued his efforts to find equations that could accurately describe the cosmos.
    Before the war ended, Einstein achieved success—arguably his greatest of all. In November 1915, he produced his general theory of relativity, which describes how space and matter interact: Matter tells space how to bend, and space tells matter how to move. To replace Isaac Newton’s mysterious “action at a distance,” Einstein viewed gravity as a local warp in the fabric of space. The Sun, for example, creates a sort of dimple, bending space most noticeably at distances closest to it. The planets tend to roll into this dimple, but their inertia keeps them from falling all the way in. Instead, they move in orbits around the Sun that keep them at a nearly constant distance from the dimple in space. Within a few weeks after Einstein published his theory, the physicist Karl Schwarzschild, diverting himself from the horrors of life in the German army (which gave him a fatal disease soon afterward), used Einstein’s concept to demonstrate that an object with sufficiently strong gravity will create a “singularity” in space. At such a singularity, space bends completely around the object and prevents anything, including light, from leaving its immediate vicinity. We now call these objects black holes.
    Einstein’s theory of general relativity led him to the key equation he had been seeking, one that links the contents of space to its overall behavior. Studying this equation in the privacy of his office, creating models of the cosmos in his mind, Einstein almost discovered the expanding universe, a dozen years before Edwin Hubble’s observations revealed it.
    Einstein’s basic equation predicts that in a universe in which matter has a