Tuesday, 27 March 2012

How It Began

How It Began: A Time-Traveller's Guide to the Universe by Chris Impey

Wall Street Journal, 28 March 2012

'The theory is beautiful beyond comparison" is how Albert Einstein modestly described his theory of gravity, known as general relativity. He believed that "scarcely anyone who fully understands this theory can escape its magic." In the years since 1916, when he published a paper setting out the theory, few have disagreed, yet buried within his greatest achievement was also what Einstein considered "my greatest blunder."

The equations of general relativity can be solved in a number of different ways, with each solution representing a model of a possible universe. Like everyone else at the time, Einstein believed that the universe was eternal and unchanging, so he incorporated a mathematical term, the "cosmological constant," to ensure that that was exactly how it remained. This fixing of the equations was Einstein's great blunder, for he failed to grasp the full magic of his theory. It was left to others to take seriously the solutions that pointed to a universe that was not static but expanding.

In his previous book, "How it Ends" (2010), Chris Impey, a professor of astronomy at the University of Arizona in Tucson, outlined the ultimate fate of our expanding universe. It was a somber scenario in which galaxies sail apart at an ever-increasing rate as the stars within them fade to dark embers, with the structure of the universe eventually unraveling.

In his latest book he tackles the more difficult question of how the universe began. In clear, enthusiastic and occasionally lyrical prose, Mr. Impey takes the reader on a mind-blowing tour back through eons, stopping along the way to explain the formation of the solar system, the birth and death of stars, white dwarfs, supernovas, spiral galaxies, cosmic inflation, string theory, black holes and M-theory—an extension of string theory featuring 11 space-time dimensions that can be curled up into the four we're familiar with in 10500 (1 followed by 500 zeros) ways. Each such solution leads to another potential universe, but ours is the one that interests Mr. Impey.

 Georges Lemaître

It was the Russian mathematician Alexander Friedmann who, in 1922, first showed that general relativity predicted an expanding universe. But Mr. Impey rightly chooses to highlight the work of Georges Lemaître, a Belgian priest who was also a physicist. In 1927 Lemaître realized that "running the clock backwards" on an expanding universe led to one that was smaller, denser and hotter in the past. "The fireworks are over and just the smoke is left," he said of the quest to imagine the initial state of the universe. "Cosmology must try to picture the splendor of the fireworks." Lemaître eventually arrived at a model that he called the "primeval atom" and that we call the Big Bang.

When Lemaître discussed the idea with Einstein, he was unimpressed. "Your math is correct, but your physics is abominable," he told Lemaître. Yet six years later, in 1933, Einstein heard Lemaître lecture and afterward admitted it was "the most beautiful and satisfactory explanation of creation I've ever heard."

The reason Einstein changed his mind was that by the late 1920s the American astronomer Edwin Hubble had discovered two remarkable facts. The first was that what we had long assumed to be the universe was only our host galaxy, the Milky Way, which in turn was just one of many such "island universes." Second, Hubble found that light from these distant galaxies was stretched toward the red end of the visible spectrum. This so-called redshift is evidence that these galaxies are moving away from our own and that the universe, therefore, is expanding. This supported Lemaître's theory that if the expansion could be rewound, the universe would steadily contract and eventually reach what he called "a day without yesterday."

According to the Big Bang model that has grown from Lemaître's insight, the moment of instantaneous creation was 13.75 billion years ago and began with a singularity, a point of infinite mass and density where our present understanding of physics simply breaks down. Yet "the Big Bang is all around us," Mr. Impey notes, in the form of cosmic microwave background radiation, which suffuses the entire universe. Soon after it was discovered in 1964, scientists recognized this radiation as the echo of the Big Bang, an afterglow from the era when the universe was hotter and denser. "There are tens of thousands of microwaves from creation in every breath you take," Mr. Impey delights in revealing.

It had long been assumed that gravity would act as a brake on cosmic expansion, but astronomers were horrified to discover in the 1990s that the expansion is speeding up. "Dark energy" is the mysterious culprit, but the name is more of a sign of ignorance than a physical description of something that makes up approximately 73% of the mass-energy of the universe. If that wasn't surprising enough, an analysis of the motion of galaxies reveals that approximately 23% of the universe is made up of something dubbed "dark matter." This means we know nothing about roughly 96% of our universe.

The Big Bang model also says nothing about what banged, why it banged or what happened before it banged. So in the 1990s cosmologists began to take seriously the idea that our universe is but part of a "multiverse" of different universes, each with its own laws of physics. It's a step too far for Mr. Impey, who suggests that "with the multiverse we seem to have taken leave of our senses and entered into wild speculation." There are some excellent books on this new multiverse cosmology, such as John Barrow's "The Book of Universes" and Brian Greene's "Hidden Reality," but "How It Began" deserves to be a well-thumbed guidebook for—and in—this universe.