The Universe

This Article was from the May 2005 issue of National Geographic 

Origin 
Infinite Beginnings 
Pushing the limits of theory and imagination in true Einsteinian fashion, cosmologists are daring to speculate that ours is not the only universe. The big bang that created everything we know of space and time could be just one of an infinite number of beginnings, yielding a never-ending sequence of universes. 

The scenario emerges from inflation theory, a descendant of Einstein’s general theory of relativity. Relativity implies that space and time can stretch to vast dimensions from a tiny starting point; inflation describes how our own universe ballooned in its first moments and suggests that the same thing can happen anywhere, at any time. The result: an eternal expanse of space erupting with bubbles of energy, or big bangs, each the seed of a universe. Not all universes will be alike. While a cosmos like our own glows with galaxies, others may contain more dimensions or different forms of matter. In some, even the laws of physics work differently. 

Structure 
The Invisible Web 
Something out there holds swarms of galaxies together and keeps their stars from flying apart, but scientists still haven’t learned what this invisible substance is. Known as dark matter, it gathers to form a colossal cosmic scaffolding. Astronomers believe that galaxies formed at the densest points in this web-like structure, and the dark matter continues to hold them in place with its gravity. Its bulky presence can be detected by tracking stars on the outskirts of galaxies, which move at speeds that would be impossible if only visible matter – a galaxy’s other stars and gas – were pulling on them. Astronomers have also mapped this unseen substance with the help of an effect predicted by Einstein’s general relativity: dark matter’s gravity wrinkles space-time, bending light rays as they pass. Such measurements indicate that dark matter could make up 90 percent of the universe’s total mass. These days, cosmologists are searching for the identity of dark matter, trying to detect the elusive substance responsible for arranging everything we see in the sky. 

Expansion 
Fast Forward: the Bid Rip? 
The death of the universe could rival its birth in explosive drama if a puzzling form of energy continues to accelerate the expansion of space-time. Since the 1920s astronomers have thought the expansion was slowing down, but recent observations of distant stars reveal that the stretching of space is actually speeding up. If it picks up even more, the universe could be headed for a ‘big rip.” One of the many possible fates – shows how some 20 billion years from now, unchecked expansion could tear matter apart, from galaxies all the way down to atoms. The driving force is a mysterious “dark energy” that counteracts gravity’s pull and might ultimately defeat all the forces that bind matter. Einstein was the first to introduce the notion of repulsive gravity, but he later disavowed it. Dark energy, says cosmologist Michael S. Turner, who coined the term, “has the destiny of the universe in its hands.” Although we live in the best of times, under a sky full of stars, it will grow ever darker and emptier as space-time expands.