The amount of dark matter in space is a key factor that controls the fate of our expanding universe. Cosmologists describe three possible fates, illustrated here as simple shapes that depict the curvature of the space-time fabric of the cosmos. If enough dark matter existed to raise the average density of matter in the universe to three atoms of hydrogen per cubic meter of space, space-time would be flat (left). The relentless pull of this matter's gravity would exactly oppose the force of expansion, creating a universe that gradually slows down but never quite stops expanding. More matter than this would result in a victory for gravity, leading to a closed universe (center) that collapses back in on itself after many billions of years. However, astronomical evidence shows that the universe contains just 30 to 40 percent of the matter needed to counteract its expansion. As a result, most cosmologists believe we live in an open universe (right) that will expand forever. A curious repulsive force within the fabric of space itself may even cause the cosmos to grow ever more rapidly as time passes.

The Sun is a mellow middle-aged star about halfway through its expected 10-billion-year lifetime. As the star continues to age, however, it will consume hydrogen more quickly in order to maintain its internal temperature and pressure. The extra work will cause the Sun to expand and grow bigger, brighter, and hotter. In about 4 billion years, Earth will experience a global warming beyond even the direst of today's predictions concerning greenhouse gases. When the Sun turns to helium as a fuel source in its core, it will swell even further into a red giant looming over the doomed Earth's horizon.