When we gaze into the peaceful, twinkling night sky, we see much the same vistas witnessed by our ancestors thousands of years ago. Yet centuries of scientific progress have so transformed our understanding that it cannot be said we share their same simplistic view. We now know that the world is not flat, that the sky is not a dome, that the stars are not pinholes through which flickering fires shine.
Many mysteries remain, of course. But with every year, new discoveries bring a better understanding of how our cosmos emerged, how it evolved and where it is going. Today we know that the stars are suns and that Earth is just one of billions of planets in our Milky Way galaxy. Scientists are racing to learn whether or not we are cosmically alone, looking for Earth-like planets and studying them for signs of life.
Life here on Earth can be traced back to events more than 4.6 billion years ago, when our sun was born as part of a larger brood in a cluster of stars. Some astronomers are on the verge of tracking down our long-lost starry siblings and reconstructing the solar system's deepest history. Meanwhile others are exploring the possibilities for life in the distant future, attempting to glimpse the universe hundreds of billions of years from now.
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On larger scales, the universe offers other surprises of great importance for us. Galaxies are held together by the invisible, gravitational glue of dark matter and are moving away from one another at an accelerating pace that is the result of a force known only as dark energy. A similar force may have been active a trillionth of a second after the big bang, when the universe's expansion briefly accelerated in a process called primordial inflation. If the properties of dark matter, dark energy or inflation had been slightly different, galaxies and stars could not have formed, precluding the possibility of life as we know it.
These discoveries are pushing science into unexplored frontiers. Many cosmologists believe that inflation led not only to our entire observable universe but also to an infinite “multiverse” of parallel universes. And a few maverick researchers are investigating the idea of a cyclic universe, a cosmos eternally oscillating between death and rebirth. Physicists are also studying whether information, rather than matter and energy, forms the fabric of spacetime.
The articles in this special edition of Scientific American delve into all of these mind-bending ideas and more. The night sky will never seem simple again.