50 Greatest Mysteries in the Universe

We live in a golden age of astronomy. Consider the history of modern humans: We struggled for 1,000 centuries to secure food and shelter. We spent another 50 centuries forming early civilizations and building cities. And in just the last century alone, technology has helped us unravel the mysteries of the cosmos in ways our ancestors could not have imagined. That's pretty humbling. Despite the fact that 5,000 generations of modern humans have pondered the heavens above, we’re really just beginning to comprehend the story of the universe — and it’s mind-boggling. Drawing on Astronomy magazine's long heritage as the leading publication in its field, 50 Greatest Mysteries of the Universe explores the cosmos’ biggest questions. We published the first iteration of this special issue just over a decade ago,…

Over the past century, astronomers have deduced several ways to estimate the age of the universe. Yet at the dawn of this century, the universe’s age remained far from certain. Fortunately, the launch of NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) in 2001 and the European Space Agency’s Planck satellite in 2009 changed all that. Still, astronomers’ attempts during the previous century to narrow the age estimates makes for a fascinating detective story. Before WMAP and Planck, the best approach for determining the universe’s age relied on the much-debated Hubble constant, a figure that describes the rate at which the universe is expanding. To find the Hubble constant, astronomers observe distant galaxies and measure their distances (by using Cepheid variable stars or other objects of known intrinsic brightness) as well as…

Two great debates have taken center stage in the search to answer this age-old question. In April 1920, Harlow Shapley and Heber Curtis argued over the scale of the universe in the great auditorium of the Smithsonian Institution’s Natural History Museum in Washington, D.C. In this discussion, which preceded Edwin Hubble’s discovery of the nature of galaxies by just a few years, Curtis argued that the cosmos consists of many separate “island universes,” claiming that the so-called spiral nebulae were distant systems of stars outside our Milky Way. Meanwhile, Shapley argued that spiral nebulae were merely gas clouds in the Milky Way. Shapley further placed the Sun toward the edge of our galaxy — which, in his view, was the entire universe — whereas Curtis believed the Sun to be…

Virtually all astronomers and cosmologists agree the universe began with a “big bang” — a tremendously powerful genesis of space-time that sent matter and energy reeling outward. The evidence is clear, ranging from the underpinnings of Albert Einstein’s general theory of relativity, to the detection of the cosmic microwave background by Arno Penzias and Robert Wilson in the 1960s, to the confirmation of ripples in the fabric of ancient space-time from the Cosmic Background Explorer (COBE) satellite in 1992. But the devil is in the details, and that’s where figuring out how Big Bang cosmology really works gets interesting. The Big Bang model is typically broken down into a few key eras and events. Standard cosmology, the set of ideas that are most reliable in helping decipher the universe’s history,…

Astronomers might be more confident about their picture of the universe were it not for dark matter. Observations show that the universe is populated with some unseen form of matter — and plenty of it. Astronomers attempt to “weigh”the universe in a variety of ways. They observe the effects of dark matter on astronomical objects that vary from small to large. Dark matter was posited by Dutch astronomer Jan Oort in the 1930 when he studied star motions in the Sun’s neighborhood. Because the galaxy was not flying apart, he reasoned, enough matter must reside in the disk to keep the stars from moving away from the galaxy’s center. Oort postulated that in the Sun’s neighborhood, three times as much dark matter existed as bright matter. Stronger evidence came later…

While observational tests on the details of cosmology proceed apace, astronomers are focusing on the mechanics of how matter came together in the early universe. The fundamental question is: Did galaxies, stars, or black holes come first? The infant universe was a relatively uniform sea of several-thousand-degree gas and dark matter — the unseen, mysterious, and predominant form of matter that is indirectly known to exist because of its gravitational influence. But how galaxies, stars, and black holes came together is the key to understanding the puzzle of the early universe. Based on cosmic microwave background data, astronomers think matter coalesced when the universe cooled and became “transparent” 380,000 years after the Big Bang. And according to recent studies, structures like stars and galaxies formed as early as 200 million…