following decades. Among them are the most profound questions that human beings have ever posed about the cosmos. The fact that they are ripe now, or soon will be, further highlights how exciting the possibilities of this moment are. The 11 questions are these:

What Is Dark Matter?

Astronomers have shown that the objects in the universe, from galaxies a million times smaller than ours to the largest clusters of galaxies, are held together by a form of matter different from what we are made of and that gives off no light. This matter probably consists of one or more as-yet-undiscovered elementary particles, and aggregations of it produce the gravitational pull leading to the formation of galaxies and large-scale structures in the universe. At the same time these particles may be streaming through our Earth-bound laboratories.

What Is the Nature of Dark Energy?

Recent measurements indicate that the expansion of the universe is speeding up rather than slowing down. This discovery contradicts the fundamental idea that gravity is always attractive. It calls for the presence of a form of energy, dubbed “dark energy,” whose gravity is repulsive and whose nature determines the destiny of our universe.

How Did the Universe Begin?

There is evidence that during its earliest moments the universe underwent a tremendous burst of expansion, known as inflation, so that the largest objects in the universe had their origins in subatomic quantum fuzz. The underlying physical cause of this inflation is a mystery.

Did Einstein Have the Last Word on Gravity?

Black holes are ubiquitous in the universe, and their intense gravity can be explored. The effects of strong gravity in the early universe have observable consequences. Einstein’s theory should work as well in these situations as it does in the solar system. A complete theory of gravity should incorporate quantum effects—Einstein’s theory of gravity does not—or explain why they are not relevant.

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