shadow aliens could be conducting their daily business, eating their shadow food, and basking in the unseen energy of their shadow stars. They would likewise be oblivious to our own type of matter—until, someday, pulsating gravitational signals link our disparate civilizations. Such an event would be one of the most outlandish resolutions of the dark-matter dilemma.

THE ESSENCE OF QUINTESSENCE

The discovery of cosmic acceleration has triggered a search for yet another type of missing substance—more precisely, an unknown source of energy. In many ways, dark energy is even more mysterious than dark matter. No material with which we’re familiar exhibits an antigravitational force. We have discussed the repulsive properties of hypothetical objects with negative mass—but imagine a force with enough muscle to push all the mass in the universe apart!

In H. G. Wells’s classic novella, The First Men in the Moon, he describes a substance called “Cavorite” that enables spacecraft to overcome gravity and effortlessly lift off from Earth. In the tale an inventor named Cavor “believed that he might be able to manufacture this possible substance opaque to gravitation out of a complicated alloy of metals and something new…. If one wanted to lift a weight, however enormous, one had only to get a sheet of this substance beneath it and one might lift it with a straw.”

Could quintessence represent a kind of Cavorite that is able to counteract universal gravitation and accelerate the universe? Is it possible that this substance dominates certain phases of the universe but not others? Could its influence even be growing in strength?

Many physicists don’t think of dark energy as a substance at all, in the traditional sense. Rather, they view it as a vacuum energy— the impact of the sea of virtual particles that pop in and out of the froth. From this perspective—rather than an independent, detectable quintessence—it is simply the lambda term, an essential quantum feature of space.



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