What in the universe is dark energy?

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What in the universe is dark energy?

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Banned Books Week Day of Action Suggestions:https://bannedbooksweek.org/wp-content/uploads/2023/10/ALA_DayofAction_OneSheet_Final.pdfDo you or someone you know need professional tech guidance for your author, coaching, or independent publishing business? If so, check out our sponsor Cloudessy, providing fractional Chief Technology Officer services. Simplify your tech stack, prevent launch failures, and build systems that actually support growth.https://marketing.cloudessy.com/30-minute-tech-stack-update/The expansion of the universe isn’t slowing down, but it should be.Every galaxy pulls on every other galaxy, so the expansion that began with the Big Bang ought to have been decelerating for thirteen billion years. In 1998, two rival teams of astronomers measured the deceleration using distant supernovae as standard candles1—and found the opposite. The supernovae were fainter than they should have been, which meant they were farther away than expected—so the expansion of the universe had been speeding up for the past five billion years or so.2 The discovery resulted in a Nobel Prize. It also introduced a name for the phenomenon: “dark energy.”3Regrettably, “dark energy” is a label rather than an explanation. Some form of energy with negative pressure, spread uniformly throughout space, is pushing everything apart. Taken together, the supernova data, cosmic microwave background, and clustering of galaxies put dark energy at roughly 68 percent of all mass-energy in the universe. Dark matter accounts for 27 percent. Ordinary matter—the stuff of stars, planets, nebulae, etc.—is about 5 percent.4 Of these, we understand dark energy, the most abundant component, the least. It’s one of the greatest unsolved mysteries of science.A Cosmological Constant?The oldest consideration of what we now call dark energy preceded its discovery by eighty years. In 1917, Einstein added a term to the equations of general relativity—the cosmological constant, Λ—to hold the universe static against its own gravity.5 He dropped the term in 1931, after Georges Lemaître (in 1927) and Edwin Hubble (in 1929) demonstrated that the universe is expanding.6 But it turns out an accelerating universe requires something like a cosmological constant—a fixed energy density built into space itself, which doesn’t attenuate as space expands.Quantum field theory tells us what Λ could be. Empty space isn’t empty. It seethes with virtual particles winking in and out of existence. Such activity carries energy, and vacuum energy behaves exactly like a cosmological constant.7 The trouble is how little we observe. The simplest estimates from particle physics predict a vacuum energy density 120 orders of magnitude greater than what astronomers measure.8 Either some unknown cancellation wipes out almost all of it, leaving the small amount we find, or vacuum energy is not the answer.So what could dark energy be? Physicists suggest some answers, each implying a different fate for the universe. It shouldn’t be surprising that science fiction writers have published stories addressing at least two of them.The Big FreezeIf Λ is the answer, the future is bleak. The universe never stops accelerating. Galaxies beyond our local group recede faster and faster until their light can no longer reach us, and the rest of the observable universe disappears into darkness. As Lawrence Krauss and Robert Scherrer have pointed out, astronomers a hundred billion years from now, living in the merged remnant of the Milky Way and Andromeda, will see a single island of stars in a void and no evidence the universe is expanding at all.9 Cosmology will vanish along with the distant galaxies.Eventually, life, too, must vanish. Freeman Dyson once argued that an intelligence could persist forever in an ever-expanding universe by thinking ever more slowly.10 Each thought costs energy, but the cost falls as the universe cools, and Dyson showed that a mind which cools along with it could stretch a finite reserve across infinite time—an endless series of ever-cheaper thoughts. Lawrence Krauss and Glenn Starkman showed in 2000 that an accelerating universe closes this loophole. It has a minimum temperature of its own, absurdly low but never zero, so the cooling stops and every thought has a minimum cost. In an accelerating universe, finite energy means a finite number of thoughts.11Gregory Benford’s “The Final Now,” published on Tor.com in 2010, is Krauss and Starkman’s paper rewritten as a creation myth.12 Two beings, He and She, have made a universe and now sit at its end conversing with the One—the merged remnants of all mortal minds that ever lived. Benford’s He explains: “The accelerating expansion of space-time, which was essential in the planning of all this, none the less yielded a more constricted long-term future.” Life has tried Dyson’s remedy—it has kept “ever-cooler,” thought slowly, and hibernated “for ever-longer ...

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