What is dark energy?

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.”3 Regrettably, “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 Freeze If 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.11 Gregory 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 periods”—and run into Krauss and Starkman’s wall: “A finite system may be capable of infinite computation, but it can only store a finite number of memories.” Even the creators are bound by their own rules. But Benford gives his story the “wrong” ending. His universe finishes in a rip, with protons “popping crimson in the sky,” which a plain cosmological constant would not produce. For that, we need something more. The Big Rip Physicists characterize dark energy by a single number, w, the ratio of its pressure to its energy density. A cosmological constant has w exactly equal to −1. Current measurements put w near −1.03, with an uncertainty of about 0.03—so it’s consistent with Einstein’s constant, but not proof of it.13 And the number could, in principle, sit on the far side of −1. Dark energy with w less than −1, which Robert Caldwell named “phantom energy” in 2002, has a property nothing else in physics shares: its density increases as space expands.14 Caldwell, Marc Kamionkowski, and Nevin N. Weinberg worked out the consequences in a 2003 paper, “Phantom Energy and Cosmic Doomsday.”15 If w were −1.5, the repulsion would strengthen without limit and reach infinity in finite time—about 22 billion years from now. Sixty million years before the end, the stars of the Milky Way would fly apart from one another. Three months before, the planets would drift from the sun. Thirty minutes before, Earth would disintegrate. In the last 10⁻¹⁹ of a second, atoms would dissolve. Caldwell and his colleagues called it the Big Rip. Few cosmologists expect this. A field with w below −1 is unstable, but at this time theorists can’t exclude it based on the data.16 Stephen Baxter’s “Last Contact,” a 2008 Hugo nominee, is the Big Rip told from a garden in England.17 Baxter compresses the timeline mercilessly—his astronomers get seven months’ warning, not twenty billion years—but he follows the physics of the paper in order: superclusters first, then galaxies, then the sun, then the ground underfoot. The story belongs to Maureen, a widow tending her vegetables, and her daughter Caitlin, the astrophysicist who has to tell the world that “dark energy is pulling the universe apart.” Around them, the radio telescopes pick up a flood of signals from civilizations that have never spoken before. As the sky goes dark, Maureen understands why: “They were just saying good-bye.” It’s an unusual apocalypse story—no villain and no escape. Its only lesson is that people plant tomatoes anyway. The Big Ambiguity The Big Freeze and the Big Rip assume w remains constant. But what if this isn’t the case? If dark energy is a field rather than a constant—a scalar field slowly rolling down a potential, in the models physicists call quintessence—then its strength can change over cosmic time, and so can w.18 In 2021, the Dark Energy Spectroscopic Instrument (DESI) began mapping millions of galaxies to measure how the expansion rate has changed over eleven billion years.19 In March 2025, DESI reported that its map of more than 14 million galaxies and quasars fit a weakening dark energy better than a constant one. Combined with the cosmic microwave background, evolving dark energy was preferred over the standard model at 3.1 sigma; adding supernova data pushed the preference to between 2.8 and 4.2 sigma, depending on which supernova catalog was used—suggestive, but short of the 5-sigma standard for discovery.20 Then, this July, a separate DESI analysis using absorption lines from hydrogen gas along quasar sightlines came back in agreement with the plain cosmological constant, a result the collaboration acknowledged “could indicate that the current hints of evolving dark energy may fade away.”21 DESI has finished its planned five-year survey ahead of schedule, with 47 million galaxies and quasars; the full analysis is expected in 2027.22 Euclid, the European Space Agency’s dark-universe telescope, expects its first cosmology results the same year, and NASA’s Nancy Grace Roman Space Telescope launched on August 30 to join them.23 Within a few years, we should know with greater confidence whether w is changing. This is important because different models of quintessence predict different futures for the universe. In some models, dark energy doesn’t merely weaken; it turns negative, with expansion giving way to contraction. A 2022 analysis by Cosmin Andrei, Anna Ijjas, and Paul Steinhardt found that under current constraints, the acceleration could end in less time than has passed since the asteroid that killed the dinosaurs, with contraction following within roughly 100 million years.24 Cosmologically speaking, that’s just around the corner. Stories Waiting to Be Written Of the three possibilities discussed above, the Big Ambiguity alone seems to have no representation in science fiction. This makes it fertile ground for new stories: The Draw-Down. In a nod to Larry Niven’s “The Missing Mass,” astronomical surveys confirm dark energy is weakening, just not uniformly.25 It is weakest in one direction, toward a supervoid a billion light-years across. A cosmologist realizes something is consuming the vacuum at an accelerating rate. What will this mean for the universe, and whom, if anyone, should she tell? Turnaround. Steinhardt’s model was right. The expansion has already stopped—we just don’t know it yet. A far-future civilization, built on the expectation of a Big Freeze, detects blueshifted light from a distant galaxy, forcing them to reimagine their entire civilization. A society built on patience, and designed for indefinite time, now faces a deadline. 31 Candles. A graduate student on an overnight shift at Rubin Observatory detects a Type Ia supernova over six billion light-years away, and it’s dimmer than it should be—as if w has moved. This is the first evidence since DESI’s hints faded that dark energy isn’t constant after all. It will take thirty more supernovae and four years to be sure. If confirmed, it’s not the end of the world, it’s the beginning of not knowing when it ends. We may learn in 2027 whether Einstein’s constant is really constant. Or the answer may wait for Roman’s five-year survey, or for physics we haven’t imagined.26 No matter which way we find out, it is likely to change our understanding of the universe in significant ways. N.B. The claim that no published story yet centers on evolving dark energy is a conclusion from a survey of the Fraknoi index, the Encyclopedia of Science Fiction, and related searches. This is a public episode. If you would like to discuss this with other subscribers or get access to bonus episodes, visit www.thecosmiccodex.com