The Universe Keeps Rewriting CosmologyNEWS | 24 August 2025To astronomers in the 1990s, these three facts were self-evident: The universe is expanding; all the matter in the universe is gravitationally attracting all the other matter in the universe; therefore, the expansion of the universe is slowing.
Two scientific collaborations assigned themselves the task of determining the rate of that deceleration. Find that rate, they figured, and they would know nothing less than the fate of the universe. Is the expansion slowing just enough that it will eventually come to a halt? Or is it slowing so much that it will eventually stop, reverse itself and result in a kind of big bang boomerang?
The answer, which the two teams reached independently in 1998, was precisely the opposite of what they expected.
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The expansion of the universe isn’t slowing down. It’s speeding up.
Cosmology has often lent itself to unthinking assumptions that turned out to be exactly wrong. The ur-example is geocentrism. Over the couple of millennia before the invention of the telescope in the early 1600s, the occasional philosopher suggested Earth orbits the sun and not the other way around. But the vast majority of astronomers could simply look up and see for themselves. The sun orbits Earth. The evidence was, well, self-evident.
But then, most of the history of astronomy had relied on an unthinking assumption: The heavens would always be out of reach. Like the prisoners in Plato’s parable, we would forever be at the mercy of our perceptual limitations, trying to make sense of the motions in a two-dimensional celestial realm that was the cosmic equivalent of a cave wall. The invention of the telescope in the first decade of the 17th century overturned both those assumptions: Earth orbits the sun; the heavens are at our fingertips.
More telescopic discoveries followed that, to varying extents, contradicted one self-evident “fact” after another: mountains on the moon, moons around Jupiter, new stars, new planets. Some assumptions turned out to have been not just unthinking but unthinkable. How could anyone in the history of civilization ever have looked at Saturn and thought, “I’m assuming it doesn’t have rings”?
That the universe is expanding—the major premise leading to the 1990s search for the deceleration rate—was a revelation that nobody saw coming, including the two theorists who made the discovery not only conceivable but inevitable.
The first, Isaac Newton, would have had to make two counterintuitive leaps of logic to reach such a shocking conclusion. He would have needed to imagine that the universe was capable of doing what it self-evidently was not doing: collapsing. Then he would have needed to conceive of it as doing the opposite: getting bigger.
Albert Einstein, the second theorist who paved the way for the expansion discovery, did conceive of it. In November 1915 he presented the equations underlying his general theory of relativity; 15 months later he applied those equations to, as he phrased the topic in the paper’s title, “cosmological considerations.” According to his math, the universe should be volatile over time, either expanding or contracting. To avoid that unsettling implication, he introduced a variable, L, the Greek symbol for lambda, to balance his equation. The value of lambda would be whatever it needed to be to satisfy Einstein’s preference for a universe in perfect balance.
Each theorist’s “blunder,” as Einstein characterized his own refusal to trust his math, was understandable. Newton and Einstein, however intellectually exceptional, were still only human. The universe was static. If evidence to the contrary existed, it certainly wasn’t obvious.
And then it was. In the early 1920s American astronomer Edwin Hubble deployed the new 100-inch telescope atop Mount Wilson in California to observe some of the nebulous smudges at the farthest reaches of previous telescopes. Using Cepheid variables (stars that brighten and dim with clockwork regularity) as a measure of distance, he inferred that at least some of those nebulae were actually “island universes”—galaxies—beyond our own Milky Way. Next he used the redshifts of those galaxies to infer not only that the galaxies are moving away from us and from one another—itself a science-redefining discovery—but also their rate.
When Hubble plotted those distances against those velocities on an x/y graph, he found a direct correlation: the more distant the galaxies, the faster they were moving away from us. Thus, the universe must be expanding. Belgian astronomer Georges Lemaître independently reached the same conclusion, working not from his own data but from Einstein’s equations. Trace the expansion backward, he argued, and you would arrive at a “primeval atom.”
Evidence supporting the existence of such a “big bang” didn’t come until 1964, in the form of a background of microwave radiation that seems to pervade all of space. Theorists had predicted the existence of such a background as the relic of an explosive origin, although the two Bell Labs astronomers who first detected the radiation initially dismissed it as noise, possibly the result of pigeon droppings lining the giant horn of their radio antenna. Four physicists at nearby Princeton University, however, recognized that the observation matched the key prediction of the big bang theory.
Six years later American astronomer Allan Sandage cast cosmology as “the search for two numbers.” One number was the “rate of expansion” now. The other, however, harbored the unthinking assumption that would motivate two teams of researchers a quarter of a century later: “the deceleration in the expansion” over time.
Both teams trying to measure cosmic deceleration followed Hubble’s methodology of plotting velocity versus distance on a graph (using the magnitudes of a type of exploding star, or supernova, rather than Cepheid variables). Both collaborations expected to find the same direct correlation that Hubble did—at least at first. At some distance, though, they assumed that the line would depart from its 45-degree trajectory and dip, indicating that the apparent magnitudes of the supernovae were brighter, and therefore nearer, than they would be in a universe expanding at a constant rate.
And depart from its 45-degree trajectory the line did. Only it didn’t dip. It rose. The supernovae were dimmer, and thus farther away, than they would be in a universe expanding at a constant rate. The expansion of the universe, the rival teams concluded, isn’t slowing down. It’s somehow speeding up.
Dark energy—as cosmologists came to call whatever was causing the acceleration—soon became part of the standard cosmological model, along with dark matter and “regular” matter, the stuff of us. Observations of the same cosmic microwave background that, back in the 1960s, helped to validate the big bang interpretation of cosmology have revealed the universe’s ingredients. By studying the patterns in the radiation, scientists have refined the contributions to the mass-energy density of the universe to an exquisite level of precision: 4.9 percent of it must be ordinary matter, 26.8 percent dark matter, 68.3 percent dark energy. The model, cosmologists believe, is solid.
But not flawless. Not even complete. What is dark energy? What is dark matter? Indeed, even after all these years: What is the fate of the universe? Just this year the Dark Energy Spectroscopic Instrument in Arizona provided evidence that dark energy may have changed over the course of the evolution of the universe. Cosmologists have found the evidence compelling, though its meaning—let alone its implications for the standard model of cosmology—remains elusive.
So: Is cosmology on the precipice of another reversal? Another revolution? If history is any guide, the answer is: Maybe. For all today’s cosmologists know, they might be laboring under a seemingly unassailable, self-evident, yet incorrect assumption. Perhaps even an unthinking one.
It’s happened before.Author: Clara Moskowitz. Richard Panek. Source
