NASA’s Roman Space Telescope could reveal dark energy’s deepest secrets — Scientific American

RESOLVING THE LEDGER

NASA’s Roman Space Telescope could reveal dark energy’s deepest secrets — Scientific American | AIPROPX

Can NASA's Roman Space Telescope solve the biggest problem in the universe?

The more scientists try to understand dark energy, the more its mysteries seem to grow. NASA's newest space telescope is custom-built to crack the case

Scientists know so little about the most dominant force in the cosmos that they refer to it with a catch-all placeholder name: dark energy .

This is the stubbornly mysterious something that's causing the universe's expansion to accelerate over time. Now, nearly three decades after dark energy's discovery, astronomers are letting themselves hope that a breakthrough is almost at hand.

That hope comes courtesy of NASA's Nancy Grace Roman Space Telescope, which the agency will launch as soon as August 30. When Roman becomes fully operational early next year, it will round out a trio of massive survey projects tackling the enigma of dark energy—which is just in time, as recent work has shown that dark energy may be more complicated than scientists have thought.

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"We're starting to see tensions between measurements of the early universe and measurements of the late universe," says Jason Rhodes, a cosmologist at NASA's Jet Propulsion Laboratory (JPL) in California, who works on the Roman team. "Some of us think these tensions may be indicating that we're ripe for another revolution in cosmology."

Dark energy's mark on the cosmos was discovered in 1998, when astronomers charting the motions and distances of faraway galaxies clocked that the universe wasn't just expanding but expanding faster and faster over time. This was a revolutionary finding, in no small part because no one had expected anything of the sort—yet, time after time, tests that could've dismissed dark energy as a mirage instead reinforced its reality. So cosmologists dutifully factored in dark energy in addition to cold dark matter—which they had already surmised outweighs the ordinary matter we can touch and see—in their appraisals of what constitutes the vast majority of the cosmos.

For years, in cursory follow-up studies of large-scale cosmic structures, dark energy looked to be equivalent to Lambda, the notoriously messy "cosmological constant" hastily enlisted from Albert Einstein's early work on his general theory of relativity. Thus was born the Lambda cold dark matter model (Lambda-CDM), the so-called standard model of cosmology, which attempts to boil the universe down to a fundamental recipe that faithfully replicates most everything astronomers can see.

That said, not everyone was fully satisfied. "Lambda is, like, the most boring dark energy model," says Kevork Abazajian, a cosmologist at the University of California, Irvine. "Nothing changes."

And things never quite added up perfectly for Lambda-CDM. The most persistent problem has been the infamous Hubble tension , by which the rate of cosmic expansion appears inexplicably different when calculated for the near universe versus the distant universe. In recent years additional discrepancies have arisen for theorists to swat at like a game of cosmic whack-a-mole. There's also a host of small hiccups in the universe's timeline, as reconstructed by James Webb Space Telescope observations, with supermassive black holes and complex organic molecules forming earlier in the universe's history than scientists thought possible . And then there's the sigma-eight tension , which notes that matter appears to be more evenly distributed on cosmic scales than the standard model predicts.

The newest snag comes from a pair of massive ground-based initiatives, the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI), each designed to map huge numbers of galaxies and allow scientists to better see the effects of dark energy on the universe. DES and DESI are largely independent of each other, but in recent years both have uncovered similar tiny-but-tantalizing hints that dark energy may be changing over time. "That's gotten everyone in a bit of a tizzy," says Tamara Davis, an astrophysicist at the University of Queensland in Australia, who is a member of both collaborations.

All in all, cosmologists are increasingly questioning the validity of their vaunted standard model, although no one can decide exactly what comes next. "It's not surprising that it's starting to break," says Alexie Leauthaud, a cosmologist at the University of California, Santa Cruz, and a member of the DESI collaboration. "And that's good news because we actually have some kind of shadow to chase."

But theorists have struggled to pull together a new approach that matches the extraordinary success of Lambda-CDM while also eliminating the discrepancies. "The type of cracks we're seeing don't look to me so much as fundamental shifts in Lambda-CDM as much [as] kind of random fractures showing up in weird places," says Adrienne Erickcek, a theoretical cosmologist at the University of North Carolina at Chapel Hill.

The most likely conclusion may be simply that the " dark sector " is more complicated than expected. Perhaps there are other "dark" enigmas waiting to be detected, or perhaps dark matter and dark energy indulge in secret shenanigans, either on their own or via interactions with each other. "The way we describe the dark things we know is very vanilla," says Adam Riess, a cosmologist at Johns Hopkins University, who shared the 2011 Nobel Prize in Physics for dark energy's discovery. "They have no sprinkles; they have no chocolate chips; they have no other textures or things going on; they're just really simple—and maybe that is not the case."

Although Roman has only been in active development for about a decade, the core concept for the mission dates back to just a few years after dark energy's detection. And w...