The Universe May Not Be Accelerating After All—And It Could Mean Dark Energy Never Existed

4 min read

Here’s what you’ll learn when you read this story:

• Cosmologists have long assumed that our expanding universe is isotropic, meaning it looks the same when viewed from any angle.
• Researchers found that there were errors in some of these previous theories when they examined observations of Type Ia supernovae.
• Because of these errors, the researchers determined that the universe is probably anisotropic, meaning that everything does not accelerate at the same rate from every direction.

The universe is supposedly expanding (and accelerating in that expansion) because of dark energy—an enigmatic component of the cosmos whose existence has long been theorized, but has never been directly detected. What dark energy actually is remains a mystery. Some see it as background energy in the vacuum of space, while others argue that it might be an energy field or take the form of wrinkles in spacetime.

But what if the nature and behavior of dark energy are far from anything we Earthlings could have possibly imagined? If so, the key to understanding the truth of dark energy may lie in Type Ia supernovae.

Type Ia supernovae have long been used to infer the existence and acceleration of dark energy. These blasts originate from binary star systems that include at at least one white dwarf—the dim, dense core left behind by a dying star. The white dwarf either devours material from its companion, eventually becoming massive enough to ignite a runaway thermonuclear explosion, or spirals towards another white dwarf until the two merge and detonate. The resulting supernovae reach a predictable brightness level that makes it relatively simple for experts to figure out their distances from us (thier redshifts, in turn, reveal how fast they're moving away from us). These calculations can then be used to measure the expansion of the universe.

But now, new questions have raised doubts about this picture of an expanding universe. Theoretical physicist Subir Sarkar of the University of Oxford has long been skeptical about whether the universe is really accelerating in its expansion, and if he’s right, it would turn the hypothesized nature of dark energy on its head. He’s now challenging existing theories after correcting for an element that he says has been overlooked.

The standardized brightness of Type Ia supernovae was thought to be independent of the age of the stars that produced them. But after poring through supernova data in the Pantheon+ catalog, Sarkar—along with Animesh Sah and Mohamed Rameez from the Tata Institute of Fundamental Research—came to an important insight. The applied a correction (developed by astronomers at Yonsei University) for the age of the progenitor stars, and concluded that after standardization, supernovae from younger progenitors are systematically fainter than those from older progenitors. And because more distant supernovae have younger progenitors, they appear faint.

This faintness makes them look extra far away, mimicking acceleration. As a result, Sarkar argues that the apparent acceleration itself was anisotropic—uneven depending on which direction you look.

“It has been suggested that the cosmic acceleration inferred from Type Ia supernovae could be illusory, due to our being ‘tilted’ observers embedded in a bulk [cosmic] flow,” Sarkar wrote in a study recently published in Monthly Notices of the Royal Astronomical Society. “The inferred acceleration should then be directed mainly along the local bulk flow,” and die out at a certain distance.

More telling was where the lopsidedness pointed. The strongest apparent acceleration lines up roughly with the direction of our own local motion through the cosmos, as marked by the hotspot in the cosmic microwave background, and it weakens the further out the team looked. That combination is hard to square with dark energy, as a cosmological constant would have to look the same in every direction and at every distance. A signal that has a preferred direction and dies out with distance looks instead like a side effect of sitting inside a moving patch of the universe.

This would imply that while the universe continues to expand, that expansion isn’t speeding up as it did at the beginning of the period of cosmic acceleration that began 5-6 billion years ago. Rather, it is actually slowing down. Acceleration, the team claims, is also not happening at the same rate at every angle. The asymmetry revealed in these findings implies the universe could actually be lopsided, which would also mean that the very nature of dark energy is not as scientists have long theorized.

It’s increasingly clear that we don’t live in the perfectly symmetric universe often assumed in standard cosmology. While Einstein’s theory of general relativity provided the mathematical framework, modeling cosmic expansion required the additional assumption that spacetime is isotropic and homogeneous on large scales. This assumption, formalized in the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, is the foundation of the standard Lambda-Cold Dark Matter (ΛCDM) model. Yet even this standard model faces challenges. Data from the Hubble Space Telescope and the Gaia satellite have created what’s known as the “Hubble tension”—a sharp discrepancy between the universal rate of expansion as derived from direct measurements of the local universe, and the rate as derived from early-universe observations.

“It is notable that correcting for [progenitor age] leaves [the local dipole] unchanged within uncertainties,” the authors wrote. “There is thus no evidence for isotropic accelerated expansion of the Universe.”