The universe is in a state of transformation. While not yet at its conclusion, one day all we know will fade away.

Everything we know—the cities, lakes, planets, solar systems, and the stars—are on a path to an ultimate finale.

What lies ahead? Some experts speculate that the universe’s expansion will eventually reverse, gathering everything tightly until it culminates in a big crunch, only to start anew in a big bounce. This idea, known as cyclic cosmology, has resurfaced, partly fueled by groundbreaking data from the Dark Energy Spectrograph (DESI)’s comprehensive 3D map of the universe.

Proponents of periodic cosmology often advocate for its aesthetic simplicity. If the universe follows this cycle, we may not need to grapple with what caused the Big Bang or what existed before it—these questions may have been resolved already. Scottish astronomer Katherine Heymans eloquently summarized during a recent lecture hosted by New Scientist: “The universe undergoes a big bang, expands, slows down, and gravity pulls it back, culminating in another big bang.”

Nobel Prize winner Adam Riess, who contributed significantly to the discovery of dark energy, highlights why many cosmologists favor this concept. He states, “This suggests we are not in a unique universe, implying that the periodic nature of the cosmos makes we, as existences, less coincidental.” However, this perspective may be seen as anthropocentric rather than purely physics-based.

For decades, periodic cosmology lost momentum, especially after Riess’s findings indicated that the universe is expanding at an accelerating rate. Should dark energy outweigh gravitational forces, the likelihood of the universe collapsing decreases. Heymans noted, “Current evidence points towards a desolate, cold demise for our universe,” referring to heat death, which is currently the prevalent theory concerning the universe’s fate.

This notion isn’t without challenges, particularly when exploring how energy, matter, and entropy behave between cosmic cycles.

The second law of thermodynamics complicates the scenario. It posits that disorder, or entropy, never declines in a closed system like the universe. While entropy rises overall as the universe expands, it would seemingly decrease if contraction occurs—an apparent contradiction lies therein. Although some theoretical work has aimed to circumvent this, the ultimate cycle still reverts to a Big Bang followed by heat death, albeit through a convoluted path.

Prominent theoretical physicist Roger Penrose introduced a model called conformal periodic cosmology to navigate these complexities. His theory posits that the universe remains seemingly ever-expanding until the end, where matter disintegrates entirely into photons. Here’s the novel aspect: the uniformity at the new cycle’s start mirrors the emptiness at the previous cycle’s conclusion, potentially allowing a new universe to emerge.

While intriguing, this paradigm remains hard to empirically test, though Penrose has suggested potential measurable evidence. However, skepticism persists in the cosmological community, yet its avoidance of the entropy quandary means it shouldn’t be disregarded outright.

DESI’s expansive cosmic map indicates that dark energy—a previously unstoppable force—may be losing strength. This suggests that while the universe’s expansion continues, its acceleration might be slowing down. As Heymans pointed out, this doesn’t imply a cosmic contraction but marks a significant shift in our understanding of dark energy.

The possibility that dark energy can weaken over the next ten billion years could usher in a new phase for periodic cosmology. “The transformation of dark energy may pave the way for a universe that can reverse its expansion one day,” noted Heymans.

Understanding the universe’s fate hinges on comprehending dark energy, which constitutes nearly 70% of the universe’s matter and energy. The nature of dark energy remains elusive, complicating efforts to theorize regarding the universe’s long-term trajectory. As Reese contended, “Extrapolating into the future without knowing more about dark energy renders predictions difficult.” While the cold death of the universe may seem the most probable outcome, the prospect of a big bounce-back is more conceivable than it has been in decades.