When the Big Bang ignited the cosmos some 13.8 billion years ago, it set in motion a chain of events of unimaginable scale and complexity. In the initial inferno, the lightest elements – hydrogen and helium – were forged. Much later, within the fiery hearts of stars and the cataclysmic explosions of supernovae, heavier elements, including the precious metals like gold, silver, platinum, and palladium, would be created and scattered. Over eons, this enriched cosmic dust and gas coalesced. Some of it formed vast, swirling masses that, under gravity’s patient hand, found stars whose orbits suited them well, beginning the slow transformation from mere collections of rock and ice into planets. As one such planet, circling a very particular star, began to develop, it eventually gave rise to life. That life took various forms, from water-based creatures to dinosaurs, and ultimately, it evolved into a bipedal and insatiably curious creature that called itself ‘human.’
There are legions of books dedicated to discovering how life evolved on this particular planet. Yet, the planet itself certainly doesn’t seem too emotionally attached to these fleeting inhabitants. It throws things like bacteria and infections at them, sometimes wipes them out with massive storms and horrible hurricanes. It starts its own fires and lets them burn wildly. The truth of the matter is that the planet itself is still evolving, even over billions of years since it first formed. From the planet’s perspective, humans are perhaps just a symptom of a thing called life.
As humans, we rarely look up from our own meager existence. Our lives flash before our eyes, come and gone, usually within a short 100 years – hardly a blip on the timeline of the universe. We focus on what makes us rich, or powerful, or comfortable, while actively encountering disease after disease until something finally takes us to the edge of life’s door and kicks us out, returning our energy to the universe from which we came.
Humans truly are insignificant when viewed from a cosmic level that observes whole galaxies, black holes, dark energy, dark matter, and everything in between. Are there forms of life on other planets? Perhaps, but if there are, they are as insignificant to the grand sweep of the universe as we are.
So, some of us choose to try and understand that cosmic perspective, away from human limitations, considering not only the beginning of the universe but how it continues to evolve. We learn to think in terms of billions of years that go by as if they were nothing. And as we look, as we measure and ponder, we begin to wonder: Where is all this stuff going? We can tell, finally, that it’s moving. Some use the word ‘expanding’ and in terms of how humans observe matter, ‘expanding’ is close enough to what is actually taking place above and around us.
Part II: The Standard Story and Its Cracks – Echoes from the Edge of Understanding
For much of the late 20th and early 21st centuries, humanity began to settle into a “standard model of cosmology.” This model, often referred to by scientists as Lambda-CDM (Lambda Cold Dark Matter), painted a picture of a universe born in a Big Bang, composed mostly of mysterious dark matter and even more enigmatic dark energy, and expanding at an ever-accelerating rate. This acceleration, a shocking discovery in 1998, was co-pioneered by a then 27-year-old Adam Riess, work that would earn him the Nobel Prize in Physics. His meticulous observations of distant, exploding stars (Type Ia supernovae) revealed a startling truth: the farther away a galaxy is, the faster it recedes from us. Something, the theorists proposed, was acting as a cosmic accelerator – a faint, repulsive force pervading all of empty space, which they dubbed “dark energy.”
This dark energy, in the standard model, is often represented by Einstein’s cosmological constant, Lambda (Λ) – an unchanging energy density inherent to space itself. As space expands, more of this energy appears, relentlessly pushing galaxies apart. This model became the dominant paradigm, explaining a vast array of cosmological observations, from the large-scale structure of galaxy clusters to the faint afterglow of the Big Bang, the Cosmic Microwave Background (CMB). It even offered a glimpse into the universe’s ultimate fate: a “heat death,” where endless acceleration would drive all galaxies out of sight, diluting matter and energy into a cold, featureless equilibrium. Adam Riess, through his groundbreaking work, had helped cement this grand, if somewhat bleak, cosmic narrative.
But science, at its best, is a restless endeavor. As The Atlantic recently chronicled, Adam Riess, now a seasoned cosmologist in his early 50s at Johns Hopkins University, is among a growing number of scientists who suspect this standard story, particularly its understanding of dark energy and the universe’s expansion, might be wrong, or at least critically incomplete. The first major “crack in the cosmic egg,” as Riess describes it, is a persistent and vexing discrepancy known as the “Hubble Tension.”
The Hubble constant (H₀) measures how fast the universe is expanding today. There are two primary ways to determine its value. One method, championed by Riess and his teams, involves building a “cosmic distance ladder.” They meticulously measure the distances to relatively nearby galaxies using pulsating stars called Cepheid variables, then use those galaxies to calibrate the true brightness of Type Ia supernovae. Because these supernovae are believed to explode with a consistent peak luminosity, their apparent brightness as seen from Earth reveals their distance. By comparing these distances to the speeds at which the host galaxies are receding (measured by their redshift), a value for H₀ can be derived. Riess’s painstaking work, leveraging the Hubble Space Telescope and now the James Webb Space Telescope, consistently yields a value around 73-74 kilometers per second per megaparsec (km/s/Mpc). Other teams, like Wendy Freedman’s at the University of Chicago, using different types of stars (Tip of the Red Giant Branch), also find values in this higher range, though sometimes with slightly smaller tension.
The other method looks back to the very early universe, specifically to the patterns imprinted in the Cosmic Microwave Background radiation. Satellites like Planck have mapped these ancient light patterns with incredible precision. By feeding this data into the standard Lambda-CDM model, which includes assumptions about dark energy, dark matter, and normal matter, cosmologists can predict what the expansion rate should be today. This method consistently yields a lower value for H₀, around 67-68 km/s/Mpc.
For years, many hoped this “Hubble Tension” would fade away with more precise data or the discovery of subtle systematic errors in one of the measurement techniques. But as Riess and others have refined their local universe measurements, the discrepancy has not only persisted but has grown in statistical significance, reaching a level that makes it difficult to dismiss as mere observational noise. “It smelled like something might be wrong with the standard model,” Riess told The Atlantic. The implications are enormous: if the standard model is indeed broken, our understanding of 96% of the universe (dark energy and dark matter) and its ultimate destiny could be upended.
Naturally, such a profound challenge has led to, as the article notes, “a bit of tension among cosmologists.” Some prominent figures, like David Spergel of the Simons Foundation, remain skeptical, suggesting it’s premature to discard a model that explains so much else so well. Sean Carroll, a cosmologist at Johns Hopkins, while praising Riess’s “heroic job” in reducing errors, also urges caution. Riess attributes some of this resistance to the “sociology” of the field, a tendency for the “old guard,” particularly those focused on the early universe, to be dismissive of conflicting local data.
Part III: New Clues from the Deep Sky – Is Dark Energy Running Out of Steam?
The Hubble Tension is not the only crack appearing in the standard model. Recent data from the Dark Energy Spectroscopic Instrument (DESI), a new observatory in Arizona, is providing even more intriguing, and potentially revolutionary, clues. DESI is meticulously mapping the positions and distances of tens of millions of galaxies to trace the history of cosmic expansion with unprecedented detail.
Its second major data release, based on three years of observations and unveiled around March 19, 2025, strengthened earlier, more tentative hints: dark energy may not be constant after all. The data suggests that dark energy was stronger in the early universe and that its power – its repulsive “kick” – has been subtly fading over the last several billion years. Adam Riess, who had an advanced look, was reportedly “delighted,” seeing it as another significant challenge to the simple Lambda-CDM model.
If this finding of weakening dark energy holds up to further scrutiny and confirmation from other experiments (like the upcoming Vera Rubin Observatory), the consequences, as Columbia cosmologist Colin Hill told The Atlantic, would necessitate a “wholesale revision” of our cosmological understanding. “The textbooks that I use in my class would need to be rewritten,” he stated.
The most immediate casualty would be the “heat death” scenario. If dark energy is indeed losing its punch, the universe’s expansion might not accelerate forever. If it weakens all the way to zero, expansion could simply stop, leaving the universe in a static, though still vast, configuration. In such a scenario, life, especially intelligent life, might have a much longer cosmic horizon than previously imagined. More dramatically, if dark energy continued to fade and somehow flipped its nature, becoming attractive rather than repulsive (a negative dark energy), it could lead to a “Big Crunch” – a recollapse of the universe into a hot, dense singularity, perhaps hinting at a cyclical pattern of cosmic birth, death, and rebirth. As The Atlantic eloquently puts it, “the deep future of the universe is wide open.”
Part IV: Grappling with the Invisible – Human Analogies for a Shifting Cosmos
These cosmic revelations, born from incredibly complex mathematics and mind-bending observations, can feel remote from everyday human experience. Yet, as we grapple with concepts like an expanding universe and the mysterious nature of dark energy, our intuition, honed by life on this planet, often seeks analogies to make sense of the immense and the invisible.
Consider the simple act of flying high above the American Midwest. From 30,000 feet, Kansas or Iowa unfolds as a vast, seemingly uniform tapestry of farmland, “acre after acre,” a view that, after a while, can appear almost monotonously homogeneous. This mirrors the cosmological principle: on the grandest scales, the universe and the dark energy driving its expansion appear statistically the same everywhere and in every direction. The Cosmic Microwave Background, that baby picture of the universe, is astonishingly smooth.
But, as anyone who has driven I-70 across Kansas for what feels like an eternity knows, the ground-level view is entirely different. Each farm is unique, each small town has its character, and the landscape undulates with subtle variations. This is the universe on smaller scales – a rich, “lumpy” tapestry of individual galaxies, clusters, and voids, each with its own unique history and composition, all shaped by the intricate dance of gravity acting on tiny primordial fluctuations. Dark energy, while orchestrating the grand expansion between these cosmic structures, doesn’t dictate their internal uniformity. The Milky Way is not Andromeda; our “local farm” has its own story.
This difference in perspective, the “fly-over” versus the “drive-through,” is akin to the challenge cosmologists face with the Hubble Tension. Measurements of the local universe, like meticulously charting individual supernovae (the “drive-through” landmarks), yield one expansion rate. Extrapolations from the early, smooth universe (the “fly-over” view), when filtered through our current standard model, yield another. The discrepancy forces us to ask: Is our understanding of the “terrain” flawed, or is the “vehicle” of our model itself not quite right for the journey?
One might even question the absolute smoothness of dark energy itself. Is it truly like a perfectly calm, high-altitude flight in a massive jet, where turbulence is but the slightest bump? Or, if we could somehow “fly a commuter plane” through the cosmic neighborhood, “experiencing it closer,” might we find that dark energy has more “wrinkles” and “formidable turbulence” than our current distant observations reveal? While standard models assume dark energy is incredibly smooth on large scales (its local effects within galaxies being swamped by gravity), the very existence of tensions and unexpected data like DESI’s hints at evolving dark energy keeps the door open for more complex behavior. Scientists are actively searching for such subtle variations, knowing that new physics often hides in these “bumps” in our understanding.
The idea of an evolving dark energy also resonates with our human experience of systems that change over time. Consider a child, full of boundless energy, running in new sneakers – a burst of rapid initial activity. As that child ages, or as we ourselves experience, that same “run” might be more measured, the initial explosive energy perhaps tempered. Could the universe, in its own cosmic timescale, be experiencing something similar? If dark energy is “losing its kick,” it’s as if the universe’s own “new sneakers” are showing their age, or the “runner” is encountering a different phase of its existence.
Imagine dark energy, or the spacetime it influences, as a vast expanse of stretchy spandex. As it expands, driven by this intrinsic property, perhaps it approaches a limit to its “stretchiness.” It might slow down, seem to stop, but then, as one thoughtful observer mused, still manage to expand at a significantly smaller rate as it “pulls out all the wrinkles/waves in the fabric.” This analogy, while metaphorical, captures the essence of a dynamic system potentially transitioning to a new state, perhaps one where expansion continues very slowly, smoothing out the largest-scale structures even further, or leading to that static configuration the article mentions.
Then there’s the ultimate question of “limits.” If this cosmic spandex is stretched to its absolute maximum, what happens if a “kitten” – some unforeseen perturbation, or just the cumulative stress – is introduced? Could it “tear” or “split”? This brings us to speculative scenarios like the “Big Rip,” where a runaway form of dark energy (phantom energy, whose repulsive force grows over time) could indeed tear apart spacetime itself. Thankfully, current hints from DESI point away from this, towards a weakening dark energy. But the question of ultimate limits, and what lies “beyond what we know,” remains.
When we ask what the expanding universe is “replacing” or expanding into, standard cosmology offers a counterintuitive answer: space itself is expanding. It’s not like a paper airplane flying through pre-existing air, pushing molecules aside. The “fabric” of spacetime between galaxies is stretching. If the universe is infinite, it simply becomes more infinite. There’s no “outside” for it to expand into. Dark energy is the property of this expanding space that causes the expansion to accelerate. Its “limit,” if one exists in the way DESI suggests, is about a change in its own intrinsic behavior over cosmic time, not about hitting an external wall.
Part V: The Human Element in Cosmic Discovery – A Field Reborn
What makes the current moment in cosmology so thrilling, as Adam Riess conveyed to The Atlantic, is precisely this ferment, this challenge to long-held certainties. Riess, a key architect of the standard model, now finds himself “gleeful” at the prospect of its potential unraveling, driven by his unwavering deference to new data. His decision to remain a “frontline investigator” rather than an administrator speaks to a deep-seated scientific curiosity.
His frustration with theorists sometimes being slow to grapple with new empirical results (“Sometimes, I feel like I am providing clues and killing time while we wait for the next Einstein to come along”) is a candid glimpse into the human dynamics of scientific progress. The “sociology of the field,” the resistance from some “graybeards,” the cautious skepticism of colleagues – these are all part of how science inches, and sometimes leaps, forward.
This human quest to understand resonates with a deeply felt obligation, as one observer put it, “to understand to our fullest capability as humans this ‘thing’ of which we are a part.” It is this drive that fuels the construction of ever more powerful instruments like DESI and the Vera Rubin Observatory, instruments designed to peer deeper into the cosmic mystery.
An Invitation to Wonder in a Universe Wide Open
The universe, it seems, is not done surprising us. The “cracks” Adam Riess sees opening in the standard model of cosmology, amplified by the intriguing data from DESI, suggest we may be on the cusp of a new understanding of dark energy, cosmic expansion, and the ultimate fate of everything. The simple narrative of an endlessly accelerating expansion into a cold, empty “heat death” may need to be rewritten.
Instead, the future might hold a universe that slows, perhaps comes to rest, or even recollapses in a “Big Crunch,” potentially leading to new cycles of creation. As The Atlantic concludes, and as the current scientific buzz affirms, “The field is hot again. A new universe suddenly seems possible.”
While the mathematical and observational details are complex, the core story is one of fundamental human endeavor: the relentless pursuit of knowledge, the courage to question established truths in the face of new evidence, and the profound sense of wonder that comes from contemplating our place in a cosmos whose deepest secrets are still unfolding. The universe is speaking, and a new generation of “Einsteins,” aided by remarkable new tools and the persistent curiosity of “thoughtful observers,” is listening intently. The final chapter of the cosmic story is far from written; indeed, we may just be realizing we’ve only read the preface.
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