Chapter 1: The Loneliness of Existence

In the poignant words of poet Ocean Vuong, eyes could be seen as the loneliest creations. They absorb the vibrant colors and forms of the world yet can never truly interact with them. Light from Earth’s flora and the vivid hues of art flood our vision, presenting beauty that remains forever out of reach. Each eye represents a fragment of our being, yet they exist alongside others, equally hungry for connection, yet isolated in their longing.

This mirrors our own experience on Earth as we strive to construct increasingly sophisticated technologies in the quest to determine our solitude in the cosmos. This endeavor is known as the Search for Extraterrestrial Intelligence. What if alien life does exist, but much like our isolated eyes, we cannot connect with them? They may attempt to reach out, but their signals could be destined to remain unanswered, lost beyond the cosmological horizon. This scenario offers a compelling perspective on the Fermi Paradox.

Chapter 2: The Nature of the Cosmological Horizon

The cosmological horizon serves as the boundary separating the known from the unknown. Within a vast expanse of 47 billion light-years lies everything we can observe with our telescopes, encompassing all we may ever comprehend. This enormous region hosts approximately 10²² stars, each illuminating a myriad of planets, including the life-sustaining Earth. Beyond this horizon, however, lies an infinite universe, potentially brimming with additional stars, as suggested by inflationary cosmology.

The existence of this horizon is dictated by the finite speed of light, which travels at an impressive 186,000 miles per second. Despite this speed, the farther an object is, the longer its light takes to reach us, meaning we see distant celestial bodies as they were in the past. For instance, gazing at a star ten light-years away reveals its appearance from a decade ago—a poignant glimpse into the past. This relationship between distance and time illustrates why our universe, thought to be only 13.8 billion years old, has a cosmological horizon that extends far beyond that age. As time progresses, the horizon continues to expand, allowing light from ever-distant objects to reach our instruments.

As our observational capabilities improve, however, the event horizon—the limit of our interaction—grows smaller.

Chapter 3: The Implications of Infinite Existence

If the universe is indeed infinite, then it stands to reason that alien lifeforms must inhabit some of these distant realms. If life arose in our corner of the universe, it’s logical to assume it could develop repeatedly across an infinite cosmos.

The event horizon of the universe mirrors that of a black hole. Beyond this threshold, we cannot communicate with or influence any celestial object. While a galaxy may fall within our cosmological horizon, it could exist beyond our event horizon due to the universe’s expansion, which propels it beyond our communicative reach. This expansion is driven by the mysterious phenomenon known as dark energy—an enigmatic force that continues to elude our understanding.

Ultimately, this means we can observe worlds we will never access. As the universe continues to expand over billions of years, distant worlds will fade from our sight. Eventually, the observable universe may shrink to just a few local galaxies, leaving behind a vast emptiness in space. In this context, we find ourselves in a moment of abundance.

Chapter 4: The Fermi Paradox and Its Mysteries

Astrophysicist Tomonori Totani has utilized the concept of the cosmological horizon to address the Fermi Paradox—pondering why we have yet to encounter signs of extraterrestrial life. The notion of abiogenesis—the emergence of life from non-living matter—is a challenging concept to accept. While it seems remarkable that all living organisms stem from non-living chemicals, the probability of such occurrences in our observable universe may be low. However, in a much larger universe with countless stars, the likelihood increases significantly.

Abiogenesis could be frequent enough to yield life beyond our cosmological horizon but rare enough that we remain the sole example within our limited region. Our sample size is simply too small to discover additional life. This suggests an intriguing duality to our existence: we may be insignificant in the grand scheme of infinite possibilities, yet we are also a unique instance of life in our observable universe.

As we focus our efforts on detecting signals from stars, it is equally plausible to search other galaxies for alien communications. A single galaxy might host an estimated 10¹? civilizations. The potential of a Fast Radio Burst (illustrated here) could represent a transmission sent to Earth by extraterrestrial entities.

Yet, it’s also possible that the emergence of life follows a specific, yet-to-be-discovered pattern. If this is the case, we may still encounter alien life forms within our observable universe. This reasoning is complex; the likelihood of life evolving must be higher than we currently believe, but not so high that it negates the original paradox.

Ultimately, we must ponder the most challenging scenario to accept: Is it preferable to be the only instance of life in the entire cosmos, or would we rather know that we are part of a larger community? Somewhere out there exist civilizations with architectures we cannot fathom and beings whose languages would sound like incantations to us. Yet, the reality remains that we cannot connect or communicate with them. We can only observe, aware of the mysteries that lie beyond our reach, resigned to the notion that some truths may forever remain hidden.

Neil deGrasse Tyson discusses the limits of our understanding of the cosmos and what lies beyond our reach.

Dr. Paul Sutter explores the boundaries of our cosmic understanding and the implications for extraterrestrial life.