The Deadliest Thing in the Universe

13.8 billion years; that's how long the universe has existed. Older than the planets, stars, as old as time itself. The universe is measurably vast. To put it into perspective, if we reduce that time scale down to a single year, the entirety of recorded human history— you, me, and everyone else who has ever lived— would take up a minute at best.

Our lives, the long decades of joy and sorrow, pride and regret, every dreary morning and sunny afternoon, would represent around one-fifth of a second. This cosmic calendar, first popularized by Carl Sagan in the 1970s, is meant to show us just how precarious our position is in the universe. We have not been here long, and if the last few months of life on this cosmic calendar are any indication, we won't be here much longer.

Over the past four weeks, our planet has been wiped clean of life nearly six times over. Of all the billions of species to ever exist, more than 99 percent have gone extinct. It would probably be a little arrogant to assume that we, who came along only a few minutes ago, are somehow different from all those who came before us; that our species could and should be the first to endure past our time.

But the fact that we're even talking about this at all suggests that maybe we are different. We built telescopes, searched the stars, and pieced together the story of our universe. We engineered tools, dug pits, and unearthed the stories of our ancestors. And in all that searching, we found no sign that there’s ever been anyone or anything like us.

The fact is, humans are adaptable. Most humans are uniquely suited to the environments they evolved in, and if their habitat changes, they don't change with it; they go extinct. Humans are different. When it gets cold, we don't freeze— we knit ourselves sweaters and light a fire. When it rains, we aren't swept away by the flood; we build barriers and divert rivers. The list goes on. Forests burn, and we douse the flames with water. Earthquakes topple cities, and we rebuild them stronger.

We've gotten so good at adapting to our surroundings that at this point, most natural disasters don't pose much of a threat to our species as a whole. But what couldn't we withstand? What if it happened tomorrow?

These could end the human story instantly and infinitely. These are the biggest threats our species faces; the threats which seek to make us history, like the 99 who came before us. These are the most dangerous things in the universe. There are many theories as to how the universe might end, and all of them posit that we won't be around to see it happen.

One theory suggests that the universe, which once began by expanding ever outward from a single point, will continue to do so for all time. The evidence for this is clear: every day, our own Milky Way galaxy grows more and more distant from the neighboring galaxies as the space between them expands. So, it makes sense to conclude that eventually, they'll disappear from our view altogether, leaving us alone in the dark.

We can live like this for billions and billions of years. The galaxies, though spread apart, will hold together. But eventually, the stellar nurseries, which add new stars to our galaxy, will run dry. The remaining stars will age and burn into nothingness, leaving only black holes to suck up everything that remains.

A hyper-advanced civilization could cobble together distant stellar remnants to create new stars, but in time, these too will run out. There will be no material left for us to use, no energy to sustain us, and eventually, the black holes will decay too. This is known as the big freeze, and it is one threat we cannot escape.

While the galaxies are, on the whole, growing apart, there are a few exceptions. Two and a half million kilometers, or about one and a half million miles, from us lies the Andromeda galaxy, and it's on a collision course with the Milky Way. In four and a half billion years, this collision will tear both galaxies apart. Surprisingly, this in itself won't hurt us.

On the cosmic scale, the stars and planets of our two galaxies are actually so distant from one another that no star will directly collide with another. Black holes, however, are a different story. They can grow far, far larger than any star in almost every spiral galaxy, including Andromeda. The Milky Way holds one at its core. If our sun comes too close to either one of these supermassive black holes, we'll be consumed or, at best, ejected into interstellar space.

While we already know about the eventual death and rebirth of our galaxy, there's one smaller, more local danger that could threaten our solar system in the same way: the appearance of a rogue black hole. Black holes form when a massive star runs out of fuel at the end of its life cycle. Unable to burn as hot as it once could, the star can no longer support its own weight, so it collapses inward until the gravity is so great it warps the fabric of space-time itself.

While black holes normally stay in the same place as their original star, violent stellar events like the merging of two separate black holes can send them hurtling out into interstellar space at incredible speeds. It's not easy to spot objects drifting in the lightless void between stars, and an object that absorbs light itself is even more impossible to spot.

If one wandered close enough to our sun, its massive gravity would disrupt the orbits of every object in the solar system, slowly but surely throwing the planets into disarray. As it consumes the planets and asteroids in the outer solar system, it’ll only grow larger until finally, Earth gets caught in the path of the singularity. After stripping the planet of its atmosphere and us along with it, the black hole's gravity would create tidal waves as large as Mount Everest, stretching the Earth's crust like a ball of dough until finally, the tectonic plates rip apart, exposing our planet's mantle and core.

As we pass the event horizon, everything that was once our home will be crushed into rubble, then dust, then atoms. In this scenario, the best strategy would be to move ourselves. There are millions of potentially habitable worlds in our galaxy alone, and by establishing settlements there, we give humanity a backup plan, saving us from being done in by any one galactic calamity.

This is the same strategy we’d use to avoid extinction through the expansion, contraction, and eventual death of our own sun, which will inevitably cook and consume the Earth just as readily as a black hole would. The hardest threats for us to avoid are those that could happen at any time, with very little warning: gamma-ray bursts and asteroid impacts.

While neither of them will destroy life on Earth forever, it may very well destroy us completely. We know this because they’ve wreaked havoc before, causing two of the past five mass extinction events in our planet's history. Gamma-ray bursts are massive blasts of electromagnetic radiation emitted by collapsing stars. They emit so much energy that for the brief moment of their existence, they're brighter than every other star in the universe.

If the Earth happened to be squarely in the path of one of these narrow bursts, the effects would be immediate and terrible. Seventy-five percent of the upper atmosphere, specifically the ozone layer, would be destroyed in an instant. Without ozone, all life on Earth would be exposed to the harsh ultraviolet radiation of the sun, causing DNA damage and cancers. Long-term effects are even more devastating.

For several years, acid rain would fall, and photochemical smog would darken the skies, reflecting more of the sun's rays. If this were to happen today, given how unstable our climate currently is, the darkness might trigger a runaway cooling effect, leading to a decade-long winter. These same effects were seen during the Cretaceous-Paleogene extinction, or the death of the dinosaurs, approximately 66 million years ago.

A 10-kilometer, or 6-mile-long asteroid struck the surface of our planet, carving a near 180-kilometer, or 112-mile-long crater in present-day Mexico. The impact instantly vaporized the water beneath it and scattered hot debris across the planet, setting the world aflame. Soot from the blast turned to aerosol and blackened the skies, causing a very real decades-long winter that slowly killed off all the life on the surface.

While humans today might survive such an impact by hiding away in a remote underground bunker, the vast majority would be left to die. If an even larger asteroid struck, say 30 to 60 kilometers, it would completely eradicate all life on Earth, no matter the hiding place. Essentially, if an asteroid or gamma-ray burst aimed itself at Earth, there would be very little we could do about it.

As gamma-ray bursts are impossible to predict, our only immediate solution would be to deal with the effects as best as we can. Unlike gamma-ray bursts, which occur in our local group of galaxies only once or twice every 5,000 million years, rogue asteroids are a real and present threat and the only one that humans have so far taken steps to counter. Since the 1990s, NASA has identified over 25,000 near-Earth asteroids.

While many smaller ones may be out there, they've located more than 90% of the asteroids large enough to devastate the Earth. While no large asteroids are currently on a collision course with our planet, NASA scientists have proposed and even tested a solution to counter them. In September 2022, NASA deliberately flew the Double Asteroid Redirection Test satellite, or DART, into the asteroid Dimorphus, measurably changing its orbit.

Should a world-ending asteroid come barreling towards the Earth, we'll simply nudge it out of the way long before it ever reaches us. Well, the dangers we've discussed so far are possible, inevitable, and devastating. They only come along every 100 million years or so, if that. The odds that one will happen in the foreseeable future are slim to none.

In that sense, the most dangerous thing in the universe isn't simply that which could cause the most destruction, but the one that's most likely to. For that reason, we are the most dangerous thing in the universe, at least to ourselves. If we are to die, the odds are overwhelmingly clear that it will be at our own hands.

There will be no death from above, no gamma-ray burst or rogue black hole. It'll be because we chose to die, either through the negligence of human-caused climate change or the short-sighted malice of nuclear war. We invented the machines that warmed our planet, causing droughts, floods, and massive storms. We invented the bombs that can annihilate millions in an instant, and each day we choose to keep them armed and aimed squarely at our own heads.

At this moment, there's over 12,000 active nuclear warheads on our planet. A full nuclear exchange will instantly wipe out entire cities, and in the aftermath, radioactive fallout will eventually kill billions. The skies will darken, plunging our planet into a nuclear winter, slowly starving everyone else unfortunate enough to survive, just as it happened 66 million years ago and 400 million years before that.

The Bulletin of Atomic Scientists keeps a symbolic clock that measures humanity's proximity to doomsday. In 1947, when the clock was first unveiled, an expert panel of scientists and academics judged that we were seven minutes to midnight to our own extinction. When the Soviet Union fell, ending the Cold War, they turned the clock back to 17 minutes.

But now, as we fail to deal with climate change, as nuclear arms treaties fall apart, and as Russia once again threatens war with the West, we're 100 seconds from midnight. At this moment, we have two paths forward: we can listen to our collective inner voices of reason and survive our own present together, or we can all join those who went before us. Only this time, our extinction would be by our own hands.