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  • Time is flying by on this busy, crowded planet as life changes and evolves from second to

  • second. At the same time, the arc of the human lifespan is getting longer: 67 years is the

  • global average, up from just 20 years in the Stone Age.

  • Modern science provides a humbling perspective. Our lives, indeed even that of the human species,

  • are just a blip compared to the Earth, at 4.5 billion years and counting, and the universe,

  • at 13.7 billion years.

  • It now appears the entire cosmos is living on borrowed time. It may be a blip within

  • a much grander sweep of time. When, we now ask, will time end?

  • Our lives are governed by cycles of waking and sleeping, the seasons, birth and death.

  • Understanding time in cyclical terms connects us to the natural world, but it does not answer

  • the questions of science.

  • What explains Earth’s past, its geological eras and its ancient creatures? And where

  • did our world come from? How and when will it end? In the revolutions spawned by Copernicus

  • and Darwin, we began to see time as an arrow, in a universe that’s always changing.

  • The 19th century physicist, Ludwig Boltzmann, found a law he believed governed the flight

  • of Time’s arrow. Entropy, based on the 2nd law of thermodynamics, holds that states of

  • disorder tend to increase.

  • From neat, orderly starting points, the elements, living things, the earth, the sun, the galaxy.

  • are all headed eventually to states of high entropy or disorder. Nature fights this inevitable

  • disintegration by constantly reassembling matter and energy into lower states of entropy

  • in cycles of death and rebirth.

  • Will entropy someday win the battle and put the breaks on time’s arrow? Or will time,

  • stubbornly, keep moving forward?

  • We are observers, and pawns, in this cosmic conflict. We seek mastery of time’s workings,

  • even as the clock ticks down to our own certain end. Our windows into the nature of time are

  • the mechanisms we use to chart and measure a changing universe, from the mechanical clocks

  • of old, to the decay of radioactive elements, or telescopes that measure the speed of distant

  • objects.

  • Our lives move in sync with the 24-hour day, the time it takes the Earth to rotate once.

  • Well, it’s actually 23 hours, 56 minutes and 4.1 seconds if youre judging by the

  • stars, not the sun. Earth got its spin at the time of its birth, from the bombardment

  • of rocks and dust that formed it. But it’s gradually losing it to drag from the moon’s

  • gravity.

  • That’s why, in the time of the dinosaurs, a year was 370 days, and why we have to add

  • a leap second to our clocks about every 18 months. In a few hundred million years, well

  • gain a whole hour.

  • The day-night cycle is so reliable that it has come to regulate our internal chemistry.

  • The fading rays of the sun, picked up by our retinas, set our so-calledcircadian rhythms

  • in motion. That’s when our brains begin to secrete melatonin, a hormone that tells

  • our bodies to get ready for sleep.

  • Finally, in the light of morning, the flow of melatonin stops. Our blood pressure spikes

  • body temperature and heart rate rise as we move out into the world. Our days, and our

  • lives, are short in cosmic terms. But with our minds, we have learned to follow time’s

  • trail out to longer and longer intervals.

  • We know from precise measurements that the Earth goes around the sun every 365.256366

  • days. Much of the solar energy that hits our planet is reflected back to space or absorbed

  • by dust and clouds. The rest sets our planet in motion.

  • You can see it in the ebb and flow of heat in the tropical oceans, the annual melting

  • and refreezing of ice at the poles, or seasonal cycles of chlorophyll production in plants

  • on land and at sea. These cycles are embedded in still longer Earth cycles. Ocean currents,

  • for example, are thought to make complete cycles ranging from four to around sixteen

  • centuries.

  • Moving out in time, as the Earth rotates on its axis, it makes a series of interlocking

  • wobbles called Milankovich cycles. They have been blamed for the onset of ice ages about

  • every one hundred thousand years. Then there’s the carbon cycle. Plants capture it from the

  • air or the sea. It finds its way into soils or ocean sediments as plants decay, or as

  • waste passes out of the food chain.

  • It can take a volcanic explosion, or a dramatic lowering of sea levels, to release this carbon

  • back into the air, often after millions of years. The processes that shape a planet like

  • ours play only the smallest of roles in the evolution of the universe.

  • So to glimpse time’s broader arcs, we must look to cycles that govern the larger cosmos.

  • The reigning theory is that the universe began in a sudden expansion of space, the big bang.

  • With entropy uniformly low, this was the time of the tiny, subatomic particles like quarks

  • and leptons stirred into a hot soup.

  • Within microseconds, they combined into atoms, setting in motion the primordial era. The

  • universe cooled as it ballooned, growing dim and falling into what’s known as the cosmic

  • dark ages. All the while, though, gravity pulled particles together, fighting the expansion.

  • After several hundred million years, larger clumps of matter had drawn together. These

  • isolated pockets of gas became dense enough to heat up and ignite. So began the era of

  • stars.

  • In this glorious age, the universe seeded the rich cosmic landscapes we see in our telescopes.

  • Trillions upon trillions of stars lit up galaxies all across the cosmos. The arc of this era

  • is defined by the life cycles of stars, which vary according to their sizes.

  • Stars shine because gravity crushes matter into their cores. The energy released pushes

  • outward and balances the inward force of gravity. This battle between energy and gravity is

  • raging in stars all around the universe. But in large stars, about ten million years after

  • their birth, gravity begins to gain the edge and tips the balance.

  • When the mass concentrating in the core of the star reaches a critical threshold, the

  • core collapses in on itself. The energy released in the collapse causes the star to explode

  • in a blast of light and debris that’s visible across the cosmos.

  • In the wake of this supernova, shock waves can cause nearby clouds of dust and gas to

  • collapse and ignite, to form generations of smaller stars like our Sun. A byproduct of

  • star formation, solar systems form in the collapse of the surrounding solar nebula.

  • The life cycle of planets, especially those in close, is tied to that of their parent

  • stars.

  • As stars like our sun age, they grow hotter and more luminous. Billions of years from

  • now, that will spell the beginning of the end for our home planet. As raging solar winds

  • begin to blast away at our atmosphere, surface water will gradually disappear, rendering

  • Earth uninhabitable.

  • Finally, the sun will begin to swell, growing so large that it actually envelops the Earth.

  • Friction with the Sun’s outer edges will cause this once blue world to gradually spiral

  • inward. Unless they are large enough to go supernova, most stars end their lives in more

  • of a whimper than a bang, as shown in this gallery of dying stars captured by the Hubble

  • Space Telescope.

  • In time, solar winds push their outer layers so far out that they blossom in spectacular

  • displays. That’s just what happened about 12,000 years ago to the star that spawned

  • the famed Helix Nebula. A vast glowing ring is the dying star’s outer layers. On the

  • inside, spokes of denser gas stubbornly resist the star’s relentless winds.

  • The star itself is now a dim, cooling remnant called a white dwarf. It’s the size of Earth,

  • but about two hundred thousand times more dense. This is likely what’s in store for

  • our sun. A distant civilization may scan it for planets, but by then they won’t see

  • Earth.

  • This battle between energy and gravity repeats in every corner of a galaxy like ours, with

  • gravity drawing gas clouds into stars, and stars burning themselves out on a variety

  • of time scales, depending on their size.

  • In time though, as the mass of the galaxy collects in successive generations of small

  • stars, it will grow dimmer and dimmer. Some galaxies will see a temporary rebirth, if

  • their mass gets stirred up and combined with another.

  • That’s what’s destined to happen to our Milky Way. At just about the time our sun

  • begins to swallow our planet, any remaining Earthlings will see the stars of the Andromeda

  • galaxy looming above the plane of our Milky Way.

  • As shown in this simulation, the two are likely to tear each other apart. If it’s a direct

  • hit, the stars in both galaxies will gradually join together in a gigantic galactic puffball

  • known as an elliptical galaxy. All the turbulence of the merger could stimulate a wave of new

  • stars being born, reinvigorating the new larger galaxy.

  • Dust-ups like this, in which galactic neighbors merge, will be common as the era of stars

  • moves into its later stages. But a wholesale thinning out of the universe is inevitable.

  • On a grand scale, recent studies of the cosmic expansion rate show that the universe as a

  • whole is in no danger of succumbing to gravity, or of ending in a Big Crunch.

  • In fact, over the last 6 billion years, the universe has begun to accelerate outward.

  • Gravity is losing its grip to an unseen force called dark energy. You can see evidence of

  • this now, out in the huge voids of space between filaments of galaxies. These voids are like

  • ever-expanding bubbles. Where the bubble walls touch you can see filaments of galaxies.

  • As the bubbles grow, the filaments will stretch and break. The distance between galaxies will

  • widen at a faster and faster pace. Eventually, no matter where you are in the universe, you

  • will see only a few isolated clusters of galaxies huddled together, with little connection to

  • anything else, and few clues to how they got there.

  • At more distant reaches of time, tens of billions of years from now, the sky will grow darker

  • and darker as everything recedes away from everything else. A good place to be, in those

  • long twilight years of the stellar era, is a place where gravity and energy have forged

  • an extended truce.

  • Perhaps a place like this: not much larger than our planet Jupiter, a Red Dwarf is one

  • of the smallest and dimmest stars in our universe. They have been shown to harbor planets close

  • enough that their dim rays can sustain liquid water, and life.

  • Brown dwarfs and red dwarfs form the vast majority of stars in our galaxy. In fact,

  • combined, their mass exceeds that of all the large stars. Because they burn so slowly,

  • theyll be the final beacons of the majestic age of stars, an era that will extend out

  • to one hundred trillion years.

  • Even as their host galaxies grow dim, another process will begin to transform these small

  • outposts. Over time, chance encounters between objects will perturb their orbits, sending

  • some toward the center of the galaxy, and others out into the void.

  • In this way, galaxies may gradually evaporate, with ever-denser concentrations of matter

  • accumulating in their cores. As that happens, the universe begins to take on a new character.

  • Welcome to the degenerate era, in which the universe is populated by red and white dwarf

  • stars, steadily cooling, and by the charred remains of supernova explosions: neutron stars.

  • Even though these dead stars have used up their nuclear fuels, they continue to produce

  • small amounts of energy. They scoop up and annihilate dark matter particles that manage

  • to stray into their grasp. Here is where cosmic evolution slows to a crawl. It’s expected

  • that protons, the building blocks of all atoms, will slowly degrade, turning into sub-atomic

  • particles that then decay into photons.

  • All the protons in existence date back to the early moments of the universe. Their eventual

  • decay will mark the end of the degenerate era, around a billion, billion, billion, billion

  • years after the big bang. That’s a one followed by 40 zeros.

  • Our picture of what happens after that depends on what we learn in the coming years beneath

  • the border of France and Switzerland, in one of the largest physics experiments ever undertaken.

  • 100 meters underground, the Large Hadron Collider was built to accelerate particles in opposite

  • directions through a giant ring 27 kilometers around. When they reach nearly the speed of

  • light, scientists will bring them into ferocious collisions.

  • One goal: to define the final time horizons of our universe, as well as the final moments

  • of its most persistent <