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 you’re 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, we’ll

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-called “circadian 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,

they’ll 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 objects. Black holes, ranging from million to tens of billions of

times the mass of our sun, occupy the centers of large galaxies today. As those galaxies

age, over trillions of years of time, much of their mass will spiral towards the center

and into the jaws of ever more ravenous black holes.

Conceivably, these black holes could end up weighing as much as a galaxy. But when they

finally stop growing, will they too be subject to the ravages of time? According to the physicist

Stephen Hawking, the answer is yes.

He proposed a theoretical process of decay that scientists are hoping to test in high-energy

particle collisions at the Large Hadron Collider. The idea is that, throughout our universe,

particles of opposite charge constantly well up in the vacuum of space. They normally destroy

each other.

But when this happens at the event horizon of a black hole, one particle can be pulled

in while the other escapes. That has the effect of slowly siphoning energy and mass from the

hole. If this is true, then even black holes are eventually doomed.

But finding out for sure is not easy. Creating a micro black hole, it seems, will take more

energy than any Earth-bound collider can pack. That is, unless there’s more to nature and

to gravity than we’ve thought.

The key lies in whether the universe we know is part of a more complex cosmic reality,

beyond the three spatial dimensions - plus time - that we experience in our everyday

lives. We may be like an insect living on the two-dimensional surface of a pond, unaware

of the deep and complex reality below it. It may be possible that an unseen extra dimension

could intersect our world on an extremely tiny scale.