second.

And yet the arc of human lifespan is getting longer: 65 years is the global average ...

way up from just 20 in the Stone Age.

Modern science, however, provides a humbling perspective. Our lives... indeed the life

span of the human species... is just a blip compared to the age of the universe, at 13.7

billion years and counting.

It now seems that our entire universe is living on borrowed time...

And that even it may be just a blip within the grand sweep of deep time.

Scholars debate whether time is a property of the universe... or a human invention.

What's certain is that we use the ticking of all kinds of clocks... from the decay of

radioactive elements to the oscillation of light beams... to chart and measure a changing

universe... to understand how it works and what drives it.

Our own major reference for the passage of time is 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... approximately...

if you're judging by the stars, not the sun.

Earth acquired its spin during its birth, from the bombardment of rocks and dust that

formed it.

But it's gradually losing that rotation 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 the retinas in our eyes, 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. Long ago, this may have been an adaptation to keep us quiet and clear of night-time

predators.

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.

Over the days ... and years... we march to the beat of our biology.

But with our minds, we have learned to follow time's trail out to longer and longer intervals.

Philosophers have wondered... does time move like an arrow... with all the phenomena in

nature pushing toward an inevitable end?

Or perhaps, it moves in cycles that endlessly repeat... and even perhaps restore what is

there?

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

days.

As the Earth orbits, with each hemisphere tilting toward and away from its parent star,

the seasons bring on cycles of life... birth and reproduction... decay and death.

Only about one billionth of the Sun's energy actually hits the Earth. And much of that

gets absorbed by dust and water vapor in the upper atmosphere.

What does make it down to the surface sets many planetary processes in motion.

You can see it in the annual melting and refreezing of ice at the poles... the ebb and flow of

heat in the tropical oceans...

The seasonal cycles of chlorophyll production in plants on land and at sea... and in the

biosphere at large.

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 completes a series of interlocking

wobbles called Milankovic cycles every 23 to 41,000 years.

They have been blamed for the onset of ice ages about every one hundred thousand years.

Then there's the carbon cycle. It begins with rainfall over the oceans and coastal waves

that pull carbon dioxide into the sea.

There, it's captured by ocean plants... including tiny organisms called plankton. They are eaten

by fish and other creatures, and the carbon is passed on up the food chain.

Eventually, when plants and animals die or expel waste, the carbon falls to the ocean

bottom where it's impounded in layers of sediment.

Without people, it takes a volcanic explosion... or a dramatic lowering of sea levels... to

send the carbon back into the air, often after millions of years.

The idea that Earth and life changes on deep time scales emerged in revolutions of thought

associated with Copernicus and Darwin.

But the processes that shape a planet like ours play only the smallest of roles in the

evolution of the cosmos. So to glimpse time's broadest arcs we must look to the universe

beyond.

The reigning theory is that it all began in a sudden expansion of space... the big bang.

This was the time of the tiny: The first microseconds set in motion the primordial era. Atoms formed

within a hot soup of subatomic particles.

The universe cooled as it ballooned outward... growing dim... and falling into what's known

as the cosmic dark ages. But gravity was always at work; particles pulling on one another.

And after several hundred million years, considerable 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 planted the rich cosmic landscapes we see in our telescopes...

where hundreds of billions of stars light up galaxies all across the universe.

The arc of this great era of stars 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 still raging in stars all around the universe.

But in large stars, about ten million years after their birth, gravity gains the edge...

and tips the balance.

When the core of such a star crosses a critical mass threshold, it collapses in on itself.

The energy released 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 catch fire... to form a generation of smaller stars like our Sun.

When sun-like stars go, the end will be more of a whimper than a bang, as shown in this

gallery of dying stars captured by the Hubble Space Telescope.

As their cores gets heavier and heavier, over billions of years time, fierce winds will

begin to push on their outer layers... causing them to blossom out in spectacular displays.

That's just what happened about 12,000 years ago to the star that's become the famed Helix

Nebula. Today we see the dying star's outer layers form a vast glowing ring. On the inside

of the ring, spokes of more dense gas are being exposed by its winds.

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

Earth, but about 3 million times more dense.

This is likely what's in store for our sun. A civilization distant in time may scan the

Sun for planets, but they won't see Earth.

Our home planet's end will have begun long before that, as rising solar luminosity gradually

blasts away its atmosphere, rendering it uninhabitable. Surface water will disappear, evaporated by

all that heat and stripped off by solar winds.

Finally, as the sun blows off its outer layers, they will envelope the Earth. Friction with

those gases will cause this once blue world to gradually spiral home, melting into its

mother sun.

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

gravity drawing gas clouds into stars...

The stars will burn themselves out on wide variety of time scales... depending mostly

on how large they are.

They'll leave remnants that slowly grow cooler.

As a result, galaxies like ours will grow dimmer over time... unless they get seriously

stirred up.

That's what's going to happen to our galaxy. At just about the time our sun begins setting

its sights on swallowing Earth, any remaining inhabitants here 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...or at least severely

jumble each other up. If it's a direct hit, all the stars in both galaxies will gradually

join together in a gigantic galactic puffball known as an elliptical galaxy.

All this turbulence of the merger could stimulate a wave of new stars being born, reinvigorating

the new larger galaxy.

Dust-ups like this - where galactic neighbors come together - will be common as the epoch

of stars grows toward 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 is in no danger of

succumbing to gravity...it won't all end in a Big Crunch.

In fact, over the last 6 billion years, it's begun to accelerate outward... as gravity

loses its grip on the universe to an unseen force called: dark energy.

You can see evidence of this now, out in the huge voids of space between clusters of galaxies.

Think of the voids as ever-expanding bubbles ... where the bubble walls touch are filaments

of galaxies.

As the bubbles grow, these filaments will stretch and break. The distances between galaxies

will widen at a faster and faster pace. Eventually, most observers will see only a few isolated

clusters of galaxies huddled together... with little connection to anything else ... and

few clues as to how they got there.

A good place to be, in those long twilight years of the stellar era, would be a place

where gravity and energy have forged an extended truce.

Perhaps a place like this:

It's one of the smallest and dimmest stars in our universe. And yet brown dwarfs like

this have been shown to harbor planets close enough to bask in their dim rays.

Brown dwarfs - and their hotter cousins the red dwarfs - form the vast majority of stars

in our galaxy.

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 galaxies like ours grow dim, another process will begin to transform them. Over

time, chance encounters between objects will perturb their orbits... sending some towards

the center of those galaxy, and others out into the void.

In

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

accumulating in their cores.

The universe now begins to take on a new character.

Welcome to the degenerate era... in which the cosmos is populated by red and white dwarf

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

stars and black holes... slowly spinning down.

Even though these dead stars have used up their nuclear fuels, the universe continues

to produce small amounts of energy. They scoop up and annihilate dark matter particles that

manage to stray into their grasp.

Now here is where change slows to a crawl. It's expected that protons, the building blocks

of all atoms, will slowly degrade... turning back into sub-atomic particles that then decay

into photons, particles of light.

All the protons in the universe date back to the earliest moments. Their decay marks

the end of the degenerate era... around a billion, billion, billion, billion years after

it all began. That's a one followed by 40 zeros.

Our picture of what happens after that will come to depend 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 ring of tubes, 27 miles 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.

And the final moments of its most persistent objects.

Black holes, ranging from millions to billions of times the mass of our sun, occupy the centers

of most large galaxies today. As those galaxies age, much of their mass will spiral towards

the center... and over trillions of years that mass will fall into ever more ravenous

black holes.

Conceivably, these ultra-massive black holes could end up weighing as much as a galaxy.

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 charges 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

yet conceived can pack.

That is, unless there's more to our universe and to gravity than we've thought.

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

beyond the three spatial dimensions plus time that we experience in our daily lives.

If so, we would be like insects living on the two-dimensional surface of a pond, completely

unaware of the deep and complex reality below it.