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Cosmic Journeys: When Will Time End?
Poziom:
Temat: Nauka i technologia
Time is flying by on this busy, crowded planet...
as life changes and evolves from second to
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.
It may be possible that one of these of extra
dimensions could intersect our world on an
extremely tiny scale.
According to some scientists, when particles
collide at very high energies the additional
gravity needed to create a micro black hole
could come from this extra dimension.
They'll know a black hole is there when they
see the shower of particles predicted by Hawking's
theory.
That shower will open a brief window to a
deeper cosmic reality... while shedding light
on the ultimate future of our universe.
Based on Hawking's theory, the last black
holes will disappear when the cosmic clock
strikes 10 to the hundredth years from now...
that's a number known as a googol.
That's the end of our universe. And yet, it's
still far short of forever.
What will happen, say, in 10 to the googol,
a googolplex years? If you wrote all those
zeroes out in tiny 1 point font, it would
stretch beyond the observable universe.
Will the great arrow of time ever come to
rest? Or does that arrow fly a curved path,
destined to cycle back again and again as
whole new universes come into being in a way
similar to our own?
The numbers that describe the time horizons
of our universe are incomprehensible, yet
they may well be relatively insignificant
in the grand scheme of things.
Earth and humanity are products of the great
era of stars, and we have been witness to
its great spectacles of gravity and energy.
Yet it's fair to say that we occupy a truly
tiny period within the vast cosmic sweep of
deep time.
We can easily imagine there are others out
there somewhere who also look out and attempt
to comprehend the changes they see. They too
may invent the idea of "time"...and develop
their own theories on where it's all leading.
Will their discoveries - and ours - somehow
survive... as we all gradually go the way
of the stars that made life possible...?
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