How the Universe Will End
Scientists think they know how the universe began, but what happens at the
other end of the space—time continuum was a deep, dark mystery—until
now
BY MICHAEL D. LEMONICK
For those who live in a city or near
one, the night sky isn't much to look
at—just a few scattered stars in a
smoggy, washed-out, light-polluted
expanse. In rural Maine, though, or
the desert Southwest or the high
mountains of Hawaii, the view is quite
different. Even without a telescope,
you can see thousands of stars
twinkling in shades of blue, red and
yellow-white, with the broad Milky
Way cutting a ghostly swath from one
horizon to the other. No wonder our
ancient ancestors peered up into the
heavens with awe and reverence; it's
easy to imagine gods and mythical
heroes inhabiting such a luminous realm.
Yet for all the magnificence of the visible stars, astronomers know they are only the
first
shimmering veil in a cosmos vast beyond imagination. Armed with ever more
powerful
telescopes, these explorers of time and space have learned that the Milky Way is a
huge, whirling pinwheel made of 100 billion or more stars; that tens of billions of
other
galaxies lie beyond its edges; and, most astonishing of all, that these galaxies are
rushing headlong away from one another in the aftermath of an explosive cataclysm
known as the Big Bang.
That event—the literal birth of time and space some 15 billion years ago—has been
understood, at least in its broadest outlines, since the 1960s. But in more than a third
of
a century, the best minds in astronomy have failed to solve the mystery of what
happens
at the other end of time. Will the galaxies continue to fly apart forever, their glow
fading
until the cosmos is cold and dark? Or will the expansion slow to a halt, reverse
direction
and send 10 octillion (10 trillion billion) stars crashing back together in a final,
apocalyptic Big Crunch, the mirror image of the universe's explosive birth? Despite
decades of observations with the most powerful telescopes at their disposal,
astronomers simply haven't been able to decide.
But a series of remarkable discoveries announced in quick succession starting this
spring has gone a long way toward settling the question once and for all. Scientists
who
were betting on a Big Crunch liked to quote the poet Robert Frost: "Some say the
world
will end in fire,/ some say in ice./ From what I've tasted of desire/ I hold with those
who
favor fire." Those in the other camp preferred T.S. Eliot: "This is the way the world
ends/
Not with a bang but a whimper." Now, using observations from the Sloan Digital Sky
Survey in New Mexico, the orbiting Hubble Space Telescope, the mammoth Keck
Telescope in Hawaii and sensitive radio detectors in Antarctica, the verdict is in: T.S.
Eliot wins.
For that reason alone, the latest news from space would be profoundly significant;
understanding where we came from and where we are headed have been obsessions
of
thinking humans, probably for as long as we've walked the earth. But the particulars
of
these discoveries shed light on even deeper mysteries of the cosmos, lending
powerful
support to radical ideas once considered speculative at best. For one thing, the new
observations bolster the theory of inflation: the notion that the universe went through
a
period of turbocharged expansion before it was a trillionth of a second old, flying
apart
(in apparent, but not actual, contradiction of Albert Einstein's theories of relativity)
faster
than the speed of light.
An equally bizarre implication is that the universe is pervaded with a strange sort of
"antigravity," a concept originally proposed by and later abandoned by Einstein as the
greatest blunder of his life. This force, which has lately been dubbed "dark energy,"
isn't
just keeping the expansion from slowing down, it's making the universe fly apart
faster
and faster all the time, like a rocket ship with the throttle wide open.
It gets stranger still. Not only does dark energy swamp ordinary gravity but an
invisible
substance known to scientists as "dark matter" also seems to outweigh the ordinary
stuff
of stars, planets and people by a factor of 10 to 1. "Not only are we not at the center
of
the universe," University of California, Santa Cruz, astrophysical theorist Joel
Primack
has commented, "we aren't even made of the same stuff the universe is."
These mind-bending discoveries raise more questions than they answer. For example,
just because scientists know dark matter is there doesn't mean they understand what it
really is. Same goes for dark energy. "If you thought the universe was hard to
comprehend before," says University of Chicago astrophysicist Michael Turner, "then
you'd better take some smart pills, because it's only going to get worse."
ECHO OF THE BIG BANG
Things seemed a lot simpler back in 1965 when two astronomers at Bell Labs in
Holmdel, N.J., provided a resounding confirmation of the Big Bang theory, at the
time
merely one of several ideas floating around on how the cosmos began. The discovery
happened purely by accident: Arno Penzias and Robert Wilson were trying to get an
annoying hiss out of a communications antenna, and after ruling out every other
explanation—including the residue of bird droppings—they decided the hiss was
coming
from outer space.
Unbeknownst to the duo, physicists at nearby Princeton University were about to turn
their own antenna on the heavens to look for that same signal. Astronomers had
known
since the 1920s that the galaxies were flying apart. But theorists had belatedly
realized a
key implication: the whole cosmos must at one point have been much smaller and
hotter.
About 300,000 years after the instant of the Big Bang, the entire visible universe
would
have been a cloud of hot, incredibly dense gas, not much bigger than the Milky Way
is
now, glowing white hot like a blast furnace or the surface of a star. Because this
cosmic
glow had no place to go, it must still be there, albeit so attenuated that it took the
form of
feeble microwaves. Penzias and Wilson later won the Nobel Prize for the accidental
discovery of this radio hiss from the dawn of time.
The discovery of the cosmic-microwave background radiation convinced scientists
that
the universe really had sprung from an initial Big Bang some 15 billion years ago.
They
immediately set out to learn more. For one thing, they began trying to probe this
cosmic
afterglow for subtle variations in intensity. It's clear throughout ordinary telescopes
that
matter isn't spread evenly through the modern universe. Galaxies tend to huddle
relatively close to one another, dozens or even hundreds of them in clumps known as
clusters and superclusters. In between, there is essentially nothing at all.
That lumpiness, reasoned theorists, must have evolved from some original lumpiness
in
the primordial cloud of matter that gave rise to the background radiation. Slightly
denser
knots of matter within the cloud—forerunners of today's superclusters should have
been
slightly hotter than average. So scientists began looking for subtle hot spots.
FIRE OR ICE?
Others, meanwhile, attacked a different aspect of the problem. As the universe
expands,
the combined gravity from all the matter within it tends to slow that expansion, much
as
the earth's gravity tries to pull a rising rocket back to the ground. If the pull is strong
enough, the expansion will stop and reverse itself; if not, the cosmos will go on
getting
bigger, literally forever. Which is it? One way to find out is to weigh the cosmos—to
add
up all the stars and all the galaxies, calculate their gravity and compare that with the
expansion rate of the universe. If the cosmos is moving at escape velocity, no Big
Crunch.
Trouble is, nobody could figure out how much matter there actually was. The stars
and
galaxies were easy; you could see them. But it was noted as early as the 1930s that
something lurked out there besides the glowing stars and gases that astronomers
could
see. Galaxies in clusters were orbiting one another too fast; they should, by rights, be
flying off into space like untethered children flung from a fast-twirling merry-goround.
Individual galaxies were spinning about their centers too quickly too; they should
long
since have flown apart. The only possibility: some form of invisible dark matter was
holding things together, and while you could infer the mass of dark matter in and
around
galaxies, nobody knew if it also filled the dark voids of space, where its effects would
not
be detectable.
So astrophysicists tried another approach: determine whether the expansion was
slowing down, and by how much. That's what Brian Schmidt, a young astronomer at
the
Mount Stromlo Observatory in Australia, set out to do in 1995. Along with a team of
colleagues, he wanted to measure the cosmic slowdown, known formally as the
"deceleration parameter." The idea was straightforward: look at the nearby universe
and
measure how fast it is expanding. Then do the same for the distant universe, whose
light
is just now reaching us, having been emitted when the cosmos was young. Then
compare the two.
Schmidt's group and a rival team led by Saul Perlmutter, of Lawrence Berkeley
Laboratory in California, used very similar techniques to make the measurements.
They
looked for a kind of explosion called a Type Ia supernova, occurring when an aging
star
destroys itself in a gigantic thermonuclear blast. Type Ia's are so bright that they can
be
seen all the way across the universe and are uniform enough to have their distance
from Earth accurately calculated. That's key: since the whole universe is expanding at
a
given rate at any one time, more distant galaxies are flying away from us faster than
nearby ones. So Schmidt's and Perlmutter's teams simply measured the distance to
these supernovas (deduced from their brightness) and their speed of recession
(deduced by the reddening of their light, a phenomenon affecting all moving bodies,
known to physicists as the Doppler shift). Combining these two pieces of information
gave them the expansion rate, both now and in the past.
DARK ENERGY
By 1998 both teams knew something very weird was happening. The cosmic
expansion
should have been slowing down a lot or a little, depending on whether it contained a
lot
of matter or a little—an effect that should have shown up as distant supernovas,
looking
brighter than you would expect compared with closer ones. But, in fact, they were
dimmer—as if the expansion was speeding up. "I kept running the numbers through
the
computer," recalls Adam Riess, a Space Telescope Science Institute astronomer
analyzing the data from Schmidt's group, "and the answers made no sense. I was sure
there was a bug in the program." Perlmutter's group, meanwhile, spent the better part
of
the year trying to figure out what could be producing its own crazy results.
In the end, both teams adopted Sherlock Holmes' attitude: once you have eliminated
the
impossible, whatever is left, no matter how improbable, has got to be true. The
universe
was indeed speeding up, suggesting that some sort of powerful antigravity force was
at
work, forcing the galaxies to fly apart even as ordinary gravity was trying to draw
them
together. "It helped a lot," says Riess, "that Saul's group was getting the same answer
we were. When you have a strange result, you like to have company." Both groups
announced their findings almost simultaneously, and the accelerating universe was
named Discovery of the Year for 1998 by Science magazine.
For all its seeming strangeness, antigravity did have a history, one dating back to
Einstein's 1916 theory of general relativity. The theory's equations suggest that the
universe must be either expanding or contracting; it couldn't simply sit there. Yet the
astronomers of the day, armed with relatively feeble telescopes, insisted that it was
doing just that. Grumbling about having to mar the elegance of his beloved
mathematics, Einstein added an extra term to the equations of relativity. Called the
cosmological constant, it amounted to a force that opposed gravity and propped up
the
universe.
A decade later, though, Edwin Hubble discovered that the universe was expanding
after
all. Einstein immediately and with great relief discarded the cosmological constant,
declaring it to be the biggest blunder of his life. (If he had stuck to his guns, he might
have nabbed another Nobel.)
Even so, the idea of a cosmological constant wasn't entirely dead. The equations of
quantum physics independently suggested that the seemingly empty vacuum of space
should be seething with a form of energy that would act just like Einstein's disowned
antigravity. Problem was, this force would have been so powerful that it would have
blown the universe apart before atoms could form, let alone galaxies—which it
clearly
did not. "The value particle physicists predict for the cosmological constant," admits
Chicago's Turner, "is the most embarrassing number in physics."
Aside from that detail, the Einstein connection made the idea of dark energy, or
antigravity, seem somewhat less nutty when Schmidt and Perlmutter weighed in. Of
course, some astrophysicists had lingering doubts. Maybe the observers didn't really
have the supernovas' brightness right; perhaps the light from faraway stellar
explosions
was dimmed by some sort of dust. The unique properties of a cosmological constant,
moreover, would make the universe slow down early on, then accelerate. That's
because dark energy grows as a function of space. There wasn't much space in the
young, small universe, so back then the braking force of gravity would have reigned
supreme. More recently, the force of gravity fell off as the distance between galaxies
grew and that same increase made for more dark energy. Nobody had probed deeply
enough to find out what was really going on in the distant past.
Or rather, nobody had got enough data. Back in 1997, astronomers Mark Phillips of
the
Space Telescope Science Institute and Ron Gilliland of the Carnegie Institute of
Washington had used the Hubble to spot a distant supernova designated SN 1997ff
and, with the help of Peter Nugent, a Lawrence Berkeley astronomer on Perlmutter's
team, had determined its speed of recession from Earth. Nugent couldn't figure out
the
distance, though: determining the brightness of a Type Ia calls for not just one but
several measurements, spread over time.
On the rival team, Riess knew of the discovery, but he learned soon afterward that
other
Hubble photos had also caught the supernova, completely by chance. So one day last
summer, he recalls, "I called up Peter and began fishing around for information. I
guess I
wasn't especially cagey. He said almost right away, ÔAre you asking about 1997ff?'"
Rather than try to scoop each other, the friendly rivals decided to cooperate—and
soon
realized they had stumbled onto something truly astonishing. The new supernova,
some
50% closer to the beginning of the universe than any supernova known before, was
far
brighter than had been predicted. That neatly eliminated the idea of dust, since a more
distant star should have been even more dust-dimmed than nearer ones. But the level
of brightness also signaled that this supernova was shining when the expansion of the
cosmos was still slowing down. "Usually," says Riess, "we see weird things and try
to
make our models of the universe fit. This time we put up a hoop for the observations
to
jump through in advance, and they did—which makes it a lot more convincing."
PROBING THE COSMIC FIREBALL
What makes it still more convincing is that an entirely different kind of observation—
the
long-standing search for lumpiness in the cosmic background radiation—now
suggests
independently that dark energy is real. The lumps themselves were first detected
about
a decade ago, thanks to the Cosmic Background Explorer satellite. At the time,
astrophysicist and cobe spokesman George Smoot declared that "if you're religious,
it's
like seeing God."
But it was more like seeing God through dirty Coke-bottle glasses: the satellite saw
lumps but couldn't determine much about them. In April, though, scientists offered up
much sharper images from a balloon-borne experiment called boomerang (Balloon
Observations of Millimetric Extragalactic Radiation and Geophysics), which lofted
instruments into the Antarctic stratosphere; from another named maxima (Millimeter
Anisotropy Experiment Imaging Array, which did the same over the U.S.); and from
a
microwave telescope on the ground at the South Pole, called dasi (Degree Angular
Scale Interferometer).
All these measurements pretty much agreed with one another, confirming that the
lumps
scientists saw were real, not some malfunction in the telescopes. And just two weeks
ago, astronomers from the Sloan Digital Sky Survey confirmed that this primordial
lumpiness has carried over into modern times. The five-year mission of the survey, to
make a 3-D map of the cosmos, is far from complete, but scientists reported at the
American Astronomical Society's spring meeting in Pasadena, Calif., that it is clearer
than ever that galaxies cluster together into huge clumps that reflect conditions that
existed soon after the Big Bang.
To the unaided eye, the images are meaningless. A statistical analysis, however,
shows
that the early lumps—actually patches of slightly warmer or cooler radiation—don't
come
at random but rather at certain fixed sizes. "It's as though you're studying dogs," says
University of Pennsylvania astrophysicist Max Tegmark, "and you find out that they
come
in just three types: Labrador, toy poodle and Chihuahua."
That turns out to be enormously important. Knowing the characteristic sizes and also
the
temperatures, to a millionth of a degree, of these warm and cool regions gives
theoretical physicists all sorts of information about the newborn cosmos. They were
already pretty sure, from the equations of nuclear physics and from measurements of
the relative amounts of hydrogen, helium and lithium in the universe, that protons,
neutrons and electrons (the building blocks of every atom in the cosmos) add up to
only
about 5% of the so-called critical density—what it would take to bring the cosmic
expansion essentially to a halt by means of gravity.
But when you add Tegmark's "dogs," plus the more esoteric equations of sub-nuclear
physics, it turns out that an additional 30% of the needed matter most likely comes in
the
form of mysterious particles that have been identified only in theory, never directly
observed—particles with quirky names like neutralino and axion. These are the
mysterious dark matter, or most of it anyway. The cosmic background radiation itself
began to shine when the universe was 300,000 years old, but the temperature
fluctuations were set in place when it was just a split-second old. "It's pretty cool,"
says
Tegmark, "to be able to look back that far."
THE FLAT UNIVERSE
The dogs also yield another key bit of information: they tell theorists how the
universe is
curved, in the Einsteinian sense. There's no way to convey this concept to a
nonphysicist except by two-dimensional analogy (see How Does the Universe Curve?
diagram). The surface of a sphere has what's called positive curvature; if you go far
enough in one direction, you will never get to the edge but you will eventually return
to
your starting point. An infinitely large sheet of paper is flat and, because it's infinite,
also
edgeless. And a saddle that extends forever is considered edgeless and negatively
curved. It also turns out that any triangle you draw on the paper has angles that add
up
to 180º, but the sphere's angles are always greater than 180º, and the saddle's always
less.
Same goes for the universe, but with one more dimension. According to Einstein, the
whole thing could be positively or negatively curved or flat (but don't try to imagine
in
what direction it might be curved; it's quite impossible to visualize). "What the new
measurements tell us," says Turner, "is that the universe is in fact flat. Draw a
triangle
that reaches all the way across the cosmos, and the angles will always add up to
180º."
According to Einstein, the universe's curvature is determined by the amount of matter
and energy it contains. The universe we evidently live in could have been flattened
purely by matter—but the new discoveries prove that ordinary matter and exotic
particles add up to only about 35% of what you would need. Ergo, the extra curvature
must come from some unseen energy—just about the amount, it turns out, suggested
by
the supernova observations. "I was highly dubious about dark energy based only on
supernovas," says Princeton astrophysicist Edwin Turner (no relation to Michael,
though
the two often refer to each other as "my evil twin"). "This makes me take dark energy
more seriously."
The flatness of the universe also means the theory of inflation has passed a key test.
Originally conceived around 1980 (in the course of elementary-particle, not
astronomical, research), the theory says the entire visible universe grew from a speck
far smaller than a proton to a nugget the size of a grapefruit, almost instantaneously,
when the whole thing was .000000000000000000000000000000000001 sec. old.
This
turbo-expansion was driven by something like dark energy but a whole lot stronger.
What we call the universe, in short, came from almost nowhere in next to no time.
Says
M.I.T.'s Alan Guth, a pioneer of inflation theory: "I call the universe the ultimate free
lunch." One of the consequences of inflation, predicted 20 years ago, was that the
universe must be flat—as it now turns out to be.
If these observations continue to hold up, astrophysicists can be pretty sure they have
assembled the full parts list for the cosmos at last: 5% ordinary matter, 35% exotic
dark
matter and about 60% dark energy. They also have a pretty good idea of the
universe's
future. All the matter put together doesn't have enough gravity to stop the expansion;
beyond that, the antigravity effect of dark energy is actually speeding up the
expansion.
And because the amount of dark energy will grow as space gets bigger, its effect will
only increase.
THE FATE OF THE COSMOS
That means that the 100 billion or so galaxies we can now see though our telescopes
will zip out of range, one by one. Tens of billions of years from now, the Milky Way
will be
the only galaxy we're directly aware of (other nearby galaxies, including the Large
Magellanic Cloud and the Andromeda galaxy, will have drifted into, and merged
with, the
Milky Way).
By then the sun will have shrunk to a white dwarf, giving little light and even less
heat to
whatever is left of Earth, and entered a long, lingering death that could last 100
trillion
years—or a thousand times longer than the cosmos has existed to date. The same will
happen to most other stars, although a few will end their lives as blazing supernovas.
Finally, though, all that will be left in the cosmos will be black holes, the burnt-out
cinders
of stars and the dead husks of planets. The universe will be cold and black.
But that's not the end, according to University of Michigan astrophysicist Fred
Adams.
An expert on the fate of the cosmos and co-author with Greg Laughlin of The Five
Ages
of the Universe (Touchstone Books; 2000), Adams predicts that all this dead matter
will
eventually collapse into black holes. By the time the universe is 1 trillion trillion
trillion
trillion trillion trillion years old, the black holes themselves will disintegrate into
stray
particles, which will bind loosely to form individual "atoms" larger than the size of
today's
universe. Eventually, even these will decay, leaving a featureless, infinitely large
void.
And that will be that—unless, of course, whatever inconceivable event that launched
the
original Big Bang should recur, and the ultimate free lunch is served once more.
Astronomers and physicists are a cautious crew, and they insist that the mind-bending
discoveries about dark matter, dark energy and the flatness of space-time must be
confirmed before they are accepted without reservation. "We're really living
dangerously," says Chicago's Turner. "We've got this absurd, wonderful picture of the
universe, and now we've got to test it." There could be surprises to come: an
Einstein-style cosmological constant, for example, is the leading candidate for dark
energy, but it could in principle be something subtly different—a force that could
even
change directions someday, to reinforce rather than oppose gravity.
In any case, new tests of these bizarre ideas will not be too long in coming. Next
week a
satellite will launch from Cape Canaveral to make the most sensitive observations
ever
of the cosmic background radiation. Supernova watchers, meanwhile, are lobbying
nasa
for their own dedicated telescope so they won't have to queue up for time on the
badly
oversubscribed Hubble. And lower-tech telescopes and microwave detectors, both on
the ground and lofted into the air aboard balloons, will continue to refine their own
measurements. If the latest results do hold up, some of the most important questions
in
cosmology—how old the universe is, what it's made of and how it will end—will
have
been answered, only about 70 years after they were first posed. By the time the final
chapter of cosmic history is written—further in the future than our minds can
grasp—humanity, and perhaps even biology, will long since have vanished. Yet it's
conceivable that consciousness will survive, perhaps in the form of a disembodied
digital
intelligence. If so, then someone may still be around to note that the universe, once
ablaze with the light of uncountable stars, has become an unimaginably vast, cold,
dark
and profoundly lonely place.
BIG BANG
About 15 billion years
ago, the universe
bursts into existence
in the Big Bang,
which gives birth to
space, time and all
the matter and energy
the universe will ever
hold
INFLATION ERA
The universe undergoes a
brief, explosive period of
inflation, growing from
smaller than an atom to the
size of a grapefruit. The
inflationary expansion stops
when the force driving it is
transformed into matter and
energy as we know them
RADIATION-DOMINATED ERA
Most of the energy is in the form of
electromagnetic radiation‹visible light, X
rays, radio waves and ultraviolet rays.
Quarks clump into protons and neutrons,
which later combine to make the nuclei
of all atoms. The lightest nuclei‹helium,
deuterium and lithium‹are forged in the
first three minutes of cosmic history
STELLIFEROUS ERA
Electrons combine with existing
nuclei to form atoms, mostly
hydrogen and helium. This raw
material condenses into the first
generation of stars during the
first billion years. The galaxies
also take shape during this
window of time. Our sun and
solar system were formed 4.6
billion years ago, and the first
life-forms appeared on Earth a
surprisingly short time
afterward. Modern humans
show up only 100,000 years
before the present. Earth
should remain habitable for
another few billion years
DEGENERATE ERA
This era extends to 10 trillion
trillion trillion years after the Big
Bang. Planets detach from
stars; stars and planets
evaporate from galaxies. Most
of the ordinary matter in the
universe is locked up in
degenerate stellar
remnants‹dead stars that have
withered into white dwarfs or
blown up and collapsed into
neutron stars and black holes.
Eventually, over spans of time
greatly exceeding the current
age of the universe, the
protons themselves decay
BLACK-HOLE ERA
This era extends to 10,000
trillion trillion trillion trillion
trillion trillion trillion trillion
years after the Big Bang.
After the epoch of proton
decay, the only large objects
remaining are black holes,
which eventually evaporate
into photons and other types
of radiation
DARK ERA
Now only waste
products
remain: mostly
photons,
neutrinos, electrons and
positrons, wandering
through
a universe bigger than
the
mind can conceive.
Occasionally, electrons
and
positrons meet and
form
"atoms" larger than the
visible
universe is today. From
here
into the infinite future,
the
universe remains cold,
dark
and dismal
The End of the Universe
John Baez
July 26, 2000
It's interesting to ponder the end of the universe. For a long time, the big question was whether there
was enough matter in the
universe to make it recollapse, or whether it would expand forever. But in the late 1990's,
astronomical observations began to suggest
that the expansion of the universe is actually speeding up!
Let's suppose this is true, and let's assume the most popular explanation for it: namely, that there is
a nonzero cosmological constant.
A cosmological constant with the right sign makes the energy density of the vacuum positive, but
makes its pressure negative - and 3
times as big. This makes the universe tend to expand. Normal matter makes the universe tend to
recollapse. If the effect of the
cosmological constant ever beats out the effect of normal matter, the universe will keep expanding,
making the density of normal
matter less... so the cosmological constant will ultimately win hands down, and the universe will
eventually expand at an almost
exponential rate.
Let's suppose this happens. What will be the ultimate fate of the universe?
First let me set the stage. What happens in the short run, i.e. the first 1023 years or so?
First, galaxies will keep colliding. These collisions seem to destroy spiral galaxies - they fuse into
bigger elliptical galaxies. We can
already see this happening here and there, and our own Milky Way may collide with Andromeda in
only 3 billion years or so. If this
happens, a bunch of new stars will be born from the shock waves due to colliding interstellar gas,
but eventually we will inhabit a large
elliptical galaxy. Unfortunately, elliptical galaxies lack spiral arms, which seem to be a crucial part
of the star formation process, so
star formation may cease even before the raw materials run out.
Of course, even if this doesn't happen, the birth of new stars must eventually cease, since there's a
limited amount of hydrogen,
helium, and other stuff that can undergo fusion.
This means that all the stars will eventually burn out. They'll either become either black dwarfs,
neutron stars, or black holes. Stars
become white dwarfs - and eventually black dwarfs when they cool - if they have mass less than 1.4
solar masses. In this case they
can be held up by the degeneracy pressure caused by the Pauli exclusion principle, which works
even at zero temperature. If they are
heavier than this, they collapse: they become neutron stars if they are between 1.4 and 2 solar
masses, and they become black holes if
they are more massive.
The black holes will suck up some of the other stars they encounter. This is especially true for the
big black holes at the galactic
centers, which power radio galaxies if they swallow stars at a sufficiently rapid rate. But most of the
stars, as well as interstellar gas
and dust, will eventually be hurled into intergalactic space. This happens to a star whenever it
accidentally reaches escape velocity
through its random encounters with other stars. It's a slow process, but computer simulations show
that about 90% of the mass of
the galaxies will eventually "boil off" this way - while the rest becomes a big black hole.
(It may seem odd that first the galaxies form by gravitational attraction of matter and then fall apart
again by "boiling off", but the point
is, intergalactic matter is less dense now than it was when galaxies first formed, thanks to the
expansion of the universe. When the
galaxies first formed, there was lots of gas around. Now the galaxies are essentially isolated intergalactic space is almost a vacuum.
And you can show in the really long run, any isolated system consisting of sufficiently many point
particles interacting gravitationally even an apparently "gravitationally bound" system - will "boil off" as individual particles randomly
happen to acquire enough kinetic
energy to reach escape velocity. Computer calculations already suggest that the solar system will
fall apart this way, barring other
interventions. With the galaxies it's even more certain to happen, since there are more particles
involved, so things are more chaotic.)
How long will all this take? Well, the white dwarfs will cool to black dwarfs with a temperature of
at most 5 Kelvin in about 1017
years, and the galaxies will boil away by about 1019 years. Most planets will have already been
knocked off their orbits by then, but
any still orbiting stars will spiral in thanks to gravitational radiation in about 1020 years.
Then what? Well, in about 1023 years the dead stars will actually boil off from the galactic clusters,
not just the galaxies, so the
clusters will disintegrate. At this point the cosmic background radiation will have cooled to about
10-13 Kelvin, and most things will be
at about that temperature unless proton decay or some other such process keeps them warmer.
Okay, so now we have a bunch of isolated black dwarfs, neutron stars, and black holes together
with atoms and molecules of gas,
dust particles, and of course planets and other crud, all very close to absolute zero.
As the universe expands these things eventually spread out to the point where each one is
completely alone in the vastness of space.
So what happens next?
Well, everybody loves to talk about how all matter eventually turns to iron thanks to quantum
tunnelling, since iron is the nucleus with
the least binding energy, but unlike the processes I've described so far, this one actually takes quite
a while. About 101500 years, to be
precise. (Well, not too precise!) So it's quite likely that proton decay or something else will happen
long before this gets a chance to
occur.
For example, everything except the black holes will have a tendency to "sublimate" or "ionize",
gradually losing atoms or even electrons
and protons, despite the low temperature. Just to be specific, let's consider the ionization of
hydrogen gas - although the argument is
much more general. If you take a box of hydrogen and keep making the box bigger while keeping
its temperature fixed, it will
eventually ionize. This happens no matter how low the temperature is, as long as it's not exactly
absolute zero - which is forbidden by
the 3rd law of thermodynamics, anyway.
This may seem odd, but the reason is simple: in thermal equilibrium any sort of stuff minimizes its
free energy, E - TS: the energy
minus the temperature times the entropy. This means there is a competition between wanting to
minimize its energy and wanting to
maximize its entropy. Maximizing entropy becomes more important at higher temperatures;
minimizing energy becomes more
important at lower temperatures - but both effects matter as long as the temperature isn't zero or
infinite.
Think about what this means for our box of hydrogen. On the one hand, ionized hydrogen has more
energy than hydrogen atoms or
molecules. This makes hydrogen want to stick together in atoms and molecules, especially at low
temperatures. But on the other hand,
ionized hydrogen has more entropy, since the electrons and protons are more free to roam. And this
entropy difference gets bigger and
bigger as we make the box bigger. So no matter how low the temperature is, as long as it's above
zero, the hydrogen will eventually
ionize as we keep expanding the box.
(In fact, this is related to the "boiling off" process that I mentioned already: we can use
thermodynamics to see that the stars will boil
off the galaxies as they approach thermal equilibrium, as long as the density of galaxies is low
enough.)
However, there's a complication: in the expanding universe, the temperature is not constant - it
decreases!
So the question is, which effect wins as the universe expands: the decreasing density (which makes
matter want to ionize) or the
decreasing temperature (which makes it want to stick together)?
In the short run this is a fairly complicated question, but in the long run, things may simplify: if the
universe is expanding exponentially
thanks to a nonzero cosmological constant, the density of matter obviously goes to zero. But the
temperature does not go to zero. It
approaches a particular nonzero value! So all forms of matter made from protons, neutrons and
electrons will eventually ionize!
Why does the temperature approach a particular nonzero value, and what is this value? Well, in a
universe whose expansion keeps
accelerating, each pair of freely falling observers will eventually no longer be able to see each other,
because they get redshifted out of
sight. This effect is very much like the horizon of a black hole - it's called a "cosmological horizon".
And, like the horizon of a black
hole, a cosmological horizon emits thermal radiation at a specific temperature. This radiation is
called Hawking radiation. Its
temperature depends on the value of the cosmological constant. If we make a rough guess at the
cosmological constant, the
temperature we get is about 10-30 Kelvin.
This is very cold, but given a low enough density of matter, this temperature is enough to eventually
ionize all forms of matter made of
protons, neutrons and electrons! Even something big like a neutron star should slowly, slowly
dissipate. (The crust of a neutron star is
not made of neutronium: it's mainly made of iron.)
But what about the black holes?
Well, they probably evaporate due to Hawking radiation: a solar-mass black hole should do so in
1066 years, and a really big one,
comparable to the mass of a galaxy, should take about 1099 years.
Actually, a black hole only shrinks by evaporation when it's in an enviroment cooler than the
temperature of its Hawking radiation otherwise, it grows by swallowing thermal radiation. The Hawking temperature of a solar-mass
black hole is about 6 x 10-8 Kelvin,
and in general, it's inversely proportional to the black hole's mass. The universe should cool down
below 10-8 Kelvin very soon
compared to the 1066 years it takes for a solar-mass black holes to evaporate. However, before that
time, such a black hole would
grow by absorbing background radiation - which makes its temperature decrease and help it grow
more!
If a black hole ever grew to about 1022 solar masses, its Hawking temperature would go below 1030 Kelvin, which would allow it to
keep growing even when the universe has cooled to its minimum temperature. Of course, 1022 solar
masses is huge - about the mass
of the currently observable universe! But it would take a nontrivial calculation to show that
reasonable-sized black holes have no
chance of getting this big. I think it's true, but I haven't done the calculation.
For now, let's assume it's true: all black holes will eventually shrink away and disappear - none of
them grow big enough to stick
around when it gets really cold.
As black holes evaporate, they will emit photons and other particles in the process, so for a while
there will be a bit of radiation like
this running around. That livens things up a little bit - but this process will eventually cease.
What about the neutron stars? Well, if they don't ionize first, ultimately they should quantum-tunnel
into becoming black holes, which
then Hawking-radiate away.
Similarly, if the black dwarfs and planets and the like don't evaporate and their protons don't decay,
they may quantum-tunnel into
becoming solid iron - as I already mentioned, this takes about 101500 years. And then, if this iron
doesn't evaporate and nothing else
happens, these balls of iron will eventually quantum-tunnel into becoming black holes, which then
Hawking-radiate away. This would
take about 10100000000000000000000000000 years - that's 26 zeros.
This is a much longer time than any I've mentioned so far, so I wouldn't be surprised if some other
effect we haven't thought about
happens first. Indeed, this whole discussion should be taken with a grain of salt: future discoveries
in physics could drastically change
the end of this story. It's also possible that the intervention of intelligent life could change things I've avoided discussing that here.
Cosmology has been full of surprises lately, and there will probably be more to come.
But the overall picture seems to lean heavily towards a far future where everything consists of
isolated stable particles: electrons,
neutrinos, and protons (unless protons decay). If the scenario I'm describing is correct, the density
of these particles will go to zero,
and eventually each one will be cut off from all the rest by a cosmological horizon, making them
unable to interact. Of course there
will be photons as well, but these will eventually come into thermal equilibrium forming blackbody
radiation at the temperature of the
cosmological horizon - perhaps about 10-30 Kelvin or so.
This is why it's really a bad idea to keep putting things off for tomorrow.
References
Many of the figures here are taken from table 10.2 in this book:
John D. Barrow and Frank J. Tipler, The Anthropic Cosmological Principle, Oxford U. Press,
Oxford, 1988.
The hypothesis of proton decay is less fashionable now than when this book was written, since
people have looked very hard for it
and not found it - pushing up the lower bound on the proton lifetime to 1032 years. Also, the
nonzero mass of the neutrino was
discovered after this book was written, as was the apparent nonzero cosmological constant.
Nonetheless, it includes some dicussion
of the effects of a nonzero cosmological constant, and notes that in a universe with everaccelerating expansion due to a cosmological
constant, all computation must eventually cease due to a heat death, as explained above.
Tipler subsequently wrote a book of speculations about the fate of intelligent in the far future. He
assumed there would be a big
crunch, which no longer seems to be true, so I do not use his ideas here.
Freeman Dyson has discussed the fate of intelligent life in the far future assuming a perpetually
expanding universe, but assuming the
cosmological constant is zero. In this situation the temperature of the universe decreases ever closer
to absolute zero, and Dyson
figured out that in principle, intelligent life could last forever and think an infinite number of
thoughts, although slower and slower.
This idea seems to be ruined by the presence of a nonzero cosmological constant and the resulting
nonzero lower bound on the
temperature.
For more, try:
F. C. Adams and G. Laughlin, A dying universe: the long-term fate and evolution of
astrophysical objects, Rev. Mod. Phys. 69
(1997), 337-372. (See also their Sky & Telescope article, Aug. 1998, and their book Five Ages
of the Universe, 1999.)
L. M. Krauss and G. D. Starkman, Life, the universe, and nothing: life and death in an everexpanding universe Astrophys. J.
(2000) 531, 22-30. (See also their Scientific American article, Nov. 1999.)
The notion of a nonzero lower bound for the temperature of the universe are discussed in Scientific
American articles appearing in the
April 1999 and November 1999 issues, the latter written by Krauss and Starkman.
The rough estimates of 10-30 Kelvin for the limiting temperature of the universe and 1022 solar
masses for the smallest black hole that
would never evaporate are derived here:
Neal Dalal and Kim Griest, Black holes must die, preprint available as http://xxx.lanl.gov/abs/astroph/0008260.
I don't think any of these sources mention the "ionization" effect I discuss here. I would like to
know the rate of this process, but I'm
busy, so if you can figure it out it, go ahead and let me know the answer.
This article arose from a discussion among the contributors to sci.physics.research, and I thank all
these people for their help in
putting this together, especially Ted Bunn and Keith Ramsay.
The sun's not eternal. That's why there's the blues. - Allen Ginsberg
baez@math.ucr.edu
© 2000 John Baez
home
TIME April 10, 2000
How Will the Universe End? (with a bang or a whimper?)
by TIMOTHY FERRIS
The fate of the cosmos is not fiery cataclysm,
say the latest telescopic observations, but a
gradual descent into eternal, frigid darkness.
We Earthlings are newcomers to cosmology, the study of the universe as a whole, and there is no
cosmological questions about which
we have more to learn than the riddle of where it's all ultimately headed. But we have glimpsed at
least a few clues to cosmic destiny,
some of them hopeful and others bleak.
The good news is that we're not going to be evicted. The universe is likely to remain hospitable to
life for at least an additional 100
billion years. That's 20 times as long as the earth has existed, and 5 million times as long as Homo
sapiens has lasted so far. If we're
not around to shoot off fireworks on New Year's Eve of the year 100,000,000,000, it won't be the
universe's fault.
The bad news is that nothing lasts forever. The universe may not disappear, but as time goes by it
may get increasingly
uncomfortable, and eventually become unlivable. Calculating how and when this will happen is a
genuinely dismal science, but not
without a certain grim fascination. The classic Big Bang theory, refined over the decades since the
astronomer Edwin Hubble
discovered the expanding universe in 1929, suggests that cosmic destiny will be decided through a
tug-of-war between two opposing
forces. One is the expansion of space, which for more than 1- billion years has been carrying
galaxies ever farther apart from one
another. The other is the mutual gravitational force apart from one another. The other is the mutual
gravitational force exerted by
those galaxies and all the other stuff in the universe: it acts as a brake, slowing down the expansion
rate.
In this simple picture, if the gravitational force is strong enough to bring expansion to a halt, the
universe is destined to collapse,
ultimately dissolving into a fireball- a Big Crunch that amounts to the Big Bang run in reverse. If
it's not, and expansion wins out, then
the universe will eventually grow unpleasantly dark and cold. Starts produce energy by fusing light
atomic nuclei, mainly hydrogen
and helium, into heavier ones. When the hydrogen and helium run low, old stars will sputter out
without any new ones to take their
place, and the universe will gradually fade to black. Such were the gloomy alternatives the Robert
Front wrote about after being
briefed on the theory of the cosmic endgame by the astronomer Harlow Shapley:
Some say the world will end in fire,
Some say in ice...
I think I know enough of hate
To know that for destruction ice
Is also great
And would suffice.
Either fate looks like curtains for life. If the end comes in fire, the Big crunch would melt down
everything, even subatomic particles.
If, on the other hand, the universe winds up cold and dark, life might hang on for a long time - say,
by extracting gravitational energy
from black holes. But trying to make a living once everything has subsided to pretty much the same
temperature - a tad above
absolute zero - is like trying to run a water mill on dead-still pond.
Our ultimate fate, though, remains in doubt, in part because the jury is still out on whether the
expansion or gravity will triumph in the
end. Most observations point toward the former, but many uncertainties persist. One is the galling
"dark matter" issue. Studies of
how galaxies are moving around indicate that there's lots of extra gravity out there, suggesting that
the stars and nebulae we can see
constitute only 1% to 10% of the matter in the universe. The rest is invisible; it emits no light.
Nobody yet knows what this dark
matter is. One possibility is that it's made of WIMP - weakly interacting massive particles. Until
the dark matter is identified and
tallied, predicting the future of the universe on the basis of what we can see will be as uncertain as
trying to predict an election by
polling a few golfers down at the country club.
Meanwhile, ironists and fatalists draw austere satisfaction from the fire-or-ice scenario, which
reflects the quintessential human
perception that nobody gets out of life alive. And that's just what makes me suspicious of it. The
great lesson of scientific cosmology
is that the universe does not usually conform to our time-honored ways of thinking - that to
understand it, we need to think in new
ways. Integral to modern cosmology are mind-bending 20th century concepts like Einstein's curved
space, Heisenberg's uncertainty
principle and the realization that exotic subatomic particles sail through our bodies by the trillions
without laying a glove on us, and I
see no reason to suppose that doors won't open onto even stranger notions in the century to come.
So perhaps we can glimpse a few
shafts of light, shining under doors as yet unopened, that promise to refine our predictions of cosmic
destiny.
One unknown has to do with inflation - the theory that the universe began as a bubble of empty
space that initially expanded at a
velocity much faster than that of light. Cosmologists take inflation seriously because it resolves
problems that bedeviled older versions
of the Big Bang, but inflation also has implications for the study of cosmic destiny. Among them is
that the force that drove the
inflationary spasm, sometimes tagged with the Greek letter lambda after its designation in Einstein's
general relativity equations, might
not have subsided altogether back when the inflationary hiccup ended. Instead, it might still be
there, lurking in empty space and
urging expansion along, like an usher politely shooing playgoers back into the theater at
intermission's end. Some observations of
exploding stars in distant galaxies suggest the presence of just such an ongoing inflationary
impulse. If so, the tug-of-war over the
future of the universe involves not only expansion and gravitational braking but also the subtle
turbocharging of lingering inflation,
which acts to keep the universe expanding indefinitely.
Perhaps the most intriguing unknown, however, concerns the cosmic role played by intelligent life
itself. As the physicist Freeman
Dyson notes, "It is impossible to calculate in detail the long-range future of the universe without
including the effects of life and
intelligence." Much of the earth has been transformed, for better and worse, by the presence here of
an intelligent species capable of
manipulating its environment for its own benefit.
Similarly, advanced civilizations in the far future might be able to melt down stars and even entire
galaxies to make gigantic campfires,
or otherwise tilt the long-term odds in the favor. Life in the waning cosmic twilight might jejune,
but it could last a long time.
Consider the marshaled resources of all the natural and artificial intelligences in the observable
universe over the next, say, trillion
years. Which would you bet on to prevail - that level of smarts or a claims, based on 19th century
thermodynamics, that they're
doomed?
So let's stay tuned, heeding the words of Einstein, who wrote to a child anxious about the fate of the
world, "As for the question of
the end of it I advise: Wait and see!"
http://www.pbs.org/deepspace/timeline/index.html