January/February 2000 Table of Contents
present epoch is important to us for obvious reasons. But what of
those in the far distant future? Can the excitement and structure
of now be perpetuated into the dark emptiness of then?
Adams and Greg Laughlin
this dawn of a new century, the latest advances in physics and astronomy
allow us to understand our Universe with unprecedented clarity.
The entire life story of the cosmos, from its singular inception
at the Big Bang to its long and gradual slide into the far future,
now rests on a basic but nonetheless solid foundation.
continuing theme within this intricately detailed biography of the
Universe is the underlying conflict between the attractive force
of gravity and the tendency for physical systems to evolve toward
more disorganized conditions. Entropy provides a measure of disorder
within a physical system-whenever entropy is generated, the amount
of disorder increases. In the broadest sense, gravity pulls things
together and thereby organizes physical structures. Entropy production
opposes this order and acts to make physical systems more disorganized
and spread out. The interplay between these two competing tendencies
provides much of the drama in astrophysics and ultimately drives
the evolution of the entire Universe, including the life cycles
of its constituent stars and galaxies.
Universe is now ten to fifteen billion years old. In the grander
perspective provided by this long-running cosmic saga, the past
history of our Universe represents an utterly insignificant fragment
of time. To face the formidable challenge of establishing a time
line for the far future, we need a convenient way to denote aggressively
large time intervals. For this role, we use a time unit called a
"cosmological decade." When an interval of time, in years, is expressed
in scientific notation, say 10h years, the exponent h (the Greek
letter Eta) is called the cosmological decade. Since the Universe
is just over ten billion or 1010 years old, we are now living in
the 10th cosmological decade. When the Universe is ten times older,
1011 years, the Universe will experience its 11th cosmological decade.
For this journey into the future, we report on the battle between
gravity and entropy over the next 100 cosmological decades.
past and future history of the Universe can be organized into five
distinct eras of time. As the Universe passes from one era to the
next, its inventory and character change dramatically, and in many
ways almost completely. These eras are analogous to the geological
eras that describe the history of our planet, and they delineate
a broad general outline for the life of our Universe. As time unfolds,
a series of natural astronomical disasters punctuates this cosmic
newsreel and shapes the subsequent development of the Universe.
And throughout this evolutionary history, both past and future,
the force of gravity continually engages in a cosmic battle with
cannot actually describe the beginning of time, when our Universe
burst into existence (we would need a theory that incorporates both
quantum mechanics and general relativity at the same time). However,
we can pick up the story just after our expanding Universe is violently
launched on its trajectory towards the future.
its first moments of existence, the Universe experienced a period
of fantastically rapid expansion. The rate at which space was created
was so great that regions which were initially in causal contact
found themselves being separated much faster than they could be
connected by light signals. This inflationary epoch (see "Armchair
Astrophysics," Nov/Dec 1999, p. 6, and in this issue, p. 8) explains
many of the observed properties of the Universe, including its large
size, its striking uniformity, and the precise flatness of its space-time
geometry. During these earliest instants of creation, entropy and
disorder initially gain the upper hand. When the Universe inflates,
the force of gravity loses major ground as everything in the Universe
flies apart at fantastic speeds. This first engagement between gravity
and entropy is over long before the Universe is even a nanosecond
old, but this initial one-sided outcome ensures that gravity cannot
recollapse the Universe for at least billions of years, and most
likely, not ever.
Development of structure in an
early epoch of the Universe. Image courtesy of Michael Norman,
inflation runs its course, the Universe settles down into a state
of more leisurely expansion, and complex physical processes set
up a tiny excess of ordinary baryonic matter over antimatter. At
extreme temperatures, matter lives in the form of microscopic particles
called quarks and antiquarks, rather than the more familiar protons
and neutrons we know today. The asymmetry between matter and antimatter
is quite modest-for every 30 million antiquarks in its inventory,
the Universe has 30 million and one quarks of "ordinary" matter.
But the antimatter annihilates almost completely with most of the
matter and prodigious amounts of entropy are produced. Only a small
residue of ordinary matter survives to build the stars and galaxies
in the Universe today.
few minutes later, the synthesis of light elements-such as helium,
deuterium, and lithium-helps shape the future nuclear inventory
of our Universe. During this short-lived epoch, the strong nuclear
force acts as a surrogate for gravity and gains an important victory
in the name of consolidation. The strong force combines about one
fourth of the available protons and neutrons into nuclei of elements
heavier than hydrogen. This advantage is soon compromised, however,
when entropy is produced by the annihilation of electrons and positrons,
and by the radioactive decay of the remaining neutrons. After these
cosmic birth throes are completed, the Universe enters an extended
phase of smooth and peaceful expansion, which continues for many
the first 10,000 years, most of the Universe resides in the form
of radiation. The radiation fields are so pervasive and energetic
that the formation of astronomical structures is seriously inhibited;
no planets, stars, or galaxies grace the cosmos during this early
era. As the Universe ages, the density of radiation grows more diffuse
relative to the matter, which then begins to consolidate into astronomical
entities. When the temperature of the Universe grows cool enough
for electrons to attach themselves to atomic nuclei, neutral atoms
spring into existence and budding cosmic structures can grow larger
without being affected by the background sea of radiation. Gravity
thus begins its organizational efforts and makes vital progress
in its continuing struggle against entropy. The Primordial Era ends
as stars and galaxies start to form for the first time.
means "filled with stars." Most of the energy generated in our Universe
today arises from nuclear fusion in conventional stars. At the current
cosmological age of ten billion years, we now live in the middle
of the Stelliferous Era, when stars are actively forming, living,
the Universe reaches its adolescence in the early Stelliferous Era,
gravity finally makes some headway against the universal tendencies
toward disorganization. During the first billion years, galaxies
are created as gravity overcomes the background expansion of the
Universe. Gravity also organizes these galaxies into bound clusters
and cosmic structures on even larger size scales. Many freshly formed
galaxies experience violent early phases in connection with their
rapacious central black holes, which represent gravitational forces
so strong that even light cannot escape their wrath. As these black
holes rip apart stars and surround themselves with whirlpool-like
disks of hot gas, vast quantities of energy and entropy are released.
Inside the galaxies, huge clouds of gas are brought together by
gravity and new stars are forged within them. Gravity wins yet another
battle as planets coalesce within nebular disks that orbit about
the nascent stars.
Stelliferous Era thus witnesses the formation of many types of cosmic
structures-planets, stars, galaxies, and clusters-all the result
of gravity's relentless and constructive efforts. The general theme
of competition between gravity and entropy underlies the formation
of each of these astronomical structures. The very existence of
these astrophysical systems is ultimately due to gravity, which
acts to pull material together. Yet in each case, the tendency toward
gravitational collapse is opposed by disruptive forces and the formation
of astrophysical structures is never completely efficient. On every
scale, the indefatigable competition between gravity and entropy
ensures that victory is often temporary, and never absolute. A successful
formation event marks a local triumph for gravity, whereas failed
attempts at formation represent victories for disorganization and
entropy. When star formation takes place within an interstellar
cloud of gas and dust, for example, only a small fraction of the
available material is incorporated into a new generation of stars.
Young and forming stars produce intense jets of hot gas which inject
energy into the parent cloud and prevent most of its mass from making
A galactic arrangement. This image,
only a small section of one collected by the first telesope of
ESO's VLT project, details life in the vicinity of quasar PB5763;
most notable is the presence of this peculiar, distant cluster
of galaxies. Indeed, in the Universe's hierarchy, galaxy clusters
are the overwhelming norm. Image courtesy of the European Southern
an astronomical body forms, entropy production and the force of
gravity regroup and continue their struggle on a redrawn field of
battle. Our own Sun provides an immediate example of this ever-present
competition (see "Of Solar Matters"). The great
war between gravity and entropy guides the future evolution and
the ultimate fate of all stellar objects; indeed, the interplay
between these two competing tendencies drives most of stellar evolution.
After a star burns through its nuclear fuel and reaches the end
of its conventional life, the ongoing conflict reaches a more desperate
level of competition. The impoverished stellar body must face astronomical
death with radical adjustments to its internal structure. Gravity
pulls the star inwards, whereas the tendency for increasing entropy
favors dispersal of the stellar material. This stellar endgame can
have many different outcomes, depending on the mass of the star
and its other properties. For the vast majority of stars, the two
warring parties reach a kind of armistice by producing a degenerate
stellar remnant called a white dwarf. Within these dense stellar
objects, the inward pull of gravity is exactly balanced by pressure
forces arising from Heisenberg's uncertainty principle acting on
electrons. The resulting white dwarfs live in this deadlock for
many cosmological decades to come.
creation of a massive star, with more than ten times the mass of
the Sun, is a rare event. Soon after its birth, a massive star quickly
depletes its store of nuclear fuel and then explodes in a fiery
burst called a supernova (see "Unveiling
Black Holes in a Supernova Cauldron," Nov/Dec 1999, p. 8). During
these highly dramatic death scenes, another type of compromise is
negotiated. A sizable majority of the massive star is dispersed
across interstellar space by a shock wave resulting from the supernova
detonation. The remaining material is tightly concentrated into
a dense stellar remnant bound by a strong gravitational field. In
most cases, the resulting remnant is a neutron star supported by
the degeneracy pressure of its constituent neutrons. Under special
circumstances, a space-warping black hole is forged and gravity
achieves a more decisive victory. In still other cases, explosive
stellar death leaves behind no remnant whatsoever and thermodynamics
claims a clean decision.
Spiral stellar show. In this VLT
UTI image of the spiral galaxy NGC 2997 in the southern constellation
Antlia (The Air Pump), the spiral arms are clearly overdencse
with bright stars and are, in fact, regions of vigorous star formation
activity. Such is true of all spiral galaxies. Image courtesy
of the European Southern Observatory.
future portion of the Stelliferous Era belongs to the low mass stars
known as red dwarfs, by far the most common stars. Although they
contain less than half the mass of the Sun, red dwarfs are so numerous
that their combined mass easily exceeds that of all the larger stars
in the Universe. These red dwarfs are the utmost misers in fusing
their hydrogen into helium. They hoard their energy and will still
be around ten trillion years from now, long after larger stars have
exhausted their nuclear fuel reserves and condensed into white dwarfs
or exploded as supernovae. The Stelliferous Era comes to a close
when the galaxies run out of hydrogen gas, star formation ceases,
and the longest lived red dwarfs slowly fade away. The stars finally
stop shining during cosmological decade 14, when the Universe is
about 100 trillion years old.
gravity is comfortably winning its battle at the current cosmological
epoch, thermodynamics and entropy production catch up as the Universe
continues to age. After the end of star formation and conventional
stellar evolution, most of the ordinary mass in the Universe is
locked up in degenerate stellar remnants. The resulting cast of
degenerate characters includes brown dwarfs, white dwarfs, neutron
stars, and black holes.
an astronomical context, degeneracy connotes a peculiar state of
matter, rather than a state of moral depravity. In the dense interior
of a degenerate stellar remnant, the quantum mechanical uncertainty
principle (due to Heisenberg) compels particles to experience a
kind of quantum claustrophobia, which induces particle motions that,
in turn, provide the pressure that supports these bizarre objects.
this Degenerate Era, the Universe grows colder, darker, and more
diffuse. A dearth of stellar radiation remains to light up the night
skies, warm the planets, or endow galaxies with the faint glow they
have today. Nevertheless, gravity continues to induce events of
astronomical interest that sparkle against the darkness. A rare
beacon of light emerges when two brown dwarfs collide to create
a new low-mass star. The resulting red dwarf subsequently lives
as an "ordinary" hydrogen-burning star for trillions of years. On
average, at any given time, a few such stars will be shining in
a galaxy the size of our Milky Way. Every so often, as two white
dwarfs collide, the galaxy is rocked by a supernova explosion. Still
other collisions can fabricate strange new types of stars that burn
helium and carbon.
fill up an extraordinarily tiny volume of interstellar space. The
density of our local galactic environment is akin to that of individual
sand grains surrounded by miles of empty space. And because stars
traverse the interstellar gulfs at a glacial pace, close passages
between stars are exceedingly rare. In the vast and desolate stretches
of time available during the Degenerate Era, however, chance close
encounters will scatter the orbits of dead stars, and the Galaxy
must gradually readjust its dynamical structure. As scattering events
redistribute the Galactic wealth of energy, most stellar remnants
are ejected far beyond the Galaxy, while an unfortunate few fall
towards the center. The apparent gravitational victory represented
by galaxy formation thus turns out to be fleeting when viewed within
the grand scheme of time. During the 19th and 20th cosmological
decades, as the scattering of stellar remnants approaches completion,
galaxies evaporate most of their stars into intergalactic voids.
Eventually, the galaxies cease to exist as distinct astronomical
entities, and gravity's work is largely undone on the galactic scale.
dwarfs-the most common stellar remnants-contain most of the ordinary
baryonic matter during the Degenerate Era. While the galaxy remains
intact, these white dwarfs sweep up dark matter particles, which
orbit the galaxy in an enormous diffuse halo. Once trapped within
the interior of a white dwarf, the dark matter subsequently annihilates
and thereby provides an important power source for the Universe.
In this dim future, the annihilation of dark matter replaces nuclear
burning as the dominant source of energy in stars. But as the galaxies
are destroyed and the dark matter supply becomes depleted, this
line of energy generation and entropy production shuts down its
The creation of a white dwarf.
The expanding cloud of gas shown in this Hubble Space Telescope
image surrounds a dying star and its brighter companion. near
the end of its life, the now fainter star ejected much of its
outer layers to reveal its hot core, which will become a white
dwarf. The layers of gas surrounding the stellar "remnant" are
energized by the intense radiation emitted by the bared core.
Image courtesy of the Hubble Heritage Team (STScI/AURA/NASA).
the ongoing war between gravity and thermodynamics, white dwarfs
and other degenerate objects represent a stalement of stellar evolution.
But even these tightly bound remnants are transient.
the end of Degenerate Era, the mass-energy stored within white dwarfs
and neutron stars dissipates into radiation as their constituent
protons and neutrons decay into smaller particles. The idea of proton
decay is predicted in general terms by theoretical physics, but
experiments have thus far only set lower bounds on the process.
For a reasonable proton lifetime of 37 cosmological decades, a white
dwarf fueled by proton decay generates approximately 400 watts,
enough power to run a few light bulbs. An entire galaxy of these
erstwhile stars shines with less light than one ordinary hydrogen-burning
star like our Sun. The net result of proton decay is that stellar
remnants evaporate away and colossal amounts of entropy are generated.
As proton decay grinds to completion, gravity loses this conflict
in the end and the Degenerate Era quietly draws to a close. The
Universe grows darker and more rarefied.
Black Hole Era
the protons decay, the only stellar-like objects remaining are the
black holes, which doggedly push forward into the next era. These
fantastic objects have such strong gravitational fields that even
light cannot escape from their surfaces. As a result, black holes
are unaffected by proton decay and survive unscathed through the
end of the previous Degenerate Era. As white dwarfs evaporate and
disappear, black holes slowly sweep up material and grow larger.
the formation of these dark stellar corpses apparently marks a definitive
score for the gravitational forces, this victory also turns out
to be illusory. Even black holes cannot last forever. They eventually
evaporate away through a painstakingly slow quantum mechanical effect
known as Hawking radiation. In spite of their name, black holes
are not completely black. In reality, they shine ever so faintly
by emitting photons, neutrinos, gravitons, and other decay products.
After the protons are gone, the evaporation of black holes, almost
by default, provides the Universe with its primary source of energy.
black hole with the mass of the Sun lasts for "only" about 65 cosmological
decades. A larger black hole with the mass of a million Suns will
be destroyed in 83 cosmological decades. Even an enormous black
hole with the mass of an entire galaxy evaporates into oblivion
within 98 to 100 cosmological decades. All black holes are thus
slated for destruction. When the mass-energy of these bodies radiates
away, large quantities of entropy are generated, and gravity loses
further ground. As black holes evaporate, their effective temperatures
rise, and their demise accelerates. In the final moments, the Hawking
evaporation process releases so much energy in such a short time
that black holes leave the cosmos with a veritable explosion. These
spectacular events put an exclamation point on the eventual defeat
of gravity on this black hole battlefield. After the largest black
holes have made their explosive exits from the Universe, the Black
Hole Era is over and gravity's magnificent work is completely obliterated.
100 cosmological decades, protons have long since decayed and black
holes have evaporated. In the final enveloping desolation of the
Dark Era, only the leftover waste products from previous astrophysical
processes remain: photons of colossal wavelength, neutrinos, electrons,
and positrons. Perhaps weakly interacting dark matter particles
and other exotica are present as well. The Universe is empty, dark,
this frigid and far-distant future, astrophysical activity in the
Universe tails off dramatically. Energy levels are low and the expanses
of time are mind boggling. Electrons and positrons drifting through
the barren wasteland of space occasionally encounter one another.
A last-minute rally by the forces of organization can then lead
to the formation of positronium atoms, which consist of electrons
and positrons in orbit about each other. Following the now familiar
theme of non-permanence, these late-forming structures are unstable.
The giant atoms slowly spiral into ever smaller configurations until
the two particles annihilate in a burst of radiation. Other low-level
annihilation events can also take place and generate more entropy,
albeit very slowly. Gravity thus fumbles its final opportunity for
achieving order and its fourth quarter comeback is destined to fail.
As the Universe reels through the shadows of the Dark Era, entropy
wins yet again.
to its profligate past, the Universe now lives a conservative and
low-profile existence. Or does it? The apparent poverty of this
distant epoch could be due to our limited powers of extrapolation,
rather than a true case of deterioration and dilapidation. It remains
possible that this uncertain future could allow for the development
of new types of complexity, and perhaps even new types of life.
As our dying Universe cascades through its five cosmological eras,
only time will tell.
ADAMS and GREG LAUGHLIN
are co-authors of the recent book The Five Ages of the Universe:
Inside the Physics of Eternity (New York: The Free Press, 1999).
Adams is a physics professor at the University of Michigan in Ann
Arbor, and Laughlin is a staff scientist at NASA Ames Research Center
in Moffett Field, California.Their email addresses are firstname.lastname@example.org
Sun lives in a state of delicate balance between the action of gravity
and entropy. The force of gravity holds the Sun together and pulls
all of the solar material toward the center. In the absence of competing
forces, gravity would rapidly crush the Sun into a black hole only
a few kilometers across. This disastrous collapse is prevented by
pressure forces which push outward to support the Sun. This pressure
that holds up the Sun arises from the energy of nuclear reactions
taking place deep within the solar interior. These reactions generate
both energy and entropy, leading to random motions of the particles
in the solar core, and ultimately supporting the structure of the
Image courtesy of SoHO/EIT consortium.
SoHO is a project of international cooperation between ESA and
the other hand, if the force of gravity was somehow shut off, the
Sun would no longer be confined and would quickly expand. This dispersal
would continue until the solar material was spread thinly enough
to match the very low densities of interstellar space. The rarefied
ghost of the Sun would then be several lightyears across, about
100 million times its present size.
evenly matched competition between gravity and entropy allows the
Sun to exist in its present state. If this balance is compromised,
and either gravity or entropy overwhelms the other, the Sun would
end up either as a small black hole or a very diffuse wisp of gas.
This same state of affairs—a finely-tuned balance between
gravity and entropy—determines the structure of all the stars
in the sky.
expansion of the Universe itself provides an intensely dramatic
example of the ubiquitous struggle between the force of gravity
and entropy. As the Universe expands and becomes more spread out,
gravity resists this trend and tries to pull the expanding Universe
back together. The particular fate which our future holds depends
on whether gravity wins or loses this cosmic battle, whose outcome
depends on the total amount of mass and energy contained within
the Universe. Current astronomical data strongly suggest that gravity
has already lost this critical conflict and our fate will be determined
by a continued and unending expansion.