The expansion of the universe may be the most important fact we have
ever discovered about our origins. You would not be reading this
article if the universe had not expanded. Human beings would not exist.
Cold molecular things such as life-forms and terrestrial planets could
not have come into existence unless the universe, starting from a hot
big bang, had expanded and cooled. The formation of all the structures
in the universe, from galaxies and stars to planets and
Scientific American articles, has depended on the expansion.
Forty years ago this July, scientists announced the discovery of
definitive evidence for the expansion of the universe from a hotter,
denser, primordial state. They had found the cool afterglow of the big
bang: the cosmic microwave background radiation. Since this discovery,
the expansion and cooling of the universe has been the unifying theme
of cosmology, much as Darwinian evolution is the unifying theme of
biology. Like Darwinian evolution, cosmic expansion provides the
context within which simple structures form and develop over time into
complex structures. Without evolution and expansion, modern biology and
cosmology make little sense.
The expansion of the universe is like Darwinian evolution in
another curious way: most scientists think they understand it, but few
agree on what it really means. A century and a half after On the Origin of Species,
biologists still debate the mechanisms and implications (though not the
reality) of Darwinism, while much of the public still flounders in
pre-Darwinian cluelessness. Similarly, 75 years after its initial
discovery, the expansion of the universe is still widely misunderstood.
A prominent cosmologist involved in the interpretation of the cosmic
microwave background, James Peebles of Princeton University, wrote in
1993: "The full extent and richness of this picture [the hot big bang
model] is not as well understood as I think it ought to be ... even
among those making some of the most stimulating contributions to the
flow of ideas."
Renowned physicists, authors of astronomy textbooks and
prominent popularizers of science have made incorrect, misleading or
easily misinterpreted statements about the expansion of the universe.
Because expansion is the basis of the big bang model, these
misunderstandings are fundamental. Expansion is a beguilingly simple
idea, but what exactly does it mean to say the universe is expanding?
What does it expand into? Is Earth expanding, too? To add to the
befuddlement, the expansion of the universe now seems to be
accelerating, a process with truly mind-stretching consequences.
What Is Expansion, Anyway?
When some familiar object expands, such as a sprained ankle or the
Roman Empire or a bomb, it gets bigger by expanding into the space
around it. Ankles, empires and bombs have centers and edges. Outside
the edges, there is room to expand into. The universe does not seem to
have an edge or a center or an outside, so how can it expand?
A good analogy is to imagine that you are an ant living on the
surface of an inflating balloon. Your world is two-dimensional; the
only directions you know are left, right, forward and backward. You
have no idea what "up" and "down" mean. One day you realize that your
walk to milk your aphids is taking longer than it used to: five minutes
one day, six minutes the next day, seven minutes the next. The time it
takes to walk to other familiar places is also increasing. You are sure
that you are not walking more slowly and that the aphids are milling
around randomly in groups, not systematically crawling away from you.
This is the important point: the distances to the aphids are
increasing even though the aphids are not walking away. They are just
standing there, at rest with respect to the rubber of the balloon, yet
the distances to them and between them are increasing. Noticing these
facts, you conclude that the ground beneath your feet is expanding.
That is very strange because you have walked around your world and
found no edge or "outside" for it to expand into.
The expansion of our universe is much like the inflation of a balloon.
The distances to remote galaxies are increasing. Astronomers casually
say that distant galaxies are "receding" or "moving away" from us, but
the galaxies are not traveling through space away from us. They are not
fragments of a big bang bomb. Instead the space between the galaxies
and us is expanding. Individual galaxies move around at random within
clusters, but the clusters of galaxies are essentially at rest. The
term "at rest" can be defined rigorously. The microwave background
radiation fills the universe and defines a universal reference frame,
analogous to the rubber of the balloon, with respect to which motion
can be measured.
This balloon analogy should not be stretched too far. From our
point of view outside the balloon, the expansion of the curved
two-dimensional rubber is possible only because it is embedded in
three-dimensional space. Within the third dimension, the balloon has a
center, and its surface expands into the surrounding air as it
inflates. One might conclude that the expansion of our
three-dimensional space requires the presence of a fourth dimension.
But in Einstein's general theory of relativity, the foundation of
modern cosmology, space is dynamic. It can expand, shrink and curve
without being embedded in a higher-dimensional space.
In this sense, the universe is self-contained. It needs neither
a center to expand away from nor empty space on the outside (wherever
that is) to expand into. When it expands, it does not claim previously
unoccupied space from its surroundings. Some newer theories such as
string theory do postulate extra dimensions, but as our
three-dimensional universe expands, it does not need these extra
dimensions to spread into.
Ubiquitous Cosmic Traffic Jam
In our universe, as on the surface of the balloon, everything
recedes from everything else. Thus, the big bang was not an explosion in space; it was more like an explosion of
space. It did not go off at a particular location and spread out from
there into some imagined preexisting void. It occurred everywhere at
once.
If one imagines running the clock backward in time, any given
region of the universe shrinks and all galaxies in it get closer and
closer until they smash together in a cosmic traffic jam--the big bang.
This traffic-jam analogy might imply local congestion that you could
avoid if you listened to the traffic report on the radio. But the big
bang was an unavoidable traffic jam. It was like having the surface of
Earth and all its highways shrink while cars remained the same size.
Eventually the cars will be bumper to bumper on every road. No radio
broadcast is going to help you around that kind of traffic jam. The
congestion is everywhere.
Similarly, the big bang happened everywhere--in the room in
which you are reading this article, in a spot just to the left of Alpha
Centauri, everywhere. It was not a bomb going off at a particular spot
that we can identify as the center of the explosion. Likewise, in the
balloon analogy, there is no special place on the surface of the
balloon that is the center of the expansion.
This ubiquity of the big bang holds no matter how big the
universe is or even whether it is finite or infinite in size.
Cosmologists sometimes state that the universe used to be the size of a
grapefruit, but what they mean is that the part of the universe we can
now see--our observable universe--used to be the size of a grapefruit.
Observers living in the Andromeda galaxy and beyond have their
own observable universes that are different from but overlap with ours.
Andromedans can see galaxies we cannot, simply by virtue of being
slightly closer to them, and vice versa. Their observable universe also
used to be the size of a grapefruit. Thus, we can conceive of the early
universe as a pile of overlapping grapefruits that stretches infinitely
in all directions. Correspondingly, the idea that the big bang was
"small" is misleading. The totality of space could be infinite. Shrink
an infinite space by an arbitrary amount, and it is still infinite.
Receding Faster Than Light
Another set of misconceptions involves the quantitative description of
expansion. The rate at which the distance between galaxies increases
follows a distinctive pattern discovered by American astronomer Edwin
Hubble in 1929: the recession velocity of a galaxy away from us (v) is directly proportional to its distance from us (d), or v = Hd. The proportionality constant, H,
is known as the Hubble constant and quantifies how fast space is
stretching--not just around us but around any observer in the universe.
Some people get confused by the fact that some galaxies do not
obey Hubble's law. Andromeda, our nearest large galactic neighbor, is
actually moving toward us, not away. Such exceptions arise because
Hubble's law describes only the average behavior of galaxies. Galaxies
can also have modest local motions as they mill around and
gravitationally pull on one another--as the Milky Way and Andromeda are
doing. Distant galaxies also have small local velocities, but from our
perspective (at large values of d) these random velocities are swamped
by large recession velocities (v). Thus, for those galaxies, Hubble's
law holds with good precision.
Notice that, according to Hubble's law, the universe does not
expand at a single speed. Some galaxies recede from us at 1,000
kilometers per second, others (those twice as distant) at 2,000 km/s,
and so on. In fact, Hubble's law predicts that galaxies beyond a
certain distance, known as the Hubble distance, recede faster than the
speed of light. For the measured value of the Hubble constant, this
distance is about 14 billion light-years.
Does this prediction of faster-than-light galaxies mean that
Hubble's law is wrong? Doesn't Einstein's special theory of relativity
say that nothing can have a velocity exceeding that of light? This
question has confused generations of students. The solution is that
special relativity applies only to "normal" velocities--motion through
space. The velocity in Hubble's law is a recession velocity caused by
the expansion of space, not a motion through space. It is a general
relativistic effect and is not bound by the special relativistic limit.
Having a recession velocity greater than the speed of light does not
violate special relativity. It is still true that nothing ever
overtakes a light beam.
Stretching and Cooling
The primary observation that the universe is expanding emerged
between 1910 and 1930. Atoms emit and absorb light of specific
wavelengths, as measured in laboratory experiments. The same patterns
show up in the light from distant galaxies, except that the patterns
have been shifted to longer wavelengths. Astronomers say that the
galactic light has been redshifted. The explanation is straightforward:
As space expands, light waves get stretched. If the universe doubles in
size during the waves' journey, their wavelengths double and their
energy is halved.
This process can be described in terms of temperature. The
photons emitted by a body collectively have a temperature--a certain
distribution of energy that reflects how hot the body is. As the
photons travel through expanding space, they lose energy and their
temperature decreases. In this way, the universe cools as it expands,
much as compressed air in a scuba tank cools when it is released and
allowed to expand. For example, the microwave background radiation
currently has a temperature of about three kelvins, whereas the process
that released the radiation occurred at a temperature of about 3,000
kelvins. Since the time of the emission of this radiation, the universe
has increased in size by a factor of 1,000, so the temperature of the
photons has decreased by the same factor. By observing the gas in
distant galaxies, astronomers have directly measured the temperature of
the radiation in the distant past. These measurements confirm that the
universe has been cooling with time.
Misunderstandings about the relation between redshift and velocity
abound. The redshift caused by the expansion is often confused with the
more familiar redshift generated by the Doppler effect. The normal
Doppler effect causes sound waves to get longer if the source of the
sound is moving away--for example, a receding ambulance siren. The same
principle also applies to light waves, which get longer if the source
of the light is moving through space away from us.
This is similar, but not identical, to what happens to the
light from distant galaxies. The cosmological redshift is not a normal
Doppler shift. Astronomers frequently refer to it as such, and in doing
so they have done their students a serious disservice. The Doppler
redshift and the cosmological redshift are governed by two distinct
formulas. The first comes from special relativity, which does not take
into account the expansion of space, and the second comes from general
relativity, which does. The two formulas are nearly the same for nearby
galaxies but diverge for distant galaxies.
According to the usual Doppler formula, objects whose velocity
through space approaches light speed have redshifts that approach
infinity. Their wavelengths become too long to observe. If that were
true for galaxies, the most distant visible objects in the sky would be
receding at velocities just shy of the speed of light. But the
cosmological redshift formula leads to a different conclusion. In the
current standard model of cosmology, galaxies with a redshift of about
1.5--that is, whose light has a wavelength 150 percent longer than the
laboratory reference value--are receding at the speed of light.
Astronomers have observed about 1,000 galaxies with redshifts larger
than 1.5. That is, they have observed about 1,000 objects receding from
us faster than the speed of light. Equivalently, we are receding from
those galaxies faster than the speed of light. The radiation of the
cosmic microwave background has traveled even farther and has a
redshift of about 1,000. When the hot plasma of the early universe
emitted the radiation we now see, it was receding from our location at
about 50 times the speed of light.
Running to Stay Still
The idea of seeing faster-than-light
galaxies may sound mystical, but it is made possible by changes in the
expansion rate. Imagine a light beam that is farther than the Hubble
distance of 14 billion light-years and trying to travel in our
direction. It is moving toward us at the speed of light with respect to
its local space, but its local space is receding from us faster than
the speed of light. Although the light beam is traveling toward us at
the maximum speed possible, it cannot keep up with the stretching of
space. It is a bit like a child trying to run the wrong way on a moving
sidewalk. Photons at the Hubble distance are like the Red Queen and
Alice, running as fast as they can just to stay in the same place.
One might conclude that the light beyond the Hubble distance
would never reach us and that its source would be forever undetectable.
But the Hubble distance is not fixed, because the Hubble constant, on
which it depends, changes with time. In particular, the constant is
proportional to the rate of increase in the distance between two
galaxies, divided by that distance. (Any two galaxies can be used for
this calculation.) In models of the universe that fit the observational
data, the denominator increases faster than the numerator, so the
Hubble constant decreases. In this way, the Hubble distance gets
larger. As it does, light that was initially just outside the Hubble
distance and receding from us can come within the Hubble distance. The
photons then find themselves in a region of space that is receding
slower than the speed of light. Thereafter they can approach us.
The galaxy they came from, though, may continue to recede
superluminally. Thus, we can observe light from galaxies that have
always been and will always be receding faster than the speed of light.
Another way to put it is that the Hubble distance is not fixed and does
not mark the edge of the observable universe.
What does mark the edge of observable space? Here again there has been
confusion. If space were not expanding, the most distant object we
could see would now be about 14 billion light-years away from us, the
distance light could have traveled in the 14 billion years since the
big bang. But because the universe is expanding, the space traversed by
a photon expands behind it during the voyage. Consequently, the current
distance to the most distant object we can see is about three times
farther, or 46 billion light-years.
The recent discovery that the rate of cosmic expansion is
accelerating makes things even more interesting. Previously,
cosmologists thought that we lived in a decelerating universe and that
ever more galaxies would come into view. In an accelerating universe,
however, we are surrounded by a boundary beyond which occur events we
will never see--a cosmic event horizon. If light from galaxies receding
faster than light is to reach us, the Hubble distance has to increase,
but in an accelerating universe, it stops increasing. Distant events
may send out light beams aimed in our direction, but this light is
trapped beyond the Hubble distance by the acceleration of the
expansion.
An accelerating universe, then, resembles a black hole in that
it has an event horizon, an edge beyond which we cannot see. The
current distance to our cosmic event horizon is 16 billion light-years,
well within our observable range. Light emitted from galaxies that are
now beyond the event horizon will never be able to reach us; the
distance that currently corresponds to 16 billion light-years will
expand too quickly. We will still be able to see events that took place
in those galaxies before they crossed the horizon, but subsequent
events will be forever beyond our view.
Is Brooklyn Expanding?
In Annie Hall, the movie
character played by the young Woody Allen explains to his doctor and
mother why he can't do his homework. "The universe is expanding.… The
universe is everything, and if it's expanding, someday it will break
apart and that would be the end of everything!" But his mother knows
better: "You're here in Brooklyn. Brooklyn is not expanding!"
His mother is right. Brooklyn is not expanding. People often assume
that as space expands, everything in it expands as well. But this is
not true. Expansion by itself--that is, a coasting expansion neither
accelerating nor decelerating--produces no force. Photon wavelengths
expand with the universe because, unlike atoms and cities, photons are
not coherent objects whose size has been set by a compromise among
forces. A changing rate of expansion does add a new force to the mix,
but even this new force does not make objects expand or contract.
For example, if gravity got stronger, your spinal cord would
compress until the electrons in your vertebrae reached a new
equilibrium slightly closer together. You would be a shorter person,
but you would not continue to shrink. In the same way, if we lived in a
universe dominated by the attractive force of gravity, as most
cosmologists thought until a few years ago, the expansion would
decelerate, putting a gentle squeeze on bodies in the universe, making
them reach a smaller equilibrium size. Having done so, they would not
keep shrinking.
In fact, in our universe the expansion is accelerating, and
that exerts a gentle outward force on bodies. Consequently, bound
objects are slightly larger than they would be in a nonaccelerating
universe, because the equilibrium among forces is reached at a slightly
larger size. At Earth's surface, the outward acceleration away from the
planet's center equals a tiny fraction (10–30)
of the normal inward gravitational acceleration. If this acceleration
is constant, it does not make Earth expand; rather the planet simply
settles into a static equilibrium size slightly larger than the size it
would have attained.
This reasoning changes if acceleration is not constant, as some
cosmologists have speculated. If the acceleration itself increased, it
could eventually grow strong enough to tear apart all structures,
leading to a "big rip." But this rip would occur not because of
expansion or acceleration per se but because of an accelerating
acceleration.
The big bang model is based on observations of expansion, the
cosmic microwave background, the chemical composition of the universe
and the clumping of matter. Like all scientific ideas, the model may
one day be superseded. But it fits the current data better than any
other model we have. As new precise measurements enable cosmologists to
understand expansion and acceleration better, they can ask even more
fundamental questions about the earliest times and largest scales of
the universe. What caused the expansion? Many cosmologists attribute it
to a process known as inflation, a type of accelerating expansion. But
that can only be a partial answer, because it seems that to start
inflating, the universe already had to be expanding. And what about the
largest scales, beyond what we can see? Do different parts of the
universe expand by different amounts, such that our universe is a
single inflationary bubble of a much larger multiverse? Nobody knows.
Although many questions remain, increasingly precise observations
suggest that the universe will expand forever. We hope, though, the
confusion about the expansion will shrink.