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SPACE + FLIGHT
How Big Is the Universe?
How big is the universe? Could it be infinitely large? If the universe has an
edge, what is beyond the edge? And if the universe had a beginning, what
was going on before that? In this article, cosmologist Brent Tully tackles such
big questions and describes recent research that has altered our
fundamental view of the universe.
BY BRENT TULLY SATURDAY, NOVEMBER 11, 2000
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There are billions of galaxies in the visible universe.NOVA/NCSA
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Our experience of the everyday world does not prepare us to grasp the concept of an
infinite universe. And yet, trying to imagine that the cosmos actually has a boundary
does not make things any easier.
The Visible Universe
There is an edge to what we are able to see and could ever possibly see in the universe.
Light travels at 300,000 kilometers per second (186,000 miles per second). That’s top
speed in this universe—nothing can go faster—but it’s relatively slow compared to the
distances to be traveled. The nearest big galaxy to our Milky Way, the Andromeda
galaxy, is two million light-years away. The most distant galaxies we can now see are 10
or 12 billion light-years away. We could never see a galaxy that is farther away in light
travel time than the universe is old—an estimated 14 billion or so years. Thus, we are
surrounded by a “horizon” that we cannot look beyond—a horizon set by the distance
that light can travel over the age of the universe.
This horizon describes the visible universe—a region some 28 billion light years in
diameter. But what are the horizons of a civilization that inhabits the most distant
galaxies we see? And what about galaxies at the limits of their vision? There is every
reason to think that the universe extends a long way beyond the part of the universe we
can see. In fact, a variety of observations suggest that our visible patch may be a small
fraction—maybe an infinitely small fraction—of the whole universe.
This view of the universe fits with the currently popular idea that the universe began
with a vast expansion of size. The idea describes a kind of undirected energy present in
the vacuum of space, called scalar fields, that somehow got channeled into a process
called “inflation.” By conservative estimates, the universe expanded so much during this
period that something the size of an atom inflated to the size of a galaxy.
While we can never directly “see” the whole of the universe or
glimpse its farthest horizons, we can discover how it is behaving
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If this grand idea is correct, then the universe is larger than we ever could have
imagined. But the question remains: Is there a boundary, and if so, what lies in the voids
beyond? The answer, according to some cosmologists, is truly mind-boggling. If the
universe sprung forth in this manner, then probably inflation has occurred in other
places, perhaps an infinite number of places, beyond our horizon and outside of our
time. The implication is that there are other universes, perhaps similar to ours or vastly
different, each in its own space and begun in its own time.
The universe began with a vast explosion that generated space and time—a hard thing to
visualize. This image comes from a simulation of the Formation of Galaxies and Large Scale
Structure by Michael Norman, Brian O’Shea and Greg Bryan, Grand Challenge Cosmology
Consortium (GC3), and visualized by Donna Cox, Stuart Levy, and Robert Patterson,
NCSA/UIUC.
NCSA/UIUC
Cosmic Acceleration
Inflation implies a vastly expanded concept of what the universe is. But the concept is
also helping us to understand the universe we see around us. Take, for example, the
recent observation that the universe is not only expanding—a fact astronomers have
known for over seven decades—but actually accelerating outward. That discovery is the
subject of NOVA’s program “Runaway Universe.”
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While we can never directly “see” the whole of the universe or glimpse its farthest
horizons, we can discover how it is behaving—how fast it’s growing, whether its growth
will one day come to a halt, and what forces have been driving its evolution on the
largest of scales. The evidence for the cosmic acceleration—the observations of distant
exploding stars called supernovae (see Birth of a Supernova)—provides a window onto
these behaviors.
The discovery of cosmic acceleration was made by examining the light of supernovae.
We astronomers believe we know the intrinsic brightness of a particular kind of
supernovae, called “Type Ia,” so we can calculate how far such an object must be from
us by its apparent, or measured, brightness. We also know how fast the supernovae—
and the galaxies they’re in—are rushing away from us by measuring their “redshift.”
Redshift refers to a color shift in the light of galaxies toward the red end of the
spectrum as they race away from us. The faster a galaxy is moving away, the redder its
light becomes. (For more on this phenomenon, go to Moving Targets.)
What we are looking for in this combination of redshift and distance is the “growth rate”
of the universe going back in time. This growth rate tells us about the gravity of all the
matter in the universe—if there is a lot of matter it will slow down the growth rate over
time.
Take the case of a universe with so much matter that gravity arrests the expansion and
everything finally collapses in on itself. We call that a “closed” universe. In such a
universe, the expansion would have once been much faster. To get to the separations
between galaxies that we see now would have taken a relatively short time. Granted, the
numbers associated with “relatively short” might still seem daunting.
A second possibility might be a universe that is practically empty, often called an “open”
universe. Yes, there must be enough stuff in it to permit the existence of observers like
us, but suppose the total amount of matter has negligible gravitational influence on the
expansion. This universe is just cruising at the same expansion rate now as in the past.
Compared with the first possibility, the closed universe, expansions in the past would
have to have been slower to get the presently observed separations between galaxies.
And it would mean that a distant supernova observed to be rushing away from us at
such-and-such a speed (redshift) is farther away in this case, compared to the dense,
closed universe case. In the closed universe case, since expansion was faster in the
past, one doesn’t have to go so far away (back in time) to arrive at a specified redshift.
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A Puzzling Expansion
So does either of these possibilities describe our universe? No! The one that comes
closest is the “open” universe. However, the supernovae are too faint—that is, they are
so far away that even that model doesn’t allow the supernovae to travel as far away as
astronomers observe. Our universe, the real one, must have been loitering after its initial
inflationary period, but then put its foot to the accelerator recently to produce the
present separations of galaxies.
Whatever could produce that acceleration? Certainly there is nothing in our Earthly
experience that prepares us for such a possibility. This is where the theory of inflation
comes into play. Now about two decades old, inflation entertains the idea that there is a
kind of energy that causes space to expand. This energy competes with gravity, though
certainly not on local scales. However, should this form of energy come to dominate,
watch out! While gravity tries to crush, this energy—call it vacuum energy, or the scalar
field, or the energy represented by the Cosmological Constant in Einstein’s equations
describing the dynamics of the universe—tries to expand the fabric of space, pushing
everything apart. The basic proposition of the inflation model is that this form of energy
once dominated gravity and caused our universe to burst forth.
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We can actually make a direct measurement of the mass density of
the universe by looking at the motions of galaxies
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It turns out that the basic inflation picture satisfies a number of observed facts about
the universe. One fact is particularly interesting because the better our observations
become the more tightly they agree with a prediction of the inflation model. This is that
the universe should be “flat”—no overall curvature of space. Spectacularly convincing
evidence—recent measurements of irregularities in the microwave background
radiation—supports this proposition.
Microwave messages
The microwave radiation comes to us from the time in the past when the universe was a
primordial fireball. We see a “surface” like we see the “surface” of the sun. We can’t look
into the sun (or a cloud in the sky) because of scattering of light. As with the sun and its
spots, the surface of last scattering of the primordial fireball had structure caused by
localized regions that were hotter or cooler, less or more dense.
The most pronounced of these structures at the cosmological surface of last scattering
were governed by the distance that acoustic (pressure) waves could travel in the age of
the universe back then, when the universe was about a half million years old. The size of
these irregularities gives us a ruler! The radiation was emitted so long ago, so far away,
that it has been redshifted down to millimeter wavelengths. So now millimeter
experiments determine the angular size of the clumps caused by acoustic oscillations in
the cooling universe at the surface of the last scattering.
We know how big the clumps were—a couple hundred thousand light years across. The
relation between their real size, their distance, and the angular size that we observe is
governed by the geometry of the universe. A universe dense with matter will distort the
final size one way, an empty or almost empty universe will distort another way, and the
flat universe of the inflation model will produce yet a different image, which we would
intuitively call undistorted. Lo and behold, the results are in agreement with the flat
universe of inflation.
Dark energy … is the delight of physicists because it might provide a
laboratory for the moment of creation.
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This is not the full story. The theory of inflation predicts a precise recipe of how
structure would form from little things merging into big things and tells us how many
little things there should be for each big thing. The observations match with
expectations if the mix of energy and matter is the same as that suggested by the
supernovae experiments. Inflation also solves the old controversy over the Hubble
Constant, the relationship between the rate galaxies are flying apart and the distances
between them. If the Hubble Constant is large then galaxies are relatively close together
and the implied age of the universe is way too short if the universe has been briskly
expanding.
The universe cannot be younger than things in it. However, if the universe has been
loitering and is now accelerating, then it is old enough and a large Hubble Constant is
still possible. And we can actually make a direct measurement of the mass density of
the universe by looking at the motions of galaxies that slosh in the gravitational wells of
the matter. We find something that has come to be called “dark matter” there. If the
universe is “flat,” then this state is achieved through the sum of the mass and energy
density. Measurements of gravity perturbations reveal just the needed complement of
matter offsetting the repulsive energy indicated by the supernova measurements.
Dark Mysteries
The last couple of years have seen a remarkable convergence of evidence, all
suggesting that we live in a universe with a few percent of the normal matter of our
everyday experience, maybe 25% of something called “dark matter,” which is a name
given to hide our ignorance of what it is, and 75% of this energy that wants to push
space apart—call it “dark energy.” If true, then relatively recently in the history of the
universe the “dark energy” has become dominant over “dark matter.” During the
transient dominance of dark matter, it caused the collapse into all the structure of the
universe that we have come to know and appreciate.
Maybe we should be less enamored of dark energy. But it is the delight of physicists
because it might provide a laboratory for the moment of creation. It may be that the
present source of repulsion is quite different from the primordial situation. Certainly the
energy density levels and time scales are vastly different. However, if we can
understand the mechanism of the present acceleration perhaps we can get a clue
about the acceleration at the first instant of our time.
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A complicated scenario indeed! So how big is the universe in the inflation model? It
begs the question of what is going on at the boundaries and whether information could
be communicated across universes. We suppose not. It may well be that only a tiny part
of even our own universe is in our horizon, within the domain that we might hope to
know.
Editor’s Notes
This feature originally appeared on the site for the NOVA program Runaway Universe.
Receive emails about upcoming NOVA programs and related content, as
well as featured reporting about current events through a science lens.
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