So
far I have tried to describe how inflation works, but now I would like to
explain the reasons why many scientists  including certainly myself  believe
that inflation really is the way that our observed universe began. There are six reasons that I will discuss,
starting with some very general ideas and then moving to more specific ones.
The first
reason is the obvious statement that the universe contains a tremendous amount
of mass. It contains about 10^{90}
particles within the visible region of the universe. I believe that most nonscientists are somewhat puzzled to hear
anyone make a fuss over this fact, since they think, Of course the universe is
big  it's the whole universe!
However, to a theoretical cosmologist who is hoping to build a theory to
explain the origin of the universe, this number seems like it could be an
important clue. Any successful theory
of the origin of the universe must somehow lead to the result that it contains
at least 10^{90} particles. The
fundamental theory on which the calculation is based, however, presumably does
not contain any numbers nearly so large.
Calculations can of course lead to factors of 2 or π, but it would take very many factors of
2 or π
to reach 10^{90}. Inflation, however, leads to exponential
expansion, and that seems to be the easiest way to start with only small
numbers and finish with a very large one.
With inflation the problem of explaining why there are 10^{90}
or more particles is reduced to explaining why there were 100 or more doubling
times of inflation. The number 100 is modest enough so that it
can presumably arise from parameters of the underlying particle physics and/or
geometric factors, so inflation seems like just the right kind of theory to
explain a very large universe.
The
second reason is the Hubble expansion itself  the fact that the universe is
observed to be in a state of uniform expansion. An ordinary explosion, like TNT or an atomic bomb, does not lead
to expansion that is nearly uniform enough to match the expansion pattern of
the universe. But the gravitational
repulsion of inflationary models produces exactly the uniform expansion that
was first observed by Edwin Hubble in the 1920s and 30s.
Third,
inflation is the only theory that we know of that can explain the homogeneity
and isotropy of the universe  that is, the uniformity of the universe. This uniformity is observed most clearly by
looking at the cosmic microwave background radiation, which we view as the
afterglow of the heat of the big bang. The
intensity of this radiation is described by an effective temperature, and it is
observed to have the same temperature in every direction to an accuracy of
about one part in a hundred thousand, after we correct for our own motion
through the cosmic background radiation.
In other words, this radiation is incredibly smooth. As an analogy we
can imagine a marble that has been ground so smoothly that its radius is
uniform to one part in a hundred thousand.
The marble would then be round to an accuracy of about a quarter of the
wavelength of visible light, about as precise as the best optical lenses that
can be manufactured with presentday technology.
In the standard
big bang theory there is no explanation whatever for this uniformity. In fact, one can even show that within the
context of the standard big bang theory, no explanation for this uniformity is
possible. To see this, we need to
understand a little about how this cosmic background radiation originated. During the first approximately 300,000 years
of the history of the universe, the universe was hot enough so that the matter
was in the form of a plasma  that is, the electrons were separated from the
atoms. Such a plasma is very opaque to
photons, which are constantly scattered by their interactions with the free
electrons. Although the photons move of
course at the speed of light, they change directions so rapidly that they
essentially go nowhere. During the
first 300,000 years of the history of the universe, the photons were
essentially pinned to the matter.
But
after 300,000 years, according to calculations, the universe cooled enough so
that the plasma neutralized. The free
electrons combined with the atomic nuclei to form a neutral gas of hydrogen and
some helium, which is very, very transparent to photons. From then on these photons have traveled in
straight lines. So, just as I see an
image of you when I observe the photons coming from your face, when we look at
the cosmic background radiation today we are seeing an image of the universe at
300,000 years after the big bang. Thus,
the uniformity of this radiation implies that the temperature must have been
uniform throughout this whole region by 300,000 years after the big bang.
To
think about whether the temperature of the observed universe could have
equilibrated by this early time, we could try to imagine fancifully that the
universe was populated by little purple creatures, whose sole purpose in life
was to make the temperature as uniform as possible. We could imagine that each purple creature was equipped with a
little furnace, a little refrigerator, and a cell phone so that they could
communicate with each other. The
communication, however, turns out to be an insurmountable problem. A simple calculation shows that in order for
them to achieve a uniform temperature by 300,000 years, they would need to be
able to communicate at about a hundred times the speed of light. But nothing known to physics allows
communication faster than light, so even with dedicated purple creatures we
could not explain the uniformity of the cosmic background radiation.
So
in the standard version of the big bang theory, before inflation is introduced,
one simply has to hypothesize that the
universe started out uniform. The
initial uniformity would then be preserved, since the laws of physics are by
assumption the same everywhere. This
approach allows one to accommodate the uniformity of the universe, but it is
not an explanation.
Inflation
gets around this problem in a very simple way. In inflationary theories the
universe evolves from a very tiny initial patch. While this patch was very small, there was plenty of time for it
to become uniform by the same mechanism by which a slice of pizza sitting on
the table cools to room temperature: things tend to come to a uniform
temperature. Once this uniformity is
established on the scale of the very tiny patch, inflation can take over and
magnify the patch to become large enough to encompass everything that we
observe. Thus, inflation provides a
very natural explanation for the uniformity of the universe.
Reason
number four is known as the flatness problem.
It is concerned with the closeness of the mass density of our universe
to what cosmologists call the critical
density. The critical
density is best defined as that density which would cause the universe to be
spatially flat. To understand what this
means, one must understand that, according to general relativity, the geometry
of space is determined by the matter that it contains. If the mass density of the universe is very
high, the space will curve back on itself to form a closed universe, the
threedimensional analogue of the twodimensional surface of a sphere. The sum of the angles in a triangle would
exceed 180^{o}, and parallel lines would meet if they are extended. If the mass
density is very low, the space would curve in the opposite way, forming an open
universe. The sum of the angles in a
triangle would then be less than 180^{o}, and parallel lines would diverge if they
were extended. But with just the right
mass density (for a given expansion rate) the spatial geometry will be exactly
Euclidean, just like what we all learned in high school  180^{o} in every
triangle, and parallel lines remain parallel no matter how far they are
extended. This borderline case is the
critical density.
Cosmologists
use the Greek letter Ω (Omega) to denote the ratio of the average mass
density of the universe to the critical density:
Today there is
growing evidence that Ω is equal to one to within about 10%,but the issue is not completely settled.
For purposes of this discussion, therefore, I will begin with the
uncontroversial statement that Ω lies somewhere in the interval between
0.1 and 2.
You would
probably not expect that much could be concluded from such an noncommittal
starting point, but in fact it tells us a lot.
When one looks at the equations describing the evolution of the
universe, it turns out that Ω = 1 is an unstable equilibrium point, like
a pencil balanced on its tip. If the
pencil is started exactly vertical and stationary, the laws of Newtonian
mechanics imply that it will remain vertical forever. But if it is not absolutely vertical, it will rapidly start to
fall in whichever direction it is leaning.
Similarly, if Ω began exactly equal to 1, it would remain 1
forever. But any deviation from 1 will
grow rapidly as the universe evolves.
Thus, for Ω to be anywhere in the ballpark of 1 today, it must
have started extraordinarily close to 1.
For example, if we extrapolate backwards to 1 second after the big bang,
Ω must have been 1 to an accuracy of 15 decimal places. While 1 second may sound like an
extraordinarily early time at which to be discussing an 11 to
16billionyearold universe, cosmologists have pretty much confidence in the
extrapolation. The nucleosynthesis
processes, which are successfully tested by present measurements of the
abundances of the light chemical elements, where already beginning at 1 second
after the big bang.
In fact,
particle theorists like myself are attempting to push the history of the
universe back to what we call the Planck time, about 10^{43} seconds,
which is the era when quantum gravity effects are believed to have been
important. Since our understanding of
quantum gravity is still very primitive, we generally make no attempt to
discuss the history of the universe at earlier times. But if we attempt to extrapolate the history of the universe back
to the Planck time without invoking inflation, we find that Ω at the
Planck time must have been equal to 1 to an accuracy of 58 decimal places.
Without
inflation, there is no explanation for the initial value of Ω. The big bang theory is equally
selfconsistent for any initial value of Ω, so one has no a priori reason
to prefer one value over another. But if
the theory is to agree with observation, one must posit an initial value of
Ω that is extraordinarily close to 1.
With inflation,
on the other hand, during the brief inflationary era the evolution of Ω
behaves completely differently. Instead
of being driven away from 1, during inflation Ω is driven very strongly
towards 1. So with inflation you could
assume that Ω started out as 1, 2, 10, 10^{3}, or 10^{6}. It doesn't matter. As long as there was enough inflation, then Ω would have
been driven to 1 to the extraordinary accuracy that is needed.
The fifth
reason for believing the inflationary description is the absence of magnetic
monopoles. Grand unified particle
theories, which unify all the known particle interactions with the exception of
gravity, predict that there should be stable particles that have a net magnetic
charge. That is, these particles would
have a net north pole or a net south pole, which is very different from an
ordinary bar magnet which always has both a north pole and a south pole. These magnetic monopoles are, according to
our theories, extraordinarily heavy particles, weighing about 10^{16}
times as much as a proton. In the
traditional big bang theory, without inflation, they would have been copiously
produced in the early universe. If one
assumes a conventional cosmology with typical grand unified theories, one
concludes that the mass density of magnetic monopoles would dominate all other
contributions by an absurdly large factor of about 10^{12}. Observationally, however, we don't see any
sign of these monopoles.
Cosmic
inflation provides a simple explanation for what happened to the monopoles: in
inflationary models, they can easily be diluted to a negligible density. As long as inflation happens during or after
the era of monopole production, the density of monopoles is reduced effectively
to zero by the enormous expansion associated with inflation.
The sixth and
final reason that I would like to discuss for believing that the universe
underwent inflation is the prediction that the theory makes for the detailed
structure of the cosmic background radiation.
That is, inflation makes very definite predictions not only for the
uniformity that we see around us, but it also predicts that there should be
small deviations from that uniformity due to quantum uncertainties. The magnitude of these deviations depends on
the details of the underlying particle theory, so inflation will not be able to
predict the magnitude until we really understand the particle physics of very
high energies. However, the shape of
the spectrum of these nonuniformities  i.e., the way that the intensity varies
with wavelength  depends only slightly on the details of the particle
physics. For typical particle theories,
inflationary models predict something very close to what is called the
HarrisonZel'dovich, or scaleinvariant spectrum. These nonuniformities are viewed as the seeds for the formation
of structure in the universe, but they can also be seen directly in the nonuniformities
of the cosmic background radiation, at the level of about one part in 100,000.
Fig. 1 shows a graph of the recent data from the Boomerang experiment, plotted against a theoretical curve
derived for an inflationary model.
Contributed by: Dr. Alan Guth
