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From bang
to eternity
George
Ellis, professor of Applied Mathematics at the University of Cape Town (South Africa),
author of Before the Beginning (Bowerdean/Marion Boyar, 1993) |
Echoes of the early universe: a satellite image showing background radiation.
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A
hundred years of pioneers
1905: Albert
Einstein announces the theory of relativity.
1912: Ernest Rutherford discovers the atomic nucleus.
1924: The basic equations for quantum mechanics are established.
1929: Edwin Hubble reveals that the universe is expanding.
1950: The term “Big Bang” is coined by astronomer Fred Hoyle. It is meant as a put-down,
but it sticks.
1965: Cosmic microwave background radiation is discovered.
1981: Alan Guth presents the first version of the theory of cosmic inflation.
2000: First possible experimental evidence of Higgs Field—the force that gives mass
to particles.
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“The
whole theory of the universe is directed unerringly to one single individual—namely
to You”
Walt
Whitman, American poet (1819-1862)
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As the universe expanded and cooled,
successively larger and more complex units could form.
We expected the expansion of the universe to be slowing down because of gravity,
but in fact it seems to be accelerating. |
The
big bang is now the accepted scientific account of how the universe came into being
and started to evolve, but there is still much left to discover
Cosmology aims to determine
the nature of the universe on the largest observable scales, and then explain how
it got to be the way it now is. Dismissed for a long time as a largely philosophical
enterprise based on a few sparse observations, this branch of science has undergone
an extraordinary transformation in the last 50 years, becoming a compelling body
of knowledge about the universe, rich in data and tied to the most pioneering advances
in nuclear and particle physics.
On the one hand, the discipline relies on telescopes of all kinds and their associated
measuring instruments and computers, amplifying and analyzing the incredibly faint
radiation1 from very distant matter. Observations of apparent size, radiation fluxes
and numbers of distant galaxies and quasi-stellar objects can now be obtained up
to almost inconceivable distances. When twinned with theories of physics—namely mathematical
laws that characterize how matter and radiation behave—the result is something that
few scientists a century ago would have believed possible: a “physical standard model”
of cosmology, comprehensive enough to take us back to the first few seconds of the
universe’ existence, when atomic nuclei formed. Less defined and more speculative
science promises to take us even further, possibly back to the very threshold of
creation.
The basic structure of the visible universe on the largest scales is now well understood:
there are vast domains of empty space more or less uniformly populated by clusters
of galaxies, with each galaxy itself being a dynamic configuration of about 100 billion
stars interspersed with dust and gas.
Furthermore, the fundamental motion of the cosmos is known: a uniform expansion of
these clusters of galaxies, with distances between them ever-increasing equally in
all directions. If we extrapolate backwards in time, this movement would suggest
an ever higher density and temperature of matter and radiation, which at a certain
point in the distant past—in conditions of the most extreme heat—coupled tightly
together. Estimates of an origin to this expansion indicate it began around 10 billion
years ago.
At the extreme temperatures (over a billion degrees centigrade) of this initial phase,
matter existed only as the most elementary particles in equilibrium with radiation.
No more complex structures could survive the bombardment of the radiation at those
temperatures. But as the universe expanded and cooled, successively larger and more
complex units could form: first of all, within the very first second of the cosmos,
protons and neutrons from quarks, thus far among the most fundamental units of matter
yet known to exist. Then, only minutes after the universe began, these protons and
neutrons could combine into light atomic nuclei, a process known as nucleosythesis.
Some 300,000 years later, complete atoms were constructed from nuclei and electrons—an
episode called recombination. This event allowed the radiation, which had previously
been trapped by the floating electrons, to separate (or decouple) from matter and
flow freely for thousands of millions of light-years, cooling all the while due to
the expansion of the universe from a temperature of about 3,000 degrees Kelvin at
emission to 2.75 Kelvin (–270 degrees centigrade) today. This radiation, known as
the cosmic microwave background radiation, provides the best map we now have of the
very early universe.
Once complete atoms were formed—mainly in the shape of hydrogen and helium—gravitation
could pull matter together to form the first generation of stars, which clustered
together to form galaxies, which in turn bunched together to form clusters of galaxies.
Some of the first generation stars ended their lives in massive supernova explosions,
spreading through space the elements of organic life they had been forming in their
interiors through successive nuclear reactions. The resulting clouds of dust then
became the birthplace of second generation stars, surrounded by planets, on which
the molecules of life could find hospitable places to generate the first living cells
and so provide the origin of complex living beings (see also pp. 26-27).
There are three basic reasons for believing this picture of our universe’s history.
Firstly, estimates of the distances of galaxies (obtained for example from their
luminosities) can be correlated with estimates of the speed at which they are moving
away from us (deduced from their measured redshifts2). The data shows that the further
away the galaxies are, the faster they are receding from us, thus providing basic
evidence for the expansion of the universe. Secondly, the very existence of the cosmic
background radiation is evidence that there was a hot early state of the universe,
because its precise spectrum—exactly described by a theoretical formula deduced by
Max Planck 100 years ago—shows that matter and radiation were in equilibrium at early
times. Such equilibrium indicates that the early universe was very hot, for only
at extreme high temperature can this balance come into being.
A third piece of evidence comes from observation of the abundance of light elements
in the universe, namely hydrogen, helium and lithium. Our theory of how atomic nuclei
were formed in the hot early universe, based on knowledge of nuclear physics together
with the hypothesis of an expanding universe, fits all these measurements just so
long as the density of matter lies in a specific limited range—a remarkable confirmation
of theory by observation.
As a result, this cosmological history has come to be accepted by the scientific
community. We have clear evidence that the universe emerged at vast speed from an
initial fireball, though this event’s remoteness—and the enormity of space—obviously
leaves a host of questions waiting to be answered, particularly as we try to understand
its origins.
Recent observations have nevertheless filled in many details of the universe’s structure
and history. We have been able to obtain estimates of the amount of matter in the
universe, particularly from studies of the motions of galaxies and clusters of galaxies.
On the basis of these figures, we have been able to deduce the presence of a large
amount of mysterious “dark matter”—matter which can’t be detected by emitted radiation
such as light because it is simply not shining. By comparing these estimates of the
amount of dark matter (about 95 percent of the universe’s mass) with those coming
from the nucleosynthesis calculations mentioned above, we can deduce that most of
this matter is not composed of protons and neutrons: in short, that it has an entirely
different make-up to that of ordinary matter.
We have also been able to get much better distance estimates than before for faraway
galaxies, particularly by observing supernovae explosions in them and measuring the
decaying light from these death throes of burnt-out stars. This has led to another
unexpected discovery. We expected the expansion of the universe to be slowing down
because of the gravitational pull of all matter, but in fact it seems to be accelerating.
This must be attributed to some form of dark energy which, unlike the dark matter
referred to above, acts like a negative gravitational field, tending to make all
matter move ever faster apart. Consequently, it now seems clear that the universe
will expand forever.
Theories of how galaxies and galaxy clusters arose have also been subject to intensive
research. By connecting data on the gravitational effects and distribution of galaxies
with minute temperature fluctuations across the sky in the cosmic background radiation,
we have been able to construct broad pictures of how large-scale structures emerged
from small variations in density in the early universe.
Yet these findings leave a major question begging: how can we possibly explain why
the universe is so homogeneous (i.e. uniform) in all directions while also hosting
from very early on in its existence minute differences in density that served as
the seeds of future galaxies?
The remarkable concept of inflation—a period of extremely rapid accelerating expansion
in the very first fraction of a second of the universe’s life—potentially explains
both features. Such an enormous expansion might first have smoothed out space, before
quantum fluctuations3 in this early force created areas of marginally different densities.
Expansion, first inflationary and then decelerating, may then have spread such tiny
variations over regions the size of galaxy clusters. From these beginnings, matter
could then be pulled by gravity over billions of years into the stars and galaxies
we are now so familiar with.
Finally, some current studies of distant spectra give tantalizing hints that the
nature of physics itself may be different at great distances, in places whose radiation
emissions we are receiving billions of years after they were emitted. Might it be
that the constants of nature vary with time? If so, this would be a discovery of
groundbreaking significance.
The next few years and decades are certain to see a massive extension in the quantity
and quality of cosmic observations. These will be accompanied by enriched theories
of how matter clustered to form galaxies and a deeper exploration of gravity, both
of which will help determine the model that best fits our observable region of the
universe.
But even once this model is drawn, a host of elusive issues await. How should we
link what we understand of quantum gravity4 to cosmological theory—especially the
creation of the universe? What of the possibility that the laws of nature were different
in the early universe? And how common is life in the universe? Could one in any other
way create a universe allowing intelligent life to exist?
This sets the framework for considering major philosophical issues within the context
of the uniqueness of the universe. Science per se can never resolve these issues,
but it can at least provide an ever clearer physical setting in which to view them.
1. Radiation
is the flow of energy via subatomic particles. The term comprises radio waves, microwaves,
infrared rays, visible light, X rays and gamma rays.
2. Light moves to the red side of the visible spectrum when its source is moving
away. This phenomenon is called the redshift.
3. According to quantum mechanics—the science of energy and particles at subatomic
levels—energy waves will tend to fluctuate randomly.
4. Quantum gravity is the as yet unknown theory of how gravity works at the quantum
level. The theory is believed to have held at the moment of the universe’ creation. |
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The
way ahead for cosmic science
What are the
major issues that remain for us to tackle? First, we want to know more about the
geometry of the universe, both inside and outside the limits we can observe. The
part of the universe we can see seems to be remarkably simple on the largest scales,
being spatially homogeneous and isotropic (it looks the same in all directions).
But the major parameters describing this region are only loosely known. Uncertainty
in our estimates of the universe’s age is about 20 percent, and needs to be improved,
as do our estimates of the dark energy that is causing an accelerated expansion of
the universe. We also want to know if sections of space close up on themselves, and
if so, whether the scale of closure is such that we live in a “small universe” where
we see multiple mirror images of the same galaxies (see pp. 24-25).
Second, we want to know more about what the universe is made of. It is disappointing
that we don’t know what kind of matter makes up the lion’s share of the density of
the universe, nor the nature of the force that presently dominates its expansion.
Better understanding of these features is intimately tied in to a better understanding
of the creation of large-scale structure in the universe.
Third, we want to understand the very early universe better: in particular, what
caused the mighty cosmic inflation? What came before inflation? What was the nature
of creation, and what alternatives are there to creation?
Though there are a wide variety of competing proposals, it proves very difficult
to test them experimentally. To help resolve these, we need to extend our understanding
of particle physics as far as we can so as to probe the interactions at work at the
moment of creation and immediately thereafter. However far we extend experiments
probing this physics, we will nevertheless be unable to attain the energies required
to unlock experimentally the secrets of quantum gravity.
There are thus clear limits to the testable laws of physics that underlie the cosmological
account of creation. The challenge is to develop a coherent and convincing physical
theory that is supported by tests insofar as they are possible.
Fourth, we must grapple with the question of how to relate theory to observation
in the exceptional context of a science with only one object of study—the single
existing Universe. We lack a proper account of the limits of scientific proof in
this context. One attempt to break this impasse is through the idea of an ensemble
of universes (a “multiverse”), but it is not clear yet if this is a physical or a
metaphysical proposal.
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