From bang to eternity - cosmology - Statistical Data Included

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 radiation [1] 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 redshifts [2]). 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 fluctuations [3] 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 ground breaking 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 gravity [4] 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 unique ness 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.

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.

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.

COPYRIGHT 2001 UNESCO
COPYRIGHT 2004 Gale Group