Feature

A Baby Universe

Cosmologists are coming to grips with the new-born universe.

Richard Easther and Michael Lauren

Imagine that you could go back to the beginning of time, and take a picture of what you saw. What would the universe look like? There would be no planets, no stars, no galaxies.

Cosmology, the branch of science that studies the evolution of the Universe, is not as straightforward as palaeontology, where fossil bones provide direct evidence of the past, or even geology, where rocks can be used to look back into the earth's distant history.

The universe is much older than the Earth, so the search for its origins must turn to space. There, with the aid of small satellite, named COBE (for Cosmic Background Explorer), scientists have produced amazing pictures they call "the baby photo of the universe." Like people, these "baby photos" show an almost unblemished skin, with bumps and wrinkles that have grown more pronounced as the universe has aged.

The COBE mission is one of a series of developments, both experimental and theoretical, that are reshaping the way scientists understand the evolution of the universe, from the fiery cauldron of the Big Bang, to the place we see today, containing stars, galaxies, planets, and people.

The COBE satellite has been accumulating data for the last four years, but the job of analysing the data is less than half finished. As well, there are many new experiments under way to probe the beginnings of time, some at stations at the South Pole, one on a mountain top at Tenerife, and others carried high into the atmosphere by balloons. More surprises still await scientists.

Virtually all scientists now agree the universe began about 15 billion years ago in an explosion known as the "Big Bang", and is expanding with age. The initial picture of the Big Bang that existed in the 1930s gave only the broad outline of what has grown into a detailed and concrete picture of the early universe. Cosmologists' most recent ideas about the primordial universe centre around what is known as inflationary cosmology.

Putting the Big
with the Bang

The idea of inflation, invented by American physicist Alan Guth in 1981 (although others, including Russian dissident Andrei Sakharov, had discussed similar ideas), filled in many of the details brushed over in the earlier versions of the Big Bang. In a sense, inflation is what puts the big into the Big Bang. Inflation supposes that the universe underwent a massive growth spurt while it was extremely young. Without this rapid growth, the entire visible universe would be about the size of a grapefruit.

The rapid growth arose from the way a special particle -- called a Higgs particle -- interacts with gravity. In most inflationary models, the early universe is dominated by these Higgs particles. Eventually, the universe became dominated by ordinary matter, and grew more sedately. With the advent of inflationary ideas, cosmologists began to form a detailed picture of the universe when it was a billion, billion, billion, billionth of a second old.

Quantum mechanics says that on a microscopic scale, particles become slightly "fuzzy", due to random quantum oscillations. Normally, this fuzziness is restricted to the sub-atomic realm. These quantum oscillations were stretched by the massive expansion of space that occurred during inflation until they reached from one side of the universe to the other. When inflation ended, they were frozen into the universe. They became lumps that eventually grew into galaxies and clusters of galaxies.

Most cosmologists believe that these are the ripples that have been discovered by the COBE satellite pictures. The exact size and shape of the ripples depends on the details of the inflationary era -- and with this knowledge, theoretical predictions are beginning to face experimental tests.

Much of the flavour of modern cosmology has its origin in the General Theory of Relativity, published by Einstein in 1915. At its most basic level, General Relativity is a theory of gravity that supersedes the ideas conceived by Isaac Newton. In Newtonian physics, space and time are something like a stage that actors in a drama move about on. General Relativity hitches space and time together, into spacetime. Spacetime itself evolves, and interacts with the matter in it. Rather than being a static arena for the action to happen in, spacetime has become one of the actors in the drama. It is this freedom that allows cosmologists to think about an expanding universe.

Galaxies Running Away

The first person to suggest that the universe might be expanding was a Russian physicist, Alexander Friedmann, in 1922, but at the time there was no proof for the idea. Edwin Hubble (after whom the Hubble Space Telescope is named) announced in 1929 the discovery that virtually all the galaxies in the universe were rushing way from our own galaxy, the Milky Way. Shortly afterwards, a Belgian priest, Georges Lemaitre, who had not heard of Friedmann's work, rediscovered the idea of an expanding universe. He pointed out that Hubble's discovery made no sense in a universe that was not growing, but fitted naturally into an expanding model. In an expanding universe, the galaxies behave as if they are drawn on the surface of a balloon being blown up.

The next major development in the understanding of the origins of the universe came after World War II, when George Gamow and his co-workers started to think about what the early universe might have been like. When gas expands, it cools. This is the principle that is used in fridges, and a similar process operates in the universe as it expands. Since the universe is getting bigger, it is also getting colder. Thus, in the past the universe must have been much hotter than it is today. Hotter, in fact, than the centre of the sun, or any other star. At these temperatures, the elementary particles present are very energetic, and move about with such force that they cannot form heavy particles or atoms. The early universe was a sea of charged particles.

Glowing Hot

As well as matter, the universe is full of photons -- the particles that make up light. Our eyes can detect photons from visible light, but many other photons are invisible to us, such as those of radio waves, X-rays, microwaves and infra-red light. All objects glow with a light that depends on their temperature, which is why a coal in a fire glows red. At most temperatures objects glow with types of "light" we cannot see.

While the universe has cooled dramatically since the Big Bang, to about -270^oC, it is still glowing, not with ordinary light, but with microwaves. This fossil light, the afterglow of the huge explosion that marked the birth of the universe, was first detected in 1965. It has a very interesting feature. Since the early Universe was a sea of charged particles, light could not move easily through it. However, once the temperature dropped to a few thousand degrees, the electrons and nuclei in the universe could join to form atoms, which have no overall charge.

When this happened, about 300,000 years after the Big Bang, the universe became transparent overnight. This means that the microwave background has had nothing to do with the universe since then. The universe is 15 billion years old, but the photons that form the microwave background have not interacted with the universe for almost all that time. It is these photons that the COBE satellite is patiently observing, mapping out the microwave background in every part of the sky.

But these microwaves are like people who emigrated from New Zealand 40 years ago, and have not kept in touch with subsequent developments. The COBE satellite hears a story -- not about the universe today -- but about the universe when it was 50,000 times younger than it is now. What it shows is a universe not with a perfectly smooth skin, but one with the tiniest of ripples. Possibly the biggest problem that cosmologists hadn't answered before this latest data was "where do galaxies come from?"

Galaxies are huge clumps of stars. The galaxies themselves form patterns in the sky called clusters and super-clusters. Once a lump forms in the universe, its gravity causes it to pull in more matter around it, causing it to get bigger. What the COBE satellite has done is provide a map of the initial wrinkles that then seeded the formation of galaxies and clusters of galaxies. The information the COBE satellite gives has put many theories about the origins of the patterns of galaxies we see in the sky to the test, ruling some out and strengthening others.

Dr Richard Easther is an astrophysicist and Michael Lauren is a science writer.

Dr Richard Easther is an astrophysicist.
Michael Lauren is a science writer.