Existence: Where did we come from?

Source: http://www.newscientist.com/section/life

WHY are we here? Where did we come from? According to the Boshongo people of central Africa, before us there was only darkness, water and the great god Bumba. One day Bumba, in pain from a stomach ache, vomited up the sun. The sun evaporated some of the water, leaving land. Still in discomfort, Bumba vomited up the moon, the stars and then the leopard, the crocodile, the turtle, and finally, humans.

This creation myth, like many others, wrestles with the kinds of questions that we all still ask today. Fortunately, as will become clear from this special issue of New Scientist, we now have a tool to provide the answers: science.

When it come to these mysteries of existence the first scientific evidence was discovered about 80 years ago, when Edwin Hubble began to make observations in the 1920s with the 100-inch telescope on Mount Wilson in Los Angeles County.

To his surprise, Hubble found that nearly all the galaxies were moving away from us. Moreover, the more distant the galaxies, the faster they were moving away. The expansion of the universe was one of the most important intellectual discoveries of all time.

This finding transformed the debate about whether the universe had a beginning. If galaxies are moving apart now, they must therefore have been closer together in the past. If their speed had been constant, they would all have been on top of one another billions of years ago. Was this how the universe began? At that time many scientists were unhappy with the universe having a beginning because it seemed to imply that physics had broken down.

One would have to invoke an outside agency, which for convenience one can call God, to determine how the universe began. They therefore advanced theories in which the universe was expanding at the present time, but didn't have a beginning. Perhaps the best known was proposed in 1948, and called the steady state theory.

According to this theory, the universe would have existed for ever and would have looked the same at all times. This last property had the great virtue of being a prediction that could be tested, a critical ingredient of the scientific method. And it was found lacking.

Observational evidence to confirm the idea that the universe had a very dense beginning came in October 1965, with the discovery of a faint background of microwaves throughout space. The only reasonable interpretation is that this background is radiation left over from an early hot and dense state. As the universe expanded, the radiation would have cooled until it is just the remnant we see today.

Theory backed this idea too. With Roger Penrose I showed that if Einstein's general theory of relativity is correct, there would be a singularity, a point of infinite density and space-time curvature, where time has a beginning.

The universe started off in the big bang, expanding faster and faster. This is called inflation and it turns out that inflation in the early cosmos was much more rapid: the universe doubled in size many times in a tiny fraction of a second.

Inflation made the universe very large and very smooth and flat. However, it was not completely smooth: there were tiny variations from place to place. These variations caused minute differences in the temperature of the early universe, which we can see in the cosmic microwave background.

The variations mean that some regions will be expanding slightly less fast. The slower regions eventually stop expanding and collapse again to form galaxies and stars. And, in turn, solar systems.

We owe our existence to these variations. If the early universe had been completely smooth, there would be no stars and so life could not have developed. We are the product of primordial quantum fluctuations.

As will become clear (see "Existence special: Cosmic mysteries, human questions"), many huge mysteries remain. Still, we are steadily edging closer to answering the age-old questions. Where did we come from? And are we the only beings in the universe who can ask these questions?