It is astonishing that the 鈥渂ig bang鈥� concept is now so firmly based. Cosmologists believe they know what our universe was like a few seconds after its beginning. The temperature was 10 billion degrees; radiation, protons, neutrons and 鈥渄ark matter鈥� were present everywhere in known proportions. And everything was cooling very smoothly: the deviations from uniformity amounted to no more than 1 part in 100,000. These statements are based on compelling and precise observations of 鈥渇ossils鈥� from that era: the radiation itself, and the proportions of hydrogen, helium and deuterium in the universe.
I am at least 99 per cent confident that the above inferences are correct. They are better established, and based on more precise evidence, than anything we can say about the early history of our Earth, or about the beginnings of life. As always in science, each advance brings a new set of great unknowns into sharper focus. But our uncovering of the big bang gives us cause to understand that these are not futile questions. Having discovered that we can grasp the basics of the origins of our universe, myriad other questions present themselves, questions that we can realistically hope to answer. And slowly but surely, against all odds, we are indeed managing to answer them.
Take, for example, our universe鈥檚 apparent contradiction of one of the most powerful laws of physics, the second law of thermodynamics. If our universe started off so 鈥渟imple鈥�, how did it evolve into the immensely intricate cosmos we see around us, and of which we are part? At first sight this seems to contradict the principle that there is a tendency for structures to mess up and homogenise. But there is no paradox; the answer lies in the action of gravity. A region slightly denser than average would undergo extra deceleration because of the extra gravity. It would lag behind its surroundings more and more. Density contrasts would grow. It is by this process that the slight overdensities in the early universe act as the 鈥渟eeds鈥� for galaxies today. This theory, and its validation by observations, is another triumph for astronomers. Astronomers are hoping to uncover the details of how this process unfolds by looking for 鈥測oung galaxies鈥�, objects so far away that their light set out when the universe was young and they had only recently formed.
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There is plenty left to do, of course. We still don鈥檛 know, for instance, what went bang and why. The question of why the universe was set up to expand in this way, and with this particular mix of ingredients, remains a challenge for science in the 21st century, and perhaps far beyond. The answer lies in the first tiny fraction of a microsecond, when the temperatures and densities were so high that the relevant physics is uncertain and not firmly grounded in experiment. But given our successes so far in addressing cosmological questions we can be confident about making further steps 鈥� probing the details of the very first microsecond, and perhaps even the nature of the big bang itself.
鈥�In the beginning
We still don鈥檛 know what went bang and why; or why the universe was set up to expand in this way, and with this mix of ingredients鈥�
