Why is the Universe so big?

It doesn't seem to me that this fantastically marvelous universe, this tremendous range of time and space and different kinds of animals, and all the different planets, and all these atoms with all their motions, and so on, all this complicated thing can merely be a stage so that God can watch human beings struggle for good and evil — which is the view that religion has. The stage is too big for the drama.

Richard Feynman

People frequently ask me ‘Humphrey, why is the universe so big?’. Actually that’s a lie, I wish people would quiz me like that. Invariably they ask me questions like, ‘would you like to come into your local branch of Lloyds TSB for a personal finance review?’, or ‘would you like to have wholegrain bread or Italian bread with your side salad?’; all things I have a limited opinion on and frankly couldn’t give a toss about. Here we are, spun into existence and breathed into life by a mysterious universe and all people seem to be concerned with is what motorway you take to get to Basingstoke; it’s the M3 in case you were wondering.

The short answer to the question ‘why is the universe so big?’ is that, since Erwin Hubble’s observations in the first half of the 20th century, we have come to realise that the universe is expanding and therefore its huge size is a consequence of its great age. This rate of expansion and vast expanse of time turn out to be critical for the development of complex life for a variety of reasons.

Any universe that contains the kinds of things you need to develop complexity must be sufficiently old enough for stars to form and generate the elements on which life is based. The universe throughout its history has been constantly changing and has gone through around 10-15 billion years of expansion. As the universe expanded it became a continually changing enviroment; sparser, more rariefied and cooler. As this process occurs certain conditons can then arise. The tempreture of the universe at the moment is pretty low. If we ran the tape back to when the cosmos was 300,000 years old, the conditons become very extreme and too hot for even atoms to exist. As things got cooler atoms and molecules were able to form and basic chemistry was able to begin. Once this occurred, great islands of material were able to form and get denser, until eventually their gravity became great enough to form stop them expanding. They formed great achipeligos of material where such objects as stars, planets and people could eventually form.

The first microscopic life forms appeared on earth just 3 or 4 billion years ago, making the origin of life remarkably close (in cosmic terms) to the beginning of everything 13.7 billion years ago. This emergence of life requires elements heavier than hydrogen and helium, which were formed in the first three minutes of the Big Bang. All the heavier biochemical elements, like carbon, have to made from these simpler elements by nuclear reactions in the stars. These reactions are delicately poised to produce a lot of carbon, but not too much oxygen. When stars die and explode these biochemical elements are dispersed into space and ultimately find their way into planets and into people. Its enjoyable to reflect that every carbon nucleus in your body has been through a star, probably more than once. This process of nuclear alchemy is long and slow. It takes tens of billions of years to run its course. Thus a universe that contains curious anomalies like us must be billions of years old and hence billions of light years in size due to the expansion rate.

We could not exist in a universe which was significantly smaller and even if we represent the only living creatures in the cosmos, the universe would still have to be roughly the size it is just to support us. You could have an economy sized universe the size of a galaxy (about 100 billion stars), but a universe the size of our galaxy would be only about 3 months old, which is barely enough time to pay your gas bill.

These are necessary conditions for life to be possible, but there are further advantages to a big expanding universe. The universe turns out to be a bit lumpy, in fact it has pretty much the same level of lumpiness whereever you look in the night sky at around 1 part in 100,000. In a more lumpy universe stars would form rapidly and turn into black holes, everything would be too dense and the tidal forces from other stars would disrupt our solar system. If there wasn’t enough lumpiness there wouldn’t be enough material together to form planets and stars. According to Guth’s cosmic inflation theory the universe went through a period of incredible acceleration, which smoothed out the bumps and wiggles and gave us the big smooth universe we observe today. The continued expansion of the universe ensures that it has a very low average density and so galaxies and stars are widely separated. Areas where life can develop are likely to be separated by vast astronomical distances. If there were more material it would change the rate at which the universe expands, exerting more gravity and making the universe evolve too quickly for stars and galaxies to emerge.Hence the apparent emptiness of the universe transpires to be a rather good thing.

The large amount of expansion also ensures that the universe is very cold. The temperature fall inversely to the size. This, in turn, means that the night sky appears dark as there is too little energy density in the universe to make it bright. Another curious component of the universe’s expansion is the small value of the cosmological constant (a repulsive force believed to be dark energy), which counteracts the force of gravity. If it were 10 times bigger, no complexity could exist in the universe as the acceleration would have begun too early for structure to form in the universe. As things stand the expansion will continue for ever and the universe will not end in a big crunch or a big rip.

It is an important fact to grasp that many of the aspects of the universe which appear so obviously in conflict with any interpretation of the universe as hospitable for life, turn out to be crucial features that are necessary for a universe to support complexity of any known sort.We shouldn’t be surprised to find that the universe is so big because that appears to be the only kind of universe we could in principle observe. Furthermore we should not be surprised not to observe extra-terrestrials as the distances involved are so vast. Universes that meet the necessary conditions for life are big and old, dark and cold. ‘All the worlds a stage’ as Shakespeare would say, and it has to be an unimaginably big one to have any players on it.


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