Why the
Universe is so large
The tremendous timespans involved in biological evolution offer a new perspective on
the question 'why is our
Universe so big?' The emergence of human life here on Earth has taken 4.5 billion years. Even before our Sun and its planets could form, earlier stars must have transmuted pristine hydrogen into carbon, oxygen and the other atoms of the periodic table. This has taken about ten billion years. The size of the observable Universe is, roughly, the distance travelled by light since the Big Bang, and so the present visible Universe must be around ten billion light-years across.
The galaxy pair NGC 6872 and IC 4970 indicate the vastness of the Universe. Light from the bright foreground star has taken a few centuries to reach us; the light from the
galaxies has been travelling for 300 million years. The Universe must be this big - as measured by the cosmic number N - to give intelligent life time to evolve. In addition, the cosmic numbers omega and Q must have just the right values for galaxies to form at all.
This is a startling conclusion. The very hugeness of our Universe, which seems at first to signify how unimportant we are in the cosmic scheme, is actually entailed by our existence! This is not to say that there couldn't have been a smaller universe, only that we could not have existed in it. The expanse of cosmic space is not an extravagant superiority; it's a consequence of the prolonged chain of events, extending back before our Solar System formed, that preceded our arrival on the scene.
This may seem a regression to an ancient 'anthropocentric' perspective - something that was shattered by Copernicus's revelation that the Earth moves around the Sun rather than vice versa. But we shouldn't take Copernican modesty (some-times called the 'principle of mediocrity') too far. Creatures like us require special conditions to have evolved, so our perspective is bound to be in some sense atypical. The vastness of our universe shouldn't surprise us, even though we may still seek a deeper explanation for its distinctive features.
Cosmology comes of age
The physicist Max Born once claimed that theories are never abandoned until their proponents are all dead - that science advances 'funeral by funeral'. But that's too cynical. Several long running cosmological debates have now been settled; some earlier issues are no longer controversial. Many of us have often changed our minds - I certainly have.
D = 3
The first crucial number is the number of spatial dimensions: we live in a three-dimensional Universe. Life couldn't exist if D were two or four. Time is a fourth dimension, but distinctively different from the others in that it has a built-in arrow: we 'move' only towards the future.
Cosmological ideas are no longer any more fragile and evanescent than our theories about the history of our own Earth. Geologists infer that the continents are drifting over the globe, about as fast as your fingernails grow, and that Europe and North America were joined together 200 million years ago. We believe them, even though such vast spans of time are hard to grasp. We also believe, at least in outline, the story of how our biosphere evolved and how we humans emerged.
Some key features of out cosmic environment are now underpinned by equally firm data. The empirical support for a Big Bang ten to fifteen billion years ago is as compelling as the evidence that geologists offer on our Earth's history. This is an astonishing turnaround: our ancestors could weave theories almost unencumbered by facts, and until quite recently cosmology seemed little more than speculative mathematics.
N = 1,000,000,000,000,000,000,000,000,000,000,000,000
The cosmos is so vast because there is one crucially important huge number in nature. N measures the strength of the electrical forces that hold atoms together, divided by the force of gravity between them. If it had a few less zeros, only a shorminiature universe could exist. No creatures would be larger than insects, and there would be no time for evolution to lead to intelligent life.
A few years ago, I already had 90% confidence that there was indeed a Big Bang - that everything in our observable Universe started as a compressed fireball, far hotter than the centre of the Sun. The case now is far stronger: dramatic advances in observations and experiments have brought the broad cosmic picture into sharp focus during the 1990s, and I would now raise my degree of certainty to 99%.
The most incomprehensible thing about the Universe is that it is comprehensible is one of Albert Einstein's best-known aphorisms. It expresses his amazement that the laws of physics, which our minds are somehow attuned to understand, apply not just here on Earth but also in the remotest galaxy. Newton taught us that the same force that makes apples fall holds the Moon and planets in their courses. We now know that this same force binds the galaxies, makes some stars collapse into black holes, and may eventually cause the Andromeda galaxy to collapse on top of us. Atoms in the most distant galaxies are identical to those we can study in our laboratories. All parts of the universe seem to be evolving in a similar way, as though they shared a common origin. Without this uniformity, cosmology would have got nowhere.