Putting it all together: the Big Bang model

Putting the three planks of evidence together (see below), we are led to The Big Bang model : a model that posits that the universe began as a superhot, superdense singularity that has been expanding and cooling ever since. Although we don’t know much about the initial singularity , there is very strong evidence for the evolution of the universe from this state – so the name Big Bang is a bit of a misnomer! Let’s summarize the evidence once more before we examine the cracks:

1. The expansion of the universe: it’s been known since 1929 that far-away galaxies are receding away from each other at a speed proportional to their distance (Hubble’s Law). This is surprisingly easy to measure as the light emitted by moving galaxies is red-shifted by the Doppler effect. Following the famous Hubble graph back down to the origin led Georges Lemaitre to the idea of a Universe bursting out from a tiny volume (the primeval atom) . Nowadays, we say the Universe began as a superhot, superdense explosion of space, time , matter and radiation, expanding and cooling as time goes on.

Best of all, the slope of the Hubble graph gives an immediate estimate for the age of the universe: the modern value of this figure agrees exactly with that expected from independent estimates (the age of the oldest stars etc)

2. The composition of the elements: if the universe began as some sort of tiny dense fireball, it would have been too hot for atoms to form at first. Calculations by Gamow suggested that a universe made up of about 75% Hydrogen and 25 % Helium should have evolved after a certain time. Guess what – the figures match our universe (it was later realised that all the other elements are fused in dying stars and account for about 0.1%).

3. The cosmic backround radiation: Alpher, Herman and Gamow also suggested that radiation left over from the very early universe might still be observable. (The reason is once atoms began to form, the scattering of light becomes reduced and the universe becomes transparent). Such radiation would be hugely redshifted and freezing cold, but it should be there. Just such radiation was found by Penzias and Wilson in 1965, using the world’s first radiotelescope. Since the background radiation offers a real glimpse of the very early universe, much of modern cosmology is concerned with getting more and more accurate measurements of this ‘cosmic fossil’, using satellite telescopes such as COBE and WMAP.

And that’s the model..

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Poscript

Of course, the model is not perfect. The most obvious problem is the singularity itself – can the universe once really have been infinitely hot and infinitely dense, or is this a feature of our limited understanding of gravity on the quantum scale? There are other problems too, such as the horizon and flatness problems. More on this next day..