LECTURE

planets) or because they are too small (such as the subatomic particles known as neutrinos).

Why should anyone believe this bizarre story? Because it rests on a colossal amount of carefully tested evidence. The earliest evidence came from Hubble’s studies of the “red shift,” which showed that the further away an object was, the faster it was moving away from us. This meant that the Universe must be expanding; so at some time in the past it must have been in nitely small.

What clinched the new theory was the discovery of “cosmic background radiation” (CBR). Until 1965, big bang cosmology had a rival, the “steady state” theory, supported by Fred Hoyle and others. They argued that the apparent expansion of the Universe was caused by the continuous creation of new matter. Supporters of the “big bang” theory suggested that when energy and matter separated 380,000 years after the big bang, there must have been a huge ash of energy that should still be detectable as a weak energy source coming from all parts of the Universe.

In 1964, two engineers, Arno Penzias and Robert Wilson, were trying to construct an extremely sensitive radio antenna. They could not eliminate a persistent background hum. Eventually, they realized they were hearing the cosmic background radiation. That clinched it for the big bang theory because no other theory could explain the source of this universal energy.

There are other powerful reasons for accepting big bang cosmology. No astronomical objects older than 13 billion years have ever been detected. As telescopes (like the Hubble space telescope) probe deeper in space they are also probing further into the past. What they nd is that the early Universe was different in several respects from today’s Universe, which is what big bang cosmology predicts in an evolving, historically changing Universe. Models of the big bang suggest that hydrogen and helium were created in huge amounts, and other elements in smaller amounts. Observed distributions

Models of the big bang suggest that hydrogen and helium were created in huge amounts, and other elements in smaller amounts.