The Cosmic Microwave Background
The theory that our universe began with a ‘big bang’ is one of the most famous ideas in all of science. It has seeped into popular culture, too, with one of the most famous television shows in the world named after it. Yet, despite its fame, it was only confirmed beyond reasonable doubt in the last 50 years.
The phrase itself was originally coined by one of the theory’s biggest detractors, gruff Yorkshire astronomer Fred Hoyle. During a BBC radio interview in 1949, he used the term to describe the main difference between it and his own favoured ‘steady state’ model. Those who trusted in the steady state theory held that the universe had been around for ever; Big Bang believers subscribed to a single, violent origin. As with everything in science, differences of opinion tend to be settled by experiment.
Clues to a calamitous beginning
The idea of a ‘big bang’ began to sprout up at the beginning of the 20th century, with larger telescopes allowing astronomers to probe deeper into the skies. Other galaxies – fuzzy patches of light then known as ‘island universes’ – were shown to lie beyond our own Milky Way. What’s more, they were moving away from us. By 1931, Belgian priest and physicist Georges Lemaître had suggested that this expansion meant that long ago all the galaxies were huddled very close together. In fact, he proposed there was a time when all the matter in the universe was concentrated into a ‘single quantum’, which acted as the seed for our universe – the idea that Hoyle would later call the Big Bang.
Such a dense concentration of matter would have made the infant universe an incredibly hot place, far too hot for whole atoms to exist. Initially, it was too hot for even protons and neutrons to exist. Instead, for the first millionth of a second, the universe was awash with a sea of quarks and leptons (like electrons). Only after that time would the nascent universe have expanded and cooled enough for the strong force to be able to bind up and down quarks together into protons and neutrons. It was still far too hot, however, for protons to capture electrons – they simply had too much energy to be confined by the electromagnetic force. According to cosmological models, it would take around 380,000 years of expansion and cooling for electrons to finally be snared. And in this notion lies the dagger that finally killed the steady state model.
Before electrons combined with protons to make neutral atoms, the bevy of particles floating about would have kept any photons of light trapped. The photons would only have been able to move a minuscule distance before bumping into something. With electrons now crowded close to protons, photons would suddenly have had a lot more room and they would have instantaneously been able to stream outwards, unhindered, at the speed of light. The universe itself continued to expand faster than the speed of light (this may seem to break the rule that nothing can travel faster than the speed of light, but that is only true of things travelling through space, not of the speed with which space itself can move, which has no such limitation). These ancient photons have been travelling across the expanding universe ever since.
Echo of the Big Bang
Crucially, if the Big Bang picture is true, we should still be able to see these photons today. The expansion of the universe in the intervening aeons should have robbed them of a lot of their energy, however, placing them in the microwave part of the electromagnetic spectrum. This reason, and the fact they should be seen in every part of the sky, led to it being called the Cosmic Microwave Background (CMB). It was predicted to exist by Robert Dicke and George Gamow in 1946, and finding it would be a smoking gun for the Big Bang and a death knell for the steady state.
It was eventually found, by accident, in the 1960s by American duo Arno Penzias and Robert Wilson. They were experimenting in New Jersey with a radio telescope that had been designed to capture radio waves reflected from communication balloons high in the atmosphere. They ran into trouble when trying to calibrate the system by removing all other radio interference – one source persisted. They were able to rule out many origins of the nuisance signal, including human technology, the Earth itself, the Sun or anything in the Milky Way. For a time they wondered if it might be coming from a ‘white dielectric material’ being deposited on the inside of the telescope by roosting pigeons. The pigeons were evicted and their droppings painstakingly cleaned away. The pigeons later returned and were shot. Despite these extreme efforts, the signal remained.
Penzias and Wilson did not realise what they had discovered. Their nuisance signal was actually the CMB. They were looking for the echo from balloons and instead found the echo of the Big Bang. Dicke eventually got wind of the discovery and realised its true significance. Despite this key role, it was Penzias and Wilson who won the Nobel Prize in Physics in 1978.
Mapping the CMB
Modern space-based instruments like the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck telescope have allowed astronomers to make exquisite measurements of the CMB. They have measured its temperature to be 2.7 K. As this radiation blankets all of the sky, this is the background temperature of space. Even the voids between galaxies glow faintly at this temperature.
Locked up in the CMB are tiny temperature variations that depart from 2.7 K by about 1 part in 100,000. These indicate that the early universe had tiny density variations that gave rise to slightly hot and cooler spots. As the universe swelled, matter gathered around the slightly denser regions. This explains the structure of the universe we see today with galaxies clumped together and separated by expansive voids. So the CMB is the key to understanding not only the universe’s past, but also its present form.