“You just won't believe how vastly, hugely, mind-bogglingly big space is,”said the author Douglas Adams. “I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.” By our best estimates there are around 100 billion stars in the Milky Way and at least 140 billion galaxies across the Universe. If galaxies were frozen peas, there would be enough to fill an auditorium the size of the Royal Albert Hall.
So how was this unimaginably giant Universe created? For centuries scientists thought the Universe always existed in a largely unchanged form, run like clockwork thanks to the laws of physics. But a Belgian priest and scientist called George Lemaitre put forward another idea. In 1927, he proposed that the Universe began as a large, pregnant and primeval atom, exploding and sending out the smaller atoms that we see today.
His idea went largely unnoticed. But in 1929 astronomer Edwin Hubble discovered that the Universe isn’t static but is in fact expanding. If so, some scientists reasoned that if you rewound the Universe's life then at some point it should have existed as a tiny, dense point. Critics dismissed this: the celebrated astronomer Fred Hoyle sarcastically called this concept the “Big Bang” theory, a phrase that would later be adopted by its proponents.
Undeterred by sceptics, scientists Ralph Alpher, George Gamow and Robert Herman predicted that if there had been a Big Bang, then a faint afterglow should linger somewhere in the Universe, and we should in theory be able to detect it. To do so would require one of the greatest pieces of fortune in science.
In the mid-1960s Astronomers Arno Penzias and Robert Wilson were having a tough time trying to tune into the microwave signals transmitted from the Milky Way. The radio antenna they were using kept picking up a persistent weak hiss of radio noise. Rebuilding the antenna couldn’t get rid of the noise. Nor could clearing the pigeons that had roosted in there, or their mess. That’s because the hiss they tried so hard to remove was the echo of the Big Bang, or the Cosmic Microwave Background radiation as it is known.
If the Big Bang theory is true, how did it lead to all the planets, stars and galaxies we can see today? Thanks to a series of calculations, observations from telescopes on Earth and probes in space, our best explanation is this.
Around 13.8 billion years ago, all the matter in the Universe emerged from a single, minute point, or singularity, in a violent burst. This expanded at an astonishingly high rate and temperature, doubling in size every 10-34 seconds, creating space as it rapidly inflated. Within a tiny fraction of a second gravity and all the other forces were formed. Energy changed into particles of matter and antimatter, which largely destroyed each other. But luckily for us some matter survived. Protons and neutrons started to form within the first second; within minutes these protons and neutrons could fuse and form hydrogen and helium nuclei. After 300,000 years, nuclei could finally capture electrons to form atoms, filling the Universe with clouds of hydrogen and helium gas. After around 380,000 years it left behind a bath of photons – the Cosmic Microwave Background that Penzias and Wilson accidentally detected. Within this were tiny ripples of matter that were stretched to enormous sizes during inflation, and in turn these became the seeds for the galaxies and galactic clusters we see today.
If this is how we think the Universe began, then how will it end? Well, that’s another story entirely.