But after 20 minutes, the universe was no longer hot enough for fusion. What was left was a hot, cloudy soup of electrons and hydrogen and helium nuclei. This stage lasted for about 380,000 years. Eventually, the cosmos cooled enough for electrons to pair up with nuclei and make the first atoms. It then took hundreds of millions of years for the first stars to form and light up the darkness, and even longer for the universe to start to resemble what we see today. Einstein’s fudge factor Most physicists believe the universe was born in a big bang 13.8 billion years ago. In it, the energy making up everything in the cosmos we see today was squeezed inside an inconceivably small space – far tinier than a grain of sand, or even an atom. Then, this unimaginably hot and dense cauldron – for whatever reason – ballooned at a terrifying rate. Professor Brian Cox asks the ultimate question: how did the universe come to be? It is daunting in its scale. We live on one planet of eight that orbit just one of the 400 billion stars in our galaxy, which in turn is one of trillions in the universe. Yet it is amongst those galaxies that we have been able to unravel the story of the universe’s creation. Thanks to a series of discoveries, our most powerful space missions have unravelled 13.8 billion years of cosmic evolution and revealed the story of our universe from its birth all the way to the arrival of our nascent civilisation. In the end, it took measurements not theories to finally convince many scientists that we live in an expanding universe. Astronomer Edwin Hubble was already world famous for discovering that our galaxy is not the only one. His later observations in 1929 proved that all galaxies are moving away from us and moving faster the further away they are. Space itself is expanding.

To answer these questions, we first need to understand how physics worked in those very early moments of the universe’s life, when all of space and time were crushed to a size far smaller than a proton. And this means somehow bringing general relativity and quantum theory together. If it can be done, we might finally understand the nature of reality. Test your knowledge Sheldon, Leonard, Rajesh, Howard and Penny form a really special group that is definitely never boring. And just like them you will not be bored by our Big Bang merch collection, in which you will find a diverse bunch of special posters and mugs or other gifts.

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How do we know all this? General relativity describes how space, time and gravity work throughout the universe. Albert Einstein came up with the ground breaking theory in 1915. But it was another physicist, Alexander Friedmann, who studied the equations and made a startling discovery. The fact they are both invisible explains why the universe appears to be a lot lighter than it should be. In fact, we now think that about 68% of the universe is dark energy and 27% is dark matter. But knowing what dark energy and dark matter actually are will give us a much better understanding of what went on during the big bang. A. Before the 1990s, there were two ideas. Either gravity would slow down the universe’s expansion and eventually reverse it – leading to a big crunch. Or the universe would keep on expanding forever. When astronomers finally had the technology to measure how the universe’s expansion was changing they discovered that expansion was speeding up. They named whatever was pushing the galaxies away from each other dark energy. Accelerating expansion could lead to two grim outcomes. Either we end up in a big freeze, where even the light from other galaxies won’t be able to reach us. Or we experience a big rip, where the violent acceleration rips apart all matter and trace of anything having ever existed. Friedmann found that relativity naturally describes a cosmos that is either expanding or shrinking. One possibility he considered was that everything we observe today expanded from a single infinitely dense point. Published in 1922, Friedmann’s work was largely ignored. Five years later, history repeated itself. Belgian priest and astronomer Georges Lemaître did the maths again and concluded that our universe mushroomed from a 'primeval atom'. Just like Friedmann, Lemaître was ignored. The big bang theory was thought up almost 100 years ago. And scientists and the public have accepted it as the origin of the universe for over 50 years. Yet it still holds many mysteries. Most of these revolve around the fact that what we see doesn’t match what theory tells us. If we go by the evidence, ~95% of the universe is invisible. As a result, physicists have added two dark components that make the maths add up – and must exist if relativity is right.