We know the Big Bang happened for several reasons. For starters, we can’t find anything in the Universe – either on Earth or through a telescope – that is confirmed to be older than 13.8 billion years old, which is the current estimated age of the Universe. If the Universe was infinite and eternal, we’d be tripping over stuff that was 105 billion or 802 trillion years old.

Second, the fact that normal matter in our Universe is mostly hydrogen and helium is exactly what you’d expect to see if the expanding Universe was super-hot for a few brief minutes but then cooled down quickly without time for many heavier elements to form. Again, if the Universe was infinitely old and infinitely big, we’d have no clear explanation why the chemical composition of the Universe is the way it is. In an infinite Universe with an eternity of stars blowing up in supernovas, there would be no clear reason not to expect just as much gold to exist in the Universe as hydrogen.

Third, in the 1920s, Edwin Hubble was mapping the cosmos and discovered that most galaxies are heading away from us as space expands. Logically extrapolating from that, and calculating backwards, Hubble figured out that all the galaxies in the Universe must have been smooshed together at a single fixed point. Despite this discovery, the Big Bang theory was not the dominant theory of cosmology for decades. Which leads us to the fourth and most crucial piece of evidence: the cosmic microwave background that emerged 380,000 years after the Big Bang.

If the Big Bang theory were true, then after a few thousand years of the Universe expanding, the crush of matter and plasma and radiation would be spread out enough for light to be able to travel freely, and there would be a brilliant flash across the cosmos. In the 1940s, physicists predicted we should be able to see the remnants of this flash everywhere in the sky. This is precisely what was discovered in 1964 by two radio engineers, Arno Penzias and Robert Wilson, who weren’t even looking for it. They were trying to eliminate all the static on a highly sensitive radio antenna, but they couldn’t get rid of a small hiss, and after many calibrations, and having shot the pigeons that shat on the antenna, a physicist from Princeton told them what they had found. From that point on, the Big Bang became the dominant explanation for the start of the Universe and all the work that has happened since has only confirmed or clarified the general framework of this theory.

What does the Universe look like?

In the first split second after the Big Bang, the Universe inflated from the size of a quantum particle to that of a grapefruit. Within a second, it was larger than our solar system. Four years later, it was bigger than the Milky Way.

The Universe, as we know it, is currently 93 billion light years across. Which means there are stars and galaxies that were born billions of years ago that are so far away their light hasn’t had a chance to reach us, since only 13.8 billion years have elapsed since the start of the Universe. The stuff we can see looking out from Earth is called the Observable Universe, but there are a lot of things beyond that horizon which we cannot see.

Moreover, because light takes time to travel from a distant object, the further away we look, the further we are looking into the past. For instance, a neighbouring galaxy, Andromeda, is 2 million light years away. So when you look at it through a telescope, you are seeing it as it existed roughly when Homo erectus started roaming the Earth and sabre-toothed tigers were still a cause for concern.

The Observable Universe can be seen looking in any direction from Earth; in that sense, the Observable Universe is a sphere. However, that is not the shape of the entire Universe. Physicists have determined that the Universe has “zero curvature”, which means it does not bend back on itself at some point. It stretches on and on, like a table-top in any direction, constantly expanding for all eternity. The Observable Universe is just one patch on it: like the ring left by a coffee cup on a table. And Earth is just one tiny fibre of wood lodged somewhere inside that coffee ring.

The colour of the Universe is beige, assuming we were looking at the entire Universe from a great distance with human eyes. If you were to look at the mixture of light from all the stars in the Observable Universe blended together, as if you were zoomed out and looking at the whole thing at once, the colour of our cosmic bubble would be beige. Cosmologists have tried to jazz it up by calling the tone “cosmic latte”, but it’s really just beige. Personally, I like the fact the Universe is beige; it renders the cosmos slightly less intimidating.

What is the Multiverse?

Permit me to get a little weird for a moment. One inevitable consequence of the Big Bang model (the most accepted model currently) is a phenomenon called “eternal inflation”, which means that while our coffee ring of the Observable Universe has popped out of inflation and is expanding more slowly than in the first split second, other parts of the table-top may still be expanding at that speed. And there will be other coffee rings (that is, other so-called universes) with physical laws and variations in historical events entirely different to our own. And this process would stretch on forever. This collection of diverse “universes”, each roughly the size of our “Observable Universe”, is known as the Multiverse.

But the term Multiverse is a misnomer: it’s all the same Universe, just different patches or coffee stains on the table, with different physics. There are an almost infinite number of variations of physical laws (10,500 or nearly six times the number of atoms in the Observable Universe) and each one of those sets of physical laws could yield many different historical results. This means – if the hypothesis is true – there is another “universe” out there where you are reading this sentence 1.5 seconds earlier. There is another universe where you were not born at all. There is yet another universe where no stars exist. There is a universe where World War II didn’t happen. And one where your face looks like fairy floss and the footpath looks like a pizza. Every possible variation you could imagine and quite a bit more besides. If this hypothesis is true, we should be able to confirm it once the light from the nearest other “universes” that have appeared (if they have such a thing as light) finally reaches us. . .in about 3 trillion years.

Excerpted with permission from The Shortest History of the Universe, David Baker, Pan MacMillan India.