 The shift in the distant galaxy spectra told us that because of the Doppler effect, they were moving away. Their spectra had been stretched out, moved to longer wavelengths called redshifting. And that's interesting because that tells us the universe is expanding. But then wind the clock back and back about 13.7 billion years ago, that's when everything must have been pretty much on top of each other. And that became the moment known as the Big Bang. George Lemaître, Belgian cleric and also scientist, he coined the term the concept of the Big Bang, but it was a journalist who was actually mocking him who called it the Big Bang. And to be honest, it's not a very smart name because one, it wasn't very big. In fact everything was really compressed. And two, it wasn't really a bang. It was just a whole lot of things sort of expanding. So maybe it's kind of rather than a bang. But anyway, who cares? That's what we call it these days. We don't know what caused it all to be close together. There was some quantum fluctuation. You know, that's the world of philosophy. It's before physics. So as things get smaller, it changes the way they behave. Suddenly we have a whole insight into our history, the history of the universe, that once upon a time, things were closer together, which means hotter. What happens as things get hotter? Solids melt, become liquids, liquids boil, become gases, gases boil, or strip off the electrons and become plasma. This, you know, charged gas, soup kind of thing. Go even tighter, the electrons get pushed into the nucleus. That's what happens inside a neutron star. Electrons and protons cancel each other out, form neutrons. You have incredibly dense stuff that makes up neutron stars. Even hotter still, you start to break protons and neutrons apart into quarks. They're actually so hot that quark. Well, we believe this is what happens, quark plasma forms, and that's something we've never been able to do on Earth, way too hot. So even hotter than that, when everything's pushed together into basically nothing, then we get what's called the singularity, which means we don't understand. It's a divide by zero error. That's out of the realm of physics. We think smaller than about 10 to the minus 35 meters. That's called the Planck length, Planck with a C named after Max Planck. At scales that small, we don't really understand what's going on. Physics, as we know, it doesn't really work. So that's something for you to work out in your PhD in the future. In the meantime, check out this website and have a look at just how big the universe is now and how small it was at the time of the Big Bang and how much it's expanded.