 We've spent a lot of time talking about the matter that makes up our universe, but were things always this way, or is there more to the story? In this video, we'll investigate what we know about the formation of the universe, a theory we call the Big Bang, and we'll talk about some of the evidence that has led us to form this theory. So the first thing you should know is that the Big Bang begins with a mystery. We think time and space emerge from a single point, but since space has no meaning at this stage and time doesn't yet exist, it's rather hard to get our heads around the start. So until the first 10 of the minus 43 seconds of the universe's existence then, a time interval we call Plank Time, we have to wave a little white flag and claim ignorance, because the laws of physics, as we know them, do not yet apply. What we do know is this, the universe is incredibly hot and it's expanding quite quickly. As it expands and as more time passes, the universe cools. On this diagram here, I'm going to go through the different time periods since the Big Bang and what we know about each of them. So as I said, at t equals zero, that's when the Big Bang begins. We don't know why or how. From 10 to the minus 43 seconds to 10 to the minus 35 seconds, we believe all the fundamental forces except gravitation act as a single grand unified force. From 10 to the minus 35 seconds to 10 to the minus 10 seconds, the strong force separates from the electric weak force, then the electromagnetic and weak forces separate. Quarks, leptons, and their antiparticles are created. From 10 to the minus 10 seconds to 10 to the minus 5 seconds, the universe consists of a hot mix of quarks, gluons, leptons, and photons. In this incredibly hot soup, quarks and gluons were only weakly bound, so quarks could float around freely in space without the constraints they typically suffer today. From 10 to the minus 5 seconds to 3 minutes, as the universe continued to cool, quarks and gluons began to form hadrons. Matter and antimatter began to annihilate, leaving a slight excess of matter in its wake. Excess of matter is what forms our universe. From 3 minutes to 10 to the 5 years, nucleosynthesis began. This is the process where protons and neutrons joined to form atomic nuclei. The lightest nuclei, deutrons, isotopes of helium and lithium, were formed. 10 to the 5 years to today, electrons began to orbit atomic nuclei in order to form atoms, and for the first time, the universe was not full, free electrons. Now these electrons scattered light and other electromagnetic waves pretty readily, so up until this point, the universe has been pretty opaque. When this change occurs, all of a sudden the universe is transparent to electromagnetic waves, and light begins to have the ability to travel long distances, without interacting with anything. During this time, matter also clumps together to form stars, planets, and galaxies. And today, the temperature of the universe is a cool 3 kelvin. So you can see how this theory, which we call the standard cosmology, neatly leads to the standard model as we know it today. The four forces and the subsequent evolution of matter as the temperature of the universe cools, is consistent with our picture of what matter is and how it interacts. But what evidence is there to support this theory, and what questions does this theory leave unanswered? Let's have a look at what we do know.