 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 0, 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 electro-weak force, then the electromagnetic and weak forces separate. Quarks, leptons, and their anti-particles 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 anti-matter began to annihilate, leaving a slight excess of matter in its wake. The 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. This idea that the universe is expanding is something that we've known about for quite a while. In the late 1920s, Edwin Hubble first showed that the universe was expanding using astronomical observations. His findings led to Hubble's law, which states that the speed at which distant galaxies move away from us is approximately proportional to their distance from the Earth. More recently, Brian Schmidt, Saul Perlmutter, and Adam Rice won the Nobel Prize in Physics for work using observations of supernovae, that's massive explosions of stars that mark the end of their lives, showing that the expansion of the universe is accelerating. So they won their Nobel Prize in 2011 for the work they did on this in the 1990s. In terms of what we actually see out in the universe, our understanding of galaxy and large-scale structure formation is ongoing work. Astronomers have been observing and classifying galaxies and large-scale structures in our universe at various stages in their lives in order to understand how these structures are formed. One really good example of a mission aimed at this is the Hubble Telescope Mission. This was launched in 1990 into orbit around Earth, and it's been sending back observations of distant galaxies for more than 25 years. This has allowed researchers to peer back in time to galaxies more than 13.4 light-years from Earth. So effectively, these researchers are looking at a galaxy as it was not too long after the start of the universe. And by looking at galaxies at various stages of their life cycle and various distances from Earth, they're able to actually get a pretty good picture of how galaxies are formed. Now the next evidence that we have has to do with the stage where all of a sudden atoms were formed, and free electrons no longer rendered the universe opaque. At the point when atoms were formed, the universe became transparent to electromagnetic radiation, and the cosmic radiation that remains today has essentially traveled through space freely from that point in time on. Now, this radiation has cooled as the universe has continued to expand. Because of this, the radiation, which is known today as the cosmic microwave background, bears signatures of this point in our universe's history. And here's a picture of what this radiation looks like from a NASA mission known as WMAP. One other set of obreservations that has contributed to our understanding of Big Bang nucleosynthesis or the formation of those light elements in the early stages of the universe also comes from astronomy. Stars have been studying the composition of the universe for a long time now. They know elements can be produced in one of two ways, either via the Big Bang or via nuclear reactions happening in stars. The Big Bang is thought to account for most of the abundance of light elements in the universe, and that's simply based on our understanding of what nuclear reactions are possible within stars. By measuring the abundances of these elements in the universe, we've been able to confirm that our theory of Big Bang nucleosynthesis is largely consistent with observation. Basically, the abundances that we observe and the abundances we predict with Big Bang nucleosynthesis theory match up pretty well. Now if we want to test our understanding of the Big Bang at periods even earlier than this, we actually can no longer rely on observation out into the universe. We can't directly observe signs of the early hot soup of quarks and gluons and other particles that are thought to have occurred in the early universe, but back on Earth, we've been using accelerators like the LHC at CERN or RIC at Brookhaven National Lab in the US to produce and study a state of matter called quark-gluon plasma. So this is a state of matter that is hot enough, has enough energy where the quarks and gluons are not tightly bound together, but are instead floating in that kind of hot soup like they would have been in this particular time in the universe's formation. In the next video, we'll learn more about an experiment at one of these accelerators, LHC at CERN, and get a glimpse of how major particle physics experiments are carried out today.