WEBVTT 99:59:59.999 --> 99:59:59.999 Looking up at the night sky, 99:59:59.999 --> 99:59:59.999 we are amazed at how it seems to go on forever. 99:59:59.999 --> 99:59:59.999 But what will the sky look like 99:59:59.999 --> 99:59:59.999 billions of years from now? 99:59:59.999 --> 99:59:59.999 A particular type of scientist, 99:59:59.999 --> 99:59:59.999 called a cosmologist, 99:59:59.999 --> 99:59:59.999 spends her time thinking about that very question. 99:59:59.999 --> 99:59:59.999 The end of the universe is intimately linked 99:59:59.999 --> 99:59:59.999 to what the universe contains. 99:59:59.999 --> 99:59:59.999 Over 100 years ago, 99:59:59.999 --> 99:59:59.999 Einstein developed the Theory of General Relativity, 99:59:59.999 --> 99:59:59.999 formed of equations that help us 99:59:59.999 --> 99:59:59.999 understand the relationship 99:59:59.999 --> 99:59:59.999 between what the universe is made of 99:59:59.999 --> 99:59:59.999 and its shape. 99:59:59.999 --> 99:59:59.999 It turns out that the universe 99:59:59.999 --> 99:59:59.999 could be curved like a ball or sphere. 99:59:59.999 --> 99:59:59.999 We call this positively curved or closed. 99:59:59.999 --> 99:59:59.999 Or it could be shaped like a saddle. 99:59:59.999 --> 99:59:59.999 We call this negatively curved or open. 99:59:59.999 --> 99:59:59.999 Or it could be flat. 99:59:59.999 --> 99:59:59.999 And that shape determines 99:59:59.999 --> 99:59:59.999 how the universe will live and die. 99:59:59.999 --> 99:59:59.999 We now know that the universe is very close to flat. 99:59:59.999 --> 99:59:59.999 However, the components of the universe 99:59:59.999 --> 99:59:59.999 can still affect its eventual fate. 99:59:59.999 --> 99:59:59.999 We can predict how the universe will change with time 99:59:59.999 --> 99:59:59.999 if we measure the amounts or energy densities 99:59:59.999 --> 99:59:59.999 of the various components of the universe today. 99:59:59.999 --> 99:59:59.999 So, what is the universe made of? 99:59:59.999 --> 99:59:59.999 The universe contains all the things we can see, 99:59:59.999 --> 99:59:59.999 like stars, gas, and planets. 99:59:59.999 --> 99:59:59.999 We call these things ordinary or baryonic matter. 99:59:59.999 --> 99:59:59.999 Even though we see them all around us, 99:59:59.999 --> 99:59:59.999 the total energy density of these components 99:59:59.999 --> 99:59:59.999 is actually very small, 99:59:59.999 --> 99:59:59.999 about 5% of the total energy of the universe. 99:59:59.999 --> 99:59:59.999 So, now let's talk about the other 95% is. 99:59:59.999 --> 99:59:59.999 Just under 27% of the rest 99:59:59.999 --> 99:59:59.999 of the energy density of the universe 99:59:59.999 --> 99:59:59.999 is made up of what we call dark matter. 99:59:59.999 --> 99:59:59.999 Dark matter is only very weakly interacting with light, 99:59:59.999 --> 99:59:59.999 which means it doesn't shine or reflect light 99:59:59.999 --> 99:59:59.999 in the way that stars and planets do, 99:59:59.999 --> 99:59:59.999 but, in every other way, 99:59:59.999 --> 99:59:59.999 it behaves like ordinary matter -- 99:59:59.999 --> 99:59:59.999 it attracts things gravitationally. 99:59:59.999 --> 99:59:59.999 In fact, the only way we can detect this dark matter 99:59:59.999 --> 99:59:59.999 is through this gravitational interaction, 99:59:59.999 --> 99:59:59.999 how things orbit around it, 99:59:59.999 --> 99:59:59.999 and how it bends light 99:59:59.999 --> 99:59:59.999 as it curves the space around it. 99:59:59.999 --> 99:59:59.999 We have yet to discover a dark matter particle, 99:59:59.999 --> 99:59:59.999 but scientists all over the world are searching 99:59:59.999 --> 99:59:59.999 for this elusive particle or particles 99:59:59.999 --> 99:59:59.999 and the effects of dark matter on the unvierse. 99:59:59.999 --> 99:59:59.999 But this still doesn't add up to 100%. 99:59:59.999 --> 99:59:59.999 The remaining 68% 99:59:59.999 --> 99:59:59.999 of the energy density of the universe 99:59:59.999 --> 99:59:59.999 is made up of dark energy, 99:59:59.999 --> 99:59:59.999 which is even more mysterious than dark matter. 99:59:59.999 --> 99:59:59.999 This dark energy doesn't behave 99:59:59.999 --> 99:59:59.999 like any other substance we know at all 99:59:59.999 --> 99:59:59.999 and acts more like anti-gravity force. 99:59:59.999 --> 99:59:59.999 We say that it has a gravitational pressure, 99:59:59.999 --> 99:59:59.999 which ordinary matter and dark matter do not. 99:59:59.999 --> 99:59:59.999 Instead of pulling the universe together, 99:59:59.999 --> 99:59:59.999 as we would expect gravity to do, 99:59:59.999 --> 99:59:59.999 the universe appears to be expanding apart 99:59:59.999 --> 99:59:59.999 at an ever-increasing rate. 99:59:59.999 --> 99:59:59.999 The leading idea for dark energy 99:59:59.999 --> 99:59:59.999 is that it is a cosmological constant. 99:59:59.999 --> 99:59:59.999 That means that it has the strange property 99:59:59.999 --> 99:59:59.999 that it expands as the volume of space increases 99:59:59.999 --> 99:59:59.999 to keep its energy density constant. 99:59:59.999 --> 99:59:59.999 So, as the universe expands 99:59:59.999 --> 99:59:59.999 as it is doing right now, 99:59:59.999 --> 99:59:59.999 there will be more and more dark energy. 99:59:59.999 --> 99:59:59.999 Dark matter and baryonic matter, 99:59:59.999 --> 99:59:59.999 on the other hand, 99:59:59.999 --> 99:59:59.999 don't expand with the universe 99:59:59.999 --> 99:59:59.999 and become more diluted. 99:59:59.999 --> 99:59:59.999 Because of this property 99:59:59.999 --> 99:59:59.999 of the cosmological constant, 99:59:59.999 --> 99:59:59.999 the future universe will be more and more dominated 99:59:59.999 --> 99:59:59.999 by dark energy, 99:59:59.999 --> 99:59:59.999 becoming colder and colder 99:59:59.999 --> 99:59:59.999 and expanding faster and faster. 99:59:59.999 --> 99:59:59.999 Eventually, the universe will run out of gas 99:59:59.999 --> 99:59:59.999 to form stars, 99:59:59.999 --> 99:59:59.999 and the stars themselves will run out of fuel 99:59:59.999 --> 99:59:59.999 and burn out, 99:59:59.999 --> 99:59:59.999 leaving the universe with only black holes in it. 99:59:59.999 --> 99:59:59.999 Given enough time, 99:59:59.999 --> 99:59:59.999 even these black holes will evaporate, 99:59:59.999 --> 99:59:59.999 leaving a universe that is completely cold and empty. 99:59:59.999 --> 99:59:59.999 That is what we call the heat death of the universe. 99:59:59.999 --> 99:59:59.999 While it might sound depressing 99:59:59.999 --> 99:59:59.999 living in a universe 99:59:59.999 --> 99:59:59.999 that will end its lifetime cold 99:59:59.999 --> 99:59:59.999 and devoid of life, 99:59:59.999 --> 99:59:59.999 the end fate of our universe 99:59:59.999 --> 99:59:59.999 actually has a beautiful symmetry 99:59:59.999 --> 99:59:59.999 to its hot, fiery beginning. 99:59:59.999 --> 99:59:59.999 We call the accelerating end state 99:59:59.999 --> 99:59:59.999 of the universe a de Sitter phase, 99:59:59.999 --> 99:59:59.999 named after the Dutch mathematician 99:59:59.999 --> 99:59:59.999 Willem de Sitter. 99:59:59.999 --> 99:59:59.999 However, we also believe the universe 99:59:59.999 --> 99:59:59.999 99:59:59.999 --> 99:59:59.999 had another phase of de Sitter expansion 99:59:59.999 --> 99:59:59.999 in the earliest times of its life. 99:59:59.999 --> 99:59:59.999 We call this early period Inflation, 99:59:59.999 --> 99:59:59.999 where, shortly after the Big Bang, 99:59:59.999 --> 99:59:59.999 the universe expanded extremely fast 99:59:59.999 --> 99:59:59.999 for a brief period. 99:59:59.999 --> 99:59:59.999 So, the universe will end 99:59:59.999 --> 99:59:59.999 in much the same state as it began, 99:59:59.999 --> 99:59:59.999 accelerating. 99:59:59.999 --> 99:59:59.999 We live at an extraordinary time 99:59:59.999 --> 99:59:59.999 at the life of the universe 99:59:59.999 --> 99:59:59.999 where we can start to understand 99:59:59.999 --> 99:59:59.999 the universe's journey 99:59:59.999 --> 99:59:59.999 and view a history that plays itself out on the sky 99:59:59.999 --> 99:59:59.999 for all of us to see.