We're living in the middle of a remarkable cosmic coincidence, an incredibly brief period of time where both matter and dark energy both play a part in the fate of our universe. To better understand this video watch our other Dark Energy episodes: Part 1 - Will the Universe Expand Forever https://www.youtube.com/watch?v=xZTb6sfHEX8 Part 2 - Why the Universe Needs Dark Energy https://www.youtube.com/watch?v=-4PayaEgEZc Part 3 - What Does Dark Energy Really Do? https://www.youtube.com/watch?v=RUE_LUqcTvM Get your own Space Time t­shirt at http://bit.ly/1QlzoBi Tweet at us! @pbsspacetime Facebook: facebook.com/pbsspacetime Email us! pbsspacetime [at] gmail [dot] com Comment on Reddit: http://www.reddit.com/r/pbsspacetime Support us on Patreon! http://www.patreon.com/pbsspacetime Help translate our videos! null In the past few episodes we have delved deep into cosmology. We have been building up the tools we need to understand dark energy and its influence on the universe. In this episode we will discuss the balance between dark energy and matter. If you add up the energy in any large volume of space, about 70% is dark energy and the remaining 30 % is “regular” matter. However the universe has been dark for a relatively short period. At some point in the past there was a perfect balance between energy and matter. Dark Energy has dominated the universe only during the tenure of life on Earth, although it’s effect has been felt for longer than that. Here’s your Challenge question: For How many of those 100 past doublings has dark energy had any significant effect? Let’s say at least 10% of the energy density. And how for many of those infinite future doublings will regular matter and energy have a significant effect – again, at least 10% of the energy density? Extra Credit: Also tell me how many billion years ago dark energy started to have a significant effect, and how many billion years in the future matter will cease to … matter? Prizes: 3 Correct Answers for the “simple” question and 3 Correct Answers for the extra credit parts will receive Space Time t-shirts. To be eligible please explain your reasoning and show your working. Feel free to propose any solutions you come up with for the apparent cosmological coincidence. Do NOT give answers in the comment section – that’ll disqualify you for this and future challenge questions. Email your answers to pbsspacetime@gmail.com with the subject line Dark Energy Challenge within two weeks. We will filter by subject line, so make sure sure you use exactly this phrase to be considered. Written and hosted by Matt O’Dowd Made by Kornhaber Brown (www.kornhaberbrown.com)