Sign up for Brilliant FOR FREE at http://brilliant.org/minutephysics - the first 200 people get 20% of a premium subscription. This video is about how Russian physicist Aleksandr Fridman corrected Albert Einstein about the expansion of the universe. Einstein thought that general relativity implied that space had to be static and unchanging, but he had made a technical error regarding the differentiation of the metric (in particular, I believe he mistook the determinant of the metric for a scalar rather than a tensor density of weight 2). Friedmann didn't make this differential geometric mistake, and the cosmologies he found from the Einstein Equations were more varied in their properties - they could be expanding, or contracting, or (with the cosmological constant), static. Support MinutePhysics on Patreon! http://www.patreon.com/minutephysics Link to Patreon Supporters: http://www.minutephysics.com/supporters/ REFERENCES Alexander Friedmann https://en.wikipedia.org/wiki/Alexander_Friedmann Einstein Wrongly Criticizes Alexander Friedmann https://einsteinpapers.press.princeton.edu/vol13-trans/301 Alexander Friedmann Corrects Einstein https://einsteinpapers.press.princeton.edu/vol13-trans/363 Einstein Admits his Mathematical Mistake https://einsteinpapers.press.princeton.edu/vol14-trans/77 Interrogating the Legend of Einstein’s “Biggest Blunder” https://arxiv.org/abs/1804.06768 Cosmological Considerations in the General Theory of Relativity https://einsteinpapers.press.princeton.edu/vol6-trans/433 On the General Theory of Relativity (Einstein) https://einsteinpapers.press.princeton.edu/vol6-trans/110 The Field Equations of Gravitation https://einsteinpapers.press.princeton.edu/vol6-trans/129 The Einstein Field Equations https://en.wikipedia.org/wiki/Einstein_field_equations Presentation about the Sequence of Events http://web.mit.edu/8.286/www/slides07/Einstein-and-Friedmann.pdf Tensor Densities: https://en.wikipedia.org/wiki/Tensor_density MinutePhysics is on twitter - @minutephysics And facebook - http://facebook.com/minutephysics And Google+ (does anyone use this any more?) - http://bit.ly/qzEwc6 Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute! Created by Henry Reich ... https://www.youtube.com/watch?v=Va5T2KcYiOw

Thanks to @AstroKatie (http://astrokatie.com, http://twitter.com/astrokatie) for the collaboration! Go to http://www.brilliant.org/minutephysics for free daily science puzzles Here's the original twitter thread https://twitter.com/astrokatie/status/1066526148149108736 Support MinutePhysics on Patreon! http://www.patreon.com/minutephysics Link to Patreon Supporters: http://www.minutephysics.com/supporters/ This video is about the cosmic distance scale and how we see objects farther away in space (ie at higher red shift) farther back in time because light takes time to reach us. Thus we can see not only stars and galaxies, but also the primordial stars & proto-galaxies, and even the remnants of the beginning of the universe itself: the CMB cosmic microwave background left over from the big bang. ... https://www.youtube.com/watch?v=Cfg11qQwPzQ