This is a question that’s bugged me for a while.<\/p>\n
First, here’s why I think this is a reasonable question to ask.<\/p>\n
Suppose you cram a mass M into a spherical volume of radius R such that R is less than the Schwarzschild radius<\/a>, i.e., R \\leq 2GM\/c^2. Then it’s a pretty well understood consequence of general relativity that the mass will collapse to form a black hole.<\/p>\n Current estimates of the mass of the (observable) Universe vary quite a bit. This webpage<\/a> seems pretty representative, though, giving a value for the mass of 3 x 10^52 kg.<\/p>\n This gives a value for the corresponding Schwarzschild radius of about 6 billion light years.<\/p>\n The radius of the observable Universe is, of course, quite a bit bigger than this. But the Universe is also expanding, and at some point in the past its radius was quite a bit less than six billion light years.<\/p>\n If that was the case, why didn’t it collapse to form a singularity? In short, how come we’re still here?<\/p>\n Any cosmologists out there who can enlighten me?<\/p>\n Update:<\/b> In comments, Dave Bacon points to an enlightening essay<\/a> from John Baez, explaining some of what’s going on.<\/p>\n My interpretation of the essay is that the standard lore I learned as an undergraduate (namely, that if you take a mass M and compress it into a smaller radius than the Schwarzschild radius then a black hole must inevitably form) is wrong, and that the FRW cosmology provides a counterexample.<\/p>\n This begs the question of when<\/em>, exactly, a black hole can be guaranteed to form.<\/p>\n","protected":false},"excerpt":{"rendered":" This is a question that’s bugged me for a while. First, here’s why I think this is a reasonable question to ask. Suppose you cram a mass M into a spherical volume of radius R such that R is less than the Schwarzschild radius, i.e., R \\leq 2GM\/c^2. Then it’s a pretty well understood consequence… Continue reading Was the Universe formerly a black hole?<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[20],"tags":[],"class_list":["post-255","post","type-post","status-publish","format-standard","hentry","category-physics","entry"],"_links":{"self":[{"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/posts\/255","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/comments?post=255"}],"version-history":[{"count":0,"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/posts\/255\/revisions"}],"wp:attachment":[{"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/media?parent=255"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/categories?post=255"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/michaelnielsen.org\/blog\/wp-json\/wp\/v2\/tags?post=255"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}