Comments on: Susskind’s lectures on cosmology http://www.nonequilibrium.net/susskinds-lectures-cosmology/ Cosmology, turbulence, markets, non-equilibrium QFT and much more. No nonsense, just science Wed, 13 Jan 2010 17:43:51 +0000 http://wordpress.org/?v=2.9.1 hourly 1 By: BlackGriffen http://www.nonequilibrium.net/susskinds-lectures-cosmology/comment-page-1/#comment-9220 BlackGriffen Wed, 05 Aug 2009 08:55:09 +0000 http://www.nonequilibrium.net/?p=4162#comment-9220 Ah, he got to the constant later in the lecture. Ah, he got to the constant later in the lecture.

]]> By: BlackGriffen http://www.nonequilibrium.net/susskinds-lectures-cosmology/comment-page-1/#comment-9219 BlackGriffen Wed, 05 Aug 2009 08:47:48 +0000 http://www.nonequilibrium.net/?p=4162#comment-9219 There was an omission in his solution to that differential equation and a mistake in the second lecture. He uses the guess and check method, but the equation is separable, so he misses his free parameter. The full answer is: [tex]a = \left( a_0^{3/2} + K * (t - t_0) \right)^{2/3}[/tex] The form of K depends on the choice of the form of M, and the free parameter is, of course, [tex]c = a_0^{3/2} - K * t_0[/tex]. Basically, he assumed that a = 0 corresponds to t = 0. The mistake is kind of funny because error in the video is that he said, offhandedly, at some point that an object at escape velocity goes logarithmically. Well, this is the solution to the equation for how an object at escape velocity behaves, he just didn't notice. There was an omission in his solution to that differential equation and a mistake in the second lecture. He uses the guess and check method, but the equation is separable, so he misses his free parameter. The full answer is:

a = \left( a_0^{3/2} + K * (t – t_0) \right)^{2/3}

The form of K depends on the choice of the form of M, and the free parameter is, of course, c = a_0^{3/2} – K * t_0. Basically, he assumed that a = 0 corresponds to t = 0.

The mistake is kind of funny because error in the video is that he said, offhandedly, at some point that an object at escape velocity goes logarithmically. Well, this is the solution to the equation for how an object at escape velocity behaves, he just didn’t notice.

]]>