Hi, I'm new to this forum, but saw that everyone was so kind and helpful, so was wondering if anyone could help me with a couple of things.. How does one calculate the age of the universe using Hubble's constant, if the constant is in km/s/Mpc? Do we know the critical density of the universe? I always thought that experts had a few pretty good estimates, but then someone told me today that this wasn't true.. Does anyone know a thought experiment for length contraction that does not involve a train and a tunnel? For some reaosn this thought experiment always confuses me...I knew one once with a rocket travelling to alpha centauri at 0.8c, but am not sure if this is allowed if it assumes time dilation.. If anyone could help I would be so grateful! I have an exam on all of this tomorrow!