EducationQ

All of the Friedmann solution have the features that at some time in the past, between ten and twenty thousand millions years ago, the distance between neighbouring galaxies must have been zero. At that time, which we call the big bang, the density of the universe and the curvature of space time would have been infinite. This means that the general theory of relativity- on whic Friedmann's solutions are based- predicts that there is a sigular point in the universe. All our theories of science are formulated on the assumption that space-time is smooth and nearly flat, so they would all break down at the big bang singu-larity, where the curvature of space-time is infinite. This means that even ifthere were events before the big bang, one could not use them to determine what would happen afterword, because predictability would break down at the big bang. Corresponding, if we could not determine what happened beforehand. As far as we are concerned, events before the big bang can hav...

AS long as ago as 340 B.C.Aristotle, in his book On the Heavens, was able to put forward two good arguement for believing that the Earth was a round ball rather than flat plate. First, he realised that eclipses of the moon weren caused by the Earth coming between the sun and the moon. The Earth's shadow on the moon was always round , which would be true only if the Earth was would have been elongated and elliptical, unless the eclipses always occurered at a time when the sun was directly above the centre of the disk. Second, the Greeks knew from their trafels that the Pole Star appeared lower in the sky when viewed in the south than it did in more northerly regions.From the difference in the apparent position of the Pole Star in Egypt and Greece, Aristotle even quoted an estimate that the distance around the Earth was four hundred thousand stadia. It is not known exactly what length a stadium was, but it may have been about two hundred yards. This would make Aristotle's est...