Wednesday, June 24, 2015

critical density in stationary universes

imagine that the scale factor does not change, because we're at the point of maximum extend of a recollapsing universe. would it be possible to define a critical density?

Wednesday, June 17, 2015

conservation laws in relativity

why can you formulate energy-momentum conservation in general relativity in a differential form but not in all cases in integral form?

Wednesday, June 10, 2015

$\Lambda$ in 2 dimensions

relativity does not allow gravitational effects in two dimensions - but what about a cosmological constant?

Wednesday, June 3, 2015

cosmology and chemistry

can you estimate how many stars there are in the observable universe and give your answer in units of Avogadro's number?