Wednesday, July 1, 2015

too many Maxwell equations?

why are there 2 scalar and 2 vectorial Maxwell equations in electrodynamics, which provide 8 relations in total for only two vector fields, i.e. 6 field components? is the system of equations overdetermined?

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?

Wednesday, May 27, 2015

balance dark matter and dark energy

can you compute the scale factor $a$ at which a $\Lambda$CDM-cosmology has a total deceleration of zero? is there an equivalent more general result for dark energy components with a constant dark energy equation of state parameter $w$?

Wednesday, May 20, 2015

thermal vs. kinetic SZ-effect

imagine there's turbulent motion in the ICM inside a cluster of galaxies: would that generate a thermal or a kinetic SZ-signal?