## 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?