## Wednesday, July 26, 2017

### CQW holiday project: ocean level around Greenland

the ocean level is elevated close to Greenland because (believe it or not!) of gravitational attraction of all the ice acting on the sea water. can you estimate how large the effect is?

bonus question 1: would a ship bound for Greenland need additional energy for sailing uphill or less energy because it's as well gravitationally attracted?

bonus question 2: if the ice sheet covering Greenland melts due to global warming, would the sea level at Greenland rise or fall?

with that, CQW wishes all our readers a nice summer vacation and hope to see you again in october. if you would like to suggest a cosmology question yourself, please send a message to cosmologyquestionoftheweek"[AT]"gmail"[DOT]"com.

## Wednesday, July 19, 2017

### horizons in relativity

an accelerated object disappears behind a horizon as well as an object at the Schwarzschild radius of a black hole: is there a difference between the two cases? is there an analogous effect in cosmology?

## Wednesday, July 12, 2017

### tidal shear invariants

the gravitational shear field could be quantified by the trace $\sum_{i}\partial^2_{ii}\Phi$, which is proportional to the density $\rho$ due to Poisson's equation $\Delta\Phi=4\pi G\rho$ or by the quantity $\sum_{ij}\partial^2_{ij}\Phi\partial^2_{ji}\Phi$. in what way do the two quantifiers differ? can you construct more of them? what would be their relation to the Ricci-scalar $R$ or the invariant curvature $R_{\mu\nu\rho\sigma}R^{\mu\nu\rho\sigma}$ in general relativity?

## Wednesday, July 5, 2017

### clocks in acceleration

can you explain how different types of clocks (string pendulum, spring pendulum, quartz oscillator, $LC$-circuit, atomic clock, Einstein-clock) are affected by an accelerated frame of reference? how is symmetry established between acceleration of the clock and acceleration of the observer, and in what way are the situations equivalent to a gravitational field?