Friday, December 30, 2011


there is a gap in element production: big bang nucleosynthesis produce everything up to lithium, and stars start at producing carbon, by triple-alpha fusion. in what way are beryllium ($Z=4$) and boron ($Z=5$) produced?

Friday, December 23, 2011

coldest place

is there a place in the Universe, which is colder than the CMB? (Planck's cryostats don't count as an answer)

for bonus points: which is the hottest place in the Universe?

Friday, December 16, 2011


when defining cosmological distances, on has a choice of 4 definitions: proper, comoving, angular diameter and luminosity distance. normally, distances out to a redshift z appear in this order: angular diameter distance $<$ proper distance $<$ comoving distance $<$ luminosity distance. can you think of a cosmological model, where the order is inversed?

Friday, December 9, 2011

change of redshift

it is planned to observe spectral lines of distant quasars with very high accuracy and see if their redshift changes with time. why does it change with time and would there be cosmological models where the redshift stays constant?