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 $w$?

# cosmology question of the week

CQW is an educational resource for theoretical astrophysics and cosmology. we post a new question every week for students to get and for teachers to stay in shape.

## Wednesday, May 27, 2015

## 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?

## Wednesday, May 13, 2015

### mechanical similarity and the moon landing

the astronauts of Apollo 14 measured the surface gravity of the Moon with a pendulum. of course you could argue that all of that was faked, that the experiment was carried out on Earth and that the footage was shown in slow motion. by how much would everything need to be slowed down? would there be another experiment like the golf ball shot by the same Apollo 14 astronauts or the dropped hammer and feather from Apollo 15 that would convince you?

## Wednesday, May 6, 2015

### FLRW-fluids

ideal cosmological fluids are characterised by their equation of state. can you present an argument why the equation of state is the only free function present in the field equation under the symmetry assumptions of a FLRW-model?

## Wednesday, April 29, 2015

### neutrinos everywhere!

please estimate how many cosmic neutrinos are in the observable Universe. are there more or less photons than neutrinos?

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