Wednesday, July 31, 2013

Planck's summer: varying electron masses

fifth post by Youness in the "Planck's summer"-series:

How can we test whether the electron mass changes with time (as compared to the other particle masses)? Give a list of astrophysical or cosmological observations that are sensitive to the electron mass (from the early to the late Universe). Do you have an idea how to measure the present time variation of the electron mass in the lab?

Wednesday, July 24, 2013

Planck's summer: observation of the CNB

fourth post in the "Planck's summer"-series by Youness

When did the cosmic neutrino background form, why is its temperature lower than that of the CMB and how could an experiment to detect it look like?

Wednesday, July 17, 2013

Planck's summer: gauge independence of cosmological surveys

third post in the "Planck's summer"-series by Youness:

Power spectra in linear perturbation theory depend on the gauge chosen. The differences are small on scales well within the horizon, but they grow as we go to very large scales. However, observers apparently measure galaxy positions unambiguously and calculate power spectra from this. How would you interpret their data on the largest scales?

Wednesday, July 10, 2013

Planck's summer: particle size matters!

second post in the "Planck's summer"-series by Youness:

Let us assume that dark matter is made of approximately collisionless particles evolving under the laws of gravity (just one type, for simplicity). Is there a minimum mass $m_\text{min}$ these particles need to have in order to fit observational constraints? Conversely, is there an upper limit $m_\text{max}$? 

Wednesday, July 3, 2013

Planck's summer: CMB observations without CMB photons

CQW presents "Planck's summer", a series of posts composed by recent graduate Youness!

Imagine a planet with an atmosphere that is, for some reason, completely opaque for low-energy photons such that a direct ground-based detection of the CMB is impossible. Furthermore, the inhabitants of the planet do not yet possess the technology to build spacecraft that leaves the atmosphere.  However, they are very good experimentalists on the ground. How could they test whether there is a CMB confirming a basic prediction of big bang cosmology?