how do astronomers measure the density of the universe:
Newton’s gravitational constant G:
Universe’s age:
Planck Collaboration (2015). “Planck 2015 results. XIII. Cosmological
parameters”
Bennett, C.L.; et al. (2013). ”Nine-Year Wilkinson Microwave
Anisotropy Probe (WMAP) Observations: Final Maps and
Results”.arXiv:1212.5225 [astro-ph.CO].
Critical density:
Planck Collaboration (2015). “Planck 2015 results. XIII. Cosmological
parameters”
Zombeck’s Handbook of Space Astronomy and Astrophysics.
Chapter 1, Page 11.
Chapter 10, Page 249.