how do astronomers measure the density of the universe:

http://www.astro.ucla.edu/~wright/density.html
Newton’s gravitational constant G:
”The 2014 CODATA Recommended Values of the Fundamental Physical Constants” ( Last update: June 25 2015). Availablephttp://physics.nist.gov/constants

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”
”The 2014 CODATA Recommended Values of the Fundamental Physical Constants” ( Last update: June 25 2015). Available: http://physics.nist.gov/constants
Zombeck’s Handbook of Space Astronomy and Astrophysics.
Chapter 1, Page 11.
Chapter 10, Page 249.