The evolution of the number density of galaxies in the universe , and thus also the total number of galaxies , is a fundamental question with implications for a host of astrophysical problems including galaxy evolution and cosmology .
However there has never been a detailed study of this important measurement , nor a clear path to answer it .
To address this we use observed galaxy stellar mass functions up to z \sim 8 to determine how the number densities of galaxies changes as a function of time and mass limit .
We show that the increase in the total number density of galaxies ( \phi _ { T } ) , more massive than M _ { * } = 10 ^ { 6 } M _ { \odot } , decreases as \phi _ { T } \sim t ^ { -1 } , where t is the age of the universe .
We further show that this evolution turns-over and rather increases with time at higher mass lower limits of M _ { * } > 10 ^ { 7 } M _ { \odot } .
By using the M _ { * } = 10 ^ { 6 } M _ { \odot } lower limit we further show that the total number of galaxies in the universe up to z = 8 is 2.0 ^ { +0.7 } _ { -0.6 } \times 10 ^ { 12 } ( two trillion ) , almost a factor of ten higher than would be seen in an all sky survey at Hubble Ultra-Deep Field depth .
We discuss the implications for these results for galaxy evolution , as well as compare our results with the latest models of galaxy formation .
These results also reveal that the cosmic background light in the optical and near-infrared likely arise from these unobserved faint galaxies .
We also show how these results solve the question of why the sky at night is dark , otherwise known as Olbers ’ paradox .