The observable universe is depicted by a celestial sphere of the Universe. It comprises all matter that can be observed from Earth at the present time, because electromagnetic radiation from astronomical objects has had time to reach Earth since the beginning of the cosmological expansion. There are at least 2 trillion galaxies in the observable universe, containing more stars than all the grains of sand on planet Earth. Assuming the Universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical volume (a ball) centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth.
- ↑ Christopher J. Conselice et al (2016). "The Evolution of Galaxy Number Density at z < 8 and its Implications". The Astrophysical Journal 830 (2): 83. doi:10.3847/0004-637X/830/2/83. Bibcode: 2016ApJ...830...83C.
- ↑ Fountain, Henry (17 October 2016). "Two Trillion Galaxies, at the Very Least". New York Times. https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html. Retrieved 17 October 2016.
- ↑ Mackie, Glen (1 February 2002). "To see the Universe in a Grain of Taranaki Sand". Centre for Astrophysics and Supercomputing. http://astronomy.swin.edu.au/~gmackie/billions.html. Retrieved 28 January 2017.
- ↑ "CNN.com - Star survey reaches 70 sextillion - Jul. 23, 2003". http://www.cnn.com/2003/TECH/space/07/22/stars.survey/.
- ↑ "Which Is Greater, The Number Of Sand Grains On Earth Or Stars In The Sky?" (in en). NPR.org. http://www.npr.org/sections/krulwich/2012/09/17/161096233/which-is-greater-the-number-of-sand-grains-on-earth-or-stars-in-the-sky.