matter to be crushed inwards under its own gravity. This instability was clarified in 1902 by the Jeans instability criterion. One solution to these paradoxes is the Charlier Universe, in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system, ad infinitum) in a fractal way such that the universe has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 by Johann Heinrich Lambert. During the 18th century, Immanuel Kant speculated that nebulae could be entire galaxies separate from the Milky Way, and in 1850, Alexander von Humboldt called these separate galaxies Weltinseln, or "world islands", a term that later developed into "island universes". In 1919, when the Hooker Telescope was completed, the prevailing view still was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope, Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that Andromeda Nebula and Triangulum among others, were entire galaxies outside our own, thus proving that universe consists of a multitude of galaxies. The modern era of physical cosmology began in 1917, when Albert Einstein first applied his general theory of relativity to model the structure and dynamics of the universe. The discoveries of this era, and the questions that remain unanswered, are outlined in the sections above. See also Cosmic Calendar (scaled down timeline) Cosmic latte Detailed logarithmic timeline Earth's location in the universe False vacuum Future of an expanding universe Galaxy And Mass Assembly survey Heat death of the universe History of the center of the Universe Illustris project Non-standard cosmology Nucleocosmochronology Parallel universe (fiction) Rare Earth hypothesis Space and survival Terasecond and longer Timeline of the early universe Timeline of the far future Timeline of the near future Zero-energy universe References Footnotes Citations Bibliography External links NASA/IPAC Extragalactic Database (NED) / (NED-Distances). There are about 1082 atoms in the observable universe – LiveScience, July 2021. This is why we will never know everything about our universe – Forbes, May 2019. Articles containing video clips Astronomical dynamical systems Concepts in astronomy Environments Main topic articles Physical cosmology