The Hubble Ultra-Deep Field image shows some of the most remote galaxies visible with present technology, each consisting of billions of stars. (Apparent image area about 1/79 that of a full moon)[1]
Age (within Lambda-CDM model) 13.799 ± 0.021 billion years[2]
Diameter At least 156–554 billion light-years (47.8–170 billion parsecs)[3][4][5] (or infinite[6])
Mass (ordinary matter) At least 1053 kg[7]
Average density 4.5 x 10−31 g/cm3[8]
Average temperature 2.72548 K[9]
Main Contents Ordinary (baryonic) matter (4.9%)
Dark matter (26.8%)
Dark energy (68.3%)[10]
Shape Flat with only a 0.4% margin of error[11]

The Universe is all of space and time (spacetime) and its contents,[12] which includes planets, moons, stars, galaxies, the contents of intergalactic space and all matter and energy.[13][14][15] While the size of the entire Universe is still unknown,[6] it is possible to measure the observable universe.

The earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the centre of the Universe.[16][17] Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the centre of the Solar System. In developing the law of universal gravitation, Sir Isaac Newton built upon Copernicus's work as well as observations by Tycho Brahe and Johannes Kepler's laws of planetary motion.

Further observational improvements led to the realization that our Solar System is located in the Milky Way galaxy, which is one of many galaxies in the Universe. It is assumed that galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. Discoveries in the early 20th century have suggested that the Universe had a beginning and that it is expanding[18] at an increasing rate.[19] Roughly eighty percent of mass in the Universe appears to exist in an unknown form called dark matter which cannot be directly observed.[20]

The Big Bang theory is the prevailing cosmological description of the development of the Universe. Under this theory, space and time emerged together 13.799±0.021 billion years ago[2] with a fixed amount of energy and matter that has become less dense as the Universe has expanded. After the initial expansion, the Universe cooled, allowing the first subatomic particles to form and then simple atoms. Giant clouds later merged through gravity to form galaxies, stars, and everything else seen today. It is possible to see objects that are now further away than 13.799 billion light-years because space itself has expanded. This means that objects which are now 46 billion light years away can still be seen in their distant past, because at that time they were much closer to us.

There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which the Universe might be one among many universes that likewise exist.[6][21][22]

  1. ^ "Hubble sees galaxies galore". spacetelescope.org. Retrieved April 30, 2017. 
  2. ^ a b Planck Collaboration (2015). "Planck 2015 results. XIII. Cosmological parameters (See Table 4 on page 31 of pfd)". Astronomy & Astrophysics. 594: A13. arXiv:1502.01589Freely accessible. Bibcode:2016A&A...594A..13P. doi:10.1051/0004-6361/201525830. 
  3. ^ "MSU researcher recognized for discoveries about universe". December 21, 2004. Retrieved February 8, 2011. 
  4. ^ Itzhak Bars; John Terning (2009). Extra Dimensions in Space and Time. Springer. pp. 27ff. ISBN 978-0-387-77637-8. Retrieved May 1, 2011. 
  5. ^ SPACE.com – Universe Measured: We're 156 Billion Light-years Wide!
  6. ^ a b c Greene, Brian (2011). The Hidden Reality. Alfred A. Knopf. 
  7. ^ Paul Davies (2006). The Goldilocks Enigma. First Mariner Books. p. 43ff. ASIN 0547053584. ISBN 978-0-618-59226-5. 
  8. ^ NASA/WMAP Science Team (January 24, 2014). "Universe 101: What is the Universe Made Of?". NASA. Retrieved February 17, 2015. 
  9. ^ Fixsen, D. J. (2009). "The Temperature of the Cosmic Microwave Background". The Astrophysical Journal. 707 (2): 916–20. arXiv:0911.1955Freely accessible. Bibcode:2009ApJ...707..916F. doi:10.1088/0004-637X/707/2/916. 
  10. ^ Cite error: The named reference planck2013parameters was invoked but never defined (see the help page).
  11. ^ NASA/WMAP Science Team (January 24, 2014). "Universe 101: Will the Universe expand forever?". NASA. Retrieved April 16, 2015. 
  12. ^ Zeilik, Michael; Gregory, Stephen A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. ISBN 0030062284. The totality of all space and time; all that is, has been, and will be. 
  13. ^ "Universe". Encyclopaedia Britannica online. Encyclopaedia Britannica Inc. 2012. Retrieved 17 February 2018. 
  14. ^ "Universe". Merriam-Webster Dictionary. Retrieved September 21, 2012. 
  15. ^ "Universe". Dictionary.com. Retrieved September 21, 2012. 
  16. ^ Dold-Samplonius, Yvonne (2002). From China to Paris: 2000 Years Transmission of Mathematical Ideas. Franz Steiner Verlag. 
  17. ^ Thomas F. Glick; Steven Livesey; Faith Wallis. Medieval Science Technology and Medicine: An Encyclopedia. Routledge. 
  18. ^ Hawking, Stephen (1988). A Brief History of Time. Bantam Books. p. 125. ISBN 0-553-05340-X. 
  19. ^ "The Nobel Prize in Physics 2011". Retrieved April 16, 2015. 
  20. ^ Redd, Nola. "What is Dark Matter?". Space.com. Retrieved 1 February 2018. 
  21. ^ Cite error: The named reference EllisKS032 was invoked but never defined (see the help page).
  22. ^ Palmer, Jason. (August 3, 2011) BBC News – 'Multiverse' theory suggested by microwave background. Retrieved 2011-11-28.

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