List of unsolved problems in astronomy
Some of the unsolved problems in astronomy are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. Some unresolved questions in astronomy pertain to one-off events, unusual occurrences that have not repeated and whose causes therefore remain unclear.
- Planetary systems: How does accretion form planetary systems? Where did Earth's water come from?
- Are there any planets beyond Neptune? What is the explanation for the elongated orbits of a group of Kuiper belt objects?
- Rotation rate of Saturn: Why does the magnetosphere of Saturn exhibit a (slowly changing) periodicity close to that at which the planet's clouds rotate? What is the true rotation rate of Saturn's deep interior?
- Solar cycle: How does the Sun generate its periodically reversing large-scale magnetic field? How do other solar-like stars generate their magnetic fields, and what are the similarities and differences between stellar activity cycles and that of the Sun? What caused the Maunder Minimum and other grand minima, and how does the solar cycle recover from a minimum state?
- Coronal heating problem: Why is the Sun's corona (atmosphere layer) so much hotter than the Sun's surface? Why is the magnetic reconnection effect many orders of magnitude faster than predicted by standard models?
- What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses – the initial mass function – apparently regardless of the initial conditions?
- Supernovae: What is the exact mechanism by which an implosion of a dying star becomes an explosion?
- p-nuclei: What astrophysical process is responsible for the nucleogenesis of these rare isotopes?
- Fast radio bursts: Transient radio pulses lasting only a few milliseconds, from emission regions thought to be no larger than a few hundred kilometres, and estimated to occur several hundred times a day. While several theories have been proposed, there is no generally accepted explanation for them. The only known repeating FRB emanates from a galaxy roughly 3 billion light years from Earth.
- The Oh-My-God particle and other ultra-high-energy cosmic rays: What physical processes create cosmic rays whose energy exceeds the GZK cuttoff?
- Nature of KIC 8462852, commonly known as Tabby's Star: What is the origin of unusual luminosity changes of this star?
Galactic astronomy and astrophysicsEdit
- Galaxy rotation problem: Is dark matter responsible for differences in observed and theoretical speed of stars revolving around the centre of galaxies, or is it something else?
- Age–metallicity relation in the Galactic disk: Is there a universal age–metallicity relation (AMR) in the Galactic disk (both "thin" and "thick" parts of the disk)? Although in the local (primarily thin) disk of the Milky Way there is no evidence of a strong AMR, a sample of 229 nearby "thick" disk stars has been used to investigate the existence of an age–metallicity relation in the Galactic thick disk, and indicate that there is an age–metallicity relation present in the thick disk. Stellar ages from asteroseismology confirm the lack of any strong age-metallicity relation in the Galactic disc.
- Gravitational singularities: Does general relativity break down in the interior of a black hole due to quantum effects, torsion, or other phenomena?
- No-hair theorem: Do black holes have an internal structure? If so, how might the internal structure be probed?
- Black hole information paradox and black hole radiation: Do black holes produce thermal radiation, as expected on theoretical grounds? If so, and black holes can evaporate away, what happens to the information stored in them (since quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants?
- Firewalls: Does a firewall exist around a black hole?
- Final parsec problem: Supermassive black holes appear to have merged, and what appears to be a pair in this intermediate range has been observed, in PKS 1302-102. However, theory predicts that when supermassive black holes reach a separation of about one parsec, it would take billions of years to orbit closely enough to merge - more than the age of the universe. 
- Dark matter: What is the identity of dark matter? Is it a particle? Is it the lightest superpartner (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?
- Dark energy: What is the cause of the observed accelerated expansion (de Sitter phase) of the universe? Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the right time? Is dark energy a pure cosmological constant or are models of quintessence such as phantom energy applicable?
- Baryon asymmetry: Why is there far more matter than antimatter in the observable universe?
- Cosmological constant problem: Why does the zero-point energy of the vacuum not cause a large cosmological constant? What cancels it out?
- Size and shape of the universe: The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe? What is the 3-manifold of comoving space, i.e. of a comoving spatial section of the universe, informally called the "shape" of the universe? Neither the curvature nor the topology is presently known, though the curvature is known to be "close" to zero on observable scales. The cosmic inflation hypothesis suggests that the shape of the universe may be unmeasurable, but, since 2003, Jean-Pierre Luminet, et al., and other groups have suggested that the shape of the universe may be the Poincaré dodecahedral space. Is the shape unmeasurable; the Poincaré space; or another 3-manifold?
- Cosmic inflation: Is the theory of cosmic inflation in the very early universe correct, and, if so, what are the details of this epoch? What is the hypothetical inflaton scalar field that gave rise to this cosmic inflation? If inflation happened at one point, is it self-sustaining through inflation of quantum-mechanical fluctuations, and thus ongoing in some extremely distant place?
- Horizon problem: Why is the distant universe so homogeneous when the Big Bang theory seems to predict larger measurable anisotropies of the night sky than those observed? Cosmological inflation is generally accepted as the solution, but are other possible explanations such as a variable speed of light more appropriate?
- Axis of evil: Some large features of the microwave sky at distances of over 13 billion light years appear to be aligned with both the motion and orientation of the solar system. Is this due to systematic errors in processing, contamination of results by local effects, or an unexplained violation of the Copernican principle?
- Origin and future of the universe: How did the conditions for anything to exist arise? Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch, or a Big Bounce? Or is it part of an infinitely recurring cyclic model?
- Carnegie Institution (16 June 2014). "Making Earth-Like Planets: Five Great Mysteries". YouTube.
- See Planets beyond Neptune#Orbits of distant objects for details.
- "Scientists Find That Saturn's Rotation Period is a Puzzle". NASA. June 28, 2004. Retrieved 2007-03-22.
- Michael J. Thompson (2014). "Grand Challenges in the Physics of the Sun and Sun-like Stars". arXiv:1406.4228v1 [astro-ph.SR].
- Kroupa, Pavel (2002). "The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systems". Science. 295 (5552): 82–91. arXiv:astro-ph/0201098. Bibcode:2002Sci...295...82K. doi:10.1126/science.1067524. PMID 11778039.
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- Devlin, Hannah (2018-01-10). "Astronomers may be closing in on source of mysterious fast radio bursts". The Guardian. Retrieved 2018-01-22.
- Wolchover, Natalie (2015-05-14). "The Particle That Broke a Cosmic Speed Limit". Quanta Magazine. Retrieved 2018-05-04.
- Casagrande, L.; Schönrich, R.; Asplund, M.; Cassisi, S.; Ramírez, I.; Meléndez, J.; Bensby, T.; Feltzing, S. (2011). "New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s)". Astronomy & Astrophysics. 530: A138. arXiv:1103.4651. Bibcode:2011A&A...530A.138C. doi:10.1051/0004-6361/201016276.
- Bensby, T.; Feltzing, S.; Lundström, I. (July 2004). "A possible age-metallicity relation in the Galactic thick disk?". Astronomy and Astrophysics. 421 (3): 969–976. arXiv:astro-ph/0403591. Bibcode:2004A&A...421..969B. doi:10.1051/0004-6361:20035957.
- Gilmore, G.; Asiri, H. M. (2011). "Open Issues in the Evolution of the Galactic Disks". Stellar Clusters & Associations: A RIA Workshop on Gaia. Proceedings. Granada: 280. Bibcode:2011sca..conf..280G.
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- Peres, Asher; Terno, Daniel R. (2004). "Quantum information and relativity theory". Reviews of Modern Physics. 76 (1): 93–123. arXiv:quant-ph/0212023. Bibcode:2004RvMP...76...93P. doi:10.1103/revmodphys.76.93.
- Ouellette, Jennifer (21 December 2012). "Black Hole Firewalls Confound Theoretical Physicists". Scientific American. Archived from the original on 9 November 2013. Retrieved 29 October 2013. Originally published Archived 3 June 2014 at the Wayback Machine in Quanta, December 21, 2012.
- D'Orazio, Daniel J.; Haiman, Zoltán; Schiminovich, David (17 September 2015). "Relativistic boost as the cause of periodicity in a massive black-hole binary candidate". Nature. 525 (7569): 351–353. arXiv:1509.04301. Bibcode:2015Natur.525..351D. doi:10.1038/nature15262.
- Milosavljević, Miloš; Merritt, David (October 2003). "The Final Parsec Problem" (PDF). AIP Conference Proceedings. American Institute of Physics. 686 (1): 201–210. arXiv:astro-ph/0212270. Bibcode:2003AIPC..686..201M. doi:10.1063/1.1629432.
- Brooks, Michael (March 19, 2005). "13 Things That Do Not Make Sense". New Scientist. Issue 2491. Retrieved March 7, 2011.
- Steinhardt, P. & Turok, N. (2006). "Why the Cosmological constant is so small and positive". Science. 312 (5777): 1180–1183. arXiv:astro-ph/0605173. Bibcode:2006Sci...312.1180S. doi:10.1126/science.1126231. PMID 16675662.
- Wang, Qingdi; Zhu, Zhen; Unruh, William G. (2017-05-11). "How the huge energy of quantum vacuum gravitates to drive the slow accelerating expansion of the Universe". Physical Review D. 95 (10): 103504. arXiv:1703.00543. Bibcode:2017PhRvD..95j3504W. doi:10.1103/PhysRevD.95.103504.
This problem is widely regarded as one of the major obstacles to further progress in fundamental physics [...] Its importance has been emphasized by various authors from different aspects. For example, it has been described as a “veritable crisis” [...] and even “the mother of all physics problems” [...] While it might be possible that people working on a particular problem tend to emphasize or even exaggerate its importance, those authors all agree that this is a problem that needs to be solved, although there is little agreement on what is the right direction to find the solution.
- Podolsky, Dmitry. "Top ten open problems in physics". NEQNET. Archived from the original on 22 October 2012. Retrieved 24 January 2013.
- "Rare Earth: Complex Life Elsewhere in the Universe?". Astrobiology Magazine. Archived from the original on 28 June 2011. Retrieved 12 August 2006.
- Sagan, Carl. "The Quest for Extraterrestrial Intelligence". Cosmic Search Magazine. Archived from the original on 18 August 2006. Retrieved 12 August 2006.
- Kiger, Patrick J. (2012-06-21). "What is the Wow! signal?". National Geographic Channel. Retrieved 2016-07-02.