This article is a
list of notable unsolved problems in
astronomy. Problems may be theoretical or experimental. Theoretical problems result from inability of current
theories to explain observed phenomena or experimental results. Experimental problems result from inability to test or investigate a proposed theory. Other problems involve unique events or occurrences that have not repeated themselves with unclear causes.
Are the mountains the remnant of hot and fast-rotating young Iapetus?
Are the mountains the result of material (either from the rings of Saturn or its ring) that over time collected upon the surface?[3][4]
Extra-solar
How common are
Solar System-like planetary systems? Some observed
planetary systems contain
Super-Earths and
Hot Jupiters that orbit very close to their stars. Systems with Jupiter-like planets in Jupiter-like orbits appear to be rare. There are several possibilities as to why Jupiter-like orbits are rare, including that data is lacking or the
grand tack hypothesis.[5]
How does the Sun generate its periodically reversing large-scale magnetic field?
How do other
Sol-like stars generate their magnetic fields, and what are the similarities and differences between stellar activity cycles and that of the Sun?[6]
What caused the
Maunder Minimum and other grand minima, and how does the solar cycle recover from a minimum state?
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?[8]
Supernova: What is the 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 (FRBs): What causes these transient radio pulses from distant galaxies, lasting a few milliseconds each? Why do some FRBs repeat at unpredictable intervals but many others do not? Several models have been proposed but no one theory has become widely accepted.[9]
Nature of
KIC 8462852, commonly known as Tabby's Star: What is the origin of the unusual
luminosity changes of this star?
Galactic astronomy and astrophysics
Galaxy rotation problem: Is
dark matter (solely) responsible for differences in observed and theoretical speed of stars revolving around the center of galaxies?
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)? In the local (primarily thin) disk of the
Milky Way, there appears to be no evidence of a strong AMR.[11] 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 indicates that there is an age-metallicity relation present in the thick disk.[12][13] Stellar ages from
asteroseismology confirm the lack of any strong age-metallicity relation in the Galactic disc.[14]
What is the origin of the
Galactic Center GeV excess?[15] Is it due to the annihilation of dark matter particles or a new population of millisecond pulsars?
The infrared/TeV crisis: Lack of attenuation of very energetic gamma rays from extragalactic sources.[16][17][18]
How do the most distant
quasars grow their
supermassive black holes up to 1010 solar masses so early in the history of the universe (with redshift greater than 6 to 7)?
If so—meaning black holes can evaporate away—what happens to the information stored in them? This appears to be an issue because the
unitarity of quantum mechanics does not allow for the
destruction of information. Does the radiation stop at some point for black hole remnants?
Firewalls: Do firewalls exist around black holes?[21]
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.[22] However, theory predicts that when supermassive black holes reach a separation of about one parsec, it may take billions of years to orbit closely enough to merge—greater than the age of the universe.[23]
Is the
CMB dipole purely kinematic, or does it signal
anisotropy of the universe, resulting in the breakdown of the
FLRW metric and the cosmological principle?[24]
Is the
Hubble tension evidence that the cosmological principle is false?[24]
If the cosmological principle is correct, is the
FLRW metric the correct metric describing the universe?[27][24]
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 this observation be a coincidence of
timing?
Is dark energy a pure cosmological constant or are models of
quintessence such as
phantom energy applicable?
The diameter of the
observable universe is approximately 93 billion light-years; what is the size of the whole universe? Is it infinite?
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. 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?
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?[30]
Hubble tension: If ΛCDM is correct, why are measurements of the Hubble constant failing to converge?[35]
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?
Is there a
multiverse and is such a concept relevant? Are such ideas
scientifically-testable or will they forever remain in the realm of
pseudoscience? Are such
metaphysical questions interpretable in the fields of cosmology, astronomy, physics, or any other
scientific discipline?
Is there potentially an infinite amount of extraterrestrial genera throughout our universe? If so, what is the explanation for the
Fermi paradox?[36][37]
^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.
^Casagrande, L.; Silva Aguirre, V.; Schlesinger, K. J.; Stello, D.; Huber, D.; Serenelli, A. M.; Scho Nrich, R.; Cassisi, S.; Pietrinferni, A.; Hodgkin, S.; Milone, A. P.;
Feltzing, S.; Asplund, M. (2015). "Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA". Monthly Notices of the Royal Astronomical Society. 455 (1): 987–1007.
arXiv:1510.01376.
Bibcode:
2016MNRAS.455..987C.
doi:
10.1093/mnras/stv2320.
S2CID119113283.
^
abcdeAbdalla, Elcio; Abellán, Guillermo Franco; Aboubrahim, Armin (11 Mar 2022), "Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies", Journal of High Energy Astrophysics, 34: 49,
arXiv:2203.06142v1,
Bibcode:
2022JHEAp..34...49A,
doi:
10.1016/j.jheap.2022.04.002,
S2CID247411131
^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.
S2CID119076077. 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.
^Kiger, Patrick J. (2012-06-21).
"What is the Wow! signal?". National Geographic Channel. Archived from
the original on March 13, 2015. Retrieved 2016-07-02.
This article is a
list of notable unsolved problems in
astronomy. Problems may be theoretical or experimental. Theoretical problems result from inability of current
theories to explain observed phenomena or experimental results. Experimental problems result from inability to test or investigate a proposed theory. Other problems involve unique events or occurrences that have not repeated themselves with unclear causes.
Are the mountains the remnant of hot and fast-rotating young Iapetus?
Are the mountains the result of material (either from the rings of Saturn or its ring) that over time collected upon the surface?[3][4]
Extra-solar
How common are
Solar System-like planetary systems? Some observed
planetary systems contain
Super-Earths and
Hot Jupiters that orbit very close to their stars. Systems with Jupiter-like planets in Jupiter-like orbits appear to be rare. There are several possibilities as to why Jupiter-like orbits are rare, including that data is lacking or the
grand tack hypothesis.[5]
How does the Sun generate its periodically reversing large-scale magnetic field?
How do other
Sol-like stars generate their magnetic fields, and what are the similarities and differences between stellar activity cycles and that of the Sun?[6]
What caused the
Maunder Minimum and other grand minima, and how does the solar cycle recover from a minimum state?
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?[8]
Supernova: What is the 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 (FRBs): What causes these transient radio pulses from distant galaxies, lasting a few milliseconds each? Why do some FRBs repeat at unpredictable intervals but many others do not? Several models have been proposed but no one theory has become widely accepted.[9]
Nature of
KIC 8462852, commonly known as Tabby's Star: What is the origin of the unusual
luminosity changes of this star?
Galactic astronomy and astrophysics
Galaxy rotation problem: Is
dark matter (solely) responsible for differences in observed and theoretical speed of stars revolving around the center of galaxies?
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)? In the local (primarily thin) disk of the
Milky Way, there appears to be no evidence of a strong AMR.[11] 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 indicates that there is an age-metallicity relation present in the thick disk.[12][13] Stellar ages from
asteroseismology confirm the lack of any strong age-metallicity relation in the Galactic disc.[14]
What is the origin of the
Galactic Center GeV excess?[15] Is it due to the annihilation of dark matter particles or a new population of millisecond pulsars?
The infrared/TeV crisis: Lack of attenuation of very energetic gamma rays from extragalactic sources.[16][17][18]
How do the most distant
quasars grow their
supermassive black holes up to 1010 solar masses so early in the history of the universe (with redshift greater than 6 to 7)?
If so—meaning black holes can evaporate away—what happens to the information stored in them? This appears to be an issue because the
unitarity of quantum mechanics does not allow for the
destruction of information. Does the radiation stop at some point for black hole remnants?
Firewalls: Do firewalls exist around black holes?[21]
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.[22] However, theory predicts that when supermassive black holes reach a separation of about one parsec, it may take billions of years to orbit closely enough to merge—greater than the age of the universe.[23]
Is the
CMB dipole purely kinematic, or does it signal
anisotropy of the universe, resulting in the breakdown of the
FLRW metric and the cosmological principle?[24]
Is the
Hubble tension evidence that the cosmological principle is false?[24]
If the cosmological principle is correct, is the
FLRW metric the correct metric describing the universe?[27][24]
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 this observation be a coincidence of
timing?
Is dark energy a pure cosmological constant or are models of
quintessence such as
phantom energy applicable?
The diameter of the
observable universe is approximately 93 billion light-years; what is the size of the whole universe? Is it infinite?
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. 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?
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?[30]
Hubble tension: If ΛCDM is correct, why are measurements of the Hubble constant failing to converge?[35]
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?
Is there a
multiverse and is such a concept relevant? Are such ideas
scientifically-testable or will they forever remain in the realm of
pseudoscience? Are such
metaphysical questions interpretable in the fields of cosmology, astronomy, physics, or any other
scientific discipline?
Is there potentially an infinite amount of extraterrestrial genera throughout our universe? If so, what is the explanation for the
Fermi paradox?[36][37]
^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.
^Casagrande, L.; Silva Aguirre, V.; Schlesinger, K. J.; Stello, D.; Huber, D.; Serenelli, A. M.; Scho Nrich, R.; Cassisi, S.; Pietrinferni, A.; Hodgkin, S.; Milone, A. P.;
Feltzing, S.; Asplund, M. (2015). "Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA". Monthly Notices of the Royal Astronomical Society. 455 (1): 987–1007.
arXiv:1510.01376.
Bibcode:
2016MNRAS.455..987C.
doi:
10.1093/mnras/stv2320.
S2CID119113283.
^
abcdeAbdalla, Elcio; Abellán, Guillermo Franco; Aboubrahim, Armin (11 Mar 2022), "Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies", Journal of High Energy Astrophysics, 34: 49,
arXiv:2203.06142v1,
Bibcode:
2022JHEAp..34...49A,
doi:
10.1016/j.jheap.2022.04.002,
S2CID247411131
^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.
S2CID119076077. 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.
^Kiger, Patrick J. (2012-06-21).
"What is the Wow! signal?". National Geographic Channel. Archived from
the original on March 13, 2015. Retrieved 2016-07-02.