The father of modern observational cosmology, Allan Sandage, passed away on November 13th at the age of 84. He is remembered for arriving at the first credible value of the Hubble constant, the age of the universe, and his Hilbert-esque list of important outstanding problems in cosmology.
In 1995, at a conference in the Canary Islands, Sandage formulated “23 astronomical problems for the next three decades“.
Sandage’s 23 Problems for Astrophysics/Cosmology
Understanding the Hubble Sequence
1. Is the sequence due to evolution or initial conditions?
2. Parameters that do and do not vary along it.
3. Cause of the sequence width (van den Bergh luminosity classes).
4. Is spiral structure totally kinematic? What is the role of rotation?
5. Is in initial star formation rate (SFR) the principle driver?
6. Cosmogony of the density-morphology relation.
7. Role (if any) of mergers.
8. Origin and age of dust (AGB stars)?
Stellar Evolution and the Galaxy
9. Age, kinematics, and distribution functions of all Galactic components.
10. The cosmogony of the distributions.
11. The sequence of events (appearance of the early galaxy).
12. The age-metallicity relation at different Galactic positions (the “wave of death” from the centre outwards, “the galaxy is a living thing”).
13. The mass function from stars to rocks. Rocks between the stars?
14. Star counts to map the halo and the thick disk (vast modern database capabilities).
15. Is the expansion real? The Tolman SB (1 + z)⁴ test, time dilation with SNe: t(z) = t(0)/(1+z), and the MWB temperature: T(z) = T(0)/(1+z).
16. Evolution in the look-back time (primeval galaxies).
17. The distance scale: Calibration of all indicators, the bias problems of all indicators, and why some methods are wrong.
19. Explain the excess N(m) counts.
20. The nature of the dark matter: Ω.
21.The Hubble-de Vaucoleurs Paradox: Are there significant velocity deviations from the pure cosmological expansion?
22. The IGM dust (gad, dust, and rocks between galaxies?)
23. The formation time for large-scale structure (are clusters and groups old or young?)
These problems have helped shaped the current searches in observational cosmology [2007, 2008] and none have been completely resolved yet, so Sandage leaves behind great work to be done by future generations.