Red nuggets are the early universe's impossible objects: quiescent galaxies at z of 2 to 3 carrying 10^10 to 10^11 solar masses of old stars inside effective radii of only 1 to 2 kpc, as much mass as a modern giant elliptical packed into a fifth to a tenth of the radius, already finished forming stars when the universe was three billion years old (Damjanov et al. 2009; van Dokkum et al. 2015).
Each property strains ΛCDM and the combination compounds them. The mass requires early vigorous star formation; the compactness requires dissipative collapse more extreme than hierarchical models produce; the quiescence requires that the vigorous engine then shut off completely and stay off; and the subsequent evolution requires these dense relics to inflate by factors of several into today's extended ellipticals through a carefully balanced diet of minor dry mergers that add outskirts without disturbing cores. A final wrinkle sharpens the test: a few compact relics survive to low redshift apparently un-puffed, so the growth mechanism must operate on most of the population while sparing some (Hopkins et al. 2009; de la Rosa et al. 2016).
The standing is a complete life-cycle puzzle, formation, quenching, growth, and survivors, each stage individually modelable and the ensemble resistant to a unified account. JWST spectroscopy is now resolving the stellar populations and formation epochs of nuggets at their birth redshifts.