The cosmological constant, dark energy, and inflation

Partly settled. Some of it is understood, some is still argued about.

Ginzburg called this 'perhaps the most crucial' item on his list (#23), combining the mystery of the Lambda-term (vacuum energy) with inflation theory. Just as he was writing, observations of distant Type Ia supernovae (Riess et al. 1998, Perlmutter et al. 1999) revealed the universe's expansion is accelerating, implying a nonzero cosmological constant/dark energy (2011 Nobel Prize). Since then, precision cosmology (WMAP, then Planck, 2013-2018) has pinned down the LCDM parameters to percent-level accuracy (dark energy density about 68%, matter about 32%), firmly establishing dark energy's existence. Its fundamental origin remains completely unexplained (the notorious ~120-order-of-magnitude mismatch between naive quantum-field-theory predictions and the observed value), and inflation, while the leading paradigm, still lacks direct confirmation — the 2014 BICEP2 claim of primordial gravitational-wave B-modes was retracted in 2015 after being attributed to galactic dust.

The vacuum of empty space itself seems to be pushing the universe apart, and the mismatch between the predicted and observed size of that push is the single largest quantitative failure in the history of physics.

References

• What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
• Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, A. G. Riess et al., The Astronomical Journal 116, 1009, 1998
• Measurements of Omega and Lambda from 42 High-Redshift Supernovae, S. Perlmutter et al., The Astrophysical Journal 517, 565, 1999
• Planck 2018 results. VI. Cosmological parameters, Planck Collaboration, Astronomy & Astrophysics 641, A6, 2020