12 Epilogue
We have now covered most of cosmology briefly, starting from the theoretical scenar-
ios in Figure 5.1 and arriving at Figure 12.1, where the scenarios have been populated
by data which permit a selection. Thus we know now that the cosmic expansion is
accelerated since recently, since the time corresponding to redshift푧≈ 0 .8. But we
still do not know what causes the acceleration—therefore the mysterious term ‘dark
energy’, which represents 71.4% of the energy density of the Universe. In Chapter 11
we met a multitude, albeit not exhaustive set of candidates. The observational knowl-
edge has improved greatly, but the solution remains unknown. The simplest candi-
date, the cosmological constant휆, begs an answer as to its origin and its value which
has to be fine-tuned to within the 52nd decimal of zero (in units of푐=1). Determi-
nations of the equation of state of dark energy,푤, point to phantom models, but the
cosmological constant value푤=−1isstillclosetothe1휎contour, see Figure 8.7 and
the cover of this book.
The knowledge of the equally mysterious dark matter has gone from a disbelieved
curiosity in 1933 to the important field of study today described in Chapter 8, much
thanks to the enormous development in gravitational lensing. It represents 23.4% of
the energy density of the Universe, yet its composition is unknown, except that it is
cold. What has happened most recently is, that the long favored WIMP candidates
have reached the useful search limit, and that the minimal supersymmetry model
appears unlikely. In any case one should keep in mind, that the discovery of a new
particle in a terrestrial accelerator does not prove that it is dark matter. That should
be discovered at galactic scales.
Cosmic inflation is a field of vigorous study, but we are still relying on essentially
the same model as 20 years ago, as described in Section 7.2. One should construct a
renormalized field theory of quantum gravity to cope with the initial Big Bang singu-
larity and inflation, but that is beyond the level of this monography. Models of infla-
tion will be tested in the near future with the advent of more and better evidence for
gravitational waves.
Introduction to Cosmology, Fourth Edition. Matts Roos
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.