Andrei Linde: Inflationary Cosmology after Planck 2013

I give a general review of inflationary cosmology and of its present status, in view of the 2013 data release by the Planck satellite.
A specific emphasis is given to the new broad class of theories, the cosmological attractors, which have nearly model-independent predictions converging at the sweet spot of the Planck data in the (ns, r) plane.
I also discuss the problem of initial conditions for the theories favored by the Planck data.

Three decades ago, in inflationary theory looked like an exotic product of vivid scientific imagination.
Some of us believed that it possesses such a great explanatory potential that it must be correct; some others thought that it is too good to be true.
Not many expected that it is possible to verify any of its predictions in our lifetime.
Thanks to the enthusiastic work of many scientists, in inflationary theory is gradually becoming a broadly accepted cosmological paradigm, with many of its predictions being confirmed by observational data.
The new data release by Planck 2013 stimulating the development of new cosmological
theories, by changing the goal from finding various complicated models capable of describing
large local non-Gaussiantiy to the development of new elegant models of inflation capable of
explaining increasingly precise data in the (ns, r) plane.
Just few months ago, we did not know many good inflationary models which would naturally predict the data favored by Planck 2013.
Now the situation has changed. The existence of the universal attractor regime for a large set
of different inflationary models does not guarantee that we are on the right track, but it is
hard to ignore that all of these cosmological attractors point in the same direction, and their
predictions converge at the “sweet spot” in the (ns, r) plane preferred by WMAP9 and Planck 2013 (…)
…

Collisions on the sky

First Observational Tests of Eternal Inflation

Stephen M. Feeney, Matthew C. Johnson, Daniel J. Mortlock, and Hiranya V. Peiris – Phys. Rev. Lett. 107, 071301 (Published August 8, 2011)

First observational tests of eternal inflation: Analysis methods and WMAP 7-year results

Stephen M. Feeney, Matthew C. Johnson, Daniel J. Mortlock, and Hiranya V. Peiris – Phys. Rev. D 84, 043507 (Published August 8, 2011)

A rapid expansion of the early universe—cosmological inflation—offers the best explanation so far of what astronomers observe. One flavor of this concept, called eternal inflation, involves nucleation of bubble universes (one of which we are in) in a continually inflating background. The collection of bubbles is known as the multiverse. Many aspects of this model would be difficult, if not impossible, to test, but one approach is to look for imprints of bubble behavior in the cosmic microwave background radiation. In a paper in Physical Review Letters, and a longer technical exposition in Physical Review D, Stephen Feeney at University College London and his colleagues analyze seven years of cosmic microwave background data from the Wilkinson Microwave Anisotropy Probe (WMAP) to hunt for signatures of eternal inflation.
The authors zeroed in on what might be the best target of opportunity, namely the effects of bubble collisions. Although direct observation of other bubble universes is not possible, the bubbles might have collided with one another and this bashing together might have left imprints on the cosmic microwave background, essentially itself a palimpsest of cosmic history. Feeney et al. carry out a statistical analysis of the full sky WMAP data, searching for specific distortions that may have been the wreckage of bubble crashes. They find no signatures of collisions, but are able to use this null result to put an upper limit on the number of bubble collisions the theory could predict and still be consistent with the data. The hope now is to continue the search for eternal inflation by applying the same tests to better data expected from the Planck satellite, which was launched in 2009. – David Voss