COSMIC INFLATION

There are a number of features of the universe that are difficult to understand, if the early universe had expanded at a constant rate. The top two are probably (1) Flatness/Critical Density, and (2) Isotropy/Homogeneity.  Each part of the universe very precisely has escape speed with respect to any other part. The space-time geometry of the universe is apparently flat, to an astonishing degree of precision... the universe has precisely the critical density required for that. And parts of the universe which could never have been in causal contact appear essentially identical. Indeed, what appear to have been quantum fluctuations in the very first instants of the existence of the universe evidently are somehow stamped onto the Last Scattering Surface!  In the period 1979 - 1990, an idea which solved these and other problems was fleshed out. The idea was that very early in its history, the entire universe underwent a phase transition which caused a huge expansion in the size of the currently observable universe, during a very, very brief time interval.

To understand how the current universe looks, we need the very early universe to undergo a first-order phase transition at the age of about 10−36 to 10−33 seconds!  In this interval the radius of the currently observable universe would have increased from extreme sub-quantum size to roughly 1 cm.  This inflation would be due to an inflation field (quanta: inflatons) with a huge, constant vacuum energy... a split-second de Sitter universe [a universe completely empty but having a cosmological constant]. At the end of the transition, that enormous vacuum energy would have to be zero.  Where did it go? The huge expansion would drastically cool the universe, but the required decay of the field bosons, "inflatons," into particles with enormous kinetic energy would reheat things. The scale factor during expansion would be like eHt, where H is the Hubble parameter at that time. and the key fact is that the quantum fluctuations existing during the inflationary era would thus be magnified to macroscopic size at the end of the era, and should still be seen in the surface of last scattering's temperature and density variations.  In fact the level of variation seen by WMAP and Planck on the last scattering surface is around 10−4 to 10−5, precisely as expected from the inflationary scenario. These density variations were the seeds of all the structure currently seen in our universe, augmented by the driven standing waves existing in the later soup of fermions and photons.  This era of intense particle creation in a compact space-time would also create distinctive gravitational radiation, and would produce a unique quadrupole signature of that radiation on the surface of last scattering, which it is very, very important, but also very, very difficult, to observe.


Since Inflation was introduced in the early 1980s, it has been the subject of sharp criticism and endless variations, but no convincing competing idea has emerged. The inflatons of the field must decay completely in the “reheat” phase at the end of inflation, and it seems logical that whatever particles the inflatons decay to must subsequently decay ultimately to matter, photons and neutrinos. A very big remaining question is where quarks and leptons came from! Many recent publications on the topic of generation of quarks and leptons seem to start with the decay of the inflation field... for example... and this does seem to be a logical place to start. Unfortunately, if you start there you are so far beyond the Standard Model, you might as well start in Disneyland.  Anyway, kudos to the work of these three great pioneers. The inflationary universe was, as we saw earlier,  originally the brainchild of Willem de Sitter (1872 - 1934), who found a solution for a universe dominated by vacuum energy which remained otherwise empty and flat while accelerating in expansion forever. When it became clear in the 1970s that our universe appears asymptotically flat and behaves almost as if it were completely empty, de Sitter's solution was revived, with the understanding that the period of inflation should be very brief, and turn off via some mechanism very early in the history of the universe... hence the current Inflation Model.  Recent studies of the last scattering surface (summarized in late 2021) have succeeded in ruling out many proposed versions of inflation, and future work based on observations by the just-launched SPHEREx telescope holds the promise of narrowing down the actual mechanisms of inflation to a few, or even just one.  No Nobel Prizes are going to be awarded until experimental observations give firm, direct evidence for a specific type of inflation actually happening in the extremely early universe.

The only phase transition we can actually study in some detail is the electroweak phase transition, which occurred at about 160 GeV in the early universe.  The exact time of this transition is not known, but it is often placed either before the start of the inflation transition or immediately afterward.  We can study it most directly in the lab by investigating processes happening around 246 GeV, where Higgs bosons become important, and learning as much about the Higgs boson as possible... it is the direct consequence of the symmetry breaking.  Cosmological phase transitions may not be likely in general to resemble this particular example very closely, but it's all we have at present.






a(t) is a dimensionless scale factor indicating the time evolution of the universe, introduced by Friedmann.



NO STRINGS ATTACHED!

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