INFLATION

There are two features of the universe that are difficult to understand, if the early universe had expanded at a constant rate. (1) Flatness/Critical Density, and (2) Isotropy/Homogeneity. Each part of the universe very precisely has escape speed with respect to any other part. And parts of the universe which could never have been in causal contact appear essentially identical. 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 size of the currently observable universe, during a very, very brief time interval.

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"The smoking gun," inflation definitely did happen.

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 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.  At the end of the transition, that vacuum energy would have to be zero.  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, 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 Big Flash 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 in the Last Scattering Surface, which it is very, very important to be able to observe.

In the early days it was hoped that the spectrum of the last scattering surface would be sensitive to inflation. It is actually not, alas.

Inflation could have occurred by a scalar field rolling down a potential energy hill. When the field rolls very slowly compared to the expansion of the Universe, inflation occurs. However, when the hill becomes steeper, inflation ends and reheating can occur.



Simulated data show how the SPHEREx survey should be able to pen down the type of inflation that occurred.

Multiple-field Inflation!


Inflation has the bizarre property of being unbounded in time from below!

THE SOUTH POLE OBSERVATORIES


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. Many recent publications on the topic of Baryogenesis 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 holds the promise of narrowing down the actual mechanisms of inflation to a few, or even just one.




GRAVITATIONAL RADIATION

• Pulsar Timing--- Rapidly rotating neutron stars, if continuously monitored, show changes in the timing of their radio pulses if a gravitational wave passes between the star and the earth. Pulsar timing arrays (PTAs) provide a network of millisecond pulsars that can be observed simultaneously to detect gravitational waves passing through the array. There are six different arrays currently being monitored.

• Laser Interferometers--- It has been a great triumph finally to observe gravitational waves directly. The current efforts using ground-based interferometers are just sensitive enough to detect binary black hole collisions, and binary neutron star collisions... but upgrades are feasible. 

• Astronomical Interferometers--- Currently the prospects for space interferometers with astronomically long baselines seem very remote indeed.

Attempts to study the quadrupole polarization of the last scattering surface are proving very difficult, but are still easier than constructing huge interferometers.  And as Niels Bohr said, “It is difficult to predict, especially the future.”






The huge number of gravitational-wave generating events observed as of the end of 2021.  Many, many more since...

THE UNIVERSE COULD HAVE EXISTED FOR AN INFINITE TIME BEFORE INFLATION!


THE UNIVERSE CANNOT HAVE BEEN EXPANDING FOREVER!

ORIGIN OF MATTER?

IS THE UNIVERSE IN A FALSE VACUUM?

CHIRAL SYMMETRY BREAKING?