Cosmic inflation

Maps of the sky’s faint microwave glow show tiny differences in temperature. This light comes from the early universe, long before galaxies formed. A proposed earlier burst of extremely rapid expansion, called inflation, could have stretched tiny fluctuations into the seeds of later structure. What physical process, if any, caused that expansion, and how did it end?[1] [2]

The sky’s microwave temperature and polarization patterns carry information about the disturbances from which cosmic structure grew. Which physical dynamics, if any, produced those disturbances through inflation, and connected that phase to the hot expanding universe?

In a common class of models, a field called the inflaton changes slowly while its energy drives accelerated expansion. Quantum fluctuations seed density disturbances. The shape of the field’s potential—how its stored energy depends on its value—affects the predicted fluctuation pattern. Planck’s analysis tests such choices under specified assumptions; its temperature and polarization results constrain models rather than directly observe this field. Reheating, the transfer of inflationary energy into particles, also affects how a model is compared with data.

Some models predict primordial gravitational waves that could leave a distinctive curl-like pattern, called B-mode polarization, in the microwave sky. This is a test, not a unique interpretation of every B-mode signal: gravitational bending of later light and polarized Galactic emission also contribute. The BICEP/Keck analysis using observations through 2018 models these contributions separately. Its constraints must be interpreted with that foreground and cosmological modeling in view.[2] [3]

Many inflation models fit important observations. Those fits do not identify a single cause or rule out every alternative.[1] [2]

Cosmic microwave background patterns support inflationary models without selecting a unique one. Their origin, onset and ending remain uncertain; consistency with existing data is not direct detection of an inflaton.[1] [2]

The fluctuations that preceded galaxies may record physics at energies beyond terrestrial accelerators. Explaining how they arose would connect the structure we can map today to those otherwise inaccessible conditions.[1] [2]

Different field dynamics predict different fluctuation spectra and possible gravitational-wave amplitudes. Distinguishing them would test high-energy physics through the observable seeds of galaxies; understanding reheating would connect that inference to the later hot universe.[2] [3]