Starts With A Bang

Is the inflationary Universe a scientific theory? Not anymore (Synopsis)

The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. Alternatives to the Big Bang, like the Steady-State theory, fell out of favor due to the overwhelming observational evidence, but the Steady-State adherents never changed their mind, not until the day they died. Image credit: C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47).

“I know of no other scientist, no other theoretical physicist alive who has a clearer focus on whether our theories and ideas are relevant to the real world. And that’s always what he’s after.” -Neil Turok, on Paul Steinhardt

The inflationary Universe is one of the most revolutionary new ways of looking at the cosmos to come out of the last 40 years of science. Instead of going all the way back to a singularity from which time, space, matter, and energy all emerged, cosmic inflation posits a different state that gave rise to our hot, dense, matter-and-radiation-filled Universe. With energy inherent to space itself, brought about via coupling to a new field known as the inflaton, this exponentially expanding epoch preceded what we presently know as the observable Universe.

The quantum fluctuations that occur during inflation do indeed get stretched across the Universe, but the larger feature of inflation is that the Universe gets stretched flat, removing any pre-existing curvature. Image credit: E. Siegel / Beyond The Galaxy.

Inflation has its attractive features, and can explain many features that are observed to be true. In addition, it has made successful predictions that were borne out, years or even decades later, by detailed observations of the Big Bang’s leftover glow. But there’s a darker side to inflation: the phenomenon of “infinite model-building,” where theorists churn out model after model after model, predicting every imaginable outcome, and therefore, predicting nothing at all.

Three possible ‘hills-and-valleys’ potentials that could describe cosmic inflation. Though they give somewhat different results for the various parameters of the Universe, there is no motivation for choosing one model over another. Created with Google’s graph tool. Image credit: E. Siegel.

Although inflation has some incredible features that no other competitor can match, it’s not a theory without its flaws. Sabine Hossenfelder gives us a peek behind the curtain on Starts With A Bang today.