Elusive evidence
With such a successful string of consistent claims, inflation should be widely accepted by all practicing cosmologists, right? Not entirely. There are other models, aside from inflation, that predict the same jitters that would lead to the large-scale structures observed today.
But inflation’s most significant shortcoming is failing to explain how it got started. Even Guth only considered its consequences, assuming inflation was somehow initiated. This led some critics to claim that inflation and its consequences aren’t testable, which is a major requirement of the scientific method. What is needed is a way to test the unique predictions inflation makes, allowing cosmologists to differentiate between a universe in which inflation took place and alternate theories.
By the early 1990s, cosmologists had found just such a “smoking gun” of inflation. They showed that, if inflation took place, it would inevitably result in primordial gravitational waves. These waves propagate at light-speed, endure forever, and pervade all matter, making them unique messengers of the inflationary epoch. They are the ideal evidence of inflation — if such waves could be detected. If measured, these waves could reveal information about the inflationary epoch much the same way that photons, massless messengers themselves, encode the properties of the cosmos 380,000 years after BBN.
In the early 1980s, Russian physicist Alex Polnarev predicted these gravitational waves would distort space-time in a way that induces specific patterns in the light of the CMB. These patterns in the light’s orientation, or polarization, were later called B-modes and their properties fully elucidated by other researchers in the late 1990s. If detected, B-modes would confirm inflation beyond a reasonable doubt. By 2001, my experimental colleagues and I decided to test whether we could detect these inflationary relics.
Detecting gravitational waves via their imprint on the CMB’s polarization would falsify alternatives to inflation, cementing it once and for all as cosmology’s touchstone. But our apparent detection of B-mode polarization using the Background Imaging of Cosmic Extragalactic Polarization, or BICEP2, instrument in 2014 was later retracted. Definitive evidence remains elusive.
The search continues
Inflation is consistent with many pieces of cosmological data, but consistency doesn’t constitute proof. Several upcoming CMB experiments hope to change that. They include the Simons Observatory, the BICEP Array, and the “Stage-4” CMB-S4 experiment. These efforts will either detect primordial B-mode polarization arising from inflation-generated gravitational waves or drastically winnow the allowable number of inflationary models.
Nevertheless, B-mode detection is not guaranteed. Or inflation may not have happened at all. Frustratingly, many alternatives to inflation are similarly difficult to prove. So, unless the B-modes signals potentially awaiting astronomers are sufficiently big, we might never be able to say for sure whether the universe underwent inflation or not.
To some cosmologists, that would be deflating news. To others, it would fascinate and inspire — impelling us to create more refined models of our cosmic origins.