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B-modes: hunting inflation's gravitational waves
1997· Discovery / experiment
Open questionNobody knows the answer.
A swirling pattern in the microwave background's polarization that would be the fingerprint of gravitational waves from inflation. Not yet detected.
See it on the Science map →How it works
The afterglow's light is slightly polarized: its waves vibrate more in one direction, set when it last bounced off electrons. Across the sky this polarization forms patterns. Density ripples make only 'E-modes', lines pointing straight toward or around hot and cold spots. Gravitational waves can also make 'B-modes', which curl like a pinwheel. Two lookalikes complicate the hunt: dust in our own galaxy, and lensing by matter along the way, which twists some E-modes into B-modes.
Why it matters
A detection would strongly support inflation, measure its energy directly, and give the first evidence that gravity itself has quantum jitters.
How we know
The method was proposed in 1997. B-modes made by lensing were found by the South Pole Telescope in 2013, proving the technique. BICEP2's 2014 claim of inflation's B-modes turned out to be dust (2015). No primordial signal yet.
Still a mystery
Many models predict a signal just below today's limits; the Simons Observatory in Chile and Japan's planned LiteBIRD satellite aim to reach it. Some models predict a signal too faint ever to see.
Connections (2)
▸Cosmic inflation predicts gravitational waves sought as B-modes: hunting inflation's gravitational wavesOpen question
The relationship
Inflation stretched quantum jitters of spacetime itself, not just of the inflaton, into gravitational waves spanning the cosmos. Their strength depends directly on inflation's energy: the higher the energy, the stronger the waves. They would twist the afterglow's polarization into B-modes on scales of a degree and larger, so measuring them would read inflation's energy off the sky.
How the link was found
Alexei Starobinsky predicted gravitational waves from the early universe in 1979. Valery Rubakov and colleagues (1982) and Larry Abbott and Mark Wise (1984) showed their strength measures inflation's energy.
Is it proven?
Not yet seen. BICEP/Keck (2021) limits the waves to under about 3.6% of the density ripples, ruling out some of the simplest models. Starobinsky's model predicts about 0.3–0.4%, a target for coming experiments.
Open questions and mysteries
Some workable inflation models make waves far too faint ever to detect, so a non-detection cannot rule inflation out. Critics see that flexibility as a weakness.
▸Gravitational waves (LIGO, 2015) from inflation would leave B-modes: hunting inflation's gravitational wavesOpen question
The relationship
A gravitational wave stretches space one way and squeezes it the other. At 380,000 years, an electron inside such a wave saw the light around it slightly hotter in some directions, and scattering polarized that light in a twisting pattern density ripples cannot make. Today these waves would be billions of light-years long, far beyond LIGO's range; only the afterglow can record them.
How the link was found
In 1997 Uroš Seljak with Matias Zaldarriaga, and Marc Kamionkowski with Arthur Kosowsky and Albert Stebbins, showed that gravitational waves leave B-modes. The DASI telescope first detected the afterglow's polarization in 2002.
Is it proven?
The polarization physics is textbook, and lensing B-modes behave as predicted. What is missing is the signal: after dust and lensing are removed, nothing primordial remains at today's sensitivity.
Open questions and mysteries
Galactic dust and lensing must be subtracted with extreme precision. Other early-universe sources, such as cosmic strings, could also make B-modes, complicating interpretation.