khoji parkDiscovery · Amundsen-Scott South Pole Station · 2015
A Claimed Signal From the Early Universe Turned Out to Include Dust
How interstellar dust eclipsed an apparent cosmic microwave background milestone
Read in slides1. South Pole Telescopes
Cryogenic telescopes deployed near the South Pole measure the polarization of the cosmic microwave background. These instruments target specific signals from primordial gravitational waves predicted by slow-roll models of cosmic inflation. Researchers utilize arrays of sensitive detectors mounted on specialized structures to map microwave radiation. BICEP1 initiated observations in the dark sector from 2006 to 2008 as a technical prototype. Subsequent generations expanded detector capacity to record subtle curl components across the sky.
BICEP2 began its three-season observing run in 2009.
2. The 2014 Announcement
Four co-principal investigators announced that BICEP2 detected B-modes from gravitational waves in the early universe. This landmark result was publicized by the Center for Astrophysics on March 17, 2014. The reported detection measured B-modes at a specific ratio that disfavored the null hypothesis significantly. Researchers utilized an improved focal-plane transition edge sensor bolometer array operating at 150 GHz. The findings generated immediate worldwide scientific interest regarding cosmic inflation.
Independent teams soon scrutinized the published polarization data.
3. Doubt Emerges
Astronomers questioned whether the detected polarization originated from inflation or intervening galactic material. David Spergel argued at an American Astronomical Society conference that the signal could stem from Milky Way dust. Reviewers of the discovery paper included an appendix discussing potential dust production of the signal. The large tensor-to-scalar ratio reported by BICEP2 contradicted strict limits previously established by Planck data. Scientific confidence in the inflation interpretation declined within months.
A formal reassessment of the polarized emissions became necessary.
4. Planck Data Release
A preprint released by the Planck team in September 2014 provided high-accuracy measurements of galactic dust. The analysis concluded that polarized emissions from dust matched the exact strength reported by BICEP2. Researchers mapped emissions across multiple frequencies to isolate genuine cosmic microwave background signals from foreground interference. This independent space-based dataset challenged the primordial origin hypothesis. Direct comparisons between the two instruments revealed significant overlapping measurements.
Joint analytical efforts were organized to resolve the discrepancy.
5. Joint Reanalysis Published
A joint analysis of BICEP2 and Planck data was published on January 30, 2015. The European Space Agency announced that the entire detected signal could be attributed to dust in the Milky Way. Researchers combined datasets from both collaborations to perform rigorous cross-checks on the polarization maps. The refined calculations eliminated the statistical support for primordial gravitational waves at that frequency band. This definitive reassessment corrected the earlier interpretation.
The combined analysis established strict new upper limits.
6. Revised Limits Set
A March 2015 publication in Physical Review Letters established an updated limit on the tensor-to-scalar ratio. BICEP2 integrated its observational data with the Keck Array and Planck to improve sensitivity. Subsequent observing seasons utilized upgraded polarimeters and advanced pulse tube cooling systems. Researchers continued mapping specific sky regions to constrain inflationary models further. These combined instruments operated continuously under harsh polar environmental conditions.
Upper limits on cosmological B-modes decreased steadily over time.
7. Keck Array Integration
The Keck Array operated adjacent to the BICEP installation using an existing telescope mount. Five polarimeters observed the same celestial patch at multiple microwave frequencies. Observations through subsequent years gathered extensive multi-frequency data to subtract foreground contamination accurately. Combined data releases from BICEP2 and the Keck Array progressively lowered the cosmological bounds. Project scientists published successive iterations incorporating WMAP data.
New results in 2021 further reduced the allowable ratio.
8. Current Status
The joint analysis with Planck data found that dust could account for enough of the observed polarization that the claimed primordial gravitational-wave detection was not established. Researchers continued measuring the sky with additional frequency information. The revised result concerned interpretation of that signal; it did not prove that inflation had been disproved or that gravitational waves of every kind had never been detected.
The original primordial-wave interpretation was not established.