Gravitational Waves, CMB Polarization, and the Hubble Tension

Area of Science:

• Cosmology
• Astrophysics
• Particle Physics

Background:

• The Hubble tension, a discrepancy between local and CMB-derived Hubble constant measurements, necessitates re-evaluating cosmological model assumptions.
• Cosmic Microwave Background (CMB) analysis provides crucial data for understanding the early Universe.

Purpose of the Study:

• To introduce an alternative early-Universe standard ruler using the recombination peak in the CMB B-mode power spectrum.
• To provide a cross-check for the standard ruler derived from CMB acoustic peaks.
• To test various explanations for the observed Hubble tension.

Main Methods:

• Analyzing the CMB B-mode power spectrum to identify the recombination peak.
• Relating the recombination peak's location to the light horizon at the surface of last scatter.
• Utilizing this as a standard ruler for cosmological parameter inference.

Main Results:

• The recombination peak location offers an independent measure of the light horizon at last scattering.
• This provides a novel method to probe the Hubble parameter and test cosmological models.
• Potential for ~2% precision with future Stage-IV B-mode experiments.

Conclusions:

• The CMB B-mode recombination peak serves as a valuable early-Universe standard ruler.
• This method can help resolve the Hubble tension and constrain cosmological parameters.
• It also offers a new way to measure the propagation speed of gravitational waves.