New Dark Matter Physics Could Solve The Expanding Universe Controversy
“If either photons, neutrinos, or some new type of dark radiation (that interacts with dark matter but not any of the normal particles) has a non-zero cross-section with dark matter, it could bias measurements of the Hubble rate to an artificially low value, but only for one type of measurement: the kind that you get from measuring these leftover relics. If interactions between dark matter and radiation are real, they might not only explain this cosmic controversy, but could be our first hint of how dark matter might directly interact with other particles. If we’re lucky, it could even give us a clue to how to finally see dark matter directly.”
One of the biggest controversies in physics today is over the expanding Universe. Despite attempts to measure the Hubble rate for nearly 100 years, we still don’t know exactly how fast the Universe expands. Two independent classes of methods, from the cosmic distance ladder and the Big Bang’s leftover relic, give two very precise and incompatible results: 73 km/s/Mpc and 67 km/s/Mpc, respectively. There’s always the possibility that one class of methods gives a biased answer, and we simply haven’t uncovered the bias. But it’s also possible that new physics is responsible, that both teams are right, and that the discrepancy is a hint of the next great leap forward in our understanding of the fundamental properties of the Universe itself.