Scientists Have Detected the First Stars

Astronomers have detected a broad “dip” in the cosmic microwave background (CMB) spectrum that matches a predicted absorption feature from neutral hydrogen during the universe’s first star era—an observation that also points to possible new physics involving dark matter. The signal comes from the 21-centimeter line, produced when hydrogen atoms flip the spin of their electron relative to the proton. In the earliest universe, that hydrogen spin-flip radiation stayed in equilibrium with the CMB, making the 21-centimeter signature effectively indistinguishable from the background. But once the first stars ignited, ultraviolet light shifted the balance: the hydrogen’s spin temperature became coupled to the gas temperature rather than the CMB. The result was a temporary period when the universe absorbed more 21-centimeter photons than it emitted, creating a measurable imprint on the CMB spectrum.

Because the universe was expanding, absorption at a fixed rest-frame wavelength would appear stretched to longer wavelengths today. The expected outcome was therefore a broad spectral dip spanning a range of observed wavelengths corresponding to the epoch between about 180 and 270 million years after the Big Bang—roughly the time from the birth of the first stars to the onset of intense black hole growth. Bowman and collaborators reported exactly such a dip using the EDGES experiment, located at the Murchison Radio-Astronomy Observatory in Western Australia, chosen for its exceptionally radio-quiet conditions.

The key twist is not the existence of the dip, but its depth. The absorption feature appears about twice as strong as standard cosmological models predict. A deeper dip implies that the absorbing hydrogen gas was colder than expected. Colder gas absorbs 21-centimeter photons more efficiently, so the observation effectively demands extra cooling beyond what the known thermal history allows. Standard models already account for the cooling from cosmic expansion and the initial temperature set by the CMB’s creation, leaving little room for hydrogen to become that cold.

To cool hydrogen further, something must be colder than it and capable of transferring heat. The only colder component available in the relevant era is dark matter. That leads to a hypothesis: hydrogen may have lost energy to dark matter through a new, non-gravitational interaction. Dark matter is otherwise assumed to interact weakly (or only via gravity) with ordinary matter, so any additional coupling would represent a significant change to the particle-physics picture. The authors treat this as a plausible explanation for the anomalously cold hydrogen, but it remains speculative and will require more data to confirm.

After the cosmology segment, the program pivots to a trebuchet “challenge” solved with energy conservation. Two projectile shots—one landing from a shallow arc and one from a higher arc—end up equally damaging because the counterweight’s lost potential energy is the same in both cases, giving the projectile the same kinetic energy at impact. Using the provided heights and masses, the impact speed comes out to about 80 meters per second, translating to kinetic energy comparable to roughly a third of a stick of dynamite.