When hydrogen atoms first formed, they absorbed and radiated ambient radiation of 21 centimeters at equal rates, effectively making the hydrogen clouds that filled the primordial universe invisible.
Then came the cosmic dawn. Ultraviolet radiation from the first stars caused atomic transitions that allowed hydrogen atoms to absorb more waves of 21 centimeters than they emitted. Viewed from Earth, this excess absorption would appear as a decrease in brightness at a specific radio wavelength that marks the moment the stars turned on.
Over time, the first stars collapsed into black holes. The hot gases swirling around these black holes produced X-rays that heated hydrogen clouds throughout the universe, increasing emissions by 21 centimeters. We would perceive this as an increase in brightness at a slightly shorter radio wavelength than that of the older light. The net result would be a decrease in brightness over a narrow radio wavelength range, as detected by EDGES.
But the observed dip, which occurred around a 4-meter wavelength, wasn’t what theoretical cosmologists expected: The trough’s timing and shape were wrong, indicating that the first stars lit up surprisingly early and that X-rays flooded the universe shortly after. Stranger still, the dip was very pronounced, suggesting that hydrogen in the early Universe was colder than theoretical models predicted, possibly because of exotic interactions with the dark matter that fills the cosmos.
Or maybe the EDGES dip had a more mundane origin.
Emissions of 21 centimeters of hydrogen from the cosmic dawn era reach Earth at wavelengths of several meters, in the range used for FM radio and television broadcasting; that’s why EDGES was operating in such a remote location. In addition, the signal is overwhelmed by radio emissions thousands of times brighter from our own galaxy, and is distorted by the passage through the upper layers of the Earth’s atmosphere.
No less important are subtle effects of the antenna itself. The environment of a radio antenna can slightly alter the area of the night sky to which it is sensitive. In such a precise experiment, even faint reflections from surfaces tens of meters away can matter. The effect of such reflections would be amplified at certain radio wavelengths, resulting in a small variation in the antenna’s field of view – and thus possibly in the measured brightness – at different wavelengths.
The EDGES team saw these kinds of ripples in their data, and the main culprits were, perhaps fittingly, the edges of a 100-foot (30-meter) wide metal screen placed on the ground around the antenna to block radio emissions from the ground itself. The team corrected in their analysis for possible reflections from these edges, but as some astronomers noted at the time, if the correction is off even slightly, the result could be a drop in background brightness over a narrow wavelength range indistinguishable from a real cosmic dawn signal.
The SARAS team took a different approach to antenna design to achieve a more uniform sensitivity across all wavelengths. “The whole design philosophy is to maintain that spectral smoothness,” said Saurabh Singh, the lead author of the SARAS paper. The antenna – an aluminum cone propped up on a Styrofoam raft – was floated in the middle of a calm lake to ensure there would be no reflections more than 100 meters horizontally, which Parsons called “a really cool and innovative approach”. . In addition, the slow speed of light in water reduced the effect of reflections from the lake bottom, and the uniform density of the water made the environment much easier to model.
This post 4 Years Later, New Experiment Sees No Sign of ‘Cosmic Dawn’
was original published at “https://www.wired.com/story/4-years-on-a-new-experiment-sees-no-sign-of-cosmic-dawn”