4 Years On, a New Experiment Sees No Sign of ‘Cosmic Dawn’

When hydrogen atoms first fashioned, they absorbed after which emitted ambient 21-centimeter radiation at equal charges, which made the clouds of hydrogen that stuffed the primordial universe successfully invisible.

Then got here cosmic daybreak. Ultraviolet radiation from the primary stars excited atomic transitions that enabled hydrogen atoms to soak up extra 21-centimeter waves than they emitted. Seen from Earth, this extra absorption would seem as a drop in brightness at a selected radio wavelength marking the second the celebrities turned on.

In time, the primary stars collapsed into black holes. The recent gases swirling round these black holes generated x-rays that heated hydrogen clouds all through the universe, rising the speed of 21-centimeter emissions. We might observe this as an uptick in brightness at a barely shorter radio wavelength than that of the older gentle. The online consequence can be a dip in brightness over a slim radio wavelength vary, just like the one detected by EDGES.

However the noticed dip, which occurred round a wavelength of 4 meters, was not what theoretical cosmologists had anticipated: The timing and form of the trough had been off, indicating that the primary stars turned on surprisingly early, and that x-rays flooded the universe quickly afterward. Stranger nonetheless, the dip was very pronounced, suggesting that hydrogen within the early universe was colder than theoretical fashions predicted, probably due to unique interactions with the darkish matter that fills the cosmos.

Or maybe the EDGES dip had a extra mundane origin.

Hydrogen’s 21-centimeter emissions from the cosmic daybreak period attain Earth with wavelengths of a number of meters, within the vary used for FM radio and tv broadcasts; that’s why EDGES operated in such a distant location. What’s extra, the sign is overwhelmed by radio emissions hundreds of occasions brighter from our personal galaxy, and it’s distorted by its passage via the higher layers of the Earth’s environment.

No much less necessary are refined results from the antenna itself. A radio antenna’s surroundings can barely change the world of the evening sky to which it’s delicate. In such a exact experiment, even faint reflections off surfaces tens of meters away can matter. The impact of such reflections can be enhanced at sure radio wavelengths, leading to a small variation within the antenna’s observing space—and thus probably within the measured brightness—at completely different wavelengths.

The EDGES workforce did see this sort of ripple of their knowledge, and the prime culprits, maybe fittingly, had been the sides of a 30-meter-wide steel display positioned on the bottom surrounding the antenna to dam radio emissions from the bottom itself. The workforce corrected for attainable reflections off of those edges of their evaluation, however as some astronomers famous on the time, if the correction was even barely off, the consequence could possibly be a dip in background brightness over a slim wavelength vary indistinguishable from an actual cosmic daybreak sign.

The SARAS workforce took a special strategy to antenna design in pursuit of extra uniform sensitivity throughout all wavelengths. “The whole design philosophy is to protect that spectral smoothness,” mentioned Saurabh Singh, the lead writer on the SARAS paper. The antenna—an aluminum cone propped on a Styrofoam raft—was floated in the course of a placid lake to make sure there can be no reflections for greater than 100 meters in any horizontal route, which Parsons known as “a extremely cool and revolutionary strategy.” Furthermore, the gradual velocity of sunshine in water diminished the impact of reflections from the lake backside, and the uniform density of the water made the surroundings a lot simpler to mannequin.