When the hydrogen atoms first formed, they absorbed and then emitted around 21 cm of radiation at an equal rate, effectively rendering the hydrogen clouds that covered the primordial universe. fruit.
Then came cosmic dawn. Ultraviolet radiation from the first stars stimulated an atomic transition that allowed the hydrogen atoms to absorb more 21-centimeter waves than they emitted. Viewed from Earth, this excess absorption would appear as a decrease in luminosity at a particular radio wavelength marking the moment the stars lit up.
Over time, the first stars collapsed into black holes. The hot gases swirling around these black holes produce X-rays that heat up clouds of hydrogen throughout the universe, accelerating the emission rate by 21 centimeters. We observe this as an increase in brightness at radio wavelengths slightly shorter than those of older light. The net result will be a decrease in brightness in a narrow radio wavelength band, like the one detected by EDGES.
But the observed drop, which occurred around a wavelength of 4 meters, was not what theoretical cosmologists had expected: The timing and shape of the groove was off, suggesting that early stars The moon has turned on surprisingly early, and the X-rays flooding the universe will soon follow. More strangely, the drop was very pronounced, suggesting that hydrogen in the early universe was colder than theoretical models had predicted, possibly due to strange interactions with the dark matter that filled the universe. .
Or perhaps the EDGES embedding has more mundane origins.
Hydrogen’s 21-centimeter emission from the cosmic dawn era reaches Earth with a wavelength of several meters, in the range used for FM radio and television programmes; that’s why EDGES operates in such a remote location. Furthermore, the signal is overwhelmed by radio emission thousands of times brighter from our galaxy, and it is distorted as it travels through the upper layers of Earth’s atmosphere.
No less important are the subtle effects from the antenna itself. The environment of the radio antenna may vary slightly the areas of the night sky to which it is sensitive. In such a precise experiment, even faint reflections off the surface tens of meters away could be problematic. The effect of such reflections will be enhanced at certain radio wavelengths, resulting in a small change in the antenna’s field of view – and thus potentially at the measured luminosity. – at different wavelengths.
The EDGES team saw this kind of ripple in their data, and the main culprit, perhaps fittingly, was the edges of a 30-meter-wide metal screen placed on the ground around the antenna to block radio emissions. line from the ground. The team corrected for possible reflections from these edges in their analysis, but as several astronomers noted at the time, if the correction is even slightly off, the results can be negative. It is possible that the background luminosity decreases in a narrow band of wavelengths indistinguishable from the real universe. dawn signal.
The SARAS team took a different approach to antenna design in pursuit of more uniform sensitivity across all wavelengths. “The whole design philosophy is to maintain the smoothness of the spectrum,” says Saurabh Singh, the lead author of the paper on SARAS. The antenna — an aluminum cone placed on top of a foam raft — is floated in the middle of a calm lake to ensure no reflections for more than 100 meters in any horizontal direction, which Parsons calls “a really interesting and innovative approach”. Furthermore, the slow speed of light in water reduces the effect of reflections from the lake bottom, and the uniform water density makes the environment much easier to model.
