Last week, researchers announced signals of gas in the early universe that seemed to be far colder than we expected, possibly due to dark matter. But instead of dark matter making the gas colder, black holes may be making the background temperature hotter.
On 28 February, a group of astronomers working on the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) radio telescope found that as the first stars began to form, the gas around them absorbed radio waves about twice as strongly as was expected. One explanation was that the gas was twice as cold as predicted, due to even colder dark matter.
鈥淭he other way to get a stronger absorption signal is just to increase the background radiation temperature,鈥� says at Massachusetts Institute of Technology, who was part of the EDGES research.
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Heating things up
Instead of dark matter making the gas ultra-cold, maybe black holes warmed up the background, making the gas simply appear cold in contrast. That鈥檚 what at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, and his colleagues found using computer models of the first black holes鈥� radiation.
Black holes are some of the strongest emitters of radio waves in the sky and the first ones were likely forming around the same time as the first stars. 鈥淭hey form from the very first stars and feed on the surrounding gas, and they鈥檙e eventually going to grow up to become these supermassive black holes that we see today,鈥� says Ewall-Wice.
The researchers calculated that these early black holes could have heated up the background enough to boost the absorption signal by a factor of three, more than accounting for what EDGES observed. That eliminates the need for frigid dark matter to cool down the gas.
Beyond the veil
There is one big problem: growing black holes don鈥檛 just emit radio waves. They also radiate X-rays and ultraviolet light. 鈥淚f the hydrogen gas is heated by the X-rays, it can reduce the absorption signal and basically erase the gains that we make from the black holes in the first place,鈥� says Ewall-Wice.
And the ultraviolet light could ionise the gas, but we know from measurements of the cosmic microwave background that this ionisation process did not happen until later on.
One solution for this is a veil of dust, Ewall-Wice says. If the black holes were engulfed in dust, much of the extra energy would get deposited in the dust cloud rather than in the hydrogen gas. This would allow the black holes to co-exist with the cold, non-ionised early universe that other measurements indicate.
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