In a vast dark hall, someone invisible plucks the strings. The cosmic gas, the tenuous hydrogen that filled the young universe, resembles stretched strings that can resonate at a radio wavelength of 21 centimeters. This is hydrogen whispering about its temperature. But through this gas rush massive clumps of dark matter — dark matter, which no one sees but everyone feels through its gravity. They don't emit light, they don't collide, but they gravitationally tug at the gas, like a spoon dragged through honey. This dynamical friction heats the hydrogen, and its "melody"—the global 21-centimeter signal — changes.
The authors turned this signal into an ultra-sensitive detective. They calculated how much the radio picture would be distorted if invisible objects (known as MACHOs, massive compact halo objects) have masses ranging from thousands to millions of suns. It turns out that even a small number of such "weights" can heat the gas by tens of millikelvins. That's already a noticeable deviation from the standard scenario where darkness is cold and calm. The scientists imposed strict conditions: the signal must not heat up beyond a certain threshold, otherwise we would have noticed. Thus, new upper limits were born — how many MACHOs could be hiding in the universe.
But the most interesting part is the shape of the mass distribution. If MACHOs are not identical, like bullets of a single caliber, but are spread across masses (for example, as in the critical collapse model), then the constraints become even tighter. Nature abhors monotony: diversity among the invisible gives away their presence faster. So the universe either contains almost no such massive dark bodies, or they are very well hidden. And our radio receiver, the very history of the cosmos, continues to listen to the silence.
as written by the authors arXiv:2604.17083
We derive a complementary cosmological upper bound on the fraction of dark matter residing inside massive compact halo objects (MACHOs) using the cosmic dawn and dark ages global 21-cm signal $$(T_{21})$$. MACHOs of masses $$M\gtrsim 10^3~M_\odot$$ moving through the post-recombination baryonic fluid transfer kinetic energy to the intergalactic medium via dynamical friction, raising the gas temperature and distorting the 21-cm signal from the $$\Lambda$$CDM prediction. We consider both a monochromatic and two extended MACHO mass distributions: log normal and critical collapse. Imposing the conditions that the deviation in the global 21-cm signal $$\Delta T_{21}$$ does not exceed $$50~\rm mK$$ at $$z\sim 17$$ or $$15~\rm mK$$ at $$z\sim 89$$, and that no emission signal appears at $$z \gtrsim 300$$, we derive upper bounds on the MACHO fraction $$f_M$$ across the mass range $$10^3 \lesssim M_c/M_\odot \lesssim 10^7$$. The dark ages criterion yields constraints that are both tighter and free from astrophysical uncertainties associated with star formation, providing a complementary cosmological window. Extended distributions produce bounds that are generally more stringent than their monochromatic counterpart, with the critical collapse models yielding the strongest constraints at intermediate masses.
The hydrogen signal at 21 centimeters is so faint that it could be drowned out by an ordinary car spark plug working a few kilometers away.
In Ivan Yefremov's novel "Andromeda Nebula," the heroes encounter invisible gravitational traps — dark objects that distort the paths of ships, reminiscent of the MACHO idea.