The universe is full of mysteries and perhaps none bigger than dark matter, the hypothetical stuff that is said to make up most of the cosmos but doesn't interact with light or normal matter. According to our current model of the Universe, its contents are 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy. Scientists have proposed many ideas for what dark matter could be, from tiny particles we haven't discovered yet to exotic objects like black holes born in the very early universe, called primordial black holes (PBHs).
One big challenge for the PBH idea is that smaller black holes are expected to evaporate away over time through a process called Hawking radiation. This would mean that only very massive PBHs could still exist today. But what if some PBHs had a magnetic charge?
Physicists have explored the possibility that primordial black holes carrying a magnetic charge or magnetic black holes(MBHs) might escape this evaporation fate. If they can hold onto their magnetic charge, their Hawking temperature could drop, allowing even smaller ones to survive until now. This means they could make up a significant part of the dark matter we observe. The problem, however, is that we have never detected or even know how to detect an MBH.
To address this, new research from the Indian Institute of Science Education and Research (IISER) Pune and the University of Oxford, UK, looked to identify unique signs that can prove magnetic black holes exist and help us detect them. They also developed a method that allows us to place an upper limit on the number of MBHs. They focused on two main approaches: using the magnetic fields spread throughout the cosmos to find how many MBHs there could be and identifying unique signatures based on how these MBHs might affect light travelling through space.
First, they used the Parker bound, which limits the number of magnetic monopoles or magnets with a single pole that can exist in the universe. When a magnetic black hole moves through a cosmic magnetic field, it gains energy by interacting with the magnetic field and draining some of the energy of the magnetic field. For the cosmic magnetic fields to still exist today, they must regenerate their energy at least as fast as the MBHs are draining it. The Parker bound uses this knowledge to limit how many MBHs can exist in the universe. By calculating how much energy MBHs would steal from different cosmic magnetic fields, the researchers could limit how many MBHs could be around.
The researchers didn't just look at magnetic fields in galaxies, which had been observed before. They also considered the vast, faint magnetic fields found in the enormous empty spaces between galaxies, called cosmic voids and the filament-like structures that connect galaxies, known as the cosmic web. Viewed from a galactic scale, the universe looks like a giant sponge – the galaxies are in the dense parts, the filaments are the connections, and the voids are the holes. The magnetic fields in these voids and filaments are much weaker but spread over enormous distances.
The researchers found that using the magnetic fields in these cosmic voids and filaments gives us much tighter limits on the population of magnetic black holes than previous studies based only on galactic magnetic fields. For example, they found that in cosmic voids, magnetic black holes could make up no more than ten-millionth of the dark matter, and in cosmic web filaments, no more than about one hundred-thousandth. These are very strict limits, suggesting that if magnetic black holes exist, they can't be too common, especially the fast-moving ones. However, these limits depend on a key assumption: these cosmic magnetic fields exist and haven't already been completely drained by MBHs. If a void or filament had its magnetic field depleted, these bounds wouldn't apply there.
The researchers' second approach was to look for a unique signature that magnetic black holes might leave on light. When light travels through a magnetised gas or plasma, its plane of polarisation, or the direction in which its waves wiggle, gets twisted. This is called the Faraday rotation. The twist amount is measured by the Rotation Measure (RM).
They calculated how much Faraday rotation would be caused by a magnetic black hole and compared it to the effect from a neutron star, a dense, compact star with a strong magnetic field. Neutron stars have magnetic fields like bar magnets, with north and south poles (dipole fields). Because magnetic black holes carry a single magnetic charge, they would have a magnetic field more like a single pole (a monopole field).
They found that this difference is crucial. The monopole magnetic field of an MBH creates a very distinctive pattern in how it twists light, especially in the map of polarisation angles around it. This pattern is unique and different from the pattern created by a neutron star's dipole field. They also calculated that for magnetic black holes with a certain amount of magnetic charge, the Faraday rotation they cause is large enough that current radio telescopes on Earth could potentially detect it. This was specifically true for those MBHs with charges greater than about 10²² A-m, which corresponds to masses greater than one million times the Sun's mass. The effect is significantly stronger than a neutron star with a similar magnetic field strength at its surface would produce.
To help scientists identify this unique pattern, the researchers also developed a simple mathematical tool called an integral measure that can help distinguish the polarisation maps of MBHs from those of other objects like neutron stars. This measure would be zero for a pure monopole field like an MBH but non-zero for other magnetic fields.
This work significantly improves our understanding of potential constraints on magnetic black holes. The bounds derived from cosmic voids and filaments are much tighter than previous limits based on galactic magnetic fields. However, as mentioned, these bounds rely on the assumption that MBHs haven't destroyed these large-scale cosmic magnetic fields. The study also highlights that the unique magnetic field structure of MBHs provides a potentially observable way to find them, which is a new avenue compared to just setting limits on their population. A limitation mentioned is that the analysis primarily focuses on non-spinning MBHs, and the effects of plasma non-uniformity arising from a spinning MBH, could alter the unique polarisation patterns.
Today, dark matter is one of the biggest puzzles in physics and cosmology. Understanding what it is could revolutionise our understanding of the universe. By providing tighter constraints on one possible dark matter candidate – magnetic black holes – this research helps narrow down the possibilities. Furthermore, identifying a unique observational signature gives astronomers a specific target to look for using powerful radio telescopes. If magnetic black holes are found, it would be a monumental discovery. If they are not found despite looking for this signature, it would further strengthen the case against them as a significant component of dark matter, pushing us closer to solving the cosmic mystery.
This research article was written with the help of generative AI and edited by an editor at Research Matters.