As cosmic strings – exotic ripples in spacetime left behind by the first moments of the Big Bang – travel, they may trigger the formation of magnetic fields in their wake, new research suggests. These magnetic fields would then soak the universe, thus explaining the magnetization of galaxies and clusters, scientists propose in a new article.
Almost every substantial object in the universe harbors a magnetic field. Smaller objects, like planets and stars, generate their own magnetic fields from dynamo actions inside them, where swirling streams of electrically charged plasma force weak magnetic fields to fold in on them themselves.
On a larger scale, astronomers have observed magnetic fields inside nebulae, supernova remnants and protoplanetary discs. In these cases, complex streams of charged particles can generate weak fields.
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Some of the largest objects in the universe, like galaxies and clusters of galaxies, also maintain magnetic fields. They are usually very weak – about a millionth of the strength of the Earth’s magnetic field – but they are huge, in some cases spanning millions of light-years.
Astronomers don’t know exactly how galaxies and clusters get their magnetic fields. To create a magnetic field, you need charged particles moving together. But in the early stages of the universe’s development, before the first stars and galaxies appeared, the cosmos was electrically neutral. A neutral gas cannot generate magnetic fields on its own, so somehow the universe must have created a magnetic field.
Once the universe had this initial seed magnetic field, it was able to amplify it when the evolution of the universe changed the neutral gas into an electrically charged plasma. But the source of the first magnetic field has been a lingering mystery in astronomy for decades.
Tangled Cosmic Strings
In a new article published on the preprint server arXivthe researchers offer what is perhaps the most exotic explanation for the source of the universe’s seed magnetic field: cosmic strings.
Cosmic strings are theoretical objects that many astronomers believe formed at the very beginning of the universe. When our cosmos was less than a second old, it went through several stages of violent phase transitions. As soon as possible, all four forces of nature were united into one force. These phase transitions took the unified force and, one by one, split it into the forces of gravitythe strong nuclear forcethe weak nuclear force and electromagnetism.
With each duplication of forces, the fundamental void of space-time was reconfigured. But this process may not have been completely smooth or perfect, and flaws may have appeared in spacetime. Some of these flaws appeared as one-dimensional creases in space, like wrinkles on a sheet of paper. These are the cosmic strings.
Astronomers have searched for cosmic strings since they were theorized in the 1970s. So far all searches have proven fruitless, and yet cosmic strings seem to be a generic prediction of all our theories about the early universe. .
If cosmic strings exist, they would be very strange indeed. For example, due to the unique way they bend spacetime, if you were to travel in a circle around one, when you have completed your journey and returned to your starting point, you would find that you have traveled less than 360 degrees. Cosmic strings can also vibrate, the ripples moving along their length at the speed of light, and sometimes forming loops which then vibrate to death in a frenzy of radiation.
The making of magnetism
The study authors took advantage of the unique properties of cosmic strings to turn them into generators of magnetic fields. The idea is that as the cosmic strings traveled, they would leave behind ripples in the fabric of spacetime, like wakes dragging a speedboat.
If a cosmic string passed through a plasma, these ripples in spacetime could alter the temperature and density of small pockets in the plasma. These differences would set electric charges in motion and could become the beginnings of a magnetic field. These seed fields would not be very strong — less than a millionth of a millionth of Earth’s magnetic field – but that would be enough.
Once the cosmic strings leave the area, the remaining plasma could compress and cool to form stars, galaxies and clusters. As the plasma compressed, it could have amplified that initial field into the forces astronomers see today.
Although this hypothesis is interesting, it poses a major problem: we do not yet know if the cosmic strings exist. Fortunately, the authors addressed this point and noted a potential observational signature of cosmic strings. These same wakes that generate magnetic fields in plasmas continue to persist long after the departure of the cosmic cord. Eventually, the wakes roll over the Earth in the form of gravitational waves.
If there were enough cosmic strings in the early universe, we might be able to observe remnants of their spacetime wakes with the next generation of gravitational wave detectors. And once we know that cosmic strings actually existed at some point, we might finally know what caused the universe to magnetize.
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