As cosmic strings — exotic wrinkles in space-time left over from the earliest moments of the Big Bang — travel, they may trigger the formation of magnetic fields in their wakes, new research suggests. These magnetic fields would then soak the universe, thereby explaining the magnetization of galaxies and clusters, scientists propose in a new paper.
Almost every substantial object in the universe hosts a magnetic field. Smaller objects, like planets and stars, generate their own magnetic fields from dynamo actions inside them, where swirling flows of electrically charged plasma force weak magnetic fields to fold over themselves.
Some of the largest objects in the universe, like galaxies and clusters of galaxies, also maintain magnetic fields. They're typically very weak — around a millionth the strength of Earth's magnetic field — but they are tremendous, in some cases stretching for millions of light-years.
Astronomers aren't exactly sure how galaxies and clusters get their magnetic fields. To make a magnetic field, you need charged particles moving together. But at the early stages of the universe's development, before the appearance of the first stars and galaxies, the cosmos was electrically neutral. A neutral gas can't generate magnetic fields on its own, so somehow, the universe had to create a magnetic field.
Once the universe had that initial seed magnetic field, it could 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 an enduring mystery in astronomy for decades.
Tangled cosmic strings
Cosmic strings are theoretical objects that many astronomers believe formed in the very early universe. When our cosmos was less than a second old, it went through several stages of violent phase transitions. At the earliest times, all four forces of nature were unified into a single force. These phase transitions took the unified force and, one by one, split it into the forces of gravity, the strong nuclear force, the weak nuclear force and electromagnetism.
With each splitting of the forces, the fundamental vacuum of space-time reconfigured itself. But that process may not have been completely smooth or perfect, and flaws may have appeared in space-time. Some of these defects appeared as one-dimensional folds in space, like wrinkles in a piece of paper. These are the cosmic strings.
Astronomers have been hunting for cosmic strings ever since they were theorized in the 1970s. So far, all searches have turned up empty, and yet cosmic strings appear to be a generic prediction of all of our theories of the early universe.
If cosmic strings do exist, they would be very strange indeed. For example, because of the unique way they fold space-time, if you were to travel in a circle around one, when you completed your journey and returned to your starting point, you would find that you had traveled less than 360 degrees. Cosmic strings can also vibrate, with the ripples traveling up and down their length at the speed of light, and occasionally form loops that then vibrate themselves to death in a frenzy of radiation.
The making of magnetization
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 cosmic strings traveled, they would leave behind ripples in the fabric of space-time, like wakes trailing a speed boat.
If a cosmic string passed through a plasma, those ripples in space-time could change the temperature and density of small pockets in the plasma. Those differences would set electric charges in motion, and they could become the beginnings of a magnetic field. Those seed fields wouldn't be very strong — less than a millionth of a millionth of Earth's magnetic field — but it would be enough.
Once the cosmic strings left 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 strengths that astronomers see today.
Although this hypothesis is interesting, it has a major problem: We don't yet know if cosmic strings exist. Thankfully, the authors addressed that point and noted a potential observation signature of cosmic strings. Those same wakes that generate magnetic fields in plasmas continue to persist long after the cosmic string has departed. Eventually, the wakes wash over Earth in the form of gravitational waves.
If there were enough cosmic strings in the early universe, it might be possible for us to observe the remnants of their space-time wakes with the next generation of gravitational wave detectors. And once we know that cosmic strings actually did exist at some point, we just might finally know what caused the universe to become magnetized.