By Heather Sanders
Gazing at the night sky is a profound and humbling experience. Distances are infinite and mind-boggling, and the universe seems to defy explanation. Yet, cosmologists like SFU’s Levon Pogosian are pushing the boundaries of what is understood about the origin, evolution and the fate of the universe. Pogosian is a professor and physicist who works in theoretical cosmology – or as he describes it: “I use the universe as a laboratory for testing the laws of Nature.”
One such law concerns the Hubble constant, the unit describing how fast the universe is expanding today. It is named for pioneering astronomer Edwin Hubble (also the namesake of the space telescope) who observed that distant galaxies are all moving away from ours. However, the actual speed at which the universe is expanding is at odds with the prediction of the standard cosmological model. This discrepancy, referred to as the “Hubble tension,” is considered one of the hottest topics in cosmology right now.
When Pogosian and colleague Karsten Jedamzik from the Université de Montpellier published a paper presenting their proposal for resolving the Hubble tension, the cosmology community was quite excited. Pogosian, however, is cautious. “This research is at preliminary stages,” he stresses. “Much more work needs to be done to confirm that our idea is what Nature intended.” The paper, Relieving the Hubble Tension with Primordial Magnetic Fields, was recently published in Physical Review Letters.
Pogosian explains that cosmologists determine the expansion speed of the universe in two ways. The first is by directly observing how fast distant galaxies are moving away from us. The second is by relying on the prediction of the standard cosmological model tuned to fit the exquisitely precise observations of the Cosmic Microwave Background. This model, known as Lambda-CDM (Lambda Cold Dark Matter), describes a universe with three major components: dark energy (Lambda), cold dark matter and ordinary matter.
Lambda-CDM is considered highly successful and over the past two decades has proven applicable to various properties of the cosmos. The amount of each component in the Lambda-CDM model is not fixed; one determines them, along with a few other free parameters, by tuning the model to fit the data. Says Pogosian: “Cosmology is a data-rich science. We have amazingly precise data that allows us to determine all parameters of the Lambda-CDM model extremely well. The model can then predict the current expansion rate of the universe.” However, the Lambda-CDM predicted rate of expansion is actually slower than what is observed directly.
To understand Pogosian and Jedamzik’s paper and their proposal for resolving the Hubble tension, it helps to imagine looking far, far into space. Light takes time to travel, so when we see an image of a galaxy a billion light years away, we see it as it was a billion years ago – that is, we see its younger version. The further out a telescope reaches, the younger the observed universe. And, because the universe is expanding, its younger version is denser.
Soon after the Big Bang, the density of the universe was so high that any atom would be grinded down into its basic constituents. Light could not pass through such an environment and, as a consequence, there is a limit to how far back we can see. At around 375,000 years old, the universe expanded just enough to allow protons and electrons free to form hydrogen, the most abundant element in the universe. The formation of hydrogen atoms meant the universe became transparent.
As we look deeper and deeper into space, at some point we start seeing the moment at which the universe turns opaque – a bit like seeing the surface of a cloud. The image of this so-called “last scattering” surface, separating transparent and opaque universe, is the Cosmic Microwave Background (CMB), also known as the “glow of the Big Bang.” The moment of the transition is referred to as recombination.
The youngest universe that astronomers can see, the CMB, was first captured in the 1960s via radio telescope as a faint glow. Since then, it has been measured in great detail by NASA’s COBE and WMAP, and ESA’s Planck space telescopes. An image of the CMB taken by WMAP is available on the NASA website:
According to Pogosian, at the time of recombination, the universe was full of sound waves traveling at about 60% of the speed of light. We see evidence of their existences from the patterns of hot and cold spots in the CMB maps. The typical size of these patterns, which is key to measuring the Hubble constant from CMB, is set by the time at which recombination happened – the precise moment at which the transition from opaque to transparent occurred. Pogosian and Jedamzik pointed out that magnetic fields would make recombination happen more efficiently, placing the time of transition at an earlier point – just what one needs to reconcile the predicted Hubble constant with the one directly observed.
“The Hubble tension is still a bit of a crisis,” says Pogosian. “And there is no shortage of ideas proposing new ingredients to add to the standard cosmological model that might help resolve it. What makes our proposal so exciting, is that it uses known physics. We observe magnetic fields in galaxies, clusters of galaxies and in intergalactic space. Their origin is not fully understood, and a primordial magnetic field has long been proposed as one possible way to explain it. So, we are adding a familiar ingredient that actually solves two problems – it relieves the Hubble tension, and it explains the origin of the magnetic fields we see in space.”
“In our careers as theoretical cosmologists, we mostly explore new ideas, constrain possibilities,” adds Karsten Jedamzik. “It is however very rare that one finds something which has the potential for real discovery. So, both Levon and I are very excited about this possibility.”
The next steps are to engage in further detailed studies of primordial magnetic fields and to set some well-defined targets for further observation. “It’s always a good thing to observe and try to come up with as many ways of testing your theories as you can,” says Pogosian, who is perhaps one step closer to solving one of the most pressing cosmological questions of our time.