Hubble Trouble in Our Expanding Universe

In 1929, Edwin Hubble confirmed that our universe was expanding and galaxies were moving further away from Earth, retreating faster the more distant they were. Scientists expected that this expansion would slow down over time. However, just over twenty years ago, they discovered that the expansion was instead accelerating, with galaxies receding into the distance faster than expected. So what’s causing this accelerated expansion of the universe? Alessandra Silvestri, associate professor at the Lorentz Institute for Theoretical Physics, part of the Leiden Institute of Physics (LION), is trying to find out.

Alessandra is a theoretical cosmologist who studies the dynamics of cosmic expansion over the last five billion years, a period known as “late time.” What makes the late universe’s accelerated expansion puzzling is that it can’t be validated just using Einstein's laws of general relativity. She explains “…we are either seeking to find some unusual, very exotic source of energy that is providing this accelerated expansion or some modification to the laws of gravity that would make us understand it better.”

Alessandra’s group has been involved in the development of pioneering software called EFTCAMB, which allows researchers all over the world to test their expansion theories and see how they might affect the evolution of the universe. The outcomes can then be compared with real observational data. Recently, her team has been examining the Hubble parameter, a measure that’s important as it provides vital information about the age of the universe, scale of the distances involved and expansion rate. Alessandra refers to it as “perhaps the holy grail of cosmology” as discrepancies in the measured values using different methods have led to what is called “Hubble tension.” The team has some work under review that suggests that the “tension is not as dramatic as it’s thought to be.”

Dark energy, which is believed to play a role in accelerated expansion, is another interest of Alessandra’s. She’s exploring it in collaboration with the European Space Agency’s Euclid mission, a telescope that will shed light on the expansion history of the universe. Shortly after Alessandra moved to Leiden, she was asked to take a more prominent role in the project and lead a “work package" to look at the source of cosmic acceleration. Many of Alessandra’s colleagues, including those in the Leiden Observatory and the Astronomy Department, are involved in the project. 

Working at LION has a lot of benefits, according to Alessandra. “Leiden is one of the reference points in the world for cosmology,” she says. Not only is there excellent infrastructure for carrying out research like the Lorentz Institute’s high-performance computer cluster, XMaris, there’s also a helpful and efficient administration system. Additionally, both world-class theoretical and observational cosmologists work together at LION, something Alessandra notes is rarely found outside of American universities. “What makes Leiden I think quite peculiar, is that there are quite good theoretical cosmologists here but also world leaders at the level of observational cosmology in the Astronomy Department.” Close collaboration between the Cosmology and Astronomy Departments is common.

Alessandra’s path to Leiden via a PhD in New York, a postdoc in Boston and a junior leadership position in Trieste was made that bit easier by the fact that “LION is a very welcoming environment for families.” The institute is very aware of the problems faced by researchers with young families, including situations where both partners are academics (Alessandra’s husband is also an associate professor at the Lorentz Institute,) and actively provides support in many forms. 

She also appreciates that Leiden University is fully committed to addressing the gender imbalance in physics and is encouraged by the many initiatives LION has developed to help female faculty achieve their full potential. She notes that compared to when she did her PhD, more women are doing doctorates and postdocs in physics. She hopes these women will continue to be supported so they can progress to becoming principal investigators. “This is really where we can make a difference,” she says. 

Want to join Alessandra at Leiden? LION is looking for a new tenure-track faculty member to be appointed within the Institute Lorentz for theoretical physics. Read the full job description and apply here.