Dark sirens singing about dark energy

In August 2017 the world was disrupted by the news: for the first time in history, the LIGO-Virgo collaboration detected gravitational waves from two colliding neutron stars. Marcelle Soares-Santos remembers the day of this event: “This was absolutely awesome! As usual, life is happening at the same time as science.” As LIGO’s measurement campaign was coming to an end, Prof. Soares-Santos wasn’t expecting new results and scheduled her move from Chicago to Boston where she had received a professor position. Exactly on the day when the moving truck arrived, she received an email from LIGO about their historical discovery.

What was so special about this event? Gravitational waves—ripples in the space-time fabric—had already been detected several times since the beginning of LIGO’s operation in 2014. However, all of them came from colliding black holes. Therefore, even though we could ‘feel’ the gravitational impact of these events, we could not ‘see’ them through our telescopes, since they were… black. This is why the neutron star collision is so special: it emits a bright light that can be seen on Earth. The news from LIGO about their detection became a call for action for many astronomers. Prof. Soares-Santos was leading one of the groups, the Dark Energy Survey, who managed to catch the light from colliding neutron stars into their cameras.

Such multi-messenger detections, where the same event is observed through different signals, open new ways to study the universe. We know the universe is expanding, but to measure its rate, we need both the distance to and speed of receding objects. The neutron star collision gave us the necessary tools. Gravitational waves are very useful for measuring distances to collision events, but they are not enough to determine the speeds. Here is where light enters the game.

When a moving object emits light at a certain frequency, we observe a shift in that frequency depending on its speed—similar to how an ambulance siren sounds lower as it moves away. This effect, called redshift, allows us to measure how fast the colliding stars are receding, providing the missing piece for estimating the universe’s expansion rate. Though not yet as precise as other methods, this marked the beginning of a new era in cosmology.

Marcelle Soares-Santos’s interest in gravitational waves developed gradually. Early on, she was fascinated by the fundamental laws of nature. So, when as a bachelor student she heard about dark energy—a mysterious entity far more abundant than anything we can see in the universe—she decided that it’s a perfect topic to study. We still don’t know what dark energy is, but we know that it strongly affects our universe. Normally, gravity should slow the universe’s expansion over time, but observations reveal the opposite: for the last 5 billion years, expansion has been accelerating. To solve this paradox, scientists introduced the concept of dark energy—a uniform repulsive gravity that pushes galaxies apart and accelerates the expansion.

Many theories attempt to explain dark energy—it could be energy associated with empty space, a new so far unknown field, or something even more exotic. During her masters, Marcelle Soares-Santos was excited to tackle this puzzle by developing alternative models and predicting their effects on the universe. However, she soon realized that the ball seems to be in the experimentalists’ courtyard: many of these models fit our current measurements of cosmic expansion, making it impossible to determine which of them is correct. This means we need much more precise and detailed measurements to reveal the nature of dark energy. Driven by this idea, Prof. Soares-Santos joined the Dark Energy Survey project to gather detailed data on galaxies, supernovae, and other cosmic structures.

In 2014, after developing and installing a new Dark Energy Camera (a high-performance, wide-field CCD imager) in the Chilean mountains, Prof. Soares-Santos was looking for new directions for her establishing research group. Meanwhile, the LIGO collaboration was preparing to measure gravitational waves from cosmic collisions and needed astronomers to capture the accompanying light signals. Prof. Soares-Santos quickly saw the Dark Energy Camera’s potential for this task—it was designed to scan large sky areas rapidly, ideal for pinpointing events that gravitational wave detectors locate only imprecisely. She signed up for the program and, thanks to this decision, was among the first to see the collision of two neutron stars in 2017. Reflecting on the moment, she said, “Participating in a discovery like that, you see the purpose of the whole endeavor actually coming together.” From that moment, her research became firmly connected to gravitational waves and their optical counterparts.

Unfortunately, since the 2017 neutron star collision, no similar event has been observed. Most detected gravitational waves come from black hole mergers, which emit little to no light. Can we still extract information about the universe’s expansion from these “dark sirens”? Drawing on her expertise in galaxy surveys, Prof. Soares-Santos proposed a novel statistical approach: instead of measuring the speed of a single event, she estimated it probabilistically. By analyzing the approximate location of a collision, she assigned each nearby galaxy a probability of hosting the event and used light from these galaxies to infer the expansion rate. However, to get precise results with this method we need a much larger set of gravitational wave data.

Since coming to the University of Zurich, Prof. Soares-Santos has been working on next-generation detectors to gather the desired data. Building on her expertise in optical instruments, she is developing new diodes for interferometers that detect gravitational waves. Next-generation detectors will allow us to go beyond snapshots of the universe at specific moments and instead track its history over time. The reason is that, unlike other instruments, gravitational wave detectors work across a vast range of distances, enabling us to observe both distant, early-time events and closer, more recent ones. This continuous timeline of the universe will help us to set stricter constraints on cosmological theories, bringing us closer to understanding the nature of dark energy.

As Prof. Soares-Santos notes, this field is full of exciting challenges and opportunities for diverse expertise. Some of her group members work in the Chilean mountains, operating and improving Dark Energy Survey cameras, while others at the University of Zurich focus on developing new diodes for gravitational wave detectors and refining data analysis to prepare for the coming wave of measurements. Prof. Soares-Santos is looking forward to the future: “If I were a student right now, I would be really excited about this field. There is a wealth of data coming in the next few years and there will be a lot of exciting new results. The first time we detect the stochastic signal of all black holes, see a cross-correlation signal between gravitational waves and galaxies, or detect the next bright emission from colliding neutron stars—each event will be amazing!”

Author: Aleksandra Nelson