Warped Light's Powerful Insights
by Adam Hadhazy
Part 1 of our 'Looking Ahead to Rubin' series explains how capturing strong gravitational lensing events will probe the abiding mysteries of dark matter and dark energy
Our understanding of the dynamics of the universe is about to be revolutionized, thanks to the Vera C. Rubin Observatory (VRO). Come 2024, this observatory is slated to begin its Legacy Survey of Space and Time (LSST), a campaign that will observe the full sky every three nights over the course of a decade. The survey will capture an unprecedented amount of astrophysical data, opening our eyes to the cosmos as never before.
Groups of researchers worldwide have banded together in forming Science Collaborations to provide scientific expertise for the VRO LSST. Many Kavli Institute-affiliated researchers belong to these Science Collaborations. This article is the first in a series profiling these researchers and the advances they expect the groundbreaking observatory will deliver.
One of the most powerful investigational tools available to cosmologists and astrophysicists can, at first glance, seem like little more than a cosmic parlor trick. Great concentrations of mass out in the universe, such as galaxy clusters, have the ability to bend and magnify light shining from background objects located much farther away. The warped light often appear as stray arcs in deep-space astronomical images; the effect is similar to that of looking down through a curved wine glass and seeing distorted patterns underneath.
Yet this play of light is no mere cosmic curiosity. The phenomenon is known as strong gravitational lensing and it is actually at the heart of many studies into the nature of dark matter and dark energy. Accordingly, gravitational lensing is a major feature of the observational campaign of the VRO LSST.
Simon Birrer, a Kavli Postdoctoral Fellow at the Kavli Institute for Particle Astrophysics and Astronomy (KIPAC) at Stanford University, works at the nexus of these studies. Formally part of the Strong Lensing Science Collaboration, Birrer is also a co-convenor of the Dark Energy Science Collaboration (DESC) Strong Lensing Working Group as well as a liaison to the DESC Dark Matter Working Group.
The throughline of strong lensing (as well as its cousin, weak gravitational lensing, which is detected statistically by computers over large datasets) to dark matter and dark energy is that the lensing serves as a unique way of indirectly gauging the amounts and effects of each dark entity on the directly observable universe. Put another way, gravitational lensing smokes out hidden forms of matter and energy.
"Strong gravitational lensing is the phenomenon of light being bent around matter as [the light] travels through space," says Simon Birrer, a Kavli Postdoctoral Fellow at the Kavli Institute for Particle Astrophysics and Astronomy (KIPAC) at Stanford University. "[This phenomenon] enables us to detect invisible matter by the distortion of light passing nearby it."
In the case of dark matter, its only detectable signature to date is the gravity it exerts. Gravitational lensing studies have shown the degree of light-bending caused by galaxy clusters falls well short of the effect ordinary matter in the cluster could plausibly produce. Such studies and other lines of evidence have reliably and repeatedly demonstrated that dark matter—whatever it may be—must outnumber normal matter about five times over.
As for dark energy, it is the poorly understood component of the universe that accounts for its accelerating expansion. Measurements of this expansion and of the earliest light observable after the Big Bang have roundly summed up dark energy's portion of the universe's composition as by far the majority stake. All told, dark energy comes in around 70% of the universe's mass-energy, compared to 25% for dark matter and 5% for the ordinary matter we perceive and of which we are humbly composed.
With its voluminous observations of the night sky over a decade, the VRO LSST will compile a staggering record of strong and weak lensing events throughout space and time. Those data will allow for more precise measurements of the properties of dark matter and dark energy, further constraining their fundamental physics and essences.
"We want to understand what the universe is made of and what the fundamental process of the observed accelerated expansion of the universe is," says Birrer. "The Strong Lensing collaboration's goal is to learn about the invisible universe through strong lensing."
Of particular interest to Birrer are the rare strong gravitational lensing events in which multiple images of the same light source can appear. Notably striking examples of these events are known as Einstein crosses, after the late-great Albert Einstein, of course, who predicted the existence of gravitational lensing more than a century ago in his theory of general relativity. These crosses can display around a foreground object as four images of a background light source, such as a quasar (a galaxy with a voraciously feeding supermassive black at its core) or a supernova (an explosion of a star).
In other events, the background object can handily appear twice with a time gap in between. That arrangement conveniently allows for a comparison of the differing light travel times for the two images as they reach the VRO, here on Earth. In the instance of a gravitationally lensed supernova, the scientific rewards can be substantial. "We see an explosion of a star twice, with a relative delay of multiple days," says Birrer. "These déjà-vu events can be used to get a handle on the depth and scales of the universe."
Birrer's path to astrophysics began in childhood, where he recalls he asked a lot of "why" questions. "I wanted to understand everything," Birrer says. By the time high school rolled around, Birrer felt that physics as a field of study was able to provide the most detailed answers to some of those vexing questions. Once within physics, he could not decide which direction to go, though after taking a range of classes, he found that the classes in cosmology and astrophysics "left the most unanswered questions," says Birrer, "and I just realized how little we actually understand."
Birrer, who will be starting a faculty position next year at Stony Brook University, eagerly looks forward to the windfall of data and the ways in which it will narrow down the answers to some of the profound "whys" that have long driven him and his colleagues.
"We will find more than ten times more strong lensing phenomena than what we currently know, and hence the amount of data and constraining power will increase substantially," says Birrer. "And who knows, perhaps some surprises are waiting for us."