Although it is impossible to see it, dark matter fills the universe. And now it seems increasingly likely that it always has. An international team from Japan used the Subaru Telescope at the controversial Mauna Kea Observatories complex to detect the earliest dark matter ever observed by tracking how it distorts measurements of the halos of millions of the oldest and most distant galaxies in the universe.
In a new article published in Physical review letters, the team reports the earliest subtle traces of dark matter’s influence on galaxies in the young universe. They made the discovery after observing 1.5 million incredibly distant galaxies and their halos of dark matter, looking back as far as 12 billion years.
What is new – For the first time, these cosmologists show that it is possible to use the cosmic microwave background itself – the radiation left over from the Big Bang – to measure dark matter halos around extremely distant galaxies. As the mass of closer, newer galaxies and their associated dark matter bends the microwave background, astronomers can pick up subtle fluctuations in the radiation to indirectly observe dark matter.
Andrés Plazas Malagón, one of the study team members and a researcher at Princeton University and the Vera Rubin Observatory in northern Chile, says Reverse that looking at just one galaxy may not reveal much distortion at all. So the team combined observations of 1.5 million galaxies and the ring of dark matter surrounding each one to find a clearer signal.
“You don’t get information for every single halo,” he says. “but we expect all these haloes to be very similar.”
“Nobody realized we could do this.”
The study differs from other galaxy lensing studies because it uses data from the cosmic microwave background instead of observing how light and radiation from more distant galaxies is affected by closer galaxies. Ultimately, the team observed dark matter from the first 1.7 billion years of the universe’s existence. The discovery tells us that the earliest galaxies had halos of dark matter even when they first formed.
One of the study’s great advantages is the huge sample size – observing many galaxies makes very small but significant fluctuations in radiation easier to detect – but also the data had already been collected. It’s possible that other astronomers could use other existing large comprehensive survey datasets to detect more early dark matter—even those researchers were shocked that they could pull this survey off.
“Looking at dark matter around distant galaxies?” Masami Ouchi, professor at the University of Tokyo and study co-author, asks in an accompanying statement.
“It was a crazy idea. Nobody realized we could do this.”
Why it is important – Dark matter cannot be observed directly, and its role in the universe is a mystery. Yet the substance is believed to make up a quarter of all existence. To better understand it, astronomers compare observations of our local universe with measurements of extremely distant and ancient objects.
Some observations tell you about the early universe, and some tell you about the current universe, and according to Plazas Malagón, “to get from one to the other you use your mathematical model of the universe, find your initial conditions, make a prediction, and that’s how you can make the comparison.”
Surprisingly, the dark matter observed in this study does not behave as the researchers had predicted based on what we know about the laws of physics, and in particular the Standard Model, which provides a framework for characterizing all matter in the universe.
The best existing measurements of the cosmic microwave background were established by ESA’s Planck mission a decade ago. And while it didn’t measure dark matter directly, the measurements put some constraints on what physicists expect to be there. According to a popular theory of cosmology called the standard Lambda-Cold Dark Matter model, dark matter should form locally dense clumps as a result of the random fluctuations of the cosmic microwave background and gravity. Instead, the team found that early dark matter is less bulky than expected.
If cosmologists’ expectations about the early universe do not match astronomical observations, it is possible that other assumptions underlying standard cosmology may break down the further back in time and space.
What’s next – Currently, this analysis involved only a third of the Galactic data set collected by the Subaru telescope. Astronomers can continue to look through the entire data set and other existing observations for traces of dark matter in extremely large samples of galaxies.
When the Vera Rubin Observatory in northern Chile where Plazas Malagón works begins observing the sky in 2023, it will be the largest digital camera in the world. The observatory’s planned legacy survey of space and time, says Plazas Malagón, will be “like this survey, but on steroids,” measuring the full half of the sky visible from Chile. This means it will capture billions upon billions of galaxies, not the relatively mere millions captured by the Subaru Telescope or even the James Webb Space Telescope.
“The James Webb Space Telescope is amazing, but too small,” says Plazas Malagón.
“The James Webb Space Telescope is amazing, but too small.”
The team is also awaiting the end of the decade for a leap in their space-based observing capabilities – this is when the Nancy Grace Roman Space Telescope is expected to launch.
“For this analysis, we need hundreds of millions of galaxies,” says Plazas Malagón. “The Roman Space Telescope is going to give us a really big view.”
That the team’s findings in this study do not match perfectly with what they expected mathematically does not mean that the dark matter theory is inherently problematic, or that the standard Lambda-CDM model is still unhelpful. While these observations are unexpected, Plazas Malagón notes that “it points us in the direction that it might be something we need to look into.”
“If you’re a good scientist, you want to break your model,” he says.