Astronomers Unveil Most Accurate Map Of Dark Matter

In August 2017, scientists with the Dark Energy Survey (DES) collaboration released the most accurate and extensive map of dark matter in the universe produced to that date. The results, based on the survey’s first year of observational data, were presented at the American Physical Society Division of Particles and Fields meeting, held at the Fermi National Accelerator Laboratory. The map charted the distribution of dark matter across a vast expanse of the sky, offering new insights into the structure and evolution of the cosmos.

The project confirmed that the universe is composed of approximately 26 percent dark matter, the invisible substance believed to provide the gravitational scaffolding for galaxies to form. This invisible mass does not emit or interact with light, making it impossible to observe directly. Its presence is inferred by its gravitational effects on visible matter and light.

Mapping by Gravitational Lensing

To create the map, DES researchers used a technique known as gravitational lensing. According to Einstein’s theory of general relativity, massive objects warp the fabric of spacetime. As light from distant galaxies travels toward Earth, its path is bent when it passes by large concentrations of mass, including clumps of dark matter. This bending effect subtly distorts the apparent shapes of the background galaxies.

By analyzing the precise, correlated distortions in the shapes of 26 million galaxies, scientists could work backward to plot the distribution of the intervening mass that caused the lensing. The resulting map revealed a complex cosmic web, with dense filaments of dark matter connecting massive clusters and large empty regions, known as voids, in between. The red-tinted areas on the map indicated regions with higher than average concentrations of dark matter, while blue areas had lower concentrations.

The Dark Energy Camera in Chile

Data for the survey was collected using the 570-megapixel Dark Energy Camera (DECam), one of the most powerful digital imaging devices in the world at the time. The camera is mounted on the 4-meter Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes, a site chosen for its clear atmospheric conditions.

The Dark Energy Survey, a collaboration of over 400 scientists from 25 institutions, began its five-year mission in 2013. The 2017 results were based only on the first year’s data, covering a fraction of the total area the survey would eventually observe. The project’s primary goal was to study the dynamics of the universe’s expansion and probe the nature of dark energy, the mysterious force thought to be driving this acceleration.

Consistency with Cosmological Models

A key finding from the 2017 data release was its strong agreement with existing cosmological theories. The structure and clumpiness of the dark matter observed by DES closely matched predictions derived from the Standard Model of cosmology. It was also consistent with data from the European Space Agency’s Planck satellite, which had previously mapped the cosmic microwave background (CMB), the faint afterglow of the Big Bang.

This consistency between observations of the early universe (from Planck) and the more recent universe (from DES) provided powerful independent confirmation of the standard cosmological model. According to reports at the time, this alignment suggested that scientists’ understanding of how cosmic structures grew over the last 14 billion years was fundamentally correct.

Future Work and Unanswered Questions

While the map was a significant achievement, it did not identify what dark matter is made of. The DES collaboration announced it was entering its fifth and final year of observations in August 2017. Scientists anticipated that the complete dataset, covering 300 million galaxies, would allow for even more precise measurements and could potentially reveal subtle deviations from the Standard Model, which might point toward a new understanding of gravity or the fundamental properties of the universe.

Questions about this record

What was the Dark Energy Survey trying to accomplish?

The primary goal of the Dark Energy Survey (DES) was to understand the nature of dark energy, the force believed to be causing the accelerated expansion of the universe. By creating a detailed map of dark matter's distribution, scientists could study how cosmic structures have formed and grown over time, providing crucial data to test cosmological models and the properties of both dark matter and dark energy.

How did scientists 'see' dark matter if it is invisible?

Scientists inferred the location of dark matter by observing its gravitational effects on light from distant galaxies. This phenomenon, called gravitational lensing, causes the apparent shapes of galaxies to be slightly distorted. By measuring these tiny, correlated distortions across millions of galaxies, researchers could map the foreground mass, mostly dark matter, that was responsible for bending the light.

Where were the observations for this dark matter map made?

The data was collected at the Cerro Tololo Inter-American Observatory, located in the Andes Mountains of Chile. Researchers used the 570-megapixel Dark Energy Camera (DECam), which was mounted on the Victor M. Blanco 4-meter Telescope. This location provides excellent atmospheric conditions for astronomical observation, allowing for the clear images needed to measure subtle galactic distortions.

Why was it significant that the map matched predictions from the Planck satellite?

The Planck satellite measured the cosmic microwave background, which is a snapshot of the universe just 380,000 years after the Big Bang. The Dark Energy Survey observed a much later universe, billions of years later. The fact that the DES map of cosmic structure matched predictions based on Planck's data provided strong confirmation for the standard model of how the universe evolved from its early, smooth state into the clumpy web of galaxies we see today.