One of the brilliant things about astrophysics is being able to take pictures of invisible stuff.
Infrared telescopes, for example, allow astronomers to see “dark” nebulas (picture), clouds of dust and gas that weakly reflect light from nearby stars, glowing mostly in thermal emissions.
Similarly, high-energy x-rays and gamma rays let scientists “see” black holes, objects that by definition are so dense not even light can escape their gravitational pull.
OK, to be fair, we don’t see the black hole directly, but rather the radiation from infalling material. But still, what we do see is pretty good proof of objects that once existed only in theory.
Dark matter is another substance drawn from theory that astrophysics is starting to make visible—although the trick to finding dark matter is a bit more complicated.
For starters, dark matter doesn’t absorb or emit light, and so far we have no proof of it interacting directly with normal matter.
We know dark matter is there based solely on gravity. Stars move within galaxies, and galaxies rotate on their axes, in ways that suggest there must be more matter present than what we can see.
In addition, clusters of galaxies bend light from objects behind them (an effect known as gravitational lensing) more than they should based on the visible matter present.
The galaxy cluster Abell 1689 is among the most powerful gravitational lensing clusters ever observed, according to NASA, making it a great place to map the distribution of dark matter.
Now, a new picture of this lens-filled cluster from the Hubble Space Telescope has allowed astronomers to create one of the sharpest and most detailed maps yet of dark matter.
—Image courtesy NASA, ESA, and Z. Levay (STScI)
“The lensed images are like a big puzzle,” team leader Dan Coe of NASA’s Jet Propulsion Laboratory said in a statement.
“Here we have figured out, for the first time, a way to arrange the mass of Abell 1689 such that it lenses all of these background galaxies to their observed positions.”
The completed puzzle presented Coe and colleagues with evidence that the core of Abell 1689 is much denser in dark matter than expected for a cluster of its size.
And Abell 1689 isn’t the only one—a handful of other clusters also seem to have meaty dark matter cores. The new find confirms this surprising result, suggesting that galaxy clusters were forming billions of years earlier than anyone thought.
“At earlier times, the universe was smaller and more densely packed with dark matter,” Coe said.
“Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today.”
But the map doesn’t stop there. Understanding dark matter’s role in the early universe should in turn help astronomers figure out dark energy, an even more mysterious force in the universe.
While dark matter pulls via gravity, dark energy pushes—it’s thought to be the force that’s causing the expansion of the universe to speed up rather than slow down.
Because dark energy is making galaxies fly apart from each other, it should have stunted the growth of galaxy clusters in the early universe, but the new dark matter map says that ain’t so.
Figuring out the rules in this ancient tug-of-war will require more studies of even bigger clusters. Enter Cluster Lensing and Supernova survey with Hubble (CLASH), a program that will, over the next three years, use Hubble data to study the “dark” interactions in 25 extremely massive clusters.