Science › Big Bang & cosmos
The Bullet Cluster: dark matter caught in the act
2006· Discovery / experiment
ProvenTested many times; part of working physics.
Two galaxy clusters that smashed through each other, leaving their hot gas behind while most of their mass sailed on, unseen.
See it on the Science map →How it works
Cluster 1E 0657-56 lies about 3.8 billion light-years away. About 150 million years ago a smaller cluster punched through a larger one; a shock wave in its gas, racing at roughly 4,500 km/s, gives it a bullet shape in X-ray images. The clusters' hot gas, which holds most of their ordinary matter, collided and lagged in the middle. Their galaxies, widely spaced, passed through. Lensing shows the bulk of the mass traveled with the galaxies, not the gas.
Why it matters
It is the clearest picture of dark matter as real stuff in a place, rather than a correction to gravity. It also limits how much dark matter can collide with itself.
How we know
Chandra X-ray Observatory images (Maxim Markevitch, 2002–2004) mapped the gas. Douglas Clowe's 2006 lensing study found the mass offset from it at 8σ, titling the paper 'A direct empirical proof of the existence of dark matter'.
Still a mystery
Some collisions are messier: in Abell 520, the 'Train Wreck', several studies found a dark clump left behind with the gas, which collisionless dark matter should not do. Others dispute the measurement.
Connections (3)
▸Gravitational lensing mapped the hidden mass in The Bullet Cluster: dark matter caught in the actProven
The relationship
Light from thousands of faint galaxies behind the Bullet Cluster is slightly stretched by its gravity, so their shapes line up in arcs around each concentration of mass. Averaging those shapes produced a map of where the mass is, regardless of whether it shines. A few strongly bent arcs pinned down the cores. Laying that map over the X-ray picture of the gas showed the mismatch.
How the link was found
Douglas Clowe, Anthony Gonzalez and Maxim Markevitch (2004), refined in 2006 with Hubble, Magellan and European Southern Observatory images; Maruša Bradač's team (2006) added the strongly lensed arcs.
Why it's proven
Lensing follows directly from general relativity and is routinely checked against clusters' X-ray and galaxy-motion masses. The independent weak and strong lensing maps agree on where the Bullet Cluster's mass sits.
Open questions and mysteries
Lensing adds up all mass along the line of sight, so unrelated foreground or background matter can blur the map slightly. How the dark matter is distributed in fine detail inside the cluster is still unclear.
▸The Bullet Cluster: dark matter caught in the act shows, separated from gas, Dark matterProven
The relationship
In a cluster collision, the hot gas clouds slam together and slow, like two water balloons meeting. Galaxies, being far apart, pass through like two crowds walking through each other. Dark matter that barely interacts should keep going with the galaxies. That is what lensing shows: most mass is with the galaxies, though the gas outweighs the stars several times. So the extra mass is neither gas nor stars.
How the link was found
Clowe and colleagues framed it this way in 2006. Markevitch's team (2004) and Scott Randall's (2008) used the same collision to measure how much dark matter can bump into itself.
Why it's proven
The mass is offset from the gas at 8σ. The dark clumps were barely slowed, which caps how strongly dark matter can collide with itself.
Open questions and mysteries
Could dark matter collide with itself weakly, just under that cap? Some astronomers favor such 'self-interacting' dark matter to explain why small galaxies' centers are less dense than simulations predict.
▸The Bullet Cluster: dark matter caught in the act is a hard test for MOND: modified gravity instead of dark matterWell supported
The relationship
In MOND, gravity comes only from visible matter, just strengthened at weak pulls. In the Bullet Cluster most visible matter is hot gas, so MOND expects lensing to peak on the gas. It peaks on the galaxies instead. MOND can survive only by adding unseen matter, such as heavy neutrinos, which gives up its main appeal of needing no invisible mass.
How the link was found
Clowe's team presented it as a test in 2006; the same year, Garry Angus, Benoit Famaey and HongSheng Zhao ('Can MOND take a bullet?') showed MOND fits only with extra unseen mass.
Is it proven?
The lensing offset is measured at 8σ. MOND already needed extra mass in ordinary clusters, so the Bullet sharpened an existing problem rather than creating a new one.
Open questions and mysteries
Could a sterile neutrino, a hypothetical heavier cousin of the known neutrinos, rescue MOND in clusters? Some newer MOND-like theories with extra fields claim to mimic the offset; tests continue.