Last week astronomers observed the most crowded collision in the Universe! Four clusters of galaxies poured into a crowded 13 million-light-year-long stream of galaxies. (eventually our Milky Way will merge with the neighboring Andromeda galaxy as well!). These galactic events can probe the existence of so called “Dark Matter”. In fact particle physicist developed a superb model with predictions confirmed at the per mille level. But how much of the Universe does the Standard Model explain? Just 4%! The rest is out there to be discovered.
I am not an expert in astronomical measurements, but these events do grab my attention. Let’s start from a simple definition of Dark Matter. Dark Matter is matter undetectable by its emitted radiation, it is not visible. As for today, astronomers measured its contribution to the total Universe mass to be ~25%.
How did we infer the existence of Dark Matter in first place ? As the Earth rotates around the Sun due to gravitational attraction, stars in galaxies rotate around the center of the galaxy. However the amount of mass visible is not enough to explain the rotational velocity, a large component of non-visible mass must exist. This is one of the proofs along with orbital velocities of galaxies in clusters of galaxies, and gravitational lensing.
Gravitational Lensing
The presence of mass can in fact be explained by the fascinating phenomena of “gravitational lensing”. Imagine a star far from the Earth. If the light from the star travels without encountering obstacles up to the Earth, we see a light spot. However, if there is a large amount of mass (say a galaxy) between us and the star, the light from the star changes its path (see the picture on the left). The gravity due to the extra galaxy acts like a lens to redirect the light rays, it bends the light. The gravitational lens does not create one single image of the star, but multiple ones. It can also distort the star disk-like shape into an ellipse. If the extra galaxy were perfectly symmetric with respect to the line between the star and the Earth, we would see a ring of stars!
Image of gravitational lensing
What happens when two clusters of galaxies collide ? By now we now that a cluster of galaxies is gravitationally bound object, and the densest part of the Universe. Stars constitute ~2% of its total mass while so called “intergalactic gas” contributes to ~15%. The remaining mass is still in the Dark.
The clusters collide at speeds of millions of miles per hour. Several are the observatories taking pictures of these titanic events.
The Hubble Space Telescope, the Magellan Telescope and Very Large Telescope provide a map of the total mass (dark and ordinary) using visible light. The gravitational lensing indicates the location of the Dark Matter component (blue). As an example you can see the pictures from the well known “Bullet Cluster” observed in 2006. The Chandra data enabled the astronomers to accurately map the position of the ordinary matter by itself, mostly in the form of hot gas, which glows brightly in X-rays (shown in pink).
Image of Dark Matter (above); Image of visible mass (below)
As the clusters travel in opposite direction, they eventually collide. The picture below shows you the mass distribution after the collision. The ordinary matter slowed down compared to the Dark Matter and the two components separate.This is due to the different forces exerted on the Dark and visible mass. Dark Matter particles interact with each other only very weakly or not at all, apart from the pull of gravity. (ordinary matter experiences larger “friction”, therefore it slows down during the collision).
The separation provides observational evidence for dark matter.
The "Bullet cluster" collision
What’s the Nature of Dark Matter ?
A variety of cosmological data suggests that Dark Matter may be relics from particles present in the early universe. Currently the best theory to explain the origin of dark matter is Supersymmetry (SUSY), which predicts the existence of a “superpartner” for each Standard Model particle. The lightest superpartner of the neutral bosons (the Z and the Higgs bosons), called the “neutralino,” is an excellent candidate for this elusive form of matter. Being able to observe the SUSY particles would be crucial for a deep understanding of the universe. Superparticles could be generated in proton-antiproton collisions at the Tevatron and in proton-proton collisions at the LHC.
The experiments at the Tevatron accelerator, CDF and D0, are desperately seeking a sign of SUSY in the collisions stored on tape, however these particles – if they exist – might be heavier than 100 times the proton. ATLAS and CMS are tuning their tools to be ready for the incoming LHC collisions!
