Dark Matter Invisibility

Dark matter has not been explained by modern physics – yet. Researchers know dark matter exists because it bends light from distant galaxies and changes galaxies’ rotations. Most scientists believe it’s composed of yet-to-be-discovered particles which almost never interact other than through gravity, making it difficult for detection.

These particles include:

The WIMP, a hypothetical particle which may be all around us in space. It would be totally different from the type of matter we know. It would interact via the electromagnetic force, therefore be mostly invisible in space. Roughly 100,000 of these would pass through every square centimeter of the Earth every second, interacting with surrounding matter. If WIMPs exist, there must be five times more than normal matter, coinciding with the large amount of dark matter in the universe.

It was independently predicted WIMPs must exist – a coincidence dubbed the “WIMP miracle.” WIMP stands for Weakly Interacting Massive Particle. This is an entire class of new fundamental particles emerging from supersymmetry. Supersymmetry is a theoretical notion by which known particles have supersymmetric partner particles. but might exist provided the Higgs boson exists. This is the WIMP presumably making up dark matter.

Axions are low-mass, slow-moving particles not having a charge and only interact weakly with other matter which makes them extremely difficult to detect. Only axions of a specific mass would be able to explain the invisible nature of dark matter. “MACHO” means “massive astrophysical compact halo object” and was the first known dark matter. MACHOs include neutron stars, and brown and white dwarfs. They are composed of ordinary matter, but are invisible. They emit very little, to no light.

One can observe them by monitoring the brightness of distant stars. As light rays bend when they pass close to a massive object, light from a distant source may be focused by a closer object to produce a sudden brightening of the distant object. This effect, known as gravitational lensing, depends on how much matter, both normal and dark, is in a galaxy.

The Kaluza-Klein theory is built around the existence of an invisible “fifth dimension” in space, in addition to the three spatial dimensions we know, and time. This string theory, predicts the existence of a dark matter particle, having a mass of 550 to 650 protons along with neutrons. This particle would interact via electromagnetism and gravity. But this particle would be in an invisible dimension. Luckily, the particle should be easy to look for in experiments since it should decay into particles we can measure. However, powerful particle accelerators like the Large Hadron Collider have not detected it yet.

Theories combining general relativity and “supersymmetry” predict a particle called the gravitino. Supersymmetry states all “boson” particles have a “super-partner.” The gravitino would be the super-partner of the hypothetical “graviton,” thought to interact with the force of gravitation. The gravitino is very light, but may account for dark matter.

Actually Seeing Dark Matter

excess gamma-ray glow at the galaxy center.
When astrophysicists model the Milky Way’s gamma-ray sources to the best of their knowledge, they are left with an excess glow at the galactic center. Some researchers have argued the signal might hint at hypothetical dark matter particles. However, it could also have other cosmic origins. Credit: NASA; A. Mellinger/Central Michigan University; T. Linden/University of Chicago

A mysterious gamma-ray glow at the center of the Milky Way is most likely caused by pulsars – the incredibly dense, rapidly spinning cores of collapsed ancient stars 30 times more massive than the sun. The findings cast doubt on previous interpretations of the signal as a potential sign of dark matter. Dark Matter accounts for 85% of all matter in the universe.

Mapping the dark matter in the core of galaxy cluster Abell 520

Douglas Clowe of Ohio University, is reporting on new Hubble observations not finding a dense clump of dark matter in the universe. “The region of interest lies at the center of a collision among massive galaxy clusters in Abell 520, located 2.4 billion light-years away,” Clowe said. “Our measurements are in complete agreement with how we would expect dark matter to behave.” Because dark matter is not visible, its presence is only found by its gravitational effects on other bodies.

Hubble has now revealed the monster “El Gordo” galaxy above is really, really huge. “El Gordo” [Spanish – “the fat one”] refers to a monstrous cluster of galaxies when the universe was just half of its current age of 13.8 billion years. It contains several hundred galaxies swarming around under a collective gravitational pull. The total mass of the cluster, is estimated to be as much as 3 million-billion stars. Actually 3,000 times larger than the Milky Way, the mass is hidden as dark matter. The cluster is huge because of a titanic collision between two galaxy clusters.

A fraction of this mass is locked up in several hundred galaxies inhabiting the cluster. The rest is tied up in dark matter, making up most of the universe. Nothing like this has ever been seen to exist so far back in time, when the universe was roughly half of its current age. The immense size of ˆ was first known in 2012. They were able to put together estimates of the cluster’s mass based on motions of the galaxies internal to the cluster.

Felipe Menanteau of the University of Illinois at Urbana-Champaign said, “We were in dire need for an independent and more robust mass estimate given how extreme this cluster is, and how rare its existence is in the current cosmological model. There was all this kinematic energy that could be unaccounted for and could potentially suggest that we were actually underestimating the mass.”

The expectation of “unaccounted energy” comes from the merger occurring tangentially to the observers’ line of sight. This means they are potentially missing a good fraction of the kinetic energy because their measurements only track the radial speeds of the galaxies.

Hubble’s high resolution allowed measurements of so-called “weak lensing,” where the cluster’s immense gravity warps images of background galaxies. The greater the warping, the more mass is locked up in the cluster.

Dark Matter Remains Elusive

The researchers believe that a recently discovered strong gamma-ray glow at the center of the Andromeda galaxy may also be caused by pulsars rather than dark matter. Although the Fermi-LAT team studied a large area of 40 x 40 degrees around the Milky Way’s galactic center the extremely high density of sources makes it very difficult to see individual ones, leaving limited room for dark matter signals to hide. The new results add to other data challenging the gamma-ray excess as a dark matter signal.

Simulated distribution of gamma-ray sources in the inner 40 degree by 40 degree region of the Milky Way.
Simulated distribution of gamma-ray sources in the inner 40 degree by 40 degree region of the Milky Way with the galactic center in the middle. The map shows pulsars in the galactic disk (red stars) and in the galaxy’s central region (black circles). Credit: NASA/DOE/Fermi LAT Collaboration

“If the signal were due to dark matter, we would expect to see it also at the centers of other galaxies,” Digel said. “The signal should be particularly clear in dwarf galaxies orbiting the Milky Way. These galaxies are held together because they have a lot of dark matter.”

This Hubble Space Telescope composite image shows a ghostly “ring” of dark matter in the galaxy cluster Cl 0024+17. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

Roughly 80 percent of the mass of the universe is made up of dark matter. It does not emit light or energy. But scientists believe it dominates the cosmos. Studies of other galaxies almost 80 years ago first indicated the universe contained more matter than was seen.

Support for dark matter has grown, but without solid direct evidence of dark matter.

Typical material in the universe is known as baryonic matter, composed of protons, neutrons and electrons. Dark matter may be made of baryonic or non-baryonic matter. To hold the elements of the universe together, dark matter must make up approximately 80 percent of its matter. [Image Gallery: Dark Matter Across the Universe]

Where’s the missing matter? Potential candidates include dim brown dwarfs, white dwarfs and neutrino stars. Supermassive black holes could also be part of the difference. But these objects need to play a more dominant role to make up the missing mass.

Bullet Cluster, where Dark Matter is clearly visible

Kevin Roeten

A former Chemical Engineer, Kevin Roeten enjoys riding the third rail of journalism: politics and religion. He is a Guest Columnist for the Asheville Citizen-Times, and the Independent (Ohio), writes for numerous blogs, is an amateur astronomer, and delves into scientific topics.