Dark Matter Invisibility

Dark matter has not been explained by modern physics---yet.

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By Kevin Roeten:

Kevin Roeten
Science Editor

No matter how much time, money, and effort we invest in further experimentation, the answer will always be right around the corner…

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 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 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.

Large Hadron Collider. No sign of Kaluza yet.
Image Editor/Flickr

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 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 protonsalong with neutrons. This particle would interact via electromagnetism and gravity. But this particle would be in an invisible dimension. Luckily, the particle should be is 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

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 – a form of matter accounting for 85% of all matter in the universe.

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.
NASA; A. Mellinger/Central Michigan University; T. Linden/University of Chicago

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 atitanic collision between two galaxy clusters.

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

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 “El Gordo” was first known 2012. They were able to put together Estimates of the cluster’s mass were 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.

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

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.

“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 in the early ’50s first indicated the universe contained more matter than seen.. Support for dark matter has grown, but there has been no solid direct evidence of dark matter—at least not yet.

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]

The missing matter is only more challenging to detect. 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.

These illustrations, taken from computer simulations, show a swarm of dark matter clumps around our Milky Way galaxy. Image released July 10, 2012.
Credit: J. Tumlinson (STScI)

Most scientists think dark matter is composed of the lead candidate, WIMPS, which can have a hundred times the mass of a proton. But their weak interactions with “normal” matter make them difficult to detect. Neutralinos, massive hypothetical particles heavier than neutrinos, are the foremost candidate, though they have not yet been seen. Laws of gravity have successfully dictated motion of objects within the solar system, but need serious revision.

Proving the unseen

Since scientists can’t see dark matter, they calculate the mass of large objects in space by studying their motion. Astronomers examining spiral galaxies in the 1950s expected to see material in the center moving faster than on the outer edges. Instead, they found the stars in both locations traveled at the same velocity, indicating the galaxies contained more mass than could be seen. Clusters of galaxies would fly apart if the only mass they contained were visible to conventional astronomy.

Albert Einstein showed massive objects in the universe bend light, and can be used as lenses. By seeing light distorted by galaxy clusters, astronomers have been able to create a map of dark matter. Scientists hope by plotting out dark matter throughout space, they will come closer to understanding it.

Dark matter versus dark energy

Dark matter makes up most of the matter, but only a quarter of the composition. The universe is dominated by dark energy. After the Big Bang, the universe began expanding outward. Scientists thought it would eventually run out of energy. But we see the universe expanding faster than in the past, showing accelerating expansion. This is only possible if the universe contains enough energy to overcome gravity, or dark energy.

This graph shows the status of searches for Weakly Interacting Massive Particles (WIMPs). The abscissa is the mass of the putative WIMP particle. For reference, the proton has a mass of about one in these units. The ordinate is a measure of the probability for WIMPs to interact with normal matter. Not much! The shaded regions represent theoretical expectations for WIMPs. The light red region is the original (Ellis et al.) forecast. The blue and green regions are more recent predictions (Trotta et al. 2008). The lines are representative experimental limits. The region above each line is excluded – if WIMPs had existed in that range of mass and interaction probability, they would have been detected already. The top line (from CDMS in 2004) excluded much of the original prediction. More recent work (colored lines, circa 2008) now approach the currently expected region. (This plot was generated by DMTools.)

Indeed, the experiments have perhaps been too successful. The original region of cross section-mass parameter space in which WIMPs were expected to reside was excluded years ago. Not easily dissuaded, theorists waved their hands, and invoked the Majorana see-saw mechanism.  This is the vertical separation of the reddish and blue-green regions in the figure.

Though set back and discouraged by this theoretical slight of hand (the WIMP “miracle” is now more of a vague coincidence, like seeing an old flame in Grand Central Station but failing to say anything because (a) s/he is way over on another platform and (b) on reflection, you’re not really sure it was him or her after all), experimentalists have been gaining ground on the newly predicted region. If all goes as planned, most of the plausible parameter space will have been explored in a few more years. (I write this in 2010. I have heard it asserted that “we’ll know what the dark matter is in 5 years” every 5 years for the past two decades.)

The Express Elevator To Hell

“We’re on an express elevator to hell – going down!”

While there is a clear focus point where WIMPs most probably reside (the blue blob) If we fail to detect WIMPs when experimental sensitivity encompasses the blob, the presumption will be that we’re just unlucky and WIMPs happen to live in the low-probability tail that is not yet excluded. This is the express elevator to hell. No matter how much time, money, and effort we invest in further experimentation, the answer will always be right around the corner. Is dark matter a scientific (falsifiable) hypothesis? The existence of dark matter is an inference, and WIMPs are falsifiable dark matter candidates.


Kevin Roeten can be reached at roetenks@CHARTER.NET.

© Copyright by Kevin Roeten, 2017. All rights reserved.

Kevin Roeten
About Kevin Roeten 159 Articles
CHO's science editor Kevin Roeten is a former Chemical Engineer. He enjoys riding the third rail of journalism: politics and religion. As an orthodox Catholic, Roeten appreciates the juxtaposition of the two supposedly incompatible subjects.   Kevin is a Guest Columnist for the Asheville Citizen-Times, and the Independent (Ohio), and writes for numerous blogs (Nolan Chart, Allvoices) and newspapers, including USA Today.   A collaborator in the book Americans on Politics, Policy, and Pop Culture (Jason Wright and Aaron Lee), he is also an amateur astronomer, and delves into scientific topics.   Kevin Roeten can be reached at roetenks@charter.net.

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