Amount Of Dark Matter

Universe Makeup

Dark matter is particularly elusive as it does not emit, absorb or reflect light, but makes itself apparent only through gravitational attraction. If asked what dark matter is, most scientists in the field will honestly respond that they do not know.

However, we "do" know from WMAP satellite data, that ordinary matter (made up of atoms) makes up only 4.6% of the universe (to within 0.1%). That "dark matter" (not made up of atoms) makes up 23.3% (to within 1.3%). And, that "dark energy" makes up 72.1% of the universe (to within 1.5%).

So there is five times as much dark matter in the universe as there is ordinary matter if Einstein's gravity equations are correct. And, there is overwhelming evidence that they are true. There does not appear to be any alternative scientific explanation that can explain dark matter on all scales, so the conclusion that dark matter exists appears unavoidable.   Top

What Might Dark Matter Be?

Football Dark matter

Shown in the photo at the left, taken by the Subaru Telescope in Hawaii, are 18 galaxy clusters. Each cluster contains thousands of galaxies with giant halos (clumps) of dark matter (shown in blue) inferred with the help of gravitational lensing. All the halos are flattened a bit like footballs, with their horizontal axes about twice as long as the vertical axes, regardless of the shape and distribution of their galaxy clusters.

In general, dark matter may be like ordinary matter but in some form which makes it invisible, or else it may be some exotic form of non-ordinary matter. Hypothetical normal dark matter objects are usually referred to as MACHOs - massive compact halo objects - since they are candidates for explaining dark matter within galaxy clusters.

On the other hand, dark matter may be non-ordinary matter. Hypothetical favorites among non-ordinary dark matter contenders are weakly interacting massive particles (WIMPs), predicted by supersymmetry particle theories, and Axions, predicted by other promising theories.

Theoretical calculations of the amount of small mass elements formed in the early Big Bang universe indicate that the total amount of ordinary matter can be no more than ten times the amount of visible matter. This is sufficient to explain the amount of ordinary dark matter on the scale of galaxies but not enough to explain the amount of dark matter in galaxy clusters. Therefore, the existence of non-ordinary dark matter appears inescapable. However to date, no particles with invisible mass, with light able to pass through them, with gravitational pull, and with zero radiation characteristics have been discovered.  Top


LUX Dark Matter Sensor

Weakly Interacting Massive Particles (WIMPs) are part of a hypothetical class of new particles that emerge from supersymmetry theory. Supersymmetry theory is a theory where known elementary particles have supersymmetric partner particles. The theory suggests that every fundamental matter particle has a massive shadow "force" carrier particle, and every force carrier particle has a massive shadow "matter" particle. For example, an electron has a "selectron' for a partner, a quark has a "squark".

On very rare occasion, a dark matter particle might just collide with a normal matter atom. The trick is to catch that signal amid the storm of outer space particles bombarding the earth so thickly that hundreds pass through our bodies each second.

Nearly a mile underground in South Dakota, the ultra-sensitive Large Underground Xenon experiment, or LUX, began searching for evidence of dark matter. LUX was the latest of several experiments trying to discover dark matter. LUX, with a two story state of the art detector, shown at the left, is sheltered in what was once North America's deepest gold mine. Scientists were looking for a flash of something far more elusive than gold - dark matter.

Scientists utilize caverns deep inside the earth to mute the outer space particle bombardment that surrounds us on the surface. The LUX tank was filled with liquid xenon that was extremely dense and is surrounded by a 70,000 gallon shield of water. Scientists then wait for dark matter particles to hit it.

Xenon is so chemically inert that electron signals from any collisions can pass freely through the liquid. These crucial signals allow scientists to eliminate noise from a true dark matter signal. Scientists were watching for a two-flash combination to occur when a dark matter particle collides with the highly purified xenon inside the detector. LUX was estimated to be at least 10 times more sensitive than all previous detectors.

LUX Dark Matter Sensor

Since running the detector from August, 2013 and looking for tiny flashes of light that could indicate a dark matter particle collision, LUX researchers found no signals beyond the background noise. However, they did so at far greater sensitivities than any previous experiment. In May of  2016, LUX completed its search for dark matter. It did not actually find any of the elusive particles.

A similar detector project, the Xenon100 experiment at Italy's National Laboratory in Gran Sasso, was also designed to search for a WIMP by watching for signals that a WIMP had recoiled off an atom in a tank of liquid xenon. But in 2017 after 9 years of testing, no WIMP particles have been detected.

In 2015 The LUX project merged with the English Zeplin project and became the LZ Project to pursue the next stage of the xenon experiment. The LZ project is a next-generation experiment. It is to be located at the same Sanford Underground Research Facility (SURF) in South Dakota (see the photo to the left). It is supported by the US Department of Energy (DOE) and the US National Science Foundation. It will be managed by DOE’s Lawrence Berkeley National Lab.

The LZ detector will consist of 10 tons of liquefied xenon to detect extremely faint interactions between dark matter and regular matter. When completed, the experiment will be the world’s most sensitive experiment for WIMPs over a large range of WIMP masses. The LZ collaboration consists of about 250 scientists in 37 institutions in the U.S., U.K., Portugal, Russia, and Korea.  Top

CDMS Searches For Dark Matter


The Cryogenic Dark Matter Search (CDMS) is a series of experiments designed to directly detect dark matter particles in the form of WIMPs. (Cryogenics is the behavior of materials at extremely low temperatures.) Using an array of semiconductor detectors at miniscule temperatures, CDMS set the most sensitive limits on the interactions of WIMP dark matter up until 2018.

The first experiment, CDMS I, was run in a shallow tunnel under the Stanford University campus in California. It was followed by CDMS II experiment in the Soudan Underground Mine on the south shore of Lake Vermilion in northern Minnesota. The mine was known as Minnesota's oldest, deepest, and richest iron mine.

The most recent experiment, SuperCDMS, was also located deep underground in the Soudan Mine and took data from 2011 to 2015. CDMS researchers observed several nuclear interactions with the detectors. However, the CDMS staff concluded that all of them were from neutrons. None of the CDMS experiments so far have resulted in any dark matter interactions being discovered.

The series of experiments continues with SuperCDMS SNOLAB, an experiment that will be located 2 km below the Earth’s surface at SNOLAB in Canada. SNOLAB is a world-class science facility located deep underground in the Vale Creighton nickel mine, near Sudbury, Ontario. SNOLAB is much deeper and cleaner than previous facilities, providing significantly more shielding from high energy cosmic ray particles, and from radioactive decay by products. CDMS DetectorConstruction started in 2018 and is expected to start taking data in the early 2020s.

A CDMS detector (pictured to the left) is designed to detect the minute signal generated within a crystal by the collision between a WIMP and the detector. The energy deposited in a detector by a WIMP could be as low as a few keV (thousand electron volts). Detection at such low energy levels requires an extremely sensitive experimental apparatus.

The main requirement is that the detector is maintained at a very low temperature to distinguish the deposited energy from the thermal energy of the detector itself. The CDMS II project and associated test facilities employed helium refrigerator techniques that were able to achieve detector temperatures as low as 10 milli-Kelvin (1/1000th of a degree Kelvin).

A WIMP that collides with a nucleus in the detector generates vibrations in the detectors crystal lattice. These vibrations are called "phonons". The phonons propagate through the crystal and some reach the surface. There, they are absorbed by the aluminum collector fins. In the aluminum, the phonons convert their energy into "quasi-particles", which are basically electrons that are then electronically detectable.

There are a lot of interested organizations supporting CDMS technology. Some of the funding sponsors are the US Department of Energy (DOE), the US National Science Foundation, and the Canadian Foundation for Innovation. The SuperCDMS collaboration involves 111 physicists at 26 institutions in the US, Canada, France and the UK.  Top

CERN - Looking For Supersymmetry Particles

CERN Tunnel

In addition to LUX, experiments at the Large Hadron Collider (LHC) in CERN have also been looking for supersymmetry particles as part of their experiments using high energy collisions. Supersymmetry dark matter particles, including WIMPS, should annihilate in a very particular way, which up until now has not been detected.

In its latest supersymmetry studies, the ATLAS and CMS collaborations have sifted through the entire proton to proton collision data collected in the experiments during the LHC’s second run. That run took place between 2015 and 2018 to look for signs of supersymmetry particles.

As of December, 2018 when the LHC shut down two years for upgrades, the data from both the Atlas and CMS detectors have ruled out supersymmetric particles up to about a thousand times the mass of a photon, a pretty hefty size. Also, no light mass supersymmetry particles have been detected.

CERN executives feel no final conclusions can be drawn yet. However, many scientists feel that if supersymmetry exists, the lighter particles should have been seen by now. Since WIMPs are part of supersymmetry theory, confidence that WIMPs exist has been shaken.  Top

University Of Washington - Azions

ADMX Detector

Axions are "hypothetical" tiny, tiny particles that have a mass about 500 million times lighter than an electron. Additionally, an axion has no spin, is electrically neutral, and interacts very weakly with other particles. However, axions do react gravitationally with other matter. Enough Axions could constitute dark matter.

Axion particles were originally suggested in 1977 to solve a complex problem in advanced quantum theory. However, axions have never been experimentally detected and remain purely theoretical particles. If axions do exist, zillions would have been produced just after the Big Bang and the universe should be full of them.

If the axion is as light as believed and interacts so weakly that it is nearly impossible to detect, that makes it an ideal dark matter candidate. The goal of the ADMX (Axion Dark Matter eXperiment) at the University of Washington, pictured at the left, is to detect this extraordinary small particle. ADMX is one of only a few experiments able to detect axions.

The concept behind ADMX is straightforward. If dark matter really consists of particles, there must be a continuous wind of them blowing through the earth and everything on it all the time. And if those particles are axions, theoretically they will very occasionally decay. The particles themselves are invisible, but in that rare decay process, they should turn into microwaves, which would produce a weak but detectable signal. Straightforward, but difficult to detect in practice.

The aim of the ADMX Experiment is to detect axions raining down from outer space by sensing the conversion of some axions into photons (light). The detector employs a powerful magnet surrounding a sensitive microwave receiver that is supercooled. The low temperature reduces noise and greatly increases the chance that the detector will see some axions converted to photons. The microwave receiver can be fine tuned to the mass of an axion, which also increases the possibility of detecting an axion. A "hit" will produce a very small amount of power in the receiver, which will then be recorded by the attached computers. The initial ADMX experiment was begun in October, 2013. However by the end of 2015, no axions had been detected.

The Large Hadron Collider (LHC) at CERN, widely known for its discovery of the Higgs boson in 2012, has not yet found any evidence to support supersymmetry. LCH evidence of small mass particles from supersymmetry theory (which was expected) would have given WIMPs a boost as a dark matter candidate. This lack of supersymmetry evidence has prompted scientists to separate the search for dark matter from the search for supersymmetry. Therefore, the concept of the ADMX is drawing interest from dark matter researchers.

The initial dark matter experiment underwent an upgrade to the ADMX G2 that will allow it to be sensitive to a broad range of plausible dark Smatter axion masses and couplings. The ADMX G2 experiment is one of the US Department of Energy's flagship dark matter searches, and the only one looking for axions. It is scheduled to run from 2016 to 2021.  Top

MIT - Axions

Football Dark matter

Because of their interaction with electromagnetism, axions are theorized to have a surprising behavior around magnetars - a type of neutron star that churns up a hugely powerful magnetic field. If axions are present, they can exploit the megastars' magnetic field to convert themselves into radio waves, which could be detected with dedicated telescopes on Earth.

In 2016, a trio of MIT theorists drew up an experiment for detecting axions, inspired by the magnetar. The experiment was dubbed ABRACADABRA. It was conceived by Jesse Thaler, who is an Associate Professor of Physics, along with Benjamin Safdi, then an MIT Fellow, and former graduate student Yonatan Kahn. The team proposed a design for a small, donut-shaped magnet kept in a refrigerator at temperatures just above absolute zero.

"This is the first time anyone has directly looked at this axion space," says Lindley Winslow, principal investigator of the experiment and an Assistant Professor of Physics at MIT. Without axions, there should be no magnetic field in the center of the donut, or, as Winslow puts it, "where the munchkin should be." However, if axions do exist, a detector should "see" a magnetic field in the middle of the donut. After the initial group published their theoretical design, Winslow, an experimentalist, set about finding a way to actually construct the experiment.

It is a challenging experiment because the expected signal is less than 20 atto-tesla (10^-18 Tesla). In building the experiment, Winslow and her colleagues had to contend with two main design challenges, the first of which involved the refrigerator used to keep the entire experiment at ultra cold temperatures. The refrigerator included a system of mechanical pumps whose activity could generate very slight vibrations that Winslow worried could mask an axion signal. The second challenge had to do with noise in the environment, such as from nearby radio stations, electronics throughout the building turning on and off, and even LED lights on computers and electronics. Any of these could generate competing magnetic fields. The team solved the first problem by hanging the entire contraption, using a thread as thin as dental floss. The second problem was solved by a combination of cold superconducting shielding and warm shielding around the outside of the experiment.

In 2018, two years later, the team carried out ABRACADABRA's first run, continuously sampling data between July and August. After analyzing the data from this period, they found no evidence of axions within the mass range of 0.31 to 8.3 nano-electronvolts. The experiment is designed to detect axions of even smaller mass, down to about 1 femto-electronvolts, as well as axions as large as 1 micro-electronvolts. The team will continue running the experiment, which is about the size of a basketball, to look for even smaller and weaker axions. Meanwhile, Winslow is in the process of figuring out how to scale the experiment up to the size of a compact car - dimensions that could enable detection of even weaker axions.  Top

CERN Approves A Hunt For New Particles - Axions

CERN Arial View

The CERN research board has approved the Forward Search Experiment, FASER. It has given a green light to the assembly, installation and use of an instrument that will look for new fundamental particles at the Large Hadron Collider (LHC) in Geneva, Switzerland.

Initiated by physicists at the University of California, Irvine (UCI), the five-year FASER project is funded by grants of $1 million each from the Heising-Simons Foundation and the Simons Foundation – with additional support from CERN, the European Organization for Nuclear Research.

FASER's focus is to find light, extremely weakly interacting particles, such as Axions, that have so far eluded scientists. This was a big disappointment from the early high-energy experiments conducted at the CERN operated LHC, the largest particle accelerator in the world.

FASER co-leader Jonathan Feng, a Professor of Physics and Astronomy at UCI, will be joined by CERN collaborators as well as other scientists from research institutions in Europe, China, Japan and the United States. The FASER team will consist of 30 to 40 members, relatively small compared to other groups conducting experiments at the LHC.

The FASER instrument is compact, measuring only about 1 meter in diameter and 5 meters long. It will be placed at a specific point along the 16-mile loop of the LHC, about 480 meters (1,574 feet) away from the hulking, six-story instrument used by the ATLAS collaboration to discover the Higgs boson.

As proton beams pass through the interaction point at the ATLAS instrument, they may create new particles that will go through the concrete in the LHC tunnel and then into the FASER instrument, which will track and measure the progress of their decay. FASER will collect data any time the ATLAS is operating.

The FASER detector, which will be one of only eight research instruments at the LHC, is being built and installed during the Collider's current 2019 hiatus and will collect data from 2021 to 2023. The LHC will be shut down again from 2024 to 2026. During that time, the team hopes to install the larger FASER 2 detector, which will be capable of unveiling an even wider array of mysterious, hidden particles.