The search for dark matter

October 27, 2016 by Shannon Brescher Shea, US Department of Energy
The search for dark matter
The Large Underground Xenon (LUX) experiment was one of the biggest efforts to directly detect dark matter. It was located a mile deep in a former gold mine to minimize radioactive "noise." . Credit: C.H. Faham. Courtesy of LUX Dark Matter experiment

At least a quarter of the universe is invisible.

Unlike x-rays that the naked eye can't see but equipment can measure, scientists have yet to detect dark matter after three decades of searching, even with the world's most sensitive instruments. But dark matter is so fundamental to physics that scientists supported by the Department of Energy's Office of Science are searching for it in some of the world's most isolated locales, from deep underground to outer space.

"Without dark matter, it's possible that we would not exist," said Michael Salamon, a DOE Office of Science High Energy Physics (HEP) program manager.

The Office of Science supports a comprehensive program in the hunt for dark matter and other phenomena that help scientists better understand how the universe functions at its most fundamental level.

Traces of Dark Matter's Influence

What we do know about dark matter comes from the ways it's influenced the universe nearly as far back as the Big Bang. Like paw prints left by an elusive animal, the cosmos is full of signs of dark matter's existence, but we haven't actually seen the creature itself.

Astronomer Fritz Zwicky discovered dark matter in 1933 when he was examining the Coma Cluster of galaxies. He noticed they were emitting much less light than they should have been, considering their mass. After running some calculations, he realized that the majority of the cluster's mass wasn't emitting light or electromagnetic radiation at all.

But it wasn't just that cluster. Today, we know that visible matter accounts for only five percent of the universe's total mass-energy. (As Einstein's famous equation, E=mc2, tells us, the concepts of matter and energy are intrinsically linked.) Dark matter makes up about a quarter of the total mass-energy, while dark energy comprises the rest.

Since Zwicky's initial discovery, scientists have found a number of other tell-tale signs. Examining the rotation of galaxies in the 1970s, astronomer Vera Rubin realized that they don't move the way they "should" if only visible matter exists. Her discovery of the galaxy rotation problem provides some of the strongest evidence for dark matter's existence. Similarly, cosmic background radiation, which has a record of the early universe imprinted on it, reflects dark matter's presence.

Scientists think dark matter is most likely made up of an entirely new elementary particle that would fall outside the Standard Model that all currently known particles fit into. It would interact only weakly with other known particles, making it very difficult to detect. There are two leading particles that theorists have postulated to describe the characteristics of dark matter: WIMPs and axions.

Weakly Interacting Massive Particles (WIMPs) would be electrically neutral and 100 to 1,000 times more massive than a proton. Axions would have no electric charge and be extraordinarily light – possibly as low as one-trillionth of the mass of an electron.

On the Hunt for Dark Matter

Not only does dark matter not emit light or electromagnetic radiation, it doesn't even interact with them. In fact, the only means by which scientists are confident dark matter interacts with is through gravity. That's why millions of dark matter particles pass through normal matter without anyone noticing. To capture even the tiniest glimpse, scientists are using some of the most sophisticated equipment in the world.

The Large Underground Xenon Experiment and Direct Detection

The Large Underground Xenon (LUX) experiment, which ran for nearly two years and ended in May 2016, was one of the most significant efforts to directly detect dark matter.

Directly detecting a dark matter particle requires it bump into a nucleus (the core of an atom) of ordinary matter. If this occurs, the nucleus would give off just a little bit of detectable energy. However, the probability of these particles colliding is staggeringly low.

The search for dark matter
The Alpha Magnetic Spectrometer on the International Space Station is supported by more than 20 different research institutions and was funded in part by DOE. It is designed to detect dark matter by measuring cosmic rays that may result from dark matter particles colliding with each other. Credit: US Department of Energy

In addition, Earth's surface has an extraordinary amount of radioactive "noise." Trying to detect dark matter interactions aboveground is like trying to hear someone whisper across the room of a noisy preschool.

To increase the chances of detecting a dark matter particle and only a dark matter particle, LUX was massive and located more than a mile underground. With a third of a ton of cooled liquid xenon surrounded by 72,000 gallons of water and powerful sensors, LUX had the world's best sensitivity for WIMPs. It could have detected a particle ranging in mass from a few times up to 1800 times the mass of a proton. Despite all of this, LUX never captured enough events to provide strong evidence of dark matter's presence.

LUX was what HEP calls a "Generation 1" experiment. Other "Generation 1" direct detection experiments currently running and supported by the Office of Science are taking a slightly different tack. The PICO 60, Darkside-50, and SuperCDMS-Soudanexperiments, for example, search for WIMPs, while the ADMX-2adetector hunted for the other potential dark matter candidate, the axion.

There are also "Generation 2" direct detection experiments currently in design, fabrication, or commissioning, including the LUX-Zeplin (LZ), Super CDMS-SNOLAB, and ADMX-Gen2.

The Alpha Magnetic Spectrometer and Indirect Detection

In addition, there are experiments focusing on indirect detection.

Some theorists propose that colliding could annihilate each other and produce two or more "normal" particles. In theory, colliding WIMPs could produce positrons. (A positron is the positively charged antimatter counterpart to the electron.) The Alpha Magnetic Spectrometer on the International Space Station captures cosmic rays, bits of atoms accelerated to high energies by exploding stars. If the AMS detects a high number of positrons in a high-energy spectrum where they wouldn't normally be, it could be a sign of dark matter.

"AMS is a beautiful instrument," said Salamon. "Everyone acknowledges this is the world's most high-precision cosmic-ray experiment in space."

So far, the AMS has recorded 25 billion events. It's found an excess of positrons within the appropriate range, but there's not enough evidence to state definitively where the positrons originate. There are other possible sources, such as pulsars.

In addition to the AMS, DOE also supports the Fermi Gamma-Ray Space Telescope, which analyzes gamma-rays as it circles the globe and may offer another route to dark matter detection.

Dark matter Production at the Large Hadron Collider

In theory, a particle accelerator could create dark matter by colliding standard particles at high energies. While the accelerator wouldn't be able to detect the dark matter itself, it could look for "missing" energy produced by such an interaction. Scientists at the Large Hadron Collider, the world's largest and most powerful particle accelerator, are taking this approach.

Lessons Learned and the Future of Research

So far, not a single experiment has yielded a definitive trace of dark matter.

But these experiments haven't failed – in fact, many have been quite successful. Instead, they've narrowed our field of search. Seeking dark matter is like looking for a lost item in your house. As you hunt through each room, you systematically eliminate places the object could be.

Instead of rooms, scientists are looking for dark matter across a range of interaction strengths and masses. "As experiments become more sensitive, we're starting to eliminate theoretical models," said Salamon.

The search for is far from over. With each bit of data, we come closer to understanding this ubiquitous yet elusive aspect of the universe.

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14 comments

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earthling98765
1 / 5 (2) Oct 27, 2016
if DM is inert or nonreactive it won't emit a photon...better look for Brownian Motion in the fluid...got any pollen in there?
cantdrive85
2 / 5 (4) Oct 27, 2016
"Without dark matter, it's possible that we would not exist," said Michael Salamon, a DOE Office of Science High Energy Physics (HEP) program manager.

What's likely is this fool has no gray matter to speak of, at the very least it's missing.
Astronomer Fritz Zwicky discovered dark matter in 1933 when he was examining the Coma Cluster of galaxies.

Zwicky didn't discover anything except his understanding of the matter involved is pathetic. This is what happens when ignorance drives science, fairies and unicorns are invented to cover up for a lack of knowledge.

Vera Rubin's "discovery", just more ignorance along with all the other morons who still believe this fairy tale.

It truly is the "dark ages" of astronomy, our descendants will laugh harder at you fools than we do with flat earthers today.
Benni
2 / 5 (4) Oct 27, 2016
if DM is inert or nonreactive it won't emit a photon...better look for Brownian Motion in the fluid...got any pollen in there?


Nope, pedesis is also off the table because the hypothesis for DM is that is 100% nonreactive when encountering ordinary matter, the theory holds that it neither attracts or repels VM, EXCEPT via gravity.
theon
not rated yet Oct 28, 2016
To the best of my knowledge, 1/12 of all the dark matter was discovered by Cowan and Reines in 1956.
Seeker2
1 / 5 (2) Oct 28, 2016
...the hypothesis for DM is that is 100% nonreactive when encountering ordinary matter, the theory holds that it neither attracts or repels VM, EXCEPT via gravity.
Not surprising, gravity being the (non-uniform) expansion of spacetime, DM being the non-uniform part. It follows from the definition of Newtonian gravity, gravity being a (non-uniform) gradient.
TimLong2001
1 / 5 (1) Oct 28, 2016
The error was interpreting photon energy loss (resulting in lengthening wavelength from various processes including gravitational lensing, refraction processes and interference, among others) as a Doppler Effect of a receding velocity. Granted, rotating galaxies exhibit actual Doppler colors depending on whether the edge observed is toward (blue) or away from (red) the observer. Of course there would be the background red shift component as well, a result of the local CMBR average from various effects on light as it traverses space.
EyeNStein
3 / 5 (2) Oct 31, 2016
DM could be unreactive if its QCD charge is completely self cancelling: Such as a glueball of gluons and anti-gluons.
The simplest example is one gluon paired with an anti-gluon. E.G. an anti-green/anti-red coupled with a red/green gluon. We don't know they would annihilate any more than a pair of N/S and S/N semi-circular magnets annihilate when placed in a ring.
Such a construct would have none of the charges or reactions with the other standard model particles (except by gravity) yet is entirely an SM particle itself.
With its QCD charges cancelled the remaining energy/mass would be small, making it stable like the neutrino of the strong force world but with integer spin.
Seeker2
not rated yet Nov 01, 2016
DM could be unreactive if its QCD charge is completely self cancelling: Such as a glueball of gluons and anti-gluons.
The simplest example is one gluon paired with an anti-gluon. E.G. an anti-green/anti-red coupled with a red/green gluon. We don't know they would annihilate any more than a pair of N/S and S/N semi-circular magnets annihilate when placed in a ring.
Such a construct would have none of the charges or reactions with the other standard model particles (except by gravity) yet is entirely an SM particle itself.
With its QCD charges cancelled the remaining energy/mass would be small, making it stable like the neutrino of the strong force world but with integer spin.
Gluons being massless would seem strange to act as a form of matter.
Seeker2
1 / 5 (1) Nov 01, 2016
Nature sure likes to make fools out of Phds. For example the filaments between galaxies. Galaxies have a higher energy density than between galaxies. That is because during the big bang nature concentrated more energy in galaxies than between. Because nothing on a macroscopic scale is perfectly uniform, especially during an event like the BB. So these excesses of energy had to come from somewhere between galaxies, namely the filaments. So the filaments are stretched-out regions of spacetime. Then under expansion the expansion will occur faster into areas of low spacetime density. This produces a gradient in spacetime density similar to what occurs in gravity. But the only thing we know about a gradient in spacetime density is gravity caused by matter. So we think the filaments must contain matter because matter ties up spacetime so regions around matter appear to have a lower spacetime density similar to the filaments. Or so it seems.
Seeker2
1 / 5 (1) Nov 01, 2016
Another fooler - black holes. Yes there is or could be a singularity around the center of a black hole, that is a singularity in spacetime. Because matter displaces, or could displace, all the spacetime around its center. The trick is for gravity to completely fold up matter and expel spacetime around the singularity.
EyeNStein
3 / 5 (2) Nov 01, 2016
Seeker2: Gluons are not massless. Most of the mass of a proton or neutron comes from the gluon field binding the quarks. The Higgs field gives the quarks themselves very little mass.
Even the photon when bound adds energy and therefore mass to the system it is bound into.
https://www.youtu...KVtScgGk
EyeNStein
5 / 5 (1) Nov 02, 2016
Gluons are quanta of the QCD force that holds quarks together in the nucleonic particles. (And in most of the particle zoo particles also)
True, the evidence for glueballs (gluons without quarks) is still sparse and the maths of the QCD force, which sets the size of the proton, is complex.
The rest of my post is conjecture: whether gluons totally annihilate like matter particles or cohere as bosons like photons in a laser, I don't think we know yet.
Seeker2
not rated yet Nov 04, 2016
Seeker2: Gluons are not massless.
Actually only the rest mass. Do any bosons have rest mass? Just wondered.
Seeker2
not rated yet Dec 08, 2016
@TimLong2001
The error was interpreting photon energy loss (resulting in lengthening wavelength from various processes including gravitational lensing, refraction processes and interference, among others) as a Doppler Effect of a receding velocity.
I don't think gravitational lensing causes energy loss. It just takes the photon longer to go from point A to point B in the stretched-out medium where lensing takes place.

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