The Absurd Search For Dark Matter

A detector is being installed one kilometer underground in a gold mine outside Melbourne to search for dark matter particles.

Dark matter makes up 85% of all matter in existence and could form a shadow universe five times more massive than everything we can see.

Over 50 experiments have tried to make direct detection of dark matter, but none has found anything except one.

• DAMA/LIBRA detector under a mountain in the Italian Alps has collected data for around 20 years • Shows a peculiar annual signal with peak detections in June and minimum in November • Some scientists believe this could be the first direct evidence of dark matter

• Galaxy is surrounded and permeated by a huge sphere of invisible dark matter particles • Dark matter particles zip around in random directions • Dark matter doesn't interact with anything including itself, except through gravity

• Solar system moves around the galaxy at 220 kilometers per second • Earth orbits the sun at 30 kilometers per second • For half the year, we move with the sun going faster through dark matter • For the other half, we move in the opposite direction going slower through dark matter

We encounter more dark matter when moving fastest through it, which happens in June, and less when moving slowest, which happens in November.

• Temperature fluctuations • Humidity and soil moisture changes • Snow on the mountain • Number of tourists in Italy • All these factors fluctuate with a one-year period

Build an almost identical experiment in the Southern Hemisphere at the bottom of the gold mine outside Melbourne where seasons are reversed.

If the same signal appears in the Southern Hemisphere despite reversed seasons, it would be strong evidence for the existence of dark matter.

Other very similar experiments don't see the same signal, leading to significant doubt about whether DAMA/LIBRA is really detecting dark matter.

Why do we think dark matter exists in the first place if the DAMA/LIBRA signal could have mundane explanations?

• In 1933, Swiss astronomer Fritz Zwicky studied the Coma Cluster of over 1,000 galaxies • Found that galaxies were moving much faster than expected • Proposed invisible matter, calling it 'dunkle materie' - the origin of the term dark matter

• Vera Ruben and Kent Ford observed stars in the Andromeda Galaxy • Found that rotational velocity stays almost constant with distance from center, contradicting expectations • Without additional mass, stars should be flung off into space

• Albert Bosma and others measured hydrogen gas far from galaxy centers using radio telescopes • Rotational velocity remained constant even at great distances • Same results found in other galaxies

Dark matter, an invisible substance, would hold galaxies together and explain why orbital velocities remain constant with distance from the center.

• A star maintains stable orbit when its centripetal force equals gravitational attraction to all mass in the galaxy • Adding dark matter increases the mass pulling the star inward, allowing it to orbit faster • This explains observations of constant rotational velocities

• MOND (Modified Newtonian Dynamics) proposes modifying gravity theory instead of invoking unseen particles • MOND suggests there's a limit to how low acceleration can go rather than additional dark matter force • Some argue we should modify laws of physics rather than invoke invisible matter

The consensus strongly favors dark matter being a physical substance rather than modified gravity, as it seems reasonable that undiscovered particles could exist.

The bullet cluster collision evidence shows dark matter is separate from ordinary matter, strongly supporting the particle hypothesis over modified gravity.

• Two clusters of galaxies collided • Most ordinary mass exists in interstellar gas • When collision occurred, interstellar gas interacted, heated up, and slowed down

Using gravitational lensing, scientists measured where most of the mass is located and found it wasn't in the middle where the gas is.

Dark matter passed right through the collision while gas got stuck in the middle, creating gravitational lensing where we see the least ordinary matter.

This is the strongest evidence that dark matter is a separate substance from ordinary matter, contradicting modified gravity theories.

• 380,000 years after the Big Bang, light could finally travel through the universe unimpeded • This is observed as the cosmic microwave background (CMB) • Red spots show where early universe was hotter, blue spots show where it was cooler

Temperature differences were tiny at just 0.01%, but they are present throughout the CMB.

• Converting the CMB picture into a graph by counting blobs of different sizes shows multiple peaks • Height of peaks depends on how much dark matter exists • Universe without dark matter produces different graph pattern than observed

• To match CMB measurements, we need about five times as much dark matter as ordinary matter • This figure agrees with dark matter amounts required to explain star motion in galaxies and galaxy motion in clusters • Dark matter hypothesis explains multiple different observations with one simple framework

• Scientists don't know what the dark matter particle is exactly • A whole bunch of different possibilities have been proposed • Detection approaches differ depending on what type of particle is being searched for

• DAMA/LIBRA and the detector in the gold mine are looking for WIMPs (weakly interacting massive particles) • These particles are expected to weigh about as much as a proton • They interact with ordinary matter extremely weakly

• Dark matter may be more than just one particle type • Could be an entire dark standard model - a dark version of everything we can see • Could be something even more complex given how much more dark matter exists than ordinary matter

To find out what dark matter is, there must be some level of interaction that scientists can probe through experiments.

At the heart of the detector are seven 77 kilogram crystals of pure sodium iodide where dark matter particles may collide with nuclei.

• When a dark matter particle hits a nucleus, it transfers energy creating a flash of light called scintillation • Flash is detected by photomultiplier tubes positioned above and below each crystal • Photomultiplier tubes are very sensitive light detectors

• Crystals are submerged in 12 tons of linear alkylbenzene liquid scintillator • Liquid emits light when exposed to gamma rays from radioactive potassium decay • Simultaneous detection in crystal and tank indicates potassium decay, not dark matter

• Mine walls contain radioactive elements like uranium and thorium that decay into radon gas • Walls are coated with special paint to contain radioactive particles • Crystals are immersed in continuous stream of pure nitrogen gas • Entire detector is shielded by 120 tons of steel and plastic

• Energetic particles from the sun and other galaxies hit Earth's atmosphere • These create muons, essentially heavy electrons, streaming toward Earth at close to light speed • Muons can create flashes of light in the crystal just like hypothesized dark matter particles

• Muon detector has three paddles of plastic separated by steel pieces • If light flash appears in all three at the same time, a muon has passed through them • At ground level, muon detector counts several muons per second

• All sensitive particle detectors are located deep underground to reduce muon background • One kilometer underground, muon count drops dramatically • Expected muon reduction is about a million times less than at surface

Muon detector is positioned immediately above the tank to eliminate false dark matter signals when muon passes through detector at same time as crystal flash.

There is a lot riding on this experiment as it will validate or disprove one of the most contentious results in physics.

• If results show nothing, it would be the death of DAMA/LIBRA signal as evidence • If results show a signal, it would validate the DAMA/LIBRA findings and support dark matter detection

In science, we must live with the possibility that at some level we may never find the answer, but at least we tried.

• Dark matter might be made of more than just one particle type • Could dark matter interact with ordinary matter at some level • Dark matter could represent an entire unknown physics standard model