Context: Scientists working on the LUX-ZEPLIN (LZ) experiment have placed the tightest restrictions on the particles that make up dark matter (i.e., they have significantly narrowed down possibilities for what dark matter could be), still, the result remains inconclusive. Despite similar global experiments, such as XENON-nT in Italy and PandaX-4T in China, there is no definitive direct evidence of dark matter.
Major Highlights
- Dark matter constitutes most of the universe's mass but interacts weakly with ordinary matter. Theories suggest it may occasionally "touch" atomic nuclei, but detecting this interaction is challenging.
- In 1985, physicists Goodman and Witten proposed using large underground detectors to catch dark matter particles as they pass through. These experiments measure the cross-section, or likelihood of interaction, between dark matter and nuclei.
- The LZ experiment pushed detection limits even further, reducing the cross-section of possible dark matter interactions by a factor of a million. However, the future progress may be hindered by interference from neutrinos, another elusive particle.
- The "neutrino fog" adds noise to detectors, complicating the identification of dark matter.
- Despite these challenges, researchers continue to explore alternative detection methods, driven by the determination to uncover dark matter's true nature.
LUX-ZEPLIN (LZ) experiment
- The LUX-ZEPLIN (LZ) experiment is a leading dark matter direct detection experiment designed to search for weakly interacting massive particles (WIMPs), a potential candidate for dark matter.
- Objective: To measure the interaction of dark matter particles with atomic nuclei of ordinary matter (known matter). This interaction, if detected, would provide critical insights into the nature of dark matter, its mass, and its interaction cross-section with ordinary matter.
- Detector: LZ employs a massive 7-tonne liquid xenon detector. The liquid xenon acts as a target for dark matter particles. If a dark matter particle collides with a xenon nucleus, it would cause a small burst of light (scintillation) and ionisation, which the detector would capture and measure.
- To minimise interference from cosmic rays and other background sources, LZ is located 1.5 kilometres below the Earth's surface at the Sanford Underground Research Facility (SURF) in South Dakota, The US.
Dark Matter and Dark Energy
- Dark matter and dark energy together make up 95% of the universe. Around 68% of the Universe is made of dark energy while dark matter makes up 27%.
- Only the remainder (5%) is composed of fermionic matter, i.e., things on the Earth, planets, stars, etc.
Dark Matter
- Dark matter is completely invisible and has not yet been observed directly. It does not interact with matter in the same way that normal matter does, meaning it does not absorb, reflect, or emit light. This makes it extremely difficult to detect using conventional telescopes or other detectors.
- In fact, researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter (galaxies and galaxy clusters).
- E.g., Galaxy Rotation Curves
- Expected Behaviour: In galaxies, stars or planets should orbit faster closer to the centre of the galaxy, due to the gravitational pull of the visible matter concentrated there.
- Observed Anomalies: However, observations show that stars and gas in galaxies continue to orbit at a relatively constant speed even at large distances from the centre. This suggests the presence of additional invisible matter exerting gravitational force.
Dark Energy
- The existence of dark energy was theorised 25 years ago, when a team of researchers found that the expansion of the Universe was speeding up or accelerating, instead of slowing down due to gravity (inwards pulling force). Scientists have hypothesised that this is happening due to a mysterious form of energy called dark energy.
Characteristics of dark energy:
- Dark energy has been hypothesised as a repulsive force or anti-gravity, i.e. while gravity tends to make objects attract, dark energy would pull them apart by increasing the space between them. Thus, dark energy has an expansionary effect. As our universe is expanding, it indicates that dark energy has a greater abundance than dark matter.
- Dark energy is a property of space, so it does not get diluted as space expands.
- Normally, as the universe expands the density of mass and radiation in it decreases.
- However, the density of dark energy remains constant throughout. This means the dark energy in the universe is ever increasing, in order to keep the energy-density constant. Thus, dark energy should be energy inherent in the fabric of space itself.