On Dark Matter

Short notes on what dark matter is, the questions it answers, the questions it raises, alternative theories, and the reasons for its prevalence.

Nothing raises controversy in the scientific community like the subject of Dark Matter. It is a concept that rouses both certainty and suspicion at the same time. So I decided to write some short notes on it. These notes will elucidate the contours of the dark matter hypothesis, its indispensable role in explaining the universe, and the challenges it presents.

Artwork - A Dark Matter Fractal

What is Dark Matter?

Dark matter, while undetected, has embedded itself in the framework of cosmology due to the gravitational anomalies it explains. Historically, as early as the 1930s, researchers like Jan Oort and Fritz Zwicky sensed that something was amiss; galaxies weren't behaving as Newtonian physics predicted. Yet, it was Vera Rubin's meticulous work on the rotation curves of spiral galaxies in the 1970s that solidified the dark matter conundrum, presenting a puzzle that has yet to be pieced together fully.

Questions Answered by Dark Matter

Galaxy Formation

Within the vast expanses of the universe, there are roughly 200 billion galaxies as per the latest estimates. Dark matter is theorised to serve as the gravitational glue, making the formation and coalescence of galaxies possible. Without its influence, the galaxies we observe wouldn't take their current forms.

Figure 1: A 3D visualisation of reconstructed dark matter clump distributions in the galaxy cluster Abell 2744.

Cosmic Microwave Background (CMB) Fluctuations

The ancient light from the universe’s infancy, the CMB, carries minuscule temperature fluctuations. These temperature variances provide an insight into the universe's early density perturbations, and dark matter's gravitational pull is pivotal in explaining the structure arising from these perturbations.

I have written in detail about CMB in the following article:

Is the Universe Flat?

Gravitational Lensing

The very fabric of space bends when influenced by mass, leading to the gravitational lensing phenomenon. Clusters of galaxies exhibit lensing effects so strong that they suggest the presence of vast amounts of unseen matter, further supporting the dark matter proposition.

Figure 2: Schematic of the gravitational lens system MG J0751+2716.

Dark Matter Candidates

Now that we have some notion of the dark matter theory and the questions it answers, let us briefly discuss the postulated candidates for the constituents of the dark matter itself. Various candidates have been proposed over the years.

WIMPs (Weakly Interacting Massive Particles)

Holding the torch as the leading candidates, WIMPs are theorised to rarely interact with regular matter, but when they do, they should leave a trace. Laboratories buried deep underground aim to detect these ghostly interactions.

Axions

These are postulated to be featherweight particles that, if they exist, could be copiously spread throughout the universe. Advanced experiments like the Axion Dark Matter Experiment (ADMX) are on a quest to detect axions by trying to observe their conversion into photons in the presence of potent magnetic fields.

Sterile Neutrinos

The mysterious siblings of regular neutrinos, they wouldn't even interact via the weak force, making their detection an intricate task. These particles, if proven, would necessitate significant adjustments to our current understanding of particle physics.

Primordial Black Holes

Unlike their stellar counterparts, these black holes could have formed soon after the Big Bang. Their contribution to the universe's unseen mass makes them intriguing dark matter candidates.

Self-interacting Dark Matter

This proposition challenges the traditional view, suggesting that dark matter isn’t entirely "dark" but can interact with itself. This idea emerged to explain some anomalies observed in the distribution of dark matter in galaxies.

Superfluid Dark Matter

This avant-garde theory hypothesises that dark matter assumes superfluid properties within galaxies. If valid, it could account for several unsolved mysteries related to dark matter's behaviour on different scales.

Questions that Dark Matter raises

Direct Detection

The perennial issue with dark matter is our inability to detect it directly. While its gravitational footprint is evident, experiments dedicated to finding it, like the Large Hadron Collider or the XENON1T project, have yet to provide conclusive evidence of its existence.

Alternative Gravitational Theories

With the absence of direct detection, theories like MOND suggest that maybe we don't understand gravity as comprehensively as we think, especially on cosmic scales. Such theories present potential alternatives to the dark matter paradigm. I intend to discuss these theories in greater detail in the coming weeks.

Galaxy Discrepancies

While dark matter offers solutions on large cosmic scales, individual galaxies present problems. The rotation curves of some galaxies and the distribution of their satellite galaxies occasionally deviate from predictions.

Nature of Dark Matter

The sheer variety of candidates underlines our uncertainty about what dark matter truly is. Each candidate presents its unique challenges for detection and integration into the broader fabric of physics.

Why is Dark Matter Theory so Resilient?

So while dark matter somewhat fits into our understanding of key concepts, there is a lot more that it leaves unanswered. And it makes me wonder, why has the scientific community discarded the dark matter theory after all? What are the reasons behind its resilience? Here are 3 thoughts I have on this.

Simplicity

Although introducing an unseen component might seem counterintuitive, it provides a straightforward explanation for various cosmic phenomena, more so than redefining our understanding of gravity.

Cosmological Observations

Models like Lambda-CDM, which account for dark matter and dark energy, consistently offer the best fits to our diverse array of cosmic observations.

Particle Physics Framework

Many dark matter candidates snugly fit into extensions of the well-established Standard Model of particle physics, making them plausible despite the challenges in detection.

Conclusion

I would like to end this article by sharing an anecdote on the discovery of Neptune. In the early 19th century, astronomers noticed something odd about Uranus: its orbital path wasn't aligning with predictions. This was not a dramatic deviation, but in the precise world of astronomy, it raised eyebrows. Could Newton's laws be flawed? Or was there an unseen planet influencing Uranus?

Independently, Urbain Le Verrier in France and John Couch Adams in England believed in the existence of an undiscovered planet. Through meticulous calculations, they pinpointed where this mysterious body might be. Le Verrier sent his findings to the Berlin Observatory, and almost immediately, Johann Gottfried Galle identified Neptune, right where it was predicted to be.

This discovery showed how indirect observations and mathematical models could unveil hidden components of our universe. It's a tale that resonates today as scientists grapple with the enigma of dark matter. Just as Neptune's gravitational effects betrayed its presence, the motion of galaxies suggests there's more mass than we can see—leading to the dark matter hypothesis.

As with Neptune, no one has directly observed dark matter. But using the same spirit of deduction, researchers are striving to uncover its nature. The hope is that, like the distant planet once did, dark matter will reveal itself through its influence on the visible universe.