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  • Writer's pictureMatt Russell

295 - Dark Matter

Introduction to Dark Matter



Today, we're diving into the mysterious and elusive world of Dark Matter. Now, when we say 'Dark Matter,' we're not talking about a sci-fi movie or a comic book series. No, this is a real scientific concept that has been puzzling astronomers and physicists for decades


Dark Matter is a hypothetical form of matter that's thought to account for approximately 26.8% of the matter in the universe. But here's the kicker - it's called 'dark' because it doesn't interact with the electromagnetic field. That means it doesn't absorb, reflect, or emit electromagnetic radiation, making it incredibly difficult to detect.


And yet, despite being so hard to observe directly, most experts think that Dark Matter is abundant in the universe and has had a strong influence on its structure and evolution. It's a topic that's as fascinating as it is complex.


So, buckle up, listeners. We're about to embark on a journey into the unseen universe as we explore the enigma that is Dark Matter. And don't worry, we'll be dedicating another episode to its elusive cousin, Dark Energy, in the future. But for now, let's focus on the matter at hand - Dark Matter.


What is Dark Matter?


Definition of Dark Matter

Dark Matter is a hypothetical form of matter.It is thought to account for approximately 26.8% of the matter in the universe. Why is it called 'Dark'? It's called 'dark' because it does not interact with the electromagnetic field. This means it does not absorb, reflect, or emit electromagnetic radiation. As a result, it's incredibly difficult to detect.


How do we know it exists?Various astrophysical observations imply Dark Matter's presence.

  • Galaxy Rotation Curves: The stars in galaxies rotate around the galactic center. According to the laws of gravity, stars further away from the center should move slower than those closer to the center. However, observations show that stars in galaxies rotate at a nearly constant speed, regardless of their distance from the center. This discrepancy, known as the "flat rotation curves," suggests that there is more mass (in the form of Dark Matter) in galaxies than what we can observe.

  • Gravitational Lensing: When the light from distant galaxies passes by an intervening mass, its path is bent, causing the galaxy to appear distorted. This effect, known as gravitational lensing, can be used to map the distribution of mass in the universe. Observations show that the mass distribution inferred from lensing is much greater than what we can observe, suggesting the presence of Dark Matter.

  • Cosmic Microwave Background (CMB): The CMB is the afterglow of the Big Bang. Tiny fluctuations in the temperature of the CMB provide a snapshot of the universe when it was just 380,000 years old. The pattern of these fluctuations can be used to determine the composition of the universe. The data suggest that a large portion of the universe is made up of a type of matter that does not interact with light, i.e., Dark Matter.

  • Large Scale Structure of the Universe: The distribution of galaxies on the largest scales can be used to infer the amount of Dark Matter. The galaxies form a "cosmic web" of filaments and voids, which is best explained by the presence of Dark Matter.

  • Galaxy Clusters: The mass of galaxy clusters can be estimated by the motion of the galaxies within the cluster. Observations show that the mass inferred from the galaxies' motion is much greater than the mass we can observe, suggesting the presence of Dark Matter.

  • Bullet Cluster: The Bullet Cluster is a pair of colliding galaxy clusters. Observations of the Bullet Cluster provide some of the most direct evidence for Dark Matter. The visible matter in the clusters (in the form of hot gas) is slowed down by the collision, but the gravitational mass (inferred from gravitational lensing) is not, suggesting the presence of Dark Matter.


These observations include gravitational effects that cannot be explained by currently accepted theories of gravity unless more matter is present than can be seen.

  • Yes, there are alternative theories of gravity that have been proposed to explain the phenomena attributed to Dark Matter. These theories aim to modify our understanding of gravity in such a way that the need for Dark Matter is eliminated. Here are a few examples:

  • Modified Newtonian Dynamics (MOND): This theory, proposed by Mordehai Milgrom in 1983, suggests that the laws of gravity behave differently at low accelerations. MOND can explain the flat rotation curves of galaxies without the need for Dark Matter.

  • TeVeS (Tensor-Vector-Scalar gravity): This is a relativistic generalization of MOND that can explain gravitational lensing observations without Dark Matter. However, it has difficulty explaining the Cosmic Microwave Background and the large scale structure of the universe.

  • f(R) Gravity: In these theories, the usual Einstein-Hilbert action in General Relativity is replaced by a function of the Ricci scalar curvature. Some versions of f(R) gravity can explain the accelerated expansion of the universe without the need for Dark Energy, and some can also explain galaxy rotation curves without Dark Matter.

  • Entropic Gravity: Proposed by Erik Verlinde, this theory suggests that gravity is not a fundamental force, but an emergent phenomenon arising from the thermodynamics of quantum information. In this theory, the effects attributed to Dark Matter are explained by a modification of the laws of gravity.

  • Conformal Gravity: This theory extends General Relativity by breaking scale invariance and can explain the flat rotation curves of galaxies without Dark Matter.

  • It's important to note that while these theories can explain some of the phenomena attributed to Dark Matter, none of them can explain all the observations as well as the Dark Matter hypothesis. Furthermore, these theories often require modifications to the laws of physics that have not been observed in laboratory experiments. For these reasons, the majority of physicists and astronomers believe in the existence of Dark Matter.

Influence on the Universe

Despite being difficult to observe directly, Dark Matter is thought to be abundant in the universe.It is believed to have had a strong influence on the universe's structure and evolution.

Composition of Dark Matter

Dark Matter is thought to be non-baryonic, meaning it may be composed of some as-yet-undiscovered subatomic particles.

The primary candidate for Dark Matter is some new kind of elementary particle that has not yet been discovered, such as weakly interacting massive particles (WIMPs) or axions.


"Baryonic" and "non-baryonic" are terms used to describe different types of matter.

Baryonic matter refers to all matter composed of baryons, which are particles made up of three quarks. Quarks are elementary particles and a fundamental constituent of matter. The most familiar baryons are protons and neutrons, which make up the atomic nuclei in the atoms that constitute stars, planets, and living beings. Therefore, all the ordinary matter that we can see, touch, and interact with is baryonic.

Non-baryonic matter, on the other hand, refers to matter that is not composed of baryons. This includes various types of elementary particles such as leptons (like electrons and neutrinos) and hypothetical particles like axions or weakly interacting massive particles (WIMPs).


Scientists believe that Dark Matter is non-baryonic for several reasons:

  1. Cosmic Microwave Background (CMB): The CMB is the afterglow of the Big Bang, and it provides a snapshot of the universe when it was just 380,000 years old. The pattern of tiny fluctuations in the CMB's temperature allows scientists to calculate the amount of baryonic matter in the universe. These calculations show that baryonic matter makes up only about 5% of the universe's total energy content, while Dark Matter makes up about 26.8%. Therefore, Dark Matter cannot be baryonic.

  2. Big Bang Nucleosynthesis: This is the theory of the production of light elements (like hydrogen, helium, and lithium) in the early universe. The observed abundances of these elements agree with the predictions of Big Bang Nucleosynthesis, but only if the amount of baryonic matter in the universe is about 5% of the total energy content. Again, this implies that Dark Matter cannot be baryonic.

  3. Structure Formation: The way that galaxies and clusters of galaxies form over time also suggests that Dark Matter is non-baryonic. Computer simulations of the universe's evolution show that structures like galaxies and galaxy clusters can only form if there is a large amount of non-baryonic Dark Matter to provide additional gravity.

For these reasons, most scientists believe that Dark Matter is made up of some as-yet-undiscovered non-baryonic particles.


Theories and Hypotheses


Weakly Interacting Massive Particles (WIMPs)

  • WIMPs are a leading candidate for Dark Matter.

  • They are hypothetical particles that interact weakly with ordinary matter and have mass.

  • Numerous experiments are underway to detect WIMPs directly.

Axions

  • Axions are another candidate for Dark Matter.

  • They are hypothetical particles that are very light and interact very weakly with ordinary matter.

  • There are also ongoing experiments to detect axions.

Sterile Neutrinos

  • Sterile neutrinos are a type of neutrino that interacts only via gravity, making them a candidate for Dark Matter.

  • They are not part of the standard model of particle physics, but they could exist if there are more than three neutrino flavors.

Primordial Black Holes

  • Primordial black holes are black holes that could have formed in the early universe and could make up some or all of the Dark Matter.

  • However, various observations have placed limits on the amount of Dark Matter that primordial black holes can account for.

Self-Interacting Dark Matter

  • This is a hypothesis that Dark Matter particles can interact with each other, not just via gravity but also via some new dark force.

  • This could help explain some astrophysical observations that are difficult to explain with the standard Cold Dark Matter model.

Warm Dark Matter and Hot Dark Matter

  • These are hypotheses that Dark Matter is made up of particles that were moving at high speeds in the early universe.

  • Warm Dark Matter and Hot Dark Matter could help explain the small-scale structure of the universe, but they have problems explaining the large-scale structure.



History

One of the earliest instances can be traced back to the 19th century:

Lord Kelvin (William Thomson) in 1884: Lord Kelvin estimated the number of dark bodies in the Milky Way from the observed velocity dispersion of the stars orbiting around the center of the galaxy. He used these calculations to arrive at an estimate of the mass of the galaxy, which he found to be different from the mass of visible stars. This discrepancy suggested the presence of a large amount of unseen matter, which could be considered an early indication of Dark Matter.

Jacobus Kapteyn in 1922 and Jan Oort in 1932: Both astronomers independently discovered stellar motions that could not be explained by the gravitational effects of visible matter alone. Kapteyn found evidence of "dark stars" when studying star motions in the local solar neighborhood, while Oort found that the mass in the galactic plane must be greater than what was observed to account for the motions of stars.

However, these early observations did not lead to the concept of Dark Matter as we understand it today. The term "Dark Matter" and the modern concept associated with it really began with Fritz Zwicky's work in the 1930s.


  1. 1933 - Fritz Zwicky's Observations: The Swiss astrophysicist Fritz Zwicky first inferred the existence of Dark Matter in 1933. He observed the Coma galaxy cluster and found that the visible matter was not enough to account for the gravitational effects he observed. He proposed the existence of "dunkle Materie" or "dark matter."

  2. 1930s - 1970s - Vera Rubin's Work: American astronomer Vera Rubin and her collaborator Kent Ford in the 1970s provided further evidence for Dark Matter. They studied the rotation curves of galaxies and found that stars in galaxies were moving at speeds that could not be explained by the gravitational pull of the visible matter alone.

  3. 1980s - Cold Dark Matter: In the 1980s, the idea of Cold Dark Matter (CDM) became popular. CDM refers to hypothetical particles that move slowly compared to the speed of light and interact very weakly with electromagnetic radiation. The CDM model has been successful in explaining the large-scale structure of the cosmos.

  4. 1990s - Present - Direct Detection Experiments: From the 1990s onwards, many experiments have been set up to directly detect Dark Matter particles, including experiments to detect WIMPs and axions. So far, these experiments have not found definitive evidence of Dark Matter particles, but they have placed important constraints on the properties of these particles.

  5. 2000s - Indirect Evidence: In the 2000s, more indirect evidence for Dark Matter has been found. For example, the Bullet Cluster, a pair of colliding galaxy clusters, has been cited as evidence for Dark Matter because the visible matter and the gravitational mass (inferred from gravitational lensing) are separated, which would not be expected if the gravitational effects were due to modified laws of gravity.

  6. Present - Ongoing Research: Today, the search for Dark Matter continues. Scientists are using a variety of techniques to try to detect Dark Matter particles, and they are also studying the cosmos on the largest scales to learn more about the properties of Dark Matter. At the same time, some scientists are exploring alternative theories of gravity that could explain the observations without Dark Matter.



In popular culture

Science Fiction Literature and Films

Dark Matter is a common theme in science fiction literature and films. It's often used as a powerful energy source, a means of faster-than-light travel, or a tool for time travel.

Examples include the novel "Dark Matter" by Blake Crouch, where it's used as a basis for exploring parallel universes, and the film "Interstellar," where a mysterious wormhole (possibly linked to Dark Matter) enables interstellar travel.

Television Series: In TV series like "Star Trek," "Doctor Who," and "Futurama," Dark Matter is often used as a plot device. For instance, in "Futurama," Dark Matter is used as a fuel for space travel.

Video Games: Dark Matter also appears in video games, often as a powerful weapon or a mysterious force. For example, in the "Mass Effect" series, Dark Matter plays a significant role in the storyline and the technology of the game universe.

Music: Dark Matter has been used as a theme or title in music as well. For example, the British rock band Porcupine Tree has a song called "Dark Matter."

Art Installations: Some artists have used Dark Matter as inspiration for their work, creating installations that attempt to visualize this invisible substance or explore its implications.



Space Missions

There are several space missions and experiments that have been designed to search for Dark Matter:

  1. Planck Satellite (European Space Agency): While not exclusively a Dark Matter mission, the Planck satellite measured the Cosmic Microwave Background (CMB) with high precision. The CMB is sensitive to the amount of Dark Matter in the universe, so these measurements provide important indirect evidence for Dark Matter.

  2. Fermi Gamma-ray Space Telescope (NASA): Fermi has been used to search for signs of Dark Matter annihilation in the gamma-ray sky. While it has not detected definitive evidence of Dark Matter, it has placed important constraints on the properties of Dark Matter particles.

  3. Alpha Magnetic Spectrometer (AMS-02) on the International Space Station: This experiment is designed to measure cosmic rays, which could contain signals of Dark Matter annihilation or decay.

  4. Dark Matter Particle Explorer (DAMPE) by China: Launched in 2015, DAMPE is a space-based observatory that aims to detect high-energy cosmic rays, gamma rays, and possibly signals of Dark Matter.

  5. Euclid Mission (European Space Agency): Euclid will map the distribution of galaxies and galaxy clusters throughout the universe. This will provide further evidence for the existence of Dark Matter and help constrain its properties.

  6. X-ray observatories like Chandra X-ray Observatory (NASA) and XMM-Newton (European Space Agency): These observatories have been used to study galaxy clusters, which are important Dark Matter probes. They observe the hot gas in galaxy clusters, which is affected by the gravitational potential of Dark Matter.

  7. LISA (Laser Interferometer Space Antenna) by ESA and NASA: While primarily designed to detect gravitational waves, LISA might also be able to detect certain types of Dark Matter, such as ultralight scalar Dark Matter.


These missions and experiments are part of the ongoing worldwide effort to understand the nature of Dark Matter.


To put it all in perspective

The composition of the universe is approximately 75% hydrogen, 23% helium, and about 2% all other elements, which is often referred to as "metals" in astronomical parlance. However, this refers to the normal (baryonic) matter in the universe.

When considering the total energy content of the universe, normal (baryonic) matter makes up only about 5% of the universe. The rest is made up of Dark Matter (~26.8%) and Dark Energy (~68.2%).

To put this in perspective, if your body was the universe, the normal matter (everything we can see, touch, and interact with) would be equivalent to just your hand. The rest of your body would represent the Dark Matter and Dark Energy.


Matt's Poems

Dark Matter is like the wind. We can't see it, but we can see its effects, and its unseen touch shapes the universe in which we sail.


Dark Matter, mysterious and deep,

Does secrets in the cosmos keep.

Much more than meets the eye,

More than all, that dance the sky

Dark Matter riddles, astronomer don’t sleep


Dark depths, mystery,

Dancing dreams, discovery,

Dark Matter, destiny.


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