The Dark Matter Mystery, Parts 1 and 2

Part One:


The Mystery of Dark Matter (1/4): Stars Are Moving Too Fast.

A mystery exists! Galaxies do not seem to have enough mass for stars to orbit at their observed speeds. Galaxies should be flying apart, but they don't. Why not? Explore the surreal world of dark matter - one of the universe's greatest mysteries.

Shedding Light on Dark Matter

Over the last few decades, physicists have discovered that around ninety percent of every galaxy in the universe is made of an unseen substance called dark matter. Damian Pope, PIs Senior Manager of Scientific Outreach, comments, Its currently one of the hottest topics in physics. The module provides teachers with tools to show how dark matter was discovered, to explain why it remains a mystery, and to share the passion of scientists who are trying to discover what its made of.

This presentation is available to educators on DVD and comes complete with specially-crafted teacher notes.

• http://www.perimeterinstitute.ca

The Mystery of Dark Matter Video Game
• http://perimeterinstitute.ca/dark_mat...

In astronomy and cosmology, dark matter is a form of matter that is undetectable by its emitted electromagnetic radiation, but whose presence can be inferred from gravitational effects on visible matter and background radiation. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter accounts for the vast majority of the mass in the observable universe.

Dark matter was postulated by Fritz Zwicky in 1934, to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequent to then, other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies.

Dark matter plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is frequently called the "dark matter component," even though there is a small amount of baryonic dark matter. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.

The vast majority of the dark matter in the universe is believed to be nonbaryonic, which means that it contains no atoms and that it does not interact with ordinary matter via electromagnetic forces. The nonbaryonic dark matter includes neutrinos, and possibly hypothetical entities such as axions, or supersymmetric particles. Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the elements in the early universe ("big bang nucleosynthesis") and so its presence is revealed only via its gravitational attraction. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves resulting in observable by-products such as photons and neutrinos ("indirect detection").

Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly). There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.

As important as dark matter is believed to be in the universe, direct evidence of its existence and a concrete understanding of its nature have remained elusive. Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theories such as modified Newtonian dynamics and tensor-vector-scalar gravity have been proposed. None of these alternatives, however, has garnered equally widespread support in the scientific community.

• http://en.wikipedia.org/wiki/Dark_matter
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Part Two - 39 Billion Missing Suns:


A mystery exists! Galaxies do not seem to have enough mass for stars to orbit at their observed speeds. Galaxies should be flying apart, but they don't. Why not? Explore the surreal world of dark matter - one of the universe's greatest mysteries.

The Dark Side of the Cosmos

Much of the picture of cosmic evolution, called standard cosmology, is well grounded in fundamental physics, but makes up only part of the story. Standard cosmology involves detailed models, whose predictions agree with, and explain, much of what astronomers see. However, there are a growing number of observations that are deeply puzzling.

For example, a number of independent astronomical observations have provided strong evidence for the existence of vast quantities of matter that do not emit or reflect electromagnetic radiation of any type (visible light, microwaves, gamma rays, etc), and thus cannot be seen. It is called dark matter. How do we know it's there? Even though we cannot see it, it exerts very clear gravitational influences on the matter and radiation we can see.

For example, Albert Einstein's theory of space, time, and gravity, called general relativity, tells us that any gravitating mass (the Sun, a galaxy, a cluster of galaxies, etc.) warps the spacetime around it in such as way that a light ray passing nearby is deflected. Gravity bends light. Astronomers find that the amount of bending around, say, a typical cluster of galaxies, is far greater than can be accounted for by the visible mass in the cluster. There appears to be a great deal of invisible mass. Current data suggests that there is more than five times as much dark matter as ordinary matter (atoms) in the universe. What is dark matter made of, and can it be detected in laboratories here on Earth? An intense, worldwide effort is currently underway to try to answer these questions.

Another profound puzzle stems from astronomical observations indicating that the cosmic expansion of space is happening at an accelerating pace. But in a universe with only matter (dark or otherwise), gravitational attraction would slow down the expansion, just like a ball, thrown upwards, slows down due to Earth's gravitational pull. The acceleration can be explained by the assumption that the universe is filled with an unusual form of energy called dark energy that makes up 70% of the universe's total energy. But what, exactly, is this dark energy, and how does it fit in with the rest of physics? To date, no one knows the answer.

• http://www.perimeterinstitute.ca/en/O...

This presentation is available to educators on DVD and comes complete with specially-crafted teacher notes.
• http://www.perimeterinstitute.ca

The Mystery of Dark Matter Video Game
• http://perimeterinstitute.ca/dark_mat...
.