the-actual-universe:UNIVERSUM INCOGNITUM : A Primer on the Dark UniverseWe often talk about dark mat
the-actual-universe:UNIVERSUM INCOGNITUM : A Primer on the Dark UniverseWe often talk about dark matter and dark energy, but just what are these spooky-sounding terms about? In this first segment, we’ll talk about dark matter.Part I: Does dark matter actually exist?Much of the mass in the Universe is unaccounted for. Our scientific instruments can only directly observe baryonic matter, a term for the familiar stuff like stars and planets and interstellar dust. Unlike baryonic matter, dark matter neither emits nor absorbs electromagnetic radiation. This makes it effectively invisible to us. Its impact on the Universe, however, is anything but invisible.It’s clear that something in the Universe is exerting gravitational force on the stuff we can see. The first observational evidence came about in the early 1930s. First, in 1932, astronomer Jan Oort noticed that stellar motion in nearby galaxies just didn’t add up considering how much matter was actually visible. This was attributed to an error in calculation, but in 1933 Fritz Zwicky came to a similar conclusion when observing the Coma Cluster of galaxies. Zwicky took two estimates of mass–one based on the total luminosity of the stars in the cluster, and another based on the movement of galaxies at the edge of the cluster. Based on the amount of visible matter in the cluster, the speed at which the galaxies moved was much too fast. Either our understanding of physics was deeply flawed, or something unknown was driving the movement of those galaxies.More evidence in the case for dark matter is visible in the accompanying picture. Abell 2218 shows galaxies warped by an effect known as gravitational lensing. This phenomenon happens when the mass of a large object (such as a galaxy cluster) causes background light to bend as it travels toward the observer. This not only serves as a galactic magnifying glass, but it allows scientists to calculate the mass of the object. The luminosity of Abell 2218 simply doesn’t account for enough mass to bend the light as it does.So, barring some discovery that upends physical principles that have held up for centuries, it’s clear that some invisible type of matter is exerting gravitational force on the stuff we can see. Without directly observing it, however, it’s difficult to pinpoint the nature of this invisible matter.We’ll cover potential candidates for dark matter in the second installment of this series. -RLOAbell 2218 Image credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASASources: 1, 2, 3 -- source link