Since Vera Rubin’s pioneering work on galactic rotation, the term Dark Matter has intrigued many people. Something else that often accompanies this term is Dark Energy. Both sound mysterious, and it’s easy to imagine them as sinister forces, I did. But then I learned Dark simply implied that it doesn’t interact with light.
A reader and physics enthusiast suggested I write about dark matter. I found it challenging because of the scientific uncertainty surrounding what dark matter actually is. Everything we know that exists: us, all living things, all nonliving things, all the stars, galaxies, asteroids, and cosmic dust. All of these collectively gather under one title, Baryonic Matter. It accounts for less than 5% of the known Universe. Under current calculation predictions, the rest of the Universe comprises dark matter and dark energy. Dark matter makes up roughly 25%, and dark energy around 70% of the Universe. This is humbling: what we know and experience comprises less than 5% of reality.
Dark matter can’t be observed. It doesn’t interact with light or the electromagnetic force. Thus, there is no direct way to detect it. How do physicists know the Universe’s mass is significantly dark matter rather than ordinary matter like dust?
Evidence of dark matter comes from Galactic Rotation Curve calculations. Rotation curves allow us to calculate the quantity and location of mass within rotating galaxies. Let’s start with our solar system. The Sun makes up roughly 99.8% of the entire mass of the Solar System. As a result, the majority of the mass is heavily concentrated at the center. The orbital velocities of the eight planets vary from each other because they are at different distances from the Sun. This suggests the force of gravity on Neptune is much weaker than on Jupiter. Jupiter is closer and feels a stronger gravitational pull. This explains the variation of orbital periods.
Jupiter’s orbital period is 12 years; Neptune’s is 165 years. A graph can be plotted with orbital velocities of all planets against their distance from the Center of mass. Here, we perceive a falling curve often called the Keplerian Decline. Near the centre (distance close to zero), a steeply rising line appears. At zero distance, orbital speed is zero because gravity pulls equally in all directions. The Keplerian Decline behavior occurs within the mass distribution. Systems that follow Kepler’s laws, like planets around a host star, show this behavior.
We expect a Keplerian Decline curve to arise in a similar calculation with the Milky Way Galaxy. This is because the mass distribution of our galaxy resembles that of the Solar System. The center of the Milky Way is more compacted with mass. In contrast, the outskirts have little to no distribution of stars compared to the center. Yet, something rather interesting is seen instead: not a falling but a flat rotation curve.
The behavior at the center resembles our earlier graph. Nevertheless, instead of the gravitational pull getting weaker, the orbital velocities are constantly high over a vast radius. The force of gravity remains strong even at the edge of the galaxy radius. The conclusion of the result is surprising. No matter how far out from the center, everything is still inside the distribution of mass. Thus, the mass continues to increase with the radius. This mass is Dark Matter, which vastly outweighs visible matter. We see the exact result with most galaxies. It exists as a spherical halo. This stands in contrast to the disk-like form of Baryonic matter.
Dark matter doesn’t absorb, emit, or show light. It interacts very weakly with ordinary matter, making it almost impossible to detect. Presently, it is only inferred by evidence like galactic rotation curves and gravitational lensing. In the next post, I will discuss how Gravitational Lensing provides evidence for the existence of dark matter. I will also talk about the leading theories of what it is.
Very Informative Post.