For some reason, despite the predictions of our current theories, the universe is expanding faster and faster in all directions. This strange fact has been attributed to dark energy, an elusive substance currently at the cutting edge of physics research.
Physicists have measured unmistakable gravitational effects in the universe where there isn’t any matter to cause it. This invisible substance, called dark matter, can be found in galaxies in large quantities, and has an unknown constitution. One theory proposes that dark matter is composed of weakly interacting massive particles (WIMPs).
The electromagnetic force is one of the four fundamental forces. It governs the interactions between charged particles (like charges repel, opposites attract) and is mediated by the photon. All of the quarks, plus the electron, muon, and tauon, all interact with the electromagnetic force. The weak bosons are also charged, and therefore themselves also interact with the electromagnetic force.
Escape velocity is a measure of the speed necessary for a projectile to escape the gravitational pull of an object. For example, the earth’s escape velocity is roughly 11 kilometers per second or about 7 miles per second; therefore (discounting air resistance) an object traveling at that speed from the surface of the earth would be able to escape the earth’s gravity and never fall back again.
If you come too close to a black hole, there comes a point when it is physically impossible to return. The imaginary sphere around a black hole that marks this boundary is called the event horizon. Nothing that crosses the event horizon can ever return, including light; therefore, the event horizon marks the region of space that we perceive as black around a black hole.
In classical mechanics, forces are thought of as a push or a pull on an object. But in modern physics, forces are thought of more generally as interactions between particles. Although we experience many different forces in our everyday lives, these can all ultimately be explained by just four fundamental interactions between the particles making up everything around us: the weak force, the strong force, electromagnetism, and gravity. The first three are incorporated into the Standard Model of particle physics, while gravity is explained by Einstein’s theory of General Relativity.
Albert Einstein’s theory of General Relativity (GR) replaced Isaac Newton’s theory of Classical Gravity as the most accurate description of gravitation. Specifically, General Gravity transforms the concept of gravity from a force between objects to a property of spacetime. Black holes are one of the many fascinating consequences of General Relativity. (See also Special Relativity.)
The gluon is a boson that mediates the strong interaction. Gluons are hypothesized to interact with each other to form transient composite structures called glueballs.