Fundamental Physics: From Optics to Cosmology
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Converging and Diverging Lenses
Converging Lenses
As long as the object is outside of the focal point, the image is real and inverted. When the object is inside the focal point, the image becomes virtual and upright.
Diverging Lenses
The image is always virtual and is located between the object and the lens.
Einstein's Static Universe
Einstein's static model is the first relativistic cosmological model. The model is static, finite, and of spherical spatial symmetry. I use the solution of Einstein's field equations in a homogeneous and isotropic universe—Friedmann's equation—to calculate the radius of curvature of the model (also known as "Einstein's universe"). Furthermore, I show, using a Newtonian analogy, the model's best-known feature, namely, its instability under small perturbations on the state of equilibrium.
Introduction to String Theory
So, what is string theory? For that matter, what are elementary particles? If this all sounds totally confusing, try this section first.
The Big Crunch
The Big Crunch is one of the scenarios predicted by scientists in which the Universe may end. Just like many others, it is based on Einstein's Theory of General Relativity. That is, if the Big Bang describes how the Universe most likely began, the Big Crunch describes how it will end as a consequence of that beginning.
The Big Rip and Dark Energy
If we look at the expansion of the universe, at first it was thought that, as things are expanding while objects have mass, the mass is going to be attracted to other mass, and that should slow the expansion. Then, in the late 1990s, supernova surveys looking deeper into space than ever before measured distances accurately to greater distances than ever seen before. Something really surprising came out, which we now use "dark energy" to explain: the acceleration is not actually slowing down—it has not even stopped. It is actually getting faster, and if you look at the most distant objects, they are actually moving away from us, and the acceleration is increasing the rate of expansion. This is actually a huge result.