What is Gravitational Lensing?
Gravitational lensing occurs when an object comes in-between the source of light and the observer and bends the light from the source affecting the amount of light that the observer sees. Sometimes multiple images of the source appear. The light rays that we would not have seen otherwise are bent from their path towards (or away from) the observer causing the far away source to change its brightness. There are several types of lensing: weak lensing, strong lensing and microlensing.
|Strong lensing: five images instead of one|
Light from far-away galaxies can be bent by closer galaxies or galaxy clusters. The sources will be stretched and magnified, but the distortions are only a few percent. Very many sources have to be analyzed by scientists to look for coherent distortions. These distortions give us information about the mass distribution of the lenses. This way clusters nearby have their mass distribution measured, i.e., are "weighted" against far way galaxies. One of the most striking of weak lensing examples is the bullet cluster, which shows a subcluster passing through the cluster. The weak lensing contours map where most of the mass is, which is the dark matter. The Bullet cluster observations show that dark matter, stars, and gas behave differently during the collision. The gas interacts electromagnetically and slows down the most. The stars collide and slow down, and the dark matter clumps just go through each other since dark matter is collisionless. This is the first direct evidence for dark matter that it makes it more likely that dark matter is formed from particles vs just a modification of gravity.
Light from stars in distant galaxies is bent by closer planets or other compact objects causing the distant stars to become brighter for a period of time, i.e., the planet acts as a magnifying glass for the distant star. This time period can be weeks, days or even hours. The duration of the event depends on the mass of the lens, the distance to the lens and its velocity. It is also important to point out that these are "one-time" events that do not repeat. Microlensing first emerged as a technique to find compact objects made from dark matter. However, no evidence for such dark stars exists to date.
Microlensing later morphed into a successful planet-finding method. It can find Earth-mass planets that are relatively far from their star. The distance dependence is stronger than the dependence on the mass of the lens. In fact, gravitational microlensing was the first method to find Earth-mass planets orbiting main sequence stars. The first extragalactic exoplanet in the Andromeda Galaxy A was also found through microlensing.
Does the title make sense?
We can use the light from distant objects to learn more about closer object though the lensing effect. So, the "behind" in the title here has an allegoric meaning, i.e., we learn about what is behind the change in brightness. However, do not be confused, the lens (closer star/planet/etc) is between Earth and the further away (and older) source, and the duration of the event depends on how long the lens spends across our line of sight.