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The concept of light cone is used in physics (including relativity) to indicate the portion of space-time from which light can reach you (the further back in time, the wider region of space is visible to you) or that light from your position can reach another position (the further into the future, the wider region of space can be reached). The light cone for a point in space-time excludes every other region of space at that instant (light hasn't had enough time to reach it or vice versa).
Though called light cones, they set limits on more than EMR: general relativity posits that no influence of any kind moves faster than the speed of light in a vacuum (c), thus portions of space-time are unknown to you. So if the Sun emits a solar flare right at this instant, there is no way to know that until the few minutes pass and the associated light reaches Earth.
With the concept of relativity, with space and time not entirely independent, and modern cosmology assuming space's growth from a point (Big Bang), there are complications and consequences.
Astrophysics takes as fact that a transient showing sudden dramatic increases in luminosity, must have a triggering event which could "turn on" the power-increase across an area facing us, but can only possibly affect any portion of this area when it is reachable at the speed of light, all the phenomena producing the observed transient must reside within the light cone of whatever mechanism started it. An observed rapid power-increase sets a maximum area on the source of this energy, i.e., it must be some object able to produce the observed energy despite the object's limited size. Through analysis based upon this, extreme transients have constituted evidence for black holes, given other types of objects are incapable of converting so much energy into observable EMR from within a small volume. Also revealed is a maximum possible size of the detected black hole.
Another consequence is the need to explain any observation that suggests some phenomenon of the early universe that appears too widespread to be triggered at or after the Big Bang. An example that has greatly influenced modern physics is the observed uniformity of the CMB.