Potential InfinityPotential infinity is infinity that can exist. This doesn't require that it actually exists such that it is easy to think of examples such as standard numbers that have no limit such that you can always make them larger from 1,2,3 ... ∞.
LimitsPotential infinity exists where anything is not given a limit. For example, computer code that contains a loop with no exit condition has potential to loop forever but this would require that the computer, electricity supply, human civilization and universe last forever.
Plato's InfinitesPlato believed that there are two infinities: small and large. For example, any line can be measured using increasing small points such that it contains infinite points. An example of infinitely large would be a line that goes off in a direction without ending.
Actual InfinityActual infinity is infinity that does exist or can truly exist in our universe. Examples of actual infinity can be designed in theory but it is often unknown if these can truly exist as they may create paradoxes or rely on unknowns. For example, numbers have potential to grow infinitely large but you wouldn't have time to count to infinity unless time itself is infinite.
TimeTime is the ordering of events in a chain of cause and effect that appears to be a fundamental dimension of the universe. Intuitively people feel that time is infinite because it is difficult to imagine a universe without time. Various theories of time dispute this notion such as the theory of unreal time that states that time is simply a human perception that doesn't exist from the perspective of the universe itself.
SpaceIt is unknown if the universe is infinite. People tend to feel intuitively that it is infinite because its hard to imagine the vast emptiness of space ending at a nothingness beyond which space doesn't exist. However, it is possible that space takes some form that is finite such as looping back into itself. As far as we can currently observe, space is enormous and unending.
Big BangWhen people speak of the universe they are typically referring to the observable universe that appears to be an explosion of matter and energy that occurred at a point of time and is still rapidly expanding. It is possible to estimate the size and age of the observable universe such that it isn't infinite. However, this is defined by how much we can observe. The big bang could be some tiny spec in a much larger or infinite universe.
Ultimate Fate of the UniverseThe ultimate fate of the universe is the question as to the future of the big bang. This depends on several unknown factors, particularly the geometry of space. One prominent possibility known as the big freeze is that the universe continues to expand forever with this rate of expansion accelerating. According to this model, all matter and energy will be torn apart into smaller and smaller particles until the remaining particles are massless, cold and moving at the speed of light. This is an infinite process assuming that space is also infinite.
LightIf space and time are infinite, this creates a bunch of actual infinities. For example, light traveling through space would travel forever as long as it didn't hit any matter that absorbed its energy.
SingularitiesA singularity is a change that occurs instantaneously and without limit such that it is essentially infinite. For example, the hypothesis that the density of a black hole at its center is infinite and that this state can occur in an instant.
Time LoopInfinities often create paradoxes or are created by paradoxes. For example, the grandfather paradox whereby you invent a time machine and go back in time to disrupt your own birth. This can result in an infinite time loop as follows. The solution above assumes that going back in time doesn't create another instance of the universe as would be predicted by multiple universe theory.
Zeno's ParadoxZeno's paradox is a simple thought experiment:
An architect finishes half the remaining work on a building each day. When will the building be complete?Many people find the answer to Zeno's paradox to be non-intuitive. By completing half the remaining work each day, the architect keeps making the remaining work smaller and smaller towards infinity but never completes. This can be seen on real projects that are impossible or infeasible to complete where progress is continually reported but the end date of the project will never come. Such a project may quickly reach 99% completion but will never reach 100%.
Something that has no limit.
That which always has something beyond itself.