The metric expansion of space is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion — that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space as, for example, an explosion of matter. The universe is not expanding "into" anything. A frequently used 2-D analogy is the expansion of the surface of an expanding rubber balloon. In this analogy the universe has two spatial dimensions (the surface of the balloon) rather than three. As the balloon expands, any two points on its surface get farther and farther apart. Another common analogy is a rising loaf of raisin bread—as the loaf expands, the raisins inside it move farther and farther apart from each other.
Metric expansion is a key feature of Big Bang cosmology and is modeled mathematically with the FLRW metric. This model is valid in the present era only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence of gravitational attraction and such bound objects clumps do not expand at the metric expansion rate as the universe ages, though they continue to recede from one another. The expansion is a generic property of the universe we inhabit, though the reason we are expanding is explained by most cosmologists as having its origin in the end of the early universe's inflationary period which set matter and energy in the universe on an inertial trajectory consistent with the equivalence principle and Einstein's theory of general relativity (that is, the matter in the universe is separating because it was separating in the past). Additionally, the expansion rate of the universe has been measured to be accelerating due to the repulsive force of dark energy which appears in the theoretical models as a cosmological constant. This acceleration of the universe, or "cosmic jerk", has only recently become measurable, and billions of years ago, the universe's expansion rate was actually decelerating due to the gravitational attraction of the matter content of the universe. According to the simplest extrapolation of the currently-favored cosmological model (known as "ΛCDM"), however, the dark energy acceleration will dominate on into the future.
While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, there is no such theoretical constraint when space itself is expanding. It is thus possible for two very distant objects to be expanding away from each other at a speed greater than the speed of light, and this is true for any object that is more than approximately 4.5 gigaparsecs away from us. We can still see such objects because the universe in the past was expanding more slowly than it is today, so the ancient light being received from these objects is still able to reach us, though if the expansion continues unabated as the model extrapolations presume, we will never see the light from such objects that is being produced today (on a so-called "space-like slice of spacetime).
Because of the changing rate of the expansion, it is also possible for a distance to exceed value of the speed of light multiplied by the age of the universe. These details are a frequent source of confusion among amateurs and even professional physicists.
Due to the non-intuitive nature of the subject and what has been described by some as "careless" choices of wording, certain descriptions of the metric expansion of space and the misconceptions to which such descriptions can lead are an ongoing subject of discussion in the realm of pedagogy and communication of scientific concepts.
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