Metric expansion of space

The metric expansion of space is the averaged increase of metric (i.e. measured) distance between distant objects in 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. (In other words, the universe is not expanding "into" anything outside of itself).

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 at relatively large scales (roughly the scale of galactic superclusters and above). At smaller scales matter has clumped together under the influence of gravitational attraction and these clumps do not individually expand, though they continue to recede from one another. The expansion is due partly to inertia (that is, the matter in the universe is separating because it was separating in the past) and partly to a repulsive force of unknown nature, which may be a cosmological constant. Inertia dominated the expansion in the early universe, and according to the ΛCDM model the cosmological constant will dominate in the future. In the present era they contribute in roughly equal proportions.

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 moving away from each other at a speed greater than the speed of light (meaning that one cannot be observed from the other). The size of the observable universe could thus be smaller than the entire universe.

It is also possible for a distance to exceed the speed of light times the age of the universe, which means that light from one part of space generated near the beginning of the Universe might still be arriving at distant locations (hence the cosmic microwave background radiation). These details are a frequent source of confusion among amateurs and even professional physicists.^[1]

Perhaps a more complete assessment is that the interpretation of the metric expansion of space continues to provide paradoxes that are still a matter of debate.^[2][3][4][5] The prevailing view^{[citation needed]} is that of Chodorowski: "unlike the expansion of the cosmic substratum, the expansion of space is unobservable"