A Four Dimensional Sphere, Just Like the Universe Seems to Be
For a sphere in three dimensions, the greater the distance from the center, the greater the area of the surface of the sphere that corresponds to that distance as a radius is. For a sphere in four dimensions, astoundingly, the exact opposite is true, the farther from the center, the smaller the area of that surface, although that 'surface' is now, having additionally, depth, a three-dimensional volume (of the momentary space), instead of an infinitely thin two-dimensional area of the surface of a sphere (corresponding to that distance as a radius). Just like any and every direction on the two-dimensional surface of a sphere in three dimensions would point to the same location at the farthest distance on the spherical surface area, so would any and every direction within the three-dimensional volume of a sphere in four dimensions also point to the opposite side of the spatial volume (instead of surface area), which here, is the region of the Big Bang.
Analogously, any and all directions, even opposite directions, at the North Pole of the globe of Earth, like meridians of longitude, point to the single location at the South Pole, with no exceptions. As if looking along these meridians of longitude radiating from the North Pole of the earth, were we to look far enough, outward into space, following "meridians" in what are now three dimensions, as if we were looking straight around the earth across Earth's two-dimensional surface, following the meridians south - outward - we would ultimately see what is effectively a single location, like the South Pole is a single location. Again, for three-dimensional space, that single location is the single tiny region that we call the Big Bang white hole that, unlike the South Pole, does not lie at a fixed distance but again, is constantly receding.
An additional, fourth dimension, that of time, provides additional "depth" to three-dimensional space, and hence provides more "room," to all that already provided by three dimensions, in fact, what is effectively an infinite amount of more, as three dimensions would provide what would effectively be infinitely more area for a two-dimensional surface. This provides enough room to accommodate the ordered "stacking" of three-dimensional, momentary 'spaces', as if they were two-dimensional "layers" instead. Each individual one of us constantly find ourselves atop what is the equivalent of the "north pole" of our own unique series of single, spherical, three-dimensional, spatial moments, each stacked consecutively, one upon the other in a circularly encompassing way (as will be explained next), which together, form a four-dimensional spherical volume, again a volume with effectively infinitely more depth than three dimensions could ever provide.
This "deeper" volume and everything filling it constitute our own, four-dimensional sphere, which is wholly unique to each individual one of us, because this sphere, like any, has a unique center, just as the location that each one of us occupies as its center is unique, being a unique location in space at an equally unique location in time. In addition to being individual and unique, our light cone is also actual, because it is made of the actual light of the moment. (To be as precise as is achievable, an actual light cone is unique to the individual particle; to be more general instead, it can be considered being individually unique to our local vicinity, like the local vicinity of the earth for example.)
This four-dimensional sphere that is individually unique to each one of us is called by science not a sphere, but a light cone. It is named a cone because, among other reasons, a four-dimensional sphere being represented in the simplicity of three dimensions is more like a cone - for example, an ice cream cone - than like a spherical globe, such as a soccer ball for example. This is no doubt why science commonly utilizes the term light 'cone' instead the term light 'sphere,' although one is synonymous to the other. Light cone includes further the term light in the name, instead of being called a space-time cone, for example, because space and time, distances and angles are mapped according to the lines established by the lines of light (see inside back cover) that constitute the cone (all photons being unique in their space and time, location and extent, with respect to cone's center point). Gravity shapes the trajectories of these lines, spherically.
The sphere of an individual and actual four-dimensional light cone is illustrated below, again, as the two-dimensional surface of a cone, like an ice cream cone made of light, in three dimensions, represented in the two dimensions of the illustration below. The illustration below shows relativity at its simplest, where the only "motion" of the observer, or of the local vicinity for the observer, again, like the observer's planet for example, (or the observer's detector) is effectively through time alone, and not through any space whatsoever (effectively).As can be imagined utilizing the illustration below, of the surface of the cone, a successive series of concentric circles of light, each corresponding to a single spatial moment alone, progressively build the cone, one spatial moment stacked upon the other, in a series of consecutive moments (as a single circle in the illustration), ordered with respect to cause and effect; that is to say, with respect to time. Each moment of three-dimensional "space" that we see, sense, experience, and/or otherwise detect in every three-dimensional direction constantly is thus really just one, in a "stack" of, by virtue of being part of a sphere, circular, three-dimensional, spatial layers, each layer corresponding to a solitary 'spatial' moment. In the illustration below, each circle constituting the cone (only a single circle is shown) is a circular layer corresponding to a single moment, that, when stacked, one moment upon the other in a series of successive (circular) moments, like a serially ordered series of concentric, progressively larger circles, arranged with respect to cause and effect, each subsequent circular layer successively encompassing the previous, but in more dimensions than the two required for a circle, form a four-dimensional sphere that spans space and time together, inseparably.
Returning to the original question of meaning for the word universe, this four-dimensional sphere of our light cone constitutes what science calls the visible universe. The visible universe is a synonym for the term light cone, and just like any light cone, is wholly unique to the individual at its center, however precise or general the specification, identically to how a center is unique to its respective sphere. This is distinct from the other scientific definition of the term universe, called the history of the universe, which, in stark contrast to a visible universe, or synonymously, to an actual light cone, that is wholly unique to the individual, is unambiguously unique to no individual anyone or single anything whatsoever, although, according to quantum theory, it is collectively unique to all life in our realm. The history of the universe excludes nothing existent in our realm, regardless of any light cone. Unlike the visible universe, which is bounded by its edge at the Big Bang, the history of the universe, being unique to no light cone whatsoever nor to its physical extent, may lack any boundary altogether and thus have no edge at all, making physical existence, in a word, infinite.