The light cone graphic below is taken from a Wiki article. The discussion therein gets the basics right, at least with regards to where the concept of a light cone comes from and the dimensional issues with the illustration.

… a light cone (or “null cone”) is the path that a flash of light, emanating from a single event (localized to a single point in space and a single moment in time) and traveling in all directions, would take through spacetime. (…)

In reality, there are three space dimensions, so the light would actually form an expanding or contracting sphere in three-dimensional (3D) space rather than a circle in 2D, and the light cone would actually be a four-dimensional version of a cone whose cross-sections form 3D spheres (analogous to a normal three-dimensional cone whose cross-sections form 2D circles)

https://en.wikipedia.org/wiki/Light_cone

That’s fine as far as it goes – with two caveats. First of all, the spacetime term should be understood as referring to a relational concept of space and time, not to Wheeler’s causally interacting spacetime. Secondly, contracting spheres of light do not exist in physical reality. Much of the rest of the article is gibberish well encapsulated by the labeling of the illustration which basically renders the image incoherent.

For starters, the image has an observer at the shared apex of the two cones but an observer is not mentioned in the text of the Wiki article. In terms of physical reality an observer is at the apex of a “past light cone” – the observer observes light emitted from distant sources, usually omnidirectional emitters like stars and galaxies.

The “past light cone” is the aggregate of all the inbound radiation from those distant sources onto the observer. Rather than calling it a “past light cone” it would be more accurate to label it a Converging Light Cone, with the understanding that the light cone is a relative, point-of-view phenomenon that has no physical relevance except with respect to the observer.

The “future light cone” does not have an observer at its apex, it has an omnidirectional emitter such as a star or galaxy there. The “future light cone” is an aggregate of the successive expanding spherical wavefronts of electromagnetic radiation emitted by the emitter. The “future light cone” should be more accurately labeled a Diverging Light Cone. The DLC is a physical entity, consisting of sequentially emitted expanding spherical wavefronts of electromagnetic radiation. That understanding flows from Maxwell and Einstein – it is standard physics

Borrowing from radio terminology the emitter/observer can be thought of as a transmitter/receiver or transceiver (TSV). The term transceiver will also be used for an observer-only by considering a non-transmitting observer (such as a human) to be a subcomponent of a transceiver such as a star or galaxy system. With respect to the space and time (relational) labels of the illustration, the apex can be labeled “Here and Now”. So the apex represents the HAN of a TSV.

The rest of the labeling is adequate with the relational nature of space and time caveat being understood. What the illustration then presents us with is a stark refutation of the modern conception of the Cosmos as a simultaneously existing Universe. The TSV (galaxy) is always and only at some unique spatio-temporal location.

The TSV is at the center of the omnidirectionally expanding spherical wavefronts of electromagnetic radiation that it emits – the Diverging Light Cone. A TSV is also at the center of all the electromagnetic radiation that is arriving at its particular place and time from all directions – the Converging Light Cone.

The following statement applies to every possible TSV – everywhere and everywhen. Every TSV is at the center of its own unique “universe” which is just its own unique view of a Cosmos that cannot be simultaneously accessed from any three dimensional HAN.

No TSV can detect the state of a remote TSV that is simultaneous with its own HAN. The finite speed of light prohibits any and all such knowledge. The nearest galaxy to our own, Andromeda, is 2.5 million lightyears distant. We see it in our frame as it existed 2.5 million years ago. We do not have and cannot have any knowledge of its “current” state. Andromeda’s “current” state is not part of the Cosmos we have access to. Andromeda’s HAN does not exist in our unique cosmological frame – Andromeda is always There and Then (TAT) in our cosmological frame.

The two dimensional projection labeled the Hypersurface Of The Present illustrates this clearly. The HAN of any TSV is always and only a local state. All other spatio-temporal locations lie outward – TAT- along the surface of the Converging Light Cone. No TSV has access to the HOTP and in fact the HOTP is only a mathematical/metaphysical construct that has no physical correlate. The HOTP does not exist in physical reality because it represents a universal simultaneity which cannot exist because lightspeed has a finite maximum. There is no physical meaning to the concept of a “universal now” – that is the reason there is no universal frame or “now” in General Relativity.

The apex point represents the only HAN available to any TSV. All remote objects exist only in the transceiver’s past – on the TAT of the Converging Light Cone.

Unfortunately, modern cosmologists are of the opinion that they do have knowledge of this simultaneous something (the HOTP) that does not have any existence in physical reality. That is what the term Universe refers to as employed by cosmologists. They believe themselves to be in possession of knowledge of this imaginary, simultaneously existing Universe that, by the known laws of physics, cannot exist. That 13.8 billion year old entity does not exist by normal scientific standards – it is not an observable.

What modern cosmologists have, of course, is just a mathematical model based on some simplifying assumptions adopted @ 100 years ago at a time when the known Cosmos barely extended beyond our own galaxy. One of the model’s assumptions is that the Cosmos has a “universal” spacetime frame (the FLRW metric) even though, in the context of General Relativity, no universal frame exists. A universal spacetime metric inherently includes a universal time with a universal now. Despite the incongruency, the FLRW metric was applied to the GR field equations. The result of this misbegotten effort speaks for itself:

The Standard Model of Cosmology is a miserable failure; it describes a Universe that looks nothing like the Cosmos we observe. To the extent that it can be said to agree with actual observations, it only arrives at such agreements by insisting that physical reality contains entities and events that physical reality, by all direct empirical evidence, does not appear to contain.

The SMC is junk science or perhaps more accurately, it is a mathematicist confabulation presented as science by people who don’t understand basic physics – that the speed of light in the Cosmos has a finite maximum of @3×10⁸ meters/second. It’s not that they don’t know that fact, they do, but rather they don’t understand what it means in the context of the vast Cosmos we observe. They only know what the SMC tells them and that model, they believe, can’t be wrong because if it were smart people like them wouldn’t believe in it.

In fact though, we have no scientific reason to think that the limited view of the Cosmos we have provides us with knowledge of an unobservable, simultaneously-existing, and expanding Universe. The consensus belief of cosmologists that they have such knowledge can be attributed to the fever dream of mathematicism that deeply infects the theoretical physics community. Modern cosmology is a mess.

Science is not perfect. Mistakes are to be expected in science. The Standard Model of Cosmology is a mistake. The model’s foundational assumption of an “expanding universe” is a mistake. It is a mistake in the same way that geocentrism was a mistake. It is fundamentally wrong about the nature of the Cosmos, It is time to move on from the expanding universe model. I’ll give the last word to the astrophysicist Pavel Kroupa:

Thus, rather than discarding the standard cosmological model, our scientific establishment is digging itself ever deeper into the speculative fantasy realm, losing sight of and also grasp of reality in what appears to be a maelstrom of insanity.

https://iai.tv/articles/our-model-of-the-universe-has-been-falsified-auid-2393

10May24 Acknowledgement: My original concept for the apex of a light cone was that it should be labeled “Here”. In an exchange with the mathematician Robert A. Wilson he made the invaluable suggestion that the apex be called “Here and Now”.