The "Horizon" theory (related to "light cones") says that the ability for an inertial object to observe a reference object is limited to the medium of observation.  In other words, an observation must be transmitted by light, sound, x-rays, gamma rays, etc.  This becomes a problem of physics which hinges on the entire concept of reality.  Our entire reality is limited to only that which can be observed through instruments which in turn can be interpreted by us.  The "Horizon" theory goes like this:

If the universe is the "cause" of its own existence, and you assume that 3 dimensional space actually exists and there are actually directions – As the hypothetical "big bang" takes place at approximately the speed of light, and the completely dense matter spews out in "different" directions, then some of those pieces of matter will never be able to be "seen" (or detected) by the others which have moved in an opposite direction - because those particles traveling at the speed of light in one direction trying to observe another which has traveled in the opposite direction would require that the relative speed of one particle to the other would be greater than the speed of light.  In other words the velocities add; one moving at the speed of light in one direction and the other moving at the speed of light in the other direction - they should be moving twice the speed of light relative to each other.  Even the "Lorentz" time constriction or dilation explanation cannot help us out here, because we are talking about a beginning point where the only reference is the spot from which the universe began.   

But if velocities of two particles each traveling the speed of light from one reference point (the location of the big bang) in two different directions, the relative speed between the two particles cannot be greater than the speed of light, or the speed of light restriction to approximately 299,792,458 meters per second has been violated - as to the two particles observing each other.  Ergo, the two particles cannot observe each other, because they are not within each others "Horizon."  The two particles have traveled away from each other faster than any information carrier, like light, sound, or electrical impulse, for example, could possibly "transmit."  Therefore, they do not exist in each others reality, yet they both exist in the reality relative to the big bang.

This becomes a physics "problem" in that one can neither enter a "speed of light" number into a calculation for the "size of the universe" nor enter a "time of creation" of the universe to actually arrive at a limit for reality. It is a question of "being" in the abstract.  Reality is an unsolvable problem because "now" or "is" does not actually exist.  Time's arrow will not allow for it.  It is only a question of time: is - "is" only what the mind remembers it to be?

In considering, all possible "physical" theories of universal causation, such as "the big bang," even in our "visible" night sky, take the best guess 13.7 billion years for the "age" of the universe.  That means anything to the one direction you see in your telescope that is more than 13.7 billion "light years" away, cannot have ever been seen or experienced by anything that is in the other direction from you – because the universe is just not old enough for them to "see" each other. Yet, you can see them both.  Is this a conundrum?

No. When you look out of your eyes, everything you see corresponds to some point of time into the past. Everything you feel even takes some time for the energy pulses to reach your brain.  A galaxy measured at ten billion light years in distance appears to us as it was ten billion years ago.  In case you are unfamiliar with the concept, it only takes you a second to just think about it for a moment, a priori, because we are thinking beings.

Light, sound, electrical impulses and really anything that can carry information, takes time to travel to the observer. If you had the money to rent time on the Hubble space telescope and were you to look at a galaxy ten billion light years away in one direction, and another in the opposite direction, the total distance between them is twenty billion light years, which is too far for them to observe each other, because the universe is simply not old enough.  Could you be observing them both while neither really exists for the other?

Or, if you also assume that the only "is" is something that had a "cause" to exist, then this means that the light from the first has not yet reached the second, because light takes a finite amount of time to travel.  This means that the entire existence of the universe simply hasn't existed for long enough time to allow it (the observation) to occur. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective "Horizons."

The New Laws of Light Speed
1. The observed speed of light in any reference frame is approximately 299,792,458 meters per second.
2. No mass, energy, or quantum particle can be observed directly by another mass energy or quantum particle that has a greater relative difference in velocity than 299,792,458 meters per second.
3. Where two masses, energies, or quantum particles are moving, spinning, or vibrating with velocities separated by greater than 299,792,458 meters per second relative to each other they must exist in a different quantum realities. However, a third mass, energy or quantum particle whose relative velocity is between the two may observe them both.
4. This duality is finite, but the number of dualities is infinite.
5. There is another reference frame in some reality that exists somewhere or sometime where this reference frame, you are in right now, is moving at the speed of light relative to that other reference frame.