I've never understood the rationale behind a holographic universe. I'm not saying it's impossible, but Occam's razor would seem to suggest it's unlikely.
If I understood correctly, the theory goes something like this: Information cannot be destroyed, yet the actions of a black hole on its surroundings (i.e. what we can observe from outside) are completely determined by its twodimensional event horizon, therefore a volume of that size can only contain as much information as what is present on that surface area. Since you can imagine any area of the universe collapsing into a black hole, and again no information can be lost there, the entire universe can only contain as much information as a twodimensional sphere, so the universe really only has two dimensions.
But why is everyone so sure information cannot be "destroyed" (rendered inaccessible) in a black hole? Many laws of physics break down in black holes, what makes you so sure the second law of thermodynamics will hold?
And it's not like this second law is really a "law" in the strictest sense. Mathematicians would never call this a law. It's like people observing a box with red and blue marbles in it. When they shake the box and then look inside, the marbles are all mixed up. Therefore, they introduce the "law" that the marbles will always be mixed up after shaking the box, and it will be impossible for all the blue marbles to be on one side and the red ones on the other.
Now this second law does turn out to be very practical. Because for all intents and purposes, given enough particles, you may reasonably expect to never see a violation of the second law. And certainly on the scale of the universe, it would be... well... "impossible".
But black holes are not like the rest of the universe. Extrapolating the second law, which isn't even really a real law but kind of a quasi-law, to black holes (which we know little about, apart from speculation) and then jumping to the conclusion that this must mean that the whole universe is twodimensional, is just silly. It's people performing "logical deductions" without asking themselves if that particular step actually makes sense.
Also, the fact that we can only see a certain amount of information from outside the black hole, does not mean that more information could not be present inside. What if you drop into the black hole? Of course you wouldn't survive, but if you disregard that detail, couldn't you find all the "lost" information inside? Applied to the universe, this might mean that you could replace a chunk of the universe with a sphere of that size, and you could calculate the effect of that chunk on its surroundings based on that twodimensional sphere, but that doesn't mean the information inside does not exist.
Also, if gravity inside a black hole is so strong that our usual laws of physics break down, it doesn't seem outrageous to assume that information could indeed really be lost there. Why not? Which is more likely? Information getting lost in a singularity, or our threedimensional world being only twodimensional? I would say Occam's razor favors the former.
Information gets lost all the time anyway, in the expanding universe. There are plenty of galaxies that are moving away from us so quickly that we will never be able to see their light. We are "losing" information all the time, yet this doesn't seem to bother anyone.
Really, maybe I'm missing something, and by all means feel free to enlighten me, but I fail to see a compelling reason to assume the universe has only two dimensions, it seems to be an unlikely solution looking for a problem.