Why on Earth would they?
In asymptotically flat spacetimes, the event horizon of a black hole is defined (roughly) as the boundary of the region of spacetime that can be seen by an immortal observer (more precisely, it's the boundary of the causal past of future null infinity). If you extend this definition to generic spacetimes, then it applies to the cosmological event horizon in a spacetime with positive cosmological constant. Because of this formal similarity, a number of results in black hole thermodynamics (specifically, the laws of black hole mechanics) can be generalized to the cosmological horizon.
Anyway, one response (Susskind's) to the sort of thing drnickbone brings up, in the case of black holes, is black hole complementarity. We seem to have this paradox when something falls into a black hole. On the one hand, we don't want information to disappear beyond the event horizon, so the information must be absorbed into the event horizon itself, theoretically readable off the structure of the horizon (or off Hawking radiation). On the other hand, from the perspective of an infalling observer, nothing special happens as she passes through the horizon. It certainly doesn't seem to her as if the information she carries has been smeared over the event horizon. Susskind's response is essentially that both of these things happen. The information is both reflected and transmitted by the event horizon. But the no-cloning theorem rules out the possibility that the reflected and transmitted information are two separate things. Instead, they are the same event described from different perspectives.
From an outside observer's perspective, the information is painted onto the horizon. From an infalling observer's perspective, the information passes right through the horizon. Extending this idea to the cosmological horizon and a spaceship leaving my horizon: From the perspective of the spaceship nothing special has happened. After all, why should they care about my horizon. From my perspective, once the spaceship hits the horizon, all the information constituting the spaceship is now smeared over the horizon. And of course, if I am to abide by the no-cloning theorem, I cannot simultaneously maintain that the spaceship continues to travel past the horizon. From my perspective, the spaceship does cease to exist (except as information encoded in the structure of the 2-dimensional cosmological horizon).
[ETA: I should have mentioned that this isn't just completely baseless speculation on Susskind's part. He bases his claim on an argument from string theory. The basic idea is this: The spatial extent of a string's wave function depends on the "resolution time", which is the time scale over which observations are made. As this time scale gets smaller and smaller, i.e. as our observations get faster, the spatial extent of the string gets larger. As long as the resolution time is significantly larger than the Planck time, the effect of this phenomenon is negligible. Now think of someone falling into a black hole while making measurements on a string. Another observer is outside the black hole looking on. The infalling observer's resolution time won't change as she falls. But the outside observer's resolution time will effectively get smaller and smaller, due to gravitational time dilation. As a consequence, the string gets bigger and bigger from his perspective. As the falling observer hits the event horizon, the string is big enough to be spread across the entire horizon, which means the information it contains is now spread across the event horizon. So from the outside observer's perspective, the string never falls into the black hole; it gets smeared across the horizon and the information is eventually radiated out. And since the infalling observer is also made of strings, she unfortunately gets spread across the horizon too. But none of this holds in the infalling observer's perspective. The string doesn't grow in size -- it remains localized and falls through the event horizon into the singularity. So, very speculative, but not baselessly so.]
On the one hand, we don't want information to disappear beyond the event horizon, so the information must be absorbed into the event horizon itself, theoretically readable off the structure of the horizon (or off Hawking radiation). On the other hand, from the perspective of an infalling observer, nothing special happens as she passes through the horizon. It certainly doesn't seem to her as if the information she carries has been smeared over the event horizon. Susskind's response is essentially that both of these things happen.
An update. I didn't reali...
Followup to: The Useful Concept of Truth
We previously asked:
What rule would restrict our beliefs to just statements that can be meaningful, without excluding a priori anything that could in principle be true?
It doesn't work to require that the belief's truth or falsity make a sensory difference. It's true, but not testable, to say that a spaceship going over the cosmological horizon of an expanding universe does not suddenly blink out of existence. It's meaningful and false, rather than meaningless, to say that on March 22nd, 2003, the particles in the center of the Sun spontaneously arranged themselves into a short-lived chocolate cake. This statement's truth or falsity has no consequences we'll ever be able to test experientally. Nonetheless, it legitimately describes a way reality could be, but isn't; the atoms in our universe could've been arranged like that on March 22nd 2003, but they weren't.
You can't say that there has to be some way to arrange the atoms in the universe so as to make the claim true or alternatively false. Then the theory of quantum mechanics is a priori meaningless, because there's no way to arrange atoms to make it true. And if you try to substitute quantum fields instead, well, what if they discover something else tomorrow? And is it meaningless -rather than meaningful and false - to imagine that physicists are lying about quantum mechanics in a grand organized conspiracy?
Since claims are rendered true or false by how-the-universe-is, the question "What claims can be meaningful?" implies the question "What sort of reality can exist for our statements to correspond to?"
If you rephrase it this way, the question probably sounds completely fruitless and pointless, the sort of thing that a philosopher would ponder for years before producing a long, incomprehensible book that would be studied by future generations of unhappy students while being of no conceivable interest to anyone with a real job.
But while deep philosophical dilemmas such as these are never settled by philosophers, they are sometimes settled by people working on a related practical problem which happens to intersect the dilemma. There are a lot of people who think I'm being too harsh on philosophers when I express skepticism about mainstream philosophy; but in this case, at least, history clearly bears out the point. Philosophers have been discussing the nature of reality for literal millennia... and yet the people who first delineated and formalized a critical hint about the nature of reality, the people who first discovered what sort of things seem to be real,were trying to solve a completely different-sounding question.
They were trying to figure out whether you can tell the direction of cause and effect from survey data.
Please now read Causal Diagrams and Causal Models, which was modularized out so that it could act as a standalone introduction. This post involves some simple math, but causality is so basic to key future posts that it's pretty important to get at least some grasp on the math involved. Once you are finished reading, continue with the rest of this post.
Okay, now suppose someone were to claim the following:
"A universe is a connected fabric of causes and effects."
vs.
(In the right-hand image we see a connected causal fabric; the sun raises the temperature, makes plants grow, and sends light into the eyes of the person eating from the plant. On the other hand, while "post-utopian" is linked to "colonial alienation" and vice versa, these two elements don't connect to the rest of the causal fabric - so that must not be a universe.)
This same someone might further claim:
"For a statement to be comparable to your universe, so that it can be true or alternatively false, it must talk about stuff you can find in relation to yourself by tracing out causal links."
To clarify the second claim, the idea here is that reference can trace causal links forwards or backwards. If a spaceship goes over the cosmological horizon, it may not cause anything else to happen to you after that. But you could still say, 'I saw the space shipyard - it affected my eyes - and the shipyard building was the cause of that ship existing and going over the horizon.' You know the second causal link exists, because you've previously observed the general law implementing links of that type - previously observed that objects continue to exist and do not violate Conservation of Energy by spontaneously vanishing.
And now I present three meditations, whose answers (or at least, what I think are the answers) will appear at later points in Highly Advanced Epistemology 101 For Beginners. Please take a shot at whispering the answers to yourself; or if you're bold enough to go on record, comments for collecting posted answers are linked.
Meditation 1:
"You say that a universe is a connected fabric of causes and effects. Well, that's a very Western viewpoint - that it's all about mechanistic, deterministic stuff. I agree that anything else is outside the realm of science, but it can still be real, you know. My cousin is psychic - if you draw a card from his deck of cards, he can tell you the name of your card before he looks at it. There's no mechanism for it - it's not a causal thing that scientists could study - he just does it. Same thing when I commune on a deep level with the entire universe in order to realize that my partner truly loves me. I agree that purely spiritual phenomena are outside the realm of causal processes, which can be scientifically understood, but I don't agree that they can't be real."
How would you reply?
Meditation 2:
"Does your rule there forbid epiphenomenalist theories of consciousness - that consciousness is caused by neurons, but doesn't affect those neurons in turn? The classic argument for epiphenomenal consciousness has always been that we can imagine a universe in which all the atoms are in the same place and people behave exactly the same way, but there's nobody home - no awareness, no consciousness, inside the brain. The usual effect of the brain generating consciousness is missing, but consciousness doesn't cause anything else in turn - it's just a passive awareness - and so from the outside the universe looks the same. Now, I'm not so much interested in whether you think epiphenomenal theories of consciousness are true or false - rather, I want to know if you think they're impossible or meaningless a priori based on your rules."
How would you reply?
Meditation 3:
Does the idea that everything is made of causes and effects meaningfully constrain experience? Can you coherently say how reality might look, if our universe did not have the kind of structure that appears in a causal model?
Mainstream status.
Part of the sequence Highly Advanced Epistemology 101 for Beginners
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