Long Beach, California—TED convened in Long Beach this morning, and in the beginning, there was science. Dubbed The Observatory, Session 1 was about how we look at our world and choose to engage it.

"Gazing out at the stars is the best way I know to evoke wonder," Chris Anderson, TED’s curator, opined. But just what are we looking at?

Brian Greene told the audience at TED that the wonder we see is not only mysterious, but a limited-run engagement. Greene is a theoretical physicist who has been engaging the public through books, PBS specials, and by organizing the World Science Festival. Here, Greene was in cosmologist mode, talking about how the Universe is going to change in ways that will fundamentally alter how it can be observed.

Astronomers in the far future will not have the beautiful night sky we have. In fact, unless they have our knowledge and scientific records, they will think that the Universe is a dark, static and unchanging place. Why will our night sky go black? The expansion of our Universe will eventually push other galaxies so far from us that we will no longer see them, even with advanced equipment. Light cannot overcome all distances, Greene said, describing a future where all we can see are the galaxies in our immediate neighborhood.

(In this part of his talk, Greene was expressing ideas similar ideas to those explored in detail by Lawrence Krauss, who has published papers on how the Universe will destroy evidence of its past. We'll have more thoughts from Krauss later this week via a review of his latest book.)

But that future is still billions of years off, and in the meantime, there's a lot to learn. "We are living through a remarkably privileged era," notes Greene. "Deep truths about the cosmos are still within reach."

And there’s a chance that, in the future, we might see something truly astonishing: proof that our Universe has collided with another universe. Some theoretical ideas on the origin of our Universe suggest it was just one of many that could form, all expanding from an inflationary fabric. Greene presents an even more intriguing idea: we may find that our universe is not the only universe but is instead part of a vast complex of universes that we call the multiverse."

It’s a strange concept. After all, "Universe" is supposed to mean everything, right? Whatever the word means, the reality is that the Universe surprises us at nearly every turn. It was only in 1929 that Hubble realized that space was expanding. Looking into the red shift, our beliefs about the Universe changed utterly. We grudgingly accepted this, but then slotted it into a new hypothesis: gravity is surely slowing its expansion down.

Of course, last year's Nobel Prize in Physics went to folks who found that not only is the Universe expanding, but its expansion is accelerating due to dark energy. The physics community has scrambled to try to find an explanation for dark energy, from returning to Einstein's cosmological constant to radical ideas coming out of string theory.

String theory posits that the four dimensions of the Universe that we experience only account for about a third of the actual dimensions, most of which remain balled up at distances far too small to be measured. The vibrations of these strings produce the fundamental particles—the electrons and quarks—that produce the visible matter in our Universe. With the right formulation, string theory unifies gravity and quantum mechanics, and produces a value for dark energy that fits nicely with the Universe we've observed.

There's just one problem: the odds of having the right formulation are vanishingly small. Green said that there are 10500 possible configurations of the extra dimensions, and only a tiny fraction of that number will produce anything that looks like our Universe. In this scenario, explaining dark energy becomes a different kind of problem. It's not how much dark energy is in our Universe—the question becomes one of why we have a particular amount of dark energy rather than any of the other possible amounts.

Why does our Universe have just the right conditions to support matter? Here, Greene made an analogy to Earth, asking why it is 93 million miles from the Sun, where water is mostly liquid. Is there some feature of the Universe that explains this? Or is this the wrong reasoning? It is, of course, the reasoning that's wrong. The right answer is that we find ourselves on Earth because Earth is hospitable to life.

We can think of our Universe in the same way. If dark energy can take on different values, it doesn’t make sense to ask what law generated this particular number. And, if the physics works out so that there is a multiverse, then we can conclude we're in one compatible with life because we wouldn't be here to contemplate the question otherwise.

Greene indicated that this is actually a scientific idea, in that it's testable. "Could we ever confirm the existence of other universes?" Green asked, before answering in the affirmative. It may be possible for us to see the effects of a collision with another universe by examining the cosmic microwave background left over from early in our own Universe's history. If our Universe isn't an only child, there's a chance one of its siblings left an imprint.

We have a few billion years before the Universe erases that evidence, so we continue to watch the sky until then.

Update: an earlier version of this article implied that Greene discussed the Multiverse in factual terms when he did not. Rather, Greene was discussing a fascinating but also controversial and theoretical framework for understanding the dark energy in our universe.