Cosmology is essentially a whodunnit but, unlike your typical 600-page detective yarn, this story doesn't unfold clue by clue until the denouement in last few pages. The puzzle — basically, "How the hell can we explain our presence here?" — is more cryptic than ever. For all the thousands of brilliant minds working on the problem and despite deep-space observations and investigations into the tiniest atomic particles, we're stuck. Notwithstanding all that science has revealed to us in the last 200-odd years, we're still essentially blind, deaf and a mystery unto ourselves. By rights, we shouldn't be here.
Three models of the universe are victims of their own success:
• Einstein's 1915 General Theory of Relativity ("mass tells space-time how to curve and space-time tells mass how to move") gives us gravity. It has passed every test (hundreds!) with flying colors.
• The Standard Model of particle physics is based on quantum field theory, which took 40 years to assemble starting in the 1960s; it's been tested to better than 10 parts in a billion. Finding the Higgs boson in 2013 only confirmed what we already knew.
• The 100-year-old Big Bang scenario — we live in an evolving, expanding universe that began in an incredibly hot and dense state nearly 14 billion years ago — fits all our observations.
Three self-consistent models, tested down to the finest details ... except they're mutually inconsistent. Nothing works to bring them into agreement. We need a bigger picture, a "Eureka moment," to get us out of the doldrums in which we've been drifting for the past few decades. At this point, such a moment appears unlikely.
To illustrate the extent of our ignorance, take the most obvious example: time. You'll agree, I think, that past and future are different in every way — that is, time has an "arrow." Cosmologists explain this by saying that the past is low entropy, an "unnatural" ordered state (imagine all the air molecules in a room clustered into a small, dense ball); the future is high entropy: disorderly (as with air molecules spread randomly throughout the room). We're now 14 billion years into randomness (fortunately, we live close to a low-entropy star that keeps us organized). Eventually, all the ordered bits of the universe will mush into a high-entropy featureless emptiness. The arrow of time, then, derives from the extremely low entropy state that the universe was in 14 billion years ago. The problem in trying to reconcile this with the Standard Model is that the laws of physics are indifferent for past and future. And — the crux of the whodunnit — nothing in the three models I outlined above explains how our ordered low-entropy early universe came about in the first place.
Hence all the current wild speculation about multiuniverses (ours is one of an infinite number of universes); many worlds (all possible alternate histories and futures are real); the anthropic principle (we're here, living in this life-friendly universe because we're here); simulations (we're self-aware bots created by a bored teenager on an iPhone 1000); Boltzmann brains (your self-awareness is a statistical fluke due to random fluctuations, nothing to do with evolution); multidimensionality (unseen dimensions exist beyond the four walls of our spacetime); etc. At this point, we have no way of checking such crazy ideas and perhaps never will.
In the past we always had some sense which way to turn, where to find answers — or at least how to look for anomalies in our present theories. Today, that doesn't seem to be the case. We have three beautiful and elegant models, each spectacularly successful, but with no clue how we might reconcile them. Welcome to stuck.
Barry Evans (firstname.lastname@example.org) can't get over the fact that the 3-pound lumps of jelly we call "brains" can even conceive of this stuff.