The observable Universe and beyond
The further we look into space, the further back in time we go and the last thing we see is left-overs from the Big Bang. This pattern in the sky could give us clues to the Universe next door.
The Universe that we can observe is fantastically large. If the entire Earth were scaled down to a nearly invisible mote of dust, even the most nearby stars would be many miles distant. Those stars are light-years away, and we're now receiving light that was emitted by them years ago.
Using state-of-the-art instruments, astronomers can see back through 13.7 billion years, viewing regions of space that — due to the cosmic expansion — are now about 45 billion light-years away.
At earlier times, the Universe was so dense that light could not propagate, so this distance forms a spherical boundary in all directions. The ball inside this boundary — our "observiball" if you will — contains all we can observe.
Astronomers peer into the distant Universe through progressively earlier concentric shells within this ball: back through the era of galaxy formation, through "dark ages" prior to the first stars, and finally to the opaque outer shell.
Light from this shell arrives unimpeded, but stretched by the cosmic expansion into microwaves. By observing this Cosmic Microwave Background (CMB), cosmologists have an amazingly clear view of the very early Universe.
Astronomers have done an amazing job at mapping out this "observiball", and over the past several decades cosmologists have assembled a very solid standard model of Big-Bang cosmology that well and accurately describes the evolution of the Universe to the present day, from the time of the CMB and even somewhat earlier.
The now fairly well-understood observiball is enormous. But it's almost certainly not everything that exists - its boundary just limits what we can see, not what is.
Inflating the ball
What's outside the ball? How big is the Universe beyond? How did it form? Potential answers are provided by a theory developed in the early 1980s known as "inflation". This theory holds that in its early history, the observable Universe underwent a period of exponential expansion, doubling in size many dozens of times, growing our observiball a tiny fraction of the size of an atomic nucleus to that of, say, a beach ball.
Such expansion would stretch space and smooth matter, while leaving small density variations that show up in the CMB.
Details of these predicted variations have been confirmed by the Wilkinson Microwave Anisotropy Probe (WMAP) and other experiments, and will be further tested by the Planck satellite, data from which is currently under analysis.
As well as explaining the state of the observiball's surface, inflation has implications for what happened far beyond that surface. For a start, if inflation produced our ball, it also predicts that the full Universe is at least a million or so times as large, and more if inflation lasted a long time.
How long would inflation have lasted? Probably forever. The same physics responsible for inflation's success causes a strange side-effect - in most cases, inflation does not end everywhere at once, but always continues somewhere.
In this "eternal inflation" picture, inflation perdures forever, and forms the backdrop for the Universe as a whole. Here and there, inflation ends and gives rise to a slower expansion, and perhaps the formation of matter, light, galaxies, stars, and beings like us. In this picture, the Universe just a short way beyond the edge of the observiball is an endless, roiling see of inflation, and all that we can see sits inside just one tiny floating bubble.
That's not just poetic imagery. A key mechanism by which space can change inflationary states, or can stop inflating, is by the formation of bubbles. Such bubbles form spontaneously, then expand at nearly the speed of light, within one might be another period of inflation, or non-inflation like what we observe. If eternal inflation is right, our observiball would be a small patch within one such bubble.
Now here's the thing - blow enough bubbles, and some of them will run into each other. In just the same way, if we reside within a bubble, it is guaranteed to encounter many others, and inter-bubble collisions just may impact our observiball.
Just as two spherical bubbles intersect in a disc, these collisions would leave disc-like bruises on our observiball forming faint warmer or cooler discs on the CMB.
While the number, intensity, and size of these discs depends on unknown details, specific enough signatures of such a collision have been worked out by several groups of cosmologists, especially Matthew Kleban of NYU and collaborators, that we can search for them.
A first study of this sort was carried out by a team led by Hiranya Peiris of University College London. They identified several candidate collisions in the CMB, but none were convincing detections.
The team is now analyzing data from the Planck mission to assess these candidates, and either confirm collisions or put stringent limits on their existence.
Detecting cosmic bubble collisions would require some luck even if eternal inflation is true - collisions must be frequent enough, strong enough, and not erased by inflation within our bubble.
Yet the potential payoff is enormous, evidence for a other universes would be an epochal expansion in our understanding.
And even if nothing is found, the very possibility reveals an amazing evolution of such enormous questions from rank speculation into the fold of solid scientific inquiry.