Observing the Multiverse (Guest Post)
The idea that there might be other universes is taken quite seriously in high energy physics and cosmology these days. This is mainly due to the fact that the laws of physics, and the various “fundamental” constants appearing in them, could have been otherwise. More technically worded, there is no unique vacuum in theories of high energy physics that involve spontaneous symmetry breaking, extra dimensions, or supersymmetry. Having a bunch of vacua around is interesting, but to what extent are they actually realized in nature? Surprisingly, when a spacetime region undergoing inflation is metastable, there are cases when all of the vacua in a theory can be realized in different places and at different times. This phenomenon is known as eternal inflation. In an inflating universe, if a region is in a metastable vacuum, bubbles containing different vacua will form. These bubbles then expand, and eat into the original vacuum. However, if the space between bubbles is expanding fast enough, they never merge completely. There is always more volume to convert into different vacua through bubble formation, and the original vacuum never disappears: inflation becomes eternal. In the theory of eternal inflation, our entire observable universe resides inside one of these bubbles. Other bubbles will contain other universes. In this precise sense, many theories of high energy physics seem to predict the existence of other universes.
In the past four years, a few groups have tried to understand if it is possible to confront this radical picture of a “multiverse” with observation. The idea is to look for signatures of a collision between another bubble universe and our own. Even though the outside eternally inflating spacetime prevents all bubbles from merging, there will be many collisions between bubbles. How many we are even in principle able to see depends in detail on the underlying theory, and given the proliferation of theories, there is no concrete prediction.
Currently, the best information about the primordial universe comes from the cosmic microwave background (CMB). A collision will produce inhomogeneities in the early stages of cosmology inside our bubble, which are then imprinted as temperature and polarization fluctuations of the CMB. One can look for these fingerprints of a bubble collision in data from the WMAP or Planck satellites.
Most of the previous work has been to establish a proof of concept that observable bubble collisions can exist, and that there are theories which predict that we expect to see them; many of the details remain to be worked out. There are however a number of generic signatures of bubble collisions that we used to guide our search. Since a collision affects only a portion of our bubble interior, and because the colliding bubbles are nearly spherical, the signal is confined to a disc on the CMB sky (imagine two merging soap bubbles; the intersection is a ring). The effect of the collision inside the disc is very broad because it has been stretched by inflation. In addition, there might be a jump in the temperature at the boundary of the disc (although the magnitude and sharpness of such a jump has yet to be worked out in detail).