The multiverse might not be the most original or even surprising idea that physicists have proposed recently, but it certainly excelled in capturing the public imagination and in stirring up discussion. On the very top of the discussion list is the question whether the multiverse is a scientific idea at all, or whether it is simply a philosophical retreat for lazy physicists who’ve gotten tired hunting answers that didn’t come knocking on their door.But there’s no simple answer to that question. Whether the multiverse is a scientific idea depends on what one means with multiverse. As I explained previously, the existence of “multiverses” in many theories is just a consequence of pushing models beyond their phenomenological reach. The only thing these types of multiverses demonstrate is that physicists don’t understand the limits of pure mathematics. In contrast, the best motivated multiverse is bubble universes in eternal inflation. And these can have observational consequences.
Eternal inflation is a variant of inflation, a phase of exponential growth in the early universe. Exactly how this phase proceeds depends on the properties of quantum fields that filled the universe back then. In eternal inflation, it is only parts of the whole space in which the rapid expansion comes to an end and structures like our galaxies form; these parts are referred to as “bubble universes”. In the rest of space, inflation continues and goes on to create new bubbles. This bubble production lasts eternally.
Most of these bubble universes are causally disconnected from ours and we have no chance to ever observe them. However, it is at least theoretically possible that some of these bubbles collide as they expand. This would mean that what is now our observable universe had, when we look back in time, not one seed, but two or more that later came to join. This type of bubble collision can have observable consequences for the formation of structures and leave imprints in the pattern of temperature fluctuations in the cosmic microwave background (CMB).
The accurate mathematical description of these bubble collisions is however difficult because Einstein’s field equations are non-linear. Solutions can normally only analytically be found in cases with many symmetries, and in the case of bubble collisions the geometry prevents one from using such a highly symmetric ansatz. Approximately valid analytical solutions have previously been put forward, but when one wants to make quantitative prediction one needs a sufficiently precise numerical simulation. Such a numerical simulation has to evolve forward in time the metric components, whose fluctuations eventually go to seed the perturbations in the CMB, which we then later measure.
Such a numerical simulation has recently been published in this paper
- Simulating the universe(s): from cosmic bubble collisions to cosmological observables with numerical relativity
Carroll L. Wainwright, Matthew C. Johnson, Hiranya V. Peiris, Anthony Aguirre, Luis Lehner, Steven L. Liebling
Roughly speaking, the bubble collision leaves a localized temperature fluctuation - a hot spot or a cold spot - in the CMB that fades off away from the center of the collision. Exactly how it fades is very important for the extraction of such a signal from the data, and yet this could only be estimated before this numerical simulation was completed. Notably, the authors found that the fall-off is faster than was expected from the analytic estimates.
The figure below shows the time-evolution for the scalar field (Φ) that drives the inflation and two functions (a and α) that quantify the behavior of the metric. The horizontal axis is one of the spatial coordinates the vertical axes a measure for the time.
|Figure 7 from arXiv:1312.1357 [hep-th]|
What you can see in the image is how the configuration starts out being initially split in two halves and then evolves towards a situation where the initial split slowly fades away. That the bubbles both originate at “the same” time can always be achieved by a suitable choice of time-coordinate. The time, or the initial distances between the bubbles, can later be changed to a free parameter by a coordinate transformation.
This numerical study demonstrates nicely that it is possible to connect the underlying model for eternal inflation to observable signatures. It is also valuable in suggesting a useful parameterization for the effects. I find this a very interesting paper which will provide a basis for further studies that are necessary to analyze cosmological data for signals that might show we live in a multiverse.