Multiple Universes,
Lead: George Clark & Morgan O’Neil

3/20/2006

Summary by Zoë Bandola

A thorough presentation on the theories connected to multiverses- or multiple universes- was given to the class on March 20th, 2006. The presentation outlined the four levels of multiple universe theories, as well as touching on the less mainstream concepts of “Membranes” and “Universal Darwinism”. This was followed by a discussion section, in which specific questions were posed to either clarify the presentation material or to present new problems, examples, and general ideas of multiverses.

The four levels of multiverse theory are derived from the well-known concepts of mathematics and physics, such as String theory, and general scientific intuition, such as thought experiments (as it is in some cases impossible to conduct physical experiments).

• Level I is an Open Multiverse,
• Level II is Andrei Linde’s Bubble theory,
• Level III is Hugh Everett’s Many-Worlds interpretation, and finally
• Level IV is the Ensemble theory of Max Tegmark.

Our universe may be just one of many billions or so solutions to these theories. All of these theories generally explain our universe in various states of “bubbles”, where they are arranged one next to another, as in a heap of bath soap bubbles. Each universe occupies a definite space of its own, but right next to it is another universe entirely, all embedded in a space with more than our 3 dimensions in space and 1 in time.

The first theory discussed was an Infinite Universe theory. This generally states that there are as many universes present as one may conceptualize there to be, or in other words any universe that is conceivable is physically present. This is the most general theory, as it deals with anything the human mind can surmise.

Secondly, Bubble theory states that the Big Bang occurs often, and that parallel universes may exist in which the laws and properties can vary greatly from our own. Universes start from bubbles in quantum “foam”, where some are barren and collapse quickly. This is further explained in terms of Andrei Linde’s physical model of a bubble universe. The landscape of Andrei Linde’s multiverses shows peaks and valleys in energy, which represent regions with different physical laws and conditions. Only the regions in valleys end up with slow expansion rates like in our universe. Such slow expansion rates must be connected with observable predictions, such as specific space time curvatures and age. The Anthropic principle addresses the fine-tuning of parameters that we observe in our universe. However, it also leaves the daunting question how such perfect physical constraints, as in our universe, come into being. This is one of the issues being addressed by the possibility of many multiverses.

The theory of Inflation was then discussed. It states that after the Big Bang, “gravity-like forces” may have acted like a super-repulsive force that pushed the universe to the huge proportions of today at an extremely rapid pace. This force of “negative gravity” is actually an energy, which pushed everything apart, and it was much stronger at the event of the Big Bang than it is today.

The next theory is the “Many Worlds” theory. This is a quantum theory, and can be graphically illustrated by the famous thought experiment of Schrödinger’s Cat. This thought experiment supposes that things may have “occurred” in two opposite ways once they are actually observed. The Many World’s theory follows this supposition in supposing that two alternative universes actually exist in the same time, and continue to split in this way to infinity.

The third theory presented was that of the “Ensemble Theory”. This theory is a mathematical theory of multiverses, which cannot be specifically contradicted, allowing there to be multiple universes occurring in the physical world. This is because their theory supposes that for any given set of physical laws, there are many mathematical solutions. However, this does not mean that each solution is necessarily an actuality, but it is a possible realization. So, the superset of all mathematical possibilities cannot be greater in number than exist as possible solutions to this equation. This is another way of saying, although there may be thousands of universes, there cannot be any more than there are answers to the aforementioned equation.

Finally, the fourth theory presented was that of the Chaotic Inflationary Theory. This theory is related to the branching universe model of Andrei Linde, where there are significant changes in the laws of physics between different universes. One “parent” universe is said to give rise to other universes in a branching pattern, but if/when the universes collide more physical changes will occur.

This theory also contains within it the concept of Universal Darwinism. Universal Darwinism is described as a universe, which contains certain “genes” (i.e. its physical laws), which eventually become engulfed in a Black Hole. The “genes” of the parent universe continue on to give rise to a new universe. The more Black Holes exist in the physical world, the more universes will be created. This may lead to a process, which is comparable to natural selection.

An additional theory presented was that of the “Membrane” model. In this concept, membranes can create multiple universes when they collide periodically in space. The calculation of total gravitational energy vs. total mass of the universe, which when complied equals a zero-sum universe. Further, this implies that our universe did not need to be created from anything at all, and could have arisen from a vacuum.