The Planck era
We now have two different but related dictionaries for how to describe what is going on at the Planck scale: string theory and LQG. And this means we now have a way to describe the conditions in the universe during and just after the Planck era. Here is one story of what the Planck era might have looked like and what it might tell us about cosmogenesis, told in the combined language of string theory and LQG.
The Nothing State: In many ways, the “beginning” of the Planck era is not a logical concept. Nor does it have an actual name, because any name presupposes a time, place, or quality, none of which may apply here. According to physicist Daniele Oriti at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, this primitive Nothing State may have consisted of ingredients that were not spacelike or timelike at all.
But as Nobel laureate and physicist Frank Wilczek once said, “The reason that there is Something rather than Nothing is that Nothing is unstable.” This instability led to a phase change in the Nothing State. In the language of LQG, Nothingness was converted into Something: a full of innumerable elemental Planck volume nodes. This occurred in perhaps the same way that a cloud of water molecules in a gas changes phase into a cloud of liquid droplets — rain — when the temperature falls.
Geometrogenesis: Our new Something State, consisting of droplets of space (nodes), did not remain random for long. The nodes were embedded in networks of links that defined the spin network’s dimensionality (N), which in turn defined the number of nearest neighbors to each node. According to Lee Smolin — one of the developers of LQG — of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, the available energy to maintain these links may have been so enormous that the Planck volumes could have been elements of a space with vastly more than the supposed 11 dimensions of the Bulk. But even this state was unstable and spawned a second phase change as the available energy declined.
Imagine a very peculiar landscape. There are mountains where links are numerous (large numbers of dimensions). These locations require huge amounts of energy to maintain themselves. And there are valleys (with fewer dimensions), where less energy is required. Phase changes occur when it is more favorable for a system to exist at a lower energy that at a higher one. This suggests that spaces with huge numbers of dimensions tend to become spaces with lower energy and fewer dimensions. The change happens by way of a process called quantum tunneling.
So, we can imagine a second phase change occurred when the new N-dimensional Something State tunneled to one of these lower-energy states of spin networks with fewer links between nodes. All but 11 of the originally numerous dimensions disconnected from the nodes and vanished. This may have formed the geometric basis for the Bulk in string theory.
Compactification: According to string theory, there must have been some event, process, or circumstance in which seven of the 11 dimensions of the Bulk were compactified to create the specific details of the Standard Model in our four-dimensional universe. We can imagine this as a third phase transition that, in the language of LQG, caused the links representing seven of the 11 dimensions to develop closed spaces, each with a specific geometry. Among the remaining four dimensions, three formed three-dimensional spacelike spin networks, or branes.
Chronogenesis: A fourth phase transition occurred as one of the four spacelike dimensions in the Bulk tunneled into a timelike dimension that tracked the changes taking place between configurations of spin networks to produce spin foams. Stephen Hawking and James Hartle proposed this idea in 1983 to solve the problem of the origin of time in cosmology. They called it the no-boundary proposal because it eliminated the need for discussing what happened before the Big Bang. Essentially, they said, the universe has no boundary (beginning), just as there is no point north of the North Pole. Once one dimension emerged as the direction of a succession of spatial states (branes in string theory; spin networks in LQG), it established cause and effect, and the Big Bang occurred.
Cosmogenesis: At first, the scope of the Big Bang was limited to the Planck scale as bubbles of new space-time emerged from within the vaster network of purely spacelike dimensions. This was an unimaginably turbulent time, perhaps resembling the topsy-turvy chaos of what theoretical physicist John Wheeler called quantum space-time foam. Collections of nodes came together to form the first primordial objects — quantum black holes — but these quickly disintegrated back into individual nodes. Other collections of nodes took on wavelike behavior and traveled through the spin foam network as gravitons.
Structures larger than the Planck scale began to form stringlike objects consisting of nodes organized along one dimension of space. With the information (provided by their links) about the seven compact dimensions, they took on the properties of the individual particles we recognize in the Standard Model. Huge ensembles of these strings began to behave as organized quantum fields. The way in which one string interacted with another is described by the way in which one enormous collection of nodes changed into another collection as part of a spin foam pattern.
During all these phase transitions, the amount of information coded into the network of nodes steadily increased. This had a profound effect upon how precisely the mathematical relationships between nodes along the emerging time axis could be specified. These relationships are what we call the physical laws of nature and include how we describe gravity and the details of the Standard Model. So, during this later stage of the Planck era, not only did time emerge, but also the laws of nature now operating across our space-time.
In his book The Mind of GodPaul Davies notes that “[Prior to] one second after the Big Bang there was less space and less information, so mathematics was in a cruder form. The computing power of the universe near the Planck time was essentially zero. All math would have been meaningless, and laws would have been nearly impossible to state.”
At first, the physical laws were very approximately specified, but as time passed and the available information grew, these patterns became more detailed. In essence, the physical laws of our universe emerged from the growing information content of the universe within which these laws could be defined. Once these patterns emerged and became more precise, the progression of the Big Bang became richer in specific patterns for how particles interact across space and time.
The bigger picture
If we were to step back and look at the big picture during the Planck era, we might envision a succession of bubbles within bubbles. The largest of these encompasses the domain of the spin networks in which other numbers of dimensions exist. Within some of these bubbles, space crystallized into 11 dimensions. We call our 11-dimensional bubble the Bulk. But there may be other Bulks with fewer or more than 11 dimensions.
Furthermore, within our Bulk bubble, one of the dimensions transitioned into time and served to organize the 3D branes (spin networks) into a recognizable, chronological order: our 4D space-time. Another transition compactified seven of the spacelike dimensions, bringing into existence our specific Standard Model particles and fields. Only the exact geometry of the compact 7D spaces defines what the Standard Model will look like for any given universe. But because string theory provides 10500 ways to do this, there are many different 11-dimensional Bulks, each with its own way of compactifying those seven dimensions.
Taken together, these are called the Landscape. You may know it as the idea of a multiverse. Continuing the analogy of our universe as a 4D collection of branes like a book, then the Bulk is like a giant library containing an infinite number of these book universes, each with different geometries for these compact spaces, leading to different Standard Models.
Back in our own space-time bubble, now vastly larger than the Planck scale, the background network of nodes defining the spin foam in four dimensions began to look smoother and smoother at larger scales as the universe became older. After a period of time, inflation occurred, ending some 10-34 seconds after the Big Bang … and here we are!
Of course, this entire story is highly speculative, even fanciful. It is based on theories or pieces of theories that remain largely unproven — or perhaps, one shudders to think, even unprovable. But our quest for the origin of the universe is a result of who we are as sentient beings.
Put together with observations, we can continue to create and improve origin stories of the universe that answer many older questions while offering new ones for future generations to explore and test