Cosmology, the subset of astrophysics that considers the origin of the universe, is a fascinating study of physics per se - and philosophy. From here we can quickly devolve into a discussion of semantics but we won’t. Current thinking that is supported by a mass of testable and tested hypotheses have settled on the “Big Bang Theory” of the universe; we even have a TV show given the same name. But a recent paper and excellent commentary on that paper have triggered a philosophical perspective worth sharing.
The Big Bang Theory posits a hot, dense, singularity that at “time” zero underwent a rapid expansion where a quark-gluon plasma violated the conservation of baryon number and allowed for baryogenesis and thus the predominance of matter over anti-matter! (Wikipedia has a very passable discussion of “Big Bang” cosmology). Unfortunately calling this model the “Big Bang Theory” almost immediately leads to a bit of cognitive dissonance - at least for me. Here’s why:
First, by necessity, physics must be guided by the “anthropic” principle - being “the philosophical consideration that observations of the physical Universe must be compatible with the conscious and sapient life that observes it.” The concept of “hot”, “dense”, and “singularity” are inherently anthropic. Each is based on a relative measurement - a “cognitive” frame of reference. Equally important is the concept of measurement: a measurement assumes that there is a “event” able to be measured - to say nothing of the events or multitude of events that would occur in the process of measurement.
Hot, dense, mass, time etc are meaningless without an event by which, utlimately, such things can be compared and measured against. If an event leaves no information - it never occurred! Dimension and time (spacetime) from which - anthropically - we measure other such things such as temperature, mass, non-relativistic dimensions, etc are ultimately based on an event.
For me, the idea of a “Big Bang” cosmology is better represented analogously (albeit a bit weak - more on that to follow) by a supercritical fluid. Most all of us have had the experience of opening a bottle of super cooled beer or soda; leaving a bottle in the freezer, for a “quick” chill a bit too long gives you a supercooled fluid. When you open the bottle, it almost instantly changes to a “slushy” and oozes out of the bottle. Or, for that quick cup of instant coffee, you superheat a cup of water in the microwave only to have it flash boil out of the cup when you drop in the instant coffee. Probably one of the most interesting examples of this “idea” occurred when preparing a stock solution of a purified glycine for use in preparing microbiological media. Mixing a fairly large quantity (I don’t remember the concentration, but most probably the glycine was from Sigma!) in a one liter flask. It was gently heated with a stirring bar on a magnetic hot plate and, once completely dissolved, placed in the lab refrigerator. A few days later when preparing that media, I grabbed the flask from the refrigerator and proceeded to measure out what I needed when to my surprise there was to be seen a large virtually perfect single pyramidal crystal sitting square in the middle of the bottom of the flask! “Crystal” clear so to speak, there were no obvious defects. But as the flask bottom wasn’t perfectly flat, the bottom of my crystal mimicked the irregularity of the flask bottom.
Clearly super critical phase transitions - much less crystal formation (also “sort” of a phase transition) - are a very rough analogy to the big bang idea but underlying the thought is a “rapid” propagation of a “phase” transition. In my model(s) we have an open bounded system with clearly defined (measurable) properties and we can easily explain and predict what is going on. Within the bounded domains we have essentially a homogeneous system - isotropic in all respects except when approaching the boundaries. Of course we pour energy into the system by either adding or removing heat (and what is heat?) Until our isotropic homogeneous system “triggers” (goes “Big Bang”), all we have is an isotropic homogeneous system (whatever those words may actually mean) that macroscopically has no events. (At the molecular and atomic level - quite busy - but we’ll neglect that for our analogy.) In this system, defining “hot”, “dense”, “singularity” etc is meaningless until the trigger occurs (what’s the trigger?) and the phase change propagates into what becomes a solid or gas or highly organized crystal. In these examples of course a “boundary” remains - but if it were without bounds conceivably the phase change propagates indefinitely. (But propagates into what? Self-propagating?)
For the amateur quantum cosmologist, this model may help resolve the weird idea of a “singularity” - singular as compared to what? (Without “events” to measure with or against, there’s nothing to measure!) “Dense” as compared to what? “Hot” as compared to what? Expanding... into where?
Is “spacetime” bounded? Open or closed? Is it a constant?