What is the branch of physics that asks the question 'what was before the Big Bang'?

Question

What is the branch of physics that asks the question 'what was before the Big Bang', assuming the Big Bang is truly what happened at the beginning of the universe? If there could be a better model that describes the start/formation of the universe, then the equivalent question would be: What is the branch of physics that asks the question 'what was before the formation of the Universe'?

What kind of knowledge do we need to answer this question? Quantum physics? Theory of relativity? Or anything else?

Answer 1

Both quantum mechanics and general relativity deal with entities whose existence depends on the very existence of the universe. In the case of general relativity, it deals with the fabric of the universe itself: spacetime. However, prior to the Big Bang (if it indeed occurred), by definition, spacetime did not exist, rendering this theory irrelevant for such purposes. Regarding quantum mechanics, it applies to objects, particles, or, in more speculative cases, to spacetime itself, but never to nothingness.

Indeed, by definition, there is no spacetime before the Big Bang, and therefore, the question of "what happened before" is not meaningful in this context. One might be interested in the first microseconds of the universe's existence. Cosmology addresses these questions, particularly in the realm of particle physics.

The early moments of the universe are explained by the Standard Model in the framework of particle physics. Subsequently, general relativity and quantum mechanics come into play. It's an extremely intense epoch, and explaining it requires the integration of various branches of physics. However, it's crucial to note that we are specifically discussing the post-Big Bang era.

Answer 2

The Big Bang is an ambiguous term (and many answers here take one or another definition of the term). It can refer to the standard cosmological theory (as in the Big Bang Theory) that the Universe started in a hotter, denser state, and gradually expanded and cooled. The "beginning" of the phase of the Universe's history that we call the "standard cosmological theory" is up for debate. Some people will put the boundary at the earliest point we have solid observational evidence (called "Big Bang nucleosynthesis"). Under this definition, "before the Big Bang" would include speculative processes like inflation, where (if it is true) the Universe would have gone through a phase of exponential expansion.

However, I think you are interested in the Big Bang Singularity, which is a common feature of many cosmological solutions within General Relativity. If you wind back the clock far enough in our current cosmological theories, you will find that in a finite amount of time, the Universe becomes infinitely dense. Now, it is normally assumed that "for some reason" the Universe doesn't actually become infinitely dense, and this is a pitfall of General Relativity. However, know one knows what does happen instead, or why, or what theory we should use to answer such a question.

Your question presupposes one possible resolution to the Big Bang Singularity. Namely, that a complete theory would describe some time before what we think of as the singularity. This is an assumption you have baked into your question. It is not known whether that is what occurs; it could be that there really is some moment of creation that occurs and there is nothing "before." A famous example of a theory like this is the Hartle-Hawking "no-boundary" proposal.

Having said that, are (speculative, not empirically confirmed, not universally accepted) theories that do consider a time before the singularity predicted by General Relativity. Here are three examples:

• Eternal inflation, which states that the Universe is much, much larger than what we observe. Our little patch of the Universe started expanding as a kind of "bubble" and effectively became disconnected from the larger whole at some time in the past. In this framework, the "Big Bang singularity" is replaced our patch joining into a much larger, "eternally inflating" spacetime.
• Bouncing Universes, which suppose that there was a Universe "before" our Universe, that underwent a collapse, then a "bounce", and is now expanding into our Universe. The Big Bang singularity here is replaced by the bounce.
• Roger Penrose's Cyclic Conformal Cosmology, which states that the far future of our Universe will join onto the far past of a future Universe in a subtle, smooth way using conformal symmetry; and similarly the far past of our Universe emerges from the remnants of an even older Universe. In this scenario, the Big Bang singularity never happens, and is replaced by this "joining" onto a past Universe. I should say (like most cosmological ideas going this far back) this idea is extremely speculative.

All of this is extremely speculative. It is not really a branch of physics that studies this question, but certain speculative sub-branches of theoretical cosmology study theories that would answer your question if they were true. However, we don't really know if your question even makes sense. It could well be that time and space themselves break down at what we think of as the singularity, and there is no clear notion of what "before" that time even means (rather like asking what takes place "inside" of the singularity of a black hole).

Answer 3

Theoretical astrophysics and cosmology.

Answer 4

It's basically called cosmology. There are multiple cosmologies, in some cosmologies, called cyclic cosmological models, our universe is thought to be originated after the collapse of a previous universe. In these models, the universe first expand from a big bang singularity point to its maximum size then starts contracting to a point called big crunch. And basic keywords are:

#cosmology #inflation #String_Theory #quantum_gravity #Quantum_cosmology

But the thing is no one came up with something to prove these models observationally, so better call them as hypothetical models (or just models without any proof). Moreover, string theorists has developed a theory called M-theory: Multiverse theory. This theory describes our universe as one of many baby universes with a different set of values for the fundamental constants like mass of elementary particles, Planck's constant etc

Answer 5

No branch of physics, because the questions you state are not scientifically well defined.

'what was before the Big Bang'

is problematic because the "Big Bang" is not an well defined event. For example, there is no consensus on whether inflation should be defined as happening before or after the Big Bang.

'what was before the formation of the Universe'

is problematic because how do you define the extent of the Universe? The mere fact that you are talking about what happened before it implies that you do not consider it to be all of spacetime!

The ambiguities matter. For example, I cannot tell whether your question includes inflationary physics, which is a pretty widely studied subfield of cosmology and does not require major new theoretical advances -- it's pretty much a business of building particle-physics models and seeing how they would affect observations.

Similar but more scientifically well defined questions might be:

• What is the history of the Universe prior to neutrino decoupling and primordial nucleosynthesis? These are the earliest events for which we have a clear theoretical understanding that is precisely confirmed by experiments.

• Why is the Universe set up the way that it is? Specifically, why are the relative abundances of different components what they are, why is the large-scale spatial structure of the Universe what it is, and sometimes even why are the fundamental particles/fields the way that they are? This is an almost philosophical question, but people attempt scientific answers, typically involving thinking about what universe is most probable.

The first question is a topic for cosmology. The second question is also cosmology, although depending on its scope, it could also be considered one of the ultimate goals of particle physics and gravitational physics.