What Happened Before the Big Bang?
Reading a few science-y books got me thinking about the question of Origin: where did everything come from? What was there before the Big Bang?
I went digging around the Internet, hoping to find some interesting theories. But to my disappointment, it seems as if physicists were quite conservative in coming up with theories to answer this question. (Perhaps I should turn to Sci-fi or mythologies to satisfy my thirst for mind-blowing conspiracies.)
I have summarised some of the main theories below:
Cyclic Model (the Big Bounce)
The ancient Hindu cosmology says that the universe is eternal, consisting of infinite series of cycles.
In modern cosmology, physicists have come up with similar models as well. One of the famous bounce cosmologist, Paul Steinhardt of Princeton University, wrote a paper in 2002 on "A Cyclic Model of the Universe" with Neil Turok of Perimeter Institue in Waterloo.
We propose a cosmological model in which the universe undergoes an endless sequence of cosmic epochs that begin with a “bang” and end in a “crunch.” Temperature and density at the transition remain finite. Instead of having an inflationary epoch, each cycle includes a period of slow accelerated expansion (as recently observed) followed by contraction that produces the homogeneity, flatness, and energy needed to begin the next cycle.
Read more about what Paul Steinhardt said.
However, one of the problems was that how a contracting universe would avoid collapsing into a point of singularity, as with a massive star which dies by shrinking to the singuar center of a black hole?
In 2017, physicists came up with 2 non-singular bounce models. One of it was by Anna Ijjas of Columbia University, whose former advisor was Steinhardt. The other model was proposed by Peter Graham, David Kaplan and Surjeet Rajendran, a well-known trio who mainly focus on particle physics questions. The main idea was that contracting universe bounces and starts expanding again before ever shrinking to a point of singularity. These bounces can therefore be fully described by the classical laws of gravity, requiring no speculations about gravity’s quantum nature.
Multiverse
Our universe arose from the Big Bang, where during the first tiny fraction of a second, it expanded exponentially (a theory known as "inflation"). Some physicists, including Andrei Linde of Stanford University, believe that whatever sparked the Big Bang could have happened many times, resulting in multiple universes. The cosmos is continuosly sprouting new universes like bubbles in a pot of boiling water.
Mirror Universe
Julian Barbour of University of Oxford, Tim Koslowski of the University of New Brunswick and Flavio Mercati of the Perimeter Institute ran computer simulations of 1000 simplified particle models and came up with this theory. In their model, there is a mirror universe to ours, in which time flows in the opposite direction. The dividing point is named the "Janus point", after the two-faced Roman god. Read more about it on the Quartz website.
No Boundary Theory
This famous theory put forth by Stephen Hawking with his frequent collaborator James Hartle in a 1983 paper, envisions the cosmos as a shuttlecock. From what I understood, the main point was that there is no before, as time only started after the Big Bang, and asking what came before Big Bang would be like asking "what lies on the South of the South pole". Hawking said that time and space are finite, but they do not have any boundaries or starting or ending points. A good way to think about it is that Earth is finite, but has no edges!
