Introduction
The question of what was there before the Big Bang is one of the most profound mysteries in cosmology. The standard classical Big Bang theory suggests that there was nothing before the Big Bang. Our universe began from a singularity with infinite density. However, alternative theories propose that the Big Bang was not the beginning but rather a transition from a previous cosmic state. Exploring these ideas leads us into the realms of quantum mechanics, cyclic universe models, and the philosophical implications of time itself.
The Classical Big Bang Theory
According to the classical Big Bang model, our universe emerged from an infinitely dense singularity. This singularity expanded in an event known as “cosmic inflation,” leading to the universe’s ongoing expansion. However, classical physics cannot explain singularities—where space and time cease to behave according to known physical laws. If time itself began with the Big Bang, then asking what was there “before” may not be meaningful.
Yet, singularities present a fundamental problem in physics, prompting researchers to seek explanations beyond classical physics. This has led to quantum cosmological models that attempt to replace singularities with more mathematically consistent descriptions of the universe’s origins.
The Hartle-Hawking No-Boundary Proposal
Many physicists have attempted to replace the singularity at the beginning of the universe with a quantum mechanical explanation. One such approach is the Hartle-Hawking proposal, which describes how the universe could have originated as a quantum system before evolving into the classical universe we observe today. This idea treats the universe as a quantum system that can be represented by a wave function, giving a probability distribution over different possible states.
In quantum mechanics, a system’s state is a mathematical description of its properties, and all possible quantum states exist within a mathematical space known as Hilbert space. For instance, in standard quantum mechanics, the possible states of a particle—such as its position and momentum—reside in a Hilbert space. When applied to quantum gravity, the entire universe is treated as a quantum system, meaning that Hilbert space contains all possible quantum states of the universe.
The Wave Function Of The Universe
A wave function, like the one described by Schrödinger’s equation in standard quantum mechanics, determines the behavior of a particle. Extending this concept to the entire cosmos, the wave function of the universe provides a probability distribution over various possible states of the universe. This implies that the universe does not necessarily follow a single, predetermined history but instead has multiple possible histories, each associated with a probability.
Hartle and Hawking proposed that the universe did not originate from a singularity but instead underwent a smooth quantum transition. According to their idea, in the earliest stages, time did not function in the manner we experience it today. Instead, it behaved more like a spatial dimension and gradually emerged from this quantum state. The Hartle-Hawking state eliminates the singularity by replacing it with a quantum description of the early universe.
Analogy
A helpful analogy is to imagine a smooth, rounded hill. As you descend the hill, the slope gradually flattens until it transitions into a straight road. The top of the hill represents the earliest quantum state of the universe, where time behaves similarly to space. The flat road at the bottom represents the classical universe, where time flows in the familiar manner. There is no abrupt beginning—no sharp boundary or singularity. Instead, the transition from quantum spacetime to classical spacetime occurs smoothly.
Furthermore, consider that multiple small paths lead down the hill in slightly different directions. This represents the idea that the universe did not have a single fixed past but rather multiple potential pasts, each with its probability. Our current universe is merely one of many possible histories that could have emerged from this quantum state. In the evolution of the quantum universe, our specific history is simply the one that materialized among many possibilities, leading to the cosmos we observe today.
Quantum Tunneling
The Hartle-Hawking no-boundary proposal provides a compelling explanation for how the universe could have emerged without a singularity. A useful way to understand this idea is through the concept of quantum tunneling. In classical physics, a particle that lacks sufficient energy to overcome a barrier cannot pass through it. However, quantum mechanics allows for exceptions due to the wave-like nature of particles.
In quantum mechanics, particles exhibit both particle-like and wave-like properties. When a quantum particle encounters a barrier, part of its wave function is reflected back, much like in classical physics. However, another part of the wave function penetrates the barrier and continues on the other side. This phenomenon, known as quantum tunneling, allows a particle to pass through a barrier even when it does not possess enough energy to surmount it. This occurs because, according to Heisenberg’s uncertainty principle, one cannot precisely determine both the position and energy of a particle simultaneously. If the position of a particle is known with high precision, its energy remains uncertain. This uncertainty allows a particle to temporarily “borrow” energy, increasing the probability of it tunneling through a barrier.
A Real-World Example
A real-world example of quantum tunneling can be found in nuclear fusion within the sun. Hydrogen nuclei, which carry positive charges, repel each other due to electrostatic forces. Classical physics suggests that the sun’s temperature is insufficient to overcome this repulsion and enable fusion. However, quantum tunneling allows protons to pass through their mutual repulsion, enabling nuclear fusion to occur and thus powering the sun.
Quantum Tunneling and Rebirth Of The Universe
The Hartle-Hawking proposal extends the concept of quantum tunneling to the birth of the universe. Before the universe became classical, it existed in a quantum state where space and time fluctuated with uncertainty. The transition from a quantum vacuum state to a classical expanding universe can be thought of as a barrier similar to those encountered in quantum tunneling. According to the Hartle-Hawking model, the universe tunneled through this barrier rather than emerging from a singularity. In this framework, just as a quantum particle can pass through an energy barrier, the universe itself transitioned from a quantum state into existence through quantum tunneling.
This idea suggests that the universe did not begin with an initial singularity, as predicted by classical general relativity, but rather emerged smoothly from a quantum vacuum state. Quantum fluctuations played a crucial role in this process, allowing the universe to transition from a probabilistic quantum realm to the expanding cosmos we observe today. In essence, the Hartle-Hawking no-boundary proposal implies that the universe spontaneously tunneled into existence, avoiding the singularity problem and providing a quantum mechanical foundation for the birth of the cosmos.
The Role of Entropy and the Arrow of Time
Once real time emerged, the universe existed in a low-entropy (highly ordered) state. As the universe expanded, entropy increased, setting the direction of time. If entropy did not increase, we would not perceive a distinction between the past and the future. Before time began to flow in the conventional sense, the universe was in a timeless quantum state. As entropy started to increase, time began to take shape, much like a ball rolling downhill.
Roger Penrose’s Cyclic Conformal Cosmology
Roger Penrose proposes that our universe is not the first and will not be the last. Instead, it is part of an infinite cycle of universes, which he calls “aeons.” Each aeon begins with a Big Bang, expands indefinitely, and eventually transitions into the next aeon. While conventional cosmology suggests that an ever-expanding universe will simply stretch out infinitely, Penrose introduces a mathematical transformation called conformal rescaling. This transformation stretches or compresses spacetime while preserving angles, making the infinitely large and stretched-out future of one universe appear like a small, dense state—similar to the conditions of a Big Bang. This allows for a smooth transition between successive universes.
There Should Be Massless Particles Before The End Of The Universe
For this process to work, only massless particles—such as photons and gravitational waves—can pass from one aeon to the next. Penrose hypothesizes that, before the end of our universe, all massive particles will decay. And black holes will fully evaporate via Hawking radiation. This will leave behind a universe composed entirely of radiation. Since photons and gravitons are massless, they do not experience time or space in the way we do. In this state, the concepts of “big” and “small” lose their meaning. As everything is so stretched out that size itself becomes irrelevant. As a result, the final state of one universe smoothly transitions into the hot, dense beginning of the next aeon. And it doesn’t even require a singularity.
The idea aligns with our understanding of the early universe. The Big Bang did not begin with atoms, stars, or solid matter; instead, it started as a state of pure energy. In the first fraction of a second, quantum fluctuations produced particles and antiparticles, eventually forming matter. Before that, the universe was just an energetic plasma. Penrose argues that, in a deep mathematical sense, the distant future of an aeon (containing only radiation) and the beginning of the next (a dense, hot state of pure energy) are fundamentally the same—just viewed from different perspectives. This challenges the conventional notion of a singular Big Bang and suggests an eternal cosmic cycle.
String Theory and the Cyclic Universe
String theory offers another perspective on what preceded the Big Bang. It proposes that our universe exists as a three-dimensional membrane (brane) floating within a higher-dimensional space known as the bulk. According to this model, multiple branes may exist, each representing a separate universe.
A related idea, the cyclic universe theory, suggests that two parallel branes are drawn together by an unknown force over trillions of years. When they collide, they release enormous amounts of energy, causing a new Big Bang. This cycle of expansion, collision, and rebirth continues indefinitely.
This concept aligns with the idea of a never-ending sequence of universes, avoiding the problem of an initial singularity. If correct, it means the Big Bang was not the first event in cosmic history. It is merely the latest transition in an eternal process.
Philosophical Implications: Is Time Fundamental?
Many of these theories challenge our understanding of time. If the universe emerged from a timeless quantum state, then “time” may not be a fundamental property of reality. It is rather an emergent phenomenon.
If time only began with the Big Bang, then asking what came “before” may not be a valid question. However, if the universe is cyclic, then time extends infinitely in both directions, rendering the concept of a “first moment” meaningless.
Conclusion
The question of what existed before the Big Bang remains open. The classical Big Bang theory suggests that time and space began at the singularity, making “before” a meaningless concept. However, alternative models propose that the universe emerged from a quantum state, transitioned through cyclic aeons, or resulted from brane collisions.
Whether the universe had a beginning or has always existed in some form is an ongoing debate. Advances in observational cosmology and theoretical physics may one day provide a clearer answer. For now, the nature of time and the origin of our universe remain among the deepest and most fascinating mysteries in science.
Sources:
https://en.wikipedia.org/wiki/Hartle%E2%80%93Hawking_state
https://www.physicsoftheuniverse.com/topics_quantum_uncertainty.html