Big Bang, the universe, and string theory

There is a widely accepted notion that the universe was born through the Big Bang. But is that truly the case? According to the prevailing theory, the Big Bang marked the very beginning of time. There's substantial evidence supporting this. However, the Big Bang theory fails to answer one fundamental question—what existed before the Big Bang?

In response to this, many cosmologists have suggested that the question itself is flawed—akin to asking what lies north of the North Pole. Once you're at the North Pole, there's no "further north"; likewise, asking what was before time began might be meaningless.

Back in the 1920s, when scientists started studying the structure of the universe, their research was largely rooted in the General Theory of Relativity. Quantum mechanics also offered some support. In other words, explanations for the universe's origin, the birth and death of stars, and so on, relied mainly on general relativity, with quantum physics playing a secondary role. If general relativity or quantum mechanics were the final say in physics, then it would be reasonable to consider the Big Bang as the absolute beginning of time.

However, string theory and quantum gravity challenge this assumption. Until the 1990s, it was widely believed that time began with the Big Bang—that before this moment, time simply did not exist.

But why didn’t time exist? Because if we trace our universe's 13.8-billion-year history backward, we arrive at a state of extreme density and heat—a single, infinitely compact point. At this point, all known physical laws break down. This point is called a singularity.

In the 1960s, Stephen Hawking and Roger Penrose introduced the theory of singularities. A singularity is a point where all the mass-energy of the universe was concentrated. Then, in a tremendous explosion, this matter began to separate. Space and time were born. The universe came into being—and it has been expanding rapidly ever since.
Why didn’t time exist before the Big Bang? Because if we trace the universe’s 13.8-billion-year history backward, we reach an extremely hot and dense point—a state where all known physical laws break down. This point is known as a singularity. In the 1960s, Stephen Hawking and Roger Penrose developed the theory of singularities.
According to this theory, the entire mass and energy of the universe were compressed into a single point. Then, through a tremendous explosion, matter separated, and space and time came into existence. This event marked the birth of the universe, which has been expanding rapidly ever since.

But singularities are troubling to scientists. In a realm where the laws of physics don’t apply, there’s nothing more you can calculate or predict. The singularity at the time of the Big Bang is now increasingly viewed with skepticism. As a result, the once-accepted notion that time began at the singularity is now an uncomfortable proposition for physicists. And much of this discomfort stems from string theory.

Currently, string theory is the leading candidate in the race to develop a “Theory of Everything.” From the microscopic world of particles to the macroscopic cosmos, string theory offers solutions to many of the toughest questions in theoretical physics, cosmology, and astrophysics. It successfully unifies Einstein’s general and special relativity with the strange laws of quantum mechanics. Though experimental evidence is still lacking, string theory is considered the most mathematically robust framework available for a unified theory of the universe.

Importantly, string theory rejects the concept of a singularity at the origin of the universe. Instead, it suggests that the universe was born from a preceding state or event—something that came before the Big Bang. What that event or state was, we still don’t know. Previously, it was claimed that time itself didn’t exist before the Big Bang, so there was no point in asking what came before. But string theory breaks that taboo, allowing us to imagine a reality beyond the Big Bang, giving us the freedom to explore what may have come before time itself.

There is another theory called Inflation Theory. It suggests that, shortly after its birth, the universe underwent a period of rapid and violent expansion—an exponential inflation. This phase lasted for only a fraction of a second, but in that instant, the universe expanded by a factor of billions upon billions. Soon after, the inflation stopped, but by then, the key features of the universe had already been set.

According to this theory, the birth of the universe still originates from the Big Bang. Inflation Theory also posits that the universe emerged from nothing, meaning that time began with the Big Bang. However, some pressing questions remain unresolved:

How exactly did inflation occur?
Where did the immense energy required for such an inflation come from?
In trying to answer these, scientists began to wonder: What if time didn’t actually begin at the Big Bang?

What if the universe’s formation process had already begun slightly before the Big Bang? If that were the case, then the primordial constituents of the universe—essentially forms of energy—would have had enough time to prepare themselves for the violent inflation.

But this leads to a deeper question: If something happened before the Big Bang, how did it happen?
To explain this, a new kind of theory was needed—a union between General Relativity and Quantum Mechanics. In short, a true Quantum Gravity theory, or a Theory of Everything, was essential.

However, when American physicists like Alan Guth and Andrei Linde were proposing the theory of cosmic inflation in the late 1970s and early 1980s, no established theory of quantum gravity existed.

Then, in the mid-1980s, two potential candidates emerged: String Theory and Loop Quantum Gravity. Over the next two or three decades, Loop Quantum Gravity made significant progress. Yet, in the race to become the definitive quantum theory of gravity, String Theory has remained the stronger contender.

String theory had its early beginnings with the Italian theoretical physicist Gabriele Veneziano. In 1968, he attempted to formulate a theory to explain the strong nuclear force. Although his efforts laid the groundwork, they were initially unsuccessful, and the theory was soon discarded.

But in the 1980s, a group of physicists revisited and revived string theory. This time, by discovering certain mathematical paths, the theory offered a way to unify quantum mechanics with general relativity—something long sought after in physics. Since then, string theory has gained increasing acceptance as a strong candidate for a unified theory of everything.

According to string theory, fundamental particles like electrons and quarks are not tiny solid spheres (like marbles), but are instead made of incredibly tiny one-dimensional strings.

"One-dimensional" means they have length, but no width or thickness.

To understand this better, imagine spinning a bangle (churī) and dropping it on the floor. It doesn't appear like a thin ring anymore—it looks more like a tennis ball. While this isn’t a perfect analogy, it helps visualize how vibrating strings might appear different based on how they behave.

If a string is shaped like a bangle, with no open ends, it is called a closed string. On the other hand, a string that’s more like a thread or a wire, with two loose ends, is called an open string.

These strings vibrate, and the manner of their vibration determines what kind of particle they manifest as. The frequency and pattern of vibration define whether the string appears as an electron, a photon, or even a graviton (the hypothetical quantum particle of gravity).

Now you might ask:
We can more or less imagine how a closed loop string vibrates—but how do open-ended strings vibrate, and where are their two ends attached?
To visualize this, don’t think of a thread anymore—think of a guitar string or a sitar wire. Its two ends are fixed to a structure, and when struck, it vibrates to create sound. Similarly, the open strings in string theory must also have their ends attached to something.

But here’s the twist: these strings are not physical wires or threads. They are vibrating strings of energy, hidden deep within the very fabric of space-time. So where are they attached?

The answer lies in a further development of string theory itself—something we’ll explore in the continuation of this explanation.
What is the relationship between vibrating strings and quantum field theory?

The founders of string theory applied the principles of quantum field theory to these vibrating strings. They observed that the waves or vibrations of these strings propagate through the quantum field at the speed of light, and as a result, new and distinct properties of the strings emerge. These strings can no longer be considered ordinary; they are known as quantum strings. When quantum effects start acting on the strings, a specific law of quantum field theory becomes active.

Quantum field theory tells us that light waves are not continuous; instead, light energy is emitted in packets or bundles. Each of these energy packets is called a photon or a particle of light. These light waves cannot be broken into smaller packets than this. Similarly, space and time also exist in such discrete packets or quanta. On the other hand, string theorists say that strings are confined to a fixed length—you can't just cut a string to create smaller ones at will. The smallest possible length of a string is about 10⁻³⁴ meters.
This constant length protects string theory from the infinities that often appear in physics, and also saves it from the problems of zero. Generally, quantum strings have no mass, but they do possess angular momentum. For those who find this surprising, consider the photon: a particle of light has no mass but does have momentum. Similarly, strings carry momentum too. Due to quantum fluctuations, strings also exhibit rotational inertia. A tiny string can gain up to two units of angular momentum without increasing its mass. This feature is significant—it helps explain how force-carrying particles are generated.

While trying to understand the behavior of quantum strings, scientists discovered that beyond the familiar four dimensions, there are more hidden dimensions. A violin string vibrates in three dimensions. But the vibrations of quantum strings are different—they are only possible if space-time is more curved or twisted than usual.

To get an idea of this concept, consider the following analogy. If you bend a sheet of paper in one direction, you can roughly represent that as a one-dimensional curvature. But if it's not paper, rather a sheet of tin, and you bend it into the shape of corrugated iron, that introduces two-dimensional curvature. Now, if you take a cylinder and bend it left-right and forward-backward, it can be said to be curved in three spatial dimensions. Add time to this, and it becomes a four-dimensional curvature.

However, string theorists have found that the way strings vibrate cannot be fully explained by four-dimensional space-time curvature. For such vibrations to be possible, space-time must bend even more—so much so that six additional spatial dimensions are needed to explain it.

In other words, though we perceive the universe as four-dimensional, it is not truly limited to four dimensions. There are six more dimensions hidden within space-time.
Another prediction of string theory is that the values of physical constants—such as Coulomb’s constant or Newton’s gravitational constant—might not always remain fixed. We call them "constants" because they are the same everywhere in the universe. But according to string theory, in certain extreme cases, these values could vary.

For instance, Coulomb’s constant is based on the nature of the electromagnetic field. String theory suggests that these fields might behave slightly differently in distant regions of the universe, causing the values of constants to fluctuate. If we ever detect such variation in a constant somewhere, it could provide strong support for string theory.

There is another special field in string theory known as the dilaton. This is considered a distinct dimension in string theory. The dilaton field determines the strength of all forces and their interactions. It exists in a special spatial dimension, which means that in addition to the ten dimensions of space-time previously suggested by scientists, the dilaton adds an extra one. As a result, the universe is considered to have a total of eleven dimensions.

Another unique feature of string theory is the concept of duality. This idea radically transforms how we understand the properties of small objects. One such duality is that smaller strings are usually lighter than larger ones, but when we try to make the strings even smaller, they start to become heavier again.

One of the most important types of duality in string theory is called T-duality (or target space duality). T-duality states that whether a dimension is small or large, the properties and behavior of these extra dimensions in string theory remain identical. String theorists believe this arises because strings move in a far more complex way than point-like particles such as photons, electrons, or quarks.

Imagine a closed string, like a ring or bracelet, placed in a space-time that resembles a cylinder or a rolled-up sheet of paper. The circular shape of the roll represents an extra dimension. The round shape of the string itself also indicates an additional dimension.

Strings can vibrate left and right, up and down, and in all directions. They can also loop around the rolled-up space-time, even winding around it multiple times—just like a rubber band can be wound around a rolled paper. This is T-duality.

Let’s take a look at why all types of strings have equal energy. When a large string winds around a rolled-up dimension (a compactified space), it requires more energy due to its size. On the other hand, a small string has a much higher frequency of vibration, so it also requires more energy to wrap around that same compact dimension. This is why strings with both large and small radii exhibit the same energy and properties.

At one time, string theorists believed that T-duality applied only to closed loop strings because of their circular shape—allowing them to wind around compact dimensions like a ring. On the other hand, open strings were thought to be excluded from this behavior, as they resemble broken rubber bands with two open ends, seemingly unable to loop.

But in 1995, Joseph Polchinski of the University of California revealed that T-duality also applies to open strings. The reason is that the open ends of the string are free only in the three spatial dimensions, but in the other seven dimensions, their ends are anchored—much like the string of an ektara (a traditional string instrument) is fixed at both ends. Because of this, open strings also follow T-duality. Therefore, all open strings—regardless of size—share the same energy, behavior, and properties.

The very nature and behavior of strings suggest that they neither favor infinity nor absolute nothingness. That’s why a string cannot be divided into an infinitely small point, nor can it collapse into a zero-volume point of infinite density. In other words, a string can never become a singularity.

If we run the history of the universe backward in time, we will see that as we move back, the curvature of space-time increases—meaning the universe becomes more compact. But eventually, this curvature reaches its maximum limit. Beyond that point, the curvature does not increase any further—in other words, the contraction stops before reaching a singularity. The point where space-time is most curved or compacted is known as the Big Bang.

However, this is not the end (or rather, not even the beginning) of the universe's history. If we continue moving further backward in time, we enter a phase where space-time expands in reverse. This suggests that the universe already existed before the Big Bang, and it contracted down to the conditions we identify as the Big Bang.

So, was there another universe before the Big Bang? And did that earlier universe contract to produce the Big Bang? To answer this, we need to understand the state of the universe at the moment of the Big Bang. But that moment was marked by extreme instability, and physicists have not yet been able to solve the equations that describe it.
Still, scientists are not the kind to sit idle.

String theorists have proposed two possible models for the pre-Big Bang universe. One of them is called the Pre-Big Bang Model, developed by Italian physicist Gabriele Veneziano and his collaborators. This model combines T-duality with time-reversal symmetry. According to this symmetry, the laws of physics apply equally in both directions of time—forward and backward.

The model shows that five seconds after the Big Bang, the universe was expanding—and in reverse time, five seconds before the Big Bang, it was also expanding, but in the opposite direction in time. The rate of expansion was much higher before the Big Bang than after it.

In short, this model suggests that the Big Bang was not the beginning of the universe or of time itself. Rather, both the universe and time existed even before the Big Bang.
However, this model supports the inflation theory, but the nature of inflation here is different from that in the Guth-Linde model. The Guth-Linde inflationary Big Bang theory states that inflation began 10⁻³³ seconds after the Big Bang; but according to the Pre-Big Bang model, the universe’s expansion or inflation started before the Big Bang, and this was a natural consequence of the new symmetries in string theory.

This Pre-Bang theory talks about symmetry. What is symmetry? Simply put, what do you feel when you look at yourself in a mirror? Is there any difference between you and your mirror image? If any, then only the left and right sides are swapped; there is no other difference. That is symmetry. Similarly, the Pre-Big Bang symmetry suggests that the universe before the Big Bang was the mirror image of the universe after the Big Bang. The universe has existed for an infinite time in the past. However, long ago, the universe was almost empty—containing only weak radiation and gaseous matter. The fundamental forces were also weak at that time. Matter barely interacted with other matter or radiation—though it would not be accurate to say there was no interaction at all.

Due to the weak forces, particles and radiation did interact slightly. These interactions strengthened the forces. Matter started to clump together. In some regions, the density increased so much that matter was pulled together by intense gravitational force to form black holes. These black holes became separate from the rest of the universe, effectively dividing the universe into different parts.

Inside these black holes, space-time changes its role. The center of a black hole is not a place but a moment in time. When particles begin falling into the black hole, the density at the center continues to increase. Matter density, temperature, and curvature of space-time reach the active limits described by string theory. At that point, time and matter begin to reverse their direction inside. Eventually, the density, temperature, and curvature of space-time start to decrease again.

This moment of reversal is what we observe as the Big Bang. In other words, the universe is born from inside the black hole.

The model proposed by Veneziano and his team caused quite a stir. Criticism quickly came from all directions. The strongest critique came from Andrei Linde, one of the founders of the inflationary Big Bang theory. He argued that this model could only be true if the black holes were extraordinarily large—much larger than the size of black holes expected at the string theory scale. But was it even possible for such huge black holes to form at that time?
In response, Veneziano’s team said that black holes at that time came in various sizes—small, large, and medium—but our universe was born from inside one gigantic black hole.
Not only Linde, but French scientist Thibault Damour and Belgian scientist Marco Henneaux also raised another question. They said that, according to the Pre-Big Bang theory, matter and space-time behaved chaotically very close to the Big Bang. Does this behavior align with the known laws of the universe? If not, then how could the current universe have emerged from such chaos?

In reply to this question, Veneziano proposed another idea. He said that during that chaotic period, a large number of string holes were created. Some of these holes were very small, while others were very large. Many of these holes were so massive and heavy that they had a strong potential to collapse into black holes.
In other words, if we accept the Pre-Big Bang model, then time did not start at the Big Bang. The universe and time existed before, and the Big Bang was simply the result of some earlier event.

Absul Gaffar Roni: Science Writer