Asking how the universe was created is one of those questions, that will probably never be answered completely. It is not one question, but a never-ending series of questions, detailing the evolution of complexity in the universe at every level. How does one even try to explain the process of how the universe has developed to the point, where beings, who question its very origin, have evolved? It is a task beyond the prowess of any mind.
Still, there are some brave souls who have tried to answer this question, in a very small measure, armed with the scientific method of '
'. As Albert Einstein has said, the universe is comprehensible, but only in one small part at a time, to an analytical and patient mind. The reason and hope for its comprehensibility lies in the fact that it evolves according to certain fundamental physical laws, which are unchangeable and eternal.
Anatomy of a Solution
This is a short introduction to things that need to be understood before we can discuss the Big Bang theory. Asking a question about the creation of the universe is easy, but thinking about a solution is certainly not. People with the patience and tenacity to look for an answer, should ask such questions. Before I tell you about Big bang theory, which is the most convincing theory yet, that can describe the evolution of the universe, let us analyze the anatomy of a solution to the problem.
To solve the biggest puzzle of all times, which is the origin of the universe, one has to go about systematically. There are three steps by which one can arrive at an answer.
Step 1: Understand the Basic Physical Laws that Govern the Universe
The universe evolved only once. There is no way we can observe it evolving again from the beginning, to understand its creation. So, we have no alternative, other than investigating its origin indirectly. One clue that can help us is the fact that physical laws which govern dynamics of the universe, are still the same, that is the laws do not change with time. They are the same since the beginning (So we believe. The day physical laws start changing with time, physics will be history.
). So, to know how the universe evolved, we must know the laws that govern its dynamics.
We have a head start here. In just two past centuries, we have figured out what are the four fundamental forces of nature, which are:
- Gravitation: The general theory of relativity, interprets gravity as not a force, but effect of curvature of spacetime itself. Einstein proved that gravity is not a force at all. It is the curvature of spacetime. To describe general relativity in short, 'Matter tells space how to bend and space tells matter how to move'. The range of gravity is infinite as it is a property of spacetime itself. Gravity is a weak force, compared to rest of the three forces. However, the evolution of the universe is ultimately dictated by gravity.
- Electromagnetic Force: This is an infinite range force, which governs almost all changes in nature. Electromagnetic force is what binds atoms into molecules and makes life possible. This force is mediated by photons. Electromagnetic radiation provides an important relic or evidence about origin of the universe, which will be discussed later.
- Strong Force: This is a very short-range force, which binds the basic building blocks of baryonic matter (quarks) together. This force is mediated by gluons.
- Weak Force: The weak force is responsible for the phenomenon of radioactivity. This force is mediated by three types of heavy vector bosons (W+, W-, Z). All bosons are force-carrying particles. The weak and electromagnetic forces have been shown to be unified at large energy scales, into a single force, that is electroweak force.
All matter content around us is now known to be made up of 6 quarks and 6 leptons, along with their antiparticles. They are all bound together by the four forces mentioned above. (Antiparticles are particles with same mass as normal particles, but opposite charge. The antiparticle of the electron is the positron, which is an electron with a positive charge. When a particle and antiparticle are brought together, they annihilate to release pure energy.) So, we know the players on the stage of the universe and we know the interactions between them.
Knowing the fundamental forces very well, doesn't make our problem any easier. Just as knowing the rules of chess, does not make you a Kasparov, similarly, knowing fundamental physical laws does not mean that we understand everything. These basic laws applied to gargantuan collections of particles show even more complex behavior.
Step 2: Observe the Current State of the Universe
Next step is observing the current state of the universe, in as much details as possible. To know what the universe was before, we need to understand what it is now. This is essential, if we want to construct a coherent picture of its past. This includes developing tools like telescopes to gather information through various bands of electromagnetic radiation (Visible, X-rays, Gamma, Radio, and Infrared). Calculating distances of objects like stars and galaxies, as well as knowing their composition is an important part of this exercise.
Step 3: Set the Clock Back in Time (Extrapolate Backwards and Simulate)
Once we know the current state of the universe in vivid detail and dynamical laws that change it, we can quantitatively model and reconstruct its past. That is, we set up differential equations that relate the dynamic quantities in the universe (including space, time, matter, and radiation) and turn them back in time, to model the past. Ideally, we would like to come up with a theory that explains the origin of the universe, from first principles. However, that is not possible, as we face theoretical limitations and complexity. So, we assume a set of initial conditions of the universe and run simulations on computers, that will create the kind of universe we live in.
This is what is studied in cosmology, which is a branch of physics, based on general theory of relativity. Cosmologists work with what is called parametrized cosmology. It creates a library of possible universes, which could be created from a given initial set of conditions. Of course, the backbone of such a simulation, is a model of our universe based on general theory of relativity, called the hot big bang model. Now, let us get to the brass tacks and confront our question and see how big bang theory answers it.
Our Best Answer Yet - The Big Bang Theory
Einstein's 'General Theory of Relativity
', which interpreted gravity as the curvature of spacetime, opened up the new science of 'Cosmology'. For the first time, we had a framework, wherein the history and fate of the universe could be analyzed theoretically. Based on the assumption that the universe is isotropic and homogeneous, Alexander Friedman developed the governing equations of what is known as cosmology. It was Georges Lemaitre, who came up with the 'Big Bang' theory (which was a pejorative name given to it much later, by one of its strongest critics, Fred Hoyle). Lemaitre called it, 'the theory of primeval atom'.
Big Bang - The Idea
The big bang idea essentially goes this way:
- It was observed by earlier astronomers and proved by Edwin Hubble that neighboring galaxies of our Milky way galaxy, are receding away. The more distant they were, the faster they were found to be moving away from us. That is, recession velocity of a galaxy is directly proportional to its distance from us. This is called Hubble's law.
- This moving away of galaxies from each other, is due to the expansion of space itself. Imagine two red dots, drawn on a deflated balloon. Now, as the balloon is inflated, the distance between the two dots will go on increasing. This is not because the two red dots are moving on the balloon surface, but because the fabric of the balloon itself, is expanding. Same thing happens in case of galaxies. Spacetime expansion itself leads to a directly proportional relation between recessional velocity and galactic distance.
- Now, if we reverse the expansion scenario, back in time, we see that if galaxies are moving rapidly apart now, then they must have been clumped together in the past. That is, the universe must have been denser (with more matter and energy packed in a unit volume) in the past.
- If you keep going back in time, the whole universe will converge to a point of infinite density and super-high temperature. This is the starting point of big bang. The universe started expanding out of an infinitely dense initial point. The essential idea to understand here is that big bang did not happen somewhere in the universe. The universe itself was an infinitely dense point, a singularity, which expanded to its present size.
The Big Bang TimelineIllustration of creation and expansion of the universe. Credit: NASA
Let us understand how the universe was created, according to the big bang theory. Universe began from a singularity. At a singularity, infinite density created infinite curvature, due to which the classical laws of general relativity, rather, all known laws of physics, break down. Therefore, what happened before the big bang is something which nobody can answer.
The age of the universe, according to the theory, is estimated to be 13.73 (± 0.12) billion years. Taking the Big Bang as the starting point of time, here I provide a brief timeline of successive events, that led to the creation of the universe (as we know it today).
The Planck Era: 0 to 10-43 second
The universe started expanding from a size, which was lesser than the size of an electron. The period from the big bang to about 10-43
second after it, is called the Planck Era
. In this period, the curvature, the energy, and the density was so high that all laws of physics, the general theory of relativity, or relativistic quantum mechanics fail to describe it. According to a still invalidated theory of supersymmetry, all four fundamental forces would have been united into a single force in this period. Gravity would be as powerful as all other forces at this time. All this, of course, is still a matter of speculation and mathematical fiction. To understand what happened in this era, we need a theory of quantum gravity, which is still a distant dream. So in the Planck Era, which is, theoretically, the shortest possible slice of time that you could have, what happened, is still a mystery as we do not have the theoretical mechanism to analyze what happened.
Grand Unification Era: Between 10-43 second to 10-36 second
During the Grand Unification Era, the temperature of the universe must have been of the order of 1027
K. This corresponds to an energy greater than 1019
GeV, which is the threshold for grand unified theories. These theories are, as of now, hypothetical theories that propound that at very high energies, the four forces of nature are combined into one supreme force. According to this theory, during this phase, gravity separated from the rest of the unified forces, due to symmetry breaking. The universe was dominated by gravity and a unified force (Strong and Electroweak force, combined into one.), dominated it. The Higgs boson is a particle which is responsible for particles gaining mass. The Higgs field pervades all of space and time and particles acquire mass due to their interaction with this particle. The Higgs boson is supposed to be the only particle that existed at this point.
What happened during this era, is unverified as yet, because terrestrial particle accelerators haven't advanced enough to create energies beyond 1019
GeV. During this period, the physical attributes of mass, charge, color, or flavor were meaningless, due to the very high energies involved.
The Inflationary - Electroweak Era: 10-36second to 10-12 second
The story of what happened in this phase is also unverified, because particle accelerators have not been able to recreate an energy scale of this order. So, what follows is a hypothesis. During this period, the temperature dropped below 1028
K, the symmetry, that was holding the strong force and electroweak force together, was broken and they were separated, becoming two distinct forces.
The phase transition that separated the strong force from the electroweak force triggered a phase of exponential inflation. That is, the universe rapidly grew, during this phase and its expansion was driven by an inflaton field. This was a scalar field, that generated a huge repulsive force, which triggered the exponential expansion of the very fabric of spacetime.
The interval between 10-36
second, is known as the inflationary era. The linear dimensions of the universe got magnified, almost 1026
times the original size. The universe became roomier, as its volume increased to almost 1078
times its present size, in a hundredth fraction of a second. That is what you call exponential growth.
The universe became smoother, roomier, and started tending towards isotropy and homogeneity. The temperature of the universe was not uniform during this phase and seeds of structure formation (structures like galaxies
), were sown by this initial inhomogeneity. The temperature fluctuations and inhomogeneities remained scale invariant, as the universe inflated rapidly. Due to this, minuscule fluctuations were magnified to larger size and ultimately, they were responsible for creating galactic structures. During this phase, huge numbers of exotic particles (all bosons), like W,Z and Higgs particles, were also created.
At about 10-32
second after the Big Bang, inflation ceased. The tremendous potential energy of the scalar inflaton field was released, creating a hot and dense relativistic plasma of quarks, anti-quarks, gluons, electrons, and neutrinos. Matter and energy are interconvertible. It was a hot dense soup of particles.
The question that remains unanswered is that why is there more matter than antimatter in the universe. It is called the problem of 'baryogenesis'. The answer lies in this period, where the first quarks and anti-quarks were formed.
At the end of the Electroweak Era, as the universe expanded, the temperature dropped down further, leading to the breaking of the electroweak symmetry. Thereafter, electroweak force split into two forces, which are the electromagnetic force, with infinite range and weak force, with a finite range.
Quark Era: 10-12 to 10-6 second
After the end of the electroweak era, the Quark Era began. In this phase, all the four forces had begun operating in an independent form. The universe was filled with a quark-gluon soup (plasma), in which quarks actually existed in free state. This particle soup contained quarks, leptons (electrons, muons, tauons, neutrinos, and their antiparticles, are collectively called leptons
) and their antiparticles. The universe was too hot for quarks to bind together and form protons, neutrons, or other hadrons (Hadrons are particles that interact via the strong force).
Hadron Era: 10-6 second to 1 second
In this period of a fraction of a second, the universe had cooled just enough, for quarks and their antiparticles to fuse together and form the first hadrons and anti-hadrons (Protons, Neutrons etc
). When matter and antimatter particles come into contact, they annihilate to form pure energy. So, as the universe cooled further, the hadron production seized and the hadron and anti-hadron pairs annihilated each other, leaving a few hadrons behind. The neutrinos were decoupled, that is freed from interaction with the hadrons and since then, they have traveled unhindered at the speed of light, rarely interacting with matter. All this happened within one second after the Big Bang. So much can happen in a second, as you can see. A universe is created within a second.
Lepton Era: 1 second to 10 seconds
A mere span of 10 seconds seems hardly enough to be called an era. However, due to the immense importance of what happened during these first few moments, it deserves to be called an era. After hadron annihilation, leptons dominated the matter content in the universe. As the universe cooled even further, lepton production stopped. Like the hadrons, leptons and anti-leptons too annihilated each other, leaving a few extra leptons behind. This is because of the inherent matter-antimatter asymmetry in the baryogenesis phase. This process of nucleosynthesis was sustained for only 17 minutes, as after that, the temperature and density of the universe fell below the critical values, that are necessary for fusion.
Photon Era: Begins 10 seconds after the Big Bang
At the end of the Lepton Era, photons dominated energy content of the universe for up to 300,000 years. Electromagnetic radiation was; however, coupled to all the charged hadrons and leptons.
Nucleosynthesis: 3 minutes to 20 minutes
Three minutes after the big bang, the universe had cooled down enough, for the protons and neutrons to bind together, to form the first nuclei in the universe. Through the process of nuclear fusion, protons, and neutrons which bumped into each other, fused together to form the very first nuclei.
In a matter of 17 minutes, all the nuclei in the universe were created. The heavier elements were synthesized from these first light elements, long afterwards, through nuclear fusion in stars. The elements that were created in the process were as follows:
- Deuterium (H-2, An isotope of Hydrogen with one proton and one neutron in the nucleus)
- Two isotopes of Helium (He-3, He-4)
- Lithium isotopes (Li-6, Li-7)
- Radioactive isotopes like Tritium (H-3), Beryllium (Be-7, Be-8) (However, they decayed into stable elements)
Of course, protons themselves are Hydrogen nuclei, which to this day, is the most abundant element in the universe. Helium-4 is also one of the most abundant elements (25% by ratio) that formed in the primordial nucleosynthesis. In fact, the abundance of helium-4 in the universe, is a strong evidence, which validates the big bang theory. The matching between observed and predicted nuclear abundance is one of the major evidences that backs the big bang hypothesis, turning it into a 'theory'.
A Dark Age: Till 377,000 years after Big Bang
For 377,000 years after the nucleosynthesis, the universe kept expanding and cooling further. After the initial tumultuous hour, post big bang, things were quiet for next 377,000 years. The universe hadn't cooled enough for the light element nuclei to form atoms and radiation was still coupled to ionic nuclei. Therefore, during this period, universe was opaque to electromagnetic radiation. Hence, this post-nucleosynthesis period of 377,000 years is known as the dark age.
The only radiation emitted during this period was the 21 cm spin line of Hydrogen, which falls in the radio band of the electromagnetic spectrum.
Meanwhile, gravity was at work, of course, clumping together matter and creating aggregates. The primordial temperature fluctuations were amplified into regions differing in density and temperatures. To put it in simple words, the rich got richer and the poor got poorer. That is, the hot regions, got hotter and denser with time and the cold regions got colder and rarer. However, these fluctuations were still comparatively small. They set up the stage for future structure formation of galaxies.
After 377,000 years, things got moving again, as the universe had cooled just enough, for the Hydrogen, Helium, and Deuteron nuclei to form neutral atoms. This meant that the primordial radiation, which was till then encumbered and coupled with ions, was decoupled and freed. Electrons got captured and bound to nuclei in the process of recombination. The universe became transparent to radiation and it freely propagated throughout the universe.
Cosmic Microwave Background Radiation
This wave of recombination swept the universe, freeing primordial radiation, which has been propagating through the universe since then, unhindered and untouched. This is known as cosmic microwave background radiation (CMBR), which pervades all space and has been recently detected and mapped. It is the premier and unarguable evidence for validity of the Big Bang theory. The map of this detected radiation is an exact image of the baby universe, as it was then.
Of course, the wavelength and frequency of this radiation is not same as it was during recombination. Imagine that the universe is a balloon and the CMBR is a wave shape drawn on it. As you go on inflating the balloon, the wavelength of the wave drawn on it, will go on increasing. This is not because the wavelength is changing, but because the balloon fabric itself is expanding. Similarly, wavelength of CMBR was stretched and elongated due to expansion of the very fabric of spacetime and today, it lies in the millimeter radio band. It was accidentally discovered by scientists working on a radio communication antenna.
In recent years, COBE (Cosmic Microwave Background Explorer) and WMAP (Wilkinson Microwave Anisotropy Probe ) have given us clear images of the baby universe, as it was then. The CMBR map is a gold mine of data for the cosmologists. It has changed cosmology, from being a purely theoretical branch of mathematical physics, to a quantitative and precisely validated science.
Reionization: 150 million to 1 billion years
After recombination, the universe kept expanding and gravity was quietly doing its job. Large clouds of gas were created through aggregation, which condensed and collapsed under gravity, to form the first objects in the universe, the quasars (quasi-stellar objects
) and the first stars (Called Type III
). Quasars emit gargantuan amounts of energy, created due to heating of matter, falling into a massive black hole in its belly. The first stars also formed due to the collapse of smaller gas clouds under gravity. The first stars and quasars lit up the universe.
The energy emitted by quasars was absorbed by neutral matter in atomic form. This released the electrons bound to nuclei, reionizing the universe. Such was the energy output of these quasars, that most of the baryonic matter in the universe was ionized.
In stars, the very first nuclear reactors ignited, synthesizing heavier elements like Carbon, from the light elements that formed from the nucleosynthesis. Stars are the furnaces powered by nuclear fusion, where all the heavier elements, that make our world, were synthesized.
First Galaxies Form
There is evidence which shows that the first galaxies were created about 500 million years after the Big Bang. All this was, of course, the handiwork of gravity. The subject of how these large-scale structures formed is a matter of intense research today and I will not attempt to explain large-scale structure formation here, as it's a complicated subject, which would well take a book.
Galaxies are islands of matter in the vast desert of dark spacetime. These galaxies are gravitationally bound to each other and they form even larger structures called galaxy clusters. They are a huge collection of galaxies, forming a giant conglomerate of sorts. These galactic clusters go on to create superclusters. All galaxies move around in a gravitational tango, around the centers of these galactic superclusters. The Earth moves around the Sun, the Sun moves around the galactic center, and our galaxy moves around another galactic supercluster.
Our Solar System is Created: 9 Billion Years Later
As galaxies like our Milky way condensed under gravity, forming billions of stars in the process, heavy elements were created. As stars burned and died, the heavier elements were sprinkled in the solar system through nuclear fusion and supernovae. Universe recycles everything. From dead remnants of previous stars, seeded with heavier elements, newer stars formed.
In such a heavy element enriched region, five billion years ago (roughly 9 billion years after the Big Bang
), our Sun, a third-generation star, was born. From accretion disk of debris, around the Sun, our solar system was created. Then eventually, Earth, as a planet came into existence about 4.5 billion years ago. Life and intelligence, evolved as we know on Earth and possibly on many other such solar systems.
There is something known as dark matter, which does not emit any kind of electromagnetic radiation and the only thing that it responds to, is gravity. It plays a major role in the evolution of the universe, through structure formation. There are indirect evidences of its existence, but its nature is unknown.
The mystery of the millennium for astrophysicists today is dark energy. Cosmic microwave background radiation predicts that there is a dark energy, which drives expansion of the universe and with time, it's only going to accelerate it. Astrophysicists today, have no idea about the nature of this energy, which constitutes 70% of the energy content of the universe. The weird thing about it is that dark energy is unclustered and exerts negative pressure.
The pressure exerted by normal matter is directly proportional to density. More the density, more is the pressure. However, an entity exerts negative pressure, when with increasing density, the pressure decreases, while with decreased density, pressure increases. As the Universe expands, it becomes rarer (less dense), due to which, negative pressure exerted by dark energy, goes on increasing. So, the universal expansion will accelerate with time.
According to the cosmic microwave background radiation data, the stuff we are made of, which is baryonic matter, forms only 4% of the total energy content of the universe. Just when we think we know everything, the universe comes up with another riddle. The ultimate fate of the universe is a matter for future inquiry.
If you look at the whole process of the evolution of the universe, you will see that it is a series of increasingly subtler energy changes that have molded complexity at every level. Every thing or action in the universe is a manifestation of energy, or an energy change.
As you can see, story of evolution of the universe, according to Big Bang, is still a largely sketchy story with blank spaces, mysteries, and enigmas. The most amazing thing, throughout this whole journey, is the fact that, matter has evolved to a point of consciousness, where it questions its own origin. Matter has self-organized itself into intelligent life.
Someday, when we know how the universe was created, next question that comes to mind is, why was the universe created. What is the ultimate purpose? Why are we here? What next?
No one can answer these questions today but to have known whatever we know today, is no humble achievement. So far, so good. I conclude this article with the hope that I gave you an overview of the way things are and how they became that way. I end this article with a quote by Stephen Hawking, which I think, puts the whole thing in perspective.
"Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe? The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. Why does the universe go to all the bother of existing? Is the unified theory so compelling that it brings about its own existence? Or does it need a creator, and, if so, does he have any other effect on the universe? And who created him?"
- Stephen Hawking (Excerpt from the Brief History of Time)