I've been thinking how to make Big Bang series of posts, but I still didn't figure out. So this might be a bit chaotic, but will give my best to introduce you to the subject. What is important to realize is that I do not plan to talk solely about Big Bang itself.  Instead, this will cover vast majority of subjects in other posts like creation and classification large scale cosmic structures, galaxies, solar systems, planets and life itself. Within each of this I will try to focus on our own neighborhood so when talking about galaxies I will also focus on Milky Way good part of the article. When talking about solar systems I will focus on ours and most likely split this into several posts explaining planets of our solar system. This initial post will cover our current understanding of first moments (referred as very early universe sometimes) and timeline as per Big Bang model. Also, posts in this series won't be regular as research on this subject takes more time.


When we look around there is hard to imagine there are things we do not know how they become to exist. I'm now on the bed writing this. This bed was assembled by as I bought it in Ikea. Ikea made in factory. Parts were done by wood and metal. Metal was made in factory and wood has been obtained from trees and later on manipulated in factory and so on. If we take a individual components and try to establish their timeline, we soon or later end up with questions regarding beginning of time or matter. While we are far away from complete theory, we do have one which seems to match most of observations and models made so far.  It is called - Big Bang. While Big Bang does not explain how life emerged on Earth, it does explain how Universe started and what conditions where there throughout the timeline. Big Bang does not say why this happened as that's outside the scope of the theory. Big Bang does not say why are you reading this now. Big Bang explains the early development of the Universe only! Everything else is outside the scope of theory as Big Bang theory never tried nor pretended to give any other answers. So when someone says "Yeah, but BB does not explain how life started" that's just as it would say "Yeah, but neuroscience does not explain economics". That's true. And what's your point? There are certain unknowns this theory too and as I said - it is not complete, but it is our best effort estimate we have today.



The Big Bang is a well-tested scientific theory which is widely accepted within the scientific community because it is the most accurate and comprehensive explanation for the full range of phenomena astronomers observe. Since its conception, abundant evidence has arisen to further validate the model. Georges Lemaître first proposed what would become the Big Bang theory in what he called his "hypothesis of the primeval atom". Over time, scientists would build on his initial ideas to form the modern synthesis. The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions (such as homogeneity and isotropy of space). The governing equations had been formulated by Alexander Friedmann. In 1929 Edwin Hubble discovered that the distances to far away galaxies were generally proportional to their redshifts - an idea originally suggested by Lemaître in 1927. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.  If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures, and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of this model. On the other hand, these accelerators have limited capabilities to probe into such high energy regimes. There is little evidence regarding the absolute earliest instant of the expansion. Thus, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the universe going forward from that point on. The observed abundances of the light elements throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.


An important feature of the Big Bang spacetime is the presence of horizons. Since the Universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. Our understanding of the Universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the Universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the Universe continues to accelerate, there is a future horizon as well.


Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast. It is popularly reported that Hoyle, who favored an alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models. After the discovery of the cosmic microwave background radiation in 1964, and especially when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiation from a black body, most scientists were fairly convinced by the evidence that some version of the Big Bang scenario must have occurred. Now, let's see now what do we know.



According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, this original state existed approximately 13.7 billion years ago, which is considered the age of the Universe and the time the Big Bang occurred. After its initial expansion from something we call singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles. It would take thousands of years for some of these particles (protons, neutrons, and electrons) to combine and form atoms, the building blocks of matter. The first element produced was hydrogen, along with traces of helium and lithium. Eventually, clouds of hydrogen would coalesce through gravity to form stars, and the heavier elements would be synthesized either within stars or during supernovae.


Most common thing people asking is - what was there before Big Bang?  Before Big Bang?  Time did not existed thus question does not make sense. There is no before - only after. Whether we live in cyclic model of universe and from that point there is before at another scale it is speculative theory. Right now, we can say that time as we know it in our current universe started with Big Bang. But you are absolutely right when thinking there might be a possibility that before Big Bang there was something. Perhaps another Universe with cyclic nature or perhaps it resets itself after membranes collide. This might all be correct, but it is important to understand this has no crucial effect on what is Big Bang telling us.  It is more sort of what caused it or what was there before in some other time. And while on this subject, this is not strange idea and new idea (see one idea here and its critic analysis here and here). Big Bang is big enough without additional complications.


OK, now that that is clear that Big Bang does not provide a theory of cosmic origins (common misconception) let us dive into it. To go through Big Bang, we will divide it in few time segments as identified by scientists so far and explain each. Actually you see some of those on previous picture. For better understanding imagine you can step out of our 4 dimensions (3 spatial and 1 time) and imagine universe does not exist. Then it starts. Universe is born and Big Bang starts. Time starts from 0. The image of the big bang as a cosmic explosion ejecting the material contents of the universe like shrapnel from an exploding bomb is a useful one to bear in mind, but it is a little misleading. When a bomb explodes, it does so at a particular location in space and at a particular moment in time. Its contents are ejected into the surrounding space. In the big bang, there is no surrounding space. As we devolve the universe backward toward the beginning, the squeezing together of all material content occurs because all of space is shrinking. The orange-size, the pea-size, the grain of sand-size devolution describes the whole of the universe - not something within the universe. Carrying on to the beginning, there is simply no space outside the primordial pinpoint grenade. Instead, the big bang is the eruption of compressed space whose unfurling, like a tidal wave, carries along matter and energy even to this day. The frequent picture people seem to have is matter flying outwards from a single point (like an explosion). However, the matter is all actually standing still while space itself expands dragging the matter with it. The general analogy for this is having a series of paperclips on a rubber band. As the rubber band is stretched, the paperclips appear to move away from one another even though they are in fact holding still with regard to the rubber band. Similarly, galaxies hold still more or less (there are small movements due to gravitational interactions) while they are carried by the expanding universe. So again, there was no "explosion" but instead, an expansion which is carrying all the rest of the universe away from us, but due to its nature we call it explosion. Twisted, huh?


When things get very hot or very cold, they sometimes change. And sometimes the change is so pronounced that you can't even recognize the things with which you began. Because of the torrid conditions just after the bang, and the subsequent rapid drop in temperature as space expanded and cooled, understanding the effects of temperature change is crucial in grappling with the early history of the universe. This is why we will have temperature indication for each of the phase.




By looking out into the universe with their most powerful telescopes, astronomers can see light that was emitted from galaxies and quasars just a few billion years after the big bang. This allows them to verify the expansion of the universe predicted by the big bang theory back to this early phase of the universe, and everything checks out to a "T". To test the theory to yet earlier times, physicists and astronomers must make use of more indirect methods. One of the most refined approaches involves something known as cosmic background radiation (see above picture).


If you've ever felt a bicycle tire after vigorously pumping it full of air, you know that it is warm to the touch. This is because when things are compressed they heat up - this is the principle, for example, behind pressure cookers, in which air is tightly compressed within a sealed pot in order for unusually high cooking temperatures to be readily achieved. The reverse is also true: When pressure is released and things are allowed to expand, they cool. If you remove the lid on a pressure cooker - or, more spectacularly, should it blow off - the air it contains will expand to its ordinary density while cooling to standard room temperature. This is the science underlying the phrase "blow off steam," a familiar approach to "cool down" a heated situation. It turns out that these simple terrestrial observations have a profound incarnation within the cosmos. Just as air in the pressure cooker cools down when the lid is removed and it is allowed to expand, the same is true for the "gas" of photons streaming through the universe as it expands. In the 1950s and 1960s, scientists realized that the present-day universe should be permeated by an almost uniform bath of these primordial photons, which, through the last 13.7 billion years of cosmic expansion, have cooled to a mere handful of degrees above absolute zero. Physicists and astronomers have confirmed to high precision that the universe is filled with microwave radiation (if our eyes were sensitive to microwaves, we would see a diffuse glow in the world around us) whose temperature is about 2.7 degrees above absolute zero, exactly in keeping with the expectation of the big bang theory. In concrete terms, in every cubic meter of the universe - including the one you now occupy - there are, on average, about 400 million photons that collectively compose the vast cosmic sea of microwave radiation, an echo of creation. A percentage of the "snow" you see on your old TV screen when you disconnect the cable feed and tune to a station that has ceased its scheduled broadcasts is, in fact, due to this dim aftermath of the big bang. This match between theory and experiment confirms the big bang picture of cosmology as far back as the time that photons first moved freely through the universe, about a few hundred thousand years after the bang. You might say few hundred years is a lot. Can we push further to even earlier times?


We can. By using standard principles of nuclear theory and thermodynamics, physicists can make definite predictions about the relative abundance of the light elements produced during the period of primordial nucleosynthesis, between a hundredth of a second and a few minutes after the Big Bang. According to theory, for example, about 23% of the universe should be composed of helium. By measuring the helium abundance in stars and nebulae, astronomers have amassed impressive support that, indeed, this prediction is right on the mark. Perhaps even more impressive is the prediction and confirmation regarding deuterium abundance, since there is essentially no astrophysical process, other than the big bang, that can account for its small but definite presence throughout the cosmos. The confirmation of these abundances, and more recently that of lithium, is a sensitive test of our understanding of early universe physics back to the time of their primordial synthesis. All the data we possess confirm a theory of cosmology capable of describing the universe from about a hundredth of a second after Big Bang to the present, some 13.7 billion years later. Nevertheless, one should not lose sight of the fact that the newborn universe evolved with phenomenal haste. Tiny fractions of a second - fractions much smaller than a hundredth of a second - form cosmic epochs during which long-lasting features of the world were first imprinted. And so, physicists have continued to push onward, trying to explain the universe at ever earlier times. Since the universe gets ever smaller, hotter, and denser as we push back, an accurate quantum-mechanical description of matter and the forces becomes increasingly important. Point-particle quantum field theory works until typical particle energies are around the Planck energy. In a cosmological context, this occurred when the whole of the known universe fit within a Planck-sized nugget, yielding a density so great that it strains one's ability to find a fitting metaphor or an enlightening analogy: the density of the universe at the Planck time was simply colossal.


Very early time of Universe we can divide in following sections:

  • Planck epoch
  • Grand unification epoch
  • Inflationary epoch
  • Electroweak epoch
  • Baryogenesis


As you will read, very early universe gives us a foundation for what universe is today; creation of forces, broken symmetry and particle zoo.





Planck Epoch

What you might heard of is inflation. Inflation explains rapid expansion of universe - you can find some details on it in previous post when I was writing on topic of multiverse. Anyway, inflation is not with what Universe (or Big Bang if you want) starts. Planck epoch (up to 10-43 seconds) is an era in traditional (non-inflationary) big bang cosmology in which the temperature is high enough that the four fundamental forces - electromagnetism, gravitation, weak nuclear interaction, and strong nuclear interaction - are all unified in one fundamental force. Little is understood about physics at this temperature, and different theories propose different scenarios. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory is based on general relativity and is expected to break down due to quantum effects. Physicists hope that proposed theories of quantum gravitation, such as string theory, loop quantum gravity, and causal sets, will eventually lead to a better understanding of this epoch. At this point, the universe spans a region of only 10-35 meters (1 Planck Length), and has a temperature of over 1032 °C (the Planck Temperature).


In inflationary cosmology, times prior to the end of inflation (roughly 10−32 seconds after the Big Bang) do not follow the traditional big bang timeline. The universe before the end of inflation is a near-vacuum with a very low temperature, and persists for much longer than 10−32 second. Times from the end of inflation are based on the big bang time of the non-inflationary big bang model, not on the actual age of the universe at that time, which cannot be determined in inflationary cosmology. Thus, in inflationary cosmology there is no Planck epoch in the traditional sense, though similar conditions may have prevailed in a pre-inflationary era of the universe.


Experimental data casting light on this cosmological epoch has been scant or non-existent until now, but recent results from the WMAP probe have allowed scientists to test hypotheses about the universe's first trillionth of a second (although the cosmic microwave background radiation observed by WMAP originated when the universe was already several hundred thousand years old). Although this interval is still orders of magnitude longer than the Planck time, other experiments currently coming online including the Planck Surveyor probe, promise to push back our "cosmic clock" further to reveal quite a bit more about the very first moments of our universe's history, hopefully giving us some insight into the Planck epoch itself. Data from particle accelerators provides meaningful insight into the early universe as well. Experiments with the Relativistic Heavy Ion Collider (RHIC) have allowed physicists to determine that the quark-gluon plasma (an early phase of matter) behaved more like a liquid than a gas, and the Large Hadron Collider (LHC) at CERN will probe still earlier phases of matter, but no accelerator (current or planned) will be capable of probing the Planck scale directly.


Grand Unification Epoch

This phase happens between 10–43 seconds and 10–36 seconds after the Big Bang. As the universe expands and cools, it crosses transition temperatures at which forces separate from each other. These are phase transitions much like condensation and freezing. The grand unification epoch begins when gravitation separates from the other forces of nature, which are collectively known as gauge forces (at the end of Planck epoch). The non-gravitational physics in this epoch would be described by a so-called grand unified theory (GUT). The grand unification epoch ends when the GUT forces further separate into the strong and electroweak forces. This transition should produce magnetic monopoles in large quantities, which are not observed. The lack of magnetic monopoles was one problem solved by the introduction of inflation. At this time the earliest elementary particles (and antiparticles) begin to be created.


If the grand unification energy is taken to be 1015 GeV, this corresponds to temperatures higher than 1027 K. During this period, three of the four fundamental interactions - electromagnetism, the strong interaction, and the weak interaction - were unified as the electronuclear force. During the grand unification epoch, physical characteristics such as mass, charge, flavour and colour charge were meaningless.


By the end of this epoch several key events took place. The strong force separated from the other fundamental forces. The temperature fell below the threshold at which X and Y bosons could be created, and the remaining X and Y bosons decayed (these are hypothetical elementary particles analogous to the W and Z bosons, but corresponding to a new type of force predicted by the Georgi-Glashow model, a grand unified theory). It is possible that some part of this decay process violated the conservation of baryon number and gave rise to a small excess of matter over antimatter. This phase transition is also thought to have triggered the process of cosmic inflation that dominated the development of the universe during the following inflationary epoch.


In modern inflationary cosmology, the traditional grand unification epoch, like the Planck epoch, does not exist, though similar conditions likely would have existed in the universe prior to inflation.


Inflationary Epoch




Inflation epoch is something we do not know in which phase it started exactly (though we suspect end of grand unification epoch), but we suspect it ends between10–33 and 10–32 seconds after the Big Bang.


Cosmic inflation is an era of accelerating expansion produced by a hypothesized field called the inflaton, which would have properties similar to the Higgs field and dark energy. While decelerating expansion magnifies deviations from homogeneity, making the universe more chaotic, accelerating expansion makes the universe more homogeneous. A sufficiently long period of inflationary expansion in our past could explain the high degree of homogeneity that is observed in the universe today at large scales, even if the state of the universe before inflation was highly disordered.


As noted before, without this super-rapid expansion our universe would not have survived for more than a moment or two, in a condition of "time" at the beginning of everything that is really difficult to understand. As a direct consequence of this expansion, all of the observable universe originated in a small causally connected region. So everything an astronomer sees now through the many different ways of observing the universe begins in this almost unbelievable super-rapid expansion event. This rapid expansion increased the linear dimensions of the early universe by a factor of at least 1026 (and possibly a much larger factor), and so increased its volume by a factor of at least 1078. The expansion is thought to have been triggered by the phase transition that marked the end of the preceding grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field. As this field settled into its lowest energy state throughout the universe, it generated a repulsive force that led to a rapid expansion of the fabric of space-time. This expansion explains various properties of the current universe that are difficult to account for without such an inflationary epoch.


Inflation ends when the inflaton field decays into ordinary particles in a process called "reheating", at which point ordinary Big Bang expansion begins. The time of reheating is usually quoted as a time "after the Big Bang" or simply ATB. This refers to the time that would have passed in traditional (non-inflationary) cosmology between the Big Bang singularity and the universe dropping to the same temperature that was produced by reheating, even though, in inflationary cosmology, the traditional Big Bang did not occur.


For cosmologists inflation answers the classic puzzle of the big bang cosmology: why does the universe appear flat (that is the geometry of space we are used to), homogeneous (or is the same everywhere) and isotropic (which means the same in all directions) and so fits with all the cosmological ideas a big bang scientist would expect if you have this inflation event. On the basis of the physics of the big bang, and without this inflation they would expect to see a highly curved, heterogeneous universe. So inflation is absolutely necessary to explain the way things appear now, even though it still full of mystery. Inflation also explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the universe, like galaxy formation.


The rapid expansion of space meant that elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe. However, the huge potential energy of the inflation field was released at the end of the inflationary epoch, repopulating the universe with a dense, hot mixture of quarks, anti-quarks and gluons as it entered the electroweak epoch.


While the detailed particle physics mechanism responsible for inflation is not known, the basic picture makes a number of predictions that have been confirmed by observation.




Electroweak Epoch

This phase happens between 10–36 seconds and 10–12 seconds after the Big Bang. In traditional big bang cosmology, the Electroweak epoch begins 10–36seconds after the Big Bang, when the temperature of the universe is low enough (1028 K) to separate the strong force from the electroweak force (the name for the unified forces of electromagnetism and the weak interaction).


Some cosmologists place this event at the start of the inflationary epoch, others place it at approximately 10–32 seconds after the Big Bang when the potential energy of the inflaton field that had driven the inflation of the universe during the inflationary epoch was released, filling the universe with a dense, hot quark-gluon plasma. Particle interactions in this phase were energetic enough to create large numbers of exotic particles, including W and Z bosons and Higgs bosons. As the universe expanded and cooled, interactions became less energetic and when the universe was about 10–12 seconds old, W and Z bosons ceased to be created. The remaining W and Z bosons decayed quickly, and the weak interaction became a short-range force in the following quark epoch.


The physics of the electroweak epoch is less speculative and much better understood than the physics of previous periods of the early universe. The existence of W and Z bosons has been demonstrated, and other predictions of electroweak theory have been experimentally verified.



The Dirac equation, formulated by Paul Dirac around 1928 as part of the development of relativistic quantum mechanics, predicts the existence of antiparticles along with the expected solutions for the corresponding particles. Since that time, it has been verified experimentally that every known kind of particle has a corresponding antiparticle. The CPT Theorem guarantees that a particle and its antiparticle have exactly the same mass and lifetime, and exactly opposite charge. Given this symmetry, it is puzzling that the universe does not have equal amounts of matter and antimatter. Indeed, there is no experimental evidence that there are any significant concentrations of antimatter in the observable universe.


There are two main interpretations for this disparity: either the universe began with a small preference for matter (total baryonic number of the universe different from zero), or the universe was originally perfectly symmetric, but somehow a set of phenomena contributed to a small imbalance in favour of matter over time. The second point of view is preferred, although there is no clear experimental evidence indicating either of them to be the correct one. The preference is based on the following point of view: if the universe encompasses everything (time, space, and matter), nothing exists outside of it and therefore nothing existed before it, leading to a total baryonic number of 0. From a more scientific point of view, there are reasons to expect that any initial asymmetry would be wiped out to zero during the early history of the universe. One challenge then is to explain how the total baryonic number is not conserved. The excess of baryons over antibaryons in the present universe is thought to be due to non-conservation of baryon number in the very early universe, though this is not well understood. While particle physics suggests asymmetries under which these conditions are met, these asymmetries are too small empirically to account for the observed baryon-antibaryon asymmetry of the universe. However, recent work might suggest we have to deal with physics beyond the current Standard model.



Credits: Wikipedia, arXiv, Brian Greene, Lubos Motl