Here is the second sequel to Big Bang series. First post deals with so called very early beginnings and covers phases from Planck Epoch to Baryogenesis. What is important to say (and repeat) is that Big Bang is not theory of how Universe begun - Big Bang is rather the theory how Universe evolved and using reverse engineering, experiments and pure theory based on mathematics we were able to set up foundation of very early evolutionary phase.  While we know little about very early phase discussed earlier, nowadays we are poking around to understand CP violations which have caused asymmetry between matter and antimatter. Without this symmetry breaking, space would be very boring place and most likely nothing as it is today. In this sequel, I will deal with what we call early universe which comes after cosmic inflation ends.  Now, the universe is filled with a quark-gluon plasma and from this point onwards the physics of the early universe is better understood and less speculative.



I already mentioned earlier states of matter, but this might be good moment to talk more about quark-gluon plasma. Today, we say universe consists of some 5% of matter. When you look at the picture above, you see that this distribution of matter makes us very small:

  • 4% free hydrogen and helium in space
  • 0.5% are stars
  • 0.3% are neutrinos
  • 0.03% are heavy elements


Yes, we are pretty much unimportant. Vast majority of mass/enery in universe goes to dark matter (some 25%) and dark energy (70%). We might have some new measure soon, but for now let's stick to this number (there are some small variations in ratio between two dark components, but they can be ignored in this story). So, when you go back in time, we start our story about Big Bang and Universe being so so so tiny that you can imagine it being size of an atom (though in very early universe it was smaller, but that also not important now). One thing which may come to your mind is: where is all this dark matter, dark energy and finally matter then?  It is compressed just like some super dense black hole (though that is not possible given the size we assumed in very early universe)? Scientists for very long time speculated that during early days universe was hot so state of universe was gas. But then, we found by running experiment that not to be truth. It was all about gluon-plasma. As for the universe itself, Einstein showed us that matter equals energy thus you can imagine very early moments of universe to be pure energy itself.



A quark-gluon plasma (QGP) or quark soup is a phase of quantum chromodynamics (QCD) which exists at extremely high temperature and/or density. This phase consists of asymptotically free quarks and gluons, which are several of the basic building blocks of matter. Experiments at CERN's Super Proton Synchrotron (SPS) first tried to create the QGP in the 1980s and 1990s: the results led CERN to announce indirect evidence for a "new state of matter" in 2000. Current experiments (2011) at the Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) at Long Island (NY, USA) and at CERN's recent LHC collider at Geneva are continuing this effort, by smashing relativistically accelerated gold ions (at LHC lead ions) into each other. Although the results have yet to be independently verified as of February 2010, scientists at Brookhaven RHIC have tentatively claimed to have created a quark-gluon plasma with an approximate temperature of 4 trillion degrees Celsius (see above picture). So, it turns around that if you’re interested in the properties of the microseconds-old universe, the best way to study it is not by building a telescope - it’s by building an accelerator. The resulting super-hot, super-dense blob of matter, about a trillionth of a centimeter across, could give scientists new insights into the properties of the very early universe. So far, they have already made the surprising discovery that QGP is a nearly frictionless liquid, not the gas that physicists had expected earlier.


With that in mind, let's see phases in which we can separate early universe phase:

  • Supersymmetry breaking
  • Quark epoch
  • Hadron epoch
  • Lepton epoch
  • Photon epoch


Let's take closer look at each of them.


Supersymmetry breaking

This part is speculative and we are still without confirmation of supersymmetry (SUSY) particles. If supersymmetry is a property of our universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak symmetry scale. The masses of particles and their superpartners would then no longer be equal, which could explain why no superpartners of known particles have ever been observed.  I already wrote about supersymmetry thus I won't do it here again.



Quark epoch

This epoch lasts from 10-12 seconds to 10-6 seconds since Big Bang. In electroweak symmetry breaking, at the end of the electroweak epoch, all the fundamental particles are believed to acquire a mass via the Higgs mechanism in which the Higgs boson acquires a vacuum expectation value. The fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction have now taken their present forms. Universe cools off to below 10 quadrillion degrees. During the quark epoch the universe was filled with a dense, hot quark-gluon plasma, containing quarks, leptons (electrons and neutrinos) and their antiparticles. Quarks and antiquarks annihilate each other upon contact, but due to baryogenesis, a surplus of quarks (about one for every billion pairs) survives, which will ultimately combine to form matter. This temperature of the universe is still too high to allow quarks to bind together to form hadrons (collisions are far too energetic for any of those). This epoch ends  when the average energy of particle interactions had fallen below the binding energy of hadrons. The following period, when quarks became confined within hadrons, is known as the hadron epoch.



Hadron epoch

This period takes place between 10–6 seconds and 1 second after the Big Bang. As you might remember, a hadron is a composite particle made of quarks held together by the strong force (as atoms and molecules are held together by the electromagnetic force). Hadrons are categorized into two families: baryons (made of three quarks) and mesons (made of one quark and one antiquark). In hadron epoch temperature of the universe cools to about a trillion degrees, cool enough to allow hadrons, including baryons such as protons and neutrons, to form (previous to that, due to thermal equilibriium, that was not possible). Electrons colliding with protons in the extreme conditions of the Hadron Epoch fuse to form neutrons and give off massless neutrinos, which continue to travel freely through space today, at or near to the speed of light. Some neutrons and neutrinos re-combine into new proton-electron pairs. The only rules governing all this apparently random combining and re-combining are that the overall charge and energy (including mass-energy) be conserved.  We believe that at approximately 1 second after the Big Bang neutrinos decoupled and begun traveling freely through space. This cosmic neutrino background, while unlikely to ever be observed in detail, is analogous to the cosmic microwave background (CMB) that was emitted much later. This is interesting part so I will spend some time on it.


Neutrinos hold the promise of providing a window that gives us views much deeper into the big bang than the window conventionally provided by photons. When the Hubble Space Telescope gives us a snapshot of a galaxy in a universe that is only 600 million years old, this feat is brought to the wider public as big news. Yet, the Hubble does not get anywhere near to exhausting the penetration depth of photons. The true capability of photons is provided by telescopes that observe at wavelengths much larger than that of visible light. The COBE, WMAP and Planck space telescopes all do so, and provide us with a view of the earliest universe accessible via photons: a universe that is only 380 thousand years old (see recombination below to understand why). With the universe at earlier times being opaque to light of any wavelength, we seem to have reached the limit of how deep we can probe into our past. On the other hand, if we find practical ways to detect ultra low energy neutrinos, we will be able to dive much deeper than ever before into the big bang. Neutrinos hold the promise of opening a window to a universe that is only second or two seconds old. Yet, the technical challenges are immense, and many believe mankind might never be able to observe directly such an extremely embryonic universe. However, we should keep in mind that neutrino astronomy is 25 years young and in its very infancy.



By the end of this era most of the hadrons and anti-hadrons were eliminated in annihilation reactions, leaving a small residue of hadrons. The elimination of anti-hadrons was completed by one second after the Big Bang, when the following lepton epoch began.


Lepton epoch

This phase takes place between 1 second and 10 seconds after the Big Bang. The majority (but not all) of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving leptons (such as electrons) and anti-leptons (such as positrons) dominating the mass of the universe. As electrons and positrons collide and annihilate each other, energy in the form of photons is freed up, and colliding photons in turn create more electron-positron pairs. Approximately 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton/anti-lepton pairs are no longer created and most leptons and anti-leptons are eliminated in annihilation reactions, leaving a small residue of leptons. This is very similar to previous epoch except it addresses process with different particle.



At the end of this epoch, the mass of the universe was is dominated by photons as it enters the following photon epoch.


Photon epoch

This epoch takes place between 10 seconds and 380000 years after the Big Bang. After most leptons and anti-leptons are annihilated at the end of the lepton epoch the energy of the universe is dominated by photons. These photons are still interacting frequently with charged protons, electrons and (eventually) nuclei, and continue to do so for the next 380000 years. This is very important, but soon you will see why. This epoch can also be divided to following sub-phases:

  • Nucleosynthesis
  • Matter domination
  • Recombination
  • Dark ages



Nucleosynthesis (or primordial nucleosynthesis, abbreviated BBN) happens between 3 minutes and 20 minutes after the Big Bang. During the photon epoch the temperature of the universe falls to the point where atomic nuclei can begin to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei in the process of nuclear fusion. As the universe expands, it cools. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. At the time at which nucleosynthesis occurs, the temperature is high enough for the mean energy per particle to be greater than the binding energy of deuterium; therefore any deuterium that is formed is immediately destroyed (a situation known as the deuterium bottleneck). Hence, the formation of helium-4 is delayed until the universe becomes cool enough to form deuterium (at about T = 0.1 MeV), when there is a sudden burst of element formation. Shortly thereafter, at twenty minutes after the Big Bang, the universe becomes too cool for any nuclear fusion to occur. At this point, the elemental abundances are fixed, and only change as some of the radioactive products of BBN (such as tritium) decay. Nucleosynthesis only lasts for about seventeen minutes, since the temperature and density of the universe has fallen to the point where nuclear fusion cannot continue. By this time, all neutrons have been incorporated into helium nuclei. This leaves about three times more hydrogen than helium-4 (by mass) and only trace quantities of other nuclei.



The key parameter which allows us to calculate the effects of BBN is the number of photons per baryon. This parameter corresponds to the temperature and density of the early universe and allows us to determine the conditions under which nuclear fusion occurs. From this we can derive elemental abundances. Although the baryon per photon ratio is important in determining elemental abundances, the precise value makes little difference to the overall picture. Without major changes to the Big Bang theory itself, BBN will result in mass abundances of about 75% of H-1, about 25% helium-4, about 0.01% of deuterium, trace (on the order of 10−10) amounts of lithium and beryllium, and no other heavy elements. Traces of boron have been found in some old stars, giving rise to the question that some boron, not really predicted by the theory, might have been produced in the Big Bang. The question is not presently yet. The observed abundances in the universe are generally consistent with these abundance numbers is considered strong evidence for the Big Bang theory. One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies are very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe.


Matter domination

This phase happens some 70000 years after Big Bang. At this time, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) are equal. The Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by free-streaming radiation, can begin to grow in amplitude.



ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy. It is frequently referred to as the standard model of big bang cosmology, since it attempts to explain:

  • the existence and structure of the cosmic microwave background
  • the large scale structure of galaxy clusters
  • the distribution of hydrogen, helium, deuterium and lithium
  • the accelerating expansion of the universe observed in the light from distant galaxies and supernovae


It is the simplest model that is in general agreement with observed phenomena; however, a small minority of astrophysicists have challenged the validity of the model. According to ΛCDM, at this stage, cold dark matter dominates, paving the way for gravitational collapse to amplify the tiny inhomogeneities left by cosmic inflation, making dense regions denser and rarefied regions more rarefied.


However, because present theories such as to the nature of dark matter are inconclusive, there is as yet no consensus as to its origin at earlier times (as currently exist for baryonic matter).



Hydrogen and helium atoms begin to form as the density of the universe falls. This is thought to have occurred about 377000 years after the Big Bang. Hydrogen and helium are at the beginning ionized, i.e., no electrons are bound to the nuclei, which (containing positively charged protons) are therefore electrically charged (+1 and +2 respectively). As the universe cools down (some 4000-3000K), the electrons get captured by the ions, forming electrically neutral atoms. This process is relatively fast (actually faster for the helium than for the hydrogen) and is known as recombination. At the end of recombination, most of the protons in the universe are bound up in neutral atoms. Therefore, the photons can now travel freely: the universe has become transparent. This cosmic event is usually referred to as decoupling. The photons present at the time of decoupling can now travel undisturbed (the photons' mean free path becomes effectively infinite) and are the same photons that we see in the cosmic microwave background (CMB) radiation, after being greatly cooled by the expansion of the Universe. Therefore the CMB is a picture of the universe at the end of this epoch including the tiny fluctuations generated during inflation (see picture below).




This detail is many times overlooked so let me try to explain it again. Before recombination happened, photons did not move freely!  Prior to recombination, photons were not able to freely travel through the universe, as they constantly scattered off the free electrons and protons. This scattering causes a loss of information, and there is therefore a photon barrier at a redshift near that of recombination that prevents us from using photons to directly learn about the universe at larger redshifts (this is why I spent more time talking about potential of neutrinos in hadron epoch above). As the temperature of the universe falls and its density also continues to fall, ionized hydrogen and helium atoms capture electrons (known as "recombination"), thus neutralizing their electric charge. With the electrons now bound to atoms, the universe finally becomes transparent to light, making this the earliest epoch observable today through CMB. It also releases the photons in the universe which have up till this time been interacting with electrons and protons in an opaque photon-baryon fluid (known as "decoupling"), and these photons (the same ones we see in today’s cosmic background radiation) can now travel freely. By the end of this period, the universe consists of a fog of about 75% hydrogen and 25% helium, with just traces of lithium.


Since most light was no longer scattered by atoms, the universe became transparent. One could have seen a long way, if there were anything to see. But there wasn't, because the universe had gone dark, and so it would remain until the first stars began to form, millions of years later.


Dark ages

Before decoupling occurs most of the photons in the universe are interacting with electrons and protons in the photon-baryon fluid. The universe is opaque or "foggy" as a result and we can't see through it even if we wanted to. There is light but not light we could observe through telescopes. The baryonic matter in the universe consisted of ionized plasma, and it only became neutral when it gained free electrons during "recombination" above, thereby releasing the photons creating the CMB. When the photons were released (or decoupled) the universe became transparent. At this point the only radiation emitted is the 21 cm spin line of neutral hydrogen. There is currently an observational effort underway to detect this faint radiation, as it is in principle an even more powerful tool than CMB for studying the early universe. The Dark Ages are currently thought to have lasted between 150 million to 800 million years after the Big Bang. Dark Ages can be easily thought as period when light could travel, but there was no stars formed yet.


The recent (October 2010) discovery of UDFy-38135539 (see below), the first observed galaxy to have existed during the following reionization epoch, gives us a window into these times. There was a report in January 2011 of yet another more than 13 billion years old that existed a mere 480 million years after the Big Bang which places duration of Dark Ages closer to lower value outlined above.



However, stars and galaxies are something belonging to next Big Bang sequel so I will stop here.



Related posts:

Big Bang: Dawn of Time


Credits: Wikipedia, Jonathan Allday, Jack Barnosky, Arizona University, Luke Mastin, NASA