Our Universe Part 8: Hadron Epoch

During this epoch matter as we know it is on its way toward existence. There are no atoms yet, no electrons or even nuclei, but  hadrons, which are protons and neutrons, are forming. However, because almost all physical processes appear to be symmetrical in nature, particles of antimatter also form. Equal amounts of antimatter should annihilate matter as it appears. But it didn't, as evidenced by everything around us today. Why?

The universe, just a fraction of a second old now, operates under the four fundamental laws as we know them today. It is an expanding bubble of energetic quarks and gluons, some of which are just now settling into particles called hadrons, as the temperature cools to about 10¹³ K.

Hadrons are composite particles made up of quarks held together by gluons (gluons carry the strong force). There are six kinds of quarks. They all have mass, they all interact with all four fundamental forces, and they all have an antiparticle twin, but only two kinds, up and down quarks, are (generally) stable and make up protons (below left) and neutrons (below right).

Up quarks have a charge of +2/3 while down quarks have a charge of -1/3. This gives a proton a charge of +1 and a neutron zero charge. The other four kinds (called flavours) of quarks (called strange, charm, bottom and top) are all very unstable and much more massive. They do not make up stable matter. Instead they decay into up and down quarks.

Not all hadrons make up matter as we know it. Hadrons come in two general kinds: baryons, made up of three quarks of which the proton and the neutron are examples, and mesons, made up of one quark and one antiquark. An antiquark is an antimatter twin of a quark, and we will explore what antimatter is in a moment. For now, just know that antimatter and matter particles annihilate each other on contact. You might think that the two particles in a meson would instantly annihilate each other, but instead they orbit each other like binary stars, with the strong force keeping them together. Mesons are very unstable, and they only exist (very briefly) in extremely high-enegy environments like this epoch. 

Baryons, on the other hand, are generally stable. The appearance of baryons marks the beginning of matter in the universe. The mass of the proton and the neutron tell us something interesting about mass/energy equivalence. Protons, for example, composed of two up quarks (3.1 MeV/c² each) and a down quark (5.7 MeV/c²), are far more massive (0.938 MeV/c²) than the sum of the masses of their quarks. The kinetic and potential energies of the quarks is converted into mass in the proton.

About seven protons will be created for every neutron because protons have a lower mass/energy and so their production is favoured. Neutrons bound inside atoms are stable but free neutrons, as they are during this epoch, decay into more stable protons with a half-life of about ten minutes. The reason for this is that down quarks themselves are slightly unstable. They last about 900 seconds before they decay into up quarks (unless they are bound in atoms). You will find out in Photon Epoch article that almost all neutrons will be bound up in stable nuclei well before they have time to decay. 

There are many kinds of mesons, composed of various kinds of quarks combined with various kinds of antiquarks but they are all very unstable. In the universe today they are created only when cosmic rays interact with matter and inside powerful colliders. Pions, for example, are a subtype of meson that is composed of only up and down quarks. A π⁺ pion is made of an up quark and an anti-down quark, shown below. 

It lasts only 2.6 x 10^-6 seconds before it decays into two new particles - an anti-muon and a muon neutrino. These two particles are examples of leptons, a family of particles that includes the electron. Anti-muons and their matter partners, muons, are very unstable themselves but muon neutrinos aren't. These particles are very interesting because they oscillate. The Sun, for example, radiates only electron neutrinos, but detectors on Earth pick up not only electron neutrinos but muon and tau neutrinos as well. This is because some electron neutrinos change into tau neutrinos and muon neutrinos along the way.

The current universe contains all flavours of neutrino. They are leptons like electrons but they have no charge, they might have no mass, and they do not make up atomic matter. At the low energy level of the current universe, only the up and down quarks, the neutrinos and the electron exist as particles of matter.

In this epoch, the electron has appeared but it is not stable at all yet. Electrons will first appear and remain when the universe cools to about 10 billion (10¹⁰) K. Electrons (e−) and their antimatter twins, positrons (e+), form when very energetic gamma photons (γ) react with each other in an equilibrium reaction, shown below.

 γ + γ ↔ e+ + e−

At 10¹³ K, the universe is a soup of photons, positrons and electrons. When the universe is about 15 seconds old, it will be cool enough to favour positrons and electrons. Most of the positrons and electrons will annihilate, releasing gamma photons in the process, and temporarily reheat the universe. But one electron in every billion will survive annihilation to seed the universe with electrons. The electron story continues in the next article, Lepton Epoch.

For now, the antiquark's (and the electron's) appearance brings up a puzzling question to which there is no easy answer. Why is there matter left over in the universe today?

What is Antimatter?

Almost every subatomic particle has an antiparticle twin with the same mass but with opposite charge. The concept of antimatter came from Paul Dirac in the 1920's. He managed to integrate quantum physics with Einstein's theory of special relativity, an enormous breakthrough in its own right. Physicists could now describe particles, such as electrons, moving near the speed of light. Unexpectedly he discovered that his formulas worked equally well for electrons with a negative charge and "electrons" with a positive charge, so he suggested that each particle should have a twin of opposite charge, an antiparticle. Since then, these "electrons," called positrons, have been observed in particle accelerators. The positron, electron's antimatter twin is produced during some kinds of radioactive decay. The laws of physics are very symmetrical with regard to matter/antimatter: An antimatter Earth would be identical to our Earth. All the laws of physics in its universe would be the same and our matter would be its antimatter. The processes involved in the formation of our universe up to this epoch also seem to have proceeded symmetrically. So why isn't the mass of our universe half matter and half antimatter, or none at all?

The distinction between charged particles and their antimatter twins seems straightforward, but the situation becomes more complex when neutral antiparticles are explored. Some neutral particles are believed to be their own antimatter twins, for example photons. However, for some the distinction between a neutral particle and its antimatter twin can be subtle. An example is the neutrino. Antineutrinos can be distinguished from neutrinos and they have been observed. Both neutrinos and antineutrinos are neutrally charged, but they are opposite each other in a quality called helicity. This is what helicity means: All particles have momentum, even those without mass, and they also have a quantum spin value. All matter particles (called fermions) have a 1/2 spin. That means electrons, quarks, protons and neutrons all have a spin of 1/2. Force particles (called bosons), have a whole-interger spin of one, or two in the case of theoretical gravitons or zero, as the Higgs boson should have. Helicity is the projection of the particle's spin onto the direction of its momentum. Neutrinos have a left-handed helicity and antineutrinos have a right-handed helicity.

CP symmetry

To figure out why matter survived this epoch, well first focus our attention on the symmetry of the universe, in particular something called CP (charge/parity) symmetry. CP symmetry is the product of two symmetries: charge (which turns a particle into its antiparticle) and parity (which creates a mirror image of a physical system). Physicists expected all physical laws to preserve CP symmetry, and the universe to observe perfect CP symmetry. Yet, they now have very good evidence that the universe is only approximately CP-symmetrical. Somehow this symmetry is violated to create an imbalance of matter over antimatter from an initial condition of balance. Does that mean that the laws of physics must have acted differently on matter versus antimatter? It turns out that this is kind of the wrong question to ask. Physicists have known for a few decades that CP symmetry is in fact violated during the natural process of radioactive beta decay, and this is how it happens: When an unstable (radioactive) nucleus is placed in a magnetic field, electrons produced from its beta decay are preferentially emitted in the direction opposite to the aligned angular momentum of the nucleus. This work was done by Chen Ning Yang and Tsung-Dao Lee in the 1950's and it earned them the Nobel prize in Physics. The consequence of this is that our Earth is in fact distinguishable from its antimatter twin, at least in terms of its weak force, which carries out radioactive decay. That's a violation of parity.

Since then, physicists have refined CP symmetry into CPT symmetry. To get a better idea of this concept, try Charge, parity and Time Reversal Symmetry, in the teacher's guide to the nuclear wall chart site. The T stands for time symmetry. Time was included because, in the realm of quantum mechanics, physicists found that equations work just fine if time runs forward or backward (this goes against our experience in which we can easily tell if a film of an event, for example, is run backward or not; we experience time's apparent one-way arrow). The reason time is so critical here is that CPT symmetry can be preserved (the CP violation can be compensated for) if time is permitted to run in either direction. This idea is called retrocausality and it is built into the very foundation of a very successful theory called quantum electrodynamics (QED), which explains the bizarre quantum behaviour of light and matter. Try Richard Feynman's book, QED, in which he explains this rather spooky quantum world in which we live, and in which CPT symmetry is explained very well with the aid of diagrams. If you are curious about the nature of time itself, try my article, Time.

So what does CP symmetry-breaking have to do with more matter being created than antimatter? One candidate explanation has to do with the genesis of quarks. This story begins with a new particle called a kaon. This very unusual type of mesonic hadron can be positively, negatively or neutrally charged. A positive kaon decays into three mesonic particles called pions, each of which consists of a quark pair, in a process that involves both the strong force (g) and the weak force (W⁺), shown below.

(Jabberwok;enWikipedia)

One positively charged kaon (composed of an up quark and an antimatter strange quark) decays into three pions, two positively charged (both consist of an up quark and an antimatter down quark) and one negatively charged (a down quark and an antimatter up quark) in the following process: The strange antiquark of the kaon decays into an up antiquark, and emits a W⁺ (weak force) boson in the process. The W⁺ boson itself then decays into a down antiquark and an up quark. Each up quark then emits a gluon (strong force) as it decays into a down quark.

A negative kaon (the antiparticle) decays in a similar manner, with opposite charges on the particles. This quark decay process may seem complicated. The point I try to make here is that the charged kaons decay in a predictable manner that conserves charge. The decays of a neutral kaon and its antimatter twin are different. To conserve charge, they should both decay into two pions, rather than three, π⁺ and π^-. This raises the question of how you prove there are is an antimatter neutral kaon. Couldn't they both be identical? Murray Gell-Mann and Kazuo Nishijima discovered that the neutral kaon could be distinguished from its antimatter partner in a way analogous to but different from neutrinos. One kaon has a quantum number, called strangeness, of +1 and its antipartner has a strangeness of -1. Keep in mind the + and - aren't charges here. Strangeness describes the decay of particles in strong and electromagnetic reactions over a period of time. Gell-mann's and Nishijima's theory meant that strangeness should be conserved in neutral kaon production but violated in the decay into the pair of pions. The -1 kaon would violate CP symmetry if it decayed into any π/π pair because any pair of pions will have two states of opposite strangeness, and this gives them together a strangeness of +1. This forces the -1 kaon to decay into three pions instead of two. This decay is much slower as a result because the mass/energy of the -1 kaon is just a tiny bit more than the mass/energy of its products. It's much more weakly favoured. What's really interesting here is that occasionally the -1 kaon actually does violate CP parity by decaying right into a π/π pair, about once every 500 decays. Christenson, Cronin, Fitch and Turlay discovered this violation in the 1960's.

Both the +1 kaon and its antimatter partner, the -1 kaon, naturally oscillate. There should be a mixture of equal parts of each particle in the universe during this epoch. The final decay product of the kaon is a stable lepton - an electron or a positron. The +1 kaon always decays into a an electron and the -1 kaon always decays into a positron. Because the decay of the -1 kaon is so much slower than the +1 kaon, electrons are created at a faster rate than positrons are.

Physicists now know of several examples of CP violation. The B meson is a another good example because it might explain why the universe experienced an excess of protons over antiprotons. The B meson contains a massive bottom quark, which makes it heavy and especially unstable. The bottom quark can decay into one of many different lighter quarks. Neutral B mesons can oscillate just like the neutral kaons do, and like the neutral kaon, these mesons show different rates of decay. In this case, the heavy bottom quark seems to decay preferentially into protons rather than antiprotons and this could account for some of the calculated mass of the universe today. This idea is currently being investigated. It is also explained well here, in the teacher's guide to the nuclear wall chart site. Also try the Cabibbo-Kobayashi-Maskawa matrix. These physicists won the Nobel Prize for this quark-mixing matrix theory, which explains CP violation. And finally, I recommend using this site as a reference for all of the subatomic particles I have mention in my articles, especially the unfamiliar unstable ones in this article.

These theories are theories in progress. None of them explains why there is so much more matter than antimatter in the universe today. For example, several particle accelerators have been designed to supply lots of B mesons. You need a lot of them in order to detect occasional CP violation out of all the possible decay products. Measurements from the SLAC accelerator in California predict one leftover proton for every 10¹⁸ protons created. but cosmologists place the ratio closer to one leftover proton in every 10⁹ protons.

There is another possibility currently being investigated, that does not require CP violation. There might not be a large matter/antimatter asymmetry at all, Instead, these two kinds of matter could be widely separated from each other in large distinct regions of the current universe. From a distance, antimatter atoms would look the same as matter atoms. Only at the boundaries between pockets of these two kinds of matter would you have any evidence that they exist, this evidence being in the form of gamma photons emitted from annihilation. This then becomes a separation problem instead. No such boundaries have been detected so far but this theory comes with a tantalizing possibility: Antimatter could have a repulsive effect on matter, acting like a kind of antigravity, much like elusive dark energy does. It could be dark energy. Antimatter, if it exists in these pockets, might explain why the universe is expanding at an accelerating rate. The Alpha Magnetic Spectrometer attached to the international space station, is currently looking for positrons, a signature for antimatter in the universe. This 24-minute Cosmic Journeys video explores the mystery of antimatter and how the AMS may be able to detect its presence in the current universe:

Meanwhile, people have come up with ways to trap and use antimatter. Positrons, electron's antimatter twin, can be trapped for use in medical (PET) imaging, for example. These particles can be kept away from matter particles inside magnetic bottles because these particles are charged. Physicists have also managed to trap some antimatter atoms as well. Storing antimatter atoms is trickier because they are neutral. Scientists made a trap lined with magnetic "mirrors" for anti-hydrogen gas, by exploiting the very tiny magnetic moment specific to anti-hydrogen atoms (this is also explored in the video above). Anti-hydrogen gas could tell researchers a lot about gravity and about the origin of matter in the universe. 

During this epoch, the temperature continues to fall toward a point where matter/antimatter pairs are no longer produced. Many particles are annihilated, releasing a large amount of gamma radiation into the universe, but not all. The remaining quarks form protons and neutrons that will seed the universe with the beginnings of what will eventually become atomic matter. The annihilation process is all but over about one second after the Big Bang, when the next epoch, the Lepton epoch begins.

Our Universe Part 9: Lepton Epoch

The early hadron-filled universe now becomes rich with the electron residue from lepton/antilepton annihilation and the decay of unstable massive leptons, all of which exist in a seething broth of high-energy photons.

The universe is one second old. It is about a thousand times the size of our solar system and it is filled with the hadron residue that survived the hadron/antihadron annihilation of the previous epoch, as well as leptons, antileptons and gamma photons.

Leptons include the familiar electron, one or more of which exist as part of every neutral atom. Below is a Bohr model of an oxygen atom. The pink nucleus contains eight protons and eight neutrons. Eight electrons orbit the nucleus.

Electrons will not exist as stable particles until the universe cools further, marking the end of this epoch. Right now, electrons and positrons exist in equilibrium with gamma photons, meaning that the universe is seething with electron/positron annihilations. You can actually "see" these annihilation bursts in this one-minute clipping from "Through the Wormhole episode 7:"



A radiation source of positrons is placed in a bubble chamber filled with superheated liquid. Each positron that is released annihilates with an electron in the liquid, releasing two gamma photons that briefly ionize a surrounding bubble of atoms in the liquid. The bright light of this ionization bubble makes the annihilation event visible.

The Feynman diagram below shows how an electron and a positron approach each other and annihilate, releasing two gamma (γ) photons.

Until the end of this epoch, the reaction above runs both directions. Gamma photons create positron/electron pairs as fast as these pairs annihilate into gamma photons. (You might have noticed that time runs backward for the positron in the Feynman diagram. I explore this further in my article, "The Behaviour of Light." Scroll down to the section, "Photons Live In Spacetime.")

There are many different kinds of leptons and they fall into two general groups: charged leptons with mass such as electrons, muons and taus, and neutral leptons such as neutrinos, which have a very small, if any, mass. The table below, clipped from the Wikipedia page on leptons, compares the mass and charge of the leptons.

In the current universe, at ordinary energies, only electrons as well as electron, muon and tau neutrinos exist and are stable. However, at very high energies, such as those which the lepton-epoch universe experienced, much more massive and unstable leptons existed as well, such as muon and the tau particles. These particles are called second and third generation leptons. Today, they can be observed only for tiny fractions of a second in the very high energy environments of particle colliders. Each of these particles is believed to exist along with its antiparticle twin for a few seconds during this epoch. These particles quickly decay (through the weak force) into lighter electrons and positrons, neutrinos and antineutrinos. The tau lepton is massive enough that it also decays into quarks as well; one tau decays into one down quark and one up antiquark, as well as a slew of other particles.

The universe is hot enough to support the creation of lepton/antilepton pairs until it is about ten seconds old. At this point, most of these pairs are annihilated, leaving a small residue of leptons (the question of why a residue of matter over antimatter remains a bit of a mystery and is discussed in the previous Hadron Epoch article). Electrons have the least mass of all the charged leptons. Unstable and massive muons and taus also form in this energetic buzzing epoch, and they begin to decay with a corresponding release of energy into a much lower energy state, into (very stable) electrons.

This online timescale of creation offers an excellent timeline diagram that sums up the birth of particles in the universe. It's a good reference for all the epochs I have covered so far.

The universe is ten seconds old as the Lepton epoch ends. It contains both hadrons and leptons, but no atoms yet. It is still far too energetic for even atomic nuclei to form. The universe is also brimming with high-energy gamma photons that were created during the hadron and lepton annihilation events.

Our Universe Part 10: Photon Epoch

As the young universe continues to expand and cool, the first atomic nuclei explode into existence in a universal fusion reaction.

The universe is about ten seconds old. It contains hadrons (protons and neutrons) as well as leptons, but it is still too energetic for atomic nuclei (protons and neutrons) to bind. The annihilation of hadron and lepton particle/antiparticle pairs contributed a tremendous number of extremely energetic gamma photons to the universe. These annihilation events are now all but over, leaving a residue of matter over antimatter. These events are explored in a previous article, "Hadron Epoch." Gamma photons now dominate the photon epoch universe. The photon epoch universe is a seething plasma of free electrons, protons and neutrons. Photons are constantly slamming into other particles, usually free electrons, and interacting with them. This collision creates an effect called Thomson scattering. When a photon strikes an energetic electron, the electric and magnetic components of the photon exert a special force called the Lorentz force on the electron. This force accelerates the electron and causes it to vibrate. This vibration sets up a new electromagnetic oscillation, which means the electron emits a new photon in a new direction. During this epoch, photons are continuously scattering off electrons in all directions and it means that the universe, for now, is opaque because photons can't stream. There is light during this epoch but it is not the kind of light you could see through a telescope. Instead, matter and radiation behave as a fluid called the photon baryon fluid. The universe will continue to cool and expand, and at about a minute old it will have cooled enough to allow the first nuclei larger than a single proton to form. This begins the process of Big Bang nucleosynthesis and it will continue for only about three minutes.

Big Bang Nucleosynthesis

The smallest and simplest nuclei are hydrogen-1 nuclei. These are simply free protons and they have been present ever since stable hadrons began to form during the hadron epoch.  Right now, deuterium and helium nuclei are beginning to form during a two-step process, called a proton-proton chain reaction. As shown in the diagram below, two protons (¹H) fuse into a deuterium (²H) nucleus. A positron and a neutrino are released as one of the protons changes into a neutron. A deuterium nucleus fuses with another proton, releasing a gamma photon, and forming a helium-3 nucleus. Two helium-3 nuclei fuse into a helium-4 nucleus, releasing two protons in the process.

(Borb;Wikipedia)

This is the same fusion reaction that happens in the Sun, converting hydrogen into helium.

Deuterium, or hydrogen-2, has a nucleus composed of one proton and one neutron. It is also stable. A helium nucleus is a very stable tightly bound arrangement of two protons and two neutrons. Free protons and neutrons are highly motivated to form a helium nucleus because it is so stable, representing a lower possible energy state. However, the production of a helium nucleus requires an intermediate step of making two deuterium nuclei. This can only happen when the energy of universe falls to a level where deuterium formation is favoured over dissolution back into free protons, and it is called the deuterium bottleneck. Once deuterium nuclei are able to form, a sudden burst of fusion occurs, creating lots of helium-4 nuclei, along with some helium-3 (a stable light form of helium with two protons and one neutron) nuclei and lithium-7 nuclei (with three protons and four neutrons). The universe is one huge fusion reactor for about two minutes. As this reaction burns out, the universe is left with all the hydrogen-1 it will ever have and almost all the helium-4 it will ever have as well. Relatively little more helium will be fused from hydrogen inside future stars.

In addition to these stable hydrogen-1, deuterium, helium-3,  helium-4 and lithium-7 nuclei, tritium (hydrogen-3) and beryllium-7 nuclei are also created but these will either quickly decay or fuse with other nuclei to create stable isotopes. The largest stable element created is lithium-7, but very little lithium is created before all free neutrons are bound up inside smaller nuclei, about three minutes after the Big Bang. About 20 minutes after the Big Bang, the energy of the universe becomes too low to allow nuclear fusion to continue. The nuclei of only three stable elements (elements 1, 2 and 3 below) are formed during this epoch. No larger nuclei exist yet.

A few minutes old, the universe contains about 75% H1, 25% Helium-4 and about 0.01% deuterium as well as trace amounts of lithium and beryllium nuclei (by mass). Much later on, as stars form, hydrogen will fuse into helium, and to a far less extent, helium and other light elements will fuse as well, but that will not change this ratio very much. Only about one percent of the original hydrogen and helium created during Big Bang nucleogenesis has been burned inside stars during the 14 billion years since the Big Bang. The universe will continue to expand and cool for another 379,000 years before the first stable atoms begin to appear. Only then will free electrons will be of low enough energy to be captured by positively charge nuclei, creating the first atoms of matter.

It will take as long as 500 million years after the first atom appears before the first stars form. They will be the furnaces in which all the larger nuclei come into existence. I explore how all the atoms formed in the article, "How Atoms Are Made."

Our Universe Part 11: Decoupling Epoch

The universe cools enough to allow free electrons to bind to nuclei and the first atoms appear. This decoupling process allows bound-up photons to stream and we can see those first streaming photons today as a map of cosmic background radiation, a first real image of the young universe that's taken almost 14 billion years to reach us.

All the forces are in place and acting on atoms of matter as they form. The universe is well on its way to becoming the star-filled vastness we are familiar with. 

A tremendous number of processes, many of which are very complex and still not well understood, have taken place to reach this epoch, from the first appearance of each fundamental force to the explosion of quarks, electrons and their antimatter twin particles. This processes reveal the fundamental nature of our current universe. We can now see how at least three of the four fundamental forces - strong, weak and electromagnetic - are very closely related to each other. In fact, they are all aspects of one single force and at high enough energy, they revert to one force. Phase changes took place as the universe cooled from its Big Bang origin, breaking various kinds of symmetry built into the fabric of the universe. These phase changes broke the single force into the different fundamental forces of our current universe. Quarks, the building blocks of protons and neutrons in atomic nuclei, first appeared as the result of yet another phase change, in which these particles decoupled from energy. Electrons, the other building blocks of atoms, decoupled from energy as well. This tells us something profound: All atoms of matter and all the different forces that act upon them, which seem so so clearly distinct from one another, are really different aspects of a single entity. They appear and behave as distinct entities only at energies that are sufficiently low.

By about 10,000 years after the Big Bang, the energy density of photons had fallen below the energy density of matter. Before this, the energy density of the universe was dominated by photon radiation, as the result of matter/antimatter annihilation. Some of those photons have since converted into particles of matter, but most of the loss of energy is in the photons themselves. Their wavelength has been stretching as the universe expands, reducing their energy. The universe is now about 379,000 years old. It has expanded to about a thousandth of its present size and it has cooled to about 4000 K. The universe has been seeded with matter but clumps of matter cannot yet collapse under their own gravity to begin to make any structure (stars, gas clouds etc.) until photons decouple from it.

In the plasma state, negatively charged electrons and positively charged nuclei are bound up in a thermal equilibrium reaction in which they combine and release a photon and alternatively, absorb a photon and dissociate. This is the equilibrium reaction for a hydrogen atom, H:

The lower-case y-like symbol is the Greek letter gamma, and it stands for a photon of gamma radiation. This ionization energy corresponds to a photon temperature of 4000 K or an energy of 13.6 eV (electron volts). Below 4000 K, the reaction can no longer support photon absorption and the nuclei irreversibly bind with electrons to create neutral atoms. At this point, photons are freed from the reaction and permitted to stream as highly energetic gamma rays. Atoms make their first appearance and the universe gradually becomes transparent.

Photons from this epoch have been traveling for almost 14 billion years. We have detected them here on Earth as cosmic microwave background radiation. The radiation that fills the universe today has been mapped and that map reveals subtle irregularities that correspond with what physicists would expect from quantum fluctuations of energy in what was once a very tiny space. Those tiny fluctuations have expanded to large-scale regions of varying energy density. The different colours in the cosmic radiation map of the universe below represent very minute variations in temperature, of around +/- 0.0002 K.

Over almost 14 billion years, the temperature of these original photons has decreased from 4000 K to 2.725 K, a factor of about 1100. This cooling off is not the same kind of cooling that happens when a cup of hot coffee gradually cools to room temperature. It is the sole result of the expansion of the universe, not through heat transfer, because there is nothing the photon energy can transfer to inside the vacuum of space. A photon's energy is measured by its wavelength. As the energy of a photon drops, its wavelength increases. Gamma photons have a very short wavelength and tremendous energy. Microwave photons photons have much less energy and a very long wavelength, as shown below.

(Philip Ronan;Wikipedia)

The expansion of spacetime caused the energy of the photons to redshift - the photon's wavelengths stretched out in other words and that caused their energy, measured as temperature, to fall so that now they fall into the microwave region of the electromagnetic spectrum. This 11-minute video explains how cosmological redshift works:



This radiation was discovered accidentally in 1965 by two researchers, Arno Penzias and Bob Wilson, trying to detect faint radio waves bounced off of satellites. Even after cleaning off pigeon droppings and eliminating all other possible sources of interference, they heard a continuous low mysterious hum, the sound of photons streaming ever since they were freed from matter, 379,000 years ago:

We are embedded in the Big Bang explosion. Photons released from that explosion bombard us from all directions.

At 379,000 years old, the first photons have begun to stream across the universe. Then, over the next 150 million years, everything gradually goes dark. Why?

Our Universe Part 12: The Dark Age

The universe gradually becomes transparent as the first simple molecules of hydrogen gas form and cosmic radiation slips into infrared. It appears dark until the first star ignites approximately 150 million years later.

A blast of gamma radiation was released as the photons decoupled from matter. This blast is detectable as cosmic background radiation. What follows, however, is a dark age that lasts until the first cloud of mostly hydrogen gas collapses gravitationally and begins to emit light, roughly 150 million years later. Meanwhile, dark matter is believed to be organizing into a universal scaffold on which matter will soon accumulate. Physicists believe that the first baryonic matter in the universe was too energetic and was under too much pressure left over from the Big Bang to collapse and form stars. They believe that dark matter, combined with a process called Jeans instability, was responsible for causing the first clouds of gas to collapse. Jeans instability works like this: A cloud of gas will remain stable as long as its (outward) internal pressure is equal to the (inward) force of gravitational attraction on its particles. As its internal pressure falls, it becomes unstable and even a tiny perturbation will trigger its collapse. The gravitational effect of abundant dark matter is believed to help overcome the internal pressure of the gas before it becomes too dispersed to collapse on its own. No one knows yet what dark matter is made of and, therefore, exactly when it first appears in the universe but most physicists believe it was present early enough to help draw baryonic matter in along its scaffold. This theory agrees with a dark matter prediction of the structure of the cosmic background radiation and it corresponds with the visible structure present in the universe today. Below is a 3-D map of the large scale distribution of dark matter in the universe, detected by measurements of weak gravitational lensing using the Hubble Space Telescope.

Meanwhile, atoms of hydrogen begin to attract each other, forming much more stable hydrogen (H₂) gas. Clouds of molecular hydrogen gas begin to collapse under their own gravity. The environment is still hazy. When decoupling began, the universe did not instantly become transparent. The first stars shine through a slowly thinning plasma fog, during which dark opaque regions will gradually become more and more interspersed with bubbles of light (forming stars) and transparent gas. This ten-minute video describes how the universe enters the dark age:



Electromagnetism had been dominating the universe because it was brimming with photons, but now gravity, by far the weakest of the fundamental forces, is taking over. It allows energy density now in the form of matter, to clump. This is an unusual example of a closed system moving away from both equilibrium and disorder. It seems to be breaking the second law of thermodynamics which states that all closed systems tend toward disorder and a state of equilibrium. Up until now, the universe maintained a fairly homogenous potential energy in terms of electric potential and matter. Now, forming stars are creating pockets of high energy density surrounded by areas of very low energy density. This appears to decrease the entropy of the universe. If you consider the long-term picture of the universe, does gravity hold the second law of thermodynamics at bay for a while or does gravity modify the law? Some theorists, considering the thermodynamics of the universe, suggest that the universe should eventually come around to obey the second law - it should succumb to heat death - a state in which there will no longer be any temperature differences or other processes that could be exploited to do work. This is an eventual return to a state of thermodynamic equilibrium, that would occur only after the last supermassive black hole evaporated away through Hawking radiation, a long time estimated to be about 10¹⁰⁰ years. The universe, prior to the effects of gravity taking over, was in a state superficially similar to heat death - thermodynamic equilibrium.

Meanwhile, the universe has continued to expand. This expansion can be measured as the redshift of the cosmic background radiation. The concept of cosmological redshift was introduced in the previous article. At this point, it is helpful to describe the universe in terms of redshift. Cosmologists use redshift, z, to designate a cosmic epoch. 1 + z is the factor by which the universe has expanded from that epoch to present day. Present time z = 0. The value of z gets larger as you go back in time. It is the factor by which the wavelength of the photons emitted during the decoupling epoch has stretched. This is what gives us direct evidence for the expansion of the universe.

Atoms started to form at z = 1100. This occurred during the decoupling epoch when the temperature was around 4000 K. Soon afterward, cosmic radiation filled the universe with a red uniform glow of blackbody radiation. This black body radiation currently fills the universe with uniform microwave radiation. A black body is an object in physics that emits photons in a spectrum that is determined only by its temperature. The graph below shows how temperature is related to the colour and intensity of the emitted light. A white hot object glows white because its temperature is around 5000 K (our Sun is an example). A red-hot object is around 3000 K (the universe during the dark age).

Some real objects are better black bodies than others, the universe is very good one. If you would like to know more about how black bodies work, try my article, Atoms Together Can Emit Light.

As the universe began this epoch at about 379,000 years old, it was opaque and almost uniformly filled with atomic gas. Now, during the dark age, a tiny fraction of this gas forms the first small molecules, most of which is hydrogen (H₂) gas. The temperature continues to drop as the universe expands and the cosmic radiation shifts to infrared. To human eyes, the universe would now appear completely dark.  It begins a while after decoupling occurs when the universe is around a million years old and it lasts for a long time, up to 200 million years, until the formation of the first stars. Galaxies and quasars have been observed up to z = 6.5 (the universe being about 500 million years old) so stars must have formed well before this, possibly at redshift as high as 18, about 150 million years after the Big Bang. Very faint objects in higher z time frames will probably be observed as technology improves.

It's hard to pinpoint exactly when the first star forms because the universe at this time is filled with random tiny primordial density fluctuations of gas and dark matter, but eventually at some random location in the universe (and likely at around 2000 K), a fluctuation in the density field will be large enough to trigger the gravitational collapse of a molecular gas cloud and eventually form a central core composed of compressed hydrogen gas. It will continue to collapse under the force of gravity. It will become hot and bright because of the energy generated by this contraction, and it will begin to spin, thanks to sum of the angular momentum of every atom involved. It will eventually grow hot enough to fuse hydrogen into helium at which point fusion begins and a star ignites, at around z = 18 (around 150 million years post-bang. This age marks the beginning of a universe filled with countless billions of stars, next.

Our Universe Part 13: Stelliferous Era

The universe appears as a homogenous dark expanding space until this era begins with the ignition of the first star, punctuating the universe with light. Once this star-forming process starts, it ushers in a boom of star births and then black holes and galaxies and all the other structures and objects we see today. This is the era we live in and it stretches into the future until the last star dies.

Stelliferous means "filled with stars." This age begins when the universe is about 150 million years old and it will continue until about 100 trillion years from now when the last star forms, leaving all stellar objects from that point on to degenerate and decay.

The universe is now about 150 million years old and the first stars have just formed from collapsing halos of mostly hydrogen molecular gas. Watch this 1.5 minute video of star forming. It is a computer model based on a cloud of hydrogen (H₂) gas fifty times more massive than our Sun, 1.2 light-years in diameter, and with a starting temperature of 10 K:



The first stars are likely to be at least as massive as this simulation suggests, perhaps closer to 100 solar masses or more. Star formation is an extremely violent process. These forming stars are drastically changing the environment in which they are formed and that's affecting the formation of subsequent stars.

The first stars are called Population III stars. Population III stars are a theoretical group of extremely massive hot stars that contain no elements except those made during the Big Bang. Heavier elements did not exist until much later on when these massive stars likely exploded as pair-instability supernovae at the end of their lifespans. These massive light-element stars grew so hot that gamma photons in the core favoured the production of electron-positron pairs, as shown below.

This would have reduced the core's pressure and led to a partial collapse of the star. It would have then led to a runaway thermonuclear explosion that would have torn the star apart, leaving nothing, not even a black hole behind.

Population III stars were hot massive stars containing no heavy elements. They would have burned very intensely, emitting intense ultraviolet light, which would have ionized the neutral hydrogen gas around them. They would have burned out rapidly, in the span of a few million years, ending their brief lives in intense supernovae blowing new heavier elements such as carbon, silicon, oxygen and iron into other collapsing gas clouds, seeding those forming stars with these new elements. This marks a trend in star composition. Older stars contain less heavy elements in them to start with, while newer younger stars are formed from a higher proportion of heavy elements. These stars are said to have a higher metallicity. All the elements heavier than hydrogen, helium and a tiny amount of lithium come from fusion reactions inside stars and from the debris left over after they die and explode. The stelliferous era is explored in this five-minute video:


Some researchers suspect that some population III stars may not have blown completely apart. They may instead have left remnant black holes after they exploded. These black holes could have clumped into more massive black holes, to form the first quasars and small galaxies.

Most if not all galaxies are believed to have black holes at their centres. They contain star systems, star clusters and interstellar gas. It is the interstellar gas that new stars are made from. At first, stars were made of only the lightest elements, those present in the gas, form. These are followed by stars made of heavier elements, as the earliest stars die and explode, seeding the interstellar medium with them. Eventually a galaxy runs out of star-forming gas, and becomes a collection of aging stars. Atomic matter, in the form of stars and gas, is only a small contribution to the makeup of any galaxy, however. Dark matter accounts for about 90% of the mass of galaxies. Our galaxy, the Milky Way, shown below, is a fairly old galaxy, about 13.2 billion years old. It is a barred spiral galaxy surrounded by a supermassive black hole called Sagittarius A*.

The Milky Way may contain up to 400 billion stars and more gas and dust in its arms than most galaxies do, especially ones this old. In fact, the Milky Way is still growing. Dust and gas is being accreted from two adjacent galaxies, the Large and Small Magellanic clouds.  New stars are being formed in the dusty gas-filled arms of the Milky Way, but this dust and gas will probably be exhausted in about five billion years. This puts the Milky Way in the green valley of the galaxy colour-magnetitude diagram, below.

(Joshua Schroeder;Wikipedia)

Spiral galaxies, with a few exceptions including our own, tend to be younger galaxies, or blue cloud galaxies while older galaxies tend to be elliptical in shape, or red sequence galaxies. Blue cloud galaxies tend to contain more young Population I stars while red sequence galaxies contain mostly older Population II stars. No population III stars have been observed. These stars are believed to be long extinct. However, there may be a way to indirectly "see" them. Below is an infrared NASA image taken by the Spitzer Space Telescope, in which all the stars, galaxies and artifacts have been masked out (grey). The remaining glow could be of the first (Population III) stars forming in the universe. NASA's James Webb Telescope will try to locate Population III stars. These star images, if they are found, will be of extremely distant long-dead stars. Remember, as you look far away into deep space you are looking backward in time.

Although the above image shows an infrared glow, the first stars shone very bright with a bluish cast, indicating their intense heat, shown in the artist's concept below. The wavelength light coming from the stars would have stretched as the universe expanded.

Hot stars like these would emit a lot of energetic ultraviolet photons that ionize gas, most of which is hydrogen, and this would create pockets of gas as hot as 10,000 K. This is the second phase change for hydrogen. The first change was recombination (or dionization) which occurred at redshift z = 1100 (the large z value indicates the size of the universe. At z = 1100 the universe was much smaller and very young). This first phase change occurred during the decoupling epoch, in which photons decoupled from matter. Recombination means that the plasma composed of electrically charged electrons and protons that filled the much younger universe combined to make electrically neutral hydrogen atoms. Now, high-energy ultraviolet photons emitted from the first stars energize hydrogen gas back into ionized plasma. This is called the reionization phase change. The ionized hydrogen gas at this stage is much less dense than when it was first ionized because the universe has continued to expand and Thomson scattering interactions are far less frequent. The universe, in this case, doesn't turn opaque, as it did during at the beginning of the dark age, but remains transparent instead.

These highly energized ions create shock waves that trigger molecular gas clouds in the region to collapse and it eventually leads to widely dispersed star formation throughout the early universe, ultimately leading to the massive array of galaxies and other objects we have in our universe today. In fact, most astrophysicists believe that galaxies formed very early in the life of the universe, at least by the time it was one billion years old, suggesting an extremely active period of star formation. The future locations and structures of galaxies might have been mapped out even before the first star formed. As the universe cooled, clumps of dark matter began to condense and within them clumps of gas began to condense. These pockets of higher dark matter/gas density could have become the seeds of future galaxies. These protogalaxies would have consisted of hydrogen and helium gases as well as dark matter. Soon after they formed, the first stars began to form within them from internal pockets of denser gas that eventually accrued enough mass to begin to form an accretion disk (I describe this process in more detail in the article, Introduction to Stars). The current distribution of galaxies in the universe is quite interesting. Rather than being evenly distributed, they are associated with what appears as giant cosmic filaments, where enormous clusters of galaxies form at the filament intersections. Processes that went on perhaps as early as the first epoch are imprinted on the distribution of galaxies we see today. They are the signature of tiny quantum fluctuations that once occurred billions of years ago, from this:

(Tiny fluctuations in temperature, shown in the cosmic microwave background radiation map above, suggest small fluctuations in the density of energy and matter in the universe when it was about 400,000 years old and the process of decoupling began.)

to this:

(The image above indicates how organized matter in the universe has become. It represents only a very small portion of the universe. The Milky Way is located in a region of galaxies organized into was is called called the Local or Virgo Supercluster)

Population II stars are the oldest extant stars in the universe. They contain a small proportion of elements heavier than hydrogen and helium,  such as oxygen, silicon and neon. These stars tend to be located in the outermost region of our galaxy, called the galactic halo, within clusters of stars called globular clusters. Population II stars are believed to contribute all the elements to the universe, except some of the very heavy unstable ones. The oldest star in our galaxy is HE 1523-0901, shown below as an artist's impression.

It is 7500 light years from Earth and is thought to be a second generation Population II star that is 13.2 billion years old and 0.8 solar mass.

Our Sun is a relatively metal-rich intermediate Population I star. Population I stars, like the Sun, are common in the spiral arms of the Milky Way. The youngest stars of all are extreme Population I stars. These are found closer to the centre of the galaxy. Population I stars are more likely to have planets orbiting them because planets, especially rocky ones, are formed from the accretion of heavy metal-rich elements. These stars form from relatively metal-rich dust left over from older Population II star supernovae. In the figure below, the Sun, an intermediate Population I star, is shown as a yellow circle.

In another 81/2 billion years from the beginning of the stelliferous era, when the universe is about 9.5 billion years old, our Sun will begin to form, and a few million years after that, Earth will have formed from  dust in the Sun's protoplanetary disk. To learn more about stars, try my article "Introduction To Stars."

The diagram below gives you an idea of the timeline of the universe's evolution:

Along with Population I and II stars and dust clouds, the universe is now populated with magnetars, quark stars, neutron stars, black holes and quasars. These fascinating objects are explored in detail in the article series, "Stellar Objects."

How and when will the universe end? Scientists haven't reached a consensus yet on the fate of the universe but a number of theories will be explored in the next article, "The End of The Universe."

Our Universe Part 14: The End of the Universe

The universe has been expanding since the Big Bang and its rate of expansion is increasing. Some mysterious component of the universe is opposing gravitational attraction and it's called dark energy. This discovery puts popular notions about the fate of the universe into question.

The discovery of cosmic background radiation in the 1960's gives tremendous support for the Big Bang theory, so much that it is now one of the most firmly rooted models in astrophysics. It also gave physicists a framework in which to measure how old the universe is, about 13.7 billion years old. This theory began as a series of solutions to Einstein's general relativity equations carried out in 1912 by Alexander Friedman. The implications of those solutions created a lot of controversy at the time. Einstein himself could not accept that the universe is not static and eternal, so he added a cosmological constant to his equations, a decision he later regretted because at the same time American astronomer Vesto Slipher discovered that all the galaxies he observed were redshifted, and thus, moving away. In 1924, Edwin Hubble confirmed these observations with his own. In 1927, the Catholic priest and physicist, Georges Lemaitre, confirmed Friedman's calculations and extrapolated backward to a point of infinite temperature and density at a particular time in the past, He called this infinite point a "primeval atom." This was not without religious controversy - a scientifically defined moment of creation! Lemaitre is sometimes credited with coining the term Big Bang but it was Fred Hoyle, who was promoting his own competing Steady State theory, who first used the term (derisively) during a 1950's radio broadcast (although he denied this). Here is what the Big Bang theory looks like (in a flat universe for simplicity):

Since the 1920's the Big Bang theory has been supported and refined by data from COBE, the Hubble Space Telescope and WMAP, as well as by the unexpected discovery that the rate of expansion is increasing (this will be explored shortly).

Questions about the workings of the universe still remain however. A number of them, the flatness problem, the magnetic monopole problem and the horizon problem, have all been more or less resolved by cosmic inflation. But baryon asymmetry (why there is more matter than antimatter) is not resolved. 

The discovery that the rate of expansion is increasing brings us to the recently formed concept of dark energy. In the late 1990's astronomers were surprised by the magnitude of redshift for type 1a supernovae. These supernovae are used as a standard candle in astronomy because they all exhibit nearly identical luminosities. They are the result of white dwarf explosions, and white dwarf stars are all confined to a very narrow mass limit called the Chandrasekhar limit. White dwarf stars are covered in my article, Introduction to the Stars. They can be used to measure both distance and redshift and because almost 97% of all stars in the Milky Way alone are destined to become white dwarfs (and then type 1a supernovae), so a lot of accurate measurements can be made. A roughly linear relationship between distance and redshift for galaxies was found, shown below, and this means that the expansion rate is increasing. But why?

(Brews ohare;Wikipedia)

Redshift velocity is plotted against distance, showing a roughly linear relationship, called Hubble's law. The Virgo Cluster of stars shows significant scatter because its redshift measurement is affected by other  effects, such as gravitational effects from other nearby galaxies. This is different from the Doppler effect, especially at higher values of z. It is a measurement of the recessional velocity of the galaxy due to the expansion of the universe. At zero z (the current universe), however, the Hubble law value and Doppler effect value are the same.

General relativity requires that, for the expansion rate to increase, much of the energy of the universe must come from a component that exhibits great negative pressure. This negative pressure has been called dark energy. Dark energy also explains why the universe is almost perfectly spatially flat (evidence for this is in the map of the cosmic background radiation), To be flat, the universe must maintain almost exactly the critical density for mass/energy. If it went out of whack even a tiny bit at any time during expansion, the universe would be either positively or negatively curved. The mass density of the universe has been measured and it's only 30% of critical density. Dark energy nicely accounts for the rest that is needed. Negative pressure is a property of vacuum energy but other than that, nothing is known about dark energy, except that while the energy density in matter is decreasing as the universe expands, the dark energy density remains constant. This means that the effect of dark energy should become more dominant in the future and the rate of the acceleration of the universe's expansion will continue to increase. Dark energy weeds out two possible universe fates: the Big Freeze and the Big Crunch. 

If the mass/energy density of the universe is below critical density, the expansion slows but never stops. Stars would gradually burn out and black holes would grow as they eventually collide with and swallow up the star remnants. The temperature of the universe would approach absolute zero and even black holes would disappear as they eventually evaporate away via Hawking radiation (explored in my article, Black Holes). This is the Big Freeze. It's also known as Heat Death. This 7-minute video paints a vivid picture of how the universe might end:



If the mass/energy density of the universe is above critical density, the universe would reach a maximum size and then contract, becoming hotter again until it perhaps re-exploded in a new Big Bang. This is the Big Crunch scenario.

With dark energy, more and more of the universe will pass beyond the visible boundary, possibly ending in what is coined the Big Rip. In this scenario, galaxies, stars, planets, all matter, will eventually be ripped apart. This is how it works: The size of the observable universe is shrinking as the rate of expansion increases because the distance to the observable edge, where everything is moving away at the speed of light, must get closer and closer. Eventually the observable (below the speed of light) size will contract to a size smaller than atoms or even subatomic particles. At this point none of the forces that hold particles together can act and the particles themselves are ripped apart. This 5-minute video explores the Big Rip possibility:

There are some problems with dark energy however. One is that while quantum field theory allows for such a force, it predicts that its density may be as much as 120 times greater than any calculated value based on astronomical observations. This is called the cosmological constant problem and it brings us back to Einstein's modification to general relativity in order to preserve his idea of a constant universe. He abandoned it back in the early 1900's but the recent discovery of accelerating expansion has brought it back out of the tool box. Physicists now think that the cosmological constant does not lead to a static universe in equilibrium because the equilibrium itself is unstable. As the universe expands, the expansion releases vacuum energy and that causes more expansion. This is a nutshell answer. If you would like to explore this interesting cosmological constant puzzle (and, really, a dark energy puzzle) in more detail, try this slide-show illustrating where the science is at right now.

There is also an idea that combines string theory and dark matter and it places our universe in a fascinating new context. String theory mathematics allow for dark energy, while leaving the value for the cosmological constant unassigned. When these calculations are done, as they were by Andrei Linde et al. in 2003, a graph is produced that is shaped like a mountainous landscape, where altitude represents the value of the cosmological constant. After the Big Bang, that value should settle on a low point of minimum energy. The catch here is that there are billions of possible low points to choose from and no obvious reason why the universe should have picked one value over another. Some physicists such as Leonard Susskind have embraced this and postulated a multiverse scenario in which there are countless universes coexisting at once, each with its own cosmological constant, whereas others point to these calculations as flawed because they can't make useful predictions. 

The idea of there being not just one universe but countless universes also finds some support from other areas of physics research, such as quantum mechanics.  For example, Richard Feynman popularized QED theory to explain the strange behaviours of photons. We experience a photon as travelling in a perfectly straight line from A to B. According to this theory, what we see is the sum of the greatest probability of its travel plan. In reality the photon travels through many multiple paths simultaneously (in a quantum state), with some of its personas even interfering with other personas in the process, to get from A to B. It is anti-intuitive but backed up by experiment, if you remember Young's famous double slit experiment where both the wave nature and particle nature of light are revealed. When a beam of photons is shot through a double slit barrier, an interference pattern shows up, just as you would expect waves to interact. When the beam is reduced to one photon shot at a time, an interference pattern still shows up. How do individual photons know where the next photon will strike? Even when photons are supposedly behaving as particles their wave nature still shows up (this experiment has been repeated with electrons showing that matter too exhibits what is called wave/particle duality). This experiment also hints at the quantum electrodynamic nature of the photon. The question becomes, is the universe a quantum system, exploring all possible physical constant values?

In 2006, Stephen Hawking and Thomas Hertog suggested that we view the universe as a quantum system, within the framework of string theory. In this case, instead of a photon, the universe itself follows all possible trajectories simultaneously as it evolves and moves forward through time. These are not multiple Susskind-like universes; they only exist in a quantum sense just as individual photon trajectories never exist in the classical sense. What exists in the classical sense, what we experience, is the maximum probability of trajectory possibilities. This is not without its difficulties. For example, how do we test the theory? One thing theorists can do is take our present universe and trace it backwards through time. There should be multiple possible branches the universe could have taken at any point, but they can weed out those that are too different from our universe. For example we know that our universe is almost perfectly flat so we can ignore all possible trajectories that rely on a cosmological constant that leads to curvatures. They could also test these results against possible universes that produce the cosmic background radiation we observe, and so on.

The fate of the universe ultimately relies on dark energy. But when physicists try to understand it, they run into a variety of problems, most of which have to do with assigning values to the cosmological constant, and that has lead researchers in a variety of directions and into notions of multiple universes. Understanding how dark energy works will be key to understanding how the universe will eventually end up.

One thing, however, seems fairly certain: dark energy's effects are growing evermore significant over time as gravity's effect's dwindle with decreasing energy/matter density in the universe. For now, this idea assumes we are heading for some kind of Big Rip, but no one knows for sure. That means that the universe's ultimate fate must remain a mystery, at least for now.