Carl Sagan once said “the cosmos is all there is, all there ever was, and all there ever will be”. Statements like this deserve to be repeated; the kind of statement that seems reasonably intuitive on a surface level, but when you think on it; ocean deep and slightly scary. The question of God seems to crop up quite a lot when you study theoretical physics; probably because of the fanciful notions and mystery that shrouds the front lines of research. I often think that if God were to exist then (s)he must love a good detective drama; leaving us sitting out on our rock, on a far flung corner of a galaxy, billions of years after the creation of the universe with nothing but small clues to piece together. God aside; this is still the situation we find ourselves in, detectives doing our best with the odds ever stacked against us. Occasionally I like to do a post where I pull together a few of the posts I have done before, along with some new material into one post. As ever, comments, additions or edits are always welcome.

The very (very) early universe

As you should well know, if school has served you well there are four interactions that physicists consider to be “fundamental”; the electromagnetic interaction, the strong interaction, the weak interaction and  the gravitational interaction. Our experience and current understanding tells us at certain energy levels some of these interactions unify; in other words they they are no longer strictly distinct from each other. The ultimate theory of unification is the Theory of Everything, where the four fundamental interactions are described in one coherent theory, otherwise known as superunification. There is no meaningful description of this as yet; there are of course theories but I want to keep this post to the things science is a little more conclusive on.

Now we journey to around 3 x 10^-44s after the big bang, in an era known as Planck time; where the mean energy available is around 10¹⁹GeV – dubbed Planck energy. At this time, a particle travelling at the cosmic speed limit of light speed could have moved 10^-35m so we are talking about a dense hot universe soup here. This is the time where gravitational reactions start to become distinct, but we can offer no meaningful discussion of what that means without a theory of quantum gravity.

Aside: The astute may well have found some difficulty with me talking about the size of the universe, since it may very well be infinite. For now, we can reconcile this difference by clarifying we speak of the observable universe. Of course there is a much richer and deeper discussion around theories and evidence supporting these ideas; but this will suffice for now.

In this time photons spontaneously create particle-antiparticle pairs and particle-antiparticle pairs annihilate into photons until a steady equilibrium is reached. X-bosons would have been in evidence; the current theories suggest. These bosons emerged from theories of grand unifications, where very massive bosons allow quarks to change to leptons and matter to change to antimatter. Mysterious indeed. These bosons are only present at very high energy levels, such as those talked about in the early stages of this post due to their very high mass energy.

As time continues a (very) small amount we reach 10^-36s; this is the time where superunification breaks. Superunification is the unification of the strong, weak and electromagnetic interactions; remember that the gravitational reactions have already become distinct. At this point strong interactions are now distinct interactions, with only an electroweak unification existing.

The universe is very very hot. Over 10²² K.

Inflation

Then comes a slightly wacky part; the inflation. You may remember me touching on this before. It is an idea put out by the American scientist Alan Guth back in 1981 and has rapidly become one of the pillars of our current understandings of the universe. Appreciate this; the universe is currently very uniform. There are certain things, that we will discuss later that are uniform from one part of the universe to the other. But how do they know? They have had no time to chat. A message cannot have gone from one end to the universe to us to the other end, there just has not been enough history for a message travelling at the speed of light to cover that ground. So how did they know to be so uniform? This is the uniformity problem.

The current best solution is inflation. In this theory, in the very early history of the universe (10^-36-10^-32s) a region of space inflated massively and rapidly. We do not know exactly by what factor; but really huge numbers in the region of 10⁵⁰ have been touted in discussions in this area. The “advantage” that this theory has is that it gets rid of the uniformity problem; there is no problem the area destined to become the observable universe started as a smaller area in which a signal could have travelled. There may well exist uniformities beyond the observable; but we have no need to worry about these things because there is no chance we will ever observe them. The cosmic speed limit again.

Shortly after this crazy inflationary period those X-bosons we talked about begin to decay. X-bosons are a little special (we think) because their matter and antimatter bosons can each decay into both matter or antimatter particles; we may witness an X-boson decay into two quarks or an antiquark and an antilepton, or an anti-X-boson decay into a quark and a lepton or two antiquarks. That isn’t vital to the understanding. This is a very special part of the universe’s history, because it is now that all six flavours of quark and all six flavours of lepton are produced in roughly equal quantities (roughly is of the upmost importance). This is the point when it is all made.

Quark-lepton ages

Let us set the scene a little further; so we have just seen the flavours of the universe decay from massive X-bosons. We now skip forwards (a bit) to 10^-11s. In the intervening time between inflation and where we are now nothing new really happened; this is often called the desert. The universe just continued to get larger and colder, which it does to this very day.

So what’s so special about 10^-11s? Well it is around this time that the temperature dropped to around 3 x 10¹⁵ K and the electroweak unification breaks. From this point onwards in the history of the universe there are four fundamental interactions, all distinct which humans will devote their lives to attempt to reunify. Take a second to think about what we have just discussed and you will have taken far longer than the history we are talking about. Everything discussed up to this point, the entire deunification of the four fundamental interactions, the creation of all the primordial leptons and quarks and the huge inflation of the universe all in little more than the blink of an eye; 10^-11s.

When you try to make sense of these phenomena; remember that the timescales only seem hard to comprehend because we are used to moving through life at a certain speed; we are conditioned to see a second as short, a day as long, 50 years as very long, 200 years beyond what we will experience first hand and so on. Nature does not appreciate these relative lengths associated to timeframes; the relativity attached is a human construct. Nature has it’s own system of working, which will not yield to human ideas – the ideas must remain malleable to natures direction.

Let us not forget the universe is still getting larger and cooler; it does not stop. As the mean temperature in the universe continues to fall, the energy starts to drop below the values that favours pair creation (photons are required to have energy equivalent to, at a minimum, the mass energy of the particles created to cause pair creation). At this point, massive quarks and leptons begin to decay into smaller ones. Many of the tauons, muons and associated anti-particles have now decayed into their less massive lepton counterparts. On the quark front the universe consists of mainly up and down quarks now; the building blocks of protons and neutrons.

Here is a very exciting quirk I like to ponder over; There are around 10 billion photons in the universe for every baryon. This implies that just before the melee of quark-antiquark annihilation took place, for every 10bn antimatter quarks there were just over 10bn matter quarks. Curious. One might be inclined to say that this is chance; accidental design and in all honestly it could be the case. I am a physicist however; and to me that is a clue to the universe. Accidental chance is just another way of saying we don’t understand it yet. A leading theory on this front goes back to the X-bosons we discussed earlier, citing that perhaps the X-boson decay has a propensity to favour the decay into particles over antiparticles. This is work for the theorists; since we are a very long way off being able to replicate the energy levels required on Earth to create the X-boson; and we might not be able to at all.

The reason I like to ponder this so much is it really is quite a startling fact. If the numbers were exactly equal, then my goodness there would be no matter just a sea of photons. I certainly would not be writing this today; my entire existence in the hands of a disturbance in the balance of one part in 10bn.

Hadron city

This next stage in our journey is actually quite a long one; this stage lasts from 10^-5s to 100s; this is an important part in the journey because this is where the neutron gets locked up. This again is an important part in the story, and a further example of which we could very well not be in existence. I hope you are starting to feel a little luck to be alive!

As the temperature begins to drop to around 3 x 10¹² K, stable baryons (protons and neutrons) begin to form from the up and down quarks that escaped annihilation. As the mass energy of the proton and neutron is approximately 1 GeV, the same as the current mean energy of the universe the confinement of quarks is exhibited from this time onwards – there is insufficient energy to escape to distances much larger than the nucleus.

Now the mass of a neutron is a little more than a proton. This could have spelled total disaster for the human race. There is a tendency within Physics for reactions to occur that flow towards “lower energy” arrangements (I use the inverted commas to highlight the fact that this energy is clearly just given out in one form or another). This phenomenon should be familiar to everyone; if you place a ball on the side of a hill it has gravitational potential energy. Let go of it and it will shed its gravitational potential energy, dissipating it as kinetic energy leaving the ball eventually at the bottom of the hill in a lower energy state. We would need to perform work on the ball to restore its former energy. Well the same can be said for decay within physics from one particle to another; since the mass of particles has an equivalence with energy. As the temperature of the universe began to cool, decay from a proton to a neutron was far less likely since it requires energy. The other way however gives out energy, which nature really does enjoy. As the cooling continues, the decays are dominated by the final weak interaction; the energetically favourable neutron decay:

 

So what’s the big deal? Well if this continues onward what are we left with? Loads of protons, loads of electrons and a bunch of antineutrinos. You try to build anything fun with those ingredients and I think you will find yourself very stuck with hydrogen. That would not be a very interesting universe at all. Again, our existence is embedded in the details. Fortunately for us the half life of a neutron is around 10 minutes, which means that before all of the neutrons could decay they were bound up in the nuclei of simple atoms; at which point they are protected from the decay described above. Again this delicate balance, where the decay was halted to allowed a ratio of 7 protons to every neutron to be achieved. If the half life of a neutron had of been tiny? Disaster. This binding in nuclei is discussed in the next section.

As the universe begins to approach 10 seconds in age, the primordial electrons and positrons begin to annihilate, which produces more photons still; although leaving an excess approximately equal to the 1 in 10bn, balancing out the charge in the universe.

Neuleosynthesis

This is the point in the universe when the first primordial nuclei are able to form; we are talking now around 100s after the big bang when the temperatures are more in the region of 10⁹ K. Initially there is a two-way reaction, as protons and neutrons combine to form nuclei emitting a gamma ray photon and nuclei splitting back into protons and neutrons with the addition of the gamma ray. However, as the temperature begins to cool, nuclei production begins to become energetically favourable.

More complex nuclei do form, however no nuclei with a mass number of 7 or more could have survived this early stage of synthesis; owing to the fact that there are no stable nuclei with a mass number of 8 (beryllium-8 has a mass energy which is more than two helium-4 nuclei). As the binding begins to finish, as described above everything is locked down in the 1:7 ratio of neutrons:protons. At this point the universe is essential made of the same things that it is today; 10 billion photons for every baryon, 7 protons and electrons for every neutron and a swarm of neutrinos and antineutrinos travelling the universe unhindered.

The rest

It is of great wonder to me, how we reach this point and we can bundle the rest into one section but truly we can. All the important exciting things happened in the first 100s or so. From this point, we are talking about a history of 300 years from the big bang to today – around 13.81 billion years afterwards. Clearly there is a little gap from where the last section left off; this is of little worry the universe simply got bigger and cooler; a common thread throughout.

As the average energy in the universe continues to fall, it reaches around 10eV; this is the last ever interaction the primordial photons have with the matter in the universe, for this is the minimum energy required to excite an atom to its next energy level. Quantum physics tells us smaller units will not be utilised. This is a special moment; as the final interactions happen, the radiation is then just left to travel through the universe; only to be picked up by Penzias and Wilson some billions and billions of years later, in a curious background radiation that has a temperature of 2.73 K, roughly equal in all directions. This is the cosmic microwave background radiation we spoke of when we talked about blackbody emission spectra. If you look at this map of the cosmic microwave background radiation you will see it is a (very slight) exaggeration to say that the radiation is totally uniform. The differences, or wrinkles, that you can see are the result of slight variations of matter distributions in the very early universe when the final interactions took place – the one I just described.

 

So what is left? Well why were the matter distributions a little off? This is where gravity starts to rule as clumping takes place. You have slightly more matter in one place and it clumps together. Well then it is more massive so it can attract smaller pieces and so on. Some time after the last interaction matter had with the photons, but before the universe was a billion years old galaxies begin to form. Clumps of gas condense together and form the early stars. This can only occur where the gas is dense enough for nuclear fusion reactions to take place, bathing the universe in electromagnetic radiation.

The rest of the history is a little more familiar, but we should fly over it anyhow. Stars continue their nuclear reactions until they run out of fuel. In the case of a star such as the sun, the amount of nuclear processes available are limited since the temperatures are not hot enough for helium fusion, however for other larger stars this is no such issue. In fact, you can cook up elements to around the size of iron within a star. Once you start to go above this limit, you can no longer extract energy by fusion (this is the territory of fission) and so the star reaches the end of its life.

There are some important processes that happen near the death of a star. For example, large numbers of free neutrons are emitted which can slowly be added one at a time to the heavy elements, some decaying into protons allowing stable nuclei of heavier elements like lead and bismuth to be created. This is called the s-process and is actually how the majority of stable non-radioactive nuclei in the world were created.

The core of the dying star, now largely iron has no energy left. The outer layers of the star begin to collapse inwards under the intense gravitational lure of the core to densities comparable to an atomic nucleus (that’s dense). However, then it stops; it halts and it rebounds. A massive supernova in which 90% of the star’s matter is strewn out across the universe. The core which is left behind will either be destined to become a neutron star or a black hole. This is how stardust gets flung all over the place, spreading the matter out.

The immense temperature in the dying star causes electrons and protons to form free neutrons which in the dying pressure cooker environment can combine with existing nuclei to build larger unstable nuclei; this is the r-process under which the naturally occurring radioactive materials which are not decay products were created.

The star cycle repeats endlessly; however next time around there are richer pickings around the gas balls of creation; with all that star dust strewn out everywhere. What does this mean? Well it means as the new stars form from gas there remains all of this dust circling it in a disc. Oxygen, silicon, carbon, iron etc; elements which start to clump up to form rocks, which clump into larger and larger rocks. Eventually they are massive enough to clear a path and pull themselves into spherical shaped objects. Of course we are now talking about planets and this is where the story ends – planets start to form, carbon begins to dance and evolve and we hand over to our biological friends.

So there we have it; we leave the story, 13.81 billion years on, sat in a very unimpressive patch of the universe confined on a ball of rock. Is there anything remarkable about us? Perhaps. I finish like I started; the greatest curiosity is our curiosity, which is surely the reason you have read this far – the inhabitants of the rock, cooked up in the centre of a star desperately seeking to understand and assign meaning to a seemingly impossible chain of events; in which statistics rule yet appear to be overcome time and time again to bring you the human race; along with, no doubt lesser and greater civilisations all with one thing in common; they exploded out of the heart of a star.