Out of nowhere, a flash of light appears. It is incredibly tiny, yet contains everything.
The tiny sparkle moves past at incredible speeds, photons racing against each other and against space itself, but losing that race hopelessly.
The race takes only a fraction of a second so tiny that if it was slowed down to be an eye-blink long, a real eye-blink would last more than the Universe itself. The duration of this race was so short that it is impossible for us to comprehend, let alone notice. However it is the cause we see the Universe as it is.
An eternity later, although still less than a blink of an eye since the end of the race, things start to appear. What was once a uniform mass of everything gets separated into parts. These parts start forming structures, which form larger structures, which, in turn, form even larger ones.
Now we are past the eye-blink limit, and even past a human lifetime. But the process still continues. Everything expands, although not as fast as in the primeval race. Particles zoom around. So far, there are only a few different ones, and it takes eons for heavier ones to form.
Then… silence. And darkness.
For eons more everything is dark. Or bright. While there is a lot of light around, it has nothing to be reflected from, therefore to anyone looking at the state of things, it might just as well be absolutely dark.
But then, after more eons have passed – BOOM! The first source of some different light appears.
It still takes more eons until the first creature on an insignificant cosmic piece of rock crawls out of the cave, looks up to the sky and asks – “How did all this appear?”
*At this point, the narrator sits back and falls silent for a moment, gently sipping his drink and looking at his audience*
Now that the lame attempt at a narrative introduction is done, let me present to you
How It All Began
I start this article with an aside that may be interesting to you. There are definitely numerous approaches how one could tackle the question of the topic. Without recourse to philosophical discussion ("What does ‘all’ mean?" etc.), I can still talk about the theory itself, about hwo it was developed, its observational support, mathematical foundations, alternative hypotheses and other things. But I chose one which is the most interesting to me, and probably illustrates (although not in the best way) one key difference between maths and physics that I hoped to portray here. You see, maths is a science of concepts. These concepts usually have something to do with numbers and other things which we intuitively think of as “mathematical”, but they very rarely have anything to do with the real, physical, world around us. As a result, mathematical concepts can be explained by starting with something very simple, and seeing what happens when new things are added to it.
Physics, on the other hand, first and foremost deals with the real world. Obviously, some far-reaching fundamental areas of physics (such as general relativity or elementary particle physics) may not seem very “real”, but they are, because they try to explain processes happening in the world around us. This is also why physics is one of the natural sciences, while maths isn’t. As a result, theoretical physics (which is what most of astrophysics happens to be) is mostly the explanation of observed phenomena. This means that most explanations of physical concepts will differ from explanations of maths topics in this aspect – while he can talk about how concepts evolved and how people developed new things, I will probably go on presenting the existing theories without detailed recourse to how they were developed.
Alright, I lie, if only a little. The reason why I will concentrate on presenting the theories rather than their development is because science history was never my strong subject. I just can find neither time nor inspiration to learn all the little details. But you are not interested in this (told you, this is going to be only funny to me ;)), so I digress.
Right, where was I? Oh yes, how it all began. At least I can pretend this is related to history. I should warn you before proceeding that I will only talk about the Big Bang model for now. It is currently accepted by the majority of the scientific community to be the best model of the Universe, but that does not mean it is correct (nothing in natural sciences is correct, by definition) or that it does not have any problems. There are some shortcomings and a certain amount of handwaving in the theory, but so far, all alternatives failed much worse (more on this in subsequent parts). The model also does not explain how the Universe started and if there was anything before the Big Bang – there are some theories dealing with that, but I neither know enough to explain them, nor consider them particularly important.
With both the introduction and the warning out of the way, I can finally move on to the topic at hand. If you know me personally, you are already familiar with my wandering mind; if you don’t, you will have to bear with it. So, the Big Bang is the current model of the origins of the Universe. Depending on who you ask, it is either the model of the origin only (the “explosion”), the origin plus something extra (the “explosion”, initial expansion and early Universe nucleosynthesis) or the cosmological model of the geometry and expansion of the Universe (which is more properly called the Concordance model). I will go down the middle path and try to explain the initial stages of how the Universe became more or less the same as we see it now.
Let’s start with the Bang. “In the beginning there was nothing, not even time, no stars, no planets, no hip-hop and no rhyme,” – says MC Hawking, and he is most likely correct. The standard variation of the Big Bang model tells us that all the possible dimensions (space, time and all other funny strings from string theory) originated at the moment of the Big Bang. Therefore asking a question “What was before the Big Bang” is like asking “What is north of the North Pole”. The answer in both cases is “nothing”. The Bang itself is impossible to describe by the laws of physics we know until something like 10^(-43) seconds after it, when the Universe became larger than the Planck length. The Planck length is the proposed theoretically smallest length scale possible (there is also a Planck temperature, Planck time and Planck mass, all derived by permuting the constants h, G and c, thought to be the fundamental constants of the Universe, in various ways; more on this also later). At that time, the fundamental interactions (there are four that we now know – gravity, electromagnetism, weak nuclear and strong nuclear) started splitting, with gravity separating from the electronuclear force, and the extra spatial dimensions collapsed onto themselves (at least that’s what string theory tells us).
What was this extremely primordial Universe like? Well, to be fair, no-one is certain. The observational evidence we have comes from the time Universe was some 300 thousand years old, so anything related to earlier times is speculation. But we think that at that time, the Universe consisted of photons, neutrinos and quarks. Quarks, contrary to the name, have nothing to do with dairy products but are rather tiny particles that protons, neutrons a whole bunch of other more exotic particles are composed of. Neutrinos are weird little things that barely interact with other kinds of matter, have no electric charge, have nearly no mass and move at speeds so close to the speed of light that we can’t yet detect any real difference.
A little bit after expanding past the Planck scale, the Universe entered what is called the inflation period. It lasted from 10^(-36) to 10^(-33) seconds and caused the Universe to expand some 10^26 times. Linear scale. That means its volume increased 10^78 times. This linear size difference is something like the scale difference between the diameter of an atom and the distance to Alpha Centauri. This inflation is probably the most handwavy part of the whole model (dark energy comes close, but is not necessary to the model I am talking about, it only comes into play a billion years after the Universe started). Inflation is necessary to explain the observed uniformity of the Universe, because if we say that the expansion was always uniform, then the observable points furthest away from us should have wildly varying temperature. They do not. In addition, the geometry of the Universe is very nearly flat, but probably not exactly flat, another feature that inflation explains. Explaining how it does that will take a few more paragraphs and a lot of maths, while I want to show you that the results of physics can be understood without maths.
After inflating, the Universe continued its expansion. This expansion is driven by the enormous pressure of matter and radiation in the nascent Universe. Pressure is sort-of defined as the energy density, i.e. total energy per unit volume. Energy of matter is conserved, while energy of radiation decreases as the Universe expands. This is because radiation energy is inversely proportional to its wavelength, and wavelength expands together with the Universe. As a result, pressure of matter and radiation both decrease, but pressure of radiation decreases faster. And even though the radiation pressure just after inflation is higher than that of matter, it will become smaller at some point in time. But we are getting ahead of ourselves.
I mentioned that the energy of radiation decreases. This is good, because it means that the temperature also decreases, as temperature is a direct measure of system energy (at this time, the thermal energy is so much bigger than the other kinds that it can be equated to the total energy). The initial temperature of the Universe is something like 10^20 K (that’s degrees Kelvin, which is just degrees Celsius + 273.15), but it soon decreases to 10^12. At this temperature, interesting things start to happen. Among these interesting things is a process called pair production. You see, when two photons collide with each other, sometimes they may have enough energy to produce an electron and a positron (which is an anti-electron). When an electron and a positron collide, they will always produce two photons (unless they start orbiting each other, but never mind this). If the temperature is too high, the chance of photons colliding is small; if the temperature is too low, the colliding photons don’t have enough energy to produce the electron – positron pairs. So at around 1 second after the Big Bang, electrons and positrons were produced.
Some time before that, something like 1 microsecond after the Bang, the quarks fall together into protons and neutrons. The weak nuclear interaction that governs quark structure behaviour decouples (that’s a fancy word for saying “becomes separate”) from the electromagnetic force (formerly called electronuclear and electroweak; the strong force decoupled from it during inflation) and starts pulling quarks together strong enough so that they cannot escape. Quarks tend to cluster in pairs or triplets (the reasons for that are detailed in the Standard Model of particle interactions, which I will not go into here). The pairs of quarks (actually, of one quark and one anti-quark) are called mesons and have some properties similar to those of photons. All of them, however, are highly unstable and decay into other quark structures before electrons start forming. Quark triplets are called hadrons (of the large collider fame) and are matter constituents. Of them, there is only one stable composition, which is the endpoint of all meson and hadron decays – that’s the proton. In addition, the neutron, while unstable, has a half-life of 10 minutes, so the neutrons effectively do not decay for the first few seconds and can participate in the early Universe nucleosynthesis, which I am slowly coming toward.
A few minutes after the Big Bang, the temperature drops to less than 10^9 K and pair production ceases. Some of you may know where one can find temperatures of 10^9 K. Yes, it’s the cores of most stars. And so in this stage of the early Universe, just as in stellar interiors, nuclear fusion happened. Protons fused with neutrons to form deuterium nuclei. Those fused with either neutrons or protons to form either tritium or helium-3. Those fused with either protons or neutrons, respectively, to form helium-4. Some helium-4 nuclei even managed to collide with more protons and neutrons and form lithium, but this was a very rare situation. The Universe at this point was composed of photons, electrons (and positrons, who later disappeared off to nowhere, i.e. we do not know why exactly anti-matter disappeared, but matter didn’t), protons, neutrons and two types of helium nuclei. There were also neutrinos and some lithium nuclei, but they are no longer all that important.
This state of affairs continued for more than 300 thousand years. The temperature was still too high for the stray nuclei to capture electrons – the Universe was effectively plasma (an ionised ball of gas). During this time, 70 thousand years after the Big Bang, matter density became greater than the radiation density – the Universe entered a matter-dominated epoch. Then, 379 thousand years after the Universe began, the particles recombined. Temperature dropped so much that protons and heavier nuclei could capture electrons. The Universe became electrically neutral.
You may notice that throughout this article, I used vague terms for most of the time periods. This is because, as I mentioned earlier, all that is speculation. It may be well grounded speculation, but still it rests upon theoretical models, which inevitably have some assumptions – for example, that the laws of physics as we know them were the same in the early Universe as they are now, – rather than observational evidence (the interpretation of which, admittedly, rests upon assumptions as well). You may also notice that the last number I gave, 379000 years, is rather more precise. That is because this transition from an ionised to neutral Universe marks the time when we can observe what it looked like. The cosmic microwave background radiation (abbreviated CMB, CMWB, CMBR, CMWBR or CBR, depending on where you look, with the first three being the most popular), discovered in the 1960s, is the relict of radiation from the time of this transition. Before it, the photons were constantly colliding with protons and electrons, scattering them and themselves, thus the Universe was opaque to any light. After recombination, photons in general did not interact with matter strongly, so we can see them now as they have been at that time. Well, this is not completely true because of the aforementioned expansion of the Universe stretching the wavelengths of those photons, so their energy decreased. The microwave background radiation is, as the name suggests, microwaves. Microwaves are somewhat more energetic than radio waves and somewhat less energetic than infrared radiation, which, in turn, is less energetic than visible light.
And this is where I shall end this instalment. I am sorry that this is all blocks of text, but finding decent pictures on the topic is more difficult than one can imagine. I tried to keep everything understandable to a non-scientifically minded person, but somehow I believe that I failed miserably. As a result, don’t hesitate to ask questions about anything you don’t understand – I may end up rewriting the whole article.
Jei netyčia visa šitai perskaitėte, greičiausiai klausiate savęs, kodėl gi viskas angliškai. Taip yra dėl to, kad kažką šia tema nusprendžiau parašyti ir viename tarptautiniame forume (ten keli mokslininkai darome visuomenės švietimo projektą), tad vienu šūviu bandau nušauti du zuikius. Teksto, aišku, daug, bet tikiuosi, kad jis suprantamas. Klausimai laukiami – tiek lietuviškai, tiek angliškai ;)