# Origin of the Universe

Looking deep inside microscopic particles, physicists need to collide them with high kinetic energies. The smaller parts of matter they want to observe, the higher energy they need. This is why they build more and more powerful accelerators. However, the accelerators have natural limitations. Indeed, an accelerator cannot be bigger than the size of our planet. And even if we manage to build a circular accelerator around the whole earth (along the equator, for example), it would not be able to reach the energy of $\sim 10^{17}$MeV at which the grand unification of fundamental interactions takes place.
So, what are we to do? How can we test the theory of everything? Is it possible at all? Yes, it is! The astronomically high values, like $\sim 10^{17}$MeV, should be looked for in the cosmos, of course. Our journey towards extremely small objects eventually leads us to extremely large objects, like whole universe.
Equations of Einstein's theory of relativity can describe the evolution of the universe. Physicists solved these equations back in time and found that the universe had its beginning. Approximately 15 billion years ago, it started from a zero size point that rapidly expanded to the present tremendous scale. This latter point will ever not sufficiently stressed: Big Bang has been an Expansion of Space, and not an expansion in space. At the first instants after the explosion, the matter was at such incredibly high density and temperature that all particles had kinetic energies even higher than the unification energy $\sim 10^{17}$MeV. This means that at the very beginning there was only one single force and no difference among fundamental particles. Everything was unified and simple.
You may ask So what? How can so distant past help us?. In many ways! The development of the universe was governed by the fundamental forces. If our theories about them are correct, we should be able to reproduce (with calculations) how that development proceeded step by step. During the expansion, all the nuclei and atoms in the cosmos were created. The amounts of different nuclei are not the same. Why? Their relative abundances were determined by the processes in the first moments after the explosion. Thus, comparing what follows from the theories with the observed abundances of chemical elements, we can judge validity of our theories.
Nowadays, the most popular theory, describing the history of the universe, is the so-called Big-Bang model. The diagram given in Fig. 15.11, shows the sequence of events which led to the creation of matter in its present form.
Nobody knows what was before the Big Bang and why it happened, but it is assumed that just after this enigmatic cataclysm, the universe was so dense and hot that all four forces of nature (strong, electromagnetic, weak, and gravitational) were indistinguishable and therefore gravity was governed by quantum laws, like the other three types of interactions. A complete theory of quantum gravity has not been constructed yet, and this very first epoch of our history remains as enigmatic as the Big Bang itself.
The ideal democracy (equality) among the forces lasted only a small fraction of a second. By the time $t\sim 10^{-43}$ sec the universe cooled down to $\sim 10^{32}$K and the gravity separated. The other three forces, however, remained unified into one universal interaction mediated by an extremely heavy particle, the so-called $X$ boson, which could transform leptons into quarks and vice versa.
When at $t\sim 10^{-35}$ sec most of the $X$ bosons decayed, the quarks combined in trios and pairs to form nucleons, mesons, and other hadrons. The only symmetry which lasted up to $\sim 10^{-10}$sec, was between the electromagnetic and weak forces mediated by the $Z$ and $W$ particles. From the moment when this last symmetry was broken ($\sim 10^{-10}$sec) until the universe was about one second old, neutrinos played the most significant role by mediating the neutron-proton transmutations and therefore fixing their balance (neutron to proton ratio).
Already in a few seconds after the Big Bang nuclear reactions started to occur. The protons and neutrons combined very rapidly to form deuterium and then helium. During the very first seconds there were too many very energetic photons around which destroyed these nuclei immediately after their formation. Very soon, however, the continuing expansion of the universe changed the conditions in favour of these newly born nuclei. The density decreased and the photons could not destroy them that fast anymore.
During a short period of cosmic history, between about 10 and 500 seconds, the entire universe behaved as a giant nuclear fusion reactor burning hydrogen. This burning took place via a chain of nuclear reactions, which is called the $pp$-chain because the first reaction in this sequence is the proton-proton collision leading to the formation of a deuteron. Nowadays, the same $pp$-chain is the main source of energy in our sun and other stars.
But how do we know that the scenario was like this? In other words, how can we check the Big-Bang theory? Is it possible to prove something which happened 15 billion years ago and in such a short time? Yes, it is! The $pp$-chain fusion,
is the key for such a proof.
As soon as the nucleosynthesis started, the amount of deuterons, helium isotopes, and other light nuclei started to increase. This is shown in Fig. 15.12 for $^{2}$H and $^{4}$He. The temperature and the density, however, continued to decrease. After a few minutes the temperature dropped to such a level that the fusion practically stopped because the kinetic energy of the nuclei was not sufficient to overcome the electric repulsion between nuclei anymore. Therefore the abundances of light elements in the cosmos were fixed (we call them the primordial abundances). Since then, they practically remain unchanged, like a photograph of the past events, and astronomers can measure them. Comparing the measurements with the predictions of the theory, we can check whether our assumptions about the first seconds of the universe are correct or not.
Astronomy and the physics of microworld come to the same point from different directions. The Big Bang theory is only one example of their common interest. Another example is related to the mass of neutrino. When Pauli suggested this tiny particle to explain the nuclear $\beta$-decay, it was considered as massless, like the photon. However, the experiments conducted recently, indicate that neutrinos may have small non-zero masses of just a few eV.
In the world of elementary particles, this is extremely small mass, but it makes a huge difference in the cosmos. The universe continues to expand despite the fact that the gravitational forces pull everything back to each other. The estimates show, that the visible mass of all galaxies is not sufficient to stop and reverse the expansion. The universe is filled with a tremendous number of neutrinos. Even with few eV per neutrino, this amounts to a huge total mass of them, which is invisible but could reverse the expansion.
Thus, the cooperation of astronomers and particle physicists has led to significant advances in our understanding of the universe and its evolution. The quest goes on. Albert Einstein once said "The most incomprehensible thing about this Universe is that it is comprehensible."