The countdown to the beginning

The question where we've all come from has been fascinating for the ages. Quite many of us have heard the scientific explanation of the Big Bang that gave the birth for the universe. Georges Lemaître, a Belgian cosmologist and Catholic priest, introduced Big Bang in his scientific paper in 1931, stating that the observable universe began at a definite point in time. Edwin Hubble had confirmed through analysis of galactic redshifts in 1929 that galaxies are drifting apart. Thus expanding universe could be traced back in time to an originating single point that is actually approximated to be happened nearly 13,8 billion years ago. There were different theories of the steady-state universe still in the following decades till 1964 when the Cosmic Microwave Background (CMB) was discovered, that had been a prediction of the Big Bang theory. The CMB is an uniform background radiation throughout the universe caused by the high temperatures and densities in the distant past. Actually, the universe was 370 000 years old when this radiation released from the matter and become observable. Anything before that has not been directly observed but there are quite solid theories with predictions till the first fractions of a second after the birth of the universe. Cosmologists have consensus that the universe was once extremely hot and dense in the beginning.

The higher are temperature and energy density the more collisions happen between particles which leads at a critical temperature to a phase transition. For instance, liquid water becomes gas at 100 Celsius. At 6000 K temperature the atom structure starts breaking to plasma of nucleons and electrons. At 10¹³ K the nucleon structure breaks to quark-gluon plasma which has been experimentally reached on the earth. This type of conditions prevailed till the universe was 10^-6 seconds old and after that universe cooled enough that hardons (nucleons) started to be created. The established standard model of the particle physics implies that at 10¹⁵ K the Higgs field went through a phase transition and acquired its current vacuum expectation value globally in the universe. The outcome from this complex transition was that all fundamental particles acquired their rest masses first time by the interaction with Higgs field, and since then were able to move below speed of the light. Also as a result, the electromagnetism and the weak interaction got separated in the electroweak symmetry braking, the age of the universe being 10^-12 seconds. What type of particle states were before that are beyond the energy ranges achievable in the particle collider experiments. 

Naturally, a variety of theories has emerged to extend the standard model to higher energy densities. One notable is the Grand Unification Theory (GUT) first proposed by Howard Georgi and Sheldon Glashow in 1974. In this model electroweak and strong interaction were unified till the symmetry got broken caused by a phase transition of a Higgs like field. That would have happened when the universe had the temperature of 10²⁷ K and the age of 10^-36 seconds. The unified electronuclear force fluctuated the early matter between quarks and leptons in a composed form. The temperature being extremely high, any matter quickly collides with other matter or energy and is converted back into energy, and hence the universe was radiation-dominated. Only very energetic (heavy) and unstable matter particles could have been pair-produced for a short time. When the temperature cooled, more stable and lighter particles were able to exist. After the GUT symmetry breaking a large amount quark-antiquark pairs would have been produced. Later in the electroweak epoch leptons become electron-positron and neutrino-antineutrino pairs. Even though the original GUT model has some shortcomings (like the misprediction of the proton decay) it has been used for a foundation for more successful modified theories like Supersymmetry (SUSY). The time before the GUT epoch is called the Planck epoch between 0-10^-43 seconds (1 Planck Time) when all fundamental forces including gravity were believed to be unified. 

There's a quite clear consensus that the early universe went through a period called cosmic inflation that can explain the homogeneity and flatness of the currently observed universe. In many theories, the inflation would have expanded the space exponentially at least by a factor of 10²⁶  which has happened between 10^-33 and 10^-32 seconds. This means that the currently observable universe with a diameter of 93 billion light years was only 7.7×10^-30 m before and 0.88 mm (a size of a grain of sand) after the inflation. New space was rapidly created spreading the wave lengths of radiation over the distances. All the energy including matter was merely in the form of radiation. It is thought that GUT symmetry braking released energy to start the inflation but the mechanism is quite obscured. If there were any particles earlier the inflation would have blown them very far away.  During the inflation the universe super cooled from 10²⁷ K to10²² K. The universe is thought to be reheated after the inflation when the field driving the inflation (Higgs like) decayed filling the whole universe with quark-gluon plasma and electroweak bosons. It is thought that dark matter emerged same time. Following the inflationary period, the universe continued to expand, but at a slower rather constant rate. The expansion of the universe started again to accelerate after 7,7 billion years and the driver for it is called as dark energy. Dark energy has also been explained by the inflation field with much lower energy density than in the early universe but strong enough to have overcome gravitational pull of the distant galaxies.

Some Big Bang theories have predicted that during Planck epoch there was the initial singularity containing all energy and space-time of the universe. Singularities are allowed in the general relativity, and they occur in a wide range of realistic solutions, notably at the beginning of an expanding universe or inside black holes. Friedmann-Robertson-Walker solutions appear to describe correctly our expanding universe. The assumption is that the early universe was perfectly uniform so that there were no places for black holes to form, even when the density was extremely high. Also inflation theory predicts temperatures and spacetime curvatures of different regions to be nearly equal. According to relativity theory, space does not remain static, but in the most cases it either expands or contracts. But why it initially chose the former is still a mystery. In some ways, you can think of the universe as a black hole turned inside out. A black hole is a singularity into which material flows. The universe is a singularity out of which material has flowed. A black hole is surrounded by an event horizon, a surface inside which we cannot see. The universe is surrounded by a cosmological horizon, a surface outside of which we cannot see. However, there are fundamental differences between the emergence of the universe and blackholes. The universe known to be homogeneous and isotropic cannot be a black hole. However, Stephen Hawking among others have suggested that the universe could be a white hole emerged from a collapse of a super massive black hole. In the Einstein–Cartan theory, the collapse reaches a bounce and forms a regular Einstein–Rosen bridge (wormhole) to a new, growing universe on the other side of the event horizon.

Using only the general relativity to predict what happened in the beginnings of the universe has been heavily criticized, as quantum mechanics becomes a significant factor in the high-energy environment. In a pure singularity space and time cease to exist and thus, it is nonsensical to ask "what came before or after this event or where time began”. How was the first Planck time unit ticked that transitioned the spacetime to emerge? Apparently there was time in the beginning in order that anything happened. But spatially there might have been no dimensions. As Stephen Hawking said, "there is nothing south of the South Pole" as lines of latitude shrinks to a single point at the South pole. On the other hand, according to the data from WMAP and Planck, the universe never achieved a temperature greater than about 10²⁹ K. This number is enormous, but it’s over 1,000 times smaller that the temperatures we’d need to equate to a singularity. 

The universe, as far as we can observe it, only contains information from the final 10^-33 seconds or so of inflation. Anything what happened prior are wiped by the nature of inflation. It could be a horizon beyond we cannot get any information like event horizon of a black hole. Moreover, it may be a critical density or temperature where known physical rules break down. Beyond it, the general relativity as such probably did not apply and matter and radiation were never in a singularity, instead all energy of the universe could have been in the inflating spacetime itself. Thus it is questionable if the Planck or GUT epoch ever actually happened as the GUT energy scale is close to the upper limit given by the current observations. Furthermore, the extrapolated zero time for the singularity might not be the beginning of the time. In the past infinite model the inflation had lasted forever. In the cyclic model, earlier universe went through a slow contracting ekpyrotic phase that transitioned to the inflation. On the other hand, our universe may have emerged from any collapse in the possible multiverse. The source of the inflation energy and a trigger for it if any are the biggest mysteries.