The universe
Author: Tomasz Jędrzejewski
Living on the surface of our so beautiful planet, we often forget about the size of the universe around us. This is the biggest space that the human is able to explore, because it contains all the material objects and processes that take or took place. But the origins, the future and the real laws that rule it are still not well known. The physical theories trying to describe it are so complicated that an ordinary person after some explainations still does not know, what is going on. Let us learn more about such ideas as M-Theory or supersymmetry.
How does it work?
The scientists explore the mysterious world of quantum physics deeper and deeper, trying to discover the fundamental laws, which other rules originate from. Quantum theories, whose effects are visible in the micro-distances, differ significantly from those ones we can observe every day. The basic tool that allows to create the theories are symmetries. They are defined as an invariance – lack of any visible change under any kind of transformation. If a property cannot be obtained with a symmetry, the theory is believed to be incomplete. Thanks to symmetries, it was possible to detect the properties of elementary particles. Most of the discovered symmetries take place only when the energy is high enough and all the scenarios concerning the history and the universe base on this fact. To illustrate it, let us imagine a couple of coins in a zero gravity state. We recognize heads and tails, but in fact the coin is symmetric with respect to the rotation. To see the heads, we simply rotate the coin. If the coins fall down a surface, some of them will show heads, and the rest – tails. We have to move them from the surface in order to change the heads into tails. If such a situation occurs, we say that the symmetry is broken.
One of the theories created with symmetries is the Standard Model describing the properties and interactions between the elementary particles. The matter is build of bosons and fermions. Four base fermions are electron, electron neutrino, up quark and down quark. To make a proton, we take two up quarks and one down. A neutron consists of two down quarks and one up. These particles belong to the first of the groups named generations. The particles from the second generation are similar to the first ones, but have different mass. Theoretically, there could be hundreds of groups, but the higher generation the less stable particles it contains. The detected generations are one, two and three. The bosons are force mediating particles. Their creation or destruction causes some interaction to exist. The Standard Model explains the origin of three forces: electromagnetic, strong and electroweak. The grativy force is not included in this model.
The experiments showed that most of the assumptions predicted by the Standard Model. The only thing that has not been detected is the Higgs boson, which is supposed to give the mass. Despite, the theory is being criticised. There are 19 free parameters that must be determined experimentally, because we cannot say, why they have exactly that value. Moreover, it does not explain some of the defects. One of them is, why the universe contains matter and no antimatter. There are new theories constructed, called GUT's (Grand Unification Theories). They assume that the three forces are kinds of one, universal force and the symmetry takes place, when there is a huge amount of energy, available in the first moments of the universe. The greatest goal is to unify the grativy, too. It is much more complicated, because it requires new mathematical instruments. The first "theories of everything" were created in 1980 thanks to John Schwarz and Michale Green. They published a model, where all the particles and forces are vibrations of supersymmetric strings, approximately 10-35 m long (The Planck length). In 1983, Edward Witten and Luis Alvarez-Gaumé proved the incoherences in the string theory. They have been removed by increasing the number of the universe dimensions to 10. It has been also showed that this is the only correct theory of quantum gravity. In 1995, Edward Witten published a new, revolutionary M-theory, whose special cases were the models by Schwarz and Green. There have been created many M-theory versions so far. The differences lie, for example, in the number of dimensions. Anyway, it is still too big for a typical person. If there are really 10 dimensions, why do we see just 3D space and the time? The answer is that the rest are shorter than a single atom, and that is why we are not able to detect them.
Neither of the M-theory assumptions has been detected in an experiment. They are supposed to require much more energy than we are able to provide. What is more, most of the existing quantum gratity theories work with every number of dimennsions and every value of physical constants. Nobody has explained, why the light speed is 299,792,458 km/s and the Planck constant equals 6.626,0693 (11) × 10-34 J/s. If they come from another theory, where to find it? According to the anithropic principle, their values are exactly measured so that the life on Earth could appear. The followers prove that the smallest change of them would change the universe. It is possible that there are many universes with different „starting conditions”. The our one is special only because we live in it. The other explaination tells that the universes evolve, like animals. Black holes are the beginning of the new universe, which inherits the properties from the parent, but with small changes. The universes, where the black holes are possible, spread faster than the rest. It does not matter, what is the answer. Probably we will never discover it.
The history of the universe
The idea of Big Bang appeared years ago. In the 60. the scientists found the proffs that allowed to approve it, like the cosmic microwave background radiation. If we assume that the universe is really spreading, we can easily say that it must have been smaller in the past. It has been even managed to calculate the age of the universe. The beginning took place 13.4 billion of years ago. We can follow its history from the first 10-43 second of the existence, that is from the moment, when the gravity separated from the other forces. The conditions were extreme. The temperature, pressure and density were reaching the infinity. 10-35 seconds after the Big Bang, the cosmic inflation began. The universe became trillions of times bigger then. The negative pressure is supposted to have caused it – the growing up did not change the density. The symmetry was totally broken 10-12 seconds after the Big Bang. The universe was built of a hot soup full of quarks and other elementary particles. As it was becoming colder and colder, the particles began to form protons and neutrons. At the same time, their opposities appeared, antimatter. The space was very dense, and the annihilation destroyed most of the particles. A small piece of matter that we see today is all that survived. During the next three minutes, the universe was a nuclear hell. The hydrogen nuclei were changing into helium in a nuclear fusion. Once the process stopped, the universe consisted of 88% hydrogen, 22% helium and rarely – lithium. Heavier atoms did not appear, because it was too cold to continue the fusion. Next 380 thousand years, the space was filled with atomic nuclei and free photons that interacted with each other. The recombination, which began then, changed the structure of the universe. The sign of that times is the cosmic microwave background radiation, which shows the universe, as it was in that moment.
The universe is growing bigger and bigger even nowadays. The light from the farthest areas was emitted 13 billion year ago. Observing such far objects, we see them, how they looked like shortly after the Big Bang. The first cosmic structures ever formed were quazars. They are supposed to have been small-sized objects, because they were able to change its state rapidly. A quazar produces a huge amount of energy, approximately 1041 W, which could be compared to the radiation power of whole galaxy. No doubt it is the brightest cosmic object known to human. If there were a planet 33 light-years from it, the quazar would have shined as strong as the Sun on its sky. By comparison, the Alpha Centauri star, 4.3 light-years from Earth, is only a yellow dot visible at night. Similar objects typical to the young universe were radio galaxies that were very luminous at radio wavelengths. However, the emission sources in the radio view are not the same, as the light emission sources. The explaination is the strong formation process of new stars.
The first stars that illuminated the space are called Population III stars. They were much more massive than today ones and they shined less than million years. There were no heavy particles in the universe, so the planets and comets could not appear. The material for them was provided by the first supernova explosions.
Planck epoch
This term describes the first 10-43 seconds of the universe. Because of the lack of any proved quantum grativy theory, we do not really know, what happened then. However we can make some suppositions using one of the M-theories. One of the models assumes that in the moment t=0, the universe was a hyperspace, 10-dimensional creation with one, big supersymmetry including all the forces. It was very unstable and at the end of the Planck epoch, it falled apart on two smaller ones, connected with the gravity. The 6-dimensional one, began to shrink, and the second one (4D) started to grow up rapidly. Many scientists say that it is impossible to guess, what was in the beginning.
What's next?
The future of the universe is as mysterius as its origins. After 13 billion years, it is a place where the stars are born and die, and a group of physicists on one of the planets tries to explain, what is going on. We know that the space is growing up and the expansion is faster in the last four billion years. We would like to present one of the possible scenarios for the future called heat death.
In approximately a hundred trillion (1014) years the last star will run out of hydrogen. All the matter will be collected in planets, white dwarfs, neutron stars, hypotetical quark stars and black holes. From time to time, two dwarfs will go into a collision and the product will be the only source of light in the universe. Then the universe will reach the age of quadrillion, planets will be flung from their orbits and thrown to black holes. Later, the same will happen with stars. In the space, there will be only sin and supermassive black holes. If estimates on the half-life of protons are correct, in 1036 one-half of the matter will decay into gamma radiation and leptons. The amount of protons is estimated at 1080 items, so the last one will disappear in 1040 year. The Black Hole Age will begin and it will continue to 10100 year. The black holes evaporate, so they will start disappearing. The end of the last one will take place in about 10150 years. The only thing in the universe will be photons then. However, this state will not continue to the distant time. One day the universe will reach the zero-state energy. Nobody knows what could happen next.
Other sources tell that the death of the universe will be much quicker – the matter will be torn apart by the expansion (called Big Rip). We have to remember that when we look at the object 13.6 billion light-years from Earth, we see it in a place, where it was then. Currently, it may be approximately 46 billion light-years from us. It is supposed that the Big Rip has already happened there and in our neighbourhood it will begin in 20 billion years. 60 million years before the end, the gravity will be too weak to hold the Milky Way together. Three months before the end, the solar system (or betters – its "ruins", because the Sun will die much earlier) will be destroyed. In the last minutes the planets will disappear and the last second will rip apart the atoms.
Theories that say that the expansion will stop one day are not very probable and popular among the scientists. The new discoveries show that there is not enough matter in the universe to reverse the history.
The end
The universe is a mysterious place, in fact. Maybe one day, the physics laws will lead us to the answer, what was at the beginning and what is the future. It does not matter, what scenario is going to happen, because our civilization will disappear. We will be able to watch everything from the heaven.