Cosmology - Referat
The size of the universe
For us humans on planet earth it is hard to imagine the size of the universe and it is the measurements are always changing, due to the universe always expanding. But to measure how far stars and other galaxies are away from us and the size of the observable universe we can use light, which is composed of photons.
But first the term “observable universe” has to be clarified: the observable universe is defined by all matter that we are able to observe from earth at this present time, due to the fact that the electromagnetic radiation from the very beginning of the cosmological expansion has had time to reach the earth. The edge of observable universe is estimated to be 46.5 billion light years (in all directions) away from the earth. But it must be noted that this distance is what is measured now (quite obviously) not from the very beginning of light emission. It is also known that the age of the universe is 13.8 billion light years. So the light we are able to see has travelled 13.8 billion light years and where it came from might even be 46.5 billion light years away.
The Big Bang
It all started about 14 billion years ago. At that time, the universe was infinitely small and infinitely hot. Then the compressed energy exploded within fractions of a second and expanded at infinite speed. Physicists confirm that it must have been like that with the observations of the Hubble telescope: The universe is still expanding today - much slower than at the beginning, but the expansion is always measurable. If you trace back the expansion, you eventually end up at an origin point where all the energy in the universe must have been bundled. Physicists refer to this moment as a singular state.
Shortly after the Big Bang, the universe is about ten trillion degrees hot. The first elementary particles are formed, including quarks and gluons. Seconds later, protons and neutrons form, the building blocks of future atomic nuclei. Over time, the universe cools down ever further. At around 2700 degrees Celsius, the first hydrogen atoms, lithium and helium, are formed. After 100 to 200 million years, the first gas clouds form - stars begin to shine. Scientists suspect that there were already planets orbiting the suns at this time. Our solar system, consisting of the Sun and the eight planets, was formed about 4.6 billion years ago.
The cosmic background radiation
The cosmic background radiation is a remnant from the time of the Big Bang. When the universe had grown to a sufficient size some 380,000 years after the Big Bang and the original gas was already cooled down far enough, the free electrons could be trapped by the ions and form neutral atoms. Immediately thereafter, the radiation barely interacted with matter and could propagate freely in the universe. This only occurs when the gas has fallen below a certain temperature and density limit. It is estimated that the transition temperature at that time was about 3000 Kelvin (2730 degrees Celsius).
The radiation field was not completely homogeneous at the time of decoupling. It is believed that during the expansion of the universe, small fluctuations in matter density (in local temperature) were formed. In the cooler areas, the free electrons were more likely to be trapped and the radiation decoupled from matter a little earlier than in the slightly hotter areas. As a result, small spatial inhomogeneities in the radiation field were created - inhomogeneities that are now visible as fluctuations in the CMB. Therefore, the pattern of these fluctuations detected by COBE tells researchers something about the conditions in the early universe. However, one must not equate the structures in the background radiation, as can be seen on the sky maps recorded by COBE, with real density structures. As there have been such structural fluctuations throughout space, and we can see the radiation over long distances in all directions, these fluctuations are added up over the entire line of sight. So you see the effect of a sum of fluctuations.
This so-called cosmological redshift is basically to be distinguished from the redshift by the Doppler Effect, which depends only on the relative velocity of the galaxies in terms of emission and absorption. The escape velocities of distant galaxies derived from the cosmological redshift are therefore directly attributable to the expansion of space-time. Even at distances of a few 100 Mega parsec, the proportion of the Doppler Effect is negligible. Furthermore, it follows from the general theory of relativity that the observed flight velocities do not produce relativistic time effects, as described by the special theory of relativity for movements in space. A cosmological time dilation nevertheless takes place, since the photons emitted later on of an object have to travel a greater distance due to the expansion. Physical processes therefore seem to slow down progressively with red-shifted objects (from our p.o.v).
If the radiation source moves toward the observer, the spectral line is shifted to smaller wavelengths. This is just the shift because the line is shifted to the blue part of the spectrum. Clearly you can imagine how the electromagnetic wave is compressed. If the radiation source moves away from the observer, the spectral line is shifted towards larger, red wavelengths. The wave is effectively pulled apart. This is called redshift. Conceptually, the blue shift is the counterpart to the redshift.
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