Why the concept of the big bang
1. Witness to the Big Bang: galaxy movement
Astronomers generally differentiate between two types of galaxy movements: On the one hand there is a purely kinematic effect: all galaxies have one Proper movement (Peculiar movement), which, as in the case of the Andromeda Galaxy, can even point towards us (and result in a blue shift of the radiation of the Andromeda Galaxy). However, this is only one local effect, i.e. such proper motions are only important if the galaxy in question is relatively close to the Milky Way. The physical cause of the proper movement is the mutual, gravitational attraction of the galaxies and galaxy clusters with one another (see also tidal forces).
On the other hand, there is a galaxy movement which, from a cosmological point of view, i.e. at great distances, clearly dominates. This movement is dictated by global spacetime, which describes the universe as a whole. Such a description is possible with Einstein's general theory of relativity, and it leads to the Friedmann world models. These models state that the universe is shaped and dynamically controlled by everything that is in it. The cosmic ingredients are baryonic matter, dark matter and dark energy. The dark energy becomes important in late development phases of the universe and especially then determines its dynamics: the dark energy in the form of a cosmological constant pulls the cosmos apart and drives an (even accelerated!) Expansion. The galaxies and galaxy clusters as part of the cosmos have to go along with this movement because they are embedded in the expanding space. These global movement of all galaxies is an escape movement because the expansion takes place equally in all directions. Astronomers refer to this phenomenon as the Hubble Effect, which was discovered astronomically in 1929. With this expanding space-time, the 'Hubble Current', all galaxies and bodies of the universe are moved with it.
2. Witness to the Big Bang: cosmic background radiation
The cosmic background radiation is the oldest signal that humans have ever measured in nature! It is about thermal radiation that reaches us uniformly from all sides from the depths of space. A temperature of only about three Kelvin can be assigned to the almost perfect black body spectrum. The radiation is interpreted in cosmology in such a way that it is the relic of the expanding fireball that was blown apart by the Big Bang. There was a hot start. Over time, the fireball expanded and cooled to the low temperature observed. This interpretation is supported by the observed Isotropy the cosmic background radiation: it comes equally from all directions. The complete distribution of the background radiation Cosmic Background Radiation, CBR or Cosmic Microwave Background, CMB) in the sky testifies to the hot state of the early universe more than 13 billion years ago.
With a cosmological redshift of z ~ 1100 the universe was in the era of recombination. In this development phase the atoms were formed and the radiation was just separating from the matter because the scattering cross-section for photons decreased drastically. In other words: the universe became transparent! In the local universe (z = 0) are still in every cubic centimeter 412 photons the background radiation included. The 3K radiation is the most important evidence of a big bang, which opponents of this model would have to explain first.
3. Witness to the Big Bang: distant star explosions
Another pillar of the big bang theory are extremely distant white dwarfs who have a fight with their companion star. For the dwarf, the liaison ends not only miserably, but fatally. What happens physically? Well, as is the case with relationships, everything is very romantic at the beginning: The neighboring star gives the dwarf presents and lovingly feeds him with matter. However, dwarfs can't take that much. They cannot grow at will through this mass influx: if the critical Chandrasekhar mass is exceeded, the dwarf becomes unstable and explodes in oneSupernova type Ia. A sensational divorce indeed! Since the Chandrasekhar limit is almost fundamental, because it only depends on the composition of the star, the explosion is more or less the same for all white dwarfs in the cosmos. It ignites with an always similar explosion energy and is therefore a very good standard candle in astronomy.
With that they are ideal distance indicators and are ideal for cosmologists to measure the universe and to determine the cosmological parameters. The supernovae that have now been observed in large numbers are also compatible with the Big Bang.
4. Witness to the Big Bang: Frequency of the first elements in the cosmos
The light chemical elements, hydrogen, helium and lithium, have already entered the cosmic stage in front of the stars. The universe was hot enough in its early stages to produce elements of its own through thermonuclear fusion. This 'reactor phase' ended abruptly when the universe became too cold as a result of expansion. This process of element production of the universe itself is called in astronomyprimordial nucleosynthesis. The measured distribution of the primordial elements fits the model of the hot big bang perfectly.
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