Horizon problem

The horizon problem is a fundamental problem of cosmology, which is raised by the standard model of the Big Bang and emerged in the 1970s. It raises the question, how can it be that different regions of the universe that are not in contact, because the distance between them is too large, yet can have the same physical properties such as a comparable temperature level.

This should not be possible if an exchange of properties ( such as energy, temperature, etc.) can be a maximum at the speed of light and the individual areas were not available more than 13.7 billion years ( as old is the universe ) to develop their own property characteristics. One explanation for the horizon problem offers the theory of inflation, which was developed not least due to this problem. Another option draws a less accepted theory into consideration, according to which the velocity value from the light may have changed over time.

Basic concept

If you look at the night sky, so you look consistently in the past. The light from a galaxy that is ten billion light years away from us, ten billion years was on the road to travel the long way to our eye. If we now consider a galaxy that is ten billion light -years away in one direction to observe and then looks to another in exactly the opposite direction of view, the distance between the two galaxies to each other is a total of 20 billion light years. This means that the light of a galaxy to date, the other galaxy may not have reached, as the universe has a detectable age of 13.7 billion years ago and this time period is not sufficient for the light, the distance between the two galaxies had to travel to. Expressed Universally speaking, that portions of the universe that are visible from us stay, for other areas of space for the time being invisible and therefore must be outside the horizon of their respective observable universe.

According to the basic physical theories, it is not possible that physical information can spread faster than light. The context information in this context means any kind of physical interaction. For example, heat flows from hotter to cooler areas usually what in physical terms, represents a type of information exchange. Following this example, it is the two galaxies is not possible to exchange any kind of physical information. You have in other words, no causal contact with each other. Therefore, it can be concluded that they differ in their physical properties, or more fundamentally, that all would be want different properties in different areas in the universe.

Contrary to this expectation, the universe is in fact shows, however, an extremely homogeneous. For example, the cosmic microwave background radiation (CMB ), which fills the universe, everywhere almost exactly the same temperature of about 2,725 K on.

The temperature difference is so small that it only recently been possible to develop tools that are able to measure these differences. This fact represents a serious problem, because even if the universe had begun with slightly different temperatures in different areas, then it should not have issued no way to achieve this uniform temperature level which can be observed at the present time. According to quantum physics, this initial temperature difference would have due to the Heisenberg uncertainty principle actually persist since the Big Bang, since it can be considered on the other side as well as impossible, that the universe has formed all exactly the same properties.

The extent of this problem is considered to be relatively substantial. According to the Big Bang model reached the universe after its density had decreased due to its expansion, eventually a point at which the photons in, mix ' of particles were not directly pressed together and they, instead, from the plasma, decoupled ' and in the universe like a fireworks of light ' spread. This birth of the background radiation had probably occurred about 300,000 years after the Big Bang. The volume of potential property exchange at that time corresponded to a diameter of 900,000 light years, shall be the speed of light and the rate of expansion of space in the early universe based. Now, the entire space has continued at the same temperature, although it has reached an extent of at least 46 billion light years.

Inflation

A solution to this phenomenon, which could also explain some other basic questions such as the problem of the flatness of the universe, provides the theory of inflation. After that there were between 10-35 and 10-32 seconds after the Big Bang, a small period with a rapid, exponential increase in expansion (titled as inflation). During this period of inflation, the universe has expanded by a huge factor faster than light and pulled the light in virtually any direction with this.

If this theory is correct, inflation solves the horizon problem in that the entire universe was causally connected before this inflationary period and the physical properties thus interact during this phase and were able to adjust. Due to the inflation it expanded so rapidly that these properties remained frozen over the whole space of time; From this point, the universe had been forced to an almost perfect homogeneity, because the information had found no causal connection more to change their properties. However, this also means that the observable universe is only a fraction of the actual cosmos.

As a consequence of such a cosmic inflation, the anisotropy would have been reduced during the big bang, would, however, not completely disappeared. The temperature differences in the cosmic background radiation were smoothed by cosmic inflation as well, however, continue to exist to a small degree. The theory says this requires a broad spectrum of the anisotropy of the microwave background radiation, which is actually mostly in line with the results that could deliver the spacecraft WMAP and COBE science.

Variable speed of light

Another solution to this problem proposed to the Portuguese João Magueijo scientists and the American physicist Andreas Albrecht ago in 1999. According to their theory the speed of light was not always the same, but had shortly after the Big Bang, by 60 orders of magnitude higher than today. This could have the different regions can contact each other in contact since their horizon would have been far more extensive before the final distance was too great. In addition, would have been much more time to so that the properties could be due to interaction are becoming more similar to each other than the measurable parameters today suggest this. In addition, detectable by this theory, that a variable speed of light is suppressed, the density fluctuation which aids the homogeneity of the universe.

However, this theory of the variable speed of light encountered in most scientists rejected because the speed of light is considered one of the most important natural constants and is the basis for Einstein's theory of relativity.

Credentials

• Cosmology (Physics)