The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and redshift of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. As noted above, the redshift arises from the metric expansion of space; as the space itself expands, the wavelength of a photon traveling through space likewise increases, decreasing its energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental physical cosmology.
Other experimental observations can be explained by combining the overall expansion of space with nuclear physics and atomic physics. As the Universe expands, the energy density of the electromagnetic radiation decreases more quickly than does that of matter, since the energy of a photon decreases with its wavelength. Thus, although the energy density of the Universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was light. As the Universe expanded, its energy density decreased and it became cooler; as it did so, the elementary particles of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then associated into atomic nuclei. At this stage, the matter in the Universe was mainly a hot, dense plasma of negative electrons, neutral neutrinos and positive nuclei. Nuclear reactions among the nuclei led to the present abundances of the lighter nuclei, particularly hydrogen, deuterium, and helium. Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.
Other observations are not clearly answered by known physics. According to the prevailing theory, a slight imbalance of matter over antimatter was present in the Universe's creation, or developed very shortly thereafter. Although the matter and antimatter mostly annihilated one another, producing photons, a small residue of matter survived, giving the present matter-dominated Universe.
Several lines of evidence also suggest that a rapid cosmic inflation of the Universe occurred very early in its history (roughly 10 seconds after its creation). Recent observations also suggest that the cosmological constant ("Λ") is not zero, and that the net mass-energy content of the Universe is dominated by a dark energy and dark matter that have not been characterized scientifically. They differ in their gravitational effects. Dark matter gravitates as ordinary matter does, and thus slows the expansion of the Universe; by contrast, dark energy serves to accelerate the Universe's expansion.
Some people think that there is more than one universe. They think that there is a set of universes called the multiverse. By definition, there is no way for anything in one universe to affect something in another.
The multiverse is not yet a scientific idea because there is no way to test it. An idea that cannot be tested or is not based on logic is not science. So it is not known if the multiverse is a scientific idea.
The future of the Universe is a mystery. However, there has a couple of theories based on the possible shapes of the Universe:
Unit of measurement
Units of measurement give "standards" so that the numbers from our measurements refer to the same thing. Measurement is a process that uses numbers to describe a physical quantity. We can measure how big things are, how warm they are, how heavy they are, and many other features.
For example, the metre is a standard unit to measure length. Before 1982, one meter was defined as the distance between two markers on a special metal rod. During that time, saying that something had a length of two meters meant that it was exactly twice as long as the rod used to define the meter. Now scientists define the meter by using the speed of light.
In the past, different units were used in different countries. Today, most units of measure fall into one of three systems:
The older two, the British imperial system and the closely related US customary system use the foot as a measure of length, the pound as a measure for weight, and the second as a measure for time. They use other units as well. The number of smaller units that make the bigger units in these two systems varies: For example, there are 12 inches in a foot and 16 ounces in a pound.
The newest and most used of the three systems is the metric system or SI system which use 10, 100 or 1000 of a smaller unit to make a bigger one. For instance, there are 100 centimetres in one metre or 1000 grams in one kilogram. This system uses the metre for length and kilogram for mass.
The common, non-metric measurement of time does not follow this pattern. The second is the basis for time measurement, and it is based on the sexagesimal system: 60 seconds make one minute, and 60 minutes make one hour.
The property of the thing being measured is given as a number of units of measure. The number only has sense when the unit of measurement is also given. By that number it represents a measurement of something.
For example, The Eiffel Tower in Paris, France is tall. That is, the distance from the top to the bottom of the Eiffel Tower is 300 metres. The property of the Eiffel Tower being measured is a distance. The number measured is 300. 300 of what? The unit of measurement is the metre.
Standards are usually special objects used to make measurements. A metre stick is an example of a standard. When you measure something with a metre stick, you can compare that measurement to anything else that is also measured with a metre stick. This makes measurement easier and comparisons between measurements easier.
Science, medicine and engineering use smaller units of measurement to measure small things with less error. It is easy to measure large things using larger units of measurement. Astronomical measurements like the width of a galaxy use light years and parsecs.
Small measurements like the mass of an atom use special units of measurement.
There are many different standards and units used all over the world. Some became less used during the 19th and 20th centuries.
The metric system is a system of measurement used in most of the world. It is also called the International System of Units, or SI.
Units of measure in the metric system include:
Imperial units were defined in the United Kingdom in 1824. These units were based on similar units that were in use before 1824. Imperial units were used in countries that were part of the British Empire. While many of these countries, including the United Kingdom, have officially adopted SI, the older system of units are still used.
US customary units are the official units used in the US. These are similar to the British imperial units and also based on the units used in the United Kingdom from before American Independence. Some of the units are different to the British ones. For example, there are 20 imperial fluid ounces in an imperial pint, but 16 US fluid ounces in a US pint. Additionally, the US fluid ounce is slightly bigger than the imperial fluid ounce. The result is that US pints and gallons are smaller than imperial pints and gallons. In the United States, the metric system has been legal for trade since 1866 but other measurements such as the gallon, inch, and the pound are still widely used.
Imperial and US units of measurement include:
The ounces for weight and volume are different. Even when measuring water, the number of ounces of weight is not the same as the number of fluid ounces.