Assignment 7 P.2 The Universe.

The Universe is huge and possibly infinite in volume. The matter which can be seen is spread over a space at least 93 billion light years across.^[12] For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years.^[13] As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter,^[14] and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away.^[15] There are probably more than 100 billion (10¹¹) galaxies in the observable universe.^[16] Typical galaxies range from dwarf galaxys with as few as ten million^[17] (10⁷) stars up to giants with one trillion^[18] (10¹²) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one sextillion (10²¹) stars in the observable universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 x 10²²).^[19]
The matter that can be seen is spread throughout the universe, when averaged over distances longer than 300 million light-years.^[20] However, on smaller length-scales, matter is observed to form 'clumps', many atoms are condensed into stars, most stars into galaxies, most galaxies into galaxy groups and clusters and, lastly, the largest-scale structures such as the Great Wall of galaxies.
The present overall density of the Universe is very low, roughly 9.9 × 10⁻³⁰ grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter and 4% ordinary matter. The density of atoms is about a single hydrogen atom for every four cubic meters of volume.^[21] The properties of dark energy and dark matter are not known. Dark matter slows the expansion of the Universe. Dark energy makes its expansion faster.
The Universe is old, and changing. The best good guess of the Universe's age is 13.798±0.037 billion years old, based on what was seen of the cosmic microwave background radiation.^[22][23][24] Independent estimates (based on measurements such as radioactive dating) agree, although they are less precise, ranging from 11–20 billion years^[25] to 13–15 billion years.^[26]
The universe has not been the same at all times in its history. This getting bigger accounts for how Earth-bound people can see the light from a galaxy 30 billion light years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; the photons emitted have been stretched to longer wavelengths and lower