The universe cooled at the same rate all over, so this process happened at the same time everywhere.
Light could pass through them and on through the universe as if it were completely empty. These atoms have no electric charge, so they don’t affect light in the same way that electrons and protons do on their own. Eventually, once the universe became cool enough, the electrons and protons began to bind together and form atoms of hydrogen. Over time though, the universe expanded and cooled down. This stopped the light from travelling very far. These particles have an electric charge, and when light reached one of the particles, the electric charge would send the light off in another direction.
COSMIC BACKGROUND FULL
At this point in time, the universe was very hot and dense, and full of particles called electrons and protons. The CMB is the light from the beginning of the universe. Over time, it has lost energy and become lower-energy microwaves. Microwaves are a type of light, and so are the X-rays that we use to check for broken bones, and the radio waves that let us listen to music in the car.Īt first, the CMB was very energetic X-ray light. The type of light we can see is called visible light, but other types of light exist. However, it isn’t made up of light that you or I are able to see with the naked eye. It comes from soon after the Big Bang – which is considered to be the beginning of the universe. The CMB will certainly appear slightly different from any distant galaxy, but the statistical distribution of hot/cold spots (and the cosmological information contained within) should be exactly the same.The Cosmic Microwave Background (CMB for short), is light: the oldest and most distant light that we can see in the entire universe. Homogeneity means that the universe appears statistically identical no matter where you are. While the CMB appears different at varying sightlines, the universe evolves in a uniform manner such that the CMB is the same distance in all directions.
Isotropy means that the universe appears statistically identical in every direction. The cosmological principle postulates that the universe, as a whole, is isotropic and homogeneous. Everything we can learn about the CMB is encoded in the distribution of the numbers and sizes of these hot/cold spots. Although higher density regions are considered the seeds for large-scale structures like galaxy clusters, individual locations of hot/cold spots in the observed CMB do not tell us anything particularly insightful. We relate these to small (a few parts in a million) differences in the density of matter at that location at the time the CMB formed. From Earth, we observe slightly hotter and colder spots in the CMB across the sky. Observations of the CMB convert the light signal into a map of the relative temperature of the radiation. So, the CMB source is 40 billion light-years away and not 14 billion light-years away, as one might expect.
This expansion of the universe describes the phenomenon whereby the distance between any two points in space gradually increases over cosmic time. This CMB radiation was mostly in the form of near-infrared light, but the wavelength of the CMB light has been stretched by the expansion of space so much it now falls in the microwave range we see today. The cosmic microwave background (CMB) is the radiation allowed to freely propagate after the universe cooled enough for electrons to combine with atomic nuclei to form neutral atoms some 300,000 years after the Big Bang, or roughly 14 billion years ago.
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