Download Children of the stars, children of the Universe…

Children of the stars
Olbers’ paradox
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Why is the sky dark at night?
If the Universe were static, infinite, eternal and uniformly filled with stars, and we looked it any
direction, our line of sight would eventually encounter a star
Stars get fainter as distance squared, but their number increases as distance squared
The sky should be uniformly bright, just like the surface of a star, but it is obviously not!
The reason why the sky is dark is that the Universe is not infinitely old, or that the look-back time is
greater than the age of the observable Universe
Expansion of the Universe
• Around 1920, Edwin Hubble discovered that most nebulae are galaxies similar to ours
and that they run away from each other
• This is, in fact, an expansion of space itself. The larger the distance between two
galaxies the faster they recede from each other, as described by the Hubble law
• Uniform expansion also means that there is no center to it!
Geometry of space
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Geometry of the Universe is described with the aid of Einstein’s General Relativity (GR)
GR assumes that the presence of mass curves space around it and we sense this effect as gravity
GR also assumes that the Universe has a curvature that determines its geometry and expansion
Curved space can be finite but have no bounds. This can be explained by the example of a twodimensional creature living on a three-dimensional surface
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The Big Bang theory predicts that the hot gas left over from the time of creation of the Universe should be
observable as a leftover background radiation coming from all directions
Because the Universe expands, the CBR should be observable in infrared and at radio wavelengths and have a
temperature of only ~ 3 K
The CBR was accidentally discover by Penzias and Wilson in mid-nineteen sixties and then very accurately
measured and mapped by the orbiting Cosmic Background Explorer (COBE)
The CBR temperature and other characteristics follow theoretical predictions extremely well
CBR is commonly interpreted as radiation from the Big Bang
The cosmic background radiation (CBR)
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The Big Bang
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The Universe was dominated by radiation over the first million years. Free electrons from ionization by gamma
rays made matter opaque for photons
After 1 million years, the Universe became cool enough for neutral atoms to form. The gas became transparent for
radiation in this recombination phase
At this time, the temperature of the Universe was ~3000 K. We observe photons from this epoch as the CBR
After recombination, the Universe became matter dominated and first galaxies were formed out of gas
condensations
Children of the stars, children of the Universe…
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10-5 sec after the Big Bang, the Universe was filled with gamma-ray photons at well over 1 trillion K and at density
comparable to that of an atomic nucleus. Photons produced particles of matter and antimatter which annihilated and
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became photons again
Protons, neutrons and electrons that matter is made of were produced in the first 4 sec of the Universe’s history.
H and He were made by the end of the third minute. Only some lithium (Li) and beryllium (Be) was made because
stable elements with atomic weights of 5 and 8 do not exist
In 30 min., the Universe expanded and cooled enough, so that all nuclear reaction stopped. All elements heavier
than Be were made later by nuclear fusion in stars or supernova explosions