Question

1. Explain how Hubble's discovery of a relationship between the distance of other galaxies and their redshifts led to the idea of the Big Bang.
2. What are the two key observational facts, other than Hubble's law, that led to widespread acceptance of the Big Bang model? Explain how these observations support the Big Bang model.
3. What is the observational evidence the universe underwent a brief but very rapid period of inflation at the end of the grand unified theory (GUT) era early in the history of the universe? Does any of this evidence count as direct observational evidence? Does the validity of the Big Bang depend on inflation being correct?
4. Why is the era of nucleosynthesis so important in determining the chemical composition of the universe?
5. What is Olbers' Paradox, and what is its resolution?

Answer 1

1)Redshift is the defined as the change in frequency or wavelength of light as the stars or other celestial bodies move away from each other. This is due to Doppler effect that is wavelength of light reaching (or sound) depends upon how fast the source or the observer is moving relative to each other. So, Hubble observed that stars are being redshifted , which means the wavelength of light reaching us is increased, this can only happen if the stars are moving away from us at considerable velocity. So at some time in the past all these celestial bodies should be at one single spot , which later expanded to form the universe that we see now. This event in the past where everything we observe now was at a single point spot of infinite density and then suddenly expanded its size is termed as BIG BANG.

2)(answer to the third question is also given here)

the first one is cosmic microwave background radiation or CMB, which can only be explained through big bang. CMB is the first light that was observed after big bang that is now uniformly spread across the universe . George Gamov predicted in 1948 that there should be a faint glow left over from when the universe was much hotter and denser. Since the universe is observed to be expanding, it means that the galaxies were originally right on top of each other. Also, the energy of the universe was concentrated in a smaller volume. The entire universe would have glowed first in the gamma ray band, then the X-ray band, then to less energetic bands as the universe expanded. By now about 13.8 billion years after the start of the expansion, the cold universe should glow in the radio band. The expansion rate has slowed down over time because of the force of gravity. This means that the early expansion was faster than it is now.At the start of the expansion, the expansion rate was extremely rapid. This phenomenon is termed as inflation. Inflation is the sudden and rapid expansion of space( even faster than speed of light) that existed from about 10^-36 seconds to 10^-32 seconds. The uniformity in  temperature of CMB is a direct observational evidence of inflation. CMB and inflation added yet another concrete evidence for big bang. So inflation is a strong theory that holds the big bang theory and if it failed to explain, then big bang theory may also go into its demise.

there are few more evidences for big bang. Another important evidence is that the observed abundance of hydrogen, helium, deuterium, lithium agrees with that predicted by the Big Bang theory. The abundances are checked from the spectra of the oldest stars and gas clouds which are made from unprocessed, primitive material. Even better observations are those made of light from very distant quasars that have passed through gas in regions of the universe where there are no stars that could have contaminated the gas.

4)When the universe was only 1 millisecond old, nuclei were hot enough and dense enough to fuse to create heavier elements, but it was so dense that the nuclei broke apart again as soon as they formed. This fusion and breakup continued until about 3 minutes after the Big Bang, when the universe cooled enough (10⁹ K) that fusion ended. At this point, 75% of baryonic matter was in the form of hydrogen, 25% in the form of helium, and trace amounts were in the form of other atoms, mostly lithium. At the end of the era of nucleosynthesis, the universe contained the "primordial" mix of hydrogen, helium, and lithium that went into making the first stars. All heavier elements have been created by fusion inside of stars and during supernova explosions. So the chemical composition of the universe was determined in the era of nucleosynthesis.

5) Olber's paradox is the conflict between the fact that the observed night sky is dark and the logical implication that in a universe that is infinite, uniform in its large-scale properties, and unchanging, the night sky should be as bright as the surface of an average star because the light from every stars should have reached us by now due to infinite age of universe. Olber's paradox is resolved by saying that since the universe started at some finite time in the past, there has not been sufficient time for the light from every star to reach us ,hence the night sky looks dark .