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According to Hydrogen-Helium Abundance "Schramm's figures for relative abundances indicate that helium is about 25% by mass and hydrogen about 73% with all other elements constituting less than 2%."

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Hydrogen and helium account for nearly all the nuclear matter in today's universe. This is consistent with the standard or " big bang" model. The process of forming the hydrogen and helium and other trace constituents is often called " big bang nucleosynthesis". Schramm's figures for relative abundances indicate that helium is about 25% by mass and hydrogen about 73% with all other elements constituting less than 2%. Carroll & Ostlie give 23 to 24% helium. There is a window of uncertainty, but it is clear that hydrogen and helium make up 98% plus of the ordinary matter in the universe. This high percentage of helium argues strongly for the big bang model, since other models gave very small percentages of helium. Since there is no known process which significantly changes this H/He ratio, it is taken to be the ratio which existed at the time when the deuteron became stable in the expansion of the universe. This ratio is significant as a test of cosmological models since it will be affected by the time period from the time when the temperature dropped below that necessary to produce neutrons from protons to the time when the deuteron became stable, halting the decay of the free neutrons.

Basically , the hydrogen-helium abundance helps us to model the expansion rate of the early universe. If it had been faster, there would be more neutrons and more helium. If it had been slower, more of the free neutrons would have decayed before the deuterium stability point and there would be less helium.

Index

References
Trefil

Schramm
Big Bang ...

Harwit
Ch 12

Carroll & Ostlie
Ch 29

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Hydrogen and helium account for nearly all the nuclear matter in today's universe. This is consistent with the standard or " big bang" model. The process of forming the hydrogen and helium and other trace constituents is often called " big bang nucleosynthesis".

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CMB is the recording of the photons at the moment of their escape from the "fog" (Extremely hot plasma). First detected by two scientist, Arno Penzias and Robert Wilson in 1965.

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According to Big Bang theory, temperatures and pressures for the first ~300,000 years of the Universe were such that atoms could not exist. Matter was instead distributed as a highly ionised plasma which was very efficient at scattering radiation. The result was that information ( photons) from the early Universe were effectively trapped in an inpenetrable 'fog' which, to this day, hides these early times from astronomers.

As the Universe expanded, however, its temperature and density dropped to a point where the atomic nuclei and electrons were able to combine to form atoms. This is known as the epoch of recombination, and it is at this time that photons were finally able to escape the fog of the early Universe and travel freely. The 'Cosmic Microwave Background radiation' (CMB) is the record of these photons at the moment of their escape.

First detected by Arno Penzias and Robert Wilson in 1965, the CMB is one of the most conclusive pieces of evidence in favour of the Big Bang. In particular, Big Bang theory predicts certain characteristics for the radiation left over from the birth of the Universe, all of which are confirmed by the CMB:

1. The multiple scattering of photons by a hot plasma in the early Universe should result in a blackbody spectrum for the photons once they have escaped at the epoch of reionisation. This is exactly what is observed for the CMB. The figure on the right plots a theoretical blackbody curve along with CMB data from the COsmic Background Explorer (COBE) satellite. The agreement is so good that it is impossible to distinguish the data from the theoretical curve.

2. The photons of the CMB were emitted at the epoch of recombination when the Universe had a temperature of about 3,000 Kelvin. However, they have been cosmological redshifted to longer wavelengths during their ~13 billion year journey through the expanding Universe, and are now detected in the microwave region of the electromagnetic spectrum at an average temperature of 2.725 Kelvin. This agrees well with what Big Bang theory predicts.

However, standard Big Bang theory does not account for all of the observed properties of the CMB. In particular, once we remove the dipole that arises due to our motion in the Universe, the CMB is incredibly uniform across the sky, varying by no more than one part in ten thousand. This suggests that regions of the Universe that are now widely separated, were once close enough to 'communicate' with each other in order to equalise their temperature. However, this is not possible given standard Big Bang theory, the age of the Universe, and the finite speed of light.

The red line in the figure on the left shows that according to Big Bang theory, the Universe had a radius of more than 10⁻¹⁰ metres at 10⁻⁴⁵ seconds after the Big Bang. Since the speed of light travels at 3×10 ⁸ m/s, information could only have travelled ~3×10⁻³⁷ metres during this time. Big Bang theory therefore makes it impossible for the whole Universe to have equalised its temperature at these early times, as not all the Universe was in communication. In everyday life we cannot receive information beyond our horizon, so this is known as the horizon problem.

To resolve the horizon problem, astronomers introduced an inflationary period into the Big Bang model (blue region in figure). This sudden increase in the rate of expansion of the Universe soon after the Big Bang, resolves not only the horizon problem, but also the flatness problem. It has therefore been accepted as part of the current concordance model of cosmology.

The presence of a background radiation which has a temperature, spectrum and uniformity consistent with Big Bang cosmology and inflation, is extremely difficult to produce by any other means. Therefore, astronomers believe that by studying the properties of the CMB, they are in fact studying the conditions of the early Universe.

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According to Big Bang theory, temperatures and pressures for the first ~300,000 years of the Universe were such that atoms could not exist. Matter was instead distributed as a highly ionised plasma which was very efficient at scattering radiation.

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It's a doppler effect but with color, the faster a star is moving away the more its electromagnetic radiation waves (light) will be on the warm side of the light spectrum.

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George Leaître thought of the idea of the Big Bang which he first shared in a paper in 1931.

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It is how most astronomers believe the universe began, that the universe began at a single point and then expanded until it was as large as it is now, it´s still expanding!