User:Amit/Preliminary Versions

From EvoWiki

(This page is meant only for my own reference.)

Contents 1 History : What led us to consider the Big Bang? 2 Facts and Misconceptions about the Big Bang 3 Evidence in support of Big Bang 3.1 Empirical Observations supporting the Big Bang 3.2 Experiemental Evidence 4 Timeline of the Big Bang 5 Alternative models? 6 References 7 See Also

History : What led us to consider the Big Bang?

1. In 1916 Einstein proposed the Theory of General Relativity. This theory uses the model that Gravitational Force is a result of curvature of space-time. This model has since been considered as a successful model for large-scale behaviour of the universe.
2. It was realised that Einstein's theory of General Relativity, as is, did not have 'static solutions', in other words, according to the theory, the universe would always either expand or contract. This was initially thought of as a problem. Einstein, himself, in order to explain the notion of a 'static universe' proposed the notion of a Cosmological Constant.
3. However, in 1929 Hubble observed, using 'red shift' method, that galaxies were moving away from the earth at a speed which was proportional to their distance from the earth. An elementary mathematical analysis of this leads us to the conclusion that everthing converges to one point as we go backwards in time, a phenomenon now known as Big Bang.
4. Even after indications from Einstein's Relativity and Hubble's observation, the idea of Big Bang met opposition when Fritz Zwicky proposed that a static universe was still possible, if 'light lost energy as it travelled'. This model is called the "Tired Light" model. However, subsequent advances in cosmological theory proved this option to be inviable, and the Big Bang is now considered to be the most probable explanation of Hubble's observations.

Facts and Misconceptions about the Big Bang

1. The Big Bang theory is not just a 'belief' but is a well tested Scientific Theory. Nothing in science is just a belief. Like any other Scientific Theory it has been critically considered. It is the evidence which makes the Big Bang seem the most probable explaination.
2. The Big Bang theory is unable to say anything about the exact point at which all matter seems to converge to. This point in space-time is sometiems referred to a 'singularity' and we currently have no satisfactory explanation about this. Thus the popular picture of the Big Bang of "explosion of a point of infinite density" is simply wrong.
3. Big Bang is an explanation of the universe "after" the singularity. In particular the Big Bang does not say anything about how the universe was created.
4. There is another concept called Big Church, related to the Big Bang. It is a point where the entire universe converges ( as opposed to diverges in the case of Big Bang). It has been proposed that the universe contracts and expands in a cyclic manner. However this is NOT a tested theory and is not a part of the standard Big Bang theory.

Evidence in support of Big Bang

Empirical Observations supporting the Big Bang

1. Hubble's Red-Shift Observation.
2. Large Scale Homogenity.
3. Abundance of light elements.
4. Cosmic Background Microwave Radiation

Experiemental Evidence

1. Tolman Tests

Timeline of the Big Bang

Going backwards in time from the present day to when the Big Bang occurred (13.7 billion years ago):

• ~4.5 billion years ago: Formation of our solar system.

• ~6 billion years ago: Formation of more modern galaxies.

• ~11 billion years ago: Peak galaxy building timeframe (mostly irregular shaped)

Quasars common; most of them have now burned out, but we see many active ones because their light has taken over 10 billion years to arrive at the Earth.

• ~13 billion years ago: the first stars and galaxies started to form in this poorly-understood phase of the Universe's history.

• ~300-400,000 years after the Big Bang: Universe has cooled enough for atoms to form. Universe becomes transparant to light. Recombination happens, with electrons combining with hydrogen and helium ions at a temperature of around 3000° K. The Universe's expansion has redshifted the recombination-era glow to the present-day 2.73°-K Cosmic Microwave Background (CMB). Density fluctuations had left their footprints on the recombination glow, producing the observed fluctuations in the CMB; these fluctuations would eventually produce galaxy formation and the large-scale distribution of matter.

Around then: decoupling, when matter (slow particles) becomes more dense than radiation (particles traveling at the speed of light). This permits overdense regions to collapse, though that only happens later.

• ~ 3 minutes after the Big Bang: Protons and neutrons combine to form the first atomic nuclei. The temperature is 10⁹° K or 100 keV (kilo electron volts).

• ~ 1 second after the Big Bang: nucleosynthesis starts when the Universe gets cold enough to allow deuterium to stick together (temperature: 10¹⁰° K or 1 MeV (mega electron volts))

• ~ 10^-6 seconds after the Big Bang: quark-hadron transition, happens when the quark soup becomes cold enough to condense into hadrons (protons, neutrons, pions, kaons, etc.) (temperature: 10¹³° K or 1 GeV (giga electron volts))

• ~ 10^-11 seconds after the Big Bang: electroweak transition: electroweak interactions split into electromagnetic and weak nuclear interactions (temperature: 10¹⁶° K or 10³ GeV).

Around that time: supersymmetry, if it exists, would become broken. Supersymmetry-related integer-spin and half-odd-spin particles would acquire masses different by a few hundred GeV or more.

The "Particle Desert"; nothing much is expected to happen

• ~10^-35 seconds after the Big Bang: grand unified theory (GUT) transition: grand-unified interaction breaks up into strong and electroweak interactions, lots of other fun things like inflation, monopoles, cosmic strings, etc. (temperature: 10²⁸° K or 10¹⁵ GeV)

• ~10^-43 seconds after the Big Bang: the Planck epoch, when quantum-gravity effects were strong (temperature: 10³²° K or 10¹⁹ GeV)

Alternative models?

Just like any other scientific theory, the Big Bang theory has questions which are still under investigation and not completely understood. In principle, since no Scientific Theory is ever "the Final explanation", it is concievable that the Big Bang is corrected to included in a bigger theory. However, those advocating alternative models should remember that

• The Big Bang theory is a well tested-model which offers a coherent and consistent explaination of a large amount of data.
• Any other proposed model should explain Hubble's red-shift observation and other data explained by the Big Bang.
• Alternative Explanations have to be scientific and in particular testable. Explanations like a "supernatural intelligent designer causes the red shift" do not belong to a Science class.

Here is a list of scientific models proposed as an alternative to the Big Bang. However none of them can explain all observations which Big Bang can explain.

1. Steady State Model (cannot explain Cosmic Microwave Background Radiation)
2. Tired Light Model (cannot explain Time dilation of supernova light curves)

References

1. http://www.talkorigins.org/faqs/astronomy/bigbang.html
2. http://www.talkorigins.org/faqs/astronomy/bigbang.html
3. http://en.wikipedia.org/wiki/Big_Bang

See Also