Cosmology: The Story Of Universe And Everything In It

Big Bang theory is the hypothesis that explains the story of universe in its 13.7 billion years evolutionary history.

Hubble’s Law and Expanding Universe

In 1920s Edwin Hubble fundamentally changed our understanding of the nature of universe.
Till then the general notion was that universe is infinite and static. However, Hubble proved that universe.
This idea was extrapolated to explain the evolutionary history of universe in its 13.7 billion-year journey.

Following from Hubble’s law if universe is expanding today it must have always expanded.
Thus, in the beginning, all that we see in the universe (matter, energy everything) today must have been confined in a very small space and thereby extremely dense. This is called singularity.
13.7 billion years ago this ultra-dense primordial space began to expand and continues to do so even today.
This marks the beginning of universe and the theory that explains this is called the Big Bang theory.
Below-given figure and table together represent a snapshot of the evolution of universe and the important landmarks along its journey.

Time since Big Bang	Event	Evidence	Observatories
t = 0-10^-43s	Planks Time	We do not know	We do not know
t = 10^-43s – 10^-35s	Cosmic Inflation	Gravitational Waves??	Gravitational Waves??
t = 10^-35s – 3 m	Quark confinement 1st Protons and Neutrons	Particle Accelerators	Large Hadron Collider (CERN), Tevitron, International Linear Collider (Japan?)
t = 3 m – 380000 years	Nucleosynthesis	Hydrogen and Helium filled universe	Spectral line of stars and galaxies
t = 380000	Atomic synthesis	Cosmic Microwave Background Radiation	COBE, WMAP, BICEP 1 and 2
t = 400 million years	1st stars and Galaxies	Direct Observation	Telescopes

Shortly after the Planck time universe experienced a brief period of extremely rapid expansion.
It expanded outward in all directions by a factor of about 1050
Note: Expansion means expansion of space. Particles did not move through space; space itself inflated.
As the universe expanded by this enormous rate it also cooled rapidly.
The temperature decreased from 1032Kelvin to 1027Kelvin giving rise to conditions suitable to form first matter.
At t=10–12s, the temperature of the universe had dropped to 1015 K.

In this period the building blocks of all matter in the universe were formed.
Elementary particles like quarks (see Standard Model) came together to form the first protons and neutrons in a process called quark confinement.
When the universe was about 1 second old, its temperature fell below 6 * 109 K, at which time electrons and positrons were formed.

As the temperatures dipped even further to about 4 *108 K, neutrons and protons came together to form the 1st nucleus. (this happened till 15 mins)
After 15 mins neutrons were not able to combine with protons. This resulted in decay of neutrons into protons, electrons, and neutrinos.
Till t=380000 years, no light could pass through as the universe filled with high-energy photons were interacting with electrons, protons, neutrons, and other matter particles.
Thus, we say universe was opaque till this time.

At t=380000 temperatures had dipped to about 3000K high-energy photons stopped interacting with matter particles like electrons, protons etc. making conditions conducive for electrons to bind to nucleus to form the 1st atoms.
Before this the radiation in the universe was made of high-energy photons which prevented electrons to bind to the nucleus.
The atom that was forming dominantly was hydrogen.
In addition, hydrogen atoms were fusing to form helium atoms. (similar to what happens in sun today)
It is like saying the entire universe was like sun emitting visible light just like how our sun does today.
The entire universe was filled with visible light radiation. As the universe continues to expand this visible light is also stretching thereby becoming microwave. (stretching of light increases the wavelength, thus visible light becomes microwave)
This is called cosmic microwave background radiation.

As we have seen around 380000 years from the beginning, 1st atoms started to form. As the universe continued to expand matter that was forming clumped together due to gravity. This left nonuniformity in the universe, some places having lot of matter, some places having no matter.
Around 400 million years from the beginning so much matter were clumped together that it gave rise 1st galaxies in the universe.
This non-uniformity in presence of matter is a characteristic feature of the universe even today. This non uniformity is characterised by clumping together of matter due to their gravitational force of attraction. (any 2 bodies with mass attracts by the force of gravity)
The clumping together of matter give rise to various structures we see in the universe .In addition clumping together, gravity also renders a hierarchical structure to Universe.
Thus stars clump together to form galaxies, galaxies clump together into clusters (local group), clusters of galaxies clump together into superclusters and so on.
Most galaxies are in gravitationally bound clusters of 10 to 10,000 galaxies.