Story of the Universe

Let us take a snapshot of the space in its current state. We see every type of enormous structure, like stars, planets, galaxies, nebulae, and many more, forming, dispersing, colliding, and giving rise to every kind of physical process we know. All this occurs in unfathomably extreme conditions, unachieved artificially, and emitting each possible frequency of the electromagnetic spectrum. However, it was not always so colourful and bustling with activity.

 

There had been a time when the universe was dark to its very core!

Evolution of the Universe

With the discovery of Hubble's expansion of the universe, The Big Bang model has become one of the prominent theories of cosmological evolution. Suppose we trace the expanding universe back in time. In that case, there will be a point when everything in the universe, spacetime, matter, and radiation, will shrink to a single point of infinite density and temperature called a singularity. We do not know much about this since the known laws of physics seem to break down at this point. We can only predict up to the time when the universe was of Plank dimensions. The Plank Epoch is an era of extreme temperatures and densities such that even the subatomic particles could not form. The four fundamental forces were combined till the temperature was reduced to appropriate values for them to separate.

Then comes one of the most wonderful times of the universe's age- Cosmic Inflation- the era of exponential expansion of spacetime stretching and distributing with itself all the matter and energy available. The exact reason for it to happen is still unknown; the inflationary model gives a very satisfactory explanation of the precise homogeneity and isotropy that we observe today. The macroscopic fluctuations in the matter smoothed out in this era, and spacetime became flat. Inflation was thought to have ended between 10^(−33) to 10^(−32) seconds after the big bang and gave rise to the normal Hubble expansion we see today. After the inflation ended and the temperature dropped to a certain level, the universe was mainly composed of ionized nuclei, and electrons spread isotropically. The electromagnetic radiation, the photons, was not free to travel long distances due to the interaction with the soup of ionized particles. Due to constant absorption and emission, the mean free path of these photons was concise.

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Therefore, we cannot see any light before this time; the universe was foggy and opaque.

Cosmic Dark Ages

The time of our interest starts with the beginning of what is called the Recombination, misnamed; it is the time when the ionized nuclei combine for the first time with the electrons to form the first neutral atoms, mostly hydrogen and helium, along with a pinch of deuterium, lithium, and beryllium. It happened because of the lowering of temperature and density in the expanding universe. Due to the still high temperatures, the electrons, when combined with nuclei, formed atoms of excited states, quickly coming down to the more stable ground state by releasing photons of a suitable wavelength. However, these photons could not travel far due to the background of ionized particles. However, once Recombination began, and this ionized background soon became neutral everywhere, it did not significantly interact with the photons. For the first time, this led to the emission of photons with infinite mean free paths; the universe had become transparent. This light, though highly redshifted, can still be observed today as the Cosmic Microwave Background (CMB) with the currently observed temperature of about 2.7 K (at z = 0) but released at a temperature around 4000 K (at z ~ 1000).

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The universe consisted of neutral atoms (hydrogen and helium) and the CMB. The minimum energy required to interact with the neutral hydrogen significantly is 10.2 eV. That is the energy to take the electron from the ground state to the first excited state: the Lyman- photon, which has an energy higher than any photons in the CMB. Therefore, the CMB hardly interacts with the neutral hydrogen background, and an almost equilibrium is established. Apart from the CMB, no new light is observed from this period, hence the name, Cosmic dark ages, since no light-producing stars had been formed yet. The dark ages stretch from z = 1000 to z = 40 − 30; the CMB temperature reduced from 4000 K to about 60 K in a little less than a billion years.

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"The Universe was homogeneous, isotropic, utterly dark, and almost stable. It could have stayed like that for the rest of its life, but it did not."

Nevertheless, the universe is not like that when we see it today; it is bustling with enormous structures forming and crumbling and not dark at all. The Dark Ages might sound like an idle, gloomy time, but something happened then that changed the universe into what it looks like today.

Formation of Large Scale Structures

All the macroscopic fluctuations left over after the big bang, which could collapse in their gravity to form structures, were smoothed out during the inflationary expansion and induced an approximate equilibrium. Nevertheless, as mentioned earlier, the universe was only almost stable. The tiny quantum fluctuations in matter density were stretched to large volumes of spacetime and became macroscopic. Still, it was far from creating radiation-emitting structures like stars.

 

The gravitational instability that emerged due to these density fluctuations led to the collapse of vast clouds of gas hydrogen. These clouds accumulate more and more gas, rising to very high temperatures. This heat could generate enough pressure to prevent any further contraction. To start the fusion, the gas needs to contract to extremely high densities; hence, the extra energy in the gas must escape somehow. Typically, this occurs by exciting the atom to higher energy states; the photon then emitted to restore the ground state can escape from the cloud unhindered, effectively lowering the temperature. However, the primordial gas mainly consisted of hydrogen and helium, both of which require humongous amounts of heat to reach their excited states. This means a tremendous amount of gas is needed to create the first generation of stars, Population-III; hence, they are believed to have been highly massive. Things get interesting when the halos reach 100000 times the sun's mass.

The extra heat generated due to the collapse was enough to form molecular hydrogen that could emit photons much more quickly than atomic hydrogen by radiative cooling. This led to the formation of condensed and fragmented gas clouds that started the fusion. The POP–III stars were too massive and very short-lived since the fuel would get used up very fast to form the first metals (elements of atomic number more than two). They weighed about 30 to 300 solar masses and lived not more than a few million years. Over the next couple hundred million years, these stars were now having some metallicity; Population II grouped to form baby galaxies bound by gravity.

The Epoch of Reionization

With the formation of stars and galaxies, the universe had started to look like today, but not quite.

 

When we observe the universe, we see that all the gas and structures are found in their ionized states with only trace amounts of neutral hydrogen. However, when forming the first stars, the universe was covered with a thick fog of neutral hydrogen that interacted with and absorbed the light emitted from the first stars; this is why we cannot see them with our telescopes. However, the situation would not remain the same for very long. The energy emitted from the stars, which is then absorbed by the neutral gas, is high enough to strip these atoms of their electrons and ionize the universe for a second time. This is called the Reionization of the Universe. If we try to picture the universe, we would see a growing transparent bubble of ionized gas around each star and galaxies in a background of opaque neutral gas; the universe would look like Swiss Cheese. The death of massive POP-III stars will leave behind massive Black holes, which by sucking the matter at near-light speeds, will produce tremendous energies and quicken the reionization process. Over time, these ionized bubbles grew in size and overlapped to cover the entire space.

 

That is pretty much what the universe looks like today.