Evolution of the Universe: From big bang to present day

Evolution of the Universe: From big bang to present day

The very early Universe was a dense, scalding hot soup of fundamental particles...within about a billion years clouds of matter formed into galaxies.

Quamrul Haider

According to the Big Bang theory, the universe came into being with a violent explosion from which all matter and energy originated. At the zero hour, the entire known universe was a mathematical point of infinite density, called the singularity, from which matter and energy burst out and the universe began to expand. This cataclysmic event occurred, as estimated by Hubble, approximately 14 billion years ago

A microsecond after the explosion, temperature of the universe was 10 trillion Kelvin and it was a dense, scalding hot soup of fundamental particles - quarks, leptons and the force carriers. Most of the radiation consisted of high energy gamma-ray photons. Photons are massless particles travelling at the speed of light. They had enough energy to make matter (electron, proton, neutrino) and anti-matter through pair production. But matter and anti-matter quickly annihilated each other. Fortunately, there was an asymmetry in favor of matter. A consequence of this asymmetry was that the universe was able to mature in a way favorable for matter to persist. The universe was also expanding at an extremely fast rate. This period is known as the Inflationary Epoch.

After one second, the universe grew older, temperature decreased to about 10 billion Kelvin, and the gamma-rays lacked sufficient energy for pair production. Hence, no matter or anti-matter was produced and their amount decreased because of the annihilation process. The quarks and gluons (carriers of the strong force) combined into composite particles like protons and neutrons. At this time, the neutrinos stopped interacting with matter and have moved freely through the universe ever since.

The universe was a sea of high-energy radiation of gamma- and X-rays. Together with some neutrons, protons and electrons, it was an ionized plasma where matter and radiation were inseparable. Neutrons spontaneously decayed into protons and electrons. Also deuteron, an isotope of hydrogen, was formed through fusion of neutrons and protons. Initially, the deuterons did not survive because the gamma- and X-rays had sufficient energy to break them up into neutrons and protons.

One hundred seconds later, the universe cooled down to 1 billion Kelvin and the photons did not have enough energy to disrupt a deuteron. Abundance of deuteron climbed swiftly; protons and neutrons became the common state of matter. After 200 seconds, protons and neutrons slowed down enough for a series of nuclear reactions (nucleosynthesis) to take place, and the chemical makeup of the universe changed from hydrogen and deuterons only to helium, lithium and beryllium. They were, however, quickly torn apart by the energetic photons.

Radiation dominated the universe during the initial few minutes. This stage in the development of the universe is known as the Radiation Era. The universe was still too hot (temperature was around 1 million Kelvin after the first few years) for atoms to be stable. Gas in the universe, mostly free electrons, was very opaque. Light and radiation never got far away from the place where they were emitted before being destroyed or rescattered again.

For the next 300,000 years, the universe was a “primordial fireball,” completely filled with a shimmering expanse of high-energy photons colliding vigorously with protons and electrons. Things gradually started to change in a fundamental way; both temperature and density started to drop and collision between particles became less violent.

The radiation that flooded the universe gradually shifted from gamma- and X-rays to relatively less energetic ultra-violet and infra-red rays. The temperature dropped to a few thousand Kelvin and the photons no longer had enough energy to keep the protons and electrons apart. They began combining to form hydrogen atoms. The universe changed from opaque to a transparent state and the atoms absorbed and scattered radiation far less efficiently. This meant that light and other forms of radiation could now stream unimpeded across space. The moment light broke free from matter is called the Decoupling Epoch or Era of Recombination.

The universe was filled with hydrogen and helium gas in the first million years after its birth. The high density regions expanded along with the rest of the universe. However, the slightly greater pull of gravity in these regions gradually slowed their expansion. Within about a billion years, the expansion of these denser regions halted and reversed, and the material within them began to contract into clouds of matter that eventually formed the galaxies.

The blinding radiation and careening particles eventually filled up the large and empty universe of today. By looking far enough into space, we can still detect the radiation that originated at the early stage in the development of the universe. They are known as Cosmic Background Radiation (CBR). Over the long period of 14 billion years, they “cooled to a faint whisper” in the microwave region. Consequently, their wavelength got red-shifted by almost 1000 times because of Doppler Effect shift in the wavelength toward larger values if the source is moving away from the observer. That is why CBR appears today as radio waves characteristic with a temperature of 2.7 Kelvin. The accidental discovery of CBR in 1977 by Penzias and Wilson of Bell Lab and the cosmological red-shift of galaxies are considered the most conclusive evidences of the Big Bang model of the universe.

We are bits of stellar matter that got cold by accident, bits of a star gone wrong. -- Arthur Eddington.

The writer is Professor and Chairman Department of Physics & Engineering Physics Fordham University, New York