The Universe

The Universe is practically everything. From space, time, and their contents, which include planets, stars, galaxies, to all other forms of matter, and energy. Everything is part of the universe, and the universe is part of everything.

Key Facts & Summary

  • In the beginning, the universe was infinitely hot – minutes after its birth estimates suggest that it had around 1 billion Kelvin degrees.
  • As the universe grew old, it also became much colder. Current average temperatures are around 2.725 degrees, Kelvin.
  • The exact size of the universe is impossible to calculate. However, the observable universe is around 93 billion light-years with a radius of around 46.5 billion light-years.
  • The universe’s density is equivalent to 5 protons per cubic meter of space.
  • The universe is made up of 4.9% ordinary (baryonic) matter, 26.8% dark matter, and 68.3% dark energy. Considering only the largest structures, the universe is made up of filaments, voids, superclusters, and galaxy groups, and clusters.
  • Dark matter and dark energy are invisible theoretical matter. The only proof of their existence lies in certain phenomena which mostly tell us that huge chunks of mass are missing, thus, in a way, this justifies their probable existence.
  • The age of the universe is predicted to be around 13.8 billion years old.
  • Since the universe is expanding, it doesn’t have a center point.
  • The universe is not only expanding, but it is also accelerating its expansion. Galaxies move away from each other but at the same time, space is also moving/expanding. 
  • It is possible that there is more than one universe.
  • There are more stars in the universe than grains of sand on Earth.
  • Most of the objects we see in the sky are in the past. 
  • There are more than 2 trillion galaxies in the observable universe.

With the further advancement of technology and on-going observation, the Universe unveiled itself to us through small little steps. Many of the earliest cosmological models of the Universe were developed by ancient Greek and Indian philosophers.

Their models were geocentric – placing the Earth at the center. Over many centuries much more precise astronomical observations were conducted and led to our current understanding of the Universe. Nicolaus Copernicus developed the heliocentric model – being the first to put the Sun in the center. Upon Copernicus’s work, Isaac Newton started to develop the law of universal gravitation – every particle attracts other particles with a force based upon their mass/weight. 

Johannes Kepler also built upon Copernicus’s work and established the laws of planetary motion. Observations conducted by Tycho Brahe also helped this. 

Galileo pioneered the use of telescopes, discovered the Galilean moons, and the conglomeration of stars in the Milky Way. He thus undermined the idea that everything revolves around the Earth. 

Many studies were conducted, ideas caught up, morphed, and finally, it was established that Earth was a planet among many others, while the Sun, was a star among billions of other stars.

After this, there was a period of time when the Universe was associated with the Galaxy – meaning – people believed that the galaxy was actually the universe, with clouds of nebulas around it. One such nebula was the Andromeda nebula and it attracted the attention of modern astronomers since it could even be seen with the naked eye.

With more modern telescopes being used, an astronomer by the name of Edwin Hubble studied the Andromeda nebula. He concluded in 1923 that the nebula was, in fact, another galaxy. 

In a sense, the discovery that we live in a galaxy among other billions of galaxies that make up the Universe, is fairly recent, less than 100 years. Hubble discovered many galaxies and devised the Hubble sequence – a system of classification for galaxies, based upon their morphology. 

Another achievement of Hubble was what is now being called the Hubble constant – Edwin Hubble’s observations concluded that objects that were farther away from Earth, were moving away faster – meaning – the Universe itself, is expanding. 

Objects in the universe cannot move faster than the speed of light, however, when it comes to pure space, these laws do not apply, and space itself is moving/expanding. Another major discovery was the cosmic background radiation.

The Big Bang theory states that the early universe was a hot place that expanded, and began to cool down. This means that the universe should be filled with radiation which is actually the relic/leftovers of the Big Bang event. And this was actually discovered! In fact, background radiation is everywhere.

It is like a cosmic echo and the most amazing fact is that old-fashioned television sets would catch a glimpse of this. Around 1% of the black and white fuzz and clacking white noise when these old TVs weren’t turned on a station – are in fact interferences made up of cosmic background radiation – the afterglow of the Big Bang.

Big Bang Theory & Physical Properties

How the Universe was created, what was before it? These questions may well be answered in time, or they will forever remain unanswered. Many scientific theories and creation myths have tried to explain the Universe’s mysterious genesis. However, currently, the most accepted explanation is the Big Bang theory.

Contrary to popular belief, the Big Bang didn’t start as an explosion but rather as a hot and infinitely dense point similar to a supercharged black hole – being just a few millimeters wide – that suddenly started to expand. Matter, energy, space, and time were thus created.

Two major stages of the Universe’s evolution followed:

1. The Radiation Era

2. The Matter Era

1. Radiation Era

These events helped shape the Universe. The first era was the Radiation era. It is named after the dominance of radiation right after the Big Bang.

The Radiation era is made up of smaller stages called epochs. These stages occurred in the Universe’s first tens of thousands of years. In chronological order:

* Planck Epoch – Matter as we know it, only existed in the form of energy. Only energy and the ancestor of the four forces of nature (Gravity, Strong Nuclear, Weak Nuclear, Electromagnetic), the super-force, were present. At the end of this stage, a key event occurred. This event split away gravity from the super-force and made way for the next epoch.

* The Grand Unification Epoch – It is named after the remaining three unified forces of nature (Strong Nuclear, Weak Nuclear, Electromagnetic). This epoch ended when the strong force or strong nuclear, broke away, leading to the next epoch.

*The Inflationary EpochIn this epoch, the Universe rapidly expanded. In an instant, it grew from the size of an atom to that of a football. It was a hot period as the Universe churned with electrons, quarks, and other particles that led to the next epoch.

* The Electroweak EpochA different event took place in this epoch. Not one, but both remaining forces of nature – electromagnetic and weak nuclear – split off as well.

* The Quark EpochAround this epoch, all of the universe’s ingredients were present. Though, it was still a period of high temperatures and density, preventing the formation of subatomic particles.

* The Hadron EpochDuring this epoch temperatures started to drop. The universe cooled down enough thus quarks were able to bind together and form protons and neutrons.

* The Lepton and Nuclear EpochsThe last two stages of the radiation era. During these epochs, the protons and neutrons underwent a significant change – they fused together and created nuclei. This led to the creation of the first chemical element in the universe, helium

2. Matter Era

The ability to form elements ended the radiation era and gave way to the matter era. It is named after the presence and predominance of matter in the universe.

This era features three epochs that span for billions of years. Actually, it engulfs the vast majority of the universe’s life span and includes the present day. In chronological order they are the:

* Atomic EpochThis stage is characterized by the universe’s cooling temperature. Because of this, electrons were able, for the first time, to attach themselves to nuclei. This process is called recombination, and it led to the creation of the universe’s second element, hydrogen.

* Galactic EpochHydrogen and helium atoms painted the universe with atomic clouds. In these atomic clouds, small pockets of gas may have had enough gravity to cause atoms to collect. Clusters of atoms formed during this epoch, and became the seedlings of galaxies.

* Stellar EpochInside these galaxies, stars began to form. This epoch is the last, yet on-going epoch of the matter era. Since stars began to form, they caused a ripple effect and helped in the shaping of the universe.

The heat within the stars ultimately caused the conversation of helium and hydrogen into almost all the remaining elements in the universe. These elements became the building blocks for planets, moons, and ultimately life. The age of the universe has been predicted to be around 13.8 billion years old. Since the universe is expanding, the initial energy and matter became much less dense over time.

The dominant fundamental force – forces of nature – out of the four, is gravity. Its effects are cumulative. When it comes to the other three, electromagnetism, due to the effects of positive and negative charges that cancel each other, is relatively insignificant on astronomical length scales.

The remaining two fundamental forces, the weak and strong nuclear forces, decline very rapidly with distance. Their effects are confined mainly to sub-atomic length scales.

Interestingly, the Universe appears to have much more matter than antimatter. This imbalance, however, is partially responsible for the existence of all matter existing today in the first place. If the Big Bang produced matter and antimatter equally, only photons would prevail as a result of their annihilating interaction.

Another physical property of the Universe is that it doesn’t have net momentum or angular momentum which follows accepted physical laws if the Universe is finite. There is no place where the Universe began, it happened everywhere at once.

Size, Structure – Regions

The universe’s exact size is difficult to predict. It does not have a center nor an edge – this is evidenced by the fact that when observed at the largest scale, galaxies seem to be distributed uniformly and at the same time they appear in all directions.

When it comes to smaller scales, these galaxies are distributed in clusters and superclusters – which are large groups of smaller galaxy conglomerations.  They form immense filaments – the largest known structures in the universe – and voids – regions of space devoid of galaxies or very few galaxies. This all, in turn, creates a vast foam-like structure.

Since the universe is expanding at a rate exceeding the speed of light, some of these regions will never be seen by us and those regions will never interact with ours.

The edge of the observable universe, on the other hand, is estimated to be around 46 billion light-years / 14 billion parsecs away from Earth. This, in turn, makes the diameter of the observable universe around 93 billion light-years / 28 billion parsecs. Since we cannot observe the space beyond the edge of the observable universe, it is unknown whether the size of the Universe, in its totality, is finite or infinite.

Age and Expansion

The Universe has expanded since the Big Bang, monotonically. It is believed that our universe has the right mass-energy density, equivalent to around 5 protons per cubic meter, which allowed it to expand for the last 13.8 billion years.

Because of this expansion, we can observe the light from a 30 billion light-years away galaxy, even though that light has traveled for only 13 billion years – the very space between them has expanded. The light from distant objects in the universe is red-shifted. This tells us that all the objects are receding away from us.

The expansion of the Universe is a key feature of the Big Bang cosmology and is a generic property of the universe we inhabit. Since Edwin Hubble”s observation in 1929, which led to the discovery of the expansion of the Universe, astronomers try to understand the behavior of it. Several models were created up today which shall explain the past and the future expansion: A decelerating universe reaches its current size in the least amount of time. The universe could eventually contract and collapse into a “big crunch” or expand indefinetly. A coasting universe is older than a decelerating universe because it takes more time to reach its present size and expands forever. An accelerating universe on the other hand is older still. The rate of expansion actually increases because of a repulsive force that pushes galaxies apart.

This can only happen if the universe is expanding. In addition to this expansion, there is also an accelerating factor. Studies determined that space expands at 72 km / 44.7 mi per second per megaparsec – around 3.3 million light-years. 

This means that for every 3.3 million light-years further away from the Earth you are, the matter where you are, is moving away from Earth 72 km / 44.7 a second faster. Many scientists attribute the current acceleration rate of expansion to dark energy. Dark energy is a mysterious yet hypothetical form of energy that has anti-gravitational properties.

Space-time & Shape

Simply put, space-time is an area in which all physical events take place. These events are the basic elements of space-time. An event is defined as a unique position at a unique time as such, space-time is the union of all events.

General relativity describes how space-time is curved and bent by mass and energy – gravity. The overall shape of the Universe has been greatly discussed and is the subject of controversy. Many scientists believe that unlike Earth, the universe may be flat. However, there are 2 other possibilities.

The density parameter – Omega – is defined as the average matter density of the universe divided by a critical value of that density. This gives birth to three possible geometries depending on whether Omega is equal to, less, or greater than 1. These are called, respectively, the flat, open, and closed universes. Observations continue in the effort of establishing the shape of the universe.


The universe is estimated to be made up of 4.9% ordinary (baryonic) matter, 26.8% dark matter, and 68.3% dark energy. Considering only the largest structures, the universe is made up of filaments, voids, superclusters, and galaxy groups, and clusters.

Dark matter and dark energy are invisible theoretical matter. The only proof of their existence lies in certain phenomena which mostly tell us that huge chunks of mass are missing, thus, in a way, justifying their probable existence.

Quasars played an important role in the investigation of helium. They are the universe’s lighthouses, and their light changes as they pass through intergalactic matter, revealing the composition of gasses.

Helium and hydrogen are the most abundant elements in the universe and are commonly found in stars and gas giant planets. Stars are mostly formed from hydrogen and helium gas. The heavier elements of the universe are commonly found in planets such as ours.

When it comes to particles, neutrinos have no charge and it is generally believed that they have little mass. They are created in energetic collisions between nuclear particles. The universe is filled with them though they rarely collide with anything. Everything with the exception of dark matter and energy is ordinary baryonic matter.

Dark matter is the second most common matter found throughout the universe. It isn’t visible to us as it doesn’t emit radiation; however, it is detectable gravitationally. Some galaxies have been found with little or no dark matter which suggests that our understanding of it is even more obscure than before. It remains theoretical.

The most common thing found in the universe is a hypothetical form of energy called dark energy. This energy is spread out throughout space and time, uniformly. 

This suggests it has anti-gravitational properties. This dark hypothetical energy has been implied to be the explanation for the universe’s accelerating rate of expansion. 

Much attention is now focused on the smaller elements in our universe in order to better understand it – particle physics. The studying of particles such as hadrons, leptons or photons may answer the bigger questions.

The Future

Since observations suggest that our universe will continue to expand, a popular theory states that our universe will cool as it expands. This will eventually lead to the end of life as we know it as everything will be too cold. Because of this, this theory is named the Big Freeze.

Another popular theory suggests that the expansion of our universe will eventually reverse. As this happens, the universe will re-collapse, and possibly lead to a reformation starting with another Big Bang. This theory is called the Big Crunch.

A third scenario named the Big Rip, hypothesizes the fact that everything will be torn to shreds, including atoms. It is said that this will happen when dark energy will become stronger than gravity. Regardless of the possible outcomes, they won’t happen for billions of years. We can use this time to our advantage, and may well unveil most, if not all the mysteries of the universe.

Did you know?

  • Many theorists suggest that our universe is but one of a set of disconnected universes, collectively denoted as the multiverse.
  • If there are 2 trillion galaxies in the observable universe, this means that over 1 billion trillion stars exist, and this is not even counting planets.
  • Since the universe is so vast, life is almost guaranteed to exist beyond our solar system. It may well be a more common thing than we may want to accept.
  • Since everything was created after the Big Bang, we ourselves are made out of star material.
  • A popular belief states that we are the universe, created to experience itself. Whatever the truth may be, in a sense, we really are the universe.
  • Using the word cosmos rather than the universe, implies viewing the universe as a complex and orderly system or deity – the opposite of chaos.
  • The word universe comes from the Latin word “universus” that later became “universum”, and the old French word “univers” – which in a sense, means whole.
  • One of the oldest and largest galaxy superclusters is the Hyperion supercluster. Is has more than 4 quadrillion solar masses, and it is believed that it formed just 2 billion years after the Big Bang.
  • There is no atmosphere in space, which means that it is a very silent “place”.


  1. Wikipedia
  2. NASA
  4. Britannica

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