The Universe

In this article, we will be discussing what the nature of the Universe is, its definition, size, properties, history, beginning, and other questions. The first question, which we will answer in the next section, is what is the definition of the Universe?

Our first question should be – what is the Universe – what is the definition of the Universe? When answering this question we will exclude the possibility of the Multiverse – a group of an almost infinite number of Universes, which could possibly exist, but is beyond the scope of this article.

Simply put, the Universe is everything that is, all of space and time and everything that is in it. This includes planets, stars, galaxies, the interstellar and intergalactic void, visible matter and energy, invisible matter and energy – the mysterious dark matter and dark energy, and anything else you can think of as well as anything that you can’t think of or imagine, as long as it actually exists – EVERYTHING!

The history of the Universe will be explored in the next sections including early conceptions of the Universe and their evolution and some of the related mythologies.

The concept of a Universe which included all reality as we know it goes back thousands of years. The ancient Greeks, Indians, and Chinese had a concept of the Universe that was geocentric; that is, the Earth was the center of the whole Universe. They had their own theories about the Universe believing in its impersonal nature and that it was governed by some kind of physical laws. Naturally, superstition also played a part in their conception of the Universe. What were some of these ancient myths?

Early myths of the Universe go back thousands of years. Ancient cultures had their own stories about the creation of the Universe, often a god of some kind who created the Universe, or at least a god who set the Universe up and natural laws took over and governed its evolution.

The ancient Indians and Chinese had stories about the Universe being hatched from a gigantic ‘world egg’. Ancient Tibetans, Greeks, Aztecs, Egyptians, and Christians had a narrative about the Universe being created from a single entity, or being, of some type.

Some other ancient cultures believed that the Universe was created from male and female deities of some type. Still other cultures thought that the Universe had been created from some sort of pre-existing substance, such as a dead god. The Hindus believed in the creation of the Universe from the most fundamental principle of Brahman, the ultimate reality and the cause of all that exists.

Ancient Greek and Indian philosophers developed some of the earliest philosophical concepts of the Universe.

The early Greek philosophers believed in the underlying cause of the phenomena being observed; that is, they thought that the appearances of some physical behavior or phenomena could be deceiving and the real cause was below the surface of observation. They wanted to find the real cause of these appearances and sought to understand the underlying reality.

Some of these early Greek philosophers proposed that all material manifestations observed in our everyday world really emanated from a primary primordial substance or material. Thales was the first to do this and proposed that this primordial material was water. His student, Anaximenes, thought that this primordial material was air based on his observations of its properties. Anaxagoras thought this material to be the mind itself, from which all material manifestations of our world came from.

Empedocles proposed a 4 element model for his primordial materials – earth, air, water, and fire which became quite popular in ancient Greece. Plato and Pythagoras believed that all physical manifestations of our world were based in abstract mathematics. Other Greek philosophers, such as Democritus and Leucippus, proposed that the Universe is made up of indivisible atoms moving through some sort of void, although Aristotle did not believe that this was feasible and thought there was a continuum which was infinitely divisible.

Parmenides had the radical thought that everything observed in the physical world, in effect all change, was an illusion and that the only true underlying reality was one of a singular nature which was always constant and unchanging.

The idea that the Universe was composed of finite atoms was also developed by the Indian philosopher Kanada who also believed that the light and heat he observed came from the same underlying substance. In the 5th century Dignaga, who was a philosopher from the school of atomistic Buddist philosophy proposed the theory that there were atoms which were infinitesimally small, without any significant duration, and made up of energy.

Babylonian astronomers are considered to be some of the very first astronomers from way back to the early history of Mesopotamia thousands of years B.C. and they had a vision of the Universe as a flat disk floating in some kind of ocean. This is something that the early Greeks picked up on as can be seen in some of their maps.

Later on, Greek astronomers were more intent to base their observations of celestial bodies on empirical evidence and Aristotle believed that there were celestial spheres which revolved eternally at a constant speed around the terrestrial sphere of Earth, which contained the material world. Aristotle also believed that the only matter which existed was inside this terrestrial sphere.

A little later on it was proposed by the Greeks that the entire Universe was in fact composed of five elements – earth, air, water, fire, and ether. The Greek astronomer Callippus expanded upon this idea and the idea of concentric spheres was eventually abandoned. The Greek-Roman astronomer Ptolemy made astronomical observations which seemed to confirm this new idea. This was in large part due to the mathematical fact that the positions of celestial bodies could be broken down and described in terms of circular functions.

Other Greek astronomers and philosophers, notably the Greek philosopher Philolaus, hypothesized that there was a type of fire in the center of the Universe around which the Earth, Sun, Moon, and planets revolved in a constant and uniform circular motion.

Another Greek astronomer, Aristarchus, is believed to be the first to develop a heliocentric model of the Universe where the Earth revolved around the Sun. He thought that the stars were at a very great distance from the Earth since they did not seem to move relative to each other as the Earth went around the Sun. Of course, it is now known that stars are much farther away than these early Greek scientists could even imagine. Seleucus was a Hellenistic astronomer who also supported the heliocentric model of the Universe through his method of logical reasoning.

Aristotle’s model of celestial spheres was to eventually replace the heliocentric model of the Universe and dominate the thinking of the Western world for about two thousand years until Nicolaus Copernicus, a Polish mathematician, and astronomer revived the heliocentric model.

Until then it was believed that the Sun and other planets all revolved around the Earth. But Copernicus came up with the radical idea that the Earth and other planets actually revolved around the Sun – according to him it was the rotation of the Earth which caused the illusion that the Earth was the center of the Universe. Copernicus himself admitted that his radical idea was actually very old, going back to the ancient Greeks around 450 B.C. This Copernican model of the Universe was accepted by Isaac Newton and other scientists.

Edmund Halley was an English astronomer and mathematician who believed that the idea of a Universe filled with an infinite number of stars would cause the night sky to be as bright as the Sun, which obviously was not the case. This was known as Olbers’ paradox in the 19th century, named after the German astronomer Heinrich Wilhelm Olbers.

Isaac Newton, on the other hand, believed that infinite space in the Universe that was uniformly filled with matter would cause virtually infinite forces and instabilities which would cause all the matter to collapse inward upon itself.

A solution to these paradoxes was called the Charlier Universe, named after the Swedish astronomer Carl Vilhelm Ludwig Charlier, which was that the matter in the Universe was organized in hierarchal systems of orbiting celestial bodies which were themselves orbiting in larger systems, and so on. This resulted in a fractal system of the distribution of matter in the Universe which in turn resulted in a much more stable Universe with a much lower average cosmological density of matter.

When the Mount Wilson Observatory was completed in 1919, it was believed that the Milky Way Galaxy was in effect the entire Universe. The famed astronomer Edwin Hubble used the Mount Wilson Observatory to conclusively prove that the Andromeda Galaxy and the Triangulum Galaxy were completely separate galaxies far outside our own Milky Way Galaxy and that in fact there were many separate galaxies in our Universe. This along with the application of Albert Einsteins General Theory Of Relativity in 1917 to model the structure and dynamics of the Universe effectively was the beginning of the modern era of cosmology.

The Big Bang Theory is the most widely accepted theory for the beginning of the Universe at the current time and seems to best explain our observations of the Universe. Simply put, time and space both emerged from some incredible explosion about 13.8 billion years ago. At the instant of the Big Bang, the Universe was incredibly dense, but an inflationary expansion then occurred, initially at an incredibly rapid rate, and the Universe gradually became less dense until it reached its current density about 13.8 billion years later.

At the earliest state of the Universe, in effect the instant of the Big Bang, the Universe was incredibly dense and hot – this is called the Planck epoch – an incredibly brief time extending from the time of zero to 10 -43 seconds, one-tenth of a tredecillionth of a second, or one ten-million trillion trillion trillionths of a second. This is a time so brief as to be beyond our ability to imagine it!

During this Planck epoch all types of matter and all of the 4 fundamental forces – gravity, weak nuclear, strong nuclear, and electromagnetic – are believed to have possibly been unified, with gravity, currently the weakest of the forces, of equal strength with the other fundamental forces. The very rapid inflation of this early Universe is thought to have occurred during the first 10 -32 seconds, one-hundred nonillionths of a second, or about 100 million trillion trillionths of a second.

It is important to understand here that physical matter itself didn’t move during this extremely intense and brief inflationary period. Rather spacetime was what moved at speeds much faster than the speed of light – this doesn’t violate the Special Theory of Relativity since this theory is concerned with a positive mass approaching the speed of light, not the empty spacetime continuum. This incredibly rapid inflationary period may also explain why space itself appears flat and why the unobservable part of the Universe is so much larger than the observable part of the Universe.

Another important point that should be made is that scientists really have no idea what happened at the very instant of the Big Bang – the singularity. What were the fundamental properties it may have had? To be able to theorize what did actually happen at the very instant of the Big Bang, the singularity of the Big Bang, would require a quantum theory of gravity, which we do not have at the present time.

The first thing to understand when starting a discussion of the chronology of the Universe is that some assumptions have been made in the Standard Model of the Universe, which is based on the General Theory of Relativity, such as that space is homogenous and isotropic in nature. The simplest model of the Universe which adequately explains the various phenomena and properties which have been observed is called the Lambda-CDM model which is also called the Standard Model.

In this model are assumptions of a cosmological constant, Lambda, and that cold dark matter, CDM, permeates the Universe. This Big Bang model can explain a number of observational phenomena of the Universe such as the microwave background radiation, the ratio of the number of hydrogen to helium atoms which occur in the Universe, and the phenomena of the correlation of the distance of galaxies(and certain other celestial objects) with their redshift values.

Simply put, the General Theory of Relativity is a geometric theory of gravity in the Universe and is the basis for the Standard Model – Lambda-CDM. This theory provides a unified theory of gravity describing it as a geometric property of space and time by generalizing two other big theories – Einstein’s Theory of Special Relativity and Newton’s Law of Universal Gravitation.

Essentially it shows that the curvature of the space-time continuum is directly related to all of the matter and radiation that is present, being described precisely by partial differential equations called Einstein’s Field Equations of the General Theory of Relativity. Therefore the distribution of the matter and energy present in a given region of space will determine the geometry, or curvature, of spacetime. This, in turn, determines the acceleration rate of matter – what we call gravity.

In the Standard Model, we make assumptions that there is a cosmological constant called Lambda and that the Universe is both homogenous and isotropic. But we know that in different regions of space there is an uneven distribution of matter which will lead to instabilities in the Universe; that is the Universe will either be contracting or expanding. Our observations currently indicate that space is stretching and galaxies are flying apart from each other. It appears that the Universe is actually expanding, possibly due to the influence of a mysterious force called dark energy.

As we have previously discussed, the Universe started with the Big Bang, which then proceeded to expand at an extremely rapid rate in the first 10 -32 seconds. In this first incredibly small amount of time, it was also extremely dense and hot – as this inflationary expansion proceeded it would gradually cool off and become less dense until about 13.8 billion years later the Universe reached the current state that we observe today.

In the first fraction of a second after the beginning of the Universe, the Big Bang, the four fundamental forces separated and manifested themselves; gravity, weak nuclear, strong nuclear, and electromagnetic. And the space-time continuum itself came into existence.

In the first ten seconds after the Big Bang, the Universe continued to cool from a state so hot and dense that it is beyond our imagination. In these first ten seconds, subatomic particles were formed in what is known as the quark epoch, the hadron epoch, and the lepton epoch. These first elementary then combined into increasingly larger and stable combinations, such as protons and neutrons.

In a process known as Big Bang nucleosynthesis, which lasted until about 20 minutes after the Big Bang, more complex atomic nuclei were formed through the process of nuclear fusion. In this process, about 75 percent of protons remained unaffected in the form of hydrogen nuclei. About 25 percent of the protons and all of the neutrons were converted into helium nuclei, with only tiny amounts of deuterium(an isotope of hydrogen with 2 neutrons in the nucleus) and even smaller amounts of lithium and some other elements.

At the end of the nucleosynthesis period, lasting about 17 minutes and ending about 20 minutes after the Big Bang, the Universe entered a period known as the photon epoch, where photons were the dominant energy in the early Universe. The photon epoch was the result of leptons and anti-leptons(effectively matter and antimatter) annihilating each other at the end of the lepton epoch, about 10 seconds after the Big Bang.

The photon epoch lasted roughly 370000 years, during which time the Universe was a very hot and dense plasma fog of nuclei(mostly hydrogen and helium), electrons, and photons – individual complete atoms could not form in this very intense plasma state.

Then after about 370000 to 377000 years, the universe cooled enough to form neutral and stable atoms from the combination of nuclei and electrons and the photon epoch ended. When this plasma state ended, neutral atoms also became transparent to the wavelengths of light, so the Universe effectively became transparent. Something that is very interesting to know is that the photons that were released when these very first atoms were formed are still visible today as the cosmic microwave background!

Eventually, the energy density of matter became greater than that of photons and neutrinos and dominated the behavior of the Universe as a whole. This effectively was the end of the radiation-dominated era of the Universe and the beginning of the matter-dominated era.

It is believed that in the early stages of this matter-dominated era that tiny fluctuations occurred in the density of the Universe causing clumping of dark matter which in turn attracted ordinary visible matter through its gravitational attraction. Gas clouds were formed, and then eventually stars and galaxies formed around the denser clumps of dark matter with voids of interstellar and intergalactic space in the less dense regions.

The first stars were formed after several hundred million years – these were probably very massive and short-lived. These early stars were responsible for the reionization of the Universe(the first ionization was the initial photon epoch period during the first 370000 years of the Universe) from around 300 million years to 1 billion years after the Big Bang. Through the process of stellar nucleosynthesis(the fusion process in a star), they also began seeding the Universe with the heavier elements – elements which were heavier in their atomic weight than helium.

This story of the chronology of the Universe would not be complete without mentioning the very mysterious force believed to exist which is known as dark energy. There is very little known about this mysterious energy other than that its density does not appear to change over long periods of time. The Universe continued to expand and after about 9.8 billion years the density of matter became less than the density of dark energy, which meant that the dark-energy-dominated era had begun. We are still in this era, which is expected to last indefinitely as the expansion of the Universe continues to accelerate due to this mysterious dark energy.

In these next sections, we will discuss the various properties of the Universe, including its age, expansion, size, shape, and spacetime.

The age of the Universe is currently believed to be about 13.8 billion years old. This estimate has been arrived at by assuming that the Lambda-CDM model(Standard Model) of the Universe is correct, which certainly seems to be the case at the current time based on its agreement with a huge amount of observational data of the Universe.

The Lambda-CDM model seems to very accurately describe cosmological parameters and the evolution of the Universe all the way from its early beginnings as an extremely hot and dense plasma state up to the present time. This Standard Model of the Universe is very well understood theoretically and is in agreement with a number of very precise observational measurements including the cosmic microwave background, brightness and redshift in relation to supernovae and galaxies, galactic clusters, and gravitational lensing to name just some.

We now know with a great deal of confidence that the Universe is rapidly expanding at an accelerating rate. Observational data of various phenomena including the redshift measurements of individual galaxies, galactic clusters, supernovae, quasars, and other celestial objects definitely indicate that space itself is expanding.

It is currently thought that this is because of the influence of a mysterious force called dark energy. While the existence of dark energy has not been proven, it is a hypothetical construct at the present time based on the Standard Model of the Universe being accurate and correct, most scientists now believe that it exists and is accountable for the accelerating expansion of our Universe.

We will now discuss what the size of the Universe is and make our very best estimate based on the most recent measurements and observational data. One thing that is important to mention here is that there will really be two parts to this answer – what is the size of the observable Universe and what is the size of the unobservable Universe? It should also be made clear that in respect to size the observable Universe is really a subset of the entire Universe as a whole – that is, the entire Universe contains the observable part of the Universe, with the unobservable part extending beyond that.

The observable Universe is that portion of the Universe for which it is possible to see, although perhaps in practice it is not easy to do so. The radius would seem to be about 13.8 billion light-years, the time since the Big Bang, and so the entire diameter would be around 27.6 billion light-years. However, if we take into account the expansion of the Universe, probably caused by dark energy, then the most accepted estimate of the size of the observable Universe is a radius of 46.5 billion light-years, or a distance across of about 93 billion light-years. This would be assuming that the shape of the Universe is that of a sphere, which as we shall see later on, is not certain at all.

One important point here is that you will see figures of around 46.5 billion light-years for the radius of the observable Universe and 93 billion light-years for its diameter. These are completely theoretical numbers based on observational data of the rapid expansion of the Universe.

The size of the unobservable Universe, that part of the Universe which is forever beyond our ability to see or observe in any way, is much more difficult to estimate, and because of its very nature, we have to use our best theoretical models to make an estimate.

There is an unobservable, or unseen part of the Universe because of the fact that the Universe is rapidly expanding at an ever faster rate and distant galaxies are moving away from us at speeds greater than the speed of light, so the light from these galaxies can never reach us – 186282 miles per second is the speed of light and the ultimate speed limit in the Universe. These remarkable speeds have been confirmed by redshift observations of galaxies, galactic clusters, and supernovae with the proper calculations from out theoretical models of the Universe and physics.

The size of the unobservable part of the Universe is believed to be far larger than that of the observable Universe – using current theoretical models estimates really vary wildly. I have seen estimates of a volume around 1000 times as much as the observable Universe with a diameter over 7 trillion light-years across. On the other hand, I have seen an estimated volume millions of times larger with a diameter over 700 trillion light-years across!

These huge estimates compared to the relatively smaller number of 13.8 billion light-years for the observable Universe is because these distant galaxies are receding at speeds MUCH greater than the speed of light. These faster than light speeds do not violate Einstein’s Theory of Special Relativity because it is the space itself which is expanding; the Theory of Special Relativity is concerned with the movement of matter at speeds approaching that of light.

The point is that at the present time with our current state of technology no one seems to really know with any degree of precision, mainly because that portion of the Universe is completely beyond our ability to observe it. The only thing about the size of the unobservable Universe that we seem to know with any degree of confidence is that it is incredibly large, much larger than that part of the Universe that we can see. For the time being and the foreseeable future, this is the best answer that we can give for the size of the unobservable Universe.

The first thing that should be made clear is that there currently a significant level of uncertainty as to the shape of the Universe. That being said we will attempt to answer that question within the constraint of the limitations of our current technology and theoretical models of the Universe and physics. Currently, the Lambda-CDM model of the Universe is the most widely accepted and, along with the Theory of General Relativity, will be the foundation upon which our estimates will be built.

Ultimately, according to present theories, the shape of the Universe will depend on the Theory of General Relativity and the matter distribution in the Universe, along with the evolution of the Universe as described by the Standard Model.

Also, cosmologists make a distinction between the observable Universe and the global Universe; the global Universe is the entire Universe consisting of both the observable and unobservable parts. First, let’s look at the shape of the observable Universe.

The observable Universe is that part of the Universe which can be seen from light reaching it from as far back as the Big Bang, about 13.8 billion light-years. But since space has been expanding at an accelerating rate over this 13.8 billion year time period, the actual radius of the observable Universe is believed to be about 46.5 billion light-years, giving it the shape of a sphere. Even this much is still not certain.

As for the shape of the global Universe, it is even more uncertain, simply because of the fact that the largest part, the unobservable Universe, can never be observed. The word estimate would really not be an accurate term to use, a guess based on our best theoretical models is really more like it.

As for the shape of the global Universe, there are 3 basic possible attributes: 1 the global Universe is either finite or infinite in size 2 it is flat with no curvature, open with negative curvature, or closed with positive curvature and 3 the topological connectivity of the global Universe.

The real shape of the global Universe is currently a matter of intense debate, although there does seem to be some precision experimental evidence from various independent sources supporting the idea of a flat Universe, infinite or nearly infinite in size. The final answer, if it is to ever come, could be in the distant future.

The spacetime continuum, or simply spacetime, is believed to consist of three spatial dimensions and one temporal dimension of time. An important note here is that I believe that to describe time as a dimension is somewhat misleading at best. Perhaps it would be better to describe it in terms of a sort of negative energy, since it seems to be associated in some way with the second law of thermodynamics, the law of entropy. Some scientists wonder if time really exists at all, perhaps it is an illusion resulting from the effects of entropy. In any case, for the purpose of this article, I will continue to refer to it as a dimension, however inadequate that may be.

Also, as for discussing spacetime, we will keep the discussion to the 4 dimensions(3 spatial and 1 temporal) – we will not attempt to get into a discussion of extra dimensions which may exist in a sort of hyperspace – that is beyond the scope of this article and deserves a treatment all of its own. We will also use the Lambda-CDM model of the Universe and the Theory of General Relativity as the foundation of our discussion. Although the Theory of Special Relativity may also be relevant in certain circumstances, applying it is best left to more specific inertial framework references.

Spacetime is in effect the foundation of the Universe and of reality itself – without it, nothing would exist. The building blocks of spacetime are events, which can be defined as a unique position of something at a unique time. Spacetime as a whole is the union and connection of all of these events. The Universe is, therefore, a smooth space-time continuum where any event can be described in terms of 4 coordinates – x, y, z, and t(time) for example.

Spacetime seems to be almost completely flat according to our best observations. This means that the curvature of spacetime would have to be very close to zero, which, if true would mean that Euclidean geometry could be used throughout most of the Universe with a very high degree of accuracy.

Observations of the Universe also seem to show that as far as the observable Universe is concerned, it may have the shape of a sphere, and so spacetime would have a very simply connected topology(the shape of the unobservable Universe is far less certain and may actually be close to flat).

This has been a simplified discussion of spacetime – there may be more dimensions in a type of hyperspace, such as the 11 dimensions in the more common version of string theory. If the Universe does, in fact, consist of a hyperspatial type of spacetime with many more dimensions, its topology would be far more complicated than the simplified Euclidean geometrical topology we have discussed here.

All kinds of possibilities might open up; perhaps it might even be possible for objects to move faster than the speed of light in hyperspace, although that is only conjectured at this time. Determining whether hyperspace actually exists, and what properties it might have, will most likely require technologies and theoretical concepts which are still well beyond our reach.

The composition of the Universe will be discussed in the next sections including visible and dark matter, energy and dark energy, and particles.

Roughly 4.9 percent of the total mass-energy of the Universe is ordinary matter – that is atoms, ions, electrons, and all of the objects that they form such as planets, asteroids, meteors, comets, stars, galaxies, and many other celestial objects.

Ordinary matter can exist in four basic states – solid, liquid, gas, and plasma – a state with very high densities and temperatures where normal atoms cannot exist, in effect electrons are stripped away from the nucleus in this plasma state. Also, ordinary matter is composed of two basic types of elementary particles – quarks and leptons.

For example, a proton is composed of two up quarks and one down quark, a neutron of two down quarks and one up quark, and the electron is composed of a lepton. The nucleus of an atom consists of protons and neutrons and is orbited by electrons to form the complete atom with a neutral charge, while an ion is basically an atom with a nonneutral charge.

A baryon is a type of composite subatomic particle containing an odd number of valence quarks. Baryonic matter is a term used to describe ordinary matter – the vast majority of the mass of an atom is in its nucleus, and this nucleus is made of baryons. Nonordinary matter, such as dark matter, is called nonbaryonic matter. Again, although all ordinary matter is described as baryonic matter, a very small portion of this total ordinary matter in the Universe is in the form of electrons, which is not made of baryons.

Dark matter is a mysterious hypothetical substance which is believed to constitute most of the matter in the Universe if we assume the Standard Model(Lambda-CDM) description of the Universe to be correct. It is a form of nonordinary matter which is also known as nonbaryonic matter – see the previous section for an explanation of this. Most of this dark matter is called cold dark matter(this is the CDM in Lambda-CDM), although some dark matter exists in the form of neutrinos, which is classified as hot dark matter.

Dark matter does not interact with the electromagnetic spectrum at all and is essentially invisible to it. The only thing dark matter is known to interact with is gravity, which we can see the effects of on a large scale, such as with galaxies, which it is believed could not exist if it were not for the fact that they consist of around 85 percent dark matter which gives them enough mass to hold together.

Cold dark matter, the vast amount of dark matter in the Universe, has never been detected directly which makes it one of the greatest mysteries, along with dark energy, in cosmology. Neutrinos, a type of hot dark matter, is the only exception to this and is believed to have been detected. It is believed that dark matter constitutes about 84.5 percent of the total matter in the Universe and 26.8 percent of the total mass-energy of the Universe.

We’ve already seen how vast the Universe is, but what about the total energy of the Universe? What is the total energy of the Universe?

The four fundamental forces of the Universe are gravity, electromagnetism, strong nuclear, and weak nuclear. In addition to this we know, thanks to Einstein, that E=mc2, that is energy is equal to mass times the speed of light(in a vacuum) squared. This famous equation tells us that, at least in theory, mass is convertible to energy, and energy to mass. We know all too well about the first case, atomic and hydrogen bombs are derived from this principle. The second case, energy to mass conversion, is implied to be possible, although we cannot achieve it at the present time.

As for the total amount of energy, what is it? Well, the answer may be surprising – the total amount of energy in the Universe comes out to be not some gigantic vast quantity, but the whopping sum total of zero – that’s right, zero! It turns out that positive and negative energies in the Universe essentially cancel each other out and obeys the law of the conservation of energy – the total energy of a closed system will remain constant over time. Thus it is possible to have an entire Universe without having any net amount of energy at all!

The dark energy believed to be present in the Universe, if we believe the Standard Model of the Universe is correct, is a hypothetical force which is every bit as mysterious as dark matter. Dark energy has played a critical role in the evolution of our Universe from the formation of galaxies, which may not be able to exist without it, to the rapidly accelerating expansion of the Universe. By the way, this accelerating expansion of the Universe essentially gives us two parts to the Universe, the observable Universe, and the unobservable Universe where distant galaxies are receding from us at greater than light speeds – here it is the space itself that is expanding so it does not violate the Theory of Special Relativity.

Some cosmologists believe there are two basic types of dark energy, the cosmological constant Lambda, and that caused as a result of scalar fields which are basically described as a relativistically invariant classical or quantum theory of scalar fields. The density of dark energy is much less than that of either matter or dark matter in the Universe and is believed to be constant and uniform over the entire Universe.

Although its density is very low(approximately 7 x 10 -30 grams per cubic centimeter) using a mass-energy equivalence, it has massive and dramatic effects on the evolution of the Universe and galaxies as described previously. The research of this very mysterious dark energy will continue for many years into the future – perhaps one day more about its nature and properties will be known.

In this section, we will be concerned mainly with elementary particles which constitute ordinary, or visible, matter in the Universe. So little is known about dark energy that there is little point in extending any discussion in that direction.

The elementary particles in the Universe are basically composed of quarks and leptons – ordinary matter is also described as baryonic matter which is composed of quarks. The nucleus of an atom, the greatest part of the mass of an atom, is made up of baryonic matter – protons and neutrons, while the electrons orbiting it are a type of lepton.

The Standard Model, Lambda-CDM, accurately predicts electromagnetic interactions as well as the strong and weak nuclear interactions. The experimental confirmation of the existence of these elementary particles – quarks and leptons, as well as their antimatter counterparts – support the Standard Model and is the reason why it is so widely accepted.

Force particles which mediate interactions, such as photons, bosons, and gluons are also an experimental confirmation of the Standard Model. Although the Standard Model is often considered as the closest thing we have to a ‘theory of everything’, it still cannot account for the force of gravity, which is mysterious in and of itself. Any elementary force particle which could be associated with gravity has never been found yet.

We have already seen how vast the Universe is, so the question arises – how many stars are there in the Universe? There are really two parts to this question, the observable Universe and the unobservable Universe, the latter which is by its very nature more of a guess than an estimate.

This where we can just get a very rough estimate because obviously there is no way to exactly count the total number of stars. But if we use the most recent estimate of 2 trillion galaxies in the observable universe and make an assumption of 100 billion stars in each galaxy on the average, we come to a figure of 200 billion trillion stars, or 200 sextillion stars. This is only intended to be a rough estimate, and may well change as our knowledge increases in the future.

Since by its very nature the unobservable Universe is forever unseeable and unknowable, we can only make a wild guess, not really an estimate, of the total number of stars that it might contain. Our best theoretical models in cosmology are the only way we can make a wild guess as to its size, although it is pretty well certain that it is far larger than the observable Universe, because of the extremely faster expansion of space at faster than light speeds.

The size of the unobservable Universe is currently thought to be anywhere from thousands to millions of times larger than the observable Universe, and from 7 trillion light-years to more than 700 trillion light-years across. It would, therefore, be meaningless to make even a wild guess as to how many stars it might contain – just suffice it to say that the number would be incredibly gigantic, beyond our ability to imagine!

Again there are two parts to this question – how many galaxies are in the observable Universe, and how many galaxies are in the nonobservable Universe?

Knowing how vast the observable Universe is, about 46.5 billion light-years in radius, or 93 billion light-years in diameter, of course, the total number of galaxies is huge. The best estimate that we have at the current time based on our most recent observational data is that there are believed to be approximately 2 trillion galaxies in the observable Universe.

We have seen that the unobservable part of the Universe is far larger than the observable, or visible, part of the Universe. And by its very nature, the contents of the unobservable Universe are unknowable. In addition to this, we don’t really know its size, only that it is probably anywhere from thousands to millions of times larger than the observable Universe. Therefore, just as in the case with stars, it is meaningless to come up with any kind of number – we just know that it is likely to be incredibly large beyond our ability to conceive of it!

Is there life in the Universe? Well, we already know the answer to that – yes, on the planet Earth. But what about elsewhere – this is the question that we will now explore.

In recent years we have made some tremendous discoveries of not only planets but potentially habitable planets within a thousand or so light-years of Earth. As of 2019, there are known to be over 4000 exoplanets in over 3000 different star systems, some of which may have extraterrestrial life or even some type of advanced intelligent life. Of course, this is just conjecture at the present time, but the chances are quite good.

With somewhere in the range of 100 to 400 billion stars, it is estimated that the Milky Way Galaxy could have as many as 800 billion to 3.2 trillion planets. And with all these planets it is estimated there could be somewhere in the range of 500 million to 10 billion or even more habitable worlds. Again, the chances of life, and even intelligent life out of such a huge number is great.

Now we get to the big question – how many habitable planets are in the Universe and what are the chances of life? Here we will be concerned only with the observable Universe, thought to be about 93 billion light-years across – the contents of the unobservable Universe are by definition unknowable so we will not venture into that territory.

We will just have to make some assumptions based on our best observational data since an exact number is obviously impossible to get. If we assume the total number of galaxies in the observable Universe is about 2 trillion with an average of 100 billion stars each, and use the low-end estimate of 800 billion planets, we come to an amazing figure of 160 billion trillion planets, about 160 sextillion planets, in the Universe.

If ten percent of these planets were habitable, we would arrive at a gigantic number of about 16 sextillion habitable planets in the Universe. Let’s say that only one out of a thousand of these planets have any kind of life, that would still give a huge number of about 160 million trillion, or about 160 quintillion, planets in the Universe with some form of life. And if only one out of a thousand of these planets had some form of intelligent life, we would still get a gigantic number of 160000 trillion, or 160 quadrillion, planets with some type of intelligent life. There seems to be a strong possibility that there could be vast numbers of intelligent civilizations in the Universe.

You may be asking if this is so, why can we not find evidence of these civilizations – surely investigations such as SETI, the Search for Extraterrestrial Intelligence would have picked up definitive signals indicative of intelligent life by now.

That is certainly a valid question, but it should be asked in the context of the primitive state of our current technology – yes primitive! We are still using radio waves as our main detection method, which have only been around for a little over a hundred years. Our civilization on Earth is hardly advanced at all when compared to what may be out there in the Universe.

After all, if there are in fact advanced alien civilizations thousands, hundreds of thousands, or even millions of years more advanced than our own, they may have stopped using radio waves for communication long, long ago. There understanding of physics might be so far beyond our own, we would not be able to distinguish it from magic! The question of intelligent life in the Universe is something we cannot answer definitively yet, only speculate about.

I think the best question to end this article with is one of the most obvious and also one of the most unanswerable – what is the fate of the Universe, how will it end?

The answer that I will give is really a nonanswer – we don’t know, we can only guess. If we assume that the Standard Model of the Universe is correct, as it seems to be at the present time, then the hypothetical presence of the very mysterious dark energy is causing a rapid and accelerating expansion of the Universe.

If there is enough matter in the Universe it is possible that this expansion could eventually slow down, and the Universe could even start to contract in a far distant future. The end result of this would be what cosmologists call the Big Crunch, where the Universe completely collapses into a singularity, and everything vanishes.

Then again, if there is an insufficient amount of matter in the Universe to halt this accelerating expansion, then the Universe will keep going on forever, although it would still eventually cease to exist as we know it – stars would run out of fuel and eventually go dark, even the entire Universe would go dark. And it has been theorized that even the elementary particles themselves which make up the Universe would eventually disintegrate in some incredibly far off future – long before this all life in the Universe would cease to exist.

Both of these scenarios are rather gloomy, but should we really concern ourselves with it? After all our civilization, and indeed humanity, as we know it, will likely cease to exist long, long before then. So why even ask the question about the fate of the Universe – because we are curious creatures, that’s why!