What Do We Really Know About the Universe?

There's so much that we've come to learn about our Universe.

From just about any standpoint, existence is pretty funky and weird. But when you get right down to the fundamental physics of it all, it gets even weirder! While many people may think that in the realm of science, everything is clear-cut and ordered. But is that the way things really work?

Throughout millennia, scholars and philosophers have debated endlessly whether life and the cosmos are orderly or chaotic. The sciences have not been immune to this debate, and many significant discoveries have been taken up by either one school of thought or the other.  

Learning about the motions of the planets, gravity, atomic theory, relativity, quantum mechanics, and the large-scale structure of the Universe has sometimes been used to add weight to ideas of both order and chaos.

At present, there's a lot of ambiguity when it comes to this question, and future discoveries may help to resolve it. But in the meantime, it's good to take stock of what we've learned and what it can tell us about life as we know it.

What Do We Really Know About the Universe?
Panoramic view of the Milky Way. Source: ESO/S. Brunier

What is the Universe?

The word “Universe” comes from the Latin “Universum”, which was used by Roman authors to refer to the cosmos as they knew them. This consisted of the Earth and all life as well as the Moon, the Sun, the planets that they knew about (Mercury, Venus, Mars, Jupiter, Saturn) and the stars.

The term “cosmos”, on the other hand, is derived from the Greek word kosmos, which means "order" or “the world”. Other words commonly used to define all of known-existence include “Nature” (from the Germanic word natur) and the English word “everything” (self-explanatory).

Today, the word Universe is used by scientists to refer to all existing matter and space. This includes the Solar System, the Milky Way, all known galaxies, and superstructures. In terms of modern science and astrophysics, it also includes all time, space, matter, energy, and the fundamental forces that bind them.

Cosmology, on the other hand, is used to describe the study of the Universe (or cosmos) and the forces that bind it. Thanks to thousands of years of scholarship, what we know about the physical Universe has grown by leaps and bounds. And yet, there is still so much that we don't understand.

To get a sense of where we are today, we must first take a look back...

History of cosmology

Human beings have been studying the nature of existence pretty much ever since they've been able to walk upright and speak. However, most of what we know about the study of the cosmos goes back only as far as the existence of written records.

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Luckily, many of these records come from oral traditions that predate writing, so a general idea of what our ancestors believed does exist. What we know indicates that the earliest accounts of the Universe's creation tended to be symbolic and metaphorical in nature.

As far as we can tell, every culture that has existed has had its own version of a creation story. In many, time and all life began with a single event, where a God or gods were responsible for creating the world, the heavens, and everything in between. Most creation stories either included or culminated with the birth of humanity.

Archaeological evidence suggests that as far back as 8000 BCE, people tracked celestial events, such as the movement of the Moon, in order to create calendars. By the 2nd millennium BCE, astronomy began to emerge as a field of study.

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Some of the earliest recorded observations of the heavens are attributed to the ancient Babylonians. These would go on to inform the cosmological and astrological traditions of the cultures in the Near East and the Mediterranean for thousands of years to come.

What Do We Really Know About the Universe?
Artist's impression of the "Arrow of Time". Source: NASA/GSFC

The notion of finite time is sometimes traced to this period and perhaps to the Zorastrian religion. At the core of this is the belief that the Universe was created, represents the unfolding of a divine plan, and has an end. 

Later doctrines espoused that time began with creation, or self-creation, and will end with a triumph of order over chaos, and a version of the Day of Judgement where all of creation will be reunited with the Creator. These concepts are likely to have been transmitted to Judaism in around the 6th century BCE with the Persian conquest of Babylon.

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The idea of time as a linear progression would go on to inform western cosmology for thousands of years, and still exists today (for example, with the "Big Bang" and the "Arrow of Time" theories.)

Between the 8th century BCE and the 6th century CE (the period often referred to as "Classical Antiquity"), the concept that physical laws governed the Universe began to gain greater traction. In both India and Greece at this time, scholars began offering explanations for natural phenomena that emphasized cause and effect.

Birth of the atom

By the 5th century BCE, Greek philosopher Empedocles theorized that the Universe was composed of the four elements of earth, air, water, and fire. Around the same time, a similar system emerged in China that consisted of the five elements of earth, water, fire, wood, and metal.

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This idea would become influential, but would soon be countered by Greek philosopher Leucippus who theorized the idea that the Universe was composed of indivisible particles known as "atomos" (Greek for "uncuttable").

The concept would be popularized by his pupil, Democritus (460 - 370 BCE), who argued that atoms were indestructible, eternal, and determined the properties of all matter.

The Greek philosopher Epicurus (341–270 BCE) would refine and elaborate on this idea. For this reason, it would come to be associated with the school of philosophy he inspired (Epicureanism).

The Indian philosopher Kanada, who is believed to have lived between the 6th and 2nd century BCE, proposed a similar idea. In his philosophy, all matter was composed of "paramanu" - indivisible and indestructible particles. He also proposed that light and heat were the same substance in a different form.

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What Do We Really Know About the Universe?
Particles of the Standard Model of particle physics. Source: Daniel Dominguez/CERN

The Indian philosopher Dignana (480 - 540 CE), who was one of the Buddhist founders of Indian logic school of thought, went even farther by proposing that all matter was made up of energy.

These theories were largely forgotten in the west but would remain popular among Islamic and Asian scholars, who translated them into Arabic and other languages. By around the 14th century, interest in "atomism" would reemerge in the west, thanks to the translation of classical works back into Latin.

Earth's place in the Solar System

Between the 2nd millennium BCE and the 2nd century CE, astronomy and astrology continued to develop and evolve. During this time, astronomers monitored the proper motions of the planets and the movement of the constellations through the Zodiac.

It was also during this time that Greek astronomers articulated the geocentric model of the Universe, where the Sun, planets, and stars revolve around the Earth.

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These traditions were summarized in the 2nd century CE mathematical and astronomical treatise, the Almagest, which was written by Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy).

This treatise and the cosmological model it contained would be considered as canon by many medieval European and Islamic scholars and would remain the authoritative source on astronomy for over a thousand years.

During the Middle Ages (ca. 5th - 15th century CE), Indian, Persian, and Arabic scholars maintained and expanded on classical astronomical traditions. At the same time, they added to them by proposing some revolutionary ideas - like the rotation of the Earth.

Some scholars went even further and proposed heliocentric models of the Universe - such as Indian astronomer Aryabhata (476–550 CE), Persian astronomers Albumasar (787 – 886 CE), and Al-Sijzi (945 - 1020 CE).

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It is possible that their works were inspired by the earlier works of Aristarchus of Samos (310 -230 BCE), Seleucus of Seleucia (190 BCE - 150 BCE), and certain Pythagorean philosophers from the 4th and 5th centuries BCE.

What Do We Really Know About the Universe?
"Figure of the heavenly bodies". Source: Bartolomeu Velho/BNF

By the 16th century, Nicolaus Copernicus published a complete model of a heliocentric Universe. He proposed this model initially in a 40-page manuscript titled Commentariolus (“Little Commentary”), which was released in 1514.

His theory resolved the lingering issues that plagued previous heliocentric models and was based on seven general principles. These postulated that:

  1. There is no single center of all the celestial orbs or spheres.
  2. The center of the Earth is the center, not of the universe, but only of gravity and of the lunar sphere.
  3. All the spheres encircle the Sun, which is as it were in the middle of them all, so that the center of the universe is near the Sun.
  4. The ratio of the Earth's distance from the Sun to the height of the firmament is so much smaller than the ratio of the Earth's radius to its distance from the Sun that the distance between the Earth and the Sun is imperceptible in comparison with the loftiness of the firmament.
  5. Whatever motion appears in the firmament is due, not to it, but to the Earth. Accordingly, the Earth together with the circumjacent elements performs a complete rotation on its fixed poles in a daily motion, while the firmament and highest heaven abide unchanged.
  6. What appear to us as motions of the Sun are due, not to its motion, but to the motion of the Earth and our sphere, with which we revolve about the Sun as [we would with] any other planet. The Earth has, then, more than one motion.
  7. What appears in the planets as [the alternation of] retrograde and direct motion is due, not to their motion, but to the Earth's. The motion of the Earth alone, therefore, suffices [to explain] so many apparent irregularities in the heaven.

Copernicus would expand on these ideas in his magnum opus – De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres) - which he finished in 1532. However, fearing persecution, Copernicus would not allow it to be published until shortly before his death (in 1534).

In this work, Copernicus would reiterate his seven major arguments and provide detailed calculations to back them up. His ideas would go on to inspire Italian astronomer, mathematician, and inventor Galileo Galilei (1564 - 1642).

Galileo would use a telescope of his own creation, his understanding of physics and mathematics, and the rigorous application of the scientific method to refine Copernicus' observations and calculations.

Galileo's observations of the Moon, the Sun, and Jupiter would prove to be very influential, and helped reveal flaws in the geocentric model. His observations of the Moon, for example, revealed a pockmarked and cratered surface, while his observations of the Sun revealed sunspots.

What Do We Really Know About the Universe?
Comparison of the geocentric and heliocentric models. Source: history.ucsb.edu

He was also responsible for the discovery of Jupiter's largest moons - Io, Europa, Ganymede, and Callisto - which would later be named the "Galilean Moons" in his honor.

These discoveries contradicted the long-held notions that the heavens were perfect spheres (consistent with Christian theology) and that no planets other than Earth had satellites.

His observations of the planets revealed that their appearances and positions in the sky were consistent with the theory that they orbited the Sun.

He shared these observations in treatese like the Sidereus Nuncius (The Starry Messenger) and the On the Spots Observed in the Sun, both of which were published in 1610.

But it was his 1632 treatise, Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems), where he advocated for the heliocentric model of the Universe.

Johannes Kepler (1571-1630) refined the model further with his Laws of Planetary Motion, which demonstrated that the orbits of the planets were elliptical, rather than perfect circles (as Galileo and previous astronomers had maintained).

This effectively settled the "Great Debate" about the nature of the Solar System and made heliocentrism the scientific consensus from the late 17th century onward.

From the Solar System to the Milky Way

Another revolutionary discovery that emerged during the 17th and 18th centuries was the realization that our Solar System was not unique. Thanks to the invention of the telescope, our understanding of the Milky Way changed drastically.

Rather than being a giant cloud in the form of a band (as was previously thought), astronomers began to understand that the nebulous structure they had been observing in the night sky for millennia was actually billions of distant stars.

What Do We Really Know About the Universe?
Map of the Milky Way. Source: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

Granted, the idea was not entirely new. In the 13th century, Persian astronomer and polymath Nasir al-Din al-Tusi (1201 - 1274) suggested this very possibility in his book, Tadhkira:

“The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color.”

However, it was not until the Scientific Revolution (ca. 16th - 18th century) that astronomers were able to directly observe this. In The Starry Messenger, Galileo described the observation he conducted of the "nebulous stars" that were contained in the Almagest's star catalog.

These observations led him to conclude that the "nebulous" sections of the Milky Way's band were actually "congeries of innumerable stars grouped together in clusters". This discovery further bolstered the case for heliocentrism, since it showed that the Universe was much larger than previously thought.

In 1755, German philosopher Immanuel Kant theorized that the Milky Way was a massive cluster of stars that were held together by the force of their mutual gravity. He further predicted that these stars (along with the Solar System) were part of a flattened disk that rotated around a common center - much like the planets around the Sun.

In 1785, astronomer William Herschel attempted to create the first map of the Milky Way. His estimates of its size and shape were thrown off by the fact that much of our galaxy is obscured by dust and gas, but his attempt was an indication of the progress that was being made.

By the 19th century, improved optics and telescopes allowed astronomers to map out more of the night sky, which led many to conclude that our Solar System was merely one of billions in the Milky Way.

By the 20th century, they would come to see that the Milky Way was merely one of billions in the Universe. But one thing at a time...

Newton and Einstein revolutionize everything

Humanity's understanding of the Universe would be revolutionized again in the late 17th century by the work of British polymath Sir Isaac Newton (1642/43 – 1727). Using Kepler's theory of motion, he developed a theory of gravity (aka. Universal Gravitation).

What Do We Really Know About the Universe?
William Blake's Newton (1795). Source: William Blake Archive

This was summarized in his major work, Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”), which was published in 1687 and contained Newton's Three Laws of Motion. These laws stated that:

  1. When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.
  2. The vector sum of the external forces (F) on an object is equal to the mass (m) of that object multiplied by the acceleration vector (a) of the object. In mathematical form, this is expressed as, F=ma
  3. When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

These laws described how objects exert forces on each other, and how motion occurs as a result. From his work, Newton was able to calculate the mass of the planets, determine that Earth is not a perfect sphere, and how Earth's interaction with the Sun and Moon influences ocean tides.

These and other detailed calculations would have a profound influence on the sciences, and would form the basis of Classical Physics (aka. Newtonian Physics), which would remain the accepted canon for the next 200 years.

This would change in the early 20th century, when a young theoretical physicist named Albert Einstein began publishing a series of papers discussing his theories of Special and General Relativity.

These theories were in part the result of attempting to resolve the inconsistencies between Newtonian physics and the recently-discovered laws of electromagnetism  - best summarized by Maxwell’s equations and the Lorentz force law).

Einstein would address this inconsistent in one of the papers he wrote in 1905 while working at a patent office in Bern, Switzerland. Titled, “On the Electrodynamics of Moving Bodies", this paper became the basis of Special Relativity (SR).

Einstein's theory challenged the previously-held working consensus that light moving through a medium would be dragged along by that medium. This meant that the speed of light (which had already been determined) was the sum of its speed through a medium plus the speed of that medium. 

This led to all kinds of theoretical complications, and experiments attempting to resolve them all obtained null results. Instead, Einstein stated that the speed of light is the same in all inertial reference frames, a theory that did away with the need for mediums or extraneous explanations.

As a theory, SR not only simplified the mathematical calculations and resolved issues between electromagnetism and physics, it also closely agreed with the speed of light and explained aberrations that had emerged in experiments.

Between 1907 and 1911, Einstein began applying his theory of SR to gravitational fields, another area where Newtonian Physics had difficulty. By 1911, these efforts culminated with the publication of “On the Influence of Gravitation on the Propagation of Light“.

This paper laid the groundwork for General Relativity (GR). In it, Einstein predicted that time is relative to the observer and is dependent on their position within a gravity field, and that gravitational mass is identical to inertial mass (aka. the Equivalence Principle).

Another thing Einstein predicted in this paper was the idea that two observers situated at varying distances from a gravitating mass would perceive the flow of time differently (aka. gravitational time dilation). These theories remain an established part of modern physics. 

The Universe is dark

Einstein's theories, which garnered widespread-acceptance, had many consequences for the sciences. In particular, his field equations for Relativity also predicted the existence of Black Holes and a Universe that was either in a state of constant expansion or contraction.

In 1915, a few months after GR became widely publicized, German physicist and astronomer Karl Schwarzschild found a solution to Einstein's field equations that gave rise to the theory of black holes decades before one was observed.

Also known as a Schwarzschild radius, this solution described how the mass of a sphere could become so compressed that the escape velocity from the surface would be the same as the speed of light. The "radius" in this case refers to the size below which the gravitational attraction between the particles of a body must cause it to undergo irreversible gravitational collapse.

In 1931, Indian-American astrophysicist Subrahmanyan Chandrasekhar expanded on this by using SR to calculate how massive a body would need to become before it collapsed in on itself - later referred to as the Chandrasekhar limit.

By 1939, the discovery of neutron stars backed up Chandrasekhar's theories by showing that white dwarf with a mass below this limit do in fact collapse. The resulting object (a neutron star) is super-dense as a result and has an incredibly powerful magnetic field.

From this, physicists like Robert Oppenheimer argued that a white dwarf of sufficient mass would continue to collapse and form a black hole. While this was another mass limit entirely (known as the Tolman–Oppenheimer–Volkoff limit), it was consistent with Chandrasekhar's theory.

By the 1960s and 1970s, astrophysicists conducted many tests of GR using black holes and large-scale structures (like galaxies and galaxy clusters). This would come to be known as the "Golden Age of General Relativity" (1960 - 1975) since it allowed Einstein's theory to be tested like never before.

However, astrophysicists noticed something particularly chilling about these tests as well. When looking at galaxies and larger concentrations of matter in the Universe, they found that the observed gravitational effects of these objects were not consistent with their apparent mass.

This led the scientific community to conclude that within galaxies, there was a whole lot of mass that they could not see. This gave rise to the theory of Dark Matter, a mysterious mass that does not interact with "normal matter" (aka. visible or baryonic matter) via the electromagnetic force.

This means it does not absorb, reflect or emit light, making it extremely hard to spot. It only interacts with matter through its gravitational force. Dark matter is believed tooutweigh visible matter roughly six to one, making up about 27% of the universe. It is also thought to have had a profound influence on its evolution.

The Universe is expanding

Another consequence of GR was the prediction that the Universe was either in a constant state of expansion or contraction. By 1927 - 1929, Belgian physicist (and Roman Catholic priest) Georges Lemaître and American astronomer Edwin Hubble confirmed that it was the former.

At the time, Einstein was still looking for a way to rationalize the idea of a static Universe. To this end, he proposed the "Cosmological Constant", which was a yet-undetected force that "held back gravity" to ensure the distribution of matter in the cosmos was uniform over time.

Using redshift measurements of other galaxies, Hubble proved Einstein wrong. These measurements showed that light coming from these galaxies had shortened wavelengths - i.e. was shifted to the red end of the spectrum - which indicated that the intervening space was expanding.

Hubble's observations also showed that the galaxies that were farthest from our own were receding faster. This phenomenon would come to be known as Hubble’s Law, and the rate at which this was happening would come to be known as the Hubble Constant.

In 1931, Georges Lemaitre would use the phenomena he co-discovered to articulate an idea that the Universe had a beginning. Upon confirming independently that the Universe was expanding, he suggested that it was progressively smaller the farther back in time one looked.

At some point in the past, he reasoned, the entire mass of the Universe would have been concentrated on a single point. These discoveries triggered a debate between physicists, who were divided into two schools of thought.

The majority still advocated that the Universe was in a steady-state (i.e. the Steady State Theory), where matter is continuously created as the Universe expands, thus ensuring uniformity over time.

On the other side, there were those who believed that the Universe was gradually expanding, and the density of matter was slowly decreasing as a result. This idea came to be known as the "Big Bang Theory", a moniker which was facetiously assigned by proponents of the Steady State Theory.

After several decades, multiple lines of evidence emerged that favored the Big Bang interpretation. This included the discovery and confirmation of the Cosmic Microwave Background (CMB) in 1965, which had been predicted by the Big Bang Theory.

CMB is basically "relic radiation" left over from the Big Bang that has been expanding at the speed of light ever since. By gauging the distance of the CMB, which is about 13.8 billion years in all directions, scientists were able to place constraints on the age of the Universe.

By the 1990s, improvements in ground-based telescopes and the introduction of space telescopes led to new and startling discoveries. Scientists had believed that gravity would eventually cause the expansion of the universe to slow. However, astronomers now observed that for the past four billion years, cosmic expansion has actually been accelerating.

This gave rise to the theory of Dark Energy, a mysterious force that somehow works against gravity and pushes the cosmos further apart. Theorists came up with different explanations for Dark Matter. Some suggested that Einstein's "cosmological constant" may have been correct all along. Others suggested that Einstein's theory of gravity was incorrect and a new theory was needed which include some kind of field that creates this cosmic acceleration. 

One leading cosmological theory today is described by the Lambda Cold Dark Matter (λCDM). It is currently the simplest model that accounts for most of the observed properties of the Universe. It states that most of the universe is made up of dark energy, dark matter, and ordinary matter and is also referred to as the standard model of Big Bang cosmology. It assumes that general relativity is the correct theory of gravity on cosmological scales and accounts for many of the properties of the cosmos, including the cosmic microwave background and the acceleration of the expansion of the universe. 

What Do We Really Know About the Universe?
The Lambda CDM model of the Universe. Source: Alex Mittelmann/Coldcreation

So what don't we know?

The answer to that question is, quite a lot really! To answer it effectively, though, we need to take a look at how scientists study the Universe from top to bottom and take note of where the gaps lie.

For starters, scientists understand how matter, time and space behave on the largest of scales. This is best summarized by GR, which accurately describes how mass and gravity are related and affect spacetime.

However, since the 1960s, astrophysicists have come to accept that there is a whole lot of mass out there that they cannot see. While this makes sense theoretically, attempts to find Dark Matter so far have yielded nothing conclusive.

So while you could say that we know how much matter is out there, we cannot conclusively account for most of it. Similarly, we have known that the Universe is in a state of expansion since the late 1920s. However, we don't know why exactly.

The rate at which the Universe is expanding can be explained by the presence of a Dark Energy. But just like Dark Matter, investigations have yet to determine what this truly is.

And there's the extent of the Universe itself. With the discovery of the CMB, astronomers and cosmologists were able to trace the evolution of the cosmos and were able to make close estimates of how old it is. The current estimate is that the cosmos is 13.799 ± 0.021 billion years old.

But as for how big it is? That remains a mystery. Based on the rate of cosmic expansion, astrophysicists estimate that the "observable" Universe is a sphere measuring about 93 billion light-years across. However, beyond that, the Universe likely extends much farther and could even be infinite.

At the other end of things, scientists have determined that there are four fundamental forces (aka. fundamental interactions) that govern all matter and energy interactions in the Universe.

These forces consist of the gravitational force (which is attributed to the curvature of spacetime and is described by GR) and the three discrete fields of quantum mechanics - collectively known as Quantum Field Theory (QFT).

These fields include the weak nuclear force, the strong nuclear force, and electromagnetism - that deal with subatomic particles and their interactions, as described by the Standard Model of particle physics.

Another way to look at it is to group these interactions into a three-category system: gravitation, electroweak forces, and strong forces. These latter two categories are subdivided into the weak nuclear and electromagnetic forces, and into fundamental and residual nuclear forces.

Whereas gravitation binds planets, stars, galaxies and galaxy clusters together (i.e. the macro-level), electroweak forces bind atoms and molecules, while strong forces bind hadrons and atomic nuclei.

Here lies the problem. Scientists understand how gravity works on the largest of scales, but not the smallest. This makes it distinct from all other known forces in the Universe which have a corresponding subatomic molecule.

For electricity and magnetism, there are electrons and photons. For weak and strong nuclear forces, there are bosons, gluons, and mesons. At present, though, there's no such thing as a "graviton", at least not outside the hypothetical.

And so far, all attempts to find a conclusive theory of quantum gravity - aka. a Theory of Everything (ToE) - have failed. Several theories have been proposed to resolve this - the top contenders being String Theory and Loop Quantum Gravity - but none have been decisively proven yet.

How will it all end?

Okay, here's the thing... we don't know that either. Granted, the notion that the Universe had a beginning naturally gives rise to the idea that it will have a possible end. If the Universe did begin as a tiny point in spacetime that suddenly started to expand, does that mean it will continue to expand forever?

Or, as has also been theorized, will it cease expanding and start contracting, eventually reducing back into a tiny, spherical mass? This question is one that has raged ever since cosmologists began to debate how the Universe began - Big Bang or Steady State?

Prior to observations that showed how the Universe has been expanding at an accelerated rate, most cosmologists were of two minds on the subject. These were known as the "Big Crunch" and "Big Freeze" scenarios.

In the former, the Universe will expand until it runs out of energy and then begin to collapse in on itself. Assuming the Universe reaches a point where its mass density is greater than its critical density, the Universe will begin to contract.

Alternately, if the density of the Universe is equal to or below the critical density, the Universe will keep expanding until star formation ceases. Eventually, all the stars will reach the end of their lifespans and become dead husks or black holes.

Eventually, the black holes would collide and form larger and larger black holes. This would ultimately lead to "heat death" in the Universe, where the last electromagnetic radiation would be consumed. The black holes themselves would eventually disappear after they shed the last of their Hawking Radiation.

Since the 1990s, observations that led to the theory of Dark Energy stimulated new discussions on the fate of the Universe. It is now theorized that as space continues to expand, more and more of the observable Universe will pass beyond the CMB and become invisible to observers.

Meanwhile, the CMB will continue to redshift until it becomes visible only in the radio wavelength. Eventually, it will disappear entirely and astronomers will see nothing but blackness beyond the edge of what's visible.

Another possibility is the "Big Rip" scenario, where continued expansion will eventually lead all galaxies, stars, planets, and even atoms themselves to be torn apart, leading to the death of all matter.

Big Crunch, Big Freeze, or Big Rip? At this juncture, we just don't know. The same is true when it comes to theories of how the Universe began - was it a Big Bang or more of a Big Bounce?

This is also the case when it comes to our attempts to unify gravity with the other fundamental forces. Right now, the best we have are theories that have a certain logical consistency but remain unproven. 

— 

As Socrates famously said: "One thing only I know, and that is that I know nothing." This knowledge, it is said, is what made Socrates the wisest man in all the land. In the same respect, humanity's grasp of the Universe is strangely paradoxical.

We know it's expanding, we're just not sure how. We know how much mass is out there, we just can't see most of it. We know how gravity works, just not how it fits with the other forces. We don't know how it began or will end, but we have some theories that fit with the observable evidence.

So while there is much that we don't know about the Universe, we at least have a pretty good idea of what we don't know. This puts us at an advantage over previous generations of humanity who were not only ignorant of the Universe at large but ignorant or their ignorance.

We are also at a point in our technological evolution where we can see more of the Universe than ever before, whether that's on the largest or smallest of scales. Between next-generation instruments, supercomputers, and particle accelerators, scientists are pushing the boundaries of what we can see.

The only way to overcome ignorance is to know where our ignorance lies and then address it. In that respect, humanity is poised to learn a great deal in the near future!

Further Reading:

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