SCIENCE21 - Primer on the Universe and its Mysteries

I have long wanted to better understand the universe, so following my primary objective in choosing blog topics - to learn something - this blog will be about the universe - what we know about it, and what we don’t know.

 

I will start out by defining the universe, and then talk about how it began, and how it operates, followed by noting the key remaining mysteries (major unknowns) about the universe.  Then I will talk about each of these mysteries.

I will list my principal sources at the end.

I will be talking a lot about theories, and computer models that try to simulate or represent the complex actions and interactions of elements of the universe - many of which cannot be observed.  We can learn a lot by observing various phenomena, but models are useful to test theories against observations, fill in the gaps, and predict and extrapolate results.  But models are only as good as our understanding of the physics involved, and as we shall see, there are a lot of major unknowns here.

 

What is the Universe?

Contrary to what many of us learned in school, the universe is a lot more than a collection of stars, planets, and moons.

The universe includes all of space, and all the physical matter and energy that space contains.  It even includes time itself and, of course, it includes us.

Matter refers to anything that occupies space and has mass.  It's the "stuff" that makes up the physical universe.  Matter can exist in different states, including solid, liquid, gas, etc. 

In the physical matter category, our Earth and the Moon are part of the universe, as are the other planets and their many dozens of moons.  Along with asteroids and comets, the planets orbit the Sun.  The Sun is one among hundreds of billions of stars in our Milky Way galaxy (large clusters of stars that are bound together by gravity), and most of those stars have their own planets. The universe contains at least 100 billion galaxies.  If galaxies were all the same size as the Milky Way, that would give us 10 thousand billion billion (or 10 sextillion) stars in the universe.

In the energy category, all forms of electromagnetic radiation - including radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays - travel through space and carry energy. 

So where did these basic elements of the universe come from, and when did they first appear?

Today’s universally-accepted Big Bang theory describes how the universe was born, rapidly expanding from a tiny point, an initial state of extremely high density and temperature, called a singularity point. 

Since the early 20^th century, scientists studying the origin and development of the universe (cosmologists) have established that the universe has been expanding continuously since the Big Bang. 

You may be having a tough time, as I do, of accepting this singularity-point birth of the universe.  This is an example of where a computer model, even if universally-accepted, is not smart enough to do any better; we just don’t understand the physics involved, and therefore have not included it in our modeling.

Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.8 billion years ago, which is considered the age of the universe.

As the universe expanded from the Big Bang, it cooled sufficiently to allow the formation of subatomic particles, and later atoms.  These primordial elements - mostly hydrogen, with some helium and lithium - then coalesced, forming early stars and galaxies.  Most of the other elements that we know from the periodic table, were formed later in stars, from violent explosions during their final evolutionary stages.  Spreading small particles from the stellar explosions collided and stuck together, eventually forming planets around young stars.

Our solar system is about 4.6 billion years old, life on Earth has existed for maybe 3.8 billion years, and our human species, Homo sapiens, appeared on Earth relatively recently, around 300,000 years ago.  In other words, the universe has existed roughly 46,000 times longer than our species has. 

 

Mysteries of the Universe

From the early satellite probes of the 1950s and 1960s, to the great telescopes of the 1990s and 21^st century, scientists have been exploring the evolution of the universe from the Big Bang to the present – making additional observations and improving theories and models.  However, there is still a great deal we don't know about the universe.

NASA’s James Webb Space Telescope produced this deep and sharp image of the distant universe in 2022.

One mystery is that observations from the 1990s show that the expansion of the universe, long believed to be at a constant rate, is accelerating over time, and scientists aren’t sure why.

In today’s widely-accepted cosmological model, the universe is made up of three primary components.  Visible ordinary matter (stars, galaxies, planets, gas clouds, etc.) comprises only about 5% of the universe.  From studying the effects of gravity on both matter and light, the universe seems to contain a bunch of matter and energy that we can’t see or directly observe.  Physicists have inferred that about 27% of the universe is dark matter, a mysterious substance that does not interact with light, making it invisible to telescopes.  However (the model says), it does exert gravitational influence, affecting the rotation of galaxies and the distribution of matter in the universe.  The remaining 68% of the universe is dark energy, an even more enigmatic component of the universe, postulated to be responsible for its accelerated expansion.  The universe as we understand it wouldn’t work (i.e., the models wouldn’t work) if dark matter and dark energy didn’t exist, and they’re labeled “dark” because we can’t directly observe them.  Neither of these “dark elements” are even remotely understood today. 

Pie chart of the primary components of the universe.

 

Another element of the universe that we don’t know much about are black holes, regions of the universe with gravitational forces so strong, that nothing, not even light, can escape.  Supermassive black holes, with masses millions or billions of times that of our Sun, are thought to reside at the hearts of all large galaxies.  Black holes may play a significant role in the structure and evolution of the universe, acting as recycling centers for cosmological debris, shaping galaxy formation, and possibly influencing the distribution of matter. 

Other unknowns about the universe include what (if anything) came before the Big Bang, the size and shape of the universe, the ultimate fate of the universe, whether there may be multiple universes, and whether there is intelligent alien life in the universe.

The following paragraphs will explore each of these mysteries.

What is Dark Matter?

In the early 1930s, two astronomers independently postulated the existence of a strange, unseen form of matter.  The Dutch astronomer Jan Oort and Swiss astronomer Fritz Zwicky both studied the motions of stars in our disk-shaped Milky Way galaxy - as the stars orbited around the center of the galaxy.  They inferred that some unseen form of matter must exist to help the stars to orbit the galaxy’s center as quickly as they do.  Zwicky named it dunkle Materie, or dark matter.

Fast-forward 40 years to the 1970s, when American astronomer Vera Rubin and her research group at the Carnegie Institution were busily studying the rotation of galaxies and discovered that some galaxies were rotating so fast that the gravitational influence of their visible matter would not be sufficient to hold them together.  They concluded that gravity of dark matter is the glue that prevents the galaxies from flying apart.  Theorists at the time proposed it must exist in the form of unseen particles. 

Fast-forward another 40 years to just the last decade or so, and satellites that map the cosmic microwave background (CMB) determined that this dark matter, whatever it is, must make up some 26% (since increased to 27%) of the mass-energy content of the universe.

CMB is leftover radiation from the Big Bang.  As the theory goes, when the universe was born, it underwent rapid inflation, expansion, and cooling. The CMB represents the heat leftover from the Big Bang.  You can't see the CMB with your naked eye, but it is everywhere in the universe.  Fluctuations in the CMB are used to measure the density and composition of the universe, including the relative amounts of normal matter, dark matter, and dark energy (see below).  Ongoing satellite missions have provided increasingly precise measurements of the CMB, allowing scientists to determine the relative proportions of ordinary matter, dark matter, and dark energy. 

Despite intense interest, dark matter remains mostly a mystery.

 

What is Dark Energy?

In the early 20^th century, American astronomer Edwin Hubble discovered that the universe is expanding, much to the shock of the physics community at the time, which had believed the universe was static.

In 1998, with scientists believing that the expansion of the universe was at a constant rate, two independent teams, monitoring the distance to exploding stars, measured the universe’s expansion rate and found that it was expanding faster than expected, indicating that the expansion is accelerating over time.

They concluded that a hypothetical unknown force - termed dark energy has a gravitationally repulsive effect (which is to say, the opposite of gravitational pull), and believe that it’s responsible for the accelerating expansion of the universe and the large-scale structure of the cosmos.

The current understanding that dark energy constitutes roughly 68% of the contents of the universe is based on observations and data collected primarily from the late 1990s onward, with ongoing refinement and confirmation.  This figure emerged from a combination of data, including observations of exploding stars, large-scale structures, and the CMB, all indicating a universe dominated by dark energy. 

 

What are Black Holes?

Black holes are some of the strangest and most fascinating objects in the universe.  They're extremely dense, with gravitational forces so strong, that nothing, not even light, can escape from them.  That means we can never hope to receive a signal from within a black hole.  They are, and will forever be, shrouded in mystery.

Scientists think that black holes play a significant role in the structure and evolution of the universe, acting as recycling centers for cosmological debris, shaping galaxy formation, and possibly influencing the distribution of matter.  Black holes engulf matter, including gas, dust, and even stars, which is then expelled as hot, energetic plasma.  This expelled matter can then be used to form new stars and planets.  Black holes are crucial for understanding fundamental physics and the behavior of extreme environments. 

The concept of black holes goes all the way back to the English philosopher and clergyman John Michell, who wrote about “dark stars” in a paper in 1783.  But confirming the existence of black holes was a long time coming.

Today, black holes are broadly classified into three main categories: stellar-mass black holes, supermassive black holes, and intermediate-mass black holes.  A fourth type, primordial black holes, is theorized but not yet definitively confirmed. 

In the 1970s, astronomers detected an extremely strong X-ray source, seemingly a black hole candidate.  Finally, by 1990, it was shown to be a stellar-mass black hole.  Stellar-mass black holes are formed from the gravitational collapse of massive stars at the end of their lives. They typically have masses ranging from a few to tens of times the mass of the Sun.  Stellar black holes are the most common type of black hole.  Millions of them must exist in our Milky Way galaxy, although we know of only a couple of dozen because they remain so hard to detect.

Soon after the discovery of stellar-mass black holes, astronomers using the Hubble Space Telescope began finding evidence for another type of black hole - supermassive black holes - in the center of many galaxies.  Within the last generation, it’s become clear that massive galaxies (including our Milky Way) have central supermassive black holes. (Smaller galaxies, however, do not.)  Supermassive black holes have masses ranging from millions to billions of times the Sun's mass.  The size of a supermassive black hole is related to the size and mass of the galaxy in which it resides.  Scientists think that supermassive black holes grow to monstrous size by merging with progressively massive black holes.  Their growth is also thought to be aided by the rapid consumption, or accretion, of gas and dust from their host galaxies.

Supermassive black holes at the center of galaxies are believed to be crucial for the formation and evolution of these galaxies.  They influence the shape and stability of galaxies. 

The first image of a black hole, a supermassive black hole, was captured in 2019 by the Event Horizon Telescope, a global network of ground-based radio telescopes that, when linked together, create a virtual telescope as large as the Earth.  The striking photo of the black hole at the center of the M87 galaxy, 55 million light-years from Earth, thrilled scientists around the world.

A light-year is the distance that light travels in one year at about 186,000 miles per second.

The first image of a (supermassive) black hole was obtained in 2019.

 

The image is not of the black hole itself, but rather its shadow, which is the region where light is trapped by the black hole's gravity. The bright ring around the shadow is caused by light being bent and emitted from the hot gas swirling around the black hole.  This image provided the first direct visual evidence of a black hole. 

Intermediate-mass black holes are a theoretical class of black holes, thought to have masses between 100 and 10,000 times the mass of the Sun, bridge the gap between stellar and supermassive black holes.  They are crucial for understanding how stellar-mass black holes evolve into supermassive black holes; they might represent a stage in that process.  They are theorized to be found in dense stellar environments like globular clusters and possibly in the centers of smaller galaxies.  While numerous candidates have been identified, confirming the existence of intermediate black holes remains challenging. 

Primordial black holes are hypothetical black holes that are thought to have formed in the very early universe, soon after the Big Bang, and could be much smaller than stellar-mass black holes.  Primordial black holes may still exist and be undetected in the universe. 

What Came Before the Big Bang?

The question of what existed before the Big Bang is one of the most profound and complex in science.  While the Big Bang theory is widely accepted as the leading explanation for the origin of the universe, it doesn't provide a description of what came before it. 

Current scientific understanding suggests that the Big Bang marks the beginning of space and time as we know them, meaning there may not have been anything "before" in the traditional sense. 

The exact physics of the Big Bang, specifically the events occurring at the very beginning of the universe, remains unknown because our current theories of physics, including Einstein’s general relativity, are thought to break down under the extreme conditions of infinite density and temperature that likely existed at the singularity.  Extrapolating back to that point using these theories leads to a singularity, which is not a scientifically useful concept. 

General relativity, which describes gravity at a large scale is not designed to handle such extreme conditions; it is incomplete or invalid at the singularity.  To understand the Big Bang, we would need a theory of quantum gravity, creating a unified theory that can describe gravity in all regimes, including the quantum realm at the small scale of atoms and subatomic particles. This combined unified theory currently does not exist.

 

How Big is the Universe?

We do not know how big the universe is because we can only see part of it. 

From Earth's perspective, the observable universe is a sphere, with Earth at its center, and is defined by the distance that electromagnetic radiation, including light, moving at 186,000 miles per second, has had time to travel from distant objects to Earth since the Big Bang, 13.8 billion years ago.  That computes to a spherical region with a diameter of approximately 93 billion light-years. 

The entire universe, however, may be much larger, possibly even infinite, and includes regions beyond our current reach. 

 

What is the Shape of the Universe?   How will our Universe End?   Is the Universe Actually a Multiverse?  

So far in this blog, I think I’ve succeeded in summarizing (at least to my satisfaction) some of the key mysteries of the universe - helped by my science and engineering background.  I’ve also taken a shot at identifying the crucial unknowns.  But these next three mysteries are way beyond my ability to understand, let alone summarize in a blog.  I personally have reached the point where the unknowns completely overwhelm the questions.

Today, scientist believe that the shape of the universe is determined by the density of matter within the universe, and how much that matter curves space according to Albert Einstein's theory of general relativity.  From recent space missions, scientists are getting a good handle on the density of space, but their resultant models of the shape of the universe are obtuse to me, and a are a long way from being certain.

Will our universe end with the very fabric of space ripping apart because of the continued increased acceleration of its expansion?  Or will matter someday become the dominant component of the universe, causing the universe to "snap back" on itself like a stretched rubber band?

A popular theory today is the Big Freeze, which would see the universe expanding into an increasingly colder, darker, lonelier cosmos.  As stars like the Sun age and die, their remnants will go cold and dark. This would eventually occur for all stars, and as billions and trillions of years pass, any remaining subatomic particles will be increasingly invisible to observation.  The universe may have started with a bang, but this scenario is that it will end with a whimper.

Some scientists speculate that our universe could be just one of an infinite number of universes making up a “multiverse.”  If the universe is compelled to repeat itself - to be part of a multiverse - could there be another version (or multiple versions) of you, each acting independently of the others?  This counts as one of the mysteries we’re unlikely to ever get an answer for, but it’s intriguing nonetheless.

Scientists have many theories, and some wild speculations, about these questions.  Too many theories, too little observational data, and too many unknowns.  Cosmologists clearly have a lot of work to do!

 

Is there Intelligent Alien Life in the Universe?

I’m going to finish with this fascinating question.

Since the early 1960s, scientists have theorized that it’s extremely likely that intelligent life exists outside of Earth.

The universe contains billions of galaxies, each with billions of stars, many of which likely have planets.  The sheer scale of the universe makes the possibility of intelligent life elsewhere extremely high, even if it's different from life as we know it. 

Today, scientists are actively searching for signs of life on other planets using telescopes like the James Webb Space Telescope. 

The recently-launched James Web Space Telescope is actively involved in searching for intelligent alien life in the universe.

 

They are looking for specific molecules in the atmospheres of distant planets that are indicative of biological activity.  We’re just beginning the search; much more observation and research is needed to find and confirm other intelligent life in the universe. 

And even though there’s a high statistical likelihood of intelligent life-forms having evolved elsewhere in the universe, there is a very low probability that we’ll be able to communicate or interact with them. The reasons are the vast distances, and travel or signal transmission times involved - based on our current understanding of the universe and its physical laws.

 

Closing

Let me close with a couple of quotes that seem to appropriately capture the state of our knowledge about the universe.

“The more you know, the more you realize you know nothing.”
- Socrates

“Imagination is more important than knowledge.  For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution.”
- Albert Einstein

Sources

My primary sources include: “Universe,” Wikipedia; “What is the Universe?” science.nasa.gov; “The Most Baffling Mysteries About the Universe," rd.com; “8 of the greatest mysteries in the universe” and “Black holes: Everything you need to know,” space.com; “10 modern mysteries of the universe” and “What shape is the Universe?” astronomy.com; plus, numerous other online sources.