Unsolved Space Mysteries

While mankind has been gazing at the night sky for untold millennia, the last few hundred years have brought us a non-stop barrage of scientific advancements related to the cosmos. From the invention of telescopes giving us up-close views of other planets, the first man in space and on the moon, and our own little robots driving around and sampling Martian soil, it’s safe to say that we’ve begun our march into the final frontier.

And when it comes to outer space, it seems that the more we learn, the more we realize we don’t understand. And so, despite incredible innovations in technology and the life’s work of some of humanity’s brightest minds, there are still a whole host of unsolved mysteries and unanswered questions in space, some of which hold the keys to changing our entire view of the universe as we know it.

What is dark matter?

Despite galaxies being one of the most fundamental and recognizable features of the visible universe, there are still many crucial things about them that we’ve yet to figure out, which is why theories of dark matter have become so popular in recent decades.

Let’s start with spiral galaxies, specifically, their rotation speed. Calculating orbital velocity is something that scientists have years upon years of experience with, since this math was first figured out in Kepler’s laws of planetary motion, which he published all the way back in the early 1600s.

What we’re mostly interested in here is Kepler’s Third Law, which describes the relationship between an orbiting object’s distance and speed. The closer a planet is to its host star, the faster it orbits, and the further away it gets, the slower it orbits, and we see this with Mercury taking just 87 days to complete one revolution around the sun, while Saturn takes 10,775 days. But more specifically, Kepler found that this decrease in speed was very precise and perfectly predictable.

Using his equations, you can easily calculate how long a year would be on a planet at any given distance from the sun. For example, if we threw a new planet into the solar system right where the asteroid belt currently lies, which is about 2.5 astronomical units from the sun, Kepler’s third law tells us that one year on this planet would be 1,443 days.

But when you scale up to the size of a galaxy, all of this neat predictability completely falls apart. For some reason, in spiral galaxies, as you move away from the center and toward the arms, orbital speed doesn’t decrease like it does in the solar system, in fact, it either stays the same or even slightly increases as you move further from the center, essentially the opposite of everything we thought we knew before.

Visualizing this on a graph, called a galaxy rotation curve, the velocity that we expect to see forms a smooth, downward slope to the right, but the data from our real-world observations shows this graph never decreasing, and instead levelling off.

This high speed on the outer edges shouldn’t be sustainable, and spiral galaxies’ arms seem like they should just break apart and dissipate into space, but they don’t, so there must be more going on than meets the eye.

It was soon realized that one way to solve these mathematical issues and explain the problems with rotation speeds was to increase the amount of mass in the galaxy, specifically around the edges of the galaxy in its halo. Obviously though, there is no mass there that we can see, and thus, the concept of dark matter was born, an invisible mass that concentrates around galaxies and helps them to keep their shapes.

The most commonly accepted idea is that dark matter is some type of subatomic particle that we’ve yet to discover. This particle would interact with mass and therefore gravity, but not with light, a behavior that we’ve yet to see elsewhere. According to the most modern estimates, dark matter makes up around 27% of all energy and mass in the visible universe, making it five times more abundant than the ordinary matter that makes up stars and planets.

There are several competing theories that have tried to nail down this mysterious particle over the years, leading to cold, hot, and warm dark matter theories, with the main difference between these having to do with the speed at which the particles travel. In hot dark matter, for example, the particles are believed to travel incredibly quickly, nearing the speed of light, while their speed is believed to be much, much slower in theories of cold dark matter.

As the years have gone by, cold dark matter, or CDM, has emerged as the leading and most promising model, and one of the best candidates for CDM are hypothetical particles called axions. Axions are very small and incredibly light, and while they would interact with other axions, they would have hardly any interaction with ordinary matter whatsoever, except through the force of gravity if enough of them were to clump up in one place. And as luck would have it, theories for axion creation predict that if they exist, the universe is absolutely filled with them, with plenty, in fact, to account for our current estimates of dark matter.

Axions hit the mark on mass, interaction, speed, abundance, and so much more, but the only problem is that we’ve yet to detect any, so despite their unique properties first being theorized more than 50 years ago, they’re still little more than ideas on paper, and it’s going to take a revolutionary discovery to change that.

Various particles or difficult-to-detect masses like black holes make up the majority of dark matter theories, but there is another category of explanations that, while admittedly far less popular, still presents some interesting points. In the 1980s, it was proposed that there isn’t any missing matter whatsoever, but that we simply need to expand our understanding of the math related to gravity. This class of theories is called Modified Newtonian Dynamics, or MOND, which essentially suggests that our previous understanding and calculation of gravitational effects need to be updated and fine-tuned.

These updated equations are centered around a new variable in Newtonian equations, representing a tiny, almost imperceptible shift in acceleration. The value of the variable is so incredibly small that its effects are not noticeable on earth or even in the solar system, which is why Kepler and Newton had no problems for hundreds of years.

In fact, the changes in earth’s gravity due to MOND would be comparable to changing your elevation by the width of a human hair. A difference of this scale can’t be reliably measured because we don’t have a stable location on earth to perform such experiments, due to the fact that things like shifting magma in the mantle and the gravitational effects of the moon produce similar differences in gravitational strength on a much larger scale. This means that even if an experiment did produce the results that we’re looking for to verify MOND, they would be absolutely overwhelmed by other factors and become indistinguishable in the data. To put this in perspective, imagine if you had a theory that predicted a single extra drop of water during a rainstorm, there’d be no way to see if your predictions came true.

However, these imperceptible changes begin to dominate over larger distances, which is why when we reach the massive scales of an entire galaxy, the effects of MOND are easily observable.

On the surface, this makes a lot of intuitive sense. After all, our current models of gravity fall apart when we shrink down to the subatomic level, so who’s to say they don’t run into problems on the other end of the scale as well? It also seems a bit more plausible to some because it’s a solution that requires us to examine and rethink facts that we might take for granted, while dark matter can appear to some critics like physicists are just a bit too quick to resort to inventing a new particle to fill in the gaps.

This makes MOND sound really promising, but there’s a reason it isn’t the prevailing theory in astronomy and still faces an uphill battle to be a serious contender. That’s because despite being able to predict a variety of galactic phenomena, there are still several problems that it fails to address that, on the other hand, dark matter does. The most damning of these is seen in the Bullet Cluster, a collision between two galaxy clusters showing lots of interacting galaxies.

Analyzing the motion of the main components of the clusters, as well as its gravitational effects on passing light, scientists discovered that the center of gravity for all this mass is not where it would be if calculated by ordinary matter alone, indicating that there is a significant amount of dark matter present that is skewing the results, an outcome almost perfectly predicted by cold dark matter. This offset center of mass, if accurate, isn’t really explainable with MOND. Neither are findings of ultra-diffuse galaxies that appear to have almost no dark matter at all and show no signs of a rotation curve predicted by modified theories of gravity.

Findings like this often force supporters of MOND to rewrite or adjust their theories, or to just claim that the conflicting findings aren’t accurate.

So, it seems that as of right now, dark matter is our best explanation for several phenomena in the universe. And yet, we still have no concrete evidence of what it actually is.

What is the Great Attractor?

The idea behind the expansion of the universe is that nearly every galaxy we can see is receding away from us, and not just receding, but accelerating. But for thousands of the galaxies nearest to us, they aren’t just heading directly away from us in a straight line, rather, it seems that they are all being gradually pulled toward a shared point in space, a region that the Milky Way appears to be heading toward as well. As far as we can tell, the entire cluster that we reside in, the Laniakea Supercluster, is converging on this point.

This mysterious region of gravitational pull has been named the Great Attractor, and the Milky Way is racing towards it at a speed of over a million miles an hour. But we can’t really tell you what the great attractor is, because, well, we can’t see it. It just so happens that this peculiar discovery is blocked from our view by our very own galaxy, sitting in what’s called the Zone of Avoidance in the night sky. All the stars, gas, and the black hole at the center of the Milky Way make it nearly impossible to see anything behind this zone in the visible spectrum, and scientists can only peer through it with X-ray analysis.

The only thing that could possibly contain enough mass to have this much attraction is a massive cluster of large galaxies, but it’s also possible that the great attractor is nothing special, and simply the gravitational center of every nearby galaxy. However, in 2005, astronomers reported from an X-ray survey that there is likely a supercluster in the expected location, the Norma Cluster, and that is potentially 10 times more massive than previously thought.

However, despite what some sources online might tell you, this doesn’t mean that every galaxy around us is going to be pulled into the Great Attractor and smash themselves into a gargantuan black hole. In fact, it’s difficult to say if the Milky Way will ever even arrive at the Great Attractor in the first place, since it’s not as simple as a destination in space, and it could just end with the merging of a couple superclusters. It also might not have enough of an effect to overcome the expansion of space, so instead of everyone directly converging on the point, their path will just be slightly altered by it.

All of these outcomes are likely trillions of years into the future, and simulations of galaxy movement is already difficult enough without trying to account for tens of thousands of them over such vast distances and time periods. And, of course, all of this is made much harder by the fact that we don’t know the attractor’s exact composition or mass.

One interesting thing to note about the Great Attractor is that while it is obscured from the earth’s view today, this wasn’t always the case. Around 100 million years ago, the solar system was on the opposite side of the Milky Way, and so any dinosaurs would have been able to easily view it with a powerful enough telescope as there was nothing obstructing their view. But we probably aren’t going to need to wait 100 million more years to get some concrete answers, as technology in this sector is rapidly improving, and the Zone of Avoidance is the subject of constant astronomical surveys.

And we’ll leave you with something even more intriguing. The same team that reported the X-ray analysis in 2005 also reported that the Great Attractor may not be the biggest fish in the pond, as their data indicated that it is itself being pulled toward something even larger, which has been named the Shapely Supercluster.

What was Oumuamua?

In 2017, astronomer Robert Weryk announced the discovery of a peculiar object, which he spotted from the Haleakalā Observatory in Hawaii. It was believed to be cigar shaped, tumbling end over end, and moving so quickly that its place of origin could only be determined to be outside of the solar system, making his discovery the first ever identification of an interstellar object. It was given the name Oumuamua, roughly translating from Hawaiian to “first distant messenger”, and immediately captured the attention of astronomers worldwide. With an estimated length between 100 and 1000 meters and a width of between 35 and 160 meters, you can imagine that it’s about the size and shape of a tall skyscraper.

As more analysis of the object was released, its existence became more and more puzzling. Not only did it have a very strange shape and rotation, but as it passed the sun, it began to accelerate. Normally this would be consistent with the behavior of a comet, but Oumuamua showed no signs of a coma, the iconic ice and dust trail that forms behind comets as the sun warms them up.

On top of this, all signs pointed to Oumuamua having a consistency more similar to that of an asteroid, which made the sudden acceleration even stranger. It was suggested by some that perhaps the object was outgassing similarly to a comet, only the volume of gas was too low to be detectable, but this was countered by other researchers who pointed out that because of its shapes and rotation, traditional outgassing had a high chance of tearing Oumuamua apart. Then came the discussions that the variations in luminosity indicated that a cigar was not the only possible shape, as a disk also fit the bill.

So, to sum up, we’ve got an object of debatable shape, pretty unknown material, and interstellar origin exhibiting a strange acceleration as it cruises through our solar system at an odd trajectory. To top it off, the Spitzer Space Telescope then reported that it was about 10 times as bright as a typical comet. It didn’t take long for alien theories to hit the news headlines, with Harvard astronomer Avi Loeb being one of their main supporters. According to Loeb, the object’s behavior is perfectly explainable as an alien probe powered by a solar sail, though this position is only held by a minority of astronomers.

Putting aside ideas of alien spacecraft, several other theories were put forward. The first of these was that Oumuamua was made of nitrogen ice, whose outgassing would be much harder to detect. An object with a lot of frozen nitrogen could have originated as a chunk of a planet similar to Pluto, and could potentially survive for hundreds of millions of years in interstellar space.

Later, it was proposed that instead of nitrogen, Oumuamua was made of frozen hydrogen. An object made of hydrogen ice would likely have originated from the core of a large gas giant, or perhaps from the infant stages of a star’s core before it gained enough mass to ignite. Hydrogen remains solid at a temperature just barely beneath the ambient temperature of space, which would explain why passing close to the sun was just enough of a temperature change to begin turning the hydrogen ice into gas, resulting in the observed acceleration.

However, both the nitrogen and hydrogen iceberg theories have been criticized on many fronts by several different researchers with very valid points, which is why today we still don’t have a consensus. The other big mystery is where Oumuamua could have possibly originated from, because tracing its path backward in time did not lead to a nearby star system. Given its speed and trajectory, it is believed that it was likely floating around in interstellar space for many millions or perhaps billions of years, and may have originated in a completely different part of the galaxy.

But this is one mystery that may be met with some concrete answers within our lifetime. Project Lyra is a proposed mission to launch a spacecraft with the capability to catch up to Oumuamua before it exits the solar system, with various proposals suggesting that the object can be reached within 10-25 years after launch. This would be a very challenging mission and would require historic levels of technological precision given how relatively small the target is, but it does seem possible. The biggest hurdle is getting enough support to get started.

Is humanity unique?

One of the core tenets of astronomy is the Copernican Principle, the idea that humans, our planet, and our sun, are not special. This principle is built upon the ideas from Copernicus, who stood behind the then heretical notion that the earth was not the center of the universe, but rather revolved around the sun. Extending this idea as the years have gone by, we now see that the sun is also not very special, as it is one of hundreds of billions in a galaxy, and even the galaxy is nothing to write home about, being one of trillions in the visible universe.

The concept that what we see around us isn’t unique isn’t just a nihilistic outlook on human insignificance, it’s actually very important in the realm of science. It means that observations about planets, stars, and things like chemistry and physics right next to us can be applied anywhere else in the universe because everything behaves around us the same way it does everywhere else.

This principle is one of the primary reasons why so many astronomers are confident that extraterrestrial life exists somewhere out there, because if the conditions and elements necessary to create life truly aren’t all that special, then intelligent life should inevitably arise elsewhere in the galaxy and the universe. This type of thinking was really hammered home by the likes of Frank Drake and Carl Sagan, who, among many others, argued that the earth is an average planet, orbiting an average star, in an average part of an average galaxy.

However, this seems to be where the Copernican Principle runs into a bit of a dead-end, because we aren’t seeing life anywhere else in the universe, we aren’t seeing their technology floating around or even receiving their messages, we appear to be alone, the essence of the Fermi Paradox.

This has led many to suggest that perhaps intelligent life, earth, and our sun, are not as commonplace as we thought, and may actually be highly unique, a sentiment known as the Rare Earth Hypothesis. For starters let’s take a look at our sun. Its place in the galaxy is anything but average, because there are only certain distances from the center of the Milky Way that are suitable for life.

Too close to the center can be met with too high of a metallicity, too many supernovas, or activity near the black hole, and too far from the center might result in not enough complex elements for life to form. On top of this, our sun is classified as a yellow dwarf, a type of star that only makes up about 6% of the population of the Milky Way, while 73% of stars in our galaxy are red dwarfs. So, because our sky isn’t red and we are in a safe distance from the center of the galaxy, we are already in the small minority of planetary systems in the Milky Way.

Then comes the problem that most planets aren’t in the habitable zone of their host star, and that many of the planets that do live at this safe distance aren’t even rocky, but are instead gaseous.

The list goes on and on. It’s been suggested that having a Jupiter sized gas giant nearby is crucial for the formation of life because it has protected our planet from numerous asteroids and comets over the last couple billion years that could have otherwise wiped us out. The earth also appears to be the only place in the solar system with plate tectonics, which are responsible for continental drift, and thus the high levels of biodiversity around the world, one of life’s best natural defenses from extinction.

Then there’s the topic of our moon, which is incredibly large compared to the earth. The moon is believed to have been created when earth was struck by a mars-sized planet called Theia in the distant past, but not only did this create the moon, this impact is also believed to have sped up the earth’s rotation. A fast rotation means that days and nights are both relatively short, which makes photosynthesis viable because you don’t have long periods without sunlight. The moon is also responsible for lunar tides, and therefore tidal pools, which are believed by many scientists to have been crucial for the development of the first life on earth.

Then, on the flip side, there is the idea of the Great Filter, which states that the reason we don’t see life elsewhere in the universe is because there is something that prevents most life from either forming or reaching intelligence. The great filter can be something as simple as the need to have a large moon that we just described, or perhaps the volatility of a planet’s star regularly bombarding it with dangerous solar flares and preventing life from ever getting started.

Adding up all of these different conditions paints the picture that our little corner of the universe is not very typical at all, and there’s a growing possibility that our star, our planet, and our species are very, very unique, a belief that would be in complete opposition to the foundations of the Copernican Principle.

As for the question of HOW unique, well, that’s still up in the air. We might be rare in the sense that circumstances similar to ours have only arisen a few times throughout our galaxy, or they could be so rare that they don’t even happen in every galaxy. These are all questions that are very difficult to answer, and the mystery of just how unique we are is one that may never be solved.

Key Takeaways

• Dark matter, an unseen mass, is hypothesized to explain galactic rotation speeds.
• The Great Attractor, a mysterious gravitational pull, is obscured by the Milky Way.
• Oumuamua’s origin and composition remain debated, with theories ranging from icebergs to alien probes.
• The Copernican Principle suggests Earth and humanity are not unique, but the Rare Earth Hypothesis challenges this.
• The Great Filter theory proposes barriers preventing life from forming or reaching intelligence elsewhere.

Sources

• Original Side Projects video: Unsolved Space Mysteries

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