The Milky Way Shouldn’t Exist, Here’s Why
If you were to take a drop of water in space and spin it on an axis, a perfectly round sphere of liquid would start to turn into an oblong ellipse. Spin it even faster, and soon, you'd have a flattened bar-shaped disk.
Spin it even faster beyond that, and the angular momentum and centrifugal forces acting on that droplet of water would tear the droplet apart and send microdroplets and even individual molecules of water flying off in every direction.
The exact same thing should happen to the Milky Way, Andromeda, and other galaxies in the universe, but that is not what we see.
Instead, we see pinwheel-shaped galaxies, globs of dwarf galaxies spinning around each other forming into clusters, and the stars along the outer rims of galaxies whipping around galactic centers at breakneck speeds in defiance of what physics might predict.
When astronomers first took all of these measurements in the second half of the 20th century, many assumed their data must have been wrong, incomplete, or that their instruments were malfunctioning.
But time after time, observation came back to the same data and the same conclusion: The mass of observable galaxies is woefully insufficient for gravity to hold everything together.
The universe should be a random, violent splatter of stars given the physical forces involved—and yet, there they are, spinning neatly into pinwheels and clumping together like condensation on the tile after a hot shower.
The only explanation anyone has managed to come up with is essentially a mathematical cheat: Dark Matter. But what is dark matter? What do scientists mean by "dark"? Has anyone ever seen it? And what does this all mean for the standard model of physics?
Dark Bodies: The Observable Universe before Dark Matter
The idea of matter we cannot see isn't particularly new. The ancient Greek natural philosophers all took a stab at the nature of the material world and whether what we saw was really all there was.
The Greek philosopher and scientist Philolaus speculated about an anti-earth — Antichthon — that revolved directly opposite of the "central fire" of the Sun from Earth, remaining ever invisible to us Earthlings.
Aristotle screwed things up for about two millennia with his geocentric model of the universe that became adopted by Catholic orthodoxy during the Middle Ages and the early Renaissance.
But that model was ultimately repudiated by Galileo's observation of other stars in the observable Milky Way and the orbit of satellites around Jupiter that were only observable with the newly invented telescope.
This is an important marker for the universe about making assumptions about matter that you cannot see and that new technology can radically alter our perspective of the universe.
Next up is the English rector and Oxford astronomer John Michell, who predicted "dark stars" in 1783 from the laws of Universal Gravitation set down by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica.
Michell, one of history's great unappreciated minds, understood that if Ole Rømer's calculation of the speed of light in 1676 was correct — especially that light didn't propagate instantaneously but actually traveled at a quantifiable speed — and Newton's "corpuscles" theory of light — that light was made of discrete, tiny particles with a physical mass — was true, then this led to an incredible conclusion.
There could be stars massive enough that their gravity would be so strong that the light they emitted could not escape and thus could not be seen. They would be incredibly massive, effectively invisible objects in the night sky.
Essentially, black holes by another name — and another physics, unfortunately, one that became outdated after general relativity. Michell's reasoning was still sound though and was essentially the same conclusion that Karl Schwarzschild reached when providing the first exact solution to Albert Einstein's gravitation field equations in 1915.
The then-invisible mass of the planet Neptune was detected in 1846 by observing the effect it had on the orbit of the planet Uranus.
The idea of "dark nebulae" came into fashion in the latter part of the 19th century, and with the advent of photography, astronomers imaging the stars saw that rather than a uniform field of evenly distributed stars, there were clumps of stars interspersed with vast expanses of dark, empty space.
Arthur Ranyard, an English astrophysicist, believed that the dark spaces in the night sky were the result of dark masses blocking out the light of the stars behind them from our perspective. He wrote in 1894:
The dark vacant areas or channels running north and south, in the neighborhood of [θ Ophiuchi] at the center .... seem to me to be undoubtedly dark structures, or absorbing masses in space, which cut out the light from the nebulous or stellar region behind them.
Lord Kelvin, the famed British scientist and mathematician, undertook the first major estimate of the mass of the universe as it was understood in his time. Observing the velocity dispersions of stars orbiting the galactic core, Kelvin deduced what he thought was the mass of the universe.
He was really measuring the mass of the Milky Way galaxy — the concept of a modern galaxy and that the Milky Way was just one of the billions of galaxies wouldn't be discovered until 1924 — he still argued that there was an incredible amount of matter in the universe that we could not see in the form of dead stars that cast no light or stars so distant that they are too dim to see:
It is nevertheless probable that there may be as many as 109 stars [inside a sphere with a radius 3.09 x 1016 kilometers] but many of them may be extinct and dark, and nine-tenths of them though not all dark may be not bright enough to be seen by us at their actual distances. [...] Many of our stars, perhaps a great majority of them, may be dark bodies.
So, even though Newton's classical mechanics has since been supplanted by the General Relativity of Einstein and Michell's dark star has been replaced by the more accurate black hole as we understand it today, the idea of matter we cannot actually clearly see isn't new.
In fact, there are nearly as many ideas about the nature of dark matter as there are astronomers writing about it through the centuries. But even though the modern concept of dark matter owes a lot to these previous investigations, how we came to it is another, well, matter.
How are Galaxies Even a Thing?
When Swiss astrophysicist Fritz Zwicky coined the term dunkle materie, or dark matter, in 1933, he certainly wasn't the only one noticing that there was a lot more gravity in the observable universe than there was observable matter.
Studying the Coma Cluster of galaxies, Zwicky used the virial theorem — a general mathematic equation for the kinetic energy of a system of particles with its total potential energy — and came up with evidence for a large amount of missing matter from his observations.
Zwicky calculated that there was upwards of 400 times more mass in the Coma Cluster of galaxies than was directly observable because the observable gravity in the cluster was far too small to account for the observed speed of the galactic orbits.
While Zwicky's figures were way off, owing to an incorrectly calculated Hubble constant in 1933, he was still correct that there was more dark matter in the Coma Cluster than there was observable matter.
While other observations of the Andromeda galaxy would add to the early evidence of unseen matter in the 1930s, it would take another 40 to 50 years for major developments in astronomy to really kickstart the investigation into the missing matter of the universe.
That work came from Dr. Vera Rubin, Kent Ford, and Ken Freeman in the early 1970s, particularly around the investigation of the rotational curves of so-called "edge-on" spiral galaxies, those galaxies where our perspective is close to 90 degrees off from its axis of rotation.
As you move from the gravitational center of a system, like our own solar system or the Milky Way galaxy, objects near the center of gravity orbit faster than objects further out.
Mercury orbits the Sun in just 87.97 days; Venus in 224.7 days; Earth in 365.25 days; Mars in 686.98 days; Jupiter in 4,332.59 days; Saturn in 10,759.22 days; Uranus in 30,688.5 days; and Neptune in 60,195 days. If you plot these figures in a graph you get a curve known as the Keplerian decline.
When Rubin, Ford, and Freeman mapped the rotational curve of spiral galaxies though, they didn't see this decline — in many cases, in fact, some stars on the outer edge of the galaxy were accelerating.
"Great astronomers told us it didn’t mean anything,” said Rubin. Rubin and her colleagues were told to keep making observations and the problem would clear itself up. Instead, they kept finding this same phenomenon.
The only way to make these observations conform to Einstein's relativity or even just the classical mechanics of Newton, was if there were several times as much mass that couldn't be seen as the mass that could.
“Nobody ever told us all matter radiated [light]”, Rubin said. “We [astronomers] just assumed it did.”
Important support for Rubin's work came from Princeton theorists Jeremiah Ostriker and James Peebles in 1973, who used supercomputer simulations to chart the evolution of spiral galaxies.
Instead of seeing the neat, water-down-a-drain development of spiral galaxies over billions of years, they found that the amount of observable mass wasn't strong enough to keep spiral galaxies like the Milky Way together.
In the end, they would either warp under the force of their own angular velocity or they would simply tear themselves apart and scatter their stars into intergalactic space.
How did these galaxies hold together then? Ostriker and Peebles started adding an additional parameter to the simulations — a halo of mass surrounding the galaxies.
They would add an arbitrary amount of mass to this halo and run the simulations again, increasing or decreasing the amount of mass in the halo until the galaxies stabilized into the galaxies we observe today.
They similarly found that for spiral galaxies to form as they clearly have in the universe, they would need to be surrounded or enveloped by several times as much mass as we can see.
It isn't just spiral galaxies that show evidence for dark matter. Astronomers observing the gravitational lensing produced by galaxy clusters find that the effects can only be explained in relativity if there is a significantly larger amount of matter present that they cannot see.
There is other indirect evidence for the existence of dark matter from microwave background radiation as well as other sources, all of which buttresses the theory of dark matter, but that still leaves a very essential, million-dollar question.
What is Dark Matter then?
Whoever answers that question will likely be offered distinguished positions at elite institutions around the world and win a wheelbarrow full of prizes, medals, and cash awards. This question is one of the great outstanding mysteries of our time, and there aren't a lot of clear leads to go on.
The one thing that does appear to be certain though is that whatever it is, it does not interact with the electromagnetic field, and so it does not radiate light on any known wavelength.
And that is about all anyone can say definitively, though there are a lot of theories filling in the yawning gap in our understanding.
Among the possible candidates are primordial black holes that are left over from the period right after the Big Bang. In the second after the Big Bang, matter was incredibly dense, but not evenly distributed. This discrepancy could have given rise to black holes of various sizes without the "modern" process of stellar collapse we typically associate with black hole formation.
According to the science site Astronomy, "Depending on when exactly they formed, primordial black holes could have masses as low as 10-7 ounces (10-5 grams), or 100,000 times less than a paperclip, up to about 100,000 times greater than the Sun."
Steven Hawking, meanwhile, calculated that black holes evaporate over time due to quantum fluctuations that give rise to what has become known as Hawking radiation.
For a stellar-mass black hole, this process would take far longer than the life of the universe to play out, but for primordial black holes, many could have already winked out of existence long ago.
Many might still remain, however. Hawking calculated that primordial black holes that formed around 13.8 billion years ago could still be around today if they weighed more than 1012 lbs (1,000,000,000,000 lbs - 453 million kg).
While this might sound incredibly massive — and it is — keep in mind that the planet Earth weighs about 1.31668 × 1025 lbs. A primordial black hole would only need to have a tiny fraction of the Earth's mass in order to still exist today.
And if you were to shrink the volume of the Earth to the point where its density would form a black hole, that black hole would be about the size of a pea. The observable universe could be filled with grain-of-sand-sized (or smaller) primordial black holes that we would have no way of directly observing.
These black holes might not have the most direct and pronounced gravitational effect on their surroundings, so they could be almost impossible to detect, but could the cumulative effect of their gravity add up to the dark matter Rubin and others hypothesize?
This could easily account for the observational evidence of dark matter, if true. Consider that if you were to press the mass of our sun into a density high enough to form a black hole, its event horizon would be just 1.86 mi (3 km) shy of the singularity at its center.
Put another way, the distance from the singularity in the center to its event horizon would be the distance it would take for you to drive through a small town of a few thousand people.
It would still have the entire mass of the sun in that sphere though, so that small town would be exerting enough gravity to capture every object in the solar system: every asteroid, every planet, even the distant objects of the Kuiper belt and beyond.
Observations made by astronomers suggest that this can't account for all of the dark matter that must be out there, however, and if such black holes do exist, they account for a small fraction of the total mass of dark matter in the universe.
Instead, many astronomers believe that throughout the universe there is a suffusion of an unknown subatomic particle that we simply cannot detect yet that accounts for all or most of the additional gravity that astronomers are observing.
In order for such particles to be the elusive dark matter, though, millions and possibly billions of these particles would need to pass through every square centimeter of the planet — and everything on it — every second.
Despite this, no one has been able to detect such a particle, and not for lack of trying. The hunt for dark matter is one of the most pressing and competitive areas of research in astrophysics and cosmology — positively identifying dark matter has "Nobel Prize" written all over it.
Still, dark matter remains elusive, and we can't even say for sure that it exists.
There is another possible solution to the problem that dark matter is trying to explain, it's just one that is the equivalent of scientific heresy: Maybe Einstein's general relativity is wrong, or at least woefully incomplete.
It wouldn't be the first time a lauded theory was supplanted by an upstart that better explained the universe as we observed it. After all, it was Einstein's General Relativity that dethroned Newton's classical physics, which had reigned over our thinking about the universe for just over two centuries; right up until the moment it didn't.
What's more, Relativity explains certain things in the universe very well, but even in Einstein's own time, it was being contradicted by discoveries in a field that Einstein, in part, helped found: quantum mechanics.
Below the atomic level, relativity simply holds no sway and can explain nothing about the nature or behavior of subatomic particles.
Quantum entanglement thumbs its quantum nose at Einstein's quaint idea that nothing can travel faster than light, and particle superposition defies the fundamental commandment of physics that matter must occupy a single point in space at a given moment in time.
So if the universe, at the scale of galaxies and clusters of galaxies, ends up operating in defiance of Einstein's relativity, it has to be pointed out that it isn't the responsibility of the universe to conform to Einstein's theory; we must develop a new theory that better conforms to the universe, as painful as that might be.
The sciences of cosmology, astronomy, and physics aren't static things, in the end. They, too, evolve with time, and dark matter — or an alternative explanation for the discrepancies from physics that we see in the night sky — is clearly a major step along that evolutionary path.
"In a spiral galaxy," Rubin said in a 2000 interview, "the ratio of dark-to-light matter is about a factor of 10. That’s probably a good number for the ratio of our ignorance to knowledge."
"We’re out of kindergarten," she added, "but only in about third grade."