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Where Is It? What Is It? Why Can’t We See It? Dark Matter & Dark Energy Explained (Well, Sort Of).

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And just like that, another Friday has appeared from nowhere. This week folks, I’m going to do a bit of a follow up on last week’s article. If the measurement problem is the biggest mystery in quantum physics, then Dark Matter and Dark Energy are the biggest mysteries in cosmology. Together they make up about 95% of the Universe, yet we still do not know exactly what either of them is. Ordinary matter, meaning stars, planets, gas, dust, trees, people and everything you’ve ever touched, accounts for only about 5% of the cosmos. Dark Matter contributes roughly 27%, while Dark Energy contributes roughly 68%.

So, are you ready? Hang on to your hats, it’s time for us to dive into another rabbit hole of truly cosmic proportions!

And we’ll start with a universe that really doesn’t add up, and as is obligatory, an analogy. Imagine you’re sat staring out of a window watching leaves swirling around in a strong wind. You can’t see the wind itself, but you know it’s there because of what it does to the leaves, and Dark Matter and Dark Energy are similar. We don’t directly see them, however, we infer their existence because of their effects on things we can observe. But that’s where the similarities end, as where Dark Matter pulls, Dark Energy pushes. One helps gravity hold the Universe together, while the other seems to be trying it’s very best to tear it apart.

The story begins in the 1930s, when Dark Matter was discovered. Swiss astronomer Fritz Zwicky was studying the Coma Cluster, a truly huge collection of galaxies, within the constellation Coma Berenices. He calculated how much visible matter the cluster contained and compared it to how fast the galaxies were moving. The numbers didn’t make sense. The galaxies were moving so fast that the cluster should have flown apart long ago as there wasn’t enough visible mass to provide the necessary gravitational glue. He proposed there must be a large amount of invisible matter. He called it dunkle Materie or “Dark Matter.”

At the time, and for many years after, most astronomers ignored the idea, until Vera Rubin discovered something even stranger in the 1970s. Galaxies had a rotation problem.

According to Newtonian gravity, stars near the centre of a galaxy should orbit quickly, and stars far from the centre should orbit more slowly, and this is exactly what happens in our Solar System. Mercury, which is very close to the sun, races around it, while Neptune, which is nearly three billion miles from the sun, crawls along in its distant orbit. But galaxies simply appeared to be refusing to obey these rules.

Rubin found that stars far from galactic centres were moving almost as fast as stars much closer in. It was as if galaxies were embedded in enormous invisible halos containing far more mass than all their stars combined, and this discovery became one of the strongest pieces of evidence for Dark Matter.

Which leads us to the question of how do we actually know Dark Matter exists?

And to answer this question, scientists actually have several independent lines of evidence. The first of which is that galaxy rotation actually curves (let me know if I’m overusing the word actually, as somebody actually pointed out to me, I actually do. Apparently). Stars move too quickly to be held by visible matter alone. Secondly, galaxies gather in clusters, which remain gravitationally bound despite insufficient signs of visible mass. Thirdly we have gravitational lensing, something else I’ve touched upon in previous articles. Einstein’s General Relativity says mass bends spacetime, that is, when light passes a massive object, it bends. Astronomers observe lensing effects that require far more mass than visible matter can supply, and it is this that allows scientists to map Dark Matter distribution directly. And lastly, we have the Cosmic Web, which shows us that on the largest scales, galaxies form immense filaments and clusters. Additionally, computer simulations can only reproduce the Universe we actually observe when large amounts of Dark Matter are included.

So, what is Dark Matter? And just like last week with the Measurement Problem, the honest answer is…

…Nobody knows.

There are some things we do know though, and they are that Dark Matter has mass, produces gravity, does not emit light, does not reflect light, and barely interacts with ordinary matter.

So, in a sense, Dark Matter is almost ghost-like as it can pass through ordinary matter seemingly without notice.

Right now, there are several ideas competing with each other as to why, and for decades the favourite candidate was WIMPs.

“WIMPs” are Weakly Interacting Massive Particles, and huge underground detectors have been searching for them, and so far, as tends to happen with the science of the cosmos, nothing definitive has turned up.

We also have Axions, which are extremely teeny-weeny hypothetical particles, and although they are hypothetical, axions remain a popular possibility with scientists. As do sterile neutrinos, which are a heavier cousin of familiar neutrinos that barely interact with anything. Or it could be something completely different and far stranger than our current theories. All we know for sure is that we don’t know what it is, and at present, Dark Matter remains unidentified.

There are some other ideas out there, some of which are pretty wild. Could gravity and everything we know and understand about it be wrong? What If Dark Matter doesn’t exist at all? Maybe gravity behaves differently on enormous cosmic scales?

The best-known attempt at describing this is another scientific theory known as MOND, which stands for Modified Newtonian Dynamics. (another article for later, methinks). And although MOND can explain some weird galactic behaviour, it is Dark Matter that generally provides a better explanation across many observations, including galaxy clusters and cosmological measurements. And this is why most cosmologists therefore regard Dark Matter as the leading explanation.

OK, if Dark Matter is pretty weird, Dark Energy is downright bizarre, and its discovery completely shocked the scientific community, as for most of the twentieth century, astronomers assumed gravity would gradually slow the Universe’s expansion. Which when you think about gravity seems obvious. Gravity attracts. Everything should slowly pull everything else back together. Right?

Well, unfortunately not. Back in the late 1990s, astronomers measured distant exploding stars called Type Ia supernovae and the result was astonishing, and it showed us that the Universe wasn’t slowing down. It was accelerating.  Some mysterious phenomenon was overcoming gravity on the largest scales, and that phenomenon became known as Dark Energy. Cool, yeah!

That leads us to the next obvious question, what exactly does Dark Energy do? And, you’ve guessed it, it’s analogy time!

Imagine throwing a ball upward and now imagine the ball speeds up instead of slowing down, and that is essentially what the Universe is doing, as galaxies, on average, are becoming separated faster and faster over time, and Dark Energy appears responsible for that acceleration.

So, that sort of answers what it does, but what actually is Dark Energy? And you’ve guessed it…

… Nobody knows.

Although several possibilities do indeed exist.

It could be vacuum energy, where empty space may not be empty, as quantum physics says that even a perfect vacuum is seething with fluctuating energy. And Einstein’s equations permit vacuum energy to act as a repulsive force.

This explanation is currently the leading candidate, but, as always when the teeny tiny stuff becomes involved, there is a problem. Theoretical calculations predict vastly more vacuum energy than astronomers observe. This mismatch is one of the largest known discrepancies in physics.

Then we have Einstein’s cosmological constant. He added a term, Λ (Lambda), to General Relativity, and many cosmologists today model Dark Energy as this cosmological constant as it behaves like a fixed energy density which is present throughout space.

Perhaps Dark Energy is a dynamic field that evolves with time, and unlike a cosmological constant, it could strengthen or weaken as the Universe ages.

At this time, however, there is no evidence that currently favours this idea over simpler models.

That’s not all. As dark as this is, there are even stranger things afoot. Dark Matter becomes diluted as the Universe expands, and Dark Energy apparently doesn’t.  As space grows, more space contains more Dark Energy, meaning that Dark Energy ultimately dominates the Universe’s future evolution.

So, with all that conflict, confusion and unknowns, what does all this actually mean for the future?

Well, we know that current observations suggest that the Universe will continue expanding, as expansion continues accelerating, distant galaxies will drift beyond visibility, and the cosmos will ultimately grow increasingly dark and cold, leading to what is known as the Heat Death or Big Freeze (pretty sure I’ve mentioned this in a previous article), trillions upon trillions of years from now, stars will eventually burn out and galaxies will grow isolated in an immense ever expanding darkness.

So why can’t we see Dark Matter and Dark Energy? The answer is actually quite simple as they don’t interact with light. No absorbing it, no emitting it, no reflecting it, nothing for a telescope to actually catch. We only know they’re there because of what they do to everything around them.

And what they do couldn’t be more different. Dark Matter shapes galaxies and galaxy clusters from the inside, holding cosmic structure together like an invisible scaffold. Dark Energy, on the other hand, barely registers on local scales at all. Our solar system, our galaxy, even our galactic neighbourhood, are all gravitationally bound tightly enough that it can’t get a grip. It only shows its hand once you zoom out to the vast, mostly empty stretches between galaxies, and that is where it dominates completely.

There is a search going on as I write this as modern observatories are attempting to solve these mysteries. One of the most important is the European Space Agency’s Euclid mission, launched in 2023. It is creating a massive three-dimensional map of billions of galaxies to investigate the nature of Dark Matter and Dark Energy and how cosmic structure evolved over time. And alongside Euclid, projects such as the Vera Rubin Observatory, James Webb Space Telescope, and the upcoming Nancy Grace Roman Space Telescope are expected to provide unprecedented data.

There is quite an astonishing bottom line to all this, as the most remarkable fact is not that Dark Matter and Dark Energy are mysterious, it’s that everything familiar is the exception, not the rule. All stars. All planets. All oceans. All people. Every great civilisation. Every work of art. Every living thing that has ever existed! Together all that and more only makes up about 5% of reality. The remaining 95% of the Universe consists of things we can detect only indirectly and do not yet fully understand. So, in other words, humanity has become extraordinarily good at studying the cosmos, only to discover that most of it is still hidden from view. And that, folks, may turn out to be one of the greatest scientific surprises of all time!

The Black Hole Glow: A Journey into Hawking Radiation

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Howdy my loyal readers. I’d like to start my saying thank you for the messages I’ve received asking if there is anything else they should know about black holes, and as they are a common theme through most of my articles, I’m going to indulge you with this one, on how Stephen Hawking discovered that the darkest objects in the universe may slowly evaporate away

By any reasonable definition, black holes should be simple. They pull things in. Nothing gets out. End of story. Yet in 1974, Stephen Hawking discovered something astonishing, and that was, black holes are not completely black. Given enough time, they glow, lose mass, shrink, and may eventually disappear altogether. That prediction is now known as Hawking radiation and remains one of the most profound (you’ve heard my ear worm already) insights in the history of physics.

This was a problem nobody expected, although I’m pretty sure Stephen Hawking had an inkling. You see, Black holes emerge naturally from Einstein’s theory of General Relativity. According to the classical picture, they are regions of space where gravity becomes so powerful that nothing, not even light, can escape once it crosses the event horizon. I know, I know, this is old news to you guys, I’ve stated it several times in previous articles.

For decades, physicists assumed black holes could only grow. Matter falls in. Energy falls in. Nothing comes out. Yeah? Then along came Stephen Hawking, and he combined Einstein’s gravity with quantum mechanics and arrived at an answer nobody could have anticipated. Black holes should emit a faint thermal glow. And this implication was extraordinary. Simply put, it meant that Black Holes could die.

As humoungously massive as it is, the Universe is never truly empty, and to understand Hawking radiation, we need to start with a strange feature of quantum mechanics, and that is what appears to be empty space isn’t actually empty.

According to quantum field theory, the vacuum of space is filled with fluctuating quantum fields. That is, teeny tiny disturbances constantly occur throughout space, causing particle-like excitations to briefly appear and disappear. Something that physicists call vacuum fluctuations.

OK. This bizarre activity actually happens everywhere in the universe, and this includes some of the most extreme environments surrounding a black hole.

And this takes us to the famous story that everyone learns. And that is, the most common explanation of Hawking radiation goes something like this:

Near the event horizon, a particle and antiparticle appear as a pair. Now then, normally they would immediately annihilate each other, but if the pair forms close enough to the event horizon, gravity can separate them before they reunite. Which means, one particle escapes into space, while the other one falls back into the black hole. To a distant observer, the escaping particle looks like radiation emitted by the black hole itself.

And this is because, the particle that falls inward, effectively carries negative energy relative to faraway observers, thus reducing the black hole’s mass, so over immense stretches of time the black hole slowly evaporates.

It is an absolutely, beautiful explanation.

But, and there’s always a but, you know this by know, surely guys, there’s one problem.

This isn’t really how Hawking derived the result.

To look at the deeper truth, among physicists, this particle-pair story is considered a useful mental picture, but is not the actual mathematics (maths again, I know. Regular readers already know my issue with maths). Anyway, Hawking’s original calculation used quantum field theory in curved spacetime. And that is, the essential idea is subtle. Different observers can disagree about what counts as empty space. A vacuum for one observer may appear to contain particles to another observer. But, near a black hole’s event horizon, spacetime is distorted so severely that these differing perspectives become physically important

When Hawking compared the state of quantum fields before and after a black hole forms, he discovered that an observer far away would detect radiation emerging from the black hole. So, in other words, Hawking radiation is less about particles popping out of nowhere, and more about the deep relationship between quantum fields, spacetime, and observation itself.

One of Hawking’s greatest discoveries was that black holes possess temperature, and the temperature of a black hole is inversely proportional to its mass, so the heavier the black hole, the colder it becomes, and this leads to a surprising consequence.

A supermassive black hole containing millions or billions of solar masses is incredibly cold, and a tiny black hole is extraordinarily hot, and this relationship means, small black holes radiate far more efficiently than large ones.

Because Hawking radiation carries energy away, the black hole loses mass, and this results in the slow death of a black hole.

As it shrinks, something curious happens. The black hole gets smaller as its temperature rises, then radiation increases which causes the mass to decrease even faster, and therefore the process accelerates.

If the theory is correct, then the final phase could end up with an enormous burst of energy, however, what exactly happens during those final moments remains unknown.

If you got the time to watch one, don’t expect to watch one evaporate as the evaporation process is incredibly slow. A black hole with roughly the mass of our Sun would survive for approximately 10⁶⁷ years, and this is one hell of a lot of zeros, and an incredibly long time. Trillions upon trillions upon trillions of times longer than the current age of the universe. Told you it was a long time.

And what’s more, supermassive black holes live even longer! Fortunately for the universe, black hole death is not an urgent problem, and nothing to worry about. Not for a verrrrrry longtime, at least!

Before Hawking’s discovery, physicist Jacob Bekenstein, argued that black holes should possess entropy, a measure of information content (remember my last article? You did read it, didn’t you?), and Hawking’s calculations confirmed this. In fact, black holes possess staggering amounts of entropy. Even more surprisingly, that entropy scales with the surface area of the event horizon rather than the black hole’s volume!

This observation helped inspire one of the deepest concepts in modern theoretical physics, and that is the Holographic Principle (you know this, I mentioned it way back in my original Black Hole article). This principle suggests that information contained within a region of space may be fully encoded on its boundary surface. It may sound like science fiction, but it has become one of the central ideas in attempts to understand quantum gravity

There’s a problem though, and that is the Information Paradox, and that is the question that changed theoretical physics forever.

Let us suppose a black hole swallows, my laptop I’m writing this article on, books, planets, stars, people, and entire civilizations and the specific details seem to vanish behind the event horizon.

If the black hole later evaporates and only thermal Hawking radiation remains, where did the information go? And that, is what has become known as the Black Hole Information Paradox.

The paradox appears to pit two fundamental pillars of physics against one another. Again, you know this, I’ve mentioned it before. It pits General Relativity against Quantum Mechanics. And this is because quantum theory normally forbids information destruction. What’s surprising is, Hawking’s original result seemed to imply exactly that.  And that is why many physicists consider this one of the deepest unresolved problems in science.

Modern research increasingly views Hawking radiation as an entanglement phenomenon. The radiation escaping from a black hole appears linked to degrees of freedom hidden behind the event horizon, and these quantum correlations may play a crucial role in preserving information.

This current line of thinking has driven decades of research involving Quantum information theory, string theory, holography (my old, now dead, dad was a big fan of holography), and quantum gravity, as well as many other areas.

Moving on, there is another remarkable discovery that Hawking radiation appears closely related to the Unruh Effect.

The what, I hear you say? Another topic for later? Sorry, yes (picture me doing an emoji grimace in apology as I type this), the Unruh Effect predicts that an accelerating observer can perceive empty space as containing thermal radiation. And that is what one observer sees as a vacuum, another observer sees as heat. Many physicists suspect both effects arise from the same underlying physics linking horizons, acceleration, and quantum fields.

Which brings us to the puzzles of tine scales. And that is, there is another challenge lurking inside Hawking’s theory.

When researchers trace Hawking radiation backward in time, some calculations appear to involve wavelengths smaller than the Planck scale (you know what that is), where known physics may stop working completely, and this issue is called the Trans-Planckian Problem. Which suggests we may need a deeper theory of nature to fully understand what is happening near the event horizon of a black hole.

Ah, who was that at the back, another question do I hear? Have we ever seen Hawking Radiation?

Surprisingly, the answer is…

No. We haven’t.

Despite being one of the most famous predictions in science, Hawking radiation has never been directly observed from a real astronomical black hole, and that is because the effect is simply too faint. Remember how long it would take to observe it? That is how faint it is. Even paint dries at the speed of light in comparison. (It doesn’t, bit that sounded cool, didn’t it!).

However, researchers have built laboratory systems that mimic the behaviour of event horizons using optical fibre, fluid systems, and Bose-Einstein condensates, and these analogue systems have produced Hawking-like radiation and, yes there’s another and, in recent experiments, even evidence of the associated back-reaction effects predicted by theory! Cool huh?

While these are not actual black holes, they do provide valuable support for the underlying physics, which leaves us with the final mystery.

And that is perhaps that the most important thing about Hawking radiation is not the radiation itself. There is a deeper lesson, that black holes sit at the intersection of humanity’s three greatest theories, General Relativity. Quantum Field Theory, and Thermodynamics. Unfortunately, those theories do not fit together perfectly, but Hawking radiation exposes the cracks.

Hawking radiation tells us that spacetime has temperature, that gravity may be linked to information, and that entropy and geometry are somehow connected. Oh, and that our deepest theories are still incomplete.

More than fifty years after Hawking’s discovery, physicists are still trying to understand what the phenomenon is really telling us.

The black hole’s faint glow may ultimately reveal not just how black holes die, but how reality itself is built! I’ve said this before at the end of my articles, and I’m going to say it again. How amazingly awesome is that!!

Black Holes: What the Hell are they Really?

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As is often the case, I’ve had a lot of things on my mind recently, mainly work stuff as there’s been a lot of uncertainty around mine, and my mate Chris who I work with, roles. All sorted now, well, sort of, I’ll tell you about it another time, if you’re lucky.

Anyway, when things start to overwhelm me, I tend to start ruminating on random stuff I know, or have learnt, that has stuck with me. Today, for no particular reason, other than I’ve been getting quite sciency recently, it has been black holes, and as I seem to have my writing mojo back, I thought to myself, why not put these thoughts to paper and bang out an article for my blog. I know I only have a handful of readers (friends and family who humour me by showing an interest), but I find it quite therapeutic. 

So, Black Holes, what the hell are they really? They’re one of those things that you hear about your whole life, mostly in passing, science programmes, random articles, the odd bit on the news, songs by Muse etc., and you sort of think you know what they are as in they’re a big gravity thing and nothing escapes from them, right?

That’s all very true, but the more I started properly looking into them, the more I realised that I didn’t really understand them at all. Or maybe a better way of putting it is, I understood the idea of them, but not what they actually are. And that’s where things start to get a bit weird.

So, what actually is a black hole?

At the most basic level, a black hole is just a place in space where gravity has become so strong that nothing can escape it. Not even light, just as I said above, but that alone is already slightly weird, because light, as we all know, is the fastest thing there is. If light can’t escape, then whatever’s going on there must be pretty extreme.

The reason this happens is actually quite simple in principle. If you take a massive object, like a star, and compress all that mass into a ridiculously small space that it fits within a critical radius, the gravitational pull increases. Keep compressing it, and eventually you reach a point where the escape velocity (the speed you’d need to get away from it) is higher than the speed of light, and at that point, you’ve got yourself a black hole.

Simple enough, in theory.

But it’s not really gravity in the way we think, and this is where it starts to shift a bit as black holes aren’t just strong gravity in the usual sense, they’re more like a distortion in spacetime itself. And although that sounds like one of those throwaway science phrases, it’s actually the key to everything.

As Einstein pointed out in his theory of relativity, space and time aren’t actually separate things, they’re part of the same fabric. Spacetime. And massive objects bend that fabric. That is they bend spacetime. The Earth bends it a bit, the Sun bends it a lot more, and a black hole basically folds it in on itself like someone’s tried to fold up a fitted sheet and given up halfway through (to this day, and despite my wife showing me numerous times, I still can’t fold a fitted sheet).

So, what is the event horizon, this so called point of no return? I hear you say

Well, the event horizon isn’t a physical surface. You won’t bump into it like a wall. It’s just a boundary in space, once you cross it, there is literally no path back out. Not because something is blocking you, but because all possible directions you could move through spacetime point further inward, cones of light tilt inward so that all future paths lead deeper in. It quite literally is the point of no return.

And what’s really strange is what it looks like from the outside. If you were watching someone fall into a black hole, they’d appear to slow down as they got closer to the event horizon, they would become slower and slower, until to the eyes of the observer, they appear to freeze there before slowly fading away, so you never actually see them going all the way in, but from their point of view, they just fall straight through.

That disconnect between what’s actually happening and what you see is where things start to feel a bit off and one of those questions that always seems to come up is, what happens if I were to fall in to one? and the honest answer is:

It depends.

I know that answer is a bit of a cop out, but if it’s a smaller black hole, you’re in a lot of trouble very quickly. The difference in gravitational pull between your feet and your head becomes so extreme that you get stretched out into a thin strand. This is what people in physics circles call spaghettification, which sounds almost funny until you think about it.

For a really massive black hole though, like the one at the centre of our galaxy (and the one in the song by Muse), it’s a bit less dramatic at first. You could actually cross the event horizon without noticing anything particularly unusual in that moment. It’s only later, the deeper in you go, that it becomes unavoidable and you’re on a one way trip.

Either way, you’re not coming back.

At the centre of a black hole is something called the singularity. In theory, this is a point where all the mass is crushed down into an infinitely small space, with infinite density, but here’s the thing, infinite in physics usually means we don’t actually know what’s going on. Physics dudes don’t like anything to be infinite as it mucks up all the other stuff we know about the universe. It’s basically our equations throwing their hands up and saying, this doesn’t make sense anymore.

So, the singularity might not actually exist in that exact form. It’s more likely that something else is happening there, something we just don’t yet have the tools or the physics to describe properly, and that’s where black holes start to become really cool.

For one thing, they’re not actually completely black, which surprised me the most when I first came across it, and black holes can actually lose energy.

There’s this process called Hawking radiation (yes, that cool dude who wrote that brilliant book, if you haven’t read it, you should), where tiny quantum effects near the edge of a black hole allow particles to escape. It’s often explained in terms of virtual particle pairs forming near the event horizon, with one falling in and the other escaping. It’s incredibly weak, but over very, very long periods of time, it means the black hole slowly shrinks and would eventually disappear entirely. Which is a strange idea in itself, something that swallows everything, but can also slowly evaporate.

Still with me? Good, as now we get to the bit we really don’t understand and where it all ties together.

Black holes are basically where two major parts of physics collide:

General relativity (which describes gravity and large-scale things) and Quantum mechanics (which describes teeny tiny things).

Both of these work extremely well on their own, but inside a black hole they don’t agree, which leads to some big problems, one of the biggest being, what happens to information?

If something falls into a black hole, all the information about it should still exist in some form as physics says information can’t just be destroyed, but black holes seem to do exactly that, either the information isn’t actually lost (and we don’t understand how it’s preserved), or our understanding of physics is incomplete. And if we’re being honest, it’s probably the latter, there’s something missing from our understanding of physics.

The part I keep coming back to is, as all this stuffs wanders through my noggin, is why does any of this matter? And it matters because black holes aren’t just weird objects sitting out there doing their own thing. They actually seem to play a huge role in how the universe works.

Most galaxies have a supermassive black hole at their centre which influences how stars form, how galaxies evolve, and possibly even how structure forms on a cosmic scale, but more than that, they’re one of the few places where we can really test the limits of our theories. They’re not just interesting in their own right, they actually do something, we just haven’t figured out exactly what that is yet.

In the end, I don’t think black holes are really about “things that suck stuff in,” even though that’s how they’re often described. They’re more like boundaries. Points where our current understanding of reality runs out and something deeper takes over. And the more you look into them, the more you start to feel like we’re still only scratching the surface.

Which, if nothing else, makes them worth thinking about properly.