Part 2: The Unknown
When you think of what makes up the universe, you will probably think of atoms. Sure, they are the building blocks to most things we see around us, but as we discovered in a previous post, atoms aren’t the end of the story. It turns out that all the things that we can see, everything we can observe, makes up a measly 4% of the universe! If you haven’t read part one, it’s all about what we do know so you can check that out here.
So what’s the other 96%??
Well, to start with, it’s practically invisible, because it doesn’t interact with light, nor other particles! This huge chunk of our universe has been split into two, unimaginatively named, sections: dark matter, and dark energy! Both fields are currently being explored by theoretical physicists, cosmologists and particle physicists- it requires so much collaboration, and thanks to this there have been more and more discoveries: most of which either confirm that we are missing something, or rule out what it isn’t! But in the future, hopefully not too far away, this will all boil down to an answer that will further our understanding of the world around us- that’s what this post is all about: illuminating the unknown!
This isn’t visible, as it emits no light on the electromagnetic spectrum. This is because it does not interact with any standard matter mentioned above, or if it does, only very weakly. And so it evaded our knowledge. However, it does have a gravitational influence/ interact with gravity (wording depends on if you use relativity or quantum theory): and this is how dark matter was discovered.
How do we know it exists? It was first theorised in the late 1800s by Lord Kelvin, and it took nearly 100 years to confirm it. The late 60s, 70s and 80s were an important time for the Dark Matter, with discoveries from the likes of Vera Rubin (a fierce advocate for women in science- specifically her field of astronomy),and Kent Ford (click on their names for a link to their Wikipedia pages if you’d like to find out more). The discoveries included:
- Galaxy Rotation/Velocity Curves measure how fast a star moves around its galaxy. This is described Kepler’s Laws, and holds true for our solar system, but seems to fail on larger scales. Stars nearer the edge of their galaxy move significantly faster than expected (see the picture for a view of this discrepancy): so fast that the galaxies should fly apart… but they don’t! This means that there must be large stores of mass both within, and surrounding the galaxy, which could be explained by dark matter.
- Gravitational Lensing is caused by the bending of space-time described by general relativity. The more mass there is, the more it bends space-time, and therefore the more it distorts the light that passes it. And if this creates more of a distortion than the visible matter predicts… we know we must be missing something!
- Measuring the velocity, density and therefore the total mass of galaxy clusters has shown us that visible matter is outweighed (haha) by 5:1!
- Remember my “Stay Curious” banner? (if not, scroll to the bottom for a reminder!) It shows the ‘cosmic web’. This shows the distribution of dark matter in our universe in its very early stages. It has been theorised that these ‘strings’ of dark matter are the reasons that galaxies and galaxy clusters formed in their current positions! (More about this in an upcoming post!)
Despite all these observational discoveries, it is often said that we know more about what dark matter isn’t than what it actually is. But that is what most of science actually is: you prove everything else wrong until you end up with the truth!
So, a dark matter particle does not interact with any of the matter found in the standard model, nor any of the force-carrying particles. We know this because it doesn’t emit light or any other radiation (so it can’t be any normal particles, or antiparticles, because these give off tell-tale radiation when interacting. What it does interact with, is gravity, so it kind of second-handly interacts with light, as any light that travels past it is bent (gravitational lensing)!
So what could it be?
One possibility, and one that is probably the most popular candidate, are WIMPs (Weakly Interacting Massive Particles), kind of like a slowly moving neutrino with mass. These could be made up of either exactly that: a new kind of slowly moving massive neutrino; or, a yet to be discovered elementary particle like an axion or flavon. These are elementary particles were first searched for to solve problems with particle symmetry, but recently have been looked into as candidates for dark matter due to their properties. I would be able to tell you more about them, but I can’t find the issue of new scientist there is a great article about them in: I have the issue before and the issue after in my stash but can’t for the life of my find that one! SO annoying.
The other popular candidate are MACHOs (MAssive Complex Halo Objects) (scientists might not always come up with the most creative names, but they do have a sense of humour!!) which consist of standard matter in weird forms, like brown dwarfs and black holes, but this would not completely solve the mystery as they would still interact with some particles, and would produce small clumps of ‘dark matter’ instead of the spread out form we actually see. Perhaps they could add more standard matter to our universe, making it a slightly less scary prospect, but personally, I think the best candidate for dark matter is some kind of WIMP. What do you think?
Have you heard of a different candidate for dark matter? Tell me in the comments!
It gets weirder… Our Universe is expanding. Like, the actual space-time is stretching in 3 dimensions! And it’s expanding at an ever increasing rate! This might be a surprise to you, and if it is, don’t worry, it surprised scientists too! How do we know this?
It’s all down to a measurable property of light known as… frequency! Imagine a police car with its siren on going past you. It sounds different as it approaches to when it speeds away, right? Light acts in the same way: if something is travelling towards you, the frequency is shortened, because as it each time it releases a photon/a wave it catches up a bit. So a short wavelength is associated with an approaching object and a longer wavelength with a receding one. On the electromagnetic spectrum (light) a shorter wavelength is blue-shifted and a longer wavelength is red-shifted. When we look out into the universe, most galaxies are red-shifted! And when I say most, the only exceptions are galaxies that are relatively close, like Andromeda, which is moving towards us and will eventually collide! And what’s even stranger is that this expansion is speeding up, contradicting the earlier existing theory, which was that expansion should be slowing down!
As I said, dark energy makes spacetime itself expand, but it seems to be more abundant in empty space, as galaxies are really tightly bound by gravity.
‘Empty space’ actually has more energy than anything else! So it could well be possible that Dark Energy is an inherent property of spacetime itself!
Alternatively, it could be something called the cosmological constant, which einsten predicted. The only problem? When it was calculated it was 10120x too big…
Another theory is that there is another fundamental force that pervades the universe that we are yet to discover, referred to as “Quintessence”, after the fifth element from the time of Greek philosophy. Perhaps this could be the same or a partner particle of the graviton? Maybe one type of charge or interaction results in gravity and the opposite expansion? Purely speculation on my behalf, if you know more about this please let me know!
The final theory, and my favourite I think, is that the energy comes from ‘virtual particles’ which pop in and out of existence, and it is their interactions that release this energy!
As you can see, we know that we must be missing something, or many of our trusted scientific theories (including those of Einstein, Newton and Kepler, just to mention a few of the big names) are wrong! But as much of today’s astronomical and cosmological work is based off of these, lots of work is being put in to find out!
What do you think?
If you can’t trust an atom… trust in science!
☆it’s like magic, but it’s true whether you believe in it or not!☆
See you next time!
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