Caoimhín's Content

Today, we are going to briefly talk about dark matter and dark energy and Modified Newtonian Dynamics. Unfortunately, these are not things that I can show. Then again, I never really show anything in these written pieces, they’re all tell, but even in the attached video I cannot show dark matter.

Dark matter, if it exists, only interacts with normal matter in the form of gravity, gravity. It cannot be seen, it cannot be touched, you can only feel it’s gravitational pull, if it exists. Modified Newtonian Dynamics or MOND is just a different way of describing physics. Newtonian Dynamics and Newton’s theories of physics are the ones that we normally use and they do generally work. In some cases the physics as we understand it doesn’t explain what we see in space, MOND is a modified version intend to explain what we see.

Last time we talked about planet 9 or planet 10 or planet X, whatever you want to call it. We talked about how this planet was used to explain the behaviour of small objects at the edge of the Solar System orbits of planets like Neptune. When there is an unknown thing in physics, sometimes the most realistic explanation is that we’ve simply missed something. For example, when Uranus’s orbit didn’t behave the way people expected, a good potential explanation was that there was another planet out beyond Uranus whose gravity was influencing Uranus’s orbit. Based on that theory people looked for another planet, they found Neptune and that solved the problem. Of course, there are peculirarities with Neptunes orbit and so the search for another large planet at the edge of the solar system is still ongoing. Of course, we don’t know if we need another large planet, but it is the most likely solution. The alternative is adjusting our understanding of physics, and in the case of these orbits there doesn’t seem to be sufficient cause, it probably is just a planet.

When we look at other galaxies things get a little bit stranger. The Whirlpool Galaxy, is a fantastic example of a spiral galaxy generally. It’s known as a Grand Design spiral galaxy, its arms are very clear and we see the full face of the galaxy from here in Earth. This galaxy is just reminder the last star in the handle of the Plough. Like other spiral galaxies, this galaxy rotates. Here in our solar system we have our rotating Sun in the middle. In the distant past there was a loose disc of dust and rock and ice orbiting the Sun. This material slowly coalesced into the Solar System we have today, and we’re all familiar with the different orbits of the different planets. Mercury, right next to the Sun gets around in just 88 days, while Neptune way out at the edge of the Solar System takes over 100 years to orbit the Sun. Based on our understanding of physics, parricularly the laws that Kepler put forth explaining orbits, their motions and speeds make sense. Looking at a galaxy like the Whirlpool, you might expect that the objects closer to the centre are orbiting faster than the objects out at the edge. You might expect that the centre of the galaxy will spin faster and sort of drag all thw other material and arms around it. However, galaxies seem to spin at a pretty consistent rate, the arms of the galaxy and the core of the galaxy almost act like one large plate, spinning around together. Stars at the edge of the galaxy still have a big circle to travel around, but they do it much faster than we’d expect. This galactic rotation just doesn’t make sense according to basic physics. Based on the rules as we currently understand them, that’s not what should be happening at all.

We know that galaxies have a lot of mass, they  are huge heavy things. One of the ways that we can tell the mass of a galaxy is by looking at how its gravity influences things around it, such as orbiting dwarf galaxies and globular clusters. The gravity of a galaxy will also pull on light, which can provide more immediately noticeable display of mass. Gravitational lensing is when the gravity of an object causes the view of objects behind them to be distorted and even magnified. Gravitational lensing really does work like a lens. When light passes through a lens it gets bent and that bending is what allows a lens to magnify things, it’s bending the light and gravity can bend light as well. The force of gravity can pull on the photons, or deflect the path of photons trying to move through the gravitational field or gravity well of a galaxy. Photons are very light, but they are still affected by gravity. A black hole is a black hole because it is pulling in all of the light. If light wasn’t affected by gravity than black holes wouldn’t be able to pull in the light and stop it from coming back out. Gravitational lensing is the gravity created by the mass of a galaxy distorting light, making things behind the galaxy appear closer, or at least distorting what they look like.

Thanks to gravitational lensing, and other methods of inferring mass, we know that galaxies have a little bit more mass than we’d expect, or at least they appear to have more mass than we’d expect. With galaxies, because of the way that they spin, because of the way that they rotate, and because of how galaxies distort the light of distant objects beyond them, it became clear that there is a problem. There seems to be missing mass. There isn’t enough gas and stars in a galaxy to give it all the mass we see it having. If the universe, or physics, works the way we expect it to, galaxies have way way more mass than they should, or way more mass than we can see. That’s where dark matter comes into play. Dark energy is the hypothetical energy driving the expansion of the universe. We still don’t know exactly how the inflation of the universe continues and accelerates, dark energy is the potential solution to that issue.

All of the bright material in a galaxy isn’t enough to explain its mass, and so dark matter was proposed to exist, filling in for the extra mass. Dark matter would help explain why galaxies rotate the way they rotate. If there was a lot of extra mass out at the edge of a galaxy, mass that we can’t see because it’s only interacts with normal matter through gravity, that would explain all the mass we can’t see. Dark matter doesn’t light up when hot as gas does, it doesn’t react with things, it may not even get hot. Photons seem to be able to pass straight to dark matter, but they do get distorted by its gravitational field. They can get pulled but they don’t actually crash into it. Gravitational lensing and the strange rotation of galaxies could be explained by dark matter, this extra matter that we don’t really get to see but it exists out there, you know, somewhere.

By observing their rotation people have been able to see that they’re rotating differently to what we’d expect. Dark matter helps explain this, but would expect galaxies to form in pretty much the way we already thought they formed, little bits of matter slowly gathering together, eventually building these big swirling clouds. According to dark matter theory,  there is this other material in space that has a gravitational effect, but it is just stuff, it’s just extra stuff that we can’t see. Physics is still expected to work the same.

Modified Newtonian Dynamics instead says that gravity doesn’t behave the way we expect it to. Gravity behaves normally when it’s strong, when you’re close to a star or close to a planet, but as gravity gets weaker its behaviour may change. Gravity most be consistent locally, as our current understanding does work for stars and planets. The idea behind Modified Newtonian Dynamics is that we don’t full understand gravity, or that gravity doesn’t behave the way Newtonian Dynamics would predict, particularly at large distances. The outer edges of the galaxy, because they’re experiencing weaker gravity than the material in the middle, more distant from the central black hole. This weaker gravity acts differently to how we’d expect, and this causes the edges of the galaxy to sort of keep up with the centre of the galaxy rather than falling behind in the way that traditional physics would predict. Modified Newtonian Dynamics does imply that galaxies would form in a different way. Young galaxies could in fact come together very quickly because when gravity is weak it behaves differently. In the early universe gravity was generally weaker, because there was less massed clusters of things, to generate strong gravitational pulls. That could allow the young galaxies to form up much quicker.

The reason I’m bringing all this up and the reason I am trying to talk about something that I can’t actually show, is because of new discoveries by the James Webb Space Telescope. The James Webb Space Telescope is able to see very early galaxies and they are already bright and lit up. When we look into the early universe we see lots of small galaxies that are in fact very well formed, small galaxies, young galaxies that are already the shape that we’d expect from a more mature galaxy, with nice clear spirals. According to dark matter and normal physics, it should take longer for galaxies to coalesce. Dark matter was the prevalent theory, it was generally accepted that dark matter and dark energy were likely to be correct, but the predictions that dark matter made about the early universe haven’t come true. Based on what the James Webb Space Telescope is seeing, the predictions made by Modified Newtonian Dynamics are turning out to be true. MOND, for a while, was a potential alternative to dark matter that most people didn’t think would actually be true. Nonetheless, its prediction of how the early universe would look seems to be correct.

Even though dark matter was seen as more likely for a while, the predictions it made haven’t come true, and even though Modified Newtonian Dynamics, or MOND, was considered to be the less likely theory, it’s predictions do seem to have come true. It looks like Modified Newtonian Dynamics is the way forward, for now at least. It could be discovered that gravity doesn’t behave strangely when it gets weak, as of yet we just don’t know and it can get a very long time to figure out the truth behind physics. Figuring out how to test theories like MOND can take a while, and building the equipment to test it can take a while as well. The general and special theories of relativity predicted the existence of gravitational waves and 100 years later the LIGO interferometer finally measured them and confirmed their existence. We may be waiting just as long before we know for sure whether MOND or dark matter are the actual true things, and it may be some third option that we have yet to even conceive.

I hope that this piece has given some sort of introduction to why dark matter and MOND are posited and why there is a controversy, or excitement, about them at the moment. I hope you enjoyed reading this piece about stuff that we cannot actually see and hopefully you will stick around for more pieces in the future. If you would like to ensure more future content, make sure to subscribe to this website and my YouTube channel. You can also like this piece directly if you enjoyed it. Thank you for reading and hopefully I’ll see you back here next time.