A lot of people have noted that astronomy is a humbling enterprise to pursue. After all, every time we make a new discovery, we find ourselves further removed from importance. The Earth is but one planet among many, orbiting a sun that is one star among hundreds of billions, out in the suburbs of a galaxy that is one among hundreds of billions more. It's easy to feel pretty small when you see all that magnificence out there. And every astronomers keep making it worse ! Because now we know that what we can see isn't even everything there is. Normal matter, the stuff that makes up you and me and all we observe in the universe ? That's only a small fraction of what's actually out there. It's time we talk about some very very dark matters .
What are dark matters?
In the 1960s and 1970s , astronomer Vera Rubin was observing spiral galaxies. She was interested in how they rotate, because you can learn a lot about a galaxy that way. Think about the solar system: back in the 1600s, Johannes Kepler figured out that the farther a planet is from the sun, the slower it orbits. Isaac Newton put numbers to that, Calculating the strength of the sun's gravity, which means we could, in turn, get the sun's mass. Same with galaxies. If you can measure how they rotate - how rapidly gas clouds move in their orbits near the edge of the galaxy for example- you can calculate the mass of the entire galaxy. Galaxies are so big that you can't physically see the nebulae move, but you can measure their Doppler shift, which gives you their velocity. What Rubin expected to see was that the farther out from the centre of the galaxy the gas cloud was, the slower it would be moving, Just like more distant planets from the sun move more slowly in their orbits. What she got through was the opposite. For many galaxies, the farther out from the centre you went the faster the clouds were moving ! Even at beast, the velocities flattened out with distance, when they should have declined. That meant the gravity of the galaxies was constant throughout the disk, not dropping from the centre as you did expect. But that's bizzare! Images of the galaxies showed that the numbers of stars and other massive objects clearly got lower the farther from the centre you went. There simply isn't enough mass far out from the centre to account for the rapid rotation rates. Or not enough mass from things we can see. The only explanation is that there must be dark material contributing to the gravity. Something besides stars, gas, and dust. Not only that, the galaxy must be embedded in a hato of this material to get the shapes of the rotation graphs right. And there must be a lot of it ! Rubin found there must be five or times as much of this invisible material than the visible matter in galaxies.
Back in the 1930s, astronomer Fritz Zwicky had drawn a similar conclusion measuring the speeds of galaxies in galaxy clusters. The member galaxies were moving too quickly to stay in the cluster : at the measured speeds they should have been flung of. Therefore, he concluded, there must be far more gravity in the clusters than just from the visible material. It turns out Zwicky's observations had way too much uncertainty in them to make any solid claims. He hugely overstimated the amount of invisible material. Rubin's observations, were far, far better and more accurate. However, the term Zwicky used to dub this mysterious material stuck, and we still use it : dark matter .
Over the years, more observations have only confirmed Rubin's measurements. We see similar behaviour in elliptical galaxies, For example. Ironically, better measurements made of galaxy cluster member velocities show they do in fact move too quickly, and clusters must have dark matter in them too. Zwicky was right for the wrong season - and in the end, Rubin is credited for making the discovery. Ofcourse, the idea that so much of the material in the universe must be dark was met with scepticism by astronomers. Everything gives of some kind of light. But more observations just kept supporting the existence of dark matter. So, what is dark matter ? That was the big question. Astronomers were methodically. They listed every single thing they could think of that dark matter could possibly be: Cold gas, dust, dead stars, rouge planets, everything. Even wired subatomic particles that were predicted to exist in quantum mechanics theories, but never seen before. Then they thought of ways they could detect these objects. Cold gas would emit radio waves, For example. But everything they tried came up empty. One by one they crossed objects of the list, and eventually everything made of normal matter atoms and molecules, protons, electrons, and neutrons - was eliminated. All that was left on the list was that truely bizzare stuff : those screwy subatomic particles no one had ever seen before. One such particles is called an axion . They have never been detected, but their properties match what we see of dark matter : axions have mass, so if you have a huge cloud of them they will have enough gravity to affect galaxies. They don't tend to emit much light, so even a huge cloud of them would be dark.
As i mentioned in our other page about Blackholes( including link) one of Albert Einstein (link) big ideas was that space wasn't just emptiness between stars. In a sense it was an actual thing, with all of matter and energy embedded in it.
We humans can get a little arrogant, thinking we occupy a special place in the universe. In a sense, we do, because most of the universe is cold, empty space, and we live in a relatively warm and desens part of it. But the stuff that makes us up, the protons, electrons, and neutrons, of normal matter- that's in a serious minority when it comes to all the matter there is.
Where we called normal matter only one kind of matter - There is also dark matter. Which we can't directly see, and which intracts with normal matter only through gravity. It affects how galaxies rotate, how galaxies move in clusters, and how large structures form in the universe.
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