For many decades after the discovery of the Big Bang, this was the theoretical expectation - that the universe would slow down as time went on. Gravity would tend to pull it indeed, it might even turn around and begin to start contracting. If there were just matter in the universe, the universe would slow down in its expansion. The matter in the universe is exerting gravity on everything else in the universe. So the universe has been expanding since the Big Bang. Last and definitely not least, dark energy: This has been the surprise of our time. We can do simulations with and without dark matter, and the universe looks entirely wrong without dark matter taken into account.
We can even show the universe at the largest scales wouldn't have the distribution of galaxies it has without dark matter being present. So it's a bit of a mystery, but at this point, dark matter is a very well characterized mystery, in the sense we know there's extra mass or something behaving an awful lot like extra mass and we can measure its distribution astonishingly accurately. But so far, it has been difficult to make a mathematically self-consistent model that would agree with general relativity and all the observations that support it. Some astronomers think it's actually not a particle, but rather a slight change in how gravity works that is causing this phenomenon. To date, none of those searches have been successful, but bigger instruments are being built and deployed, and perhaps those will find the answer. People are now making increasingly heroic efforts to directly detect a particle that could be a dark-matter particle, and there are many theories what that dark-matter particle might be. If you look at light from distant objects, it gets bent gravitationally by intervening galaxies or clusters of galaxies, and that bending is too strong for the visible mass, so there must be additional mass causing the light to bend more strongly than would be actually be seen and so forth. So far, it has only been detected by the action of gravity on visible matter.Įach individual galaxy is spinning too fast for the matter in it, implying there's dark matter inside individual galaxies and surrounding them. Since those days in the 1930s, many different lines of evidence have been found to show there's extra mass in the universe, and really quite a lot - 5 times as much as all the atoms, all the regular matter we can observe, and yet it cannot be readily detected. There's still a factor of 6 error that, to this day, has not been placed to any detectable particle or type of matter, and yet that matter is clearly there there's a mass there. It turns out, some of that matter was regular atoms in between the galaxies, hot 10-million-degree gas around the galaxies.īut that only solves part of the problem. The galaxies are moving far too fast, implying there's a lot of missing matter. So, all the way back in the 1930s, Fritz Zwicky already realized that if you look at clusters of galaxies and measure how fast the galaxies are moving and compare to how much mass is in the stars of the galaxies themselves, that there's something wrong. If you look at galaxies orbiting each other, you can play the very same game as for planets orbiting a star, and infer how massive the galaxies are by how fast they're orbiting each other. If you orbit something more massive, you move faster. If you have two objects orbiting each other, you can infer their mass from the speed with which they orbit, and that's a direct consequence of Newtonian gravity. It's an interesting question - why the universe has so much more matter than antimatter, which goes back to conditions in the extremely early universe.Īntimatter is something we can observe it shows up in our detectors and has well-defined behavior and consequences when it encounters matter, so that's why I call it the least mysterious of those three things.ĭark matter is a label on something that we have to infer because we can't detect it directly.
Antimatter turns out to be part of the nature of the universe. We can make antimatter and also can observe antimatter being made naturally in the form of high-energy particle collisions. This is the physical kernel of truth that is the basis for science-fiction antimatter drives. Matter and antimatter may meet, and destroy each other, releasing huge amounts of energy. An antielectron, sometimes called a positron, is a positively charged electron as opposed to the negative electrons in our own atoms.
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Mordecai-Mark Mac Low: Antimatter is like matter, except it has the opposite charge, so an antiproton is a negatively charged proton as opposed to the protons in our own atoms that are positively charged.
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