#normal matter

This Is Why Every Galaxy Doesn't Have The Same Amount Of Dark Matter

“It isn't the properties of one or two galaxies that will be the ultimate test of dark matter, however. Whether these galaxies are generic dwarf galaxies or our first examples of dark matter-free galaxies isn't the point; the point is that there are hundreds of billions of these dwarf galaxies out there that are presently below the limits of what's observable, detectable, or having their properties measured. When we get there, especially in the distant Universe and in post-interaction environments, we can fully expect to truly find this yet-unconfirmed population of galaxies.

If dark matter is real, it must be separable from normal matter, and that works both ways. We've already found the dark matter-rich galaxies out there, as well as isolated intergalactic plasma. But dark matter-free galaxies? They might be right around the corner, and this is why everybody is so excited!”

When the Universe was first born, everything was uniform. There was dark matter and normal matter everywhere, in the same 5-to-1 ratio in all structures. But then the Universe had to go and get messy. It formed stars and galaxies of different masses and sizes, and that’s where the trouble started. In large, massive galaxies, even cataclysms like supernovae or active supermassive black holes don’t eject very much normal matter. But in small galaxies, significant amounts of normal matter can get ejected, upping that ratio to dozens or event hundreds to one. That ejected matter doesn’t just go away, but can itself, at least in theory, form dark matter-free galaxies. Where are we in our understanding of galaxies, dark matter, and gravitation?

We Just Found The Missing Matter In The Universe, And Still Need Dark Matter

“For over 40 years, scientists have argued over dark matter's existence. Big questions arose from the motions inside galaxies, clusters of galaxies, and along the cosmic web. From their gravity, we can infer the total mass in the Universe. Yet multiple sources indicate that only 15% of that mass can be baryonic: made of normal matter.”

Is dark matter truly necessary? Many argued that, until we found the entirety of the normal matter in the Universe, we couldn’t be sure. The motions of galaxies, clusters of galaxies, and the formation of large-scale structure and the cosmic web all indicate a certain amount of mass in the Universe, and many sources such as the CMB and big bang nucleosynthesis indicate that the “normal” matter can only be about 15% of the total, implying dark matter. But finding all the normal matter has proven elusive, with the theorized WHIM (warm-hot intergalactic medium) not showing up in sufficient abundance. In particular, the hot part just wasn’t there.

How Does Our Earliest Picture Of The Universe Show Us Dark Matter?

“So all you need to do, to know whether your Universe has dark matter or not, is to measure these temperature fluctuations that appear in the CMB! The relative heights, locations, and numbers of the peaks that you see are caused by the relative abundances of dark matter, normal matter, and dark energy, as well as the expansion rate of the Universe. Quite importantly, if there is no dark matter, you only see half as many total peaks! When we compare the theoretical models with the observations, there's an extremely compelling match to a Universe with dark matter, effectively ruling out a Universe without it.”

If your young Universe is full of matter and radiation, what happens? Gravitation works to pull matter into the overdense regions, but that means that the radiation pressure must rise in those regions, too, and that pushes back against the matter. On small scales, this pushback washes out the gravitational growth, but on large-enough scales, the finite speed that light can travel means that no wash-out can happen. Dark matter, however, doesn’t collide with radiation or normal matter, while normal matter collides with both radiation and itself. If we can calculate exactly how these three species interplay, we can calculate what types of patterns we expect to see in the Big Bang’s leftover glow, and then compare it with what we observe with satellites like WMAP and Planck. And what have we seen, exactly, when we’ve done that? 

What Would Happen If You Became Dark Matter?

“What if, instead of being made out of Standard Model particles, which experience the full suite of all the fundamental forces, we transitioned to being made out of particles which, to the best of our knowledge, interact only gravitationally? The first thing that would happen to you is that you'd no longer be bound together in any way whatsoever, and that anyone watching you would immediately see you disappear. The nuclear forces that hold your nuclei and protons together would vanish; the electromagnetic forces that caused atoms and molecules to stay together (and light to interact with you) would disappear; your cells and organs and entire body would cease to hold together.”

Here on Earth, everything we know of is made of normal matter, almost exclusively in the form of atoms. But if our understanding of the Universe is correct, there’s five times as much dark matter out there as there is normal matter. Have you ever wondered what would happen to you, a human being, if you spontaneously converted from normal matter into dark matter? Dark matter is fundamentally different from normal matter in a myriad of ways, perhaps most notably for the interactions it doesn’t have. Without the strong, weak, or electromagnetic forces acting on it, it’s effectively invisible to all types of matter and energy, including other dark matter particles. But it continues to interact gravitationally, even as the normal matter making up Earth, the solar system, and the galaxy is affected by everything else. If you gave it enough time and could watch them closely enough, you’d start to see the particles that once made you illustrate this difference.

Why do the tiniest galaxies have the most dark matter?

"When you get a large burst of star formation, you create intense, ultraviolet radiation. When the most massive stars die, they create bursts of supernovae, which ionize matter and accelerate it to near-relativistic speeds. And when you funnel matter into a black hole, it can cause jets, which eject matter into the intergalactic medium. All of these factors are at play in all galaxies, and yet these matter-ejecting effects only touch the normal matter. Because dark matter is transparent to all electromagnetic phenomena, only the normal matter gets ejected whenever you have a star-formation, stellar-death or black-hole-infalling event. On the other hand, these effects simply pass through the dark matter, and so it remains in these low-mass galaxies."

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Einstein's Telescope - The Hunt For Dark Matter and Dark Energy in the Universe - Book Review

A few years ago scientist thought they were well on the way to understanding the universe. New tools allowed them to look further and to see more than ever before. One thing that they needed to do is to get some idea of the amount of mass that the universe contained. This should be easy. The matter that we have here on earth is the same kind of matter that shines in the stars. Picking a representative area of the cosmos and getting an inventory of what is visible should give us the answer. Imagine their shock when they found that the normal matter that we all know is only 5% of what the universe contains. A type of matter called dark matter is 23% and the rest is dark energy at 72%. How can we find out about dark matter? We can’t see it. As it behaves just like normal matter with respect to gravity, we can find out how much there is by how it influences the normal matter that we can see. It is the motion and speed of the things we can see that tell us it must be there. Dark energy is even harder to pin down. The explanation of why scientist believe it’s there is a bit more convoluted, so I’ll leave that one up to the author.

One thing that helps us find out about dark matter is Einstein’s telescope. We have much better telescopes and other instruments today than we ever had but Einstein’s telescope is not one that we can build. Einstein’s theory told him that when light from an far object in space passed near an object closer to us like our sun the light would bend so that the apparent position would be shifted from the actual position as observed from the earth. It took an eclipse of the sun with a very bright object slightly behind it and a few years but conditions were finally right and Einstein’s prediction was proven. It is the bending of light through a lens that makes a telescopes work. So light bent by a mass in space makes it act like a telescope. This means that, we can use massive objects in space to see images of the objects behind them. The image is somewhat distorted and sometimes multiple images of the same object are created but we can often see objects that we could never see with our normal telescopes. It’s best when the massive object is about half way between us and the distance object. Some of the most interesting results come from when the massive object is a cluster of galaxies. But the other important thing is what we learn about the lens. The angle that it bends the light tells us it’s mass and every thing that isn’t visible normal matter is dark matter. There are other little tricks to getting it right, so again, I’ll leave that to the author.