Long you live and high you fly
Smiles you'll give and tears you'll cry
All you touch and all you see
Is all your life will ever be. -Pink Floyd
In part I of this series, we talked about a number of different ways -- all using gravity -- to measure the amount of matter in galaxies, clusters of galaxies, and the entire Universe. We got the same measurement no matter which method we used, finding out that 25-30% of the total energy of the Universe is in some type of matter. But, only about 0.5% of the total energy is in stars, which means that nearly all of this matter doesn't give off light! So what is the rest of this matter?
Well, one possibility could be that it's just normal matter like us: protons, neutrons, and electrons. After all, we are massive and we don't give off light! Maybe our Solar System -- where over 99% of the mass is in the Sun -- is a rarity? Or, as one commenter put it:
Why would it be surprising that there is mass in the universe that does not emit appreciable light? If that's all that's meant by 'dark matter', it seems obvious that it exists (Phobos and Pluto don't emit much light, for instance).
Well, as a cosmologist, there are a few tricks we can use to put this to the test. In fact, off the top of my head, I can think of four tests we can do to find out how much of this gravitational matter is made up of the conventional stuff: protons, neutrons, and electrons.
1.) Gas fraction of clusters. When we look at clusters of galaxies, many of them emit X-rays. The X-rays are created by hot, energetic gas at different temperatures throughout the clusters. From this information, we can deduce what percentage of the mass in a galaxy cluster comes from gas and dust, which is all normal matter. The answer -- pretty much always -- comes out to 13-15% for every cluster. This is much more than the 2% that's in stars, but still much less than the 100% total.
2.) Cosmological clustering of galaxies. Galaxies and clusters group together due to gravity, but the way they group together is highly dependent on what they're made out of. You make everything out of normal matter (what's called baryons by physicists), and things group together differently than if they're made out of dark matter. In this graph from one of my favorite cosmology textbooks, you can see the drastic difference between a Universe made up of normal matter (with the baryons label) and a Universe made up of either Hot Dark Matter (HDM), Cold Dark Matter (CDM), or a mixture of these (MDM):
What do we actually observe? Something that matches up very well with the CDM graph, but actually has some small, tiny wiggles in it. Altogether, we can draw the conclusion that the mass in the Universe is about one-sixth baryons and about five-sixths Cold Dark Matter.
3.) Nucleosynthesis. This is my favorite method of discriminating between baryons and "other" matter, perhaps because it's been around the longest. It's also one of the simplest. In the very early Universe, things are hot and dense enough that nuclei haven't formed yet; everything is just a bath of free photons, protons, neutrons, and electrons. When the Universe cools enough, however, protons and neutrons come together to form Deuterium, Helium, and Lithium. How much of these elements will be formed? That is highly dependent on how much normal matter (i.e., baryons) is in the Universe. In fact, that's pretty much all it depends on. We do our measurements, and then our calculations, and we find that no more than about 4-5% of the total energy in the Universe can be baryons, otherwise the abundances of these elements would be off by too much.
4.) The Cosmic Microwave Background. These tiny little temperature fluctuations -- just a few millionths of a degree -- can tell us an incredible amount about what's in the Universe. You can generate for yourself (using CMBfast) what these temperature fluctuations would look like in a Universe filled with 25-30% baryons as the matter in the Universe, and what it would look like with 4-5% baryons and the rest of the matter being something else. The differences are here, and they leave no doubt: the matter we're looking for cannot all be baryons.
So you can pick any one of these observations, or you can (like me) listen to all four. Any way you slice it, all of this matter can not be made up of the normal stuff we know and love. So, you would conclude, this means that there's some new type of matter out there, exerting its gravity but not emitting any light.
But all of these observations are based on an assumption: that we've got the right theory of gravity. Do we really? Or is it possible to tweak gravity to explain these observations without introducing a new type of matter? You'll have to come back for part 3 to find out.
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Tune in next time for the exciting conclusion to ...
DAAAARK MATTER!
Oh, you're going to explain why modified gravity is not the solution? That will be a lively discussion, I just wish I could quick jump to Part III.
Ethan, these are great posts. thanks for the cosmology book recommendation - I'm actually looking. But this one seems 10+ years old, so it's missing COBE, acceleration & other fun stuff. Do you have a tip to a more recent book at about the same level? and while we're at it, good GR books for non-physicists who are willing to wrangle with Reimann.
I lurrrrrve the CMB, but the isotope distribuition is my favourite peace of evidence, too.
What does it take to make a baryon a baryon, i.e. what constitutes 'normal' matter? Is it just a matter of interacting via the strong force, or is it more? I guess I'm getting ahead of the game, but I'm curious to hear about Wimps vs. Axions.
You have not said anything about black holes. Are they included with the stars and how do you calculate the mass of any black hole. What does the matter absorbed by the black holes become?
Cold.
Dark.
Matter.
Coming soon to a blog near you this fall!
@Gerry: If you'll allow me to conjecture (and that's all this is), since most stars are less massive than the sun -- much less stars that can become black holes -- there are few black holes compared to the number of stars, contributing only a small proportion of dark mass. That's especially true when taking into account black holes that can be detected by their activity, especially the supermassive ones in the cores of galaxies.
Ok, so here's a question that's been bothering me - why is the dark matter 'out there'? Why isn't it here right now, next to me, and exerting it's influence on me as I type this?
Surely, if it's measurable 'out there', you should find it 'right here' too?
DaveM: I was thinking much the same thing. I suspect that traveling in a universe made up of mostly dark matter is a bit like driving down the highway in fog. When you're in the middle of a fog, you can see for some distance before the fog becomes opaque. But the water vapor that makes up fog is actually evenly distributed. It's not like the fog opens up in a big circle centered on your car.
I suspect that there is dark matter 'right here,' but gravity is such a weak force that all the baryonic matter in our immediate neighborhood overwhelms it. Just as you might need to look out a distance of 100 meters or so in order to see the fog that's right next to you, you have to be looking out on the order of intergalactic distances in order to see the effects of dark matter.
I think though that this raises the possibility that, if a way were theorized to detect dark matter particles, we ought to be able to detect it right here, without leaving home.
But I'm not a physicist. Ethan or any of the other physicists care to tell me if I'm on the right track?
If you calculate the total amount of dark matter in the entire Solar System, out to Pluto or so, it comes out to be less than the mass of a large asteroid.
So, HP, your explanation isn't just on the right track, it's very, very good!
When you calculate the amount of normal matter, you are calculating the known knowns, or that which you can see. When you calculate dark matter, you are calculating the unknown known, or what you believe to be known. But the unknown unknown is the Aether, which may include Dark Matter in part, nobody knows.
The Aether is simply the physical medium responsible for so many of our unknowns. Newton was right, as was Aristotle and many of the 19th century scientists. You can call it what you want, Quantum Vacuum, Dark matter (in part), whatever, but thats what they referred to. Even Einstein admitted there was an Aether after he rejected it, but he simply called it space having physical characteristics like Aether that was necessary to support his general theory of relativity, but was not needed to explain his special theory of relativity. Today we call it a Quantum Vacuum.
Nikolas Tesla referred to the Aether as a possible source of energy for man, what we refer to Zero Point Energy from the Quantum Vacuum today. Not saying it is possible with todays technology of course, or is even permitted by the laws of thermodynamics, which are not really laws, just that we have never observed them not to be true.
Imagine a world with free energy. Perhaps this explains why science has been hijacked. It is a nightmare to those who maintain power in a world where energy is made to be a costly finite resource.
Since government funding dictates what gets researched and the ability to classify patents due to National Security concerns, this technology and knowledge can stay out of the public domain.
Maybe this is why the world is in the Energy Dark Ages, and Scientific Dark Ages. Bright minds waste time on mathematical descriptions or models of esoteric string theories which can not be disproved or tested, and in fighting Global Warming said to be caused by mans CO2 which will cause the Earth to escape the ice age we have been in for 2 million years (with a handful of interglacial periods lasting 12,000 years or so). Science is trapped within the Matrix with the rest of us.
Think about it, sounds weird I know, but the truth is weird. The greatest obstacle to the truth is the illusion that one already knows it.
Nicely and clearly written. I got two simple (at least on the surface) questions for you.
1) The nucleosynthesis argument assumes a constant number of Baryons during the nucleosynthesis phase (about 10-15 minutes if I remember correctly?). How would this be modified if there was some sort of metastable "super-particle" (with mass so high that we have no hope of encountering one in an accelerator), that decayed on the sort of timescale, producing extra baryonic matter as it did so ?
2) If there is cold dark matter out there, depending on the distribution, (i.e. if there were clumps of it in free space) could it not form otherwise invisible gravitational lenses ? Have we observed any ? What would happen if a large mass of it collapsed into a black hole with little or no baryonic material around ?
3) If, on the other hand, cold dark matter clumps around the distribution of regular matter (or the other way around if there is more of it), the wouldn't expect to see some of it here ? Not of course with our eyes, but through anomalies like unexplained tidal effects ?
4) Or is the current belief that dark matter is pread out completely homogeneously ? That would seem unlikely given that it is coupled (admittedly weakly) to regular matter that is not perfectly homogeneous.
5) On that note, does any clumpiness in the distribution also depend on whether there are forces analogous to electromagnetism, that act on dark matter but not on baryons ?
sooo..... "aether-energy" or "quantum vacuum energy", which may or may not be a potential source of energy, much less possible with today's technology, has been overlooked by those who've "hijacked" science and may account for our energy problems and for the "scientific dark ages" we're in?? We've wasted our time with esoteric nonsense like string theory, holding us back from "aether energy" (i.e. "free energy", 'cuz really the laws of thermodynamics are just helpful suggestions...), and preventing us from fighting global warming, which is "said to be caused by mans [sic] CO2 which will cause the Earth to escape the ice age we have been in for 2 million years"?? The Matrix?!?
Calling that "weird" would be an understatement.
"Dark matter" is the modern day turtles upon turtles upon which the universe supposedly rests. But like the turtles, "dark matter" has never been observed. Though Ethan Siegel's credentials and analysis are excellent; the idea of "dark matter" is based upon one simple fundamentally wrong assumption. Siegel's analysis assumes that flat Euclidean/ Neutonian gravity is sufficient to describe the universe at large. By analogy, on the surface of the Earth at any local city level, the flat Euclidean assumption is correct; but at a global Earth level, the sum of the angles of a large enough triangle (e.g. Moscow, New York and Johannesberg) is not 180 degrees (i.e. the Earth is not flat). Similarly at an astronomical level, our local solar system is flat (i.e.Euclidean); but at the galactic and cosmic level, the Euclidean assumption and the Newtonian gravitational assumption are introduce gigantic errors even before any calculation is begun. Long ago Albert Einstein said, "From the latest results of the theory of relativity it is probable that our three-dimensional space is also approximately spherical, that is that the laws of disposition of rigid bodies in it are not given by Euclidean geometry, but approximately spherical, if only we consider parts of space which are sufficiently extended." Today Fred I Cooperstock in his book General Relativistic Dynamics (Extending Einstein's Lagacy Throughout the Universe) just published 2009 says, "Cooperstock asserts, "Einstein's general theory of relativity, could account for the observed flat galactic rotation curves without the requirement for vast stores of mysterious dark matter." His ideas should not be glibly dismissed; he is the Professor Emeritus of General Relativity at the University of Victoria, Canada and renowned expert on General Relativity. His book is readable for the literate amateur (e.g. a writer and physics critic like me. I am not a working physicist). By the way, in my book A Critique on Pure Physics; I give an easy to understand explanation of how spherical geometry gives a universe with approximately 20% of the mass as a flat Euclidean universe for the identical astronomical data. Thus the the "dark matter" hypothesis should be disucssed only as a very provisional and speculative hypothesis. My book is fully viewable on Google books online; read from bottom of page 66 to bottom of page 68. Now I may be wrong; but regardless, Fred I. Cooperstock's ideas need to be listened and understood; get his book and read it; and DO NOT accept unqualified dogmatic statements that "dark matter" exists. If it does, we'll all be happy to understand what it is; but for now it is just modern "turtles". By the way, I don't reference Cooperstock in my book; because I didn't stumbled upon his remarkable book in a university library until a month ago (i.e. 3 months after my book was published).
this comment by Thomas Neil Neubert, author of A Critique of Pure Physics
Oh man. Not another one who hasn't even approximately understood what zero-point energy means, and that all known quantum fluctuations follow the first law of thermodynamics precisely -- you can borrow energy ex nihilo, but you have to pay it back; the more, the sooner.
You should familiarize yourself with how science funding is done in different places.
Hint: there's more than one country in the world.
If we don't do anything, this will indeed happen.
The only problem is...
...it's not a good thing. You disagree? Evacuate Bangladesh first.
Fred Middle Initial Cooperstock speaks of himself in the third person?!?
Few people since Gaius Julius Caesar have been that pretentious.
If I may defend Fred I. Cooperstock, he is not pretentious. In the acknowledgement to his book he says, "The reader will note that the word "I" occurs only once in this book and it is in this sentence. This is not because we prefer the "royal we"... Rather, it is because much of our work has been developed over the years in collaboration with others and it is simplest to use the "we" consistently. We are indebted to these collaborations whose varied talents and interests have brought us into contact with a wide variety of research projects and from whom we learned many things." Well that's enough, you judge for yourself whether Professor Cooperstock sounds pretentious. And by the way he uses the first person plural (i.e. we) not the third person (i.e. he) to refer to himself. So please discuss Cooperstock's ideas (which you are free to agree with or not); don't trash his person. Now if you look at the full title and subtitle of my book, I must admit it does sound pretentious. But click on my name and link to the full online text of my book and decide for yourself whether I am pretentious. But I'd rather you discuss my physics ideas. Cooperstock's book you'll have to get at a university library or buy. I borrowed it from a university library, read it cover to cover and then I bought it so I could study and refer to it.
comment by Thomas Neil Neubert, author of A Critique of Pure Physics (Concerning the Metaphors of New Physics)