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Lessons From Dark Matter – Searching For Something That Ought To Be

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August 11, 2014 Tags: Earth, Universe & Time

Today's entry was written by Benjamin Shank. Please note the views expressed here are those of the author, not necessarily of The BioLogos Foundation. You can read more about what we believe here.

Lessons From Dark Matter – Searching For Something That Ought To Be

At the close of the 1920s, the theory of gravitation as outlined by Newton and Einstein was perhaps the most successful scientific theory to date, the major structures of the universe were largely understood and the art of lens-making had advanced to such a degree that astronomers could really get down to the nitty–gritty business of cataloguing stars and galaxies.

It therefore came as something of a shock in the early 1930s when various astronomers (Jan Oort and Fred Zwicky usually get the credit) pointed their powerful telescopes at various structures, both within and beyond our own Milky Way galaxy and declared that they were moving too fast. That is, the observable mass within each system was insufficient, often by several orders of magnitude, to hold themselves together gravitationally given their reckless speeds. Later measurements using gravitational lensing (the bending of light by super-massive objects) confirmed that most objects larger than star systems contain far more gravitational mass than the sum of their stars, free atoms and other known objects can explain. For those who saw it as their job to describe the Universe in detail, failure to account for 80% of its mass was considered unacceptable. The excess mass was termed ‘dark matter’ and that uninformative moniker summarized the state of our knowledge for nearly half a century.

Ordinarily, when astronomers encounter a new phenomenon, they are able to use better and better information to design better telescope searches until a coherent picture begins to emerge. When the new phenomenon is explicitly not visible to telescopes, indirect observations must be employed. The 1960s-1980s saw a number of increasingly precise maps of the speeds of different parts of a galaxy as a function of distance from the center. (The technique is based on redshifting of known chemical signatures.) They could then calculate from Newton's Law of Gravitation how much mass was enclosed within the orbit of each object. They found that dark matter is usually distributed concentrically within a galaxy and maintains substantial mass density well beyond the observable stars. Observations of satellite galaxies, such as globular clusters orbiting our own Milky Way, indicate that the dark matter halo usually extends to many times the radius of the ‘baryonic galaxy’ it supports. 'Baryon' is a category that covers protons and neutrons (and similar particles of known properties), the primary components of 'normal' matter.

With some meaningful data to go on, the 1980s saw a proliferation of theories to explain the missing mass. Many of them stem from finite visibility. Telescopes have limited resolution and there are whole classes of objects which they are simply unable to observe. If galaxies were packed with black holes or planet-sized rocks, they would be difficult to detect using light-collecting telescopes. Several comprehensive surveys of non-luminous objects have now largely ruled out this “dark normal matter” hypothesis. Additionally careful studies of light from the early Universe, when everything was much more tightly packed, suggest that most of the matter that was around then did not interact as strongly as the baryonic matter we see around us today. (See, e.g., the Wilkinson Microwave Anisotropy Probe (WMAP).) The existence of non–baryonic matter strongly suggests that some previously unknown particle account for the locally observed excess mass without interacting in a noticeable way with baryonic matter.

At present there is a strong, though not unanimous, consensus in the astrophysics community that we should be looking for at least one new kind of particle to explain dark matter. We know that it has non–zero mass and that it interacts very rarely compared to atoms. (To help us compare different theories, we quantify interaction strengths by the 'cross–sectional area' that particles would have if they were solid balls flying past each other.) Despite several decades of work, there are still quite a few categories of viable theories. Some of these are impossible to test using present technology, but a number of active experiments are designed to probe different regions of the unknown particle mass vs. cross–section space.

All dark matter detection experiments aim to take in as many potential interactions as possible and then search for very rare events, as few as a handful of interactions scattered across multiple years of operation. However the details vary considerably as different searches rely on different kinds of physics to measure an event. In accelerator-based searches, large numbers of high energy collisions are the relevant scientific currency and new particles are discovered by the mass–energy they carry away. Telescope-based searches often look for characteristic high-energy light bursts from hypothesized dark matter decays. The interaction space is the entire sky, but the events being sought are more diffuse. 'Direct detection' experiments rely on ambient dark matter, which is supposed to be sleeting continuously through the Earth, interacting very rarely with a detector in the lab. Increasing the number of potential interactions means building heavier and larger detector arrays.

Besides the observational questions of missing mass in the early and present Universe, some form of dark matter is required by many theories intended to address how the Universe evolved from a uniform, hot plasma to the structured, yet diverse place we see around us. We are grateful to live in a world where gravity, chemistry and nuclear physics can co-exist, but we strongly suspect that these seemingly unrelated processes are all derived from some unified set of physical laws. And it would be nice to know how that was done. Detecting dark matter and determining its properties would provide a powerful tool to decide which, if any, of the ideas proposed to answer this question is correct. As of yet, we have succeeded only in ruling out sections of the mass vs. cross–section space, but even these exclusion limits have helped to prune the thicket of dark matter theories.

One recent survey identified about 600 people worldwide as 'dark matter physicists', scientists prepared to spend a significant portion of their career searching for a particle which they believe ought to exist, despite precious little theoretical consensus regarding its properties. I am one of those physicists. Strictly, I do not know that there is dark matter in the same way that I know about gravity or God. Establishing the existence of particle dark matter to high confidence would require multiple detections; confidence requires repeated interaction. But if that were the only remaining goal most of us would probably invest our research time elsewhere. I think the case for particle dark matter is fairly strong, but right now I have to exercise a degree of faith. Because the interesting physics, the knowledge that will change how we think about matter and fundamental forces, lies beyond the question of existence. Far more than simple mass accountancy, it is the properties of dark matter that will inform our understanding of physical laws.

This interplay between faith and knowledge is often a part of the scientific process, but is less often explicitly discussed. All scientific knowledge comes with some measure of confidence which improves as many independent experiments verify a particular hypothesis. When viewing established theoretical frameworks supported by mountains of evidence, it is easy to lose the point of view of the people who sifted through that evidence, knowing that the truth was important and believing that this idea was worth pursuing.

The dark matter hypothesis is one case where we have remained 'fairly confident' for an extended period of time while we gather many different kinds of evidence. This situation is a good analogue for the common deity detection problem. The question “Is there a God?” is not readily addressable by observation and, for practical purposes, not nearly as interesting as “What is God like?”. To get from “There might be a God” to actively searching for a God who came to Earth and died on a cross requires an appreciation for the implications of this idea, a measure of faith and enough humility to possibly be wrong. This is not a strange feature of religious knowledge. It is part of how finite people learn new things about a creation beyond our comprehension.


Benjamin Shank earned his Ph.D. in physics in 2014 from Stanford University, where he worked on the Cryogenic Dark Matter Search (SuperCDMS). He is currently a Visiting Assistant Professor of Physics at Hope College.


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Merv - #86166

August 11th 2014

Thank you, Dr. Shank, for an illuminating overview of this mysterious field of study.

I don’t know if we have the privilege of interacting with you personally here or not; but I do have a layman’s question for you or anybody else reading here more knowledgeable about astrophysics than me.

What (if any) intersection is there between the study of / search for dark matter and dark energy? Dark matter has been invoked because of a substantial shortage of visible matter in the cosmos that can’t therefore account for all the gravity; but on the other hand it sounds like dark energy has been invoked because of (a surplus?) of matter that then fails to account for the lack of gravity as things accelerate apart more rapidly than they ought to. From my superficial perspective of these subjects it sounds like the two problems ‘cancel each other out’ at least to some little extent. I do know that gravity (in any amount) cannot account for acceleration apart since gravity only attracts, hence the need for a dark energy hypothesis. Nevertheless, doesn’t the dark matter hypothesis exacerbate the dark energy/ acceleration problem? Is dark matter more an issue related to the excess rotational momentum problem while dark energy is used to explain a linear acceleration on a larger extra-galactic scale?

On the philosophical side, it is interesting to note the parallel between this scientific investigation challenge and the origins issues. Here we have an acknowledgment of a substantial problem for Newtonian/Einsteinian gravity, and yet an understandably strong commitment to that framework prevents us from jettisoning it as we treat this as a challenge to be solved within that existing framework.

Evolution (the merely scientific kind with a small ‘e’) enjoys the same level of commitment from nearly all scientists, and therefore any apparent anomalies are also treated as problems to be solved within the evolutionary paradigm. Except this latter issue has such a polarizing religious fervor attached that scientists are afraid to be seen touching those questions (like soft dinosaur tissue), and healthy curiosity is replaced by avoidance or perhaps unwarranted, and highly postured dismissal.


bshank - #86168

August 11th 2014

To address your first question, the studies of dark matter and dark energy are sufficiently separate that i am an expert in the former but not the latter.  So the rest of my response should be taken with a grain of salt.

A sense of scale is important here.  As you suggest, dark matter has been invoked to explain the gravitational dynamics of structures within the universe.  It is widely believed to be an independent particle which clusters around visible galaxies.  Dark energy, on the other hand, arises out of general relativity as a property of free space.  It is invoked to explain the expansion of the cosmos as a whole, independent of any structures.  Mathematically, it functions like a positive pressure term, pushing everything uniformly apart.  Imagine drawing lots of little galaxies on a small balloon and then blowing it up.

Even the largest galactic superclusters that have been observed to contain dark matter are very small compared to the visible Universe.  You could say that dark matter exacerbates the dark energy problem in the sense that the parameters that describe dark energy have to be larger given the observed inflation because they also have to counteract the extra gravity of dark matter.  But space is very, very big compared to all of the stuff in it.  The amount of additional dark energy (a property of space) needed to counteract dark matter (a form of stuff) is modest.

My perception is that the backlash against Big Bang cosmology has not been as strong as that against evolution by natural selection.  I can all too ready imagine how difficult our work would be if every theory we proposed became a political hot topic.  It is not entirely clear to me why we have been spared this, but I for one am grateful.


Merv - #86169

August 11th 2014

Thanks for your response—it is exciting to be able to interact with experts in this.

As a high school physics teacher I can really appreciate your appeal to consider the scale.  It is simple to use Newton’s equation to compare the earth’s pull on us as we stand on its surface to the sun’s pull if the earth were not here and we were standing on a stationary platform that kept us this distance above the sun.  Despite its much larger mass the sun has such miniscule pull at this distance.  And it gets even more extreme when one does the same for Saggitarius A (4 million solar masses I think) in the middle of our galaxy.  The “squared” in the inverse square law dominates.

So thinking of dark matter as more of a “local effect” comparatively speaking helps me understand a difference in the effects.

If there is a “sleet” of these mysterious particles permeating our very own space (sort of like neutrinos only presumably more massive?)  then there must be some basic density calculations already well-known.  If this stuff were all turned into visible particulate matter or dust would it obscure neighboring stars?  Would the entire stellar cosmos become opaque to us?


Merv - #86170

August 11th 2014

Regarding big-bang cosmology as “being spared” the unwanted polarization, it may just be that cosmology is a smaller elephant standing next to the larger one (origins of man).  I fellowship with a wide range of Christians and some (but not all) of those who reject evolution also reject the time scales associated with big-bang cosmology.  So while they may not spend much powder and shot on you cosmologists, it may be because you are less high profile than evolutionary biology has been.  But there are some who see the acceptance of deep-time as a stepping stone to acceptance of evolution (and they are probably right to think so.) 

 

Physics doesn’t seem to have the same history of religious antagonism as biology quickly developed over the past couple centuries.  For some reason, “God pushing around planets to maintain orbits” was physically clarified into “gravitational persuasion and inertia” without ruffling many religious feathers.  But “Adam formed from the dust of the earth” being physically elaborated on as “common descent” managed to ruffle a lot more.  Thomas Huxley probably had a lot to do with that.  Perhaps if Krauss had lived over a century earlier to do for physics what Huxley did for biology, then physics would be “of the devil” right now too and you would be busier engaging in nonsensical turf wars with theologians.


Eddie - #86186

August 12th 2014

Merv:  

I agree with some of your comments above, and will add some remarks of my own.

1.  The great early modern physicists and chemists—Newton, Boyle, Kepler, Copernicus, and I believe Galileo as well—were all devout (not necessarily orthodox) believers in God, and saw physics and chemistry as a way of understanding the mind of God.  There was no “war between science and faith” in their minds.  (That may not be true of all the philosophers of the same era, but of those philosophers who had actual scientific accomplishment, most were devout.)  

2.  It was not until the 19th century that a certain attitude among physical scientists—“I had no need of that hypothesis”, attributed to Laplace—became prominent.  I don’t think it is any coincidence that this was the birth-time of modern biology.  That is, modern biology took its birth not in the 17th century, when the context of science was clearly theistic and even Christian, but in the 19th century, when the context of science was increasingly becoming agnostic, materialist, reductionist, etc.  Darwin came from a generally skeptical family, and though he had some religious faith as a young man, it waned throughout his life; many of his intellectual friends and contacts shared in this increasing agnosticism and skepticism of the era.  And he had militant, church-mocking, religion-baiting allies like Huxley; I can’t think of any parallel in the case of the 17th-century scientists.  So modern biology already had an inclination toward agnosticism, materialism, reductionism, etc. that physics and chemistry never had at their own births.

3.  To be sure, in the 20th century, physics and cosmology have often had Huxley-like defenders, and to that extent they have been criticized as evolutionary biology has by Christians.  But when I think of the non-religious physics/astronomy writers of the middle 20th century, they seem much less aggressive and overtly anti-religious.  Asimov didn’t believe in God, but he avoided direct attacks on religion; Carl Sagan was a cheery rather than a grumpy atheist; Jastrow was more of an agnostic than atheist.  And there were many leading physicists and astronomers who showed positive attitudes to religion, admittedly often to unorthodox Christianity or Eastern religions, but still, that is different from Dawkins, Coyne, etc.  Einstein seemed to support a sort of vague pantheism; Heisenberg, Bohr, and other big names speculated about connections with Eastern thought; the whole “Tao of Physics” school did the same.  Whitehead supported a process theology that was nominally Christian; he liked the Gospel of John.  Even agnostics like Hoyle spoke about someone monkeying with the fundamental laws of physics.  There simply was not the hostility we see today.  If anything, in the mid-20th century, elite physical thought was moving slightly away from hardcore 19th-century materialism and toward some form of spiritual reading of nature.  

4.  The current situation is different.  We have people like Krauss and Hawking doing their best to destroy religious belief, or show that God isn’t necessary because you can get universes out of nothing, etc.  So the reductionist, materialist-leaning, agnostic/atheist biologists (i.e., most biologists who teach at secular universities, and probably 95% of full-time evolutionary biologists) have some loud public atheists in the physics/astronomy camp to encourage them, in a way they didn’t 45-90 years ago.

5.  Creationism is divided into old and young earth groups.  The old earth group has no problem with billions of years etc.  The young earth group bases (or at least conforms) its science on (to) a literal reading of Genesis, with 24-hour days.  Obviously the old-earth group is not going to have the problems with geology/physics/cosmology that the young-earth group has.  Most of the ID leaders are old-earth.  Probably half of ID supporters are old-earth as well.  So there is wide support among Bible-focused Christians for much of modern historical science.

6.  The sticking point is human evolution.  Many old-earth folks allow for microevolution and don’t much care if tapirs are related by common descent to horses or rhinoceroses.  But even old-earth folks think that man required a special creation.  Further, even the Catholic Church, which allows for the evolution of the human body, insists on special creation of the human soul, and that goes against not only Darwin’s Descent of Man, but against modern evolutionary theory generally, and now even some TEs are now embracing the “soul isn’t anything special, just an emergent product of our apelike ancestry” approach.  Then of course there is the problem of keeping the Fall if Adam and Eve aren’t the biological ancestors of all human beings, as Paul seems to have assumed—and for an evangelical, questioning Paul (and the Augustinian/Thomistic/Calvinist reading of Paul) is questioning the truth of Christianity itself.

7.  In short, the Big Bang is not nearly as threatening to most Christians as Darwin is.  Christians don’t much care if the atoms that make up their bodies were baked in stars.  They do care if the soul is just an epiphenomenon of matter, an upgraded version of chimpanzee savagery, so to speak.  It seems to speak against the image of God in us, the Fall, etc.  It seems to reduce the dignity of the most important part of us.

8.  That said, there is plenty from a purely scientific (non-theological) point of view to be said about the Big Bang cosmology.  It keeps having to be “patched” to make it conform to observation.  Already it is much different in its modern form than it was in Lemaitre’s day.  The question is how many patches you can add before it’s no longer the same theory at all.  There does seem to be an dogmatic attachment to it that bears examining.  But that is not in itself a theological concern, except insofar as it might indicate a non-theistic or anti-theistic motivation for some physicists etc.  Certainly Krauss and Hawking display that motivation.  But in my view the scientific and theological analysis of Big Bang cosmology should be kept separate.


Merv - #86189

August 13th 2014

Thanks for these clarifications, Eddie.

Regarding your #8, what substantial patches are you thinking of that make it dissimilar to its earlier versions?  I know time scales have changed since then, but the basic concept of a universal “point origin” expanding out into the cosmos observed today would still be the fundamental concept identified with that theory both then and now, no?


Eddie - #86190

August 13th 2014

Merv:

Yes, the basic concept of expanding from a central body of tightly-packed matter-energy is still the same, but originally the idea was simply that the force of the original explosion was sufficient to explain current observations (speed of galaxies, whether they are speeding up or slowing down, etc.)  Since then, all kinds of things have been added, because observation does not agree with the original simple model.  All this stuff about repulsive forces, dark matter, dark energy, etc. over the past X years has been brought in to fix of the disagreement between theory and measurement.  If everything had matched up with observation perfectly no one would be imagining dark matter or dark energy.

I’m not saying that it is never permissible to modify a model, but there is a limit beyond which it looks as if the goal is to save the model at any cost.  If I take an old teddy bear and put a patch on its ripped elbow, it is still essentially the same teddy bear, but what if I have to replace both its eyes with green beads (to replace the lost blue beads), and re-stuff the whole thing, and replace the tattered clothing that was originally on it, etc.?  At what point is it no longer the same teddy bear, but essentially a new bear, merely occupying the same space as the old one?

If the Big Bang is a solid model, it should be able to explain not merely the apparent movement of galaxies away from a center, but their speeds and other important data, without importing all kinds of theoretical entities which have nothing to do with the Big Bang per se.

You know, Darwin famously wouldn’t allow any “tinkering” by God in the evolutionary process; it had to be all natural causes for him, all variations acted upon by selection.  He wanted one set of simple, consistent, elegant principles that could explain the whole process, not some principles from normal natural science and some others drawn in from out of left field.  Similarly, if the Big Bang is a truly elegant and coherent model, it should contain within itself all the necessary principles to explain our observations.  What has either “dark energy” or “dark matter” got to do with the original Big Bang model?  Were they predicted by it?  Demanded by it?  Or are they heterogeneous parts of the overal picture of modern cosmology, pulled in to compensate for the failure of a straight Big Bang model to explain the facts?

You know, a bull is an organic whole, and a man is an organic whole, but the Minotaur is not an organic whole, but rather a mythmaker’s jamming together of heterogeneous concepts.  Likewise griffins, chimaeras, etc.  So is modern cosmology’s universe like the Minotaur—the Big Bang and a bunch of other unrelated causes all mixed together to explain the data in an ad hoc way?  Or is there wholeness and simplicity to modern cosmology?


Merv - #86193

August 13th 2014

So is modern cosmology’s universe like the Minotaur—the Big Bang and a bunch of other unrelated causes all mixed together to explain the data in an ad hoc way?  Or is there wholeness and simplicity to modern cosmology?

Probably there is no neat, elegant, or whole package when we’re discussing something this close to the ‘bleeding edge’.  One could argue that Newton’s gravity has had some “ad hoc” additions too, with Einstein’s addition of space-time curvature.  Perhaps it is all a great reflection on the tentative nature of even what is publicly touted as established science, but without hiding behind that as an excuse to then ignore evidence that we don’t like.

Your teddy bear comment reminded me of how I’ve heard that every atom in the human body is replaced—some (such as skin) on a faster replenishment schedule than other (bone or some parts of the brain).  But nevertheless I think I’ve heard it asserted that after X number of years, 99%+ of your atoms will be different than they were.  Makes one wonder where their true identity lies!


bshank - #86171

August 11th 2014

Yes, the mass density of dark matter in our local region is known to be 0.3 proton masses per cubic centimer, with ~10% accuracy. Unfortunately, the mass of each particle is not known; commonly displayed WIMP search graphs cover a range from less than 1 to over 1000 proton masses per WIMP.  So the particle density is a bit ambiguous.  But let’s say the dark matter were removed and replaced by an equal mass density of hydrogen atoms.  Most of the “light matter” in the galaxy is hydrogen atoms and there’s about 5 times as much dark matter as light matter so the total mass of hydrogen near the galaxy would increase by a factor of 6-7.  However the dark matter is arranged spherically around the center of the galaxy, while the hydrogen has mostly collapsed down into the spiral plane.  So for people trying to look at stars in the Milky Way or extra-galactic stars on the other side, there wouldn’t be much difference.  For astronomers who are intentionally looking perpendicular to the galactic plane so they can see deep field objects, this would be a disaster.  We would notice, but the night sky would hardly be blotted out.

Ironically, we’ve run up against a scale difference the other direction.  The interstellar medium already contains 0.1-100 hydrogen atoms per cubic centimeter, but the dark matter cloud, with typical densities of 0.1-1 hydrogen masses per cubic centimeter, dominates the total mass by being much bigger.


g kc - #86172

August 11th 2014

Benjamin,

I have several questions I hope you’ll address:

1)

“Additionally careful studies of light from the early Universe, when everything was much more tightly packed, suggest that most of the matter that was around then did not interact as strongly as the baryonic matter we see around us today.”

Does this suggest that certain physical properties and predictable physical interactions that we see today were not always the case in the past?

2)

Is dark matter an integral, or even necessary, part of current Big Bang Theory (Standard Cosmological Model)?

3)

I’m not sure I understand the last paragraph, particularly the sentence

“To get from “There might be a God” to actively searching for a God who came to Earth and died on a cross requires an appreciation for the implications of this idea, a measure of faith and enough humility to possibly be wrong.”

Is this saying that the searcher could possibly be wrong about the deity of Jesus Christ?


bshank - #86175

August 12th 2014

Hi g kc,

Good questions.  I’ll try to address them in order.

1) I did not mean to imply that baryon cross-sections has changed since the emission of the Cosmic Microwave Background (the “light from the early Universe”).  Quite the opposite, i wanted to appeal to the constancy of fundamental physics to suggest that, if non-baryonic matter was around then and hasn’t subsequently decayed, we should expect it to be with us today.  Strictly, the very early Universe was energetic enough that physical interactions behaved differently than what we see today, but the CMB was emitted late enough that we can safely ignore this.  I should point out that the dark energy density is a property of free space, which has expanded significantly since the CMB epoch.  The dark energy *has* increased, coming to dominate the energy density of the Universe only recently (on cosmological time-scales).

2) The Google result for “Standard Cosmological Model” starts with the Wikipedia page for the Lambda-CDM Model.  Lambda is the variable assigned to the cosmological constant (dark energy) and CDM stands for Cold Dark Matter.  (‘Cold’ here means ‘slow enough to be bound to galaxies’.)  So i am not alone in thinking that a complete understanding of cosmology requires both dark energy and dark matter.

3) I am convinced that no seeker will be wrong about the divinity of Christ.  However it takes a measure of humility even to pursue a God in whom you have little faith.  Also, seekers will almost certainly bring misconceptions about God which need to be corrected to fully engage him.  In faith, as in science, we need to act confidently when we legitimately have great confidence without hanging on to our own ideas out of pride.

4) Regarding “~10% accuracy”: Merv’s response is right on the money in terms of how masses and mass densities are inferred.  I was quoting Jovy and Tremaine’s recent paper ‘On the local dark matter density’ (http://arxiv.org/abs/1205.4033), where they give the density near the galactic mid-plane as 0.008+-0.003 solar masses per cubic parsec, which converts to 0.3+-0.1 hydrogen masses per cubic centimeter.  Near the end of their paper, they derive two effects which they claim could increase the dark matter density by up to 30% right near the Sun (the relevant location for WIMP hunters).  So i should have said “+-40%(statistical) +-30% (systematic) margin of error”.  But compared to the factors of millions in cross-section between WIMPs predicted by various theories, 10% vs 50% uncertainty in mass density is about the same.  In this informal setting, the proper term is probably “0.3-ish hydrogens/cm^3”.


g kc - #86177

August 12th 2014

Benjamin,

Thanks for your response.

You wrote “Strictly, the very early Universe was energetic enough that physical interactions behaved differently than what we see today, but the CMB was emitted late enough that we can safely ignore this.”

I think you’re saying the short answer is Yes, the early universe behaved differently than today. But I’m not following how the CMB changes the short answer.  

 

“… the Lambda-CDM Model.  Lambda is the variable assigned to the cosmological constant (dark energy) and CDM stands for Cold Dark Matter.  (‘Cold’ here means ‘slow enough to be bound to galaxies’.)”

Where would the dark matter be, and what was it doing, before the galaxies began to form?  

 

You’ve stated that a complete understanding of cosmology requires dark matter, and, I think, that the dark matter should be comprised of at least one kind of particle. You’ve also said that you’re not sure such particles exist. Would it be fair, then, to say a complete understanding of cosmology requires belief in something which may not exist?


bshank - #86181

August 12th 2014

Hi g kc,

Regarding the changing behavior of the Universe for the purpose of this discussion, i would say the short answer is No, particle physics hasn’t changed much since the CMB epoch.  At the time the CMB was emitted (100,000 yrs after the Big Bang, which is as far back as we can see), the fundamental forces of gravity, electromagnetism and the weak and strong nuclear forces were pretty well separated and the elementary particles that are common today were common then.  This comes with a big caveat that there were epochs before the CMB epoch when this was very much not true.

Prior to the formation of galaxies everything in the Universe, including dark matter, was homogenously distributed.  Exactly how homogenous and how it got to be that way is a major part of inflationary theory.  Presumably dark matter particles fell into the slight over-densities that eventually became galaxies, which is why most of the dark matter we see today is trapped in galactic accretions.

I am reasonably confident that particle dark matter exists precisely because it explains a lot of things we don’t otherwise understand and i am unable to assemble a satisfatory understanding of cosmology in my own mind without it.  That’s not the same as saying that it absolutely must exist.  I think it would be fair to say that the modern consensus understanding of cosmology requires belief in something which may not exist.  It wouldn’t be the first time the physics community has been wrong and had to go off in a completely different direction.  The most famous example is the non-detection of the ether as a light propagation medium, which inspired the development of Einsteinian relativity.  That’s part of the reason dark matter detection is important.  The implications of dark matter are sufficiently important that its not enough to be pretty sure.


g kc - #86185

August 12th 2014

Benjamin,

Thanks for your additional replies. You’ve said a lot.

The homogeneity issue does seem to be problematic. You wrote:

“Prior to the formation of galaxies everything in the Universe, including dark matter, was homogenously distributed.  Exactly how homogenous and how it got to be that way is a major part of inflationary theory.  Presumably dark matter particles fell into the slight over-densities that eventually became galaxies…”

I would think over-dense would be decidedly un-homogenous.


bshank - #86199

August 13th 2014

‘Over-dense’ in this case means about 1 part in 100,000 above average density. The problem is homogenous mass distributions are dynamically unstable; any slight over-density will be amplified as nearby mass is gravitationally attracted to it until it eventually collapses into a distinct structure surrounded by empty space.  The article has a link to  the Wilkinson Microwave Anisotropy Probe (WMAP) which goes into this in more detail.


g kc - #86173

August 11th 2014

Benjamin,

I’m having some difficulty understanding how one can give precise measurements of something that may not exist. For example, you responded to Merv with

“Yes, the mass density of dark matter in our local region is known to be 0.3 proton masses per cubic centimer, with ~10% accuracy.”

Can you help me with this?

Also, by “~10% accuracy” do you mean ~+/-10% margin of error?


Merv - #86174

August 11th 2014

I think I can help you out with at least one of your questions, g kc, subject to further correction of course.

Regarding how we can throw numbers around about things that may not exist (or simply has not been directly observed, rather)...   Black holes may be a good example to illustrate this point.  We can’t see them directly, but we can see the effects they have on their surroundings; such as a visible star in a fast, tight orbit around something.  We may not be able to see that “something”, but we can compute its mass with simple Newtonian physics by how fast the visible objects are orbiting it at their various distances.  So when this phenomena occurs and we can’t identify anything visible at the center, we still have good confidence it is there.  Gravitational lensing would be another observed effect.

So after observing the interactions of galaxies and super clusters, the approximate mass of these things can be inferred from their effects on each other and themselves.  When the visible matter is accounted for, its mass falls substantially short of what would be necessary to have that observed effect ... so there must be some other form of mass we aren’t detecting.  And it apparently isn’t just “non-illuminated but otherwise ordinary” dust.   They’ve ruled that out, probably on issues related to Dr. Shank’s reply to me about dust.

So ... given a mass that can be calculated, and the regions where that mass would have to be in order to exert the observed effects, you have mass and volume .... everything you need for an average density calculation.  And ~10% would typically mean within a range bounded by plus or minus 10%.


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