- If dark matter has not been directly observed, why do scientists feel the need to propose its existence?
- If there is so much missing mass, why is the expansion of universe still accelerating?
- What is dark energy and what does it have to do with the universe?
- Does dark matter imply dark light?
- Could dark matter be anti-matter?
- Mass density, MACHOS and galactic rotation...
- WIMPs vs. MACHOs
- What is the theory of Dark Matter?
- Can we know baryonic content of DM?
- Does the sea of virtual particles contribute to the Dark Matter?
- Is Dark Energy related to the Casimir effect?
- What are some ways to detect dark matter?
- WIMP mass, thermal velocities, interaction properties...
- How can WIMPs interact with nuclei if they have no charge -- (particles and fields)?
- Explain the story of dark matter in a nutshell
- Could dark matter be hidden within other dimensions?
- Can dark matter radiate light like ordinary matter?
- Since dark matter is a cousin particle to our normal periodic table, does that mean that there are dark people too?
- What is the pioneer anomaly, and does it substantiate the theory of dark matter?
- Is this dark matter located near the center of our galaxy, and what causes it?
- Is there such a thing as anti-dark matter?
- Can dark matter/energy arise from interactions with other universes?
- Does the black hole at our galaxy's center consume dark matter at a similar rate to ordinary matter?
- Do we measure any gravitational effect of dark matter in our solar system?
For other Dark Matter and Cosomogy questions, check out the Archived FAQ here.
If dark matter has not been directly observed, why do scientists feel the need to propose its existence?
This is probably the most frequently asked of all questions about dark matter, and understandable so. On the face of it, the dark matter hypothesis is a hard one to swallow. It proposes that the vast majority of the matter in our universe is composed of mysterious stuff that does not resemble any terrestrial matter and has never been directly observed. Nonetheless, there is wide agreement among cosmologists today that some form of dark matter really exists. How have so many people come to such an extraordinary conclusion?
Below is a brief outline, but for more information check out our education pages, especially the essays titled ďWhy Dark Matter?Ē here are many independent lines of empirical evidence that suggest that the universe contains more than meets the eye; so far, the dark matter hypothesis is actually the simplest explanation that ties together all of these observations.
Motions of matter
We can learn about the universe today by watching the motions of its visible parts: gas, stars, galaxies, and galaxy clusters. Using the laws of motion and gravitation determined by Newton, Einstein, and others, we can estimate an object's mass by measuring the effect of its gravity on other objects nearby. We find that the orbital speeds of stars and gas clouds in galaxies are so fast that the gravity of the visible matter is not enough to hold them together. There must be additional, invisible mass to provide the extra gravitational pull needed to hold the structure together. Similarly, the motions of gas and galaxies within galaxy clusters tell us that these larger structures must also be dominated by "dark matter". Gravitational lensing When light from a distant source passes near a heavy object, its path is bent slightly by gravity. This phenomenon of "gravitational lensing" has been used to map out the mass distribution around galaxy clusters, based upon the distortions induced in the images of more distant galaxies. Again, we find that the visible objects in these clusters are only a small portion of the total mass. The universe's baby pictures We can also learn about the universe by studying evidence from its earliest history. Two sorts of data about the early universe are particularly important: Primordial nucleosynthesis: During its first few seconds of existence our universe was as hot as the interior of a star, hot enough for nuclear fusion to take place. Certain light nuclei (notably deuterium and helium) were produced in these first moments, and by studying their abundances we can learn about the conditions in that primordial fireball. These data give clear evidence that the bulk of the universe's matter is "non-baryonic" in nature - that is, it is made of something different from the protons, neutrons, and electrons that build up all matter on earth. The cosmic microwave background: Some of the heat from the early universe is still detectable in the sky today, visible as microwave radiation coming nearly uniformly from all parts of the sky. The study of this snapshot of the early universe has already lead to two Nobel Prizes in Physics. The pattern of slight hot and cold spots in the microwave background across the sky also tell us about the properties of the early universe, and lead us to the same conclusion: much of the universe's matter is non-baryonic in nature. Evolution of the universe Finally, the composition of the universe determine how the massive structures within it - galaxies and galaxy clusters - grow and change over time. The uniformity of the microwave background radiation tells us that the distribution of baryonic matter in the early universe was extremely uniform. Without the presence of dark matter to provide additional gravitational pull, there simply has not been enough time for the smooth early universe to grow into the very lumpy one we see around us today.
~Dr. Jeffrey Filippini
Observational Cosmology Group
So, first there is the dark matter problem. Many pieces of evidence suggest that the mass of the universe is constituted primarily of a yet-unidentified form of matter. The atoms which form galaxies, stars, interstellar and intergalactic dust and you and me are a cosmic minority, constituting only about 5% of the mass of the universe. There seems to be much more mass out there for sure.
Now, on top of that, the expansion of the universe is accelerating as the article you cited suggests. This does seem to contradict the first problem. If there is even MORE mass than we think, shouldn't that make the universe's gravity even stronger and slow the expansion not speed it up? Yes -- but there is something else throwing off the equation. Einstein found in his equations of general relativity that there could be an extra term floating about called the cosmological constant. It turns out that this constant would have the strange property of pushing on space-time, not attracting like gravity. Recent observations of super novae (which actually happened billions of years ago) tell us that this constant is not just a mathematical possibility but a real thing.
So, EVEN given the fact that there is much more mass out there than we thought, the cosmological constant (dark energy, quintessence, what have you...) is dominating the expansion and causing the acceleration -- despite all of the extra matter. In fact, if the value of the cosmological constant were converted to units of mass, there would be twice as much dark energy as dark matter.
Furthermore, the mass density of the universe decreases as the universe expands. This occurs just in the same way as the density, meaning mass per volume, of the atoms of gas in a box decreases as the size of the box increases. As the size of the universe increases with the expansion, the density decreases. It turns out that just now we are in an era in which the density has decreased enough to allow the dark energy to be noticed (this is what I mean by "twice as much dark energy as dark matter" in the preceding paragraph). Billions of years ago, the universe was smaller, so all of the mass was in a smaller box so its density was greater, so we might not have noticed the dark energy. The dark energy density, according to our best guess, doesn't change with time -- it is constant, which is why we call it the "Cosmological Constant". It is simply a property of the space in the box itself regardless of the size of the box.
Two big problems! Stay tuned to the news as welook for an answer....
Michael Scott Armel
Center for Particle Astrophysics, Berkeley
We believe that the universe is expanding due to the Big Bang which happened about 12 billion years ago. Using Einstein's equations of general relativity we can write down other equations explaining how the universe grows with time after this Big Bang.
It turns out that there is room in the equations for an extra parameter which was first just believed to be a trick of the mathematics. This extra parameter was called the "cosmological constant". If it should be real and not just some mathematical trick, it would create negative gravity Ė meaning that it would cause the fabric of the universe to repel against itself. Now, this would be opposite to the gravity we know of, which attracts, binding people to the earth and the earth to the sun and the sun to the galaxy and all the galaxies together. If there were no real "cosmological constant", the expansion of the universe would slow down since all of the galaxies are gravitational attracted to each other and will pull on each other. This would be like putting on the brakes and slowing down a car. But, if we add the effects of the supposed cosmological constant and its repulsive gravity, things would change. At some point the braking effect of all of the gravity of the universe would start to be overcome by the pushing of the cosmological constant, or Dark Energy. This would be like trying to stop a car with brakes, but at the same time having the gas pedal pushed down a little bit. Gravity of the galaxies tries to slow the expansion down, but the Dark Energy tries to speed it back up. At some point, as the gravitational brakes wear down, the push of Dark Energy would take over!
Is this happening?
As we look into space, the further away we look, the further back in time we are looking actually, since it takes a certain amount of time for light from each star to propagate through space. So, as we look at far away stars we are actually looking at the history of the universe. By using supernovas in distant space as a standard measuring stick, we can look at the history of the universe and see how fast things were expanding at different points in time. It turns out that when we examine the measurements of how fast things were expanding throughout the history of the universe using the supernovas, we find that we we can make the data match the theory very well by making the cosmological constant or Dark Energy a real part of nature, not just a mathematical trick! In other words, there is some property of space and time whichis described by this cosmological Dark Energy which Einstein discovered in his equations. The expansion of the universe was slowing down, but recently (over the last billion years) itis starting to speed back up! The universe keeps getting stranger...
Michael Scott Armel
Sort of, actually. Here is the story. Dark Matter is expected by many to be formed from particles which are not currently part of the Standard Model of particle physics. Many physicists believe that there are other particles out there which have not been discovered. In fact, according to the popular theory of Super Symmetry (SUSY), each particle that we know of now would have a partner in this yet- undiscovered particle realm.
The particles in this SUSY particle family are not like anti-matter or mirror-matter. They are just other particles. But, to get to your question, even the photon would have a partner in this SUSY realm called the "photino". It is not made of dark energy and is not otherwise exotic except for some different particle properties.
So, yes, if we find dark matter, we expect there also to be light particles in the same new particle family, but they would not have strange anti-properties.
Neat idea, but no.
It is "see-through" in that light does not interact with it directly (except very, very weakly). But, you are correct, it does bend light by its gravity.
Before the Big Bang is a tough question. But, in general, dark matter particles (if they are discovered) would probably be like all other particles. They would have existed in the primordial soup of particles in the unbelievably hot and dense early universe.
Michael Scott Armel
Center for Particle Astrophysics
That is a good idea and many physicists have mused about anti-matter in the universe. But, it is unlikely that the dark matter is anti-matter. Anti-matter does indeed exist -- it can be created on earth in the lab and also can exist in outer space. But, when anti-matter collides with matter, an annihilation occurs and radiation is emitted.
Now, if we are right about dark matter, there is a *huge* amount of it in the unverse -- much more than ordinary matter. It exists in spheroid "halos" in the centers of galaxies and stretches out beyond the outskirts of the galaxies. If this were anti-matter, it would annihilate with all of the ordinary matter and would have done so long ago.
But, what dark matter is, we don't know...
Michael Scott Armel
Center for Particle Astrophysics
The technical answer is that in the equations of general relativity which describe the expansion of the universe, the terms cancel out such that you are left with a term which is proportional to the overall mass density and a term which is proportional to the expansion constant. This nice form comes about since the universe has roughly the same mass density everywhere and is expanding uniformly everywhere. Thus, you can write the equations to describe the expansion emanating from any arbitrary point, since the expansion is occuring from every point in the same way. When you write the equations, you actually have first to write down an expression for the total mass of your miniature example case. But, you can reduce the equation and the total mass is reduced to just a density term and the expansion term which is the Hubble constant.
Physically, you can think of it as an explosion of a set amount of mass. If the explosion occurs and the pieces of mass are very close together (more dense), the gravity of the pieces will be stronger and will change the motion of the mass. If the explosion occurs such that the mass pieces are farther apart (less dense), then their gravitational attraction to each other will be comparatively weaker. Thus, for the same mass, the density (being how far apart the pieces are) is important.
Compact objects such as MACHOS are becoming less and less likely as dark matter candidates. They do exist and there is exciting research being conducted now as astronomers search for them, but it appears as though they can only contribute a few percent of the total missing mass.
There are at least two important sources of information about dark matter which we determine from studying galactic motions. The first and best known is the rotation curve method which you mentioned. Astronomers can measure the rotation velocity of material within a galaxy as it orbits the galactic center. This measurement is made on an individual galaxy, not on a cluster. Galaxies have structures which can be categorized. By observing the radiation from the material in a galaxy and then by noting the structure of the galaxy, the center can be determined and the velocity of the galactic stars and gas around that center can be measured.
Michael Scott Armel
Center for Particle Astrophysics
So, there have been at least two very successful theories about the nature of dark matter. Some have suggested that dark matter is not made of some strange new particle but consists of ordinary but hard-to-see failed or dead stars or even huge cold planetary objects. Since this type of dark matter is not in the form of particles strewn through space, but rather is in the form of clumpy objects we call these hypothesized masses "compact". These dark matter objects would be accumulated inside the galaxies, particularly about the center of each galaxy. We call this cluster of dark matter about the center of each galaxy a "halo". Thus, these hypothesized objects would be massive, compact and they should be found in the halo of the galaxy. So, they are called "Massive Compact Halo Objects" or "MACHOs" for short.
WIMPS are the other very popular candidate for dark matter. Unlike MACHOs, they aren't in the form of clumpy objects gathered in galactic centers but are undiscovered particles which were created in the Big Bang and exist everywhere. However, their numbers are greater in the centers of galaxies and they would exist in the form of halos joined to most every galaxy. We know that if these particles exist, they interact with ordinary matter very weakly. And, even though each individual hypothesized particle is probably very light, they are probably still massive compared to other particles with which we are familiar (perhaps 100 times heavier than a hydrogen atom) So, we call them "weakly interacting massive particles" or "WIMPs" for short.
Now, from a variety of studies like gravitational lensing observations, we know that do MACHOs exist. But, they probably don't account for much more than a few percent of the missing mass. We are guessing that the rest is WIMPS. Our experiment may tell us more within a year or two!...
Michael Scott Armel
So, the universe began with the Big Bang about 13 billion years ago.
Many particles and anti-particles were created in the Big Bang. In the first moments, most particles were annihilated with their anti-particle partner, but a small fraction remained due to a slight imbalance in the proportions of particle and anti-particle. This slight excess is responsible for the atoms which got propelled through the universe. They eventually formed galaxies, stars and you and me.
But, the catch is that this known form of mass (which formed all of the elements of the periodic table) is probably not the only set of particles which survived the Big Bang. We believe that another entire family of particles was created and survived the Big Bang. These are invisible but constitute *most of the mass of the universe*! They survived the Big Bang not due to an excess in their particle/anti-particle ratio but because the universe expanded too quickly relative to their annihilation rate and they were dispersed before they got a chance to annihilate.
The particles of ordinary (not dark) matter are part of the "Standard Model" of particles. The dark matter particles are conjectured to be members of a new family of particles called the "Super Symmetric" family. For each particle in the Standard Model, there would be a cousin particle in the Super Symmetric family. They have different properties and are NOT the same thing as anti-particles.
So, particle accelerators on Earth are being used also to try to find these SUSY (short for Super Symmetric) particles. If we can find them in the lab, we have better reason to suspect that they are indeed the culprit in the missing mass problem.
Hope this helps,
Michael Scott Armel
Scientists do indeed believe that the contributions from both non-baryonic and baryonic dark matter are able to be stated.
In general, it is thought that it is almost impossible for a significant portion of the dark matter to be baryonic. At best, a few percent of the missing mass can be ordinary baryonic matter.
The best piece of evidence for this is Big Bang Nucleosynthesis (BBN) calculations. Given the knowledge of nuclear reaction rates, the production of light elements in the first moments of the universe can be calculated. These calculated abundances can be compared to measurements of the abundances in the universe. Theory matches observation very well -- if only a few percent of the mass of the universe is baryonic. If there were much more baryonic mass, the calculations would be thrown off.
Do note, however, that even of the few percent of baryonic mass, much of it is missing too. This is believed to be in the form of dust or ionized gas or MACHO-like objects. So, there are in fact two missing mass problems -- the missing baryonic mass and the real missing mass problem, which must be solved by a non-baryonic particle.
Just for the record, about 2/3 of the mass-energy of the universe is dark energy. 1/3 is dark matter and a marginal percentage is baryonic.
Michael Scott Armel
Center for Particle Astrophysics
Given that virtual particles are constantly being created and near instantaneously destroyed, could their fleeting existence (and presumable mass) account for some or all of the dark matter in the universe?
The effect of the cloud of virtual particles which surround every real particle is already understood in this regard. In fact, the masses and charges of every particle are changed slightly by the virtual cloud. Dark matter seems to be made of real particles which would in turn have their own little clouds of surrounding virtual particles.
But, there is something in the virtual world which we do not understand and may have something to do not with dark matter but dark energy. As you may have heard, the expansion of the universe is accelerating. This is believed to be cause by an energy field which is an integral part of the fabric of space-time. The nature of this field is not understood and it is likely ot involve a more deep consideration of the flitting of virtual particles in every bit of space.
Center for Particle Astrophysics UCBerkeley
Science/Health Forums Host, NYTimes on the Web
''Is the Casimir effect ('vacuum energy' where two large parallel plates with a sufficiently small gap (ie quantum level) will be forced together by the pressure from excess virtual particles outside the gap) related to dark energy? ~Geoff''
That "vacuum energy" to which you refer is actually conceptually related to the "dark energy", but in a round-about way. So, we know that the vacuum of space-time has fluctuations in it. If you set up two parallel plates close together, only certain fluctuations are allowed between the plates. Specifically, the "modes" of vibration which are allowed can be visualized as those which from trough to next trough fit neatly within the space of the plates. Now, the space on the other side of the plates is bounded only by the deepest reaches of space (or the wall of the laboratory). More modes are allowed to vibrate there, connecting to the outside of the plates. So, the energy per every bit of volume between the plates is smaller since fewer fluctuation modes are allowed. Energy density is the same as pressure, physically speaking. So, one can imagine that the pressure between the plates is smaller than the pressure outside the plates. That forces them together. This is known as the Casimir Effect.
Now, this property of the vacuum, to generate quantum fluctuations, is distinct from the property of the vacuum which is involved in the Dark Energy issue, but similar nonetheless. It is as if the fabric of space itself is exerting a mini-Casimir experiment within its own little cells or building blocks. If space is atomized on some incredibly small scale, then there may be some Casimir-type forces pulling each individual cells together. The problem is that if we look at the scale of the size of the tiny building blocks of space (Planck Space), and the Casimir force which should result is many orders of magnitude higher than the observed Dark Energy density. The observed value is small. The natural theoretical value is HUGE. We expect the observed value either to be very big (as the thoery suggests) or zero. But, it is some sloppy small value with no justification!
That is the mystery...
Berkeley Cosmology Group
Our project pursues what is called as "direct detection" of dark matter. A leading theory suggests that dark matter consists of particles which are clustered around the centers of galaxies and spread throughout each galaxy. So, some of these particles should be passing through the earth and you and me. We have designed detectors to detect the tiny energy from a collision between a galactic dark matter particle and an atom in the body of our detectors. Several other groups around the world are pursuing similar endeavors.
The theoretical landscape is vast, and there are still many ideas floating around about how to detect dark matter and what it is. Here is another idea: Dark matter particles in our galaxy should cluster also around stars like our sun -- they get trapped by the gravity. So, there should be a higher density of dark matter particles in the sun than in the neighboring space around us. Dark matter particles may decay or annihilate with one another and this will give out a signature progeny particle or photon of light. Perhaps we can detect those signatures from the decay of DM trapped in our sun?...
Berkeley Cosmology Group
What is the mass of Wimps? We don't know their mass for certain yet, or even very well at all. Our best guess, based on the particle theories which predict their existence, is that they are about 100times heavier than the proton or 100 a.m.u.
How do Wimps move? We believe that they behave much like the individual atoms of nitrogen and oxygen in the air in a room. If the door is shut, the collection of atoms remains roughly "trapped" in the room, but they bump into the other objects in the room. WIMPs, if we are right, would be "trapped" in a big cloud called a halo. Such a halo seems to surround each galaxy -- in fact, the halo of dark matter formed first and drew in the gas which formed stars. Now, the WIMPs in a galactic halo have individual thermal motions (like the gas atoms in the air in the room) and they remain localized to the galaxy roughly (again, like the gas in a room) and they bump into the other atoms which constitute the stars and stuff of our galaxy. In fact, we believe that our sun, like every such star, has its own little special collection of WIMPs which bumped into the sun's atoms and lost energy and got pulled into the sun's gravity. Incidentally, we suspect that the random thermal velocities of the WIMPs in the galaxy are about 300km/s, which is about as fast as our sun is moving through the galaxy.
Are Wimps solitary particles or do they group together like molecules? Our best guess is that they are weakly interacting -- with atoms and with each other, so they are most likely solitary particles, like neutrinos. Some models suggest that WIMPs are self-interacting, but that would only imply they collide more easily with each other and not form WIMP atoms.
Do Wimps create electromagnetic fields or react to any normal particles?
They have no charge, so there should be no electromagnetic interactions. Some models of dark matter particles suggest that they interact with the nuclei of atoms very strongly. They might then get trapped inside of nuclei, particularly in heavy atoms with large nuclei. These theories are not really taken seriously by many. However, as I mentioned before, when they collide with atoms in the stars (rare, but after time it does happen enough), they do lose energy and "fall" into the stars forming pockets of higher WIMP density.
Could any particle be detected by its collision with an atom or do WIMPs hav especial properties that make this the preferred kind of detection? We are gambling that they interact weakly with atomic nuclei. In particle physics, we know of basically four types of interactions: strong (holds protons and neutrons together and keeps them bound in the nucleus), electromagnetic (charges and static and binds electrons to the nucleus), weak (neutrinos, decays of neutrons), and gravity. By the "W" in WIMPs we mean "weak" interaction as in the third category above. This, as you might guess, is hard to detect. Now, just because a particle has mass does not mean it can be detected-- except by gravity, of course. If you say something has mass, then it has to produce some gravitational attraction. This is actually how we know dark matter is out there. We measure gravitational motions of galaxies and stars and clusters of galaxies and realize that there must be more mass out there to be causing the motions.
But, in terms of particle-to-particle interactions, gravity is not important and we are left with strong, electromagnetic or weak. Some particles interact by more than one of the above three, and all interact gravitationally. But, the actual degree to which a particle engages in interactions depends on its constitution. It might be that they interact so rarely that we will never detect them, but it seems to be that, according to theories, they ought to have an interaction strength such that if we wait a year or so with a good set of detectors, we'll record just a few bumps.
So, in particle theory there are particles and fields. Every particle with mass interacts gravitationally, through the graviton, which is a "quantum" or "package" of energy of the gravitational field. Everything with charge interacts with the electromagnetic field through photons which are the force carriers of the electromegnetic field (light is a photon, incidentally). Every quark (which make up protons and neutrons) interacts through gluons, which are the "quanta" (packets) of the strong field. The weak field is another field, a bit more hard to visualize than gravity and it is in fact related to the electromagnetic field. Certain particles (neutrinos, WIMPs?) interact through the weak field and its "quanta" are called the "W and Z bosons". The point is just as things without charge can interact gravitationally, things without charge can also interact weakly through weak interactions. It's non-intuitive, I know, but sometimes you have to look at things mathematically to see the underlying symmetry
The universe is full of stuff -- it is made of atomic particles and it is shaped into people, planets, stars and galaxies (which are stars and dust).
All of this matter, we believe, was made in a huge outburst of energy about 13 billion years ago called The Big Bang. The energy from this Big Bang created the atomic matter and threw it out into space. There it formed me and you and Cadillac cars and stars.
We used to think that that was the whole story.
But, we are realizing that there was MORE than just atomic matter created in the Big Bang. Another form of matter was created and it got spewed out into the universe along with the atomic matter. In fact, as far as we can tell, there is more of this other form of matter than atomic matter!
This other matter, created in the Big Bang like atomic matter, is called Dark matter. We can't see it. It is invisible. But, we can detect it by seeing its gravitational effects. Where is it? Well, remember when I said that a galaxy was just stars and dust? Well, nowadays we believe that Galaxies are really big clouds of this dark matter. Just like water forms clouds in the sky, dark matter formed clouds in the universe. These clouds of dark matter, by their gravitational pull, drew swirls of atomic matter into them. This atomic matter formed stars and planets. We used to think of it as the body of the galaxy are really just the icing on the cake. A galaxy is really mostly a cloud of Dark Matter which has stars stuck in it.
The Earth orbits around the sun which is just an ordinary star in a typical galaxy and therefore we must be moving through the Dark Matter cloud. So, some of the dark matter must be going through the earth and we might be able to detect it!
In fact, some cosmologists are investigating possibilities very similar to this. Professor Andy Albrecht at UC Davis is working on models in which mass in other dimensions is responsible for the dark matter. I recommend seeing this article: http://www.sciencenews.org/20010407/bob14.asp for more information.
Regards, Scott Funkhouser
Here is the difference. "Ordinary" matter, which is the stuff of which you, planets and stars are made, is all made of the same building blocks: atoms consisting of protons and neutrons in the dense inner nucleus and electrons in orbitals around this nucleus. Now, dark matter, if it be real, is almost definitely not made of atoms. The zoo of fundamental particles, some of which constitute the pieces of atoms, is called the "Standard Model" and it used to be the comprehensive list of all matter. Dark matter changed that. Some physicists now suggest that each of the members of the Standard Model family have some heretofore unknown "cousins" in another entire family of particles. That is what we believe dark matter is made of. These cousin particles resemble their Standard Model kin, but each may be different in some important ways. As far as we can quess, this would prevent them from forming atoms like the Standard Model particles do.
So, after all that, we can see why dark matter particles don't radiate. Electromagnetic radiation is the result of charged particles getting accelerated. What does that have to do with an atom? Atoms radiate because the particles from which they are made will get "excited" in some way and will jiggle out of place, but then get accelerated back into their natural states releasing radiation. The lights in our homes and offices and the flames of a fire are generated this way: the electrons in the atoms of a material get excited (with heat or electricity) and wiggle back into place, releasing light. Dark matter may be made of a bunch of lone, weakly interacting neutral particles. There is no way for them to "get excited" and release light by "wiggling back into place" since they may be these heavy, un-charged, ghostly particles which don't form atomic structures, so there is no jiggling back into place.
Regards, Scott Funkhouser Top
Since dark matter is a cousin particle to our normal periodic table, does that mean that there are dark people too?
We canít really be sure, since we havenít yet figured out for certain what the dark matter even is, but physicists donít generally think that there are people made solely of dark matter. The basic reason for this is what you mention Ė the different properties of ordinary matter and dark matter (or, at least, WIMP dark matter - our best guess as to what the dark matter is made up of).
In order to make a person (or a planet, or any other ordinary object), you need to gather a bunch of particles into one region and get them to stick together. In your body, the electromagnetic force (attraction between positive and negative charges) makes atoms stick together to make chemicals and complicated structures. This same force holds the atom itself together Ė the positively-charged nucleus is attracted to the negatively-charged electrons. The nucleus in the center of the atom is made of protons and neutrons, held together by the strong nuclear force.
WIMPs (the particles believed to make up the dark matter) donít ďfeelĒ the electromagnetic or strong nuclear forces. This means that they canít be held together to form things like atoms (or people). If you made a pile of WIMPs the size of a person, there would be nothing to hold it together and it would just fall apart. WIMPs do feel the weak nuclear force, but thatís not very useful for holding things together (it is, however, how the CDMS experiment hopes to detect them!). Most importantly they also feel gravitational forces, which allow dark matter to affect the motions of stars and planets. This allows dark matter particles to be held together in big, loose clouds the size of entire galaxies, but isnít strong enough to make small, solid objects (like people).
Hope that helps!
Jeff Filippini Berkeley Cosmology Group Top
An excellent question about a real mystery! The Pioneer anomaly is an unexplained additional acceleration that the space probes Pioneer 10 and Pioneer 11 seem to have experienced as they flew out of our solar system.
Pioneer 10 was launched in 1972 and flew by Jupiter before it swung out of the solar system. Pioneer 11 was launched in 1973 and visited both Jupiter and Saturn before heading out of the solar system (in roughly the opposite direction from Pioneer 10). Scientists were able to monitor the motion of both probes to very great precision, and used this data to study various astronomical forces (the gravity of the sun and planets, the solar wind, etc.) as they pushed and pulled on the spacecraft.
Around 1980, when the probes reached roughly the orbit of Uranus, the scientists found that both craft were slowing down a little bit faster than they could account for. It seemed like there was a tiny extra force on the probes directed toward the Sun. This force continued until good contact was lost with the probes in the mid-1990s. After years of extremely careful study, scientists have still been unable to explain the origin of this acceleration.
Some people have suggested that this anomaly might be caused by a real force from some kind of strange gravitational effect - maybe this is an indication that we don't understand gravity as well as we think we do! Some physicists claim that this could be connected to new explanations for dark matter or dark energy, which would be a very exciting possibility.
Unfortunately, no one is quite certain if the anomaly is even real. Other spacecraft (Galileo, Ulysses, and most recently Cassini) have not definitively seen this effect - though there are some hints, they can't say for certain. This effect has also not been seen in the laboratory or in the motions of the planets themselves. It's still possible that the acceleration is caused by, say, fuel leaking from the spacecraft or some kind of weird error in the system. Even if it is real, it may not be connected with gravity or dark matter at all. At this point, nobody knows!
Thanks for the question!
Jeff Filippini Berkeley Cosmology Group
In fact, it's such a good question that some of the brightest people in the world are still trying to figure out the answer. So far, we're still not sure exactly what the dark matter really is. There is a lot of evidence that dark matter exists, and that there is a lot more dark matter than visible matter (stars, gas, etc.). There is also evidence that tells us that nearly all of this dark matter is "non-baryonic" - in other words, it's not made up of protons, neutrons, and electrons the way ordinary matter is. The current best guess of most cosmologists is that dark matter is made up of new kind of particle, called a weakly-interacting massive particle (WIMP), produced in the Big Bang. We think these particles are all around us, not just in the galactic center - in fact, billions of these particles might pass through your body every second. WIMPs are thought to form a big cloud (called a "dark matter halo"), much larger than the visible part of the galaxy. Our galaxy would be immersed in this cloud, sitting near the center. The cloud is probably densest at the center, so we expect there to be more WIMPs near the galactic center than in the outskirts of the cloud, but dark matter should be everywhere and not just at the galactic center. A similar cloud would surround each galaxy in the universe, and bigger "super-clouds" would surround entire clusters of galaxies. Of course, we are not yet sure that this is right. Many groups of scientists are currently looking for these particles, either by creating them in particle accelerators or detecting them in the world around us. Cosmologists are also busy trying to think up other good explanations for dark matter. Hopefully we'll find out more in the upcoming years.
Hope that helps, and let us know if you have any more questions.
- Jeff Filippini UC Berkeley Cosmology Group
In the most popular theories, the dark matter is composed of some new kind of elementary particle, usually just called a Weakly Interacting Massive Particle ("WIMP"). This particle would have been created in large numbers in the Big Bang, along with everything else we see in the universe around us. In many of these theories (including most models derived from supersymmetry), the dark matter particle is its own antiparticle. This means that if two WIMPs were to come unto contact with one another, they could annihilate to produce energy in just the same was as a proton and antiproton. There is no separate "anti-WIMP".
It's also possible that the WIMP is not its own antiparticle, in which case the universe could be filled with both WIMPs and anti-WIMPs. Most theorists think that the Big Bang should have produced matter and antimatter in equal quantities, so there would be roughly equal amounts of both WIMPs and anti-WIMPs. Of course, we can see that there is a lot more ordinary matter than antimatter in the world around us, so it's possible that there might also be an imbalance between WIMPs and anti-WIMPs.
Even though WIMPs should annihilate one another when they collide, WIMPs interact with one another so weakly that these collisions should be very rare. They should happen fairly often in places where lots of WIMPs clump together, such as the center of the galaxy. Many astronomers are trying to detect dark matter indirectly by looking for radiation from such annihilations coming from the galactic center.
Jeff Filippini UC Berkeley Cosmology Group
Some theorists have suggested that dark matter may not be "stuff" in our own universe, but the gravitational shadow of matter in other universes. The idea is suggested by models in which space has more than the usual four dimensions (three of space, one of time). One possibility is that the visible universe is a sort of four-dimensional sheet (technically called a "3-brane") embedded in a higher-dimensional bulk (picture the 2-dimensional surface of a soap bubble floating in three-dimensional space). We don't perceive the other dimensions in these models because the interactions responsible for particle physics (electromagnetism and the strong and weak nuclear forces) are constrained to operate only along the brane. For example, since light rays can only travel along the surface of the brane, particles in the bulk (off the surface of the "soap bubble") would be invisible to our eyes.
Gravity, however, could leak away into the bulk. This may be why gravity seems so much weaker than the other forces of nature - its strength along the brane gets diluted by leaking away into the extra dimensions. The force of gravity in 5-dimensions between two objects will fall off as the inverse-cube of the distance between them, for example, much faster than the inverse-square law of 4-dimensional spacetime. If gravitational forces can leak only a short distance into the bulk, gravity could seem to follow an inverse-square law at large distances but become much stronger at very short distances.
As for dark matter (similar models may be interesting for dark energy, though I don't immediately know of an example), it could be the result of several branes like ours lying extremely close together in the bulk. We could never see the matter in these other branes, but their gravitational pull would affect our visible matter, just as dark matter does!
This is a very interesting alternative to our usual ideas about dark matter. It's not currently the favored explanation of most cosmologists, however. For one thing it depends on a rather complicated setup (a sheaf of branes extremely near each other, gravity that can leak a little but not very much, etc.). It's also not clear whether this theory should produce the observed structure for galaxies - blobs of stars embedded in much larger clouds of invisible gravitating mass. It's a very interesting possibility that's still being pursued, however.
Jeff Filippini UC Berkeley Cosmology Group
Does the black hole at our galaxy's center consume dark matter at a similar rate to ordinary matter?
Dark matter should be absorbed alongside ordinary matter by the supermassive black hole in the galactic center. It should not be absorbed as quickly as ordinary matter, however.
To understand this, we need to consider how ordinary matter falls into black holes. Gas falling into the black hole forms an accretion disk: a rotating, disk-shaped cloud of hot gas. The disk is heated enormously by friction within this cloud, as all the gas tries to cram into a very tiny volume. The hot gas releases vast amounts of energy in the form of X-rays and other radiation. causing it to lose orbital energy and spiral inward to lower orbits around the hole. The hot gas eventually falls in entirely.
The difference comes from the very property that makes dark matter "dark": it's lack of interaction with ordinary matter (or, in fact, with other dark matter). This means that dark matter particles cannot easily lose energy - they don't experience significant friction or emit radiation. This means that dark matter does not join the accretion disk, instead remaining in relatively large orbits around the galactic center. Since it cannot lose energy as quickly, dark matter is expected to accrete into the black hole much more slowly than ordinary matter.
For further details on this subject, you may find the following links interesting:
Recent paper on dark matter and black holes: http://arxiv.org/abs/0802.2041. The authors argue that only ~10% of the mass of a supermassive black hole comes from absorbed dark matter.
Layman's summary of this paper: http://www.universetoday.com/2008/03/08/greedy-supermassive-black-holes-dislike-dark-matter/
Thanks for the question!
Jeffrey Filippini Particle Cosmology Group University of California - Berkeley
DARK MATTER IN THE SOLAR SYSTEM
Dark matter should have gravitational effects on the planets orbits and on space probes, but we are so far unable to detect them. This is not surprising, however, because they are hidden by bigger effects: the gravitational pulls of the sun and planets are much, much larger.
The average density of dark matter near the solar system is approximately 1 proton-mass for every 3 cubic centimeters, which is roughly 6x10-28 kg/cm3. The actual density might be a little lower or higher, but this is the right order of magnitude.
Based on this number, we can work out the total mass of dark matter within the radius of Earth's orbit around the sun: for an orbital radius of 100 million km, we get a total of 2.3x1012 kg of dark matter within the Earth's orbit. This sounds like a lot, but the sun's mass is 2x1030 kg. All of that dark matter only weighs 10-18 as much as the sun does, so we cannot detect the tiny pull of dark matter upon the Earth's orbit. The same story is true all over the solar system: the gravitational pulls of the sun and planets are always much larger than that of the dark matter.
DARK MATTER IN A GALAXY
Now consider the effect of dark matter upon the orbit of the sun around the galactic center. Let's suppose that the density of the dark matter is the same everywhere in the galaxy; this is NOT true (the density is much higher near the galactic center), so the dark matter mass will really be higher than we calculate.
The radius of the sun's orbit is about 2.5x1017 km, so the total mass of dark matter within that orbit is 6x1040 kg. This is the mass of 3x1010 (30 billion) stars like the sun! The entire galaxy only contains ~100 billion stars, so the dark matter does have a significant effect on the sun's orbit through the galaxy. For objects farther out near the edge of the galaxy, the dark matter is actually the main thing keeping them in their orbits. This is more or less how dark matter was discovered by astronomer Vera Rubin and others: the orbital speeds of galactic stars and gas clouds don't match our expectations from the visible matter.
In other words, a galaxy is much lower in density than the solar system, so the small dark matter density becomes much more important.
DARK MATTER DISTRIBUTION
Dark matter is not distributed uniformly in space. The galaxy is embedded in a large cloud of dark matter, and gravity makes this cloud denser in the center than at the edges. The density varies slowly over many light years, though some theories suggest that there could be "clumps" on smaller scales than that.
Hope that helps, and thanks again for the questions.
Jeffrey Filippini Particle Cosmology Group University of California - Berkeley