Tuesday, September 4, 2012

Are neutrinos the main component of dark matter? (Most likely yes)

There is a large debate right now in the physics community about whether neutrinos are the main component of the dark matter of the universe. And the idea that neutrinos can explain dark matter is starting to pick up in the physics community. The first goal of this post is to discuss experimental evidence that points to neutrinos as the main source of the dark matter in the universe. This is also called the Neutrino Minimal Standard Model. The second goal of the post is to discuss why this would have any relevance to anybody’s day-to-day life.

I’m going to start this post with a recently developed picture by other researchers of the mass/energy distribution of dark matter in a spiral galaxy. This picture was generated using data from the spiral galaxy NGC 4216. The dark matter is represented by a blue glow (even though it really does not emit photons…it really is dark.)

A few things are immediately obvious from the picture:  The distribution of the mass/energy is nearly spherically symmetric, and the radius of this spherical ball is greater than the radius of the spiral galaxy. The near perfect symmetry of the halo implies that the dark matter particles can collide with each other, i.e. that they can collide, and after the collision, the particles go in random directions. The spherical shape implies that there are irreversible processes going on between the dark matter particles. This means that the particles must interact via the weak nuclear force, which is the only of the four known forces of nature that is not time reflection symmetric. 

The homogeneity of the energy/mass density of the spherical ball rules out massive dark objects like black holes or brown dwarfs as the source of the spherical missing mass/energy density. The spread of the halos also suggests that we are looking at a fermion (i.e. spin ½ particle) that can interact with itself and that does not clump together.
The fact that we can’t see the dark matter implies that the matter does not interact via the electromagnetic force. In addition, we can rule out the heavy uncharged neutron because neutrons are not stable particles. Neutrons decay into protons with a decay time of roughly 15 minutes.

The dark material is not protons, electrons, neutrons or photons. So, what options are there:  (1) neutrinos or (2) one or more undiscovered particles.  But what undiscovered particle would have all of the same attributes as a neutrino? It’s unlikely that there’s some undiscovered particle in the universe with nearly exactly the same attributes as the neutrino (i.e. spin ½, interactions only via the weak nuclear force, and whose kinetic energy may be comparable to its rest mass.) We can also pretty much rule out the proposed particles of the theory of supersymmetry because these particles would all be extremely massive (since we haven’t foundthem yet in particle accelerators.) Massive cold particles have a problem called the "cuspy halo problem." Their density wouldn't be uniform, as it would be for a particle with a kinetic energy around the same value as its rest energy.

So, why haven’t all astrophysicists jumped on the ‘neutrino bandwagon’ ?  The main reason is that physicists haven’t nailed down the exact rest mass of the three types of neutrinos (electron neutrino, muo-neutrino, and tau-neutrino) and Big Bang models likely underestimate their density. But just because we haven’t nailed down the mass of the neutrinos and just because Big Bang models underestimate their density doesn’t mean that we should be wasting our time speculating about “new physics” or “new particles.” Why don’t we try to use our understanding of dark matter halos to estimate what the rest masses of the neutrinos should be or what the density of the neutrinos should be?

The problem with this is the fact that we only know the mass density of the particles in galaxy-size halos. We don’t know the density of each type of neutrinos, such as the number of electron-neutrinos per unit volume,  and we don’t know the mass of each type of neutrino. There are a lot of free variables. Here's a list of the free variables:  the rest mass, the kinetic energy and density of 12 different particles. The 12 different particles are the (left,right handed) (electron, muon, tau) (particle or anti-particle) neutrinos. (2x3x2) neutrinos times the 3 variables means that there are 36 unknown quantities. We simply do not have enough information about all 12 times of neutrinos. Further, the reason that we don't know the density of the neutrinos is that neutrinos can be created throughout the history of the universe. Neutrinos are made in stars all of the time, though this is likely a small number compared with the number required to account for dark matter.

But with roughly free 36 variables, this means that no physicist can rule out neutrinos as the source of dark matter yet. A physicist could adjust each of the 36 variables (such as the density, the rest mass and the kinetic energy of the neutrinos) to fit any shape of the mass density of dark matter. With that having been said, there's likely to be less than 36 free variables. (1) The so called sterile neutrinos don't actually interact via the weak nucelar force, and this might rule them out. (2) If cold electron, muon, and tau neutrinos convertly back and forworth, then the density of each type might be nearly identical. (3) There will be constraints on the kinetic energy of the neutrinos based off of their rest mass. The ratio of kinetic energy to rest mass can't be too high or else the particles wouldn't be gravitationally captured and it can't be too low, or else they would all be clumped in the middle of the galaxy...like the rest of the matter in the galaxy.

Because there are so many free variables and because the hope of symmetric particles is diminishing, there are recent results suggesting that neutrinos may be able to account for dark matter without causing problems elsewhere in cosmology. For example, Canetti, Drewes, Frossard and Shaposhnikov recent wrote the following in an article that can be download here:

"We conclude that neutrino physics can explain all confirmed detections of physics beyond the
standard model except accelerated cosmic expansion."

So, neutrinos are the most promising of the candidates for dark matter because they fit all of the generic descriptions of what we see, except for accelerated cosmic expansion (which makes sense because the cosmic expansion is most likely due to the increase in entropy due to irreversible processes associated with the weak nuclear force.) So far, the likely dark matter candidate would be a 'warm' sterile neutrino with mass around 1-20 keV (likely 2-7 keV), and if this is the dark matter, then we will be able to detect it because these sterile neutrinos supposedly will slow decay into ~1 eV neutrinos and emit a X-ray. But before continuing to discuss the evidence for neutrinos, I’d like to discuss a little bit about why physicists had to create the term “dark matter” in the first place.

The term dark matter was created in order to explain the experimental results in most galaxies in which the orbital velocity of stars in the galaxy increases monotonically with the radius of the star from the center of the galaxy. The problem is that, using Newton’s law of gravitation and assuming that the mass in the galaxy is proportional to the density of light emitted from a given volume of the galaxy, then the velocity of the stars should increase, peak, and then decrease with increasing radius from the center of the galaxy (dashed line below.) However, the velocity of the stars often continuous to increase or remains flat with increasing distance from the center of the galaxy.

Dark matter is a real and what’s fascinating is that there’s indirect evidence for dark matter from a variety of different experiments. Another example is the fact that we see gravitational lensing due to dark matter when a galaxy passes in front of galaxy that is farther away from it. The gravitational lensing refutes the idea that the solution to the problem of the orbital velocity is simply due to errors in our knowledge of the force of gravity.
So, we are left with the conclusion that dark matter is real (i.e. it’s not just that we have the wrong equations) and that the mostly like source of dark matter is neutrinos. These neutrinos can’t easily be constrained by their own gravitational interactions because their kinetic energy is comparable to their rest mass, and the fact that they can interact via the weak nuclear force means that there is a mechanism for a randomization of their velocities (which explains the near spherical symmetry of their mass/energy density.)
Many physicists have ruled out neutrinos because they estimate that there were not enough neutrinos created during the Big Bang in order to explain the amount of Dark Matter in galaxy halos. The problem is that this is a circular argument against neutrinos. There is likely a lot we still don't know about the Big Bang. (I'm personally skeptical of models that claim to precisely model the Big Bang a small time scales because these models don't include time irreversibility of the weak nuclear force. When youn include the weak nuclear force, it means that you lose the ability to predict the past or the future, given the current state of the universe.)
Another possible problem with this reasoning against neutrinos is that the density of neutrinos is not a constant with time. Neutrinos and anti-neutrinos can be created or destroyed in reactions involving the weak nuclear force (such as nuclear fission and fusion.) This means that the density of neutrinos in a galaxy is not a constant. The density is likely much greater as time increases. So, as a galaxy first forms, there might be less neutrinos and the neutrinos may not have a spherically symmetric distribution, but over time, the density will likely increase, and the shape of the halo will become more symmetric due to irreversible collisions between the neutrinos.

So, why should anybody care about dark matter in a galaxy far, far away?
The reason we should care is that we should be proud of our understanding of the laws of physics and our understanding of the particles in the universe. We can explain so much in the universe and we don’t need to make it sound like dark matter means we fundamentally don't understand the world. Instead. let’s just call dark matter by what it most likely really is: neutrinos. Likewise, in a recent post, I discussed how the acceleration of the universe is really just due to the increase in entropy of the universe. We don’t need to make up a silly term like dark energy. We actually can explain the expansion of the universe and we can explain the orbital velocity of stars in galaxies using the known forces and particles. We actually know a lot more about the universe that many physicists will give us credit for. So, let’s spend more of our time and effort on applying the known laws of physics to grow life and to look for life on other planets. Since we don’t need to waste a lot more time debating what is dark energy or dark matter, we can focus our time exploring this solar system and building satellites that can look for life on other planets. I can’t wait for the day when NASA or ESA obtain a picture of a planet with visible signs of life. I think that finding the equivalent of forests (or some other visible signs of life) on other planets should be one of our highest priorities. Finding intelligent life forms on other planets would be an even greater accomplishment.

So, why do so many physicists and astrophysicists continue to use the terms dark energy and dark matter? I think that there are a couple of reasons why. 
(1) If there is some unknown force or particle, then this justifies giving them money to research this subject.
(2) Some physicists are hoping that their pet theories (like string-theory, supersymemtry or Grand Unified Theories) will be true and many of these theories predict particles that we haven’t found yet. So, if we call dark matter "neutrinos", we are really stating that we don’t need these theories to describe the orbital velocity of stars in galaxies and we are taking away one more reason to believe in string theory or supersymmetry.
(3) Some physicists and some astrophysicists don’t like the concept of entropy or time irreversibility. If the neutrino were the source of dark matter, then they would have to admit that irreversible collisions via the weak nuclear force are the reason why the galactic halos are so symmetric. They really wish that they lived in a universe that was perfectly time reflection symmetric so that they could study the unchanging geometric manifold of the universe and predict the past and future. They wish that the source of dark matter was a particle that interacts using forces of nature that are time-reflection symmetric. Unfortunately for them, we live in a universe with one force of nature that is not time reflection symmetric. This means the entropy increases with time and you can’t predict the future of the universe. All we know is that the future will be more symmetric than the past.
(4) Some people like thinking that there is still mystery in the universe. (There is still plenty of mystery in the universe. Such as: Is there life and is there intelligent life on other planets? Let's focus on time/energy/effort on solving real mysteries rather than dwelling on past solved mysteries.)

But now that we’ve collected so much data, the answer to the question of “what is dark matter?” is staring us in the face:  left-or right neutrinos or anti-neutrinos of the electron, muo and/or tau type. My goal in writing this post is to help us focus more of our time and effort on growing life and looking for life on other planets.

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