Last week’s dramatic evidence in favor of particulate dark matter, and weighing against modifications to gravity, as an explanation for the dynamics of galaxy clusters is another terrific result of observational cosmology. Equally important however, are the implications of these observations, at some of the largest scales in the universe, for the physics of the unimaginably small – particle physics.
The Bullet cluster result, building on earlier measurements, adds a crucial discriminating data point to the already overwhelming evidence that the universe contains matter of a type other than that which we see forming galaxies, stars, planets and us (called baryons). In fact, the evidence shows that there is five times more of this so-called dark matter in the universe than there are baryons. It is observed indirectly through many different cosmological methods and, indeed, is the reason that galaxies are able to form the way they do. This is confirmed not only through observations, but by comparing those to the results of increasingly accurate and beautiful numerical simulations of how cosmic structure crystallizes out of a soup of dark and baryonic matter.
That we are now more certain than ever that a critical component of cosmic dynamics is due to an entirely new type of matter, sharpens the associated particle physics question – how do these particles fit into our greater structure of fundamental physics – what is the dark matter?
There is a good reason that the answer is not yet known. The reason the dark matter is not seen glowing along with much of the rest of the material in galaxies is that it does not experience electromagnetism, the force of nature that leads to light. We think that dark matter particles must be only weakly interacting (electromagnetism is quite a strong force) and a consequence of this is that it is hard to get them to do anything measurable to material on Earth in order to betray their presence.
There are two ways to get around this. One is to build very sensitive detectors to measure even the smallest effects of dark matter on normal matter. After all, if there is five times more dark matter than baryons around, there should be lots passing through the Earth all the time as our solar system orbits the galaxy. There are many people devoted to these efforts and there are reasons to think that success is lurking in the not too distant future. The second way is, rather than waiting for cosmological dark matter to hit something in your detector, to smash particles together hard enough to create some of it all for yourself. If one can do this, then one would be able to measure its properties (its mass and the strengths of its interactions) and study how it fits into the overall structure of particle physics. This is where our colliders are indispensable.
The mere possibility that we may be able to probe the nature of most of the matter in the universe, hitherto undiscovered, using terrestrial machines is, to my mind, breathtaking science that is crying out to be done. However, in the case of dark matter, and the possibility that is made up of weakly interacting massive particles, there is also a relatively general and quite compelling argument, arising purely from particle physics, that there should be candidate particles within extensions of the standard model of particle physics.
The relevant particle physics/cosmology connection has its roots in the hierarchy problem – the problem of reconciling two wildly disparate mass scales; the weak scale (102 GeV) and the Planck scale (1019 GeV). This hierarchy is technically unnatural in particle physics, since, in general, the effect of quantum mechanics (here known as renormalization) is to make the observable values of such scales much closer in size.
One approach to this problem is to introduce a mechanism that cancels many of the quantum corrections, allowing the scales to remain widely separated even after quantum mechanics is taken into account. An example of such a mechanism (and the most popular one, for sure) is supersymmetry (SUSY). Supersymmetry is a beautiful idea that relates seemingly unrelated types of particles – fermions (such as the electron), and bosons (such as the photon) – to each other, and also to the underlying symmetries of space and time. A remarkable property of supersymmetric theories is that subtle cancellations between the effects of all the particles mean that the quantum effects I referred to above are rendered harmless. Even though supersymmetry is not an exact symmetry of our world, if it is exact just above the energy scales of the standard model and broken below, the structure of the standard model remains stable, since quantum corrections can only be effective up to the scale at which SUSY becomes exact (much lower than 1019 GeV in this case).
Another perspective is to view the hierarchy problem no longer as a disparity between mass scales, but rather as an issue of length scales, or volumes. The general hypothesis is that the universe as a whole is 3+1+d dimensional (so that there are d extra spatial dimensions), with gravity propagating in all dimensions, but the standard model fields confined to a 3+1 dimensional submanifold that comprises our observable universe. This submanifold is called the brane (as in membrane).
This is really a superstring-inspired modification of the Kaluza-Klein idea that the universe may have more spatial dimensions than the three that we observe. As in traditional Kaluza-Klein theories, it is necessary that all dimensions other than those we observe be compactified (wrapped up nice and small), so that their existence does not conflict with experimental data. The difference in the new scenarios is that, since standard model fields do not propagate in the extra dimensions, it is only necessary to evade constraints on higher-dimensional gravity, and not, for example, on higher-dimensional electromagnetism. This is important, since electromagnetism is tested to great precision down to extremely small scales, whereas microscopic tests of gravity are far less precise (although remarkable advances have been made in recent years, prompted in part by these theoretical ideas).
Since constraints on the new scenarios are less stringent than those on ordinary Kaluza-Klein theories, the corresponding extra dimensions can be significantly larger, which translates into a much larger allowed volume for the extra dimensions. This extra volume is a big deal, because the spreading of gravitational flux into the large volume of the extra dimensions allows gravity measured on our brane to be so weak, parameterized by the Planck mass MP, while the fundamental scale of physics M* is parameterized by the weak scale, MW, say.
The problem of understanding the hierarchy between the Planck and weak scales now becomes that of understanding why extra dimensions are stabilized at a linear size (~0.1 mm, for example) that is large with respect to the fundamental length scale (1/M*). This is the rephrasing of the hierarchy problem in these large extra dimension models.
I give the two approaches above as examples, and there certainly exist other approaches to the hierarchy problem. However, an important point is that the connection between dark matter candidates and new particle physics, just above the weak scale, with the power to address the hierarchy problem, is very general one, which is independent of the particular approach one might find most compelling. Here’s the brief argument.
- In the absence of extreme fine-tuning, the stability of the standard model demands that there be new physics not far above the weak scale – usually referred to as the TeV scale.
- This new particle physics will inevitably involve new particles and symmetries relating them to the standard model particles (otherwise, how are their interactions to help us with the hierarchy problem).
- A danger with introducing such new particles is that their interactions may ruin the spectacularly precise and tested predictions of the standard model. To avoid this, one usually needs to introduce a new discrete symmetry – basically saying that all standard model particles have one charge, and all new particles the opposite – to suppress unwanted interactions.
- There will inevitably be a lightest one of the new particles and it will be stable because it can’t decay into other new particles because they are heavier than it, and it can’t decay into SM particles, because that wouldn’t conserve the new discrete symmetry.
- In large ranges of parameter space, this lightest particle can be electrically neutral.
- So now we have a new, weakly interacting, stable particle at the TeV scale (a WIMP), demanded purely from particle physics considerations, that makes an excellent dark matter candidate.
This basic structure applies to the popular ideas for addressing the hierarchy problem that I discussed above. In SUSY, the lightest superpartner of the SM particles (the LSP) can be neutral and rendered stable by the R-Parity symmetry. In extra dimensional models, the lightest Kaluza-Klein particle (the LKP) may be dark matter, and is stable by virtue of KK-Parity, and in little Higgs models, which address the hierarchy problem in a different way, and which I have not discussed, a similar situation holds, with T-Parity playing the relevant stabilizing role.
Thus, although it is important to remember that there are other well-motivated dark matter candidates, such as the axion, discovering what new physics exists at the TeV scale may play a central role in uncovering the nature of the particulate dark matter that the Bullet cluster observations have so clearly revealed. This is one reason that cosmologists, as well as particle physicists, await with bated breath the upcoming operation of the Large Hadron Collider (LHC) at CERN. The world’s largest machine is designed to take us one level deeper into the mysteries of subatomic physics, and to help answer some of the most pressing questions in particle physics, such as the origin of electroweak symmetry breaking and the nature of the solution to the hierarchy problem. But these days, particle physics and cosmology walk hand in hand, and every new discovery at the LHC will help us to sharpen and expand our understanding of cosmic evolution. The Bullet cluster observations have provided a yet clearer hint that we are on the right path.