Astronomers and physicists have suspected as early as the 1930s that electrons, protons, and neutrons, in other words the constituents that build up our bodies, are not the dominant form of matter in the Universe.
- Rotation curves of individual galaxies show the existence of mass beyond the regions with significant luminous matter density.
- Anisotropies in the cosmic microwave background radiation (CMBR) allow one to measure various cosmological
parameters. Recent CMBR measurements tell us that only 27% of the mass energy density in the Universe resides in matter. The same
measurements pinpoint the fraction of the energy density in baryons (ordinary matter) as only 5%.
- Big Bang Nucleosynthesis (BBN) places stringent limits on the density of baryons. Observations of the deuterium
to hydrogen ratio in molecular clouds backlit by quasars give a baryonic density of only 5% of the total mass-energy density of the Universe.
Hence all the matter in the Universe cannot be baryonic.
- Structure formation simulations rule out a Universe dominated by ordinary matter. Since matter could not gravitationally clump until recombination (matter was still ionized), structure could only begin to form after roughly the 300,000 year mark. Quantitatively, the anisotropy of the
temperature of the cosmic microwave background with an RMS Quadrupole of 18.4 +-1.6 microK is 10x too small if there is no
cold dark matter component in the Universe. Hence, structure formation without dark matter in inconsistent with the amount and
complexity of structure present in the Universe today.
- Probes of Type Ia supernovae show that supernovae at high redshift are actually fainter than expected, and hence farther away. This hints at an acclerating Universe in which some form of dark energy is the dominant contribution to the mass-energy density of the Universe. Disregarding the exact form of the dark energy for the moment, supernova acceleration measurements show a Universe consistent with 25% of the mass-energy density in dark matter.
Although dark matter has never been detected in the laboratory (or created in an accelerator), particle physics does offer a concrete model for dark matter. It is important to understand that there is no dark matter candidate in the Standard Model -- dark matter should be electrically neutral and weakly interacting. Only the neutrino satisfies these constraints; however, the neutrino is far from an ideal dark matter candidate. Recent results from the Wilkinson Microwave Anisoptropy Probe (WMAP) have placed the limit m < 0.23 eV on neutrinos, which correponds to a comsological density of < 0.008 (in terms of the critical density). Hence, neutrinos are simply not massive enough to make up the dark matter. So we conclude that we need to look beyond the Standard Model for a dark matter particle candidate.
So what is dark matter made up of? Well, we're not sure, but we've got some exciting ideas.
|Image courtesy PDG (Particle Adventure)|
One promising extension to the Standard Model is the theory of supersymmetry. At the barest level, in supersymmetry (SUSY) each fermion in the SM receives a bosonic superpartner, and each boson in the SM receives a fermionic superpartner.
So what is supersymmetry? Basically, it's a fermion-boson symmetry which we add to the Standard Model. Recall, in the Standard Model there is no way to turn a quark into a lepton or vice-versa -- supersymmetry allows us a way around this restriction. Remember, fermions are objects with half-integer spin, and bosons are objects with integer spin. Quarks and leptons are fermions, while force carriers like the photon and gluon are bosons.
So each elementary particle now has a superpartner
|Image courtesy PDG (Particle Adventure)|
Although it seems like an unnecessary complication to double the number of fundamental particles, supersymmetry is extremely appealing theoretically for the following (highly technical) reasons:
- Precision measurements of Standard Model parameters at LEP show that using only the Standard Model particle content SU(3), SU(2), and U(1) couplings (using Q2=MZ2 data) do not converge at a single high scale. This means that the simple SU(5) Grand Unified Theory is incorrect. However, if one adds the minimal particle content of supersymmetry, the couplings indeed seem to converge at a unification scale of M = 2 x 1016 GeV.
- Supersymmetry may solve the hierarchy and the naturalness problem in the Standard Model. Why is the vacuum expectation value (vev or
simply v) of the Higgs field in the Standard Model so small with respect to a much larger scale, the cut-off for the Standard Model? Since the masses of scalar fields are subject to quadratically divergent renormalization corrections, keeping the Higgs mass
and v small requires a fine tuning on the order of 36 orders of magnitude, which is extremely unsatisfying. Supersymmetry helps eliminate this problem in the following manner: in unbroken SUSY (supersymmetry) each scalar mass must be equal to its superpartner
fermion mass. And since boson and fermion mass corrections have opposite signs, they can cancel each other leading to a "naturally" small Higgs vev. Of course supersymmetry is broken and fermion and boson masses are not equal; in order to produce
acceptable corrections to the Higgs mass, the difference between boson and fermion masses must be of the order of 1 TeV.
- Supersymmetry is inherent in string theory, which currently is the only theory which has the possibility of unifying the quantum world with gravity.
- And finally, and this is perhaps the most appealing characteristic of supersymmetry in relation to dark matter, in many SUSY models the conservation of R-parity is assumed. The immediate consequence of R-parity conservation (esentially a conservation of
baryon and lepton number) consequence is that the Lightest Supersymmetric Partner, the LSP, is stable, and can act as dark matter.
In the two theoretical models with which I work (constrained MSSM and MSUGRA), the neutralino (a linear combination of the superpartners
of the photon, neutral Z, and two higgs states) is the LSP and thus a dark matter candidate.
Dark matter can be detected in one of two ways: directly or indirectly. Each detection method involves detecting particles in the laboratory the old-fashioned way (through scattering and collisions). What we mean by the distinction is the following: In direct detection, a neutralino is actually observed in the laboratory (usually through an inelastic collision with a nucleus in which a small amount of energy is deposited, typically a few keV) while in indirect detection, the neutralino is never seen directly; rather, the decay products of the neutralino are detected and the presence of a neutralino is inferred.
Some prominent Direct Detection Experiments:
EDELWEISS (Experience pour DEtector Les WIMPS en Site Souterrain)
CDMS (Cryogenic Dark Matter Search)
ZEPLIN (originally ZonEd Proportional scintillation in LIquid Noble gases) at UCLA
CRESST II (Cryogenic Rare Event Search using Superconducting Thermometers)
Each direct detection experiment uses slightly different means and detection mediums -- browse through their websites to get a feel for how interesting, massive, and tremendously difficult these experiments are to perform.
Some prominent Indirect Detection Experiments:
ICECUBE (at the South Pole - literally a detector of size 1 km cubed)
NESTOR (Neutrino Extended Submaire Telescope with Oceanographic Research)
ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch)
NEMO (NEutrino Mediterranean Observatory)
RICE (Radio Ice Cherenkov Experiment)
In general, indirect detection schemes use a large volume of ice of water to act as the detection medium. Neutrions from neutralino annihilations produce muons somewhere near the detector, which are then subsequently detected by the cherenkov light they emitt as they travel faster than the speed of light in water or air.
The astro-particle physics research group is currently involved in several projects:
- Dark Matter in Non-Standard Cosmologies: Standard Big Bang Cosmology provides a concrete method for theoretically determining the abundance of neutralinos (dark matter) at the present epoch. It is critical for the relic abundance of neutralinos to match the experimental observations of the dark abundance in our galaxy and universe overall. However, most dark matter models generically predict too little or too much dark matter and most of the theoretical parameter space has been ruled out by dark matter searches. A successful extension to the standard model should predict the dark matter density without too much fine-tuning. One way of evading this problem is to deal with non-standard cosmological models. For example, if the early Universe were dominated by the energy density of scalar field (which then decays leading to the era of radiation dominance), the standard production scenario of neutralinos can be altered in important ways. The purpose of this project (which builds off work of Gelmini and Gondolo) is to determine if neutralinos, produced through non-standard cosmology scenarios such late-decay of scalar particles, can act as the majority or even a sub-dominant component of the dark matter in our galaxy and universe and 2) to determine if dark matter indirect detection experiments can put limits on the scalar field properties or rule out late-decaying scalar fields entirely. This project will analyze various theoretical scenarios in which dark matter using computer simulations and analytic calculations to determine if dark matter in this form is detectable by current and future experiments.
- Kaluza-Klein Dark Matter: Theories of extra dimensions can offer a stable dark matter candidate completely independent of SUSY. In theories with extra dimensions, excited states of particles (called pyrgons or ladder states) can act as dark matter. We are currently working to incorporate Kaluza-Klein dark matter into DarkSUSY, a software package which simulates and predicts dark matter interactions and properties.
- Direct and Indirect Limits on Dark Matter in DarkSUSY: Direct detection experiments such as CDMS and ZEPLIN (and others) as well as satellites wuch as PAMELA and GLAST are placing new limits on dark matter properties. This project involves modifying DarkSUSY to use the latest detection limits to either validate or invalidate theoretical models.