Subatomic Physics

584 Nuclear and Particle Astrophysics
The peak in Fig. 19.2 indicates that on average the temperature fluctuations have
a width of, and are separated by, ∼ 1degreeinthesky.
Perhaps the most important present problem in cosmology which is closely connected
to subatomic physics is that baryonic matter constitutes less than 4% of the
mass required by the condition Ω = 1; about 22% is dark matter and about 74% of
it is vacuum energy. (12) The evidence for dark matter comes from the distribution
of galaxies and clusters and their motions, from the study of stars, and from the expansion
of the universe. The dark matter makes itself felt through its gravitational
effects, but remains invisible to other probes. Weakly interacting massive particles
(WIMPs) and axions (a particle that has been proposed to explain the smallness
of CP-breaking in the strong interaction) are candidates to solve this problem. (13)
Continuing efforts are being undertaken to search for this missing dark matter. (14)
There are two important scenarios, named ‘hot dark matter’, where dark matter is
assumed to have velocities close to the speed of light, and ‘cold dark matter’ that
assumes very low velocities, similar to the baryonic matter. Fig. 19.2 shows that a
model that assumes that dark matter is cold can fit the CMBR data very nicely, so
this is presently a favored hypothesis (see problem 19.13.)
Vacuum energy became a possible explanation for the unanticipated accelerating
expansion of the universe found by two separate investigative teams that were
studying distant supernovae. (2) Supporting evidence comes from the studies of the
CMBR and nucleosynthesis, which show that cold dark matter and baryons only
account for ∼ 26% of the mass required for a flat universe. The vacuum energy can
be represented by a cosmological constant, introduced by Einstein in general relativity
and dubbed by him as “my biggest mistake”! (15) A deeper understanding of
the birth of our universe and its transitional stages occurred with the development
of grand unified theories or GUTs. These theories, which unify the electroweak and
hadronic forces also predict baryonic decays, which together with CP or time reversal
violation, are a possible scenario for understanding the particle over antiparticle
excess and the ratio of baryons to photons (about 6 × 10 −10 ) in our universe. The
conditions for obtaining an excess of baryons over antibaryons were stated succinctly
by Sakharov. (16) They are: 1) CP nonconservation, 2) baryon nonconservation, and
3) nonequilibrium conditions. CP nonconservation permits a slight difference to develop
between the number of baryons and antibaryons. As an example consider a
particle X of mass = 10 14 GeV/c 2 . If baryon and lepton numbers are not conserved
exactly, X may decay to a quark and electron and X to a q and e + .Above
temperatures of 10 14 GeV, decay and formation of X and X were in approximate
12W.L. Freedman and M.S. Turner, Rev. Mod. Phys. 75, 1433(2003).
13K. van Bibber and L.J. Rosenberg, Phys. Today pg. 30, Aug. (2006).
14R.J. Gaitskell, Annu. Rev. Nuc. Part. Sci. 54, 315 (2004); Sadoulet, Rev. Mod. Phys. 71,
S197 (1999)
15Einstein originally introduced the constant with the intention of precluding his equations from
predicting an expanding universe, because at the time there was no evidence for the expansion.
16A.D. Sakharov, Pis’ma Z. Eksp. Teor. Fiz. 5, 32 (1967); English Translation: JETP Lett. 5,
24 (1967); L.B. Okun, Ya.B. Zeldovich, Comments Nucl. Part. Phys. 6, 69 (1976).