4.2: The Cosmos
Understand the cosmos. Other Information:
The universe began in an extremely hot, dense condition and has undergone a tremendous expansion, greatly reducing its energy
density. The early universe can be described by a unified picture of particles and forces. As it expanded and cooled, however,
this simpler universe “froze out” into the complexity we see today. In 1998, we learned that the expansion of the universe
is now accelerating rather than decelerating. This means that some unknown source is producing an antigravity force stronger
than gravity. This mysterious dark energy now composes 73% of the total matter and energy content of the universe. The second
largest fraction, 23%, is called dark matter and it has not been identified either. Ordinary matter, including all the stars
and galaxies, amounts to around 4%. Since the science of the very large and the very small are intertwined, we will develop
joint research programs with NASA and other partners to combine high energy physics research with related programs in astrophysics
and cosmology. Identify dark energy. Explaining the dark energy that is pulling the universe apart is crucial for understanding
its evolution. Our strategy includes the following emphases: • Work in partnership with NASA to observe distant supernovae
using a dedicated telescope in earth orbit. The JDEM will precisely measure the emission of light from supernovae located
at a wide range of distances, providing a history of accelerating and decelerating periods in the life of the universe. •
Develop a theoretical understanding of dark energy. Our best attempts to calculate the vacuum energy density give results
that are much too large. Identify dark matter. The nature of dark matter has not yet been determined, but we suspect that
it consists of weakly interacting massive particles. A prime candidate is the lowest mass supersymmetric particle, left as
a remnant of a very early stage of the universe. Our strategy includes the following emphases: • Search for weakly interacting
massive particles in cosmic rays. • Search for supersymmetric particles produced in accelerator experiments. • Study the large-scale
structure of the universe and infer the distribution of dark matter. Explain the matter/antimatter puzzle. There appears to
be no antimatter in the universe now, although equal amounts of matter and antimatter should have been created in the early
universe. This is one of the great mysteries of physics. Our strategy includes the following emphases: • Use the SLAC B-Factory
to provide sensitive measurements of a minute asymmetry in the weak interactions of quarks that may help explain the absence
of antimatter. • Conduct an experiment on the International Space Station to search for antimatter in cosmic rays. Study the
cosmic role of neutrinos. Neutrinos permeate the universe and hardly interact with matter, yet play a key role in the explosion
of stars. The recent discovery of neutrino mass has important consequences for these supernovae. Our strategic emphases in
this section overlap with those listed in section 4.1, for exploring unification phenomena: • Study neutrino masses and mixing
in much more detail using new accelerator beams and detectors. • Search for neutrino-less double beta decay to provide an
absolute scale of neutrino masses. Investigate high energy astrophysics. High energy physics research can help solve important
problems in astrophysics—the origin of the highest-energy cosmic rays, corecollapse supernovae and the associated neutrino
physics, and galactic and extragalactic gamma-ray sources. Our strategy includes the following emphasis: • Develop detectors
on the ground and in space that will be used to study high-energy cosmic rays and gamma rays.
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