elements carbon, nitrogen, and oxygen to effectively convert hydrogen into helium.

Cosmic microwave background (CMB) radiation:

The residual light from the big bang.

Cosmic rays:

Protons, nuclei of heavy atoms, and possibly other particles that have been accelerated to high energies by astrophysical process and then impinge upon Earth.

CP violation:

The mechanism by which matter and antimatter evolve in time differently. The C and P, standing for charge conjugation and parity, refer to so-called symmetry operations in quantum physics.

Dark energy:

An as-yet-unknown form of energy that pervades the universe. Its presence is inferred from the discovery recently that the expansion of the universe is accelerating.

Dark matter:

Matter that does not emit or absorb enough light or other radiation to be observed directly.


A theoretical framework for the introduction of particles with mass into a modern quantum field theory (named for Paul A.M.Dirac). A key feature of this framework is that the particle is distinct from its antiparticle.


A low-mass quark of negative charge one-third that of the electron. The down quark is one of the two quarks that occur in everyday matter (neutrons, protons).

Elastic scattering (interaction):

In this context, the scattering of neutrinos by electrons via the electroweak interaction. The probability with which an electron neutrino scatters differs from that for the muon or tau neutrino.

Electron-volt (eV):

A measure of energy equal to that gained by an electron passing through a potential difference of 1 volt. Einstein’s relation between mass and energy (E=mc2) is often used to define a unit of particle mass when divided by the speed of light (c) squared. The electron volt, with its internationally recognized multipliers for milli, kilo, and mega (meV, keV, MeV), respectively, is a useful unit for discussing the variety of particle masses.

Equivalence principle:

A fundamental principle of Einstein’s theory of general relativity of which one consequence is that all objects (and light) behave in a gravitational field in the same way independent of the velocity, internal structure, or other properties.

Gamma-ray burst (GRB):

High-intensity burst of gamma rays from cosmic sources first observed by detectors on satellites. Most of the gamma-ray bursts

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