Accelerating Universe
A model for the universe in which a repulsive force counteracts
the attractive force of gravity, driving all the matter in the universe apart at speeds
that increase with time. Recent observations of distant supernova explosions suggest that
we may live in an accelerating universe.
Big Bang
A broadly accepted theory for the origin and evolution
of our universe. The theory says that the observable universe started roughly
13.7 billion years ago from an extremely dense and incredibly hot initial state.
Closed Universe
A geometric model of the universe in which the overall
structure of the universe closes upon itself like the surface of a sphere. The
rules of geometry in a closed universe are like those that would apply on the
surface of a sphere.
Cosmic Microwave Background
Radiative energy filling the universe that is believed
to be the radiation remaining from the Big Bang. It is sometimes called the
“primal glow.” This radiation is strongest in the microwave part of
the spectrum but has also been detected at radio and infrared wavelengths. The
intensity of the cosmic microwave background from every part of the sky is almost
exactly the same.
Cosmological Principle
This principle states that the distribution of matter
across very large distances is the same everywhere in the universe and that the universe
looks the same in all directions. According to this principle, our view of the
universe is like the view from a boat on an ocean, which is essentially the
same for any other person on any other boat on any other ocean. Measurements
of matter and energy in the universe on the largest observable scales support
the cosmological principle.
Cosmology
The investigation of the origin, structure, and development
of the universe, including how energy, forces, and matter interact on a cosmic
scale.
Critical Density
The minimum average density that matter in the universe
would need in order for its gravitational pull to slow the universe’s
expansion to a halt.
Dark Matter
Matter that is too dim to be detected by telescopes. Astronomers
infer its existence by measuring its gravitational influence. Dark matter makes
up most of the total mass of the universe.
Flat Universe
A geometric model of the universe in which the laws of
geometry are like those that would apply on a flat surface such as a table top.
Grand Unified Theory (GUT)
A theory stating that that strong and weak nuclear forces
and electromagnetic forces are varying aspects of the same fundamental force.
Hubble’s Law
Mathematically expresses the idea that the recessional
velocities of faraway galaxies are directly proportional to their distance from
us. Hubble’s Law describes the relationship of velocity and distance by the
equation V=Ho * d, where V is the object’s recessional velocity, d is the
distance to the object, and Ho is the Hubble constant. Essentially, the more
distant two galaxies are from each other, the faster they are traveling away
from each other. American astronomer Edwin Hubble discovered this relationship
in 1929 when he observed that galaxies and clusters of galaxies were generally
moving away from each other.
Hubble Constant (Ho)
A number that expresses the rate at which the universe
expands with time. Ho appears to be between 60 and 75 kilometers per second
per megaparsec.
Open Universe
A geometrical model of the universe in which the overall
structure of the universe extends infinitely in all directions. The rules of
geometry in an open universe are like those that would apply on a saddle-shaped
surface.
Primordial Nucleosynthesis
Element building that occurred in the early universe when
the nuclei of primordial matter collided and fused with one another. Most of
the helium in the universe was created by this process.
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