Astronomy Ch 15

Front 1

antimatter

Back 1

Matter composed of antiparticles, which upon colliding with a matching particle of normal matter annihilate and convert the mass of both particles into energy. The antiproton is the antiparticle of the proton, and the positron is the antiparticle of the electron. (p. 333)

Front 2

big bang

Back 2

The high-density, high-temperature state from which the expanding universe of galaxies began. (p. 329)

Front 3

big rip

Back 3

The fate of the universe if dark energy increases with time and galaxies, stars, and even atoms are eventually ripped apart by the accelerating expansion of the universe. (p. 346)

Front 4

closed universe

Back 4

A model universe in which the average density is great enough to stop the expansion and make the universe contract. (p. 339)

Front 5

cosmic microwave background radiation

Back 5

Radiation from the hot clouds of the big bang explosion. The large red shift makes it appear to come from a body whose temperature is only 2.7 K. (p. 333)

Front 6

cosmological constant

Back 6

A constant in Einstein's equations of space and time that represents a force of repulsion. (p. 345)

Front 7

cosmological principle

Back 7

The assumption that any observer in any galaxy sees the same general features of the universe. (p. 336)

Front 8

cosmology

Back 8

The study of the nature, origin, and evolution of the universe. (p. 326)

Front 9

critical density

Back 9

The average density of the universe needed to make its curvature flat. (p. 337)

Front 10

dark age

Back 10

The period of time after the glow of the big bang faded into the infrared and before the birth of the first stars, during which the universe expanded in darkness. (p. 334)

Front 11

dark energy

Back 11

The energy believed to fill empty spaces and drive the accelration of the expanding universe. (p. 345)

Front 12

flat universe

Back 12

A model of the universe in which space-time is not curved. (p. 339)

Front 13

flatness problem

Back 13

In cosmology, the peculiar circumstance that the early universe must have contained almost exactly the right amount of matter to make space-time flat. (p. 343)

Front 14

grand unified theories (GUTs)

Back 14

Theories that attempt to unify (describe in a similar way) the electromagnetic, weak, and strong forces of nature. (p. 343)

Front 15

GUTs

Back 15

See grand unified theories.

Front 16

homogeneity

Back 16

The assumption that, on the large scale, matter is uniformly spread through the universe. (p. 336)

Front 17

horizon problem

Back 17

In cosmology, the circumstance that the primordial background radiation seems much more isotropic than can be explained by the standard big bang theory. (p. 343)

Front 18

hot dark matter

Back 18

Dark matter made up of particles such as neutrinos traveling at or nearly at the speed of light. (p. 342)

Front 19

Hubble time

Back 19

The age of the universe, equivalent to 1 divided by the Hubble constant. The Hubble time is the age of the universe if it has expanded since the big bang at a constant rate. (p. 330)

Front 20

inflationary universe

Back 20

A version of the big bang theory that includes a rapid expansion when the universe was very young. Derived from grand unified theories. (p. 343)

Front 21

isotropy

Back 21

The assumption that in its general properties the universe looks the same in every direction. (p. 336)

Front 22

large-scale structure

Back 22

The distribution of clusters and superclusters of galaxies in filaments and walls enclosing voids. (p. 346)

Front 23

MACHOs

Back 23

Massive compact halo objects. Low-luminosity objects such as planets and brown dwarfs that contribute to the mass of the halo. (p. 342)

Front 24

nonbaryonic matter

Back 24

Proposed dark matter made up of particles other than protons and neutrons (baryons). (p. 341)

Front 25

observable universe

Back 25

The part of the universe that we can see from our location in space and in time. (p. 328)

Front 26

Olbers' paradox

Back 26

The conflict between observation and theory about why the night sky should or should not be dark. (p. 326)

Front 27

open universe

Back 27

A model of the universe in which the average density is less than the critical density needed to halt the expansion. (p. 339)

Front 28

oscillating universe theory

Back 28

The theory that the universe begins with a big bang, expands, is slowed by its own gravity, and then falls back to create another big bang. (p. 340)

Front 29

quintessence

Back 29

The postulated energy that fills empty space and drives the acceleration of the universe. (p. 345)

Front 30

recombination

Back 30

The stage within 300,000 years of the big bang, when the gas became transparent to radiation. (p. 334)

Front 31

reionization

Back 31

The stage in the early history of the universe when ultraviolet photons from the first stars ionized the gas filling space. (p. 334)

Front 32

steady-state theory

Back 32

The theory (now generally abandoned) that the universe does not evolve. (p. 333)

Front 33

supercluster

Back 33

A cluster of galaxy clusters. (p. 346)