A point of view from which an observer can measure physical quantities
Einstein's first postulate
The laws of physics are the same for two observers moving at a constant speed relative to one another
Einstein'ssecondpostulate
The speed of light in a vacuum is the same for all observers
time dilation
The apparent increase in time of an event for an object that is moving relative to an observer
length contraction
The apparent decrease in length of an object that is moving relative to an observer (this only takes place in the direction that the object is travelling), (l'=l/gamma)
the doppler effect
The change in frequency observed when a source of sound (or light) waves move relative to an observer
redshift
Light from stars and galaxies that are moving away from us are shifted to longer (more red) wavelengths
dark matter
The matter in galaxies that we cannot see
dark energy
An unknown force acting against gravitational attraction, causing expansion to accelerate
the big bang
A rapid expansion, called inflation, from a singularity followed by a continually expanding Universe
