3. Match the word with its definition.

1. background

a. a very brief exponential expansion of the universe postulated to have interrupted the standard linear expansion shortly after the big bang

2. gravity

b. low-intensity radiation from radioisotopes present in the natural environment

3. inflation

c. a celestial body orbiting the earth or another planet

4. satellite

d. the force that attracts a body towards the centre of the earth, or towards any other physical body having mass

5. wave

e. a periodic disturbance of the particles of a substance which may be propagated without net movement of the particles, such as in the passage of undulating motion, heat, or sound