According to thw big bang theory of the orgin of the universe the universe has expanded from a hot dense state into a vast, cooler state. what would be considered scientific evidence supporting this theory

Dr. arenas is a scientist who has been trying to determine if increasing pressure will increase the sound that occurs when two chemicals explode. it measures the volume of the sound that occurs at the minimum pressure at which the explosion will occur. it then increases the pressure and measures the volume of the explosions that occur. what kind of investigation is the scientist conducting?