How blueshift might beat redshift

Light that escapes collapsing stars and dust clouds may sport an unusually blue hue.

Light emitted within these gravitationally unstable objects can get compressed and thus become bluer, physicists suggest in the Feb. 4 Physics Letters B. They say this blueshift could remain even after the light reddens as it moves through the vacuum of space. While the effect would be difficult to observe, it could help in understanding what happens to light when it is produced by or passes through an object caving in due to its own gravity.

Because the universe is expanding, distant objects appear to move away from each other, leading to an effect that stretches light to make its wavelength longer. Consequently light that reaches Earth from remote stars and galaxies is generally redshifted. (Red light has the longest wavelength of visible light; blue has the shortest.) But Cosimo Bambi, an astrophysicist at Fudan University in Shanghai, and colleagues calculated that a dust cloud collapsing into a star or a star caving in as it goes supernova can manipulate light waves as if they were moving through a contracting universe. As a result, the waves get compressed and bluer. If this effect outweighs the subsequent redshift as the light travels through space, an observer will measure a blueshift.