What is a red shift and what does it tell us?

Answer 1

A 'red shift' is the increase in the wavelength of light received from astronomical

objects, compared to what we know the wavelength should be. It's called "red

shift" because if the wavelength of some visible light is increased, the color of the

light changes in the direction of the color with the longest visible wavelength, which

is red. If the shift is really substantial, light that left a star at visible wavelengths

may even red-shift completely out of the visible range, and arrive here as infrared

radiation.

The only mechanism we know of that can increase the wavelength of light ...

without the light passing into a different medium ... is the receding (moving away)

of the light source from us, and it's generally agreed that if the light we receive from

an astronomical object is red-shifted, then the object is moving away from us. The

faster the object is moving away from us, the greater the increase in the wavelength

of its light.

This is a big deal in Astronomy and Cosmology. If it's true, then there have been

enough observations of red shift over the past hundred years to reveal the general

pattern that ALL of the distant objects in the universe are moving away from us, and

the farther from us they already are, the faster they're moving away. This is the basis

of the statement you've heard that "the Universe is expanding".

Notes:

1) If thelight source ismoving toward us, then its light arrives here blue-shifted ... with

shorter wavelengths than we know it had when it left the source. There's not much of

that to be seen, and it's all coming from objects that are relatively close to us.

2) Strictly speaking, the expanding Universe causes the "Cosmological Redshift"

which is similar to, but different from, the "Doppler Effect".

3) There is, in fact, another mechanism for producing a redshift. Strong gravitational fields can have this effect. Some astronomers used to argue

this was a possible explanation for the redshifts of some galaxies.

However the consensus now is that it's the expansion of space that's by far the most likely cause of the redshifts of galaxies.

Answer 2

Red shift is the phenomenon in which light from distant galaxies appears to be shifted towards longer (redder) wavelengths as they move away from us. This effect is a key piece of evidence for the expansion of the universe, as it indicates that galaxies are moving away from each other and suggests an overall increase in the universe's size over time.