What is cosmological redshift?

The dimming of light from distant galaxies due to absorption by intergalactic dust The stretching of light to longer (redder) wavelengths caused by the expansion of space itself as the light travels through the universe The Doppler shift of light from galaxies moving away from us through space The change in color of starlight as it passes through dust in a galaxy's disk

How is cosmological redshift fundamentally different from the Doppler effect?

They are identical effects — cosmological redshift is just a type of Doppler shift for very fast-moving galaxies Doppler shift changes the frequency but not the wavelength; cosmological redshift changes only the wavelength Cosmological redshift is caused by space itself stretching between the galaxy and us, not by the galaxy moving through space Cosmological redshift only applies to radio waves, while Doppler shift applies to all light

What is the redshift parameter z, and how is it calculated?

z = observed frequency divided by emitted frequency z = the speed of the galaxy divided by the speed of light z = (observed wavelength minus emitted wavelength) divided by the emitted wavelength z = the distance to the galaxy in megaparsecs divided by the Hubble constant

If a galaxy has a redshift of z=1, what does this mean for the light we receive from it?

The galaxy is moving away at exactly the speed of light The light's wavelength has doubled during its journey — the universe was half its current size when the light was emitted The light has been completely absorbed and re-emitted once on its way to us The galaxy is 1 billion light-years away, because z equals distance in billions of light-years

What does a galaxy with a redshift of z=7 tell us?

The galaxy is 7 billion light-years away and was observed 7 billion years ago The galaxy is moving away at 7 times the speed of light, violating relativity Its light wavelength was stretched by a factor of 8 during travel, and we see it as it was about 800 million years after the Big Bang The light has passed through 7 galaxy clusters that each added a redshift of 1

Why does the Andromeda galaxy (M31) show a blueshift rather than a redshift?

Andromeda is forming stars so rapidly that it emits mostly blue light Andromeda is falling toward a large mass on the other side of the universe, coincidentally pointing it at us Andromeda is close enough that its peculiar velocity (gravitational attraction toward us at ~110 km/s) overcomes the general expansion of the universe Andromeda is inside the Milky Way and has no cosmological redshift

How is redshift used as a proxy for both distance and lookback time?

Redshift can only measure velocity, not distance — a separate method is needed for distance Higher redshift means the universe was smaller when the light was emitted — so higher z corresponds to greater distance and earlier cosmic time Redshift measures distance but not time — all distant galaxies look the same age Lower redshift means an older, more distant galaxy, because nearby galaxies have had more time to accelerate

What does it mean when astronomers say a galaxy has a peculiar velocity?

A galaxy whose light is unusually bright, indicating it is a quasar A galaxy whose light has an unusual spectral type, suggesting exotic composition A motion through space due to local gravitational influences, distinct from the overall expansion of the universe A galaxy orbiting another galaxy at a velocity that seems too fast for its mass

If astronomers detect a spectral line at 700 nm (red light) that was originally emitted at 500 nm (green light), what is the redshift z?

z = 1.4, because 700 divided by 500 equals 1.4 z = 200, because the wavelength increased by 200 nm z = 0.4, because (700 - 500) / 500 = 200 / 500 = 0.4 z = 0.7, because 500 divided by 700 equals approximately 0.7

Why do cosmological recession velocities sometimes appear to exceed the speed of light, and does this violate Einstein's relativity?

It does violate relativity — cosmological recession is an exception Einstein specifically allowed It does not violate relativity because it is space itself expanding — no object is moving through space faster than light Recession velocities can never exceed the speed of light — this is a common misconception about redshift It violates special relativity but not general relativity, and Einstein was troubled by this finding