Doppler Effect and hubbles law

doppler effect

when a wave source moves towards an observer, the frequency and wavelength of waves recieved by the observer change compared to what would be observed without relative motion

.walength of waves from wave source moving towards an observer decreases (frequency increases)

.wavelength of waves from a wave source moving away from an observer increases (frequency decreases)

Doppler shift in starlight? (4)

. the doppler effect can be used to find the relative velociy of a distant galaxy

1) 1st step: the absorption spectrum of a specific element is determined in a laboratory

2) the same spectrum is observed in light from a distant galaxy

3)any difference in observed wavelength must be caused by relative motion between the earth and galaxy

4) if galaxy is moving towards the earth the absorption lines will be blue shifted, they move towards the blue end of the spectrum (as wavelength appears to be shorter)

5) if the galaxy is moving away from the earth the absorption line will be red-shifted, they move towards the red end of the spectrum, because the wavelength appears stretched

doppler equation (can use for any EM wave)

Hubbles law

the resessional velocity of a galaxy is almost directly proportional to the distance from the earth. V proportional to d

two observations hubble made:

1)the light from the vast majority of galaxies was red shifted,they had a relative velocity away from the earth

2) in general the further away the galaxy was the greater the observed red shift and so the faster the galaxy was moving

Hubbles equation

V=H_od

H_o= Hubbles constant

v= recessional speed of galaxy

d= distance from earth

what evidence does the hubbles law provide?

further evidence that the universe is expanding

cosmological principle

when viewed on a large enough scale, the universe is homogenous and isotropic and the laws of physics are universal

homogenous

matter is distributed uniformly across the universe. For a very large volume the density of the universe is uniform

Isotropic

the universe looks the same in all directions to every observer