The expanding universe

Question 1

What is the doppler effect?

Answer 1

The Doppler effect is the apparent change in wavelength (and
therefore frequency) observed when a source of a wave is moving
relative to a stationary observer.

Question 2

what is fo, fs, v and vs

Answer 2

fo-observed frequency
fa-frequency emitted by source
v-speed of sound (340ms-1)
vs-speed of source

Question 3

When do you use - and + for the doppler effect?

Answer 3

• if towards observer (negative if a car is coming at you)

+ if away from observer

Question 4

What is redshift?

Answer 4

• The Doppler effect can be applied for moving objects that emit light.

• Light from objects moving away from us is shifted to longer
wavelengths. This is known as redshift.

Question 5

How do you calculate the redshift of a galaxy?

Answer 5

• To calculate the redshift of a galaxy, you divide the change in
wavelength of light by the wavelength of light emitted by the galaxy.

𝑧 =
𝜆𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑− 𝜆𝑟𝑒𝑠𝑡/𝜆𝑟𝑒𝑠𝑡

Also, for slowly moving galaxies, redshift can be determined by the
ratio of the velocity of the galaxy to the velocity of light.

𝑧 =𝑣/𝑐

Question 6

What is the Hubble-Lemaitre Law?

Answer 6

When Hubble plotted the velocities of the galaxies against their distances
away from us, he found that they were directly proportional to each other,
with the constant of proportionality known as the Hubble constant, H0.

Ho=gradient of v against d

v=Hod

Question 7

What does the hubble-lemaitre law allow us to do?

Answer 7

estimate the age of the universe

t=1/Ho —> 13.8 billion years

Question 8

What is dark energy?

Answer 8

Dark energy is the name given to the unknown force which causes the
accelerating rate of expansion of the universe.

(dark energy acts against the force of gravity)

Question 9

What can the mass of a galaxy be estimated by?

Answer 9

the orbital speed of stars within it

Question 10

Where does evidence supporting the existence of dark matter come from?

Answer 10

estimations of the mass of galaxies

(Stars on the outside of a galaxy should travel slower than those in the
centre as they are further away from the centre. This does not
happen- there must be more mass we can’t see holding onto
the galaxy. Ewe call this dark matter.)

Question 11

What is the temperature of stellar objects related to?

Answer 11

the distribution of emitted radiation over a wide range of wavelengths

Question 12

Is the peak wavelength of this distribution shorter or longer for hotter objects?

Are their peaks higher or lower?

Answer 12

shorter (they emit shorter wavelengths)

with a higher peak (hotter stars emit more radiation per unit area per unit time than cool ones)

Question 13

What is the relationship between the peak wavelength and the surface temperature (in K) of a star approximately?

Answer 13

𝑇 × 𝜆𝑝𝑒𝑎𝑘 = 2.9 × 10−3

Question 14

What is evidence for the big bang theory?

Answer 14

1. The expanding universe- Large number of galaxies showing redshift
   (rather than blueshift)
2. Cosmic microwave background radiation (CMBR)
3. The darkness of the sky (Olber’s paradox)
4. The abundance of the elements Hydrogen and Helium in the
   universe