Cosmology

Question 1

Doppler effect

Answer 1

-the ​compression or spreading out of waves​ that are emitted or reflected by a ​moving source​.
-as source is moving, wavelengths in front of it compressed and wavelengths behind are spread out
-e.g hearing sound of a car moving past you.

Question 2

Blue shift

Answer 2

blue end of the visible spectrum when they move ​towards the Earth​

Question 3

Red shift

Answer 3

red end of the spectrum when they move ​away from the Earth​

Red-shift is used as evidence for the ​expanding universe​, as distant objects are red-shifted.
The more distant the object, the ​greater its red-shift​.

Question 4

Red shift equation

Answer 4

z = v/c = Δf/f = − Δλ/λ

v = object’s receding velocity (m/s)
c = the speed of light (m/s)
∆f = change in frequency(Hz)
f = original frequency (Hz)
∆λ = change in wavelength (m)
λ = original wavelength (m)

The ​wavelength ratio is negative​ because ​wavelength is inversely proportional to frequency – as the frequency of a wave increases, the wavelength decreases.
The ​z value (also known as red shift)​ ​is positive ​for ​red-shift​, and ​is negative​ for ​blue-shift​.

Question 5

Spectroscopic binaries​

Answer 5

binary star systems in which the stars are too close to be resolved​ by a telescope, meaning the only way to identify them is by using the Doppler shifts of each star.

Question 6

Eclipsing binaries​

Answer 6

when the plane of the orbit of the stars is in the line of sight from Earth to the system, meaning that the stars ​cross in front of each other​ as they orbit.
these can be identified from their characteristic light curves

Question 7

Quasars ​

Answer 7

objects which have ​very large red shifts​, suggesting they’re far away, however they’re also bright.
using ​inverse square law​, can show that power output of a quasar must be around that of entire galaxy.

Question 8

Typical light curve of eclipsing binaries:

Answer 8

Question 9

Hubble’s law​

Answer 9

states that ​galaxy’s recessional velocity is directly proportional to its distance from the Earth​.
essentially states that ​universe is expanding from common starting point​.

Question 10

Hubbles law formula

Answer 10

v = Hd
v is recessional velocity (km s-​ 1)​
H is Hubble constant (65 km s-​ 1​ Mpc-​ 1,​ also sometimes written as H0​ )​ ,
d is distance from Earth (​Mpc = megaparsec​)

Question 11

How to estimate age of universe

Answer 11

1. Rearrange​ v = Hd as follows:
2. Using ​distance = velocity x time​ (rearranged to​ t = dv ) ~ t=1/h
   3.convert H to ​SI units​:
   a. Multiply by 1000 ​converting from km to m
   b. Divide by 1 Mpc (megaparsec) in metres, to ​convert from parsecs to metres
3. You can now substitute your value of H in SI units, into the equation derived above.
4. Convert from ​seconds to years​: ÷ 3600 ÷ 24 ÷ 365

Question 12

Big bang evidence

Answer 12

As universe currently expanding, reasonable to assume universe began from ​one point​ – ​singularity that was infinitely small and hot​.
​Big Bang Theory​ suggests universe began with a ​huge explosion​ from this point.
When Big Bang happened, there was high-energy radiation everywhere, and as universe expanded and cooled, radiation would have lost energy and been red-shifted.
The remains of radiation is called Cosmological Microwave Background Radiation (CMBR)​, which is microwave radiation that has been detected from space.
During early stages of Big Bang,​ nuclear fusion​ converted hydrogen nuclei into helium nuclei.
However, only lasted for short period of time before universe cooled too much and nuclear fusion stopped. ​
Approximately 1⁄4 ​of all existing hydrogen nuclei fused into helium, resulting in relative abundance ratio of H:He of 3:1​.
The relative abundance by mass of different elements observed today is approx 73% hydrogen, 25% helium and 2% everything else​.

Question 13

Quasars definition

Answer 13

A ​quasar​ is ​active galactic nucleus​ – ​supermassive black hole surrounded by disc of matter​ ​which, as it falls into the black hole, causes​ ​jets of radiation​ to be emitted from poles.

Question 14

Quasars are characterised by the following features:

Answer 14

● Extremely ​large optical red-shifts
● Very​ ​powerful​ light output
● Their size being ​not much bigger than a star

Question 15

Quasars explanation

Answer 15

Quasars are thought to be some of the ​most distant measurable objects​ in the known universe.
-inverse square law shows that quasars are ​extremely powerful​ and can have ​same energy output as several galaxies​.
-first quasar discovered was 3C 273, which was thought to be dim star, but had ​greater radio emissions than expected​.
-later found to be 26 billion light years away and 1000 times more luminous than Milky Way​.

Question 16

Exoplanets​

Answer 16

planets that are not within our solar system​; they orbit other stars.
can be difficult to detect directly as they tend to be ​obscured by light of their host stars​.

Question 17

Methods : ➔ Radial velocity method​

Answer 17

-similar to method of detecting spectroscopic binaries.
- star and planet orbit ​common centre of mass​, which causes the star to ​‘wobble’​ ​slightly.
- ​causes Doppler shift in light received from star ​( ‘wobble effect’ )
- effect is most noticeable with ​high-mass planets since they have greater gravitational pull on the star.
- line spectrum of star is blue-shifted when it moves towards Earth, then red-shifted when it moves away.
- shows there is something else near star that is exerting gravitational force on it – exoplanet.
-​ time period (T)​ of planet’s orbit​ equal to time period of Doppler shift​.

Question 18

➔ Transit method​

Answer 18

• involves observing ​intensity of light output​ ​of star.
  -if planet ​crosses in front of star ​(​‘transits’​), intensity dips slightly.
  -if intensity of a star dips ​regularly​, it could be sign there is exoplanet orbiting it.
  -if there are variations in regularity of dips, may be several planets orbiting same star which have gravitational effect on transiting planet.
• ​size​ and​ orbital period​ of planet can be determined from ​amount​ that intensity falls by and ​duration​ of dip
  -the application of method is limited since ​only works if line of sight to star in plane of planet’s orbit​, which ​more likely for planets with small orbits​.
  This is because most orbits are ​inclined​ (do not pass in front of the star when observed from the Earth), and smaller orbits mean that parts of the planet are more likely to cross in front of the star and block some of its light.

Question 19

Typical light curve of a star with a transiting planet:

Answer 19