Cosmology Flashcards
Doppler effect
-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.
Blue shift
blue end of the visible spectrum when they move towards the Earth
Red shift
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.
Red shift equation
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.
Spectroscopic binaries
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.
Eclipsing binaries
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
Quasars
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.
Typical light curve of eclipsing binaries:
Hubble’s law
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.
Hubbles law formula
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)
How to estimate age of universe
- Rearrange v = Hd as follows:
- 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 - You can now substitute your value of H in SI units, into the equation derived above.
- Convert from seconds to years: ÷ 3600 ÷ 24 ÷ 365
Big bang evidence
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.
Quasars definition
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.
Quasars are characterised by the following features:
● Extremely large optical red-shifts
● Very powerful light output
● Their size being not much bigger than a star
Quasars explanation
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.
Exoplanets
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.
Methods : ➔ Radial velocity method
-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.
➔ Transit method
- 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.
Typical light curve of a star with a transiting planet:
