Chapter 9.3: Cosmology Flashcards
What is the Doppler effect?
The change in wavelength and frequency of a wave as the source moves away or towards the observer.
As the source moves towards the observer, the waves are compressed and wavelength decreases. As the source moves away from the observer, the waves spread out and the wavelength increases.
What is red shift?
Red shift (z) is the shift in wavelength and frequency of waves from a retreating source towards/beyond the red end of the EM spectrum. Cosmological red shift is evidence for the Big Bang.
State Hubble’s law.
The velocity of receding galaxies is proportional to their distance from Earth.
Use Hubble’s law to estimate the age of the universe.
Time = distance/ velocity = 1/H (since v=Hd)
Convert Hubble’s constant to SI units.
65 km/s Mpc x 1000 gives H = 65000 m/s Mpc
Divide by 1 Mpc to get the units for H as per second.
H = 2.11 x 10^-18 so 1/H = 4.74 x 10^17 s
Convert to years: 1.5 x 10^10 years
Draw a simplified light curve for a binary star system.
Look it up
What are quasars? What suggests that they are extremely distant objects?
A quasar is a nucleus of an active galaxy; a supermassive black hole surrounded by a disc of matter. As matter falls into the black hole, jets of radiation are emitted from the poles of the quasar.
Large optical red shift shows quasars are the most distant observable objects. From the inverse square law for intensity we know they are extremely powerful, with the same energy output as several galaxies. They were initially found to be powerful radio sources but with further telescope developments we know they emit all wavelengths of EM radiation.
Give 4 conclusions to draw from quasars and evidence for each claim.
- Come from supermassive black holes = we observe them at the centre of galaxies
- They are distant = large optical red shift
- They are powerful = they are brighter than their host galaxy often obscuring it
- They are small = have variations in their power output
What is the Big Bang Theory? State evidence that leads us to believe this is true.
13.8 billion years ago, universe exploded from an extremely hot and dense point and it still expanding now.
CMBR (cosmological background radiation) is the heat signature left behind from the big bang. EM radiation released in explosion shifted from extremely high energy waves into the microwave region as the universe expanded, stretching out the waves. CMBR has a black body distribution with a peak that corresponds to a temp of 2.7K. There was nuclear fusion of hydrogen into helium which explains the large abundance of helium in today’s universe.
How si the relative abundance of hydrogen and helium used as evidence for the big bang?
During the big bang, hydrogen was converted into helium via nuclear fusion. The expansion of the universe caused it to cool, meaning it was no longer hot enough to fuse elements. About 1/4 of all hydrogen was converted into helium, resulting in a ratio of H:He of 3:1. The observed distribution of matter now is 73% hydrogen, 23% helium and 2% everything else which follows the predicted ratio.
What are exoplanets?
Exoplanets are planets that are not in our solar system.
Direct observations of exoplanets are difficult as their light is often obscured by the stars they orbit. Also they tend to be too close together for the telescope to resolve them.
How can we detect exoplanets?
- Radial velocity method: A star and planet will orbit their common centre of mass, so star will have a tiny “wobble” in its distance from Earth, shown by tiny red and blue shifts in its spectrum.
- Transit method: As a planet moves between the star it orbits and the Earth, the star’s brightness appears to decrease slightly. We can detect this and use it to calculate the diameter of the planet. Mostly only useful for detecting planets with small orbits.
Draw the light curve for an exoplanet transiting in front of a star.
Look it up
