Astrophysics (Cosmology) Flashcards
Describe the Doppler effect.
A change in frequency (or wavelength) due to relative motion between a source and an observer.
If a light source is receding (moving away from us), describe what happens to its wavelength and frequency. Give the name for this.
Wavelength increases (wave stretch out).
Frequency decreases.
This is red shift.
If a light source is moving towards us, describe what happens to its wavelength and frequency. Give the name for this.
Wavelength decreases (wave compress).
Frequency increases.
This is blue shift.
Give the equation for red shift (z) in terms of recessional velocity (v) and speed of light (c). What do positive and negative values represent?
Positive z = light source moving away from us.
Negative z = light source moving towards us.
How can red and blue shift be observed?
Measure the wavelength or frequency of known absorption lines in the observed spectrum.
Compare the observed values and the laboratory values.
Give the equation for red shift (z) in terms of frequency. Define any terms.
z = Δf / f
Δf = f - fobs
f = actual frequency emitted (given by laboratory value)
fobs = observed frequency (from spectrum)
Give the equation for red shift (z) in terms of wavelength. Define any terms.
z = -Δλ / λ
Δλ = λ - λobs
λ = actual wavelength emitted (given by laboratory value)
λobs = observed wavelength (from spectrum)
Describe cosmological red shift.
Red shift from distant galaxies moving away from us due to the expansion of the Universe, as opposed to moving through space away from us.
Describe and explain the relationship between the distance a galaxy is from Earth and the cosmological red shift.
The more distant the galaxy, the greater the cosmological red shift, as they are moving away from us faster.
What is happening to the size of the Universe?
The Universe is expanding at an increasing rate.
How did Edwin Hubble calculate the distance to distant galaxies?
Using Type 1a supernovae as standard candles
How did Edwin Hubble calculate the recessional velocity of distant galaxies?
Measured red shift (z) of spectral lines.
Calculated recessional velocity using v = zc.
What relationship was found between recessional velocity and distance from Earth of distant galaxies?
Recessional velocity is directly proportional to distance from Earth.
Give the equation that represents Hubble’s law.
v = Hd
Define the Hubble constant.
H = v/d, so H is the ratio of the recessional velocity of distant galaxies to their distance from Earth.
Give the value for the Hubble constant from the data sheet including units.
H = 65 kms-1Mpc-1
How can the Hubble constant be used to approximate the age of the Universe?
At the start of time, the distance between the galaxies was zero.
Age of Universe = time = distance/speed = d/v = 1/H
Why does the calculation give an approximate age?
It assumes the expansion rate and the Hubble constant are constant over the lifetime of the Universe.
What is happening to the rate of expansion of the Universe?
Rate of expansion is increasing (i.e. an accelerating expansion).
What is dark energy?
An energy filling the whole of space that has been suggested to explain the accelerating expansion of the Universe.
Describe the Big Bang Theory.
The universe started off from an extremely hot and dense point, and has been expanding ever since.
Give three pieces of evidence for the Big Bang Theory.
Cosmological red shift of distant galaxies.
Cosmic microwaves background radiation.
Relative abundance of H and He.
What is cosmic microwave background radiation (CMBR) and how does it support the Big Bang theory?
Big Bang theory predicts lots of electromagnetic radiation was produced in the very early universe.
This cosmic background radiation leftover from the Big Bang can still be observed today.
As the universe has expanded, the wavelengths of this cosmic background radiation have been red-shifted and are now in the microwave region.
Isotropic and homogeneous which agrees with the cosmological principle.
CMBR has a black body spectrum corresponding to a temperature of 2.7 K.
What is the relative abundance of H and He in the Universe and how does it support the Big Bang theory?
Big Bang theory suggests that a very brief period of fusion occurred in the very early universe, resulting in the production of helium from fusing hydrogen.
Fusion stopped as the universe then expanded and cooled.
Therefore, relative abundance of hydrogen and helium in the ratio of 3:1
What property of quasars led to their discovery?
They are very powerful radio sources.
Why do quasars resemble stars?
They are point-like sources of light.
Where are quasars found?
At the centre of active galaxies.
Describe the structure of a quasar.
Produced by a supermassive black hole at its centre.
Surrounded by an accretion disc of rotating matter that is drawn into it, causing a very high power output of radiation
What do you know about the distance of quasars from Earth and what is the evidence for this?
Quasars are very distant.
Evidence is the large red-shift in light from the galaxy.
Why do quasars outshine their host galaxies?
Very bright absolute magnitude due to very large power output.
Approximately 1042W.
Why is the power output of quasars so high?
As matter falls into the supermassive black hole, it forms an accretion disc.
This is a rotating disc of matter which heats up due to friction and emits intense radiation.
Some quasars also emit jets from their poles - making them very powerful radio sources.
What do you know about the size of quasars and what is the evidence for this?
Quasars are small in size (approximately size of a solar system).
Evidence is the rapid fluctuations in their power output.
Why do quasars appear faint even though they have a very bright absolute magnitude?
Very distant from Earth.
Inverse square law means that the intensity is greatly reduced.
How do quasars form?
The merging of two galaxies leads to the formation of quasars.
Matter moves towards the supermassive black hole.
The supermassive black hole becomes active, drawing in nearby matter and emitting intense radiation.
