Cosmology Flashcards
What is the equation for doppler shift?
Δf/f = Δλ/λ = v/c
Δf/λ = change in frequency/wavelength (Hz)
f/λ = reference frequency / reference wavelength (frequency/ wavelength of the source)
Δv = relative velocity of the source (- the observer’s velocity)
c = speed of light
What is redshift?
The wavelength of light is stretched, so that the light is seen as ‘shifted’ towards the red part of the spectrum.
z = Δλ/λ = - v/c
(z, the measure of redshift, is positive if a star is moving away from Earth)
What is an eclipsing binary star system?
Where 2 stars orbit around a common centre of mass with their orbital plane in the Earth’s line of sight.
What does the light intensity curve of orbiting eclipsing binary stars look like?
1- intensity is at full brightness to begin with
2 - then big star blocks little star (so all its light it blocked) (primary minima)
3 - back to normal
4 - little star blocks big star so there is less of a dip in intensity (secondary minima)
5 - back to normal
What does the Hubble graph look like?
recession velocity on y axis and distance on x axis.
They are directly proportional - straight line diagram.
What is Hubble’s law? (written)
The recessional velocity of a galaxy is proportional to its distance from Earth
What is Hubble’s law? (equation)
v = Hd
v = recessional velocity of an object (km/s)
H = Hubble constant (km/sMp/c)
d = distance between the object and the Earth (Mpc)
What does Hubble’s law and diagram show us?
That the furthest galaxies are moving away faster.
What is the Hubble constant?
H = 67.4 +/- 0.5 (km/s Mp/c)
in data sheet = 65
How can Hubble’s constant be used to find the age of the universe?
Hubble’s constant tells us the speed of an object at any distance, and since the distance between all t=objects in the universe at beginning of expansion was 0, the time in this equation must be the age of the universe. so 1/H = estimate of the universe
How old is the universe?
13.7 billion years old
What is the big bang theory?
The most renowned theory for the beginning of the universe.
- All matter at t=0 was in one hot, dense singularity and since then has been expanding and cooling
What are 3 key pieces of evidence for the big bang theory?
1 - Galactic redshift & Hubble’s law = evidence for expansion of universe
2 - CMBR - evidence that the Universe had a hot beginning
3 - relative abundance of hydrogen and helium = evidence the Universe as once far hotter and denser than it is now.
How does galactic redshift provide evidence for the expansion of the universe?
1 - observations show that distance galaxies are all moving away from us = the stretch of the wavelengths due to the expansion of the universe
2 - Galaxies further away moving away faster - v ∝ d = means at some point in the past galaxies must have been at the same point.
What is cosmic microwave background radiation?
The radiation detected in all parts of the Universe. (in the microwave region of the electromagnetic spectrum)
How does CMBR provide evidence for a hot beginning?
- Theory predicts the existence of a uniform black body radiation that peaks in the microwave region - extremely uniform, meaning the universe was initially much smaller
- CMBR is consistent with radiation that has been redshifted overtime - used to be producing gamma rays (when universe was high energy and hot), wavelengths stretched overtime
- radiation left over from big bang
How does the relative abundance of hydrogen and helium prove that the universe was originally extremely hot?
- Theory predicts hydrogen fused into helium nuclei in early universe (as it was so hot)
- 3:1 ratio of hydrogen to helium
What is the process of nucleosynthesis of hydrogen and helium?
- Once protons and neutrons could exist freely in the universe, neutrons would decay into protons (so hydrogen to helium ratio is 3:1 for this reason)
- the universe would have been hot enough to fuse into helium nuclei (free neutrons became stable when bound to helium nuclei
What is the Hubble Tension problem?
The present measure of the expansion rate significantly exceeds the prediction.
What is dark energy?
An unknown source that is said to have possibly sped up the rate of expansion.
Why is dark energy a controversial solution to the Hubble Tension problem?
- It cannot be detected directly but there is evidence for its existence
- Nothing is known of its nature and origins
- No mechanisms can currently explain how it drives the accelerated expansion
What are quasars?
Extremely luminous star-like sources of radiation with very high redshifts.
How were quasars discovered and when?
IN THE 1960s
- strong radio emissions
- High luminosities
- Extremely large redshifts
- Small size
What is the formation of a quasar?
- They are supermassive black holes surrounded by an accretion disc of matter
- They are found at the centre of extremely distant galaxies
- When they become active (when mater falls into it), they become quasars
What is the structure of a quasar?
A supermassive black hole surrounded by an accretion disc and emits jets of radiation at the poles.
What are the most distant measurable objects?
quasars?
Why are quasars strong emitters of all wavelengths?
- Gravitational potential energy of infalling matter is transferred to electromagnetic radiation - so quasars aren’t just strong radio emitters
How do we know that quasars are some of the most distant measurable objects?
- They have extremely large redshifts
- This allows astronomers to see very far back into the early universe so it was not long after the big bang
What is an exoplanet?
A planet found outside out Solar System, in orbit around another star
What are 2 reasons why are exoplanets difficult to detect directly?
1- Light from host star is much brighter than
the reflected light from the planet
2 - they subtend extremely small angles compared to the resolution of telescopes
What are 3 methods to indirectly detect exoplanets?
- transit method
- radial velocity method
- Direct observation
How does the transit method work?
- Look at the points where the light from the star dips, this could be when the planet passes in front of the star and obscures some of the light
What does the transit method also reveal about an exoplanet?
- It’s size and orbital period
What are 2 limitations to the transit method?
- The accuracy can be reduced if the Earth, planet and star are not aligned in the same plane
- Only planets with short orbital periods can be detected
How does the radial velocity method work?
Detects very small changes in the star’s speed caused by gravitational pull of orbiting planets. This creates a periodic shift in the star’s spectral lines.
What is the main limitation of the radial velocity method?
Low-mass or Earth-like planets do not cause so much ‘wobble’ as high-mass planets since they have a greater gravitational pull on the star.
What is the equation for intensity?
power/area
What is the surface area if a sphere?
4πr²
