Cosmology Flashcards
Everything you need to know to get A* in OCR Physics A Level, closely following the specification and the textbook
What are the 3 main specialist measurements for distance in space?
astronomical unit (AU), light years and parsec
What is an AU?
It is the distance from Earth to Sun, 1.5 x 10^11
What is a light year?
Distance travelled by light in a vacuum in one year, 9.46 x 10^15
What is a parsec?
The distance at w hich a radius of 1 AU subtends an angle of 1 arcsecond, 3.1 x 10^16
How many arcminutes are in 1 degree?
60 arcminutes
How many arcseconds in 1 arcminute?
60 arseconds
What is 1 arcsecond in degrees?
1/3600
What is the equation with arcsecond, AU and parsecs
tan(1 arcsecond) = 1 AU/1 pc
What is stellar parallax?
A technique used to determine the distance to stars that are relatively close to the Earth by comparing their apparent positions against distant stars at times 6 months apart
What is parallax?
The apparent shift in the position of a relatively close star against the backdrop of more distant stars as the Earth orbits the Sun
What is the equation to calculate the parallax angle, and what units must each quantity be in?
p = 1/d, where p is the parallax angle measured in arcseconds, and d is the distance to the nearby star measured in parsecs
What is the Cosmological Principle?
The assumption that the universe is homogeneous, isotropic, and the laws of physics are universal
What does homogeneous mean?
Matter is distributed uniformly across the universe
What does isotropic mean?
The universe looks the same in all directions to every observer
What is the Doppler effect?
The change in frequency and wavelength of waves received from an object moving relative to an observer compared with what would be observed without relative motion
What change in wavelength is observed on the absorption lines in a spectra when a galaxy is moving towards earth?
The absorption lines will be blue-shifted - they move towards blue end of the spectrum
What change in wavelength is observed on the absorption lines in a spectra when a galaxy is moving away from earth?
The absorption lines will be red-shifted
What is the Doppler equation?
Δλ/λ = Δf/f = v/c
where λ is the source wavelength
f is the source frequency
What is Hubble’s Law?
The recessional speed, v, of a galaxy is almost directly proportional to its distance, d, from earth
What is the equation from Hubbles law?
v = H0 x d
where H0 is Hubble’s constant
What is the model of the expanding universe?
It states that the fabric of space, and time, is expanding in all directions
What is Hubble’s Law key evidence for?
The Bing Bang Theory, and the model of the expanding universe
What is the Big Bang Theory
The theory that at a moment in the past all the matter in the universe was contained in a singularity, the beginning of space and time, that expanded rapidly outwards
What are the 2 key pieces of evidence for the Big Bang Theory?
Hubble’s Law and microwave background radiation
How does Hubble’s Law provide evidence for the Big Bang?
Hubble’s Law shows that the universe is expanding. If we could run time backwards, the Universe would keep shrinking, getting smaller, denser and hotter, until it collapses into a singularity, that, according to the Big Bang Theory, expanded into the universe
What is microwave background radiation?
The microwave signal of uniform intensity detected from all directions of the sky
What two ways does microwave background radiation provide evidence for the Big Bang?
- At the beginning, space was saturated with high energy gamma photons. As universe expanded, space itself stretched over time, stretching the gamma photons which can now be observed as microwaves
- Universe was extremely hot at the beginning. Over time, temperature reduced to 2.7K. Universe may be treated as black body radiator - at this temperature, peak wavelength is in microwave region
How can you calculate the age of the universe?
age of Universe, t = 1/H0
where H0 is Hubble’s constant in units s^-1
Describe the history of the universe, from formation to now
10^-35: rapid expansion, earth full of EM radiation - gamma radiation - and energy. No matter
10^-6: first fundamental particles gain mass
10^-3: first hadrons (protons, neutrons) formed. Most mass formed through pair production
1s: Matter stops being created
100s: Lighter nuclei start forming (deuterium, helium, lithium, beryllium). 25% of matter is helium nuclei
38000 years: atoms start forming
30 million years: first stars form
What causes the expansion of the universe to accelerate?
Dark energy
How much of our universe is dark energy predicted to make up?
68% of our universe
What is dark matter?
A hypothetical form of matter spread throughout the galaxy that neither emits nor absorbs light
What could dark matter provide an explanation for?
The differences between predicted and observed velocities of stars in galaxies
