Chapter 20 - Cosmology

Question 1

Doppler Effect

Answer 1

The change in detected wavelength and frequency of a wave as it moves relative to an observer
(you appear to see more frequent wavefronts as they come from gradually closer)

Question 2

Star moving closer

Answer 2

Shorter wavelength, higher frequency, blue-shifted

Question 3

Star moving away

Answer 3

Longer wavelength, lower frequency, red-shifted

Question 4

Doppler shift in starlight

Answer 4

The shift in the spectral lines of an absorption spectra of a galaxy compared to a sample in a lab are due to the relative motion of the galaxy compared to Earth

Question 5

Doppler effect equation

Answer 5

Δλ/λ0 = v/c

where Δλ is the shift in wavelength and λ0 is the original wavelength

Question 6

Astronomical Unit (AU) and value

Answer 6

The average distance from the Earth to the Sun

1.5 x 10^11 m

Question 7

Light Year (ly)

Answer 7

Distance travelled by light in a vacuum in 1 year (9.46 x 10^15 m)

Question 8

Parsec (Pc)

Answer 8

The distance at which a radius of 1 AU subtends an angle of 1 arcsecond (3.1 x 10^16 m)

Question 9

1 arcminute

Answer 9

(1/60)°

Question 10

1 arcsecond

Answer 10

(1/3600)°

Question 11

Angle in arcseconds formula

Answer 11

θ = 1/d (where d is the distance in parsecs)

Question 12

Parallax

Answer 12

The apparent shift in position of a relatively close star compared to a background of much more distant stars as the Earth orbits the Sun

Question 13

Using stellar parallax to determine the position of the star

Answer 13

• When the Earth is at opposite sides of the Sun, a star is viewed
• The parallax angle is the angle between one of these and the line from the star to the Sun (1/2 x the difference between the angles)
• The distance in parsecs is 1/p where p is the parallax angle

Question 14

Hubble’s observations from absorption spectra

Answer 14

1. Light from the majority of galaxies was red shifted

2. In general, the red-shift observed was greater for more distant galaxies

Question 15

Hubble’s law

Answer 15

The recessional speed of a galaxy from Earth (v) is directly proportional to its distance from the Earth (d)

Question 16

Hubble’s law equation

Answer 16

v = H d

0

Question 17

Cosmological Principle

Answer 17

When viewed at a large enough scale, the Universe is both homogenous and isotropic and the laws of physics are universal

Question 18

Homogenous

Answer 18

Matter is evenly distributed across the universe. For a very large volume, the density of the universe is uniform. This means that the same type of structures are seen everywhere

Question 19

Isotropic

Answer 19

The universe looks the same in all directions to every observer. This means that there is no centre or edge to the universe

Question 20

Big Bang Theory

Answer 20

Suggests that all matter was contained in an infinitely dense and hot singularity that expanded outwards

Question 21

Cosmic Microwave Background Radiation (CMBR)

Answer 21

Emitted by the plasma that made up the universe 13.5 billion years ago
Space was saturated with high-energy gamma photons that have red-shifted to microwaves as the universe expanded
The universe is around 2.7K, giving a wavelength of approximately 1mm
Observed to have uniform intensity

Question 22

Age of the universe from Hubble’s constant

Answer 22

≈ 1/H0

Question 23

First second after the big bang

Answer 23

The universe expands rapidly, a phase of incredible acceleration called inflation - full of gamma photons at 10^29K
The first fundamental particles gain their mass and the quarks then combine to form the first hadrons in pair-production
After 1 second the creation of matter stops, 10^9K

Question 24

More than 1 second after the big bang

Answer 24

Protons and neutrons fuse into deuterium and helium nuclei + some lithium and beryllium, too fast to form any heavier elements
The universe eventually cools and atoms form, the nuclei capture electrons and CMBR is emitted
The first stars appear allowing heavier elements to fuse before clouds of hydrogen are pulled together to form galaxies

Question 25

Accelerating Universe

Answer 25

Type 1A supernovae were observed to produce a less intense kind of light than expected. There must be a type of energy to produce this “dark energy”

Question 26

Dark matter

Answer 26

The velocity of stars wasn’t observed to decrease as they moved from the centre of a galaxy
This suggests that the mass of the galaxy is not concentrated in the centre as the behaviour was not observed in other gravitational systems
Dark matter must be spread through the galaxy - cannot be seen by telescopes and neither absorbs nor emits light