Cosmology

Question 1

arcminute

Answer 1

An astronomical unit of angle equal to 1/60th of a degree

Question 2

arcsecond

Answer 2

An astronomical unit of angle equal to 1/60th of an arcminute (1/3600th of a degree)

Question 3

cosmological principle

Answer 3

The idea that the universe is homogenous and isotropic on a large scale and that the laws of physics are the same everywhere in the universe.

Question 4

homogeneous

Answer 4

The idea that matter (on a large scale) is distributed evenly across the universe, which therefore has approximately constant density (the Universe is invariant under translation)

Question 5

isotropic

Answer 5

The idea that (on a large scale) the universe looks the same in all directions to every observer (there is no centre or edge to the Universe)

Question 6

light-year (ly)

Answer 6

The distance travelled by light in a vacuum in a time of 1 Earth year.

Question 7

parsec (pc)

Answer 7

A unit of astronomical distance (equal to about 3.26 ly). It is the distance to a star with a parallax angle of exactly one arcsecond.

Question 8

Stellar parallax

Answer 8

The apparent motion of ‘nearby’ stars against the background of ‘distant’ stars due to the Earth’s motion around the Sun. Can be used to calculate distances up to 100pc. Beyond this distance, angles are too small to be measured accurately.

Question 9

parallax angle (p)

Answer 9

Half the angular displacement in the apparent position of a nearby star observed from Earth over a period of six months. (using the largest shift in any six month period)

Question 10

Doppler effect

Answer 10

The apparent change in frequency and wavelength of waves measured by an observer due the relative motion between the source of waves and the observer.

Question 11

blue-shift

Answer 11

The increase in frequency (and decrease in wavelength) of visible light observed when the source is moving towards the observer

Question 12

red-shift

Answer 12

The decrease in frequency (and increase in wavelength) of visible light observed when the source is moving away from the observer

Question 13

Big Bang

Answer 13

The leading theory to describe the origin of the Universe in which space and time expanded from a singularity approximately 13.7 billion years ago

Question 14

Hubble’s Law

Answer 14

The recessional speed of distant galaxies is directly proportional to their distance from us
v = Hₒ x d

Question 15

Hubble Constant

Answer 15

The gradient of the best-fit line for a plot of recessional speed vs distance, where speed is measured in km/s and distance in megaparsec (Mpc). Current best value Hₒ= 65kms^-1 Mpc^-1

Question 16

Cosmic Microwave Background Radiation (CMBR)

Answer 16

The largely uniform microwave radiation with the blackbody spectrum of an object at 2.7K, detected in every direction in the sky. It is consistent with a hot and dense initial state for the universe, which then cooled down to the current temperature of 2.7K. The CMBR is the echo of the Big Bang when it was 380 000 years old.

Question 17

(Cosmic) inflation

Answer 17

A period very early on (10^-35 s) in the evolution of the universe during which an unknown mechanism appears to have caused a very rapid expansion of the universe

Question 18

Dark matter

Answer 18

A hypothetical form of matter that doesn’t emit or absorb light, postulated to explain the difference between the expected and observed orbital velocity of stars around the galactic centre

Question 19

Dark energy

Answer 19

A hypothetical form of energy postulated to explain the observed acceleration in the expansion of the Universe

Question 20

Astronomical unit (AU)

Answer 20

The mean distance from the Earth to the Sun. 1 AU = 1.5 x 10¹¹ m

Question 21

Stellar parallax equation

Answer 21

p = 1/d, where p is the parallax angle in arcseconds and d is the distance in parsec

Question 22

Doppler equation

Answer 22

Δλ/λ = Δf/f = v/c, for a source of electromagnetic radiation moving relative to an observer with velocity v. v is the radial velocity (along the line of sight)

Question 23

Equation for estimation for the age of the universe

Answer 23

t ≈ 1/Hₒ

Question 24

Composition of the universe

Answer 24

The universe is made up of dark energy (68.3%), dark matter (26.8%), and a small percentage of ordinary matter (4.9%).

Question 25

Atoms could not form until 380 000 years after the Big Bang because …

Answer 25

The universe was too hot so atoms were ionised. Back then, the universe was a mixture of ionised atoms, electrons and photons, constantly scattering against each other.

Question 26

How can the observations of star velocities in galaxies be explained by the presence of dark matter?

Answer 26

Dark matter has mass and may be distributed throughout the galaxy rather than concentrated at the centre, allowing for the flattening of the graph which plots orbital speed of stars versus their distance from the galactic centre.

Question 27

State 2 pieces of evidence in support of the Big Bang

Answer 27

1. Expanding universe as shown by red shift and Hubble’s law.
2. Cosmic microwave background radiation

Question 28

Use Hubble’s law to estimate the age of the universe

Answer 28

1.5 x 10^10 years = 15 billion years

Question 29

Evolution of the Universe in steps

Answer 29

The Big Bang: Time and space are created; the universe is a dense, hot singularity.
- 10-35s after the Big Bang: The universe expands rapidly, in a period of incredible acceleration known as “inflation”. There is no matter, only high energy gamma photons and electromagnetic radiation.
- 10-6s after the Big Bang: The first fundamental particles gain mass. The mechanism behind this is not fully understood but it involves the Higgs Boson.
- 10-3s after the Big Bang: Most of the mass is created using pair production (high-energy photons combining into particle-anti particle pairs). The first hadrons come from quarks combining together.
- 1s after the Big Bang: Production of mass is halted.
- 100s after the Big Bang: Protons and neutrons fuse to form deuterium, helium, lithium and beryllium nuclei, but nothing heavier. Rapid expansion continues. 25% of matter is helium nuclei.
- 380 thousand years after the Big Bang: It is now cool enough for the first atoms to form. So photons can now freely propagate through space (this is the CMBR)
- 30 million years after the Big Bang: The first stars form, and fusion creates heavier elements.
- 200 million years after the Big Bang: Our galaxy forms as gravitational forces pull together clouds of hydrogen and existing stars.
- 9 billion years after the Big Bang: The solar system forms by a nebula left by a supernova. This is followed by the formation of our sun, and then the Earth almost 1billion years later.
- 11 billion years after the Big Bang: Primitive life begins on Earth.
- 13.7 billion years after the Big Bang: The first modern humans evolve.
(13.7 billion years is the current best estimate of the age of the Universe)

Question 30

Recessional speed

Answer 30

is the rate at which a galaxy recedes (becomes more distant) from an observer as a result of the expansion of the universe.