Universe

Question 1

Evidence for Big Bang

Answer 1

1. Redshift - spectra from galaxies shift to longer wavelengths so are moving away from us.
2. Cosmic Microwave Background Radiation: 2.7K, almost perfectly uniform, is the cooled remnants of the universe at its beginning due to expansion.
3. Primordial Helium/Lithium: Distant, young stars are only light elements produced in early universe.
4. Ripples/variations in microwave background: slight concentrations lead to galaxies etc

Question 2

Describe how a star is formed

Answer 2

There is gravitational attraction between interstellar dust and gas.
The gravitational collapse causes the gas to heat up & have more KE and lose PE.
At ~10^7K H NUCLEI fuse to form He and other light elements and this increases temp further
Stable when grav. forces balances radiation pressure from the photons

Question 3

Describe the Sun’s Evolution

Answer 3

When H runs out, outward pressure stops so core contracts & heats up more.
This increase in pressure causes more fusion (from He etc) which causes the OUTER LAYERS to expand more into a Red Giant.
The core continues to collapse and reaches about 10^8K. Then: helium flash: ejection of material as planetary nebula.
This leaves a white dwarf (stable because of Fermi pressure)

Question 4

Describe a large star’s evolution

Answer 4

Same as Sun except it becomes a super red giant.
The core then collapses to form a white dwarf where grav. pressure can overcome Fermi pressure.
Electrons combine with protons to form neutrons (& neutrinos escape).
Outer shells surrounding this core collapse against the core generating shockwave, exploding surface layers as a supernova.
This leaves either a neutron star or a black hole.

Question 5

What is the principle content of the universe?

Answer 5

Stars, Galaxies, Radiation, asteroids, planetary satellites, comets.

Question 6

Define the parsec and the value of 1pc in metres

Answer 6

1 pc is the distance that gives a parallax angle of 1 arc second. 3.1x10^16m

Question 7

State the cosmological principal

Answer 7

The universe is homogeneous: On a large scale, the density of the universe is the same everywhere.
Isotropic: Universe is the same in all directions.
Laws of Physics are universal, they apply everywhere.

Question 8

State Olbers’ Paradox

Answer 8

For an infinite, uniform and static universe, the night sky should be bright because of light received from stars in all directions.

Question 9

State Hubble’s Law

Answer 9

The speed of recession of a galaxy is directly proportional to its distance from us.

Question 10

What is the significance of the microwave background radiation?

Answer 10

It’s a continuous spectrum at about 2.7K of EM radiation from the Big Bang.

Question 11

Describe the evolution of the universe

Answer 11

At 10^-43s, all 4 forces are unified.
At 10^-6s sea of quarks and leptons
At 10^-3s quarks combine to from hadrons
At 10^2s Primordial helium nuclei form by fusion.
At 3x10^5 years, atoms form as electrons combine with nuclei.
To now, galaxies, stars, planets form, temp of 2.7K

Question 12

Define critical density

Answer 12

The density of the universe for a flat evolution of the universe: the rate of expansion will tend to zero.