5.5 Astrophysics (Cosmology, Doppler, Hubble, Expansion Timeline)

Question 1

What does the cosmological principle state?

Answer 1

The universe is isotropic and homogenous, and the laws of physics are universal.

Question 2

What does the word isotropic mean, in its usage for the cosmological principle?

Answer 2

The universe is the same in all directions to every observer, and it has no centre or edge.

Question 3

What does the word homogenous mean, in its usage for the cosmological principle?

Answer 3

Matter is uniformly distributed - for a large volume of the universe the density is the same.

Question 4

Doppler Effect

Answer 4

The apparent shift in wavelength occuring when a source of waves is moving. If the source moves towards the detector, the wavelength appears to decrease (blueshift). If the source is moving away, the wavelength appears to increase (redshift).

Question 5

What effect does the doppler effect have on the position of spectral lines?

Answer 5

It shifts them.

Question 6

Using the doppler equation, what can we determine?
How would we do this?

Answer 6

The relative speed of a star, using the shift in wavelength from a hydrogen emission spectrum.

Question 7

What is the doppler equation?

Answer 7

Δλ / λ = v / c

v: velocity of star relative to earth
λ: original wavelength of hydrogen spectral line
Δλ: change in wavelength of h spectral line from light emitted by star

Question 8

State Hubble’s law

Answer 8

The recessional velocity v of a galaxy is proportional to its distance from earth.

Question 9

Almost all light from distant galaxies is redshifted. What does this mean about the galaxies? What does this suggest?

Answer 9

The galaxies are moving away.
This suggests the universe is expanding, and any point in the universe is moving away from any other point.

Question 10

How can Hubble’s law be used to estimate the age of the universe?

Answer 10

If initially all points in the universe were together, then the distance of a galaxy from Earth, and its speed, are related to the time taken for the galaxy to reach this distance away from the Earth.

The time is given by d/v, which is equal to 1/H0, with H0 measured in seconds. The age is approximately 14 billion years.

Question 11

What are the two key pieces of evidence to support the Big Bang theory?

Answer 11

Hubble’s Law
Microwave Background Radiation

Question 12

What is Cosmological Microwave Background Radiation?

Answer 12

The radiation coming from all parts of the universe that has been red shifted into microwave radiation as the universe expanded.

Question 13

Describe the first half of the evolution of the universe, starting from the big bang and finishing with the production of mass being halted.

Answer 13

Big Bang - Time and space are created; the universe is a dense, hot singularity

10^-35s: The universe expands rapidly, in a period of incredible acceleration known as “inflation”. There is no matter, only high energy gamma photons and EM radiation

10^-16s: First fundamental particles gain mass.

10^-3s: Most of the mass is created using pair production. The first hadrons come from quarks.

1s: Production of mass is halted.

Question 14

Describe the last half of the evolution of the universe, starting from the fusion of protons and neutrons, and ending with the first modern humans.

Answer 14

100s: 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: It is now cool enough for the first atoms to form

30 million years: The first stars form, and fusion creates heavier elements.

200 million years: Our galaxy forms, as gravitational forces pull together clouds of hydrogen and existing stars.

9 billion years: Solar system is formed by a nebula from a supernova. This is followed by the sun, and then the earth almost 1 billion years later.

11 billion years: Primitive life begins on earth

13.7 billion years: First modern humans evolve.

Question 15

What is dark energy?

Answer 15

A hypothetical form of energy that fills all of space, which accelerates the expansion of the universe.

Question 16

Dark energy should make up what percentage of the total energy in the universe?

Answer 16

68

Question 17

Have experiments found dark energy?

Answer 17

No.

Question 18

How did we come to predict the existence of dark matter?

Answer 18

As an object moves away from the centre of a galaxy, we’d asssume the velocity would decrease (due to lower gfield). It was observed this was not always the case.

Some observations suggest that instead of the mass being concentrated in the centre, it is spread out. All observable mass is concentrated in the centre, so there must be another type of matter we can’t see (dark matter).

Question 19

How much of the universe should dark matter make up?

Answer 19

27%

Question 20

Why can’t dark matter be seen?

Answer 20

It doesn’t interact with light.

Question 21

What are the three possible futures of the universe?

Answer 21

Open - It continues to expand for all of time
Closed - It will stop expanding at some point, contract and collapse on itself.
Flat - The expansion ends and the universe remains a fixed size.

Question 22

What does the final outcome of the universe depend on?

Answer 22

The density of the universe.

Question 23

Why are hydrogen spectral lines important for determining red shift of galaxies?

Answer 23

Hydrogen is the most common element in the universe.

Question 24

Here is the Hubble’s constant. Convert it into SI units.
68kms-1Mpc-1

Answer 24

Multiply by 1000
68000ms-1Mpc-1
Divide by 3.1x10^16x10^6
2.2x10^-18s-1

Question 25

When determining the age of the universe, what are we assuming?

Answer 25

Linear expansion