Week 12

Question 1

Expansion of the Universe leads naturally too?

Answer 1

• The universe had a beginning
• The universe began in a very hot, very dense state
• The universe has been expanding and cooling ever since

Question 2

What does the big bang theory not explain?

Answer 2

how the initial conditions came to be.

Question 3

Observational Evidence for the Big Bang Theory

Answer 3

• Galaxies are moving away from us (universe expansion)
• Cosmic Microwave Background (universe was once hot and uniform)
• Abundances of Atomic Elements (H and He common)

Question 4

Hot plasma in early universe

Answer 4

free electrons interact strongly with light

→ opaque, no light escapes

Question 5

Cosmic Microwave Background Radiation

Answer 5

electromatic radiation emmited from opaque universe 400,000 years old.

Question 6

Why is CMB important

Answer 6

• it shows the Surface of last scattering
• i.e. the point where recombination (electrons combining protons)
• allows us to see light from these atoms and see what the universe was like around its beginning

Question 7

Inflation

Answer 7

• super-expansion of the universe
• space expanding so rapidly smoothed out matter and explains smoothness of temperature
• Smooths very curved spacetime out into the flat spacetime we see today

Question 8

Neutrons stability

Answer 8

not stable unless bound in a nucleus (and sometimes not even then)

Question 9

The earliest stages of the universe

Answer 9

extremely high temperature 10-43s

Question 10

Planck Era

Answer 10

• The earliest stages of the universe

- Energies of individual particles warp spacetime into a “quantum foam”

Question 11

Rapid growth

Answer 11

• inflation, i.e. acceleration of universe expansion at 10-32s
• Spacetime expands by 10^40 times in a brief period

Question 12

Formation of nuclei

Answer 12

t~0.001s

Question 13

Era of Nucleosynthesis

Answer 13

• 2-3mins, larger nuclei form (He, some Li)

- 25% of particles fuse into He

Question 14

Imagine an alternate universe which cooled a bit
faster than our own. How much helium would
have formed in this universe?

Answer 14

Shorter cooling → less neutrons decay → more neutrons
available to make helium
=more helium

Question 15

Imagine an alternate universe which cooled a bit
faster than our own. How much helium would
have formed in this universe?

Answer 15

Shorter cooling → less neutrons decay → more neutrons
available to make helium
=more helium

Question 16

Why are heavier elements not produced around the time of the big bang?

Answer 16

• Heavier elements require more heat and density for fusion, hence less likely to occur
• universe cools so their isn’t time for it to occur

Question 17

Isotropic

Answer 17

Same in all directions (no preferred direction)

Question 18

Homogenous

Answer 18

all places are alike (galaxies in one place are similar to another place)

Question 19

Is our universe isotropic or homogeneous?

Answer 19

both

Question 20

Flat or curved universe?

Answer 20

flat over large scales

Question 21

CMB redshift

Answer 21

The CMB photons have been redshifted by ~1000x (now

appear cold at T~3K but T~3000K initially)

Question 22

era of galaxies

Answer 22

t=50-600 million years

Question 23

Why can’t current theories describe what happened during the Planck era?

Answer 23

We do not yet have a theory that links quantum

mechanics and general relativity

Question 24

Particle era

Answer 24

(10-3 sec) “quark soup”

Question 25

Why would we expect the cosmic microwave background to look almost the same in all directions?

Answer 25

The universe at that era had the same temperature at all | locations.

Question 26

CMB bright spots

Answer 26

seeds of galaxy clusters and large-scale | structures

Question 27

Consider three periods in the history of the Universe: one million years after the Big Bang (age = 1 million years), about five billion years ago (age = 9 billion years), and today. What is the ranking in the expansion rate of the Universe in these three period, from fastest to slowest expansion

Answer 27

1 million years, today, 9 billion years

Question 28

Why can't we see past the cosmological horizon?

Answer 28

Beyond the cosmological horizon, we are looking back to a time before the universe had formed

Question 29

Based on current estimates of the value of Hubbles constant, how old is the universe?

Answer 29

between 10 and 15 billion years old

Question 30

What can change the expansion rate of the Universe over time?

Answer 30

Attractive forces such as gravity decrease the expansion rate. Repulsive forces would speed-up expansion by pushing spacetime apart.

Question 31

We receive today the light from a distant galaxy and from its spectrum we conclude that it took 400 Myr for the photons to reach us. Which one of the following statements is correct?

Answer 31

That galaxy is moving away from us, so today it is more distant from us than 400 million light years.

Question 32

What is Hubble's Law?

Answer 32

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

Question 33

What two observable properties of a Cepheid variable are directly related to one another?

Answer 33

the period between its peaks of brightness and its luminosity

Question 34

When was most of the Helium in the Universe formed?

Answer 34

A few minutes after the Big Bang Helium nuclei were formed 2-3 minutes after the big bang, when the temperature dropped to about 10-100 million degrees. Nuclear fusion in the cores of main-sequence stars is producing some addition Helium, but in small quantities compared to that initial production

Question 35

cosmic web

Answer 35

distribution of matter in the universe

Question 36

main components of cosmic web

Answer 36

Clusters, Filaments, Voids.

Question 37

inverse square law

Answer 37

physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity

Question 38

You observe the peak brightnesses of two white dwarf supernovae (a.k.a. Type Ia supernovae). Supernova A is only 1/4 as bright as Supernova B. What can you say about their relative distances?

Answer 38

Supernova A is about twice as far away as Supernova B

Question 39

What is life?

Answer 39

``` Hard to define, but some common elements are: • order • Reproduction • Growth and development • Energy utilization • Response to environment • Evolutionary adaptation ```

Question 40

Key ingredients for life

Answer 40

- Source of materials - energy gradient - liquid water

Question 41

the fine tuning problem

Answer 41

our universe appears to be precisely optimized for complex life

Question 42

Six numbers that describe the critical aspects of physics.

Answer 42

- Relative strength of gravity - neutron / proton mass ratio - matter density - dark energy density - strength of primordial density fluctuations - number and type of spacetime dimensions

Question 43

effect of different gravity

Answer 43

-stronger gravity → universe would collapse too quickly for life to evolve. -weaker gravity → stars couldn’t form

Question 44

effect of different neutron / proton mass ratio

Answer 44

-Neutrons even heavier → decay into protons faster, no helium in universe. -Protons heavier → p-p fusion wouldn’t produce energy → no stars.

Question 45

effect of different strength of primordial density fluctuations

Answer 45

i.e. slight variations in density in the early universe • too small → galaxies take too long to form. • too large → everything collapses into black holes

Question 46

effect of different number and type of spacetime dimensions

Answer 46

- fewer dimensions = chemistry very limited, too simple for life - more demensions = unpredictable

Question 47

effect of different matter and dark energy densities

Answer 47

- Too much matter → universe collapses too quickly. | - Too little matter (or too much DE) → universe expands too fast for stars, galaxies to form.

Question 48

The Strong Anthropic Principle

Answer 48

- The universe was designed with intelligent life in mind | - Sentient life is the “purpose” of the universe

Question 49

The Weak Anthropic Principle

Answer 49

- Our universe is a selection bias. | - No matter how unlikely it may be, the only universe we can inhabit is one suited to intelligent life.

Question 50

The unique universe

Answer 50

A deeper theory of everything will explain what seems to be arbitrary now and why universe is the way it is.

Question 51

the self explainig universe

Answer 51

Universes can only exist when observed (causal loop, Participatory Anthropic Principle)

Question 52

The fake universe

Answer 52

the universe is a simulation in a computer somewhere (but who programmed the computer?)

Question 53

Multiverse

Answer 53

there are a group of different universes