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Astronomy > Week 12 > Flashcards

Week 12 Flashcards

1
Q

Expansion of the Universe leads naturally too?

A
  • The universe had a beginning
  • The universe began in a very hot, very dense state
  • The universe has been expanding and cooling ever since
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2
Q

What does the big bang theory not explain?

A

how the initial conditions came to be.

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3
Q

Observational Evidence for the Big Bang Theory

A
  • 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)
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4
Q

Hot plasma in early universe

A

free electrons interact strongly with light

→ opaque, no light escapes

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5
Q

Cosmic Microwave Background Radiation

A

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

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6
Q

Why is CMB important

A
  • 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
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7
Q

Inflation

A
  • 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
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8
Q

Neutrons stability

A

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

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9
Q

The earliest stages of the universe

A

extremely high temperature 10-43s

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10
Q

Planck Era

A
  • The earliest stages of the universe

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

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11
Q

Rapid growth

A
  • inflation, i.e. acceleration of universe expansion at 10-32s
  • Spacetime expands by 10^40 times in a brief period
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12
Q

Formation of nuclei

A

t~0.001s

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13
Q

Era of Nucleosynthesis

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

- 25% of particles fuse into He

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14
Q

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

A

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

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15
Q

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

A

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

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16
Q

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

A
  • 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
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17
Q

Isotropic

A

Same in all directions (no preferred direction)

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18
Q

Homogenous

A

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

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19
Q

Is our universe isotropic or homogeneous?

A

both

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20
Q

Flat or curved universe?

A

flat over large scales

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21
Q

CMB redshift

A

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

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

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22
Q

era of galaxies

A

t=50-600 million years

23
Q

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

A

We do not yet have a theory that links quantum

mechanics and general relativity

24
Q

Particle era

A

(10-3 sec) “quark soup”

25
Q

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

A

The universe at that era had the same temperature at all

locations.

26
Q

CMB bright spots

A

seeds of galaxy clusters and large-scale

structures

27
Q

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

A

1 million years, today, 9 billion years

28
Q

Why can’t we see past the cosmological horizon?

A

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

29
Q

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

A

between 10 and 15 billion years old

30
Q

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

A

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

31
Q

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?

A

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

32
Q

What is Hubble’s Law?

A

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

33
Q

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

A

the period between its peaks of brightness and its luminosity

34
Q

When was most of the Helium in the Universe formed?

A

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

35
Q

cosmic web

A

distribution of matter in the universe

36
Q

main components of cosmic web

A

Clusters, Filaments, Voids.

37
Q

inverse square law

A

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

38
Q

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?

A

Supernova A is about twice as far away as Supernova B

39
Q

What is life?

A 
Hard to define, but some common elements are:
• order
• Reproduction
• Growth and development
• Energy utilization
• Response to environment
• Evolutionary adaptation
40
Q

Key ingredients for life

A
  • Source of materials
  • energy gradient
  • liquid water
41
Q

the fine tuning problem

A

our universe appears to be precisely optimized for complex life

42
Q

Six numbers that describe the critical aspects of physics.

A
  • Relative strength of gravity
  • neutron / proton mass ratio
  • matter density
  • dark energy density
  • strength of primordial density fluctuations
  • number and type of spacetime dimensions
43
Q

effect of different gravity

A

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

44
Q

effect of different neutron / proton mass ratio

A

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

45
Q

effect of different strength of primordial density fluctuations

A

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

46
Q

effect of different number and type of spacetime dimensions

A
  • fewer dimensions = chemistry very limited, too simple for life
  • more demensions = unpredictable
47
Q

effect of different matter and dark energy densities

A
  • Too much matter → universe collapses too quickly.

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

48
Q

The Strong Anthropic Principle

A
  • The universe was designed with intelligent life in mind

- Sentient life is the “purpose” of the universe

49
Q

The Weak Anthropic Principle

A
  • 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.

50
Q

The unique universe

A

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

51
Q

the self explainig universe

A

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

52
Q

The fake universe

A

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

53
Q

Multiverse

A

there are a group of different universes

Astronomy (12 decks)

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