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Cosmology > Definitions and Explanations > Flashcards

Definitions and Explanations Flashcards

1
Q

galaxies appear to be

A

clustered

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2
Q

evidence of galaxy clustering

A

projected distribution of galaxies in the sky

redshift surveys.

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3
Q

Redshift survey

A

reveals patterns in the galaxy distribution i.e. clusters, sheets and voids.

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4
Q

peculiar motion

A

is caused by galaxy clusters’ gravitational interaction with the other cluster members.

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5
Q

Superclusters

A

galaxy clusters are themselves clustered and are organised into larger-scale structures

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6
Q

luminosity indicators

A

RR Lyrae stars and Cepheid variable stars

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7
Q

Hubble’s constant measures

A

the expansion rate of the Universe

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8
Q

The standard model for the origin and evolution of the Universe is called

A

the Hot Big Bang model

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9
Q

Cosmological Principle

A

The universe is homogeneous and isotropic:

universe homogeneous = no matter where you are in it

universe isotropic = no matter what direction you look in

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10
Q

Galaxies can be thought of as

A

local disturbances in an otherwise perfectly homogeneous and isotropic Universe.

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11
Q

The scale factor measures

A

the characteristic size of the Universe at time t.

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12
Q

the proper distance

A

Is the physical distance between two objects in the Universe.

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13
Q

the co-moving separation

A

is the separation of two objects in a coordinate system that expands along with the expansion of space-time.

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14
Q

a(t) is not a constant in

A

time, but is a constant in space at any given time.

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15
Q

Hubble time, τ

A

sets the timescale for the expansion of the Universe

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16
Q

Hubble time : t(0) < τ is when

A

including the effects of gravity

gives an age of the universe smaller than the hubble time

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17
Q

number of things in defence of homogeneity and isotropy

A
  1. The Hubble-Lemaitre Law
  2. The cosmic microwave background radiation
  3. Primordial nucleosynthesis
  4. The age of the universe
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18
Q

Statement of Olber’s Paradox

A

‘why is the sky dark at night’

if the Universe is infinite in extent and eternal, with stars roughly uniformly distributed throughout space. Every line of sight will intercept a star, so that the whole night sky should be as bright as the surface of a star.

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19
Q

Resolution of Olber’s Paradox

A
  1. Stars have finite lifetimes
  2. The speed of light is finite
  3. The universe has a finite age.
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20
Q

Hubble-lemaitre law is a natural consequence of

A

the cosmological principle

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21
Q

The Friedmann equation describes

A

how gravity acts against the expansion of the Universe

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22
Q

the constant k defines

A

the geometry or curvature of the Universe, it determines how a(t) evolves with time

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23
Q

three classes of solutions depending on values of k:

A

k > 0: the universe is closed, with positive curvature

k < 0: the universe is open, with negative curvature

k = 0: the universe is flat, with zero curvature

24
Q

p(c) is

A

the density required to just close the Universe
If p > p(c) the universe recollapses
if p < p(c) the universe expands indefinitely

25
Q

density parameter has three classes of solutions:

A

Ω > 1 implies Universe is closed
Ω < 1 implies Universe is open
Ω = 1 implies Universe is flat

26
Q

Although Ω can change with time it can be shown that

A

its state of being closed, open or flat cannot change

27
Q

the mass continuity equation tells us

A

that in a steady state, the rate at which mass enters a system is equal to the rate at which mass leaves the system

28
Q

in a matter-dominated universe p ~
in a radiation-dominated universe p ~

A

p ~ 1/a^3
p ~ 1/a^4

29
Q

the mass continuity equation has three resulting solutions:

A

k < 0: a(dot) -> c√-k - open universe
k > 0: a(dot) = 0 at some critical radius a(c), and a collapse phase starts after a = a(c) - closed universe
k = 0: a(dot) -> 0 at t -> ∞ - flat universe

30
Q

temporal behaviour of the scale factor is given by

A

a(t) ∝ t^2/η

a(t) ∝ { t^2/3 matter-dominated
{ t^1/2 radiation-dominated

31
Q

the effect of the pressure term P is to

A

slow down the expansion

32
Q

Galaxy rotation curves

A

the observed rotation velocities are greater than those expected from the gravitational influence of the luminous stars alone,

Indicating the presence of a dark matter halo surrounding the galaxy

33
Q

virial theorem assumes

A

galaxies can be seen as being in a fluid in hydrostatic equilibrium

34
Q

other methods for identifying dark matter

A
  1. gravitational lensing
  2. nucleosynthesis
  3. studying the pattern of temperature variations in the CMBR
35
Q

for any η of interest Ω ->

A

1 as z -> ∞ in the Early Universe

36
Q

in the case where Ω(0) < 1

A

we can expect a fast decrease from Ω = 1 to Ω -> 0.

Hence any value of Ω(0) which is not ~1 or ~0 would place us at a very special time.

37
Q

Dark matter candidates

A

Baryonic:
1. Gas clumps in galaxy halos and clusters
2. MACHOS
3. Low surface brightness galaxies

Non-Baryonic:
1. WIMPs
2. Primordial black holes

38
Q

hot dark matter

A

non-baryonic dark matter was moving relativistically at the time of decoupling from baryonic matter

39
Q

cold dark matter

A

non-baryonic dark matter was moving non-relativistically at decoupling.

40
Q

Jeans mass

A

how much mass can you have in a given volume of space before it starts to collapse under its own weight

41
Q

Gravitational collapse will occur when

A

the characteristic length L of the perturbation is larger than Jean’s length λ(J).

42
Q

stable oscillations

A

If M < M(J) or L < L(J)

43
Q

Jeans instability is not

A

fast enough to create large-scale structures

44
Q

c^2(s) = γp^(γ-1)

A

γ is the ratio of specific heats
such that γ = 5/3 for adiabatic process
and γ = 4/3 for isothermal process

45
Q

Hubble flow

A

Will produce time-independent coefficients in the density equation
The exponential law is replaced by a power law
Jeans mass still persists allowing for an expanding universe
Fragmentation less plausible as an explanation for the observed mass distribution

46
Q

open universe

A

universe expands forever

47
Q

closed universe

A

the universe will stop expanding and will then contract

48
Q

flat universe

A

the expansion will slow down unit it stops at t -> ∞

49
Q

newtonian cosmology

A

the study of the evolution of the Universe using only Newton’s law to describe gravity

50
Q

matter dominated

A

corresponds to the epoch where the evolution of the Universe is dominated by ‘matter’, i.e. non relativistic particles exerting negligible pressure

51
Q

radiated-dominated

A

corresponds to the epoch where the evolution of the Universe is dominated by relativistic particles exerting a non-negligible pressure

52
Q

graph of how d(diam) varies with redshift z

A

see notes

53
Q

How the viral theorem can lead to infer the existence of dark matter

A

Measure o(vr) and the effective radius of the cluster to obtain an estimate of gravitating matter in the cluster
On comparison there is a discrepancy between the measurements and theory invokes the existence of dark matter

54
Q

Luminosity distance

A

The distance to the object implied by the observed flux

55
Q

Angular diameter distance

A

Is the distance implied by the small angle approximation

56
Q

If γ < 4/3

A

Jeans mass decreases with increasing density
Isothermal collapse

57
Q

If γ = 5/3

A

An adiabatic process and therefore jeans mass increases with density
Gravitational collapse halts.

Cosmology (7 decks)

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