Definitions and Explanations

Question 1

The cosmological principle

Answer 1

the Universe is homogeneous and isotropic - there is no privileged position in the Universe.

mass density p is uniform

Question 2

Hubble’s law

Answer 2

v = Hd. There is a linear relationship between an objects recession velocity and the distance to it.

Question 3

Does hubble’s law violate the cosmological principle

Answer 3

For observers at the ‘centre’ of an expanding sphere hubbles law and the cosmological principle hold.

for observers on the surface of an expanding 2D sphere, Hubble’s law still holds, two observers move apart from each other at a velocity in direct proportion to their distance apart.

No violation.

Question 4

the introduction of a positive cosmological constant

Answer 4

a positive value of Λ
1. Will cause acceleration.
2. This implies a smaller velocity at early times
3. The universe needs longer to reach its present size
4. The universe at the present time, would be older.

Question 5

photon number density is obtained from

Answer 5

a blackbody spectrum of the CMBR and from knowing that it’s equal to known constant x T^3. T = 2.7K.

Question 6

critical mass density is obtained from

Answer 6

H^2 = 8πGp(c)/3 so long as we know H, which we can get from redshift measurements.

Question 7

density parameter is obtained from

Answer 7

galaxy rotation measurements or measuring the deceleration parameter as Ω = p/p(c).

Question 8

Planck time

Answer 8

is the time in the universe’s history before which quantum mechanics is expected to be important.

Question 9

Decoupling

Answer 9

Refers to the point after recombination where photons fall out of thermal equilibrium with matter.

Question 10

After decoupling

Answer 10

photons do not interact with matter and this has a consequence that radiation expands and cools.

Question 11

the particle content at the earliest times of the universe

Answer 11

is unknown and Grand Unified Theories (GUT) are still speculative. Later on, the content was that of the standard model.

Question 12

At T ~ 100GeV

Answer 12

the W and Z bosons and quarks acquire mass in a phase transition

Question 13

T ~ 200MeV

Answer 13

the quarks underwent a phase transition and became confined inside hadrons

Question 14

T ~ 0.1MeV

Answer 14

the neutrons and protons combined to form light nuclei, mainly helium but with residual excess photons

Question 15

Friedmann cosmology

Answer 15

describes a universe which expands with time cooling as it goes

Question 16

the period just after planck time

Answer 16

1. the universe is far too hot to allow nucleosynthesis to proceed
2. eventually the Universe has expanded sufficiently and is cool enough that radiation no longer has enough energy to break apart newly formed matter

this is the current epoch

Question 17

theoretical basis for Friedmann cosmology

Answer 17

is a classical theory describing an expanding universe

Question 18

observations that support Friedmann cosmology

Answer 18

homogeneity and isotropy. On large scales, there is very little evidence of large-scale structure. The homogeneity and isotropy of the CMBR is one of the biggest pieces of evidence.

Question 19

Big Crunch

Answer 19

gravity wins over expansion and the universe recollpases

positive deacceleration parameter

Question 20

Big Chill or Big Rip

Answer 20

expansion wins out over gravity and the matter in the universe becomes infinitely far apart.

Question 21

a negative effective pressure

Answer 21

negative pressure would drive accelerated expansion overcoming gravity, such a thing would be consistent with a positive value of Λ

Question 22

if density p = 0

Answer 22

empty universe, devoid of matter

Question 23

Objects just beyond the edge of our local supercluster will disappear over the edge

Answer 23

represents the nearest point at which we currently observe large-scale structure

Question 24

Observations made beyond the horizon

Answer 24

little evidence to make measurements of p and q. On such scales little evidence for the Big Bang.

Question 25

3 key properties of particles that play an important role in determining how much helium-4 was produced

Answer 25

1. protons are slightly lighter than neutrons
2. free neutrons decay into protons with a relatively long half-life
3. neutrons bound into stable isotopes/don’t decay

Question 26

protons are slightly lighter than neutrons

Answer 26

in the early universe, the number of protons and neutrons will remain almost identical as long as kbT&raquo_space; (m(p)-m(n))c^2

Question 27

free neutrons decay into protons with a relatively long half-life

Answer 27

when the universe has cooled down enough, the only process which can change the relative p/n abundances is the decay of free neutrons. If the neutron decay had been much faster we would be able to form only hydrogen

Question 28

neutrons bound into stable isotopes/don’t decay

Answer 28

stable isotopes are required to build heavier elements; several steps in going from p+m to 4He.

Question 29

Flatness problem

Answer 29

if the universe has non-zero curvature, then the dimensionless density parameter diverges very rapidly from 1 as the universe expands.

Thus, to be so close to 1 today would require |Ω-1| to have been unfeasibly close to zero in the early universe.

Question 30

solving the flatness problem

Answer 30

a period of exponential expansion, with H constant, in the very early universe would drive the universe exponentially quickly towards zero curvature, solving the flatness problem.

Question 31

Concordance cosmological model

Answer 31

The concordance model is the generally consistent and tight constraints on cosmological model parameters over a wide range of redshift.

The universe contains Λ, cold dark matter and ordinary matter.

Question 32

horizon problem

Answer 32

Explains how the temperature of the CMBR can be so uniform when regions of the CMBR even just a few degrees apart are casually disconnected.

Question 33

Inflation can solve

Answer 33

the flatness and horizon problem as inflation is a short period of accelerated expansion in the early universe.

Question 34

fluid equation

Answer 34

an increase in volume - density decrease

the internal energy of the fluid changes as the universe expands.

if P = 0 non-relativistic matter no change in the internal energy

Question 35

if P>0 then the expansion of the universe

Answer 35

decelerates

Question 36

current observations indicate

Answer 36

matter density is about 30% of the critical density
the luminous matter is <1% of the critical density
dark matter is non-baryonic and cold
universe has flat geometry

Question 37

negative value of the deceleration paramater

Answer 37

universe is accelerating

Question 38

cosmic complementary

Answer 38

we combine multiple different cosmological probes and data spanning different redshift ranges

overlap between data sets will allow the values of the parameters to be more tightly constrained.

Question 39

perturb solutions

Answer 39

perturbations grow exponentially tending towards a de Sitter model in the future.

Question 40

cosmological constant problem

Answer 40

pΛ &laquo_space;p(vacuum)

should Λ be zero?

no as we have observed it

Question 41

CMBR

Answer 41

is described by a Planck spectrum

originated when the universe was in thermal equilibrium

photons interact very strongly with electrons so have a very short mean free path

as T drops

photons now travel unimpeded.

called decoupling

matter-dominated

Question 42

recombination

Answer 42

refers to the period during which electrons and protons first become bound as hydrogen nuclei

Question 43

surface of last scattering

Answer 43

1.the photons we see in the CMBR come from the surface of last scattering
2. represents the universe as it was just after it became cool enough for photons to travel without being scattered by electrons

Question 44

for T>T(eq)

Answer 44

radiation dominated

Question 45

prior to decoupling

Answer 45

the universe consists of a baryon-photon fluid

fluctuations in the CMBR imply that there were tiny differences in gravitational potential at the epoch of recombination

gravity tries to collapse the fluid and radiation pressure tries to expand it

the fluid sloshes around in the potential wells and sets up acoustic oscillations

Question 46

acoustic oscillations implies

Answer 46

a pressure or sound wave

Question 47

oscillations after decoupling

Answer 47

cease and their pattern was frozen in to the CMBR pattern we observe today

this generates a series of acoustic peaks

corresponding to oscillations that were just the right size to be at maximum compression when the photon decouple.

Question 48

sound horizon

Answer 48

the physical scale of the oscillations is determined by how far sound waves could have travelled before decoupling

Question 49

the particle horizon

Answer 49

is the limit of the region with which an observer can be in casual contact.

the proper size of the observable universe

Question 50

the height of the acoustic peaks is sensitive to

Answer 50

the baryon density

Question 51

Sakharov conditions for baryogenesis

Answer 51

1. baryon number violation
2. CP violation
3. Departure from thermal equilibrium

Question 52

How to find the Planck time

Answer 52

We can find it by setting the Schwarschild radius equal to the Compton radius.