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astro > Dark Matter, Dark Energy & the Fate of the Universe > Flashcards

Dark Matter, Dark Energy & the Fate of the Universe Flashcards

1
Q

Stars & ISM clouds are

A

visible matter

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2
Q

Existence of unseen matter

A

inferred from its gravity effects on visible matter.  Orbital velocities of visible objects can be observed & measured

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3
Q

Kepler’s Law

A

Normally, the orbital speed drops as distance from a single central mass increases.

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4
Q

Same orbital speeds even for stars far away from galactic centre

A

Most of galaxy’s mass lies beyond our Sun

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5
Q

Same orbital speeds even for stars far away from galactic centre

A

 Galactic mass NOT only at centre & most of it is located into halo and extends far out from it

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6
Q

Where is the dark matter in our galaxy?

A

This mass gives off very little light → dark matter
Total amount of dark matter could be 10× the total mass of all stars in the disk!
 Visible matter in disk only a small fraction of total mass!
 Radius of dark matter halo may be 10× as large as galaxy’s halo of stars

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7
Q

To determine the amount of dark matter in a galaxy

A

→ compare galaxy’s mass with its luminosity

 Measuring luminosity → easy if its distance can be determined

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8
Q

Total mass of other spiral galaxies also determined

A

 Measure orbital speeds as far from the galaxy’s centre as possible
 Stars are rarely found at great distances from galactic centre
  Doppler shift of the 21 cm radio emission line of H gas clouds in the receding & nearing arms of the galaxy tells us how fast the clouds are moving towards or away from us

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9
Q

Rotation curves of galaxies are flat at large distances from centres

A

 Similar to our galaxy  dark matter distributed far beyond disk & halo

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10
Q

Other spiral galaxies also have at least 10× as much mass in dark matter as they do in stars

A

 In typical spiral galaxies, 90% of matter is dark matter

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11
Q

Different techniques must be used for elliptical galaxie

A

 Very little H gas  no detectable 21 cm emission line  Must observe motions of stars

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12
Q

Mass-to-Light Ratio in dark matter in galaxy

A

Amount of dark matter in galaxy can also be quantified by its mass-to-light (M/L) ratio:
 Mass: in [MSun] units
 Visible luminosity (from apparent brightness): in [LSun] units

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13
Q

Milky Way: mass to light ratio

A

M/L = 6 if deduced from visible matter inside Sun’s orbit, but of about 50 if ALL matter of entire galaxy is considered

Other spiral galaxies have M/L ratios of ~50 or more

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14
Q

The large M/L ratios (≥50 MSun/LSun) obtained from measurements for elliptical galaxies also indicate that, just as spirals, they contain far more dark matter than visible one

A

and also predominantly located in the halo.

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15
Q

Dark matter in galaxy clusters discovered by Fritz Zwicky in 1930s.

A

 Assumed galaxies orbit about cluster centre
 Measured orbital speed of galaxies (redshifts) & their distance from centre
 Calculated cluster mass with Kepler’s law  Huge M/L ratios found (>100MSun/LSun)  More sophisticated measurements today confirmed his finding

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16
Q

2 other independent ways to determine cluster mass:

A

 Measure temperature & hot gas distribution between galaxies
 Observe how clusters bend light (gravitational lenses)

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17
Q

Measure temperature of intracluster medium between galaxies

A

Gas heated to ~10…100 m K → temp. of intracluster gas depends on the mass of the clusters

 Being so hot → emits X-rays & is nearly in gravitational equilibrium, i.e. outward pressure balances gravity’s inward pull → cluster mass can be estimated from temperature of hot gas

 50× more dark matter than the combined mass of the stars in the cluster’s galaxies

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18
Q

Gravitational lensing

A

A massive cluster of galaxies bends light like a lens  distant object behind cluster can be seen

Multiple distorted, magnified & brightened images of the background source can be generated

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19
Q

Gravitational lensing (continued)

A

Distortion strength reveals mass.

 Lens bending angle depends on mass

A different theory of gravity is used: masses distort the space-time “fabric” of the Universe

 All previous methods to find mass depended on Newton’s law of gravity

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20
Q

Cluster masses measured by 3 independent methods all agree

A

Conclusion: Clusters of galaxies hold huge amounts of dark matter

 Most galaxy clusters have >100 MSun/Lsun → contain far more dark matter mass than stars

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21
Q

At least some of dark matter can be ordinary matter:

A

 Protons, neutrons (baryons) & electrons  Only thing unusual is that it is dim  Called baryonic matter

The rest must be extraordinary matter  Made of particles we have yet to discover  Known as nonbaryonic matter

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22
Q

Our galactic halo has lots of dark baryonic matter:

A

 Low-mass M dwarfs, brown dwarfs
 Black holes & Jovian-sized planets
 Too faint to be seen at large distances  Also known as “MAssive Compact Halo Objects”  MACHOs
 Still, the number of MACHOs is not large enough to account for all Milky Way’s dark matter

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23
Q

Neutrinos from the Sun are nonbaryonic matter

A

Weakly interacting particles → interact with other particles through gravity & weak force.
 Very low mass & high speed → easily escape galaxy’s gravity.  Can account for only a very small % of dark matter observed.

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24
Q

Others are “Weakly Interacting Massive Particles” = WIMPs

A

WIMPs are still theoretical & have NOT yet been discovered

 Massive enough and/or in large enough numbers to exert gravitational influence.

 They do not emit light or are bound to any light-emitting charged matter.

 They do/will not collapse into a galaxy’s disk/body.

 Hence, they will remain gravitationally bound in a galaxy’s halo.

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25
Q

Gravitational attraction overcomes the expansion of the Universe at close range

A

 Galaxy’s velocity deviates from Hubble’s law.  Universe expands but individual galaxies attract one another

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26
Q

Structure probably began with slight enhancements in matter density during the early Universe.

A

 Regions collapsed into protogalactic clouds forming galaxies.
 Individual galaxies fell in towards one another to form clusters.
 Individual clusters now congregating to form superclusters.

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27
Q

Collapses against expansion facilitated by dark matter

A

even today, dark matter’s gravitational pull must be the primary force holding the large-scale cosmic structures together.

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28
Q

Cosmic structures studied by galaxy surveys.

A

 Once required years of effort just to map location of a few 100s galaxies (measure redshift to deduce distances)  Recent technology measures 100s of galaxies in a single night of telescopic observation

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29
Q

Hubble’s law primarily used to measure galaxy distances

A

Hubble expansion dominates beyond 300 m l.y. from Earth

 Galaxy distribution maps can be constructed

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30
Q

Galaxy distribution map reveals large scale structures much bigger even than clusters of galaxies! →

A

superclusters, and some are even LARGER !!!

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31
Q

Galaxies are not randomly scattered but arranged on a

A

scale of 100s m l.y. in gigantic chains, sheets & walls surrounded by empty regions called voids

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32
Q

 Chains come from the initial regions

A

s of density enhancement

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33
Q

Voids come from the initial

A

regions of density depletion

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34
Q

Galaxies appear evenly distributed on a 1 b l.y. scale

A

On very large scales, the Universe looks much the same everywhere → in agreement with the Cosmological Principle

The structures we see today mirror the original distribution of dark matter in the early stage of the Universe

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35
Q

How will the Universe end ?

A

Gravity pull between galaxies slows down Hubble expansion.

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36
Q

Critical density is the average mass density for its gravity pull to

A

equal the kinetic energy of expansion.

37
Q

Critical density is the average mass
density for its gravity pull to equal the
kinetic energy of expansion.

A

 IF existing total mass has density < critical density
 Universe expands forever
 If existing total mass has density > critical density
 Universe will stop expanding & contract  The current density of the Universe is very-very close but still below critical density

38
Q

Current kinetic energy of the universe given by

A

H0

39
Q

Hence, the critical density is found to be

A

10–29 g/cm3 → ~5 H atoms/m3

40
Q

Visible matter contributes only

A

~0.5% of the critical density necessary to halt the Universe’s expansion

41
Q

Dark matter in individual galaxies & galaxy clusters has

A

~10…50× the mass in stars → but it should be ~200× to stop the expansion

Overall density of matter (both baryonic & non-baryonic dark matter) of Universe = ~25% of critical density

 This suggests Universe will expand forever!  Unless much more dark matter beyond cluster boundaries exist (not probable as this would be causing large deviations from Hubble’s law, which are not observed)

42
Q

Recent observations of white dwarf supernovae in very

distant galaxies yielded unexpected results.

A

 Look-back time (distance) from standard candles

 Redshift (recession velocity) gives the Universe’s expansion rate

43
Q

Redshift (recession velocity) smaller than expected at measured look-back time.

A

 Universe expanding slower in the past

44
Q

Implies that Universe’s expansion is accelerating!

A

There must be an unknown force that repels the galaxies: Dark energy

45
Q

We do NOT know why the Universe is accelerating

A

tremendous amounts of energy required! We have NO idea what dark energy might be!!!
Hypothetical energy which permeates space & has strong negative pressure.

Effect of such pressure is qualitatively similar to an opposing force to gravity at large scales.
Explains expansion at an accelerating rate & accounts for significant portion of missing mass in the Universe.

46
Q

From observing distant galaxies

A

 Farther out we look, the farther back in time we see → as early as 1…2 b years old

 However, light could not move freely before Universe was ~380,000 years old

47
Q

Earlier conditions & expansion rate of universe deduced from running expansion backwards with mathematical models.

A

 Temperature/density predicted with basic physics  Study matter at high temperatures/densities in laboratory  Conditions as early as 10–10 s after Big Bang can be obtained

48
Q

Early Universe conditions

A

Universe extremely hot during 1st few seconds but cools as it expands.

At such extreme temperatures, photons can transform into matter (& vice versa)!

 According to E = mc2 Early universe filled with radiation (remnant heat of the Big Bang) & subatomic particles.

49
Q

2 particles created when 2 photons collide

A

 Total energy more than 2× subatomic particle mass (p+, n, e–)
 Particles of matter & antimatter
 Reaction also runs in reverse → matter-antimatter annihilation

50
Q

Matter/radiation continually converts into each other during the very first few moments.

A

 Constant total mass-energy

 Laws of physics can be used to calculate the proportions of radiation & matter at various moments in Universe’s early history → the only problem: incomplete understanding of laws of physics: could have been different under those extreme conditions!

51
Q

Each Era in Universe’s very early history

A

distinguished from the others by some major change in the physical conditions as the Universe cooled

52
Q

Planck Era (t < 10–43 s)

A

The first 10–43 s → until Universe cooled to 1031 K.

Universe consisted entirely of 31 K
radiation (very small, dense &
hot).

Substantial energy fluctuations from point to point

 rapidly changing gravitational field that must have randomly and severely warped space-time. • Planck Era (t < 10–43 s)

Unable to describe this Era.

 Our science can only adequately describe conditions after this Era.

Do we use quantum mechanics or general relativity?

 Need to unify them in order to more adequately describe the conditions and phenomena of that Era.

All four natural forces (strong, weak, electromagnetic & gravitational) were probably unified in this Era in a single “superforce”

53
Q

GUT Era (10–43 s < t < 10–38 s)

A

Until Universe cooled to 1029 K. Only 2 natural forces: Gravity & Grand Unified Theory (GUT) force

 Our current understanding of physics allows us to know only a bit more about this Era than about the Planck Era

At its end, the strong force froze out of the GUT force

 enormous amount of energy released

 a dramatic inflation (expansion by a factor of 1030) occured in only 10–36 s: the size of an atomic nucleus grew to the size of our Solar system !

54
Q

• Electroweak Era (10–38 s < t < 10–10 s)

A

Universe cooled to 1015 K when reached the age of 10–10 s.
GUT force splits into 2: strong & electroweak  with gravity they form now the 3 natural forces

Intense radiation still filled the space  spontaneous production of matter-antimatter elementary particles but all convert back to photons immediately

 As many particles as photons

Electroweak condition experimentally verified in 1983.

55
Q

• Particle Era (10–10 s < t < 10–3 s)

A

All 4 forces now distinct

T ↓ to 1012 K  too cool for the spontaneous exchange matter ↔ energy to continue

At t = 0.1 ms, photons turned into quarks, which combined to form protons (p+), neutrons (n), e–, neutrinos (ν) & perhaps WIMPs.
Since matter obviously is present in today’s Universe

 p+ must have outnumbered antiprotons (p– )

 Today photons outnumber p+ by about 109:1

in early Universe there were 1+109 p+ for each 109 p–

56
Q

The Era of nucleosynthesis (10–3 s < t < 5 min.)

A

p+ & n started fusing into heavier nuclei.  But formed nuclei also torn apart by (the still) very high T
At t = 5 min. (T = 109 K), density in expanding universe dropped significantly & fusion stopped.
When fusion stopped:
 ~75% of the ordinary (baryonic) matter = p+
 ~25% of the ordinary (baryonic) matter = fused into He nuclei
 Also traces of deuterium & Li nuclei in leftover baryonic matter

57
Q

The Era of nuclei (5 min. < t < 380,000 years)

A

Universe now a hot plasma of H nuclei (p+), He nuclei & e– :
 Fully ionized nuclei moved independently of e–
 Photons did not travel very far before bumping into an e–
 the Universe was opaque

Cooled to 3,000 K after 380,000 years:
 Nuclei captured e– to form stable H & He
  The Universe became transparent   Photons free to stream across universe → the Cosmic Microwave Background (CMB) we see today

58
Q

The Era of atoms (380,000 years < t < 109 years)

A

Universe holds neutral atoms, plasma & lots of photons
Density enhancements in gas & gravity attraction by dark matter eventually form protogalactic clouds.  1st stars formed: light up Universe & trigger galaxy formation

59
Q

Era of Galaxies (t > 1b years

A

1st galaxies appeared ~1b years after Big Bang

This is the current Era of the Universe

 Galaxies are still forming to this day and will continue for quite some time in the future

 A good scientific model makes verifiable predictions.

The Big Bang model has gained wide scientific acceptance because it makes 2 predictions that have been verified experimentally since 1960s:

 Radiation that began to flow into the suddenly transparent Universe at the end of Era of nuclei should still be present today → The Cosmic Microwave Background (CMB)

 The He content of the Universe (& that of other light elements) must be that resulted from the Era of nucleosynthesis

60
Q

• Radiation left over from the Big Bang

A

Universe immersed in a sea of radiation after Big Bang → unleashed only at the end of the Era of nuclei, when the Universe was only 380,000 years old.
 Universe cooled enough (3000 K) for H & He atoms to form

  The Universe became transparent

  Photons free to stream across universe → the Cosmic Microwave Background (CMB) we see today

61
Q

The CMB predicted by Big Bang theory was accidentally

discovered in 1965.

A

 Appeared to come from every direction  Had a perfectly thermal spectrum at 2.73 K → expected temperature due to Universe’s expansion

62
Q

Origin of CMB

A

Radiation originally identical with that of a 3,000 K blackbody.
However, the Universe expanded 1,000 times until now!
 Cosmological redshift turned this radiation into microwaves Perfect thermal radiation spectrum peak corresponds to T ~2.73 K.

63
Q

Density of the Universe

A

Measurements in the 90s showed that the CMB is NOT perfectly uniform

 Temperature variations of the 380,000 year-old universe reflect density variations that acted as a genetic code for the Universe’s structure today → the ‘seeds’ of structure formation during the Era of nuclei

The CMB indicates exactly how hot the Universe was and how much He it should have made

64
Q

• How was cosmic helium created?

A

During the Era of nucleosynthesis, when T ≥1011 K → p+-to-n nuclear particle conversions were reversible

When T↓ <1011 K → nuclear conversions favored p+
 n = more massive than p+
 p+-to-n conversion requires energy
 n-to-p+ conversion releases energy → unhindered by Universe cooling

The He nuclei created then were very short-lived

 Split apart by the intense γ radiation that filled the Universe at that time

Long-lasting He nuclei formed only when the Universe was ~1 min. old & cooled enough to contain much less γ radiation

65
Q

How much cosmic helium was created?

A

12 H nuclei for every He nucleus made 

The final p+-to-n ratio must have been ~14:2, i.e. the Big Bang model predicts Universe must have had (at the end of the Era of nucleosynthesis) a composition of 75% H & 25% He ≡ 3:1 mass ratio of H:He

66
Q

Milky Way has

A

~28% He & no galaxy has <25% He.

 Fusion in stars only produced ~10% of He observed
 The estimation of the mass/concentration of other elements, like deuterium, etc., also match the practical measured values
 Excellent agreements with Big Bang model, confirming its validity!

67
Q

Abundances of light elements

A

When the first stable & long-lived He nuclei formed, the Universe was already too cold for enabling fusion of other elements.

 Only very few reactions between p+, deuterium (2H) nuclei, tritium (3H) nuclei, 3He & 4He were possible

 3H + 4He → 7Li

 These had minor contributions because 2H & 3H were very rare Before cooling of the Universe shut off fusion entirely, only trace amounts of Li & Be were produced  Most of Be & all B were created later, by high-energy splitting up of heavier elements fused in stars

68
Q

We saw earlier that formation of deuterium was necessary for the production of He nuclei.

A

 The fusion stopped before all deuterium could be used up
 The amount of deuterium in Universe today indicates the density of baryons (p+ & n ) at the end of the Era of nucleosynthesis.
 Presently 1 in 40,000 H atoms has a deuterium nucleus
 Density of ordinary (baryonic) matter = ~4% of critical density

69
Q

Overall density of Universe

A

~25% of critical density

70
Q

The Universe contains about

A

6× more extraordinary (nonbaryonic) matter than ordinary (baryonic) matter  Most of the extraordinary matter is believed to consist mainly of WIMPs

71
Q

Inflation explains several key features of today’s Universe:

A

1) Although CMB shows that the density of the Universe at the end of the Era of nuclei was not perfectly uniform (variations of ~0.01%), the overall smoothness is remarkable → Inflation explains why distant reaches of the Universe look so similar wherever we look

Regions presently very distant (i.e. which can’t have possibly exchanged light or any information) still have the same temperature & density because they were in contact prior to inflation

72
Q

Inflation explains why

A

ρOrdinaryMatter + ρDarkMatter + ρDarkEnergy < ρCrit
⇔ ρMatter + ρDarkEnergy < ρCrit ⇔ Ωm + Ωv < 1 where
Ωm = ρMatter/ρCrit and Ωv = ρDarkEnergy/ρCrit i.e. total density of the Universe → very close to critical density ρUniverse < ρCrit

This equation was preferred instead of ρUniverse ≅ ρCrit which does not indicate clearly which of the 2 terms is bigger

The overall geometry of the Universe should be very flat (perfectly flat only if EXACT equality would occur in the relation above)

73
Q

General relativity says that matter can curve spacetime.

A

 Curvature of universe can vary from place to place…  …but the Universe as a whole has the same overall shape → flat, closed or open

74
Q

Spacetime curvature can result from

A

imbalance in the expansion kinetic energy & pull of gravity.
 If density was 10% more, Universe would have collapsed long ago
 If density was 10% less, expansion would have spread all matter too thin & galaxies had never formed

75
Q

Detailed studies of the CMB suggest that the overall geometry of the Universe is

A

remarkably flat.
 Gravity’s pull ALMOST balances the expansive kinetic energy
 TOTAL density of our Universe is very close to critical density

76
Q

Calculations show that the largest temperature

differences in the CMB should typically be

A

between sky patches separated by ~1° for a flat universe.
 Angular separation <1° if the Universe were open
 Angular separation >1° if the Universe were closed

77
Q

Largest temperature differences observed at

A

1° angular separation.  Deduced from CMB map of WMAP satellite

78
Q

Various experiments (e.g. Boomerang) done in 2000-2001

A

confirmed the presence of the largest ripples, which extend about 1o across the sky.  Hence, it provided convincing evidence that the overall Universe geometry is “flat”

79
Q

These experiments also showed that there is a series of preferred scales over which

A

the temperature fluctuations occur: harmonics peaks were found at the angles predicted by inflation theory

The first peak reveals a specific spot size for early universe sound waves, just as the length of guitar string gives a specific note. The second and third peaks are the harmonics

80
Q

Why are the Boomerang and WMAP Data Important?

A

The size of a constant-temperature region is fixed by the size of the horizon at the time of last-scattering (The horizon = the distance over which a photon can travel during the age of the Universe). The apparent angle over which the region is spread depends on the geometry of the universe

The experiments showed average angular separation of ~1o  Universe is almost flat

81
Q

Why is the geometry of the Universe so flat?

A

 Effect of rapid inflation flattened spacetime

 Inflation stretched any initial curvature to near flatness Curvature is only noticeable on scales much larger than observable universe

82
Q

Inflation —> Total matter-energy density < critical density of our Universe.

A

 Consistent with measured matter density & dark energy powering the accelerating expansion of our universe

 Dark energy can account for the shortfall in the density of dark matter

 Einstein’s theory of relativity → Energy must be able to curve space-time just like mass!

 Dark energy → associated with a large-scale repulsive force → can therefore compensate for the shortfall in the matter density and make the Universe flatter that it would otherwise be

83
Q

Density of ordinary matter is

A

~4.5% of critical density → measurements of deuterium in the Universe

84
Q

Total matter density is

A

~27% of critical density.

 Extraordinary dark matter makes up to ~23% of critical density → In line with measurements of mass in galaxy clusters

85
Q

Flat geometry

A

total mass-energy < critical density.

86
Q

Flat geometry & matter density < critical density

A

repulsive dark energy accounts for remaining 73% of mass-energy & expansion is accelerating.  In line with white dwarf supernovae observation

87
Q

~13.75 b years old at current microwave temperature of

A

2.73 K.  Agreement with Hubble’s constant & age of oldest stars observed

88
Q

Why is the night sky black if the universe is infinite & filled with stars?  Should see a star in every direction we look  The sky should be ablaze with light

A

 Possible solutions to this conundrum: 1) The Universe has a finite number of observable stars  Hence it would NOT be possible to see a star in every direction we look…
…they are different distances, with luminosities ∝ 1/d2 2) The Universe is changing over time  … in such a way that it prevents us from seeing an infinite number of stars Since modern observations have shown a (more or less) uniform distributions of galaxies at very large scales (remember the Cosmological Principle ?!)   The Universe may contain an infinite number of stars, but…  …since the Universe began at a particular moment with the Big Bang… We can see only the stars that lie within the observable Universe, i.e. inside our cosmological horizon

astro (14 decks)

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