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Title: Lecture 25 Problems with the Big Bang Inflation


1
Lecture 25Problems with the Big BangInflation
  • ASTR 340
  • Fall 2006
  • Dennis Papadopoulos


2
The cosmic concordance
  • What is our universe like?
  • Matter content?
  • Geometry (flat, spherical, hyperbolic)?
  • Anything else strange?
  • Remarkable agreement between different
    experimental techniques
  • Cosmic concordance parameters

3
Measurements of the matter content of the
Universe (recap)
  • Primordial nucleosynthesis
  • Theory predicts how present light element
    abundances (4He, 3He, D, 7Li) depend on mean
    baryon density
  • Observed abundances ? ?B?0.04
  • Galaxy/galaxy-cluster dynamics
  • Look at motions of stars in galaxies, or galaxies
    in galaxy clusters
  • Infer presence of large quantities of dark
    matter which gravitationally affects observed
    objects but cannot be seen with any telescope

4
Nucleosynthesis
5
  • Analysis of galaxy motions suggests a total
    matter density of ?Matter?0.3
  • Same conclusion from gravitational lensing by
    clusters (light from background objects is bent
    due to GR effects)

6
  • First stunning conclusion
  • Compare ?B?0.04 and ?Matter?0.3
  • Normal matter only accounts for about 1/8 of the
    total matter thats out there!
  • Dark matter provides ?DM?0.26
  • Were made of the minority stuff!

7
  • Can be confirmed by taking an inventory of a
    cluster, where diffuse gas is hot and emits
    X-rays
  • Find that about 1/8 of a clusters mass is in
    baryons
  • We believe that clusters should be representative
    samples of the universe
  • Confirms ?DM?0.26

8
MEASURING THE GEOMETRY OF THE UNIVERSE
  • Recall that universe with different curvature has
    different geometric properties
  • Adding up the angles in a triangle,
  • Flat universe(k0) angles sum to 180?
  • Spherical universe (k1) angles sum to gt180?
  • Hyperbolic universe (k-1) angles sum to lt180?
  • Similarly, for a known length L at a given
    distance D, the angular size on the sky varies
    depending on the curvature of space
  • Flat universe (k0) angular size ?L/D
  • Spherical universe (k1) angular size ?gtL/D
  • Hyperbolic universe (k-1) angular size ?ltL/D

9
L
L
L
D
k-1
k0
k1
10
Angular size of fluctuations in the CBR
  • Remember the cosmic microwave background
  • It has fluctuations, with average separations
    corresponding to a known scale L at the distance
    where light last interacted with matter
    (matter/radiation decoupling)
  • Distance D to this surface of last scattering
    is also known
  • Can use apparent angular separations of
    fluctuations compared to L/D to infer geometry of
    Universe

11
Surface of last scattering
12
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13
us
L
D
14
Flat universe!
  • Result
  • The universe is flat
  • In terms of omega curvature parameter,
  • ?k0, i.e k0
  • Recall that the sum of all three omega parameters
    as measured at present time must be 1
  • How do we reconcile ?k0 with our measurement of
    the matter density, which indicates ?M0.3?
  • There must be a nonzero cosmological constant,
    ??0.7!

15
Non-zero ?
  • Recall that with a non-zero, positive value of ?
    the universe expands more rapidly than it would
    if it contained just matter

16
  • Are there other indications of nonzero ??
  • Yes, from direct measurement of deceleration
    parameter q0
  • Recall q0 ?(d2R/dt2 )/(RH2) measures the rate
    of change of the Hubble parameter (expansion
    rate)
  • The relation between q0 , ??, and ?M is
  • q0 0.5? ?M ? ??
  • If ?M 0.3 and ??0.7, would expect q0 -0.55
  • More generally, any negative q0 means
    acceleration rather than deceleration in the
    cosmic expansion rate, and would imply ?? gt ?M
    /20.15
  • Direct measurement of q0 would be able to confirm
    finding that ? is nonzero

17
The accelerating Universe
  • Huge clue came from observations of Type-1a
    Supernovae (SN1a)
  • Very good standard candles
  • Can use them to measure relative distances very
    accurately

18
Type 1A Supernovae
19
  • In the normal life of a star (main sequence)
  • nuclear fusion turns Hydrogen into Helium
  • In the late stages of the life of a massive star
  • Helium converted into heavier elements (carbon,
    oxygen, , iron)
  • At end of stars life, get an onion-like
    structure (see picture to right)

20
  • Whats special about iron?
  • Iron has the most stable nucleus
  • Fusing hydrogen to (eventually) iron releases
    energy (thus powers the star)
  • Further fusion of iron to give heavier elements
    requires energy to be put in
  • Can only happen in the energetic environment of a
    supernova explosion
  • So, all heavier elements are created during
    supernova explosions

21
Supernovae
  • What produces a SN1a?
  • Start off with a binary star system
  • One star comes to end of its life forms a
    white dwarf (made of helium, or carbon/oxygen)
  • White Dwarf starts to pull matter off other star
    this adds to mass of white dwarf (accretion)
  • White dwarfs have a maximum possible mass the
    Chandrasekhar Mass (1.4 MSun)
  • If accretion pushes White Dwarf over the
    Chandrasekhar Mass, it starts to collapse.

22
  • White Dwarf starts to collapse
  • Rapidly compresses matter in white dwarf
  • Initiated runaway thermonuclear reactions star
    turns to iron/nickel in few seconds
  • Liberated energy blows star apart
  • Resulting explosion briefly outshines rest of
    galaxy containing it these are the SN1a events
  • SN1a
  • No remnant (neutron star or black hole) left
  • Since white dwarf always has same mass when it
    explodes, these are standard candles (i.e.
    bombs with a fixed yield, hence fixed luminosity)

23
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24
H0 and q0 with SN1as
  • The program
  • Search for SN1a in distant galaxies
  • Compare expected power with observed brightness
    to determine distance
  • Measure velocity using redshift
  • Low redshift galaxies give measurement of H0
  • High redshift galaxies allows you to look for
    deceleration of universe

25
The results
  • This program gives accurate value for Hubbles
    constant
  • H72 km/s/Mpc
  • Find acceleration, not deceleration, at large
    distance!
  • Very subtle, but really is there in the data!
  • Profound result!

26
  • What does the future hold? Increasingly rapid
    expansion!

27
Dark Energy
  • There is an energy in the Universe that is
    making it accelerate
  • Call this Dark Energy
  • This makes up the rest of the gravitating energy
    in the Universe, and causes it to be flat!
  • Completely distinct from Dark Matter
  • Remember Einsteins cosmological constant?
  • Dark Energy has precisely the same effect as
    Einsteins cosmological constant
  • So, he was probably right all along!

28
What is dark energy?
  • An energy that is an inherent component of
    space
  • Consider a region of vacuum
  • Take away all of the radiation
  • Take away all of the matter
  • Whats left? Dark energy!
  • But we have little idea what it is

29
The Age of the Universe
  • Using this cosmological model, we can figure out
    the age of the Universe.
  • Answer 13.7 billion years
  • Prediction
  • There should be no object in the Universe that is
    older than 14 Gyr.
  • This agrees with whats seen!
  • This was a big problem with old cosmological
    models that didnt include dark energy
  • e.g age of the universe in ?M 1, ?k 0, ??0
    model is 9 billion years
  • But there are globular star clusters whose
    estimated ages are 12-14 billion years!
  • This was troubling since universe must be at
    least as old as the oldest stars it contains!

30
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31
Concordance model
  • In summary, the parameters for our Universe,
    using best available data
  • Hubble constant H072 km/s/Mpc
  • Geometry ?k 0 (flat)
  • Deceleration parameter q0?0.55
  • Baryon density ?B0.04
  • Dark matter density ?DM0.26
  • Cosmological constant ??0.7
  • Age t013.7 billion years

32
Deceleration Acceleration The Saga Continues
33
  • although we are far from understanding all the
    properties of the Universe, recent observations
    are bringing us to the era of precision
    cosmology!

34
Observing the Big Bang for Yourself
  • Olbers Paradox
  • Why is the darkness of the night sky evidence for
    the Big Bang?

35
Why is the darkness of the night sky evidence for
the Big Bang?
36
Olbers Paradox If universe were 1)
infinite 2) unchanging 3) everywhere
the same Then, stars would cover the night sky
37
Olbers Paradox If universe were 1)
infinite 2) unchanging 3) everywhere
the same Then, stars would cover the night sky
38
Night sky is dark because the universe changes
with time As we look out in space, we can look
back to a time when there were no stars
39
Night sky is dark because the universe changes
with time As we look out in space, we can look
back to a time when there were no stars
40
  • Why is the darkness of the night sky evidence for
    the Big Bang?
  • If the universe were eternal, unchanging, and
    everywhere the same, the entire night sky would
    be covered with stars
  • The night sky is dark because we can see back to
    a time when there were no stars

41
What aspects of the universe were originally
unexplained with the Big Bang theory?
42
Inflation
  • What aspects of the universe were originally
    unexplained with the Big Bang theory?
  • How does inflation explain these features?
  • How can we test the idea of inflation?

43
What is Inflation
  • Power law expansion rate of change R gets
    longer as the Universe expands. i.e. if R was 50
    smaller 10 Gyars ago it will be a factor of 2
    bigger 30 Gyears later
  • Rate of change of R constant expansion
    exponential- Universe could expand by a factor of
    1050 in a fe10-30 seconds
  • In GR rate of expansionr1/2 (doubling
    time1/r1/2)

44
Mysteries Needing Explanation
  • Where does structure come from?
  • Why is the overall distribution of matter so
    uniform?
  • Why is the density of the universe so close to
    the critical density?

45
Mysteries Needing Explanation
  • Where does structure come from?
  • Why is the overall distribution of matter so
    uniform?
  • Why is the density of the universe so close to
    the critical density?
  • An early episode of rapid inflation can solve all
    three mysteries!

46
How does inflation explain these features?
1 meter
47
Inflation can make all the structure by
stretching tiny quantum ripples to enormous
size These ripples in density then become the
seeds for all structures
48
How can microwave temperature be nearly identical
on opposite sides of the sky?
49
Regions now on opposite sides of the sky were
close together before inflation pushed them far
apart
50
Overall geometry of the universe is closely
related to total density of matter energy
Density Critical
Density gt Critical
Density lt Critical
51
Inflation of universe flattens overall geometry
like the inflation of a balloon, causing overall
density of matter plus energy to be very close to
critical density
52
How can we test the idea of inflation?
53
Patterns of structure observed by WMAP show us
the seeds of universe
54
Observed patterns of structure in universe agree
(so far) with the seeds that inflation would
produce
55
Seeds Inferred from CMB
  • Overall geometry is flat
  • Total massenergy has critical density
  • Ordinary matter 4.4 of total
  • Total matter is 27 of total
  • Dark matter is 23 of total
  • Dark energy is 73 of total
  • Age of 13.7 billion years

56
Seeds Inferred from CMB
  • Overall geometry is flat
  • Total massenergy has critical density
  • Ordinary matter 4.4 of total
  • Total matter is 27 of total
  • Dark matter is 23 of total
  • Dark energy is 73 of total
  • Age of 13.7 billion years

In excellent agreement with observations of
present-day universe and models involving
inflation and WIMPs!
57
What have we learned?
  • What aspects of the universe were originally
    unexplained with the Big Bang theory?
  • The origin of structure, the smoothness of the
    universe on large scales, the nearly critical
    density of the universe
  • How does inflation explain these features?
  • Structure comes from inflated quantum ripples
  • Observable universe became smooth before
    inflation, when it was very tiny
  • Inflation flattened the curvature of space,
    bringing expansion rate into balance with the
    overall density of mass-energy

58
What have we learned?
  • How can we test the idea of inflation?
  • We can compare the structures we see in detailed
    observations of the microwave background with
    predictions for the seeds that should have been
    planted by inflation
  • So far, our observations of the universe agree
    well with models in which inflation planted the
    seeds
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