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Seeing the Beginning: The Cosmic Microwave Background and What it Can Tell Us About Our Universe
1. Seeing the Beginning: !
The Cosmic Microwave Background !
and What it Can Tell Us About Our Universe!
Blake Sherwin!
Department of Mathematics and Theoretical Physics / Kavli Institute for Cosmology!
University of Cambridge!
2. Outline!
• Reminder: the expanding universe and the big bang!
• The Cosmic Microwave Background (CMB): evidence
for the big bang!
• What we have learned about the universe from patterns
in the CMB!
• What we don’t understand and are working on now!!
2!
3. Where are we in the universe?!
3!
300 billion stars!
>>100 billion galaxies!
4. We Have Observed an !
Enormous Number of Galaxies!
4!
Image: Hubble Deep Field!
5. We Have Observed an !
Enormous Number of Galaxies!
5!
Image: Hubble Deep Field!
6. • The further away galaxies are, the faster they seem to
be moving away from us!
All Galaxies Moving Away from Us! !
6!
speed
distance
7. 7!
galaxies!
distance
between
galaxies!
1 billion years after big bang!
Explanation: All distances are expanding with time!
• Space itself is expanding. The more space in between
two objects, the more it expands, so the faster they
(seem to) move away. We are not at the center of the
universe!!
!
16. • Space itself is expanding. The more space in between
two objects, the more it expands, so the faster they
(seem to) move away.!
!
• Expansion suggests universe used to be denser and
hotter, had a beginning in a fiery big bang!
Explanation: Space is expanding!
16!
+me
17. Outline!
• Reminder: the expanding universe and the big bang!
• The Cosmic Microwave Background (CMB): evidence
for the big bang!
• What we have learned about the universe from patterns
in the CMB!
• What we don’t understand and are working on now!!
17!
18. • Very early universe: so hot that heat holds apart
electrons and protons (like interior of the sun)!
• But: cools with expansion…atoms form after 400000
years!
!
Predictions of The Big Bang !
(Universe Expanding out of a Fiery Primordial Origin)!
18!
=
light
Hot
early
universe:
opaque
CMB
Cool
late
universe:
transparent
19. • Heat radiation (“light”) from the early fireball phase (like
the surface of the sun) streams through the universe…
to us !
!
Emission of the CMB from the Primordial Fireball!
19!
20. time (+ distance light travels!)!
Early big-bang
universe is dense
and hot plasma
(fire-like) !
History of the universe!
!
Emission of the CMB from the Big Bang Fireball!
21. time (+ distance light travels!)!
“afterglow”
light should
stream through
the universe…
to us:!
!
Cosmic!
Microwave!
Background!
Early big-bang
universe is dense
and hot plasma
(fire-like) !
Emission of the CMB from the Big Bang Fireball!
History of the universe!
22. time (+ distance light travels!)!
“afterglow”
light should
stream through
the universe…
to us:!
!
Cosmic!
Microwave!
Background!
Early big-bang
universe is dense
and hot plasma
(fire-like) !
Emission of the CMB from the Big Bang Fireball!
We see faraway objects in the past because the light has taken
long to reach us. So if I look out far enough into space I should
see far enough back in time that I see the “primordial fireball”.!
23. time (+ distance light travels!)!
“afterglow”
light should
stream through
the universe…
to us:!
!
Cosmic!
Microwave!
Background!
Early big-bang
universe is dense
and hot plasma
(fire-like) !
Emission of the CMB from the Big Bang Fireball!
Big bang theory makes a clear prediction: !
we should see light from the primordial “fireball” in the sky!
24. • Look out at sky… most of it is dark. Why don’t I see the
primordial fireball wherever I look?!
!
The CMB in the Sky?!
24!
No
fireball
L
just
see
Milky
Way
stars
L
25. • Answer: expanding universe stretches wavelength of
light (by 1000 times) from visible to microwave
wavelength!
• Our eyes can’t see this stretched radiation.!
!
The CMB in the Microwave Sky!
25!
0.0005
mm
0.5
mm
26. • But can build “microwave eyes” / telescopes!!
!
! !“A Measurement of Excess Antenna Temperature at 4080 Mc/s”"
!
The CMB in the Microwave Sky!
26!
Penzias/Wilson!
1968!
27. • But can build “microwave eyes” / telescopes!!
!
! !“A Measurement of Excess Antenna Temperature at 4080 Mc/s”"
!
The CMB in the Microwave Sky!
27!
Penzias/Wilson!
1968!
28. • But can build “microwave eyes” to see this radiation!!
!
! !“A Measurement of Excess Antenna Temperature at 4080 Mc/s”"
!
The CMB in the Microwave Sky!
28!
Penzias/Wilson!
1968!
"To get rid of them, we
finally found the most
humane thing was to
get a shot gun…and at
very close range we just
killed them instantly. It’s
not something I’m
happy about, but that
seemed like the only
way out of our
dilemma.”!
Arno Penzias!
29. • Radiation that looks like the heat radiation of a 2.7
degree K object without color – i.e. blackbody radiation!
Cosmic Microwave Background Light!!
29!
30. • Compare CMB spectrum measurement with predicted
blackbody spectrum: great confirmation of big bang!
!
Cosmic Microwave Background Light!!
30!
xkcd!
Frequency!
CMBBrightness!
31. Outline!
• Reminder: the expanding universe and the big bang!
• The Cosmic Microwave Background (CMB): evidence
for the big bang!
• What we have learned about the universe from patterns
in the CMB!
• What we don’t understand and are working on now!!
31!
32. • Radiation that looks like a blackbody / heat radiation of a
2.7 degree K object!
• Increase contrast by 1000: still uniform…!
!
Is the CMB Brightness Uniform?!
32!
33. • NO! Turn up “contrast” by 100000!
• See fluctuations in brightness across the sky:!
!
Is the CMB Brightness Uniform?!
33!
COBE Satellite Data !
(Nobel Prize for Smoot/Mather)!
brighter!
less bright!
34. • Enhance: send up better satellites or use better ground
based telescopes...!
!
CMB Fluctuations – Learning More!
34!
35. !
CMB Fluctuations – !
Best Picture From Planck Satellite !
35!
CMB Brightness Fluctuations!
36. !
CMB Fluctuations – !
Best Picture From Planck Satellite !
36!
Because we are observing light that has traveled to us for
such a long time from so far away, we are seeing the past.
This CMB image is a picture of the universe as it looked
13.8 billion years ago.!
37. !
CMB Fluctuations – !
Best Picture From Planck Satellite!
37!CMB Brightness Fluctuations (an actual picture of the Big Bang fireball)!
38. • Why are there these fluctuations in brightness?!
• Very simplified explanation: some regions have more
matter and so appear brighter.!
!
!
CMB Fluctuations!
38!
39. • Regions with more matter è pull in even more matter
through gravity è collapse to form galaxies, stars etc.!
• The CMB is the universe’s “baby picture” – from 13.8
billion years ago, almost the beginning (99.997% there) !
!
Why is this Important – !
Understand Cosmic Evolution!
39!
A bit more matter
than average at
start (“density
fluctuation”)!
Over time: a galaxy!
40. • Galaxies today grow from runaway gravitational collapse
of small initial density fluctuations as seen in the CMB!
Small Fluctuations in CMB turn into Galaxies!
40!
[Video:
A.
Kravtsov++]
41. • Regions with more matter è pull in even more matter
through gravity è collapse to form galaxies, stars etc.!
• The CMB is the universe’s “baby picture” – from 13.8
billion years ago, almost the beginning (99.997% there) !
!
Why is this Important – !
Understand Cosmic Evolution!
41!
A bit more matter
than average at
start (“density
fluctuation”)!
Over time: a galaxy!
42. • Regions with more matter è pull in even more matter
through gravity è collapse to form galaxies, stars etc.!
• The CMB is the universe’s “baby picture” – from 13.8
billion years ago, almost the beginning (99.997% there) !
!
Why is this Important – !
Understand Cosmic Evolution!
A bit more matter
than average at
start (“density
fluctuation”)!
Over time: a galaxy!
Where do fluctuations in matter density come from? [See later]!
43. • Our measurements of the CMB are now so precise that
we can use it as a tool to learn about the universe!
• Make PERCENT-level measurements of its composition,
age, geometry…!
!
Why is this Important II – !
Understand Cosmic Properties / Composition!
43!
44. • Determine the distance to the CMB fireball by looking at
size of large bright spots; gives size of the universe and
age.!
!
Example: Size of the Universe!
44!
too close! too far!correct!
distance!
what we observe!
45. • We don’t actually compare the fluctuations in the CMB
brightness (~T) by eye with our models to measure
things!
• Instead, we use the power spectrum – tells us the
magnitude of fluctuations as a function of angular size!
Power Spectrum!
45!
Size of !
Fluctuations!
Angle spanned on Sky!
best fit theory (with
6 free parameters)!
Planck!
data!
46. • Best described in Fourier / Spherical Harmonic space.!
• Spherical harmonic transform:!
• Measure power spectrum (variance)!
Details 1: Measure Power Spectrum of !
CMB Temperature (T)!
46!
Size of !
Fluctuations!
Cl!
Angle spanned on Sky!
(multipole l) !
best fit theory (with
6 free parameters)!
Planck!
data!
alm =
Z
dˆnT(ˆn)Ylm(ˆn)
Cl =
X
m
1
2l + 1
|alm|2
47. • Calculate Cl(parameters – for example age of universe)!
• How? Codes that rapidly solve differential equations for
evolution / propagation of linear photon perturbations!
!
Details 2: Theory Calculation!
47!
Size of !
Fluctuations!
Angle spanned on Sky!
best fit theory (with
6 free parameters)!
Planck!
data!
48. • Can determine: P(data|parameters) –" " " "
"Gaussian likelihood !
!
• But want: P(parameters|data)! Get via Bayes theorem: !
P(parameters|data) P(data|parameters) x P(parameters)!
!
!
• Example: want to know about P(size of universe|data)!
!
Details 3: Bayesian Parameter Estimation!
/
prior (choose!)!likelihood!posterior!
/ e [Cl Ctheory
l (parameters)]2
/(2 2
l )
49. • Problem: P(many parameters), i.e. P(size,θ1,θ2,...|data),!
• To get size distribution, need to integrate /“marginalize”: """!
"- hard as high dimensional. Fast: draw samples from P!
Details 4: Deriving Parameter Constraints!
P(size) =
Z
d✓1d✓2 · · · P(size, ✓i)
50. • Problem: P(many parameters), i.e. P(size,θ1,θ2,...|data),!
• To get size distribution, need to integrate /“marginalize”: """!
"- hard as high dimensional. Fast: draw samples from P!
• Great algorithm: MCMC = Markov-Chain Monte Carlo!
Details 4: Deriving Parameter Constraints!
Age of universe!
Θ1
(Matter
density)!
!
samples!
P(size) =
Z
d✓1d✓2 · · · P(size, ✓i)
51. – Origin and evolution of structure!
– Age: 13.798±0.037 billion years!
– Geometry: flat!
– Composition: see following…!
What have we learned from the CMB!
51!
Size of !
Fluctuations!
Angle spanned on Sky!
best fit theory (with
6 free parameters)!
Planck!
data!
52. But Galaxies Only Trace !
Even More Underlying Dark Matter!
52!
Galaxies! Dark Matter!
<20% of the mass!
Composition of the universe (in mass/energy): !
5% normal matter in galaxies and gas!
Galaxies, Gas: <20% of the mass!
53. 53!
Composition of the universe: !
24% dark matter!
!
• Dark matter: an invisible, poorly understood substance!
• We know it is there because it still has gravitational pull!
• Know not just from CMB, but also many other methods!
54. 54!
Composition of the universe: !
71% dark energy!
!
• Form of energy that is accelerating the universe apart!!
• Implies that universe will expand faster and faster,
leading to cold dilute universe!
55. 55!
71% of the universe: dark energy!
! • Form of energy that is accelerating the universe apart!!
• Implies that universe will expand faster and faster,
leading to cold dilute universe!
56. We have learned an !
enormous amount about the universe!!
!
!
!
!
!
!
We live in a simple universe, evolved out of
small density fluctuations and described by
just six numbers!
56!
57. We know a lot!!!!
!
!
!
!
!
!
!
But it seems the more we know, !
the more we don’t know… !
57!
??
?
???
fluctuations!
58. Outline!
• Reminder: the expanding universe and the big bang!
• The Cosmic Microwave Background (CMB) and what
we have learned from it: evidence for the big bang and
the standard cosmological model!
• What we still don’t understand and are working on now!!
58!
59. time (+ distance light travels!)!
Many Big Questions Left – Ongoing Research!!
• What happened at the
very beginning? !
"
• What and where is dark matter?"
![ask! – topic of my research]"
"
"
• What is dark energy?"
!
60. What happened at the very beginning? !
60!
• What blew the universe up in the first second of the big
bang and made it nearly smooth?!
!
!
!
!
!
!
!
!
CMB!
61. 61!
• Where do the tiny fluctuations in density come from?!
!
!
!
!
!
!
!
!
CMB
CMB!
What happened at the very beginning? !
64. Theory of Inflation!
64!
!
• Inflation: period of REALLY
fast initial universe expansion
(1040 in just 1/1035 seconds) –!
!
• Enormous expansion inflates
the microscopic to the
macroscopic. Random
quantum fluctuations get
magnified to big density
fluctuations.!
Quantum
uncertainty: empty
space is not really
empty!
65. 65!
Inflation blows
quantum fluctuations
up, generates small
differences in density!
!
Cosmic Inflation: Expanded!
Quantum Weirdness Makes Everything!
We can see these
density differences in
the CMB, a picture of
(nearly) the beginning!
Over time, they
turn into the stars
and galaxies we
see around us
today!!
66. 66!
Inflation blows
quantum fluctuations
up, generates small
differences in density!
!
Cosmic Inflation: Expanded!
Quantum Weirdness Makes Everything!
We can see these
density differences in
the CMB, a picture of
(nearly) the beginning!
Over time, they
turn into the stars
and galaxies we
see around us
today!!
Inflation: we (and everything) exist because of random quantum
vacuum fluctuations in the early universe!
67. Inflation – How to Test Experimentally?!
67!
!
• Lots of confirmed predictions already: !
– properties of CMB fluctuations (bell-curve distribution) !
– properties of CMB power spectrum!
– flatness of the universe…!
!
!
!
!
!
!
consistent with inflationary predictions!
Size of !
Fluctuations!
Angle spanned on Sky!
best fit theory
(with 6 free
parameters)!
Planck
data!
68. • Prediction not yet
seen: leaves a
characteristic swirly
pattern in the CMB
polarization, “B-
modes”:!
!
• Find this: confirm
inflation, understand
the beginning of the
universe!!
CMB!
Smoking Gun Evidence for Inflation: !
CMB B-mode Pattern
69. Understanding the Physics of Inflation with B-modes!
• CMB B-modes with different strengths correspond to
different kinds of inflation physics!
Strength!
of!
signal!
~size of typical fluctuations!
different kinds of inflation!
71. Problems with the BICEP B-modes!
71!
• Initially claimed to see swirly B-mode patterns!
• It turns out they were instead “boring” B-modes from
dust in our galaxy mimicking the inflation signal!
!
!
!
!
!
!
!
!
But: solvable problem, search continues at high precision!!
!
distribution of !
dust in our galaxy!
72. 72!
Search ongoing, e.g., with current Atacama Cosmology Telescope (ACT)!
• Ground based CMB telescope high in the Chilean Atacama desert!
73. ALMA!
ACT!
CLASS!
POLARBEAR!Existing!
Notional Pads for Simons Observatory Phase 2 and CMB S4!
And Upcoming Simons Observatory !
– 100 times more sensitive!
Notional Simons Observatory Phase 1 !
Power!
Control !Vehicle
s!
74. time (+ distance light travels!)!
We have learned a lot from the CMB, !
but there is much we still need to understand!
• What happened at the
very beginning? !
"
• What is dark matter?"
"
• What is dark energy?"
!
75. One thing is certain:!
The CMB will continue to give us insight into the beginning !
of the universe and the composition, evolution and ultimate
fate of the cosmos!
!
!
Thank you!!
!
!
!
!
75!
76. Summary!
• The CMB is a picture of the afterglow of the fiery early
universe: our best proof of the hot big bang.!
• From the CMB picture: we live a simple yet strange
universe, with primordial density fluctuations, 24% dark
matter, and 71% dark energy!
• We are working on understanding the details of what
happened at the very beginning (and more)!
76!
77. CMB Gravitational Lensing!
• Distribution of dark matter (80% of the mass!) deflects CMB light
that passes through!
77!
78. “Light” Source for Lensing: !
The Cosmic Microwave Background (CMB)!
• CMB: leftover radiation from the hot primordial plasma – most
distant observable source of light!
• Distribution of dark matter deflects light that passes through!
78!
79. 79!
!
• Look for characteristic lensing magnification feature to map
distribution of dark matter!
Mapping Dark Matter with !
Gravitational Lensing of the CMB!
CMB with lensing feature! Map of dark matter!
80. ACTPol CMB Lensing Dark Matter Map (51 degs. long)!
Mapping all the Dark Matter!
!
• Precise lensing map of dark matter from ACTPol:!
Light spot: a real blob of dark matter!
81. ACTPol CMB Lensing Dark Matter Map (51 degs.
long)!
Mapping all the Dark Matter!
!
• Precise lensing map of dark matter from ACTPol:!
Does the dark matter look as we expect?! By removing lensing “blurring”: what
happened at the beginning of the
universe?!
82. Outline!
• The expanding universe and the big bang!
• Why the big bang leads to the Cosmic Microwave
Background (CMB)!
• Patterns in the CMB!
• Ongoing research II: seeing dark matter with CMB
lensing!
82!
83. CMB Gravitational Lensing!
• Distribution of dark matter (most of the mass!) deflects CMB light
that passes through!
83!
84. “Light” Source for Lensing: !
The Cosmic Microwave Background (CMB)!
• CMB: leftover radiation from the hot primordial plasma – most
distant observable source of light!
• Distribution of dark matter deflects light that passes through!
84!
85. 85!
• Clump of dark matter in front…!
Lensing: Effect on the CMB!
86. 86!
!
• Dark matter causes lensing magnification feature in the CMB!
Lensing: Effect on the CMB!
87. 87!
!
• Dark matter causes lensing magnification feature in the CMB!
Lensing: Effect on the CMB!
d!
d!
d!
described by
lensing
deflection
field: d!
!
(very small:
here
exaggerated
by x ~100)!
88. CMB Lensing: How to Measure!
d: lensing deflection
field!
CMB!CMB!
Measure lensing by looking for magnification of
characteristic scales in CMB correlations
89. CMB Lensing: How to Measure!
CMB!
[Seljak
1997;
Hu
2001]
d: lensing deflection
field!
Measure lensing by looking for magnification of
characteristic scales in CMB correlations
90. ACTPol CMB Lensing Dark Matter Map (51 degs.
long)!
Mapping all the Dark Matter!
!
• From first detections in temperature (ACT) and polarization
(POLARBEAR) to precise lensing maps of dark matter:!
Does the dark matter look as we expect?! By removing lensing “blurring”: what
happened at the beginning of the
universe?!