1) The document discusses the Big Bang theory of the origin and evolution of the universe from an extremely hot and dense state approximately 13.8 billion years ago. 2) It describes how small fluctuations in the cosmic microwave background from 380,000 years after the Big Bang are thought to have grown over time due to gravity to form the large scale structures we observe today like galaxies. 3) These fluctuations are theorized to have originated from quantum fluctuations during a brief period of exponential expansion called inflation within the first fraction of a second after the Big Bang, which would have stretched microscopic variations to cosmic scales.

1. THE BIG BANG
and the Origin of Structure
Big History Lecture
September 2022
Daniel Baumann
University of Amsterdam &
National Taiwan University

4. 10 000 galaxies
Hubble Ultra Deep Field

5. 100 000 light years
100 billion stars

6. 8 light minutes

7. Hubble Ultra Deep Field

8. Sloan Digital Sky Survey
1 trillion galaxies

9. The Observable Universe

10. Where did it all
come from?

11. The answer involves a fascinating connection between the physics
of the very small and the very large:

12. Outline
The Big Bang
Structure
Formation
Quantum Origin

13. Please ask
questions!

14. The Big Bang

15. 0 0.5 1 1.5 2
−0.2
0
0.2
0.4
0.6
0.8
1
1.2
×103
km/s
0 5 10 15 20 25 30 35
0
5
10
15
20
×103
Hubble (1929)
Hubble and Hummerson (1932)
In 1929, Edwin Hubble discovered the expansion of the universe:

16. The universe therefore started in a hot and dense state:
As the universe expands, it cools.
Many interesting things happened.
temperature
earlier later
time
hotter colder

17. t=0.0000000000000000001seconds
The universe is filled with almost equal
amounts of matter and antimatter
For some mysterious reason, there was initially a fraction more matter
than antimatter. This matter survived the annihilation.
10 000 000 001 10 000 000 000
matter
Without this asymmetry we wouldn’t exist.
As the universe cools, matter
and antimatter annihilate.
+ =
light

18. t=0.00001s: Quarks and gluons condense into nuclei:
temperature
time
u
d
u
u
d
d
proton
neutron
10 μs

19. t=1s: Neutrinos decouple and neutrons freeze out:
temperature
time
1 s
Free-streaming neutrinos
• 40% of the energy density
• Significant effect on the expansion

20. t=3min: Light elements (H, He, and Li) form:
temperature
time
H He
3 min
• Heavier nuclei were fused inside stars.
• Big Bang nucleosynthesis (BBN) predicts the
correct abundances of the light elements.
25%
75%

21. 380 000 yrs
e-
e-
e-
e-
e-
e-
H He
e
temperature
time
t=380 000yrs: Atoms form and the first light is released:
Free-streaming
photons

22. • 410 photons per cubic centimeter
• cooled by the expansion: 2.7 K
• faint microwave radiation: CMB
This afterglow of the Big Bang is still seen today:
0
100
200
300
400
Intensity
[MJy/sr]
100 200 300 400 500 600
−0.1
0
0.1
Frequency [GHz]
Cosmic Microwave Background

23. temperature
time
t>1billionyrs: Matter collapses into stars and galaxies:
1 billion yrs

24. This history of the universe is an observational fact:
10 μs 380 000 yrs
1 s 3 min
QCD phase
transition
Neutrino
decoupling
BBN
e
-
Photon
decoupling
Structure
formation
1 billion yrs
• The basic picture has been confirmed by many independent observations.
• Many precise details are probed by measurements of the CMB.

25. Where did it all
come from?

26. Structure
Formation

27. In 1965, Penzias and Wilson discovered the Cosmic Microwave Background:

28. The signal was the same in all directions.
If that had been the end of the story it would have been a disaster.
How would inhomogeneous structures have formed?

29. WMAP
COBE Planck
Looking more closely, the temperature of the CMB was found to vary with
direction.
1992
2001-2010
2013-2017

30. Hot spots correspond to dense regions.

31. These small density fluctuations grew over time and became the structures we
see around us: galaxies, stars, planets, …
380 000 years 13.8 billion years
gravity

32. • The growth of structure is only fast enough if the universe contains an
invisible form of dark matter.
Atoms
Dark matter
Dark energy
The nature of the dark matter and dark energy is a deep mystery, but that
would be the subject of another talk.
• The typical size of CMB fluctuations requires the existence of dark energy
(which is also seen in the acceleration of the cosmic expansion).

33. ?
380 000 years 13.8 billion years
10-32 sec
Instead I want discuss what created the initial fluctuations just fractions of a
second after the Big Bang.

34. Quantum
Origin

35. An important clue is that the CMB fluctuations aren’t random, but are correlated
over large distances:

36. Superhorizon
An important clue is that the CMB fluctuations aren’t random, but are correlated
over large distances:
90◦
18◦
0
1000
2000
3000
4000
5000
6000
Power
[µK
2
]
2◦
0.2◦
0.1◦
0.07◦
Angular separation

37. distance light travelled
since the Big Bang
In the standard hot Big Bang theory, this is impossible:
2
Big Bang
Observable
universe

38. 10-32 sec = 0.00000000000000000000000000000001 seconds
This can be explained if the early universe expanded faster than the speed of
light, doubling in size at least 80 times within a fraction of a second:
Inflation

39. The entire observable universe then originated from a small region of space
(the size of an orange):
CMB
inflation
Small fluctuations were stretched to enormous scales.

40. In quantum mechanics, empty space is full of violent fluctuations:

41. During inflation, quantum fluctuations get amplified and stretched:
The correlations observed in the afterglow of the Big Bang are inherited from
the correlations of the initial quantum fluctuations.

42. The predicted correlations are in remarkable agreement with the data:
Although the evidence for inflation is growing, the physical origin of the
inflationary expansion remains a mystery.
90◦
18◦
0
1000
2000
3000
4000
5000
6000
Power
[µK
2
]
2◦
0.2◦
0.1◦
0.07◦
Angular separation

43. Conclusions

44. 10 μs 380 000 yrs
1 s 3 min
e
-
1 billion yrs
We have a remarkably consistent picture of the history of the Universe from
fractions of a second after the Big Bang until today:
We also have tantalizing evidence that the primordial seed fluctuations for the
formation of structure were created during a period of inflation:

45. Yet, many fundamental questions remain:
• What is dark matter and dark energy?
• Did inflation really occur? And what was driving it?
• What is the origin of the matter-antimatter asymmetry?
• …
We hope that future observations will shed light on these questions.
Observations of the CMB have revolutionized cosmology:
⌦b
⌦m
⌦⇤
As , ns
We have a simple 5-parameter standard model.

46. Bedankt voor uw aandacht