2023年6月5日

1. Introduction to modern astronomy 28
島袋隼⼠(Hayato Shimabukuro)（云南⼤学、
SWIFAR）
©GETTYIMAGES

2. 17.Early universe
（早期宇宙）

3. Q
.
If the universe is expanding, wouldn't the universe be smaller when we go back in time?
A
.
Yes, the early universe is smaller, hotter, and denser!
Light elements such as hydrogen, deuterium, helium, and lithium formed in the hot,
dense early universe (Big Bang nucleosynthesis). The Big Bang is not the explosion of
the universe, but the creation of elements in a hot, dense universe
Bigbang nucleosynthesis

4. Q
.
If the universe is expanding, wouldn't the universe be smaller when we go back in time?
A
.
Yes, the early universe is smaller, hotter, and denser!
Light elements such as hydrogen, deuterium, helium, and lithium formed in the hot,
dense early universe (Big Bang nucleosynthesis). The Big Bang is not the explosion of
the universe, but the creation of elements in a hot, dense universe
Bigbang nucleosynthesis

5. p
h
o
t
o
n
s
pnD
First, deuterium (D) is synthesized from protons and neutrons.
However, deuterium is easy to be decayed and is destroyed by
photons (γ).
The first 3 minutes
p + n ⟷ D + γ

6. pnD
When the temperature of the universe reaches below
0.1MeV (about 3 minutes after the birth of the universe),
photons no longer have the energy to destroy deuterium,
and a sufficient amount of deuterium is produced.
The first 3 minutes
p
h
o
t
o
n
s
p + n ⟶ D + γ

7. H
e
D
Helium-4 is then synthesized in a series of two-body reactions.
D
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D + D ! 3
He + n
3
He + D ! 4
He + p
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D + D ! 4
He +
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D + D ! 3
H + p
3
H + D ! 4
He + n
Other paths to produce helium-4
The first 3 minutes
p
h
o
t
o
n
s

8. Bigbang nucleosynthesis
•Big bang theory predicts the abundance of light elements. In particular, the abundance
of helium-4 is important.
0.25%

9. •If the big bang really occurred in the early universe, we can observe the CMB photons!
Cosmic microwave background (CMB)
•Remember the big bang theory, there exists photons.
•The universe is filled with photons that are a remnant of the Big Bang. These photons are
called cosmic microwave background radiation (CMB).

10. •If the big bang really occurred in the early universe, we can observe the CMB photons!
Cosmic microwave background (CMB)
•Remember the big bang theory, there exists photons.
•The universe is filled with photons that are a remnant of the Big Bang. These photons are
called cosmic microwave background radiation (CMB).

11. Cosmic microwave background (CMB)
TCMB = 2.73[K]
•Yes, Penzias and Wilson first discovered the CMB photons!
•They discovered that CMB photons uniformly exist on
the whole of the sky and its temperature is ~3K
Big Bang occurs everywhere!

12. Cosmic microwave background (CMB)
•Remember the black body radiation. The spectrum
of stars are approximated by blackbody radiation.
•The CMB spectrum has almost perfect blackbody
radiation.
•You can find the error is very small.
•The COBE satellite measured the
spectrum of CMB very accurately.
COBE

13. Cosmic microwave background (CMB)
•The most important measurement by COBE is not the spectrum of CMB, but the measurement
of fluctuations of CMB temperature.

14. Cosmic microwave background (CMB)
•The most important measurement by COBE is not the spectrum of CMB, but the measurement
of fluctuations of CMB temperature. T
T
⇠ 10 5

15. Cosmic microwave background (CMB)
•The most important measurement by COBE is not the spectrum of CMB, but the measurement
of fluctuations of CMB temperature. T
T
⇠ 10 5

16. •Recent Planck satellite measures the fluctuations of CMB
temperature more accurately.
T
T
⇠ 10 5
Cosmic microwave background (CMB)

17. •Recent Planck satellite measures the fluctuations of CMB
temperature more accurately.
T
T
⇠ 10 5
Cosmic microwave background (CMB)

18. •Recent Planck satellite measures the fluctuations of CMB
temperature more accurately.
T
T
⇠ 10 5
Cosmic microwave background (CMB)

19. •Recent Planck satellite measures the fluctuations of CMB
temperature more accurately.
T
T
⇠ 10 5
Cosmic microwave background (CMB)
•The fluctuations of CMB temperatures provide us fruitful cosmological information.

20. •To evaluate the CMB fluctuations, we describe them by power spectrum
•We can find measured CMB power spectrum can be explained by theory very well!
Cosmic microwave background (CMB)

21. •To evaluate the CMB fluctuations, we describe them by power spectrum
•We can find measured CMB power spectrum can be explained by theory very well!
Cosmic microwave background (CMB)
The age of the universe is 138亿岁

22. The fluctuations and structure formation in the universe
•Not only the fluctuations of temperature, but dark matter density fluctuations also exist.
•The matter density fluctuations are seed of the structure formation in the universe
Fluctuations
Positions
If the matter density fluctuations are
beyond the criterion, structure formation
occurs.

23. The fluctuations and structure formation in the universe

24. The fluctuations and structure formation in the universe
High matter density regions
The gravitational force is strong, and thus dark
matter particles get together and form dark
matter halo.

25. The fluctuations and structure formation in the universe
High matter density regions
The gravitational force is strong, and thus dark
matter particles get together and form dark
matter halo.
Dark matter halo

26. The fluctuations and structure formation in the universe
High matter density regions
The gravitational force is strong, and thus dark
matter particles get together and form dark
matter halo.
Dark matter halo
•Stars and galaxies are formed in the dark matter halos.

27. The fluctuations and structure formation in the universe
High matter density regions
The gravitational force is strong, and thus dark
matter particles get together and form dark
matter halo.
Dark matter halo
•Stars and galaxies are formed in the dark matter halos.

28. The fluctuations and structure formation in the universe
The matter density fluctuations form the current universe!
But, why is there density fluctuations in the
universe?

29. The origin of fluctuations
•At the beginning of the universe , only quantum（量⼦） fluctuations exist.
(t = 0)
•The size of the universe was extremely small ( )
∼ 10−26
m

30. The origin of fluctuations
•The universe expands rapidly during a very short time( ). This is called cosmic inflation.
10−34
s
•After inflation, the size of the universe becomes from
∼ 102
m ∼ 10−26
m
•Currently, the size of the universe we can observe is Mpc. But, the size of the
unobservable universe extends to Mpc beyond the observable universe!
∼ 104
105
− 106
∼ 102
m
∼ 105
− 106
Mpc

31. The origin of fluctuations
•After inflation, the quantum fluctuations also grow up and become a seed of the structure
of the universe.

32. Exploring the cosmic inflation
We have not observed evidence of cosmic inflation!
Now, we have some projects to explore the cosmic inflation.
•If cosmic inflation occurs in the early universe, the gravitational wave by the inflation imprints
special polarization (B-mode) on the CMB.

33. Exploring the cosmic inflation
We have not observed evidence of cosmic inflation!
Now, we have some projects to explore the cosmic inflation.
•If cosmic inflation occurs in the early universe, the gravitational wave by the inflation imprints
special polarization (B-mode) on the CMB.

34. The history of the universe

35. Summary
• During 3 minutes, light elements were formed (Big Bang)
• The cosmic microwave background (CMB) is a remnant of
the Big Bang and the CMB tells us fruitful cosmological
information.
• The matter density
fl
uctuations are seeds of the structure
formation in the universe.
• The density
fl
uctuations are embedded during
cosmological in
fl
ation.