Public talk: Studium Generale in Groningen (31 May 2018)

1. The Scientific Legacy
of Stephen Hawking
Daniel Baumann
University of Amsterdam

2. 1993

3. 1999

4. 2006

5. 2010
letter?

6. 2014

7. Outline
Big Bang
Time
2.
3.
In this talk, I will explain Hawking’s
main scientific achievements:
Black Holes
1.

8. Hawking on Black Holes

9. Black Holes
GR
Michell, 1784
Schwarzschild, 1916

10. LIGO, 2014
Black Holes

11. Things Always Get Worse
The entropy of the universe is always increasing.
entropy (= disorder)
The Second Law

12. “Black holes must have an entropy.”
Black Hole Entropy
Bekenstein, 1972

13. “Black holes don’t radiate and
hence can’t have an entropy.”
Hawking, 1972
Black Hole Entropy

14. Empty Space isn’t Empty
Heisenberg, 1927
positive
energy
negative
energy
The uncertainty principle of quantum mechanics
allows the spontaneous creation of virtual particles:

15. Black Holes aren’t Black
Near the event horizon of a black hole
these virtual particles can become real:
Hawking
radiation
Hawking, 1975

16. Hawking Radiation
Hawking, 1975
The corresponding temperature of a black hole is

17. Hawking Radiation
Hawking, 1975
The timescale of black hole evaporation is

18. Hawking’s Formula
entropy
area
Hawking, 1975
“OK, black holes do have an entropy.”

19. Newton’s constant
Gravity
Planck’s constant
Quantum Mechanics
Boltzmann’s constant
Thermodynamics
Speed of light
Relativity
Hawking’s Formula

20. Planck length:
Hawking’s Formula
area, not volume
A successful theory of quantum gravity
needs to explain Hawking’s formula.
10-35 m

21. Information Paradox

22. Information Paradox
some
information

23. Information Paradox
some
information
Hawking
radiation

24. Information Paradox
some
information
Hawking
radiation
more
information Information seems to get lost
in black hole evaporation.

25. Information Paradox
? out
in
This loss of information would violate unitarity,
a fundamental principle of quantum mechanics.

26. Information Paradox

27. Hawking on the Big Bang

28. The Big Bang
380,000 yrs
earlier later
time
hotter colder
e-
e-
e-
e-
e-
H He
e
temperature
• At early times, the temperature was too high for stable atoms to exist:
• The Universe is expanding it was hotter and denser in the past.

29. 380,000 yrs
earlier later
time
hotter colder
e-
e-
e-
e-
e-
H He
e
temperature
• Light was released and created the cosmic microwave background:
• 380,000 yrs after the Big Bang, the first stable atoms formed.
e-
e-
The Big Bang

30. The First Light
In 1965, Penzias and Wilson discovered the Big Bang’s afterglow:

31. 2.7 K
The signal was found to be completely featureless across the sky:
The First Light

32. 2001-2010
WMAP
COBE
Planck
1992
2013-2017
Looking more closely, future experiments have found fluctuations:
The First Light

33. 2001-2010
WMAP
COBE
Planck
1992
2013-2017
Looking more closely, future experiments have found fluctuations:
The First Light

34. What created the initial correlations?
These fluctuations aren’t just random noise, but are correlated over
large distances:
The First Light
Planck, 2015

35. Inflation
10-32 sec
10-34 sec
The observed correlations are explained if the early universe went
through a period of very rapid expansion called inflation:

36. Hawking Radiation
Hawking, 1983
Inflation turns virtual particles into real particles:

37. Structure Formation
380,000 yrs 13.8 billion yrs
10-33 sec
This explains the formation of all structure in the universe:

38. Hawking on Time

39. What happened before inflation?
?
How did the universe begin?

40. In 1983, Hawking suggested that quantum mechanics allows the
creation of pairs of universes:
Hartle and Hawking, 1983
universe
mirror
universe

41. Hartle and Hawking, 1983
What came before is a meaningless question.
space
imaginary
time
Hawking also proposed that, at the “beginning”, time behaved
like space and had no boundary:

42. real time
space
imaginary
time
time
This is a possible explanation for how inflation started.
Remarkably, the Hartle-Hawking process only creates universes
that start out inflating:

43. Thanks Stephen!
1942 - 2018

44. Bedankt voor uw aandacht
http://cosmology.amsterdam

45. Credits
Books
Discussions
In the preparation of this talk I have benefited from discussions with Jan Pieter
van der Schaar, John Stout, Guilherme Pimentel, Maulik Parikh, and especially
Thomas Hertog.
S. Hawking, A Brief History of Time
L. Susskind, Black Hole Wars
K. Thorne, Black Holes and Time Warps
Videos
J. Maldacena, Black Holes and the Structure of Spacetime