multiverse partition function and maximum entropy principle · 2. anthropic principle. (one of the...
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Multiverse Partition function and
Maximum Entropy Principle
Dec. 18, 2018
at KEK
Hikaru Kawai
(Kyoto Univ.)
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Special features of SM and the Higgs field have been revealed by LHC.
In particular, they have found that(i) SM can be valid up to the Planck
scale.(ii) The effective potential of the Higgs
field is flat around the Planck scale.⇒ Critical Higgs inflation
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Bare couplings as a function of the cutoff
SM is valid up to the Planck scale.
Top mass from[1405.4781][Hamada, Oda, HK ,1210.2538, 1308.6651]
mHiggs =125.6 GeV
2
1 2
1
16I
=
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Higgs potential with
the running coupling 4
4V
mH = 125.6 GeV
mHiggs =125.6 GeV
𝜑[GeV]
MPP
MEP
MEP: Maximal entropy principleMPP: Multiple Point Criticality Principle
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If we introduce a non-minimal coupling
a realistic Higgs inflation is possible.
2R
2 2
, .1 /
h h
P
hV
h M
In the Einstein frame the
effective potential becomes
𝜉 can be as small as 10.
Critical Higgs inflation
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I will discuss the possibility thatsuch behavior of the Higgs field is understood as a consequence of quantum gravity / string theory in the context of the naturalness problem.
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Naturalness problem
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Suppose the underlying fundamental theory, such as string theory, has the momentum scale mS and the coupling constant gS .
The naturalness problem
Then, by dimensional analysis and the power counting of the couplings, the parameters of the low energy effective theory are given as follows:
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dimension 2 (Higgs mass)
dimension 4
(vacuum energy or cosmological constant)
dimension 0
(gauge and Higgs couplings)
dimension -2 (Newton constant)
2
2.S
N
S
gG
m
1 2 3
2
, , ,
.
S
H S
g g g g
g
2 22 0 .H S Sg mm
2 40 .Ss mg
222 2 2 17100GeV 10 GeVH S Sm g m
44 4 172 ~ 3meV 10 GeVSm
unnatural ! →
unnatural ! !→
tree
(cont’d)
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The real values of the cosmological constant and Higgs mass are very unnatural:
If we consider everything only in the ordinary field theory, they are free parameters. In principle we can give any values to them by hand, but still we need fine tunings in order to get our universe.
Or, if we assume the existence of the underlying fundamental theory like string theory, we need to explain the values of the parameters that look extremely unnatural from the point of view of the low energy effective local field theory.
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SUSY as a solution to the naturalness problem
Bosons and fermions cancel the UV divergences:
However, SUSY must be spontaneously broken at some momentum scale MSUSY , below which the cancellation does not work.
+
bosons fermions
⇒2 2
SUSY .Hm M
+ ⇒ 0.
⇒
2 0.Hm
⇒4
SUSY .M
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(cont’d)
Therefore, if MSUSY is close to mH , the Higgs mass is naturally understood, although the cosmological constant is still a big problem.
However, no signal of new particles is observed in the LHC below 2 TeV.
We have to think about other possibilities.
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Possible attitudes to the naturalness problem
other than SUSY
1. We do not have to mind. We should simply take the parameters as they are.
2. Anthropic principle. (one of the variations)
In some models, the wave function of the universe is a superposition of various worlds, each of which obeys different low energy effective theory:
1 2 3 .
We are sitting in one of them, where the parameters should be such that human beings can appear in the history of the universe.
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3. The parameters are fixed by some mechanisms
that are not in the ordinary field theory.
There are several attempts which are related:
• asymptotic safetyShaposhnikov, Wetterich
• multiple point criticality principleFroggatt, Nielsen.
• classical conformalityMeissner, Nicolai,
Foot, Kobakhidze, McDonald, Volkas
Iso, Okada, Orikasa.
• baby universe and multi-local actionColeman
Okada, Hamada, Kawana, Sakai, HK
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I will discuss the last one, and show that there is a universal non-perturbative effect in quantum gravity / string theory that provides a mechanism to tune the parameters automatically.
“Nature does fine tunings!”
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Low energy effective theoryof
QG / string theory
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Consider the low energy effective theory of
quantum gravity / string theory obtained after
integrating out the short distance fluctuations.
eff gauge matter ...DS d x g R
Usually people believes that it should be a
local action:
Is it true?
Basic question
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As we will see, the low energy effective
theory has the multi-local form:
eff 1 2, ,
,
( ) ( ).
i i i j i j i j k i j k
i i j i j k
D
i i
S f S S
c S c S S c S S S
S d x g x O x
Here 𝑶𝒊 are gauge invariant scalar local
operators such as
𝟏 , 𝑹 , 𝑹𝝁𝝂𝑹𝝁𝝂 , 𝑭𝝁𝝂 𝑭
𝝁𝝂, 𝝍𝜸𝝁𝑫𝝁𝝍 ,⋯ .
It is not sufficient if we take the fluctuation of
space-time topology into account.
Sugawara ‘82 Nambu ‘84 Linde ‘87 Coleman ‘89
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Start with Euclidean path integral that
involves the summation over topologies
and consider the low energy effective theory
obtained after integrating out the short-
distance configurations.
topology
exp ,dg S
One way to understand such action is
to consider the space-time wormhole like
configurations as Coleman did.
Why multi-local ?
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Among such short-distance configurations
there should be a space-time wormhole-like
configuration in which a thin tube connects
two points on the space-time. Here, the two
points may belong to either the same universe
or different universes.
Do not confuse with 3D
wormhole.
We are considering 4D
configuration such that
a baby universe is
virtually created and
absorbed.
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If we see such wormhole-like configuration
from the side of the large universe(s), it looks
like two small punctures.
But the effect of making a small puncture is
equivalent to an insertion of a local operator.
Furthermore, if we integrate over the gauge
field and the metric on the tube, each
operator is projected to a gauge invariant
scalar operator.
integrate𝑨𝝁, 𝒈𝝁𝝂
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4 4
,
( ) ( ) ( ) ( ) exp .i j i j
i j
c d x d y g x g y O x Odg Sy
Therefore, a wormhole contributes to the
path integral as
x
y
𝒄𝒊𝒋 are numbers that are determined by the
path integral over the tube, and expected to be
of order 1 in the Planck unit.
Here 𝑶𝒊 are gauge invariant scalar local
operators such as
𝟏 , 𝑹 , 𝑹𝝁𝝂𝑹𝝁𝝂 , 𝑭𝝁𝝂 𝑭
𝝁𝝂, 𝝍𝜸𝝁𝑫𝝁𝝍 ,⋯ .
𝒙 and 𝒚 are
positions in the
large universe(s).
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If we sum over the number of wormholes, we have
4 4
,
( ) ( ) ( ) ( )exp .i j
i j
i j
c d x d y g x g y O xg yS Od
4 4
0 ,
4 4
,
1( ) ( ) ( ) ( )
!
exp ( ) ( ) ( ) ( ) .
n
i j
i j
n i j
i j
i j
i j
c d x d y g x g y O x O yn
c d x d y g x g y O x O y
The wormholes induce the “bi-local” action:
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bifurcated wormholes
⇒ cubic terms, quartic terms, …
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.)()(
,eff
xOxgxdS
SSScSScScS
i
D
i
kji
kji
kjij
ji
iji
i
ii
“ The low energy effective theory of quantum
gravity that involves topology change is not
simply local but multi-local.”
Thus we have seen
In short,
“topology change induces multi-local terms
in the low energy effective theory.”
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In the case of Coleman, he considered the
wormhole as a classical solution of the
Euclidean 4D theory like instanton.
Coleman had essentially the same result in
1989.
Comment on Coleman’s
Here we are considering a Wilsonian
effective theory.
We integrate over wormhole-like
configurations no matter whether they are
classical solutions or not.
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We have seen that we have the multi-local
action irrespectively to the existence of the
classical solution.
Probably, theories with this property are more
natural than those without it.
This is because we have to introduce
additional conditions on the path integral if
we want to fix the topology of the space-time.
What we need is only the property that
topology change of space-time is included in
the theory.
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Also string theory seems to contain topology
change of space-time automatically.
In this sense the multi-local action can be
regarded as a universal form of the low energy
effective theory of quantum gravity / string
theory.
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for the multi-local action
eff ,
( ) ( ),
i i i j i j i j k i j k
i i j i j k
D
i i
S c S c S S c S S S
S d x g x O x
effexpZ d S
We want to do the path integral
Consequences of multi-local action
where 𝒄𝒊 , 𝒄𝒊𝒋 , 𝒄𝒊𝒋𝒌 ⋯ are constants of O(1) in
the Planck unit.
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we introduce the Laplace transform
effexp exp .i i
i
Z d S d w d S
Coupling constants are not merely constants,
but they should be integrated.
eff 1 2 1 2exp , , , , exp .i i
i
S S S d w S
Then the path integral becomes
Regarding 𝑺𝐞𝐟𝐟 as a function of 𝑺𝒊’s
eff eff 1 2, , ,S S S S
path integral for a local action 𝝀𝒊 𝑺𝒊.𝝀𝒊 are coupling constants of the action.
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effexp ( ),
( ) exp .i i
i
Z d S d w Z
Z d S
This contradicts with our experience.
We know coupling constants are just
constants, and we never observe a
superposition of different coupling constants.
We have seen that the partition function is
given by averaging 𝒁(𝝀) with weight 𝒘(𝝀).
However, the problem is resolved if 𝒁(𝝀) has
a sharp peak around some value 𝝀~𝝀(𝟎). In that case we can say that the nature itself
tune the parameters to 𝝀(𝟎).
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An interesting point of this scenario is that
𝒘(𝝀) is not important if it is a smooth
function of 𝝀.
We are not sure if it is the case or not, but it
is possible because exp(−Seff) may be a well-
behaved function of 𝑺𝒊’s, and so is its Laplace
transform.
In that case the so called Big Fix occurs:
All the couplings of the low energy effective
theory are fixed by themselves, and we do
not need precise information about the short
distance physics.
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Necessity of multiverse
Even if we start from a connected universe, we
have disconnected universes after integrating
out the short-distance fluctuations.
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exp i i
i
Z d w d S
single
0
single
1( )
!
exp ( ) .
n
n
d w Zn
d w Z
n
Therefore we have to sum over the number of
universes in the evaluation of the partition
function.
Here 𝒁𝐬𝐢𝐧𝐠𝐥𝐞 𝝀 is the partition function for a
single universe.
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“solution” to the cosmological constant problem
single ( ) exp .Z dg gR g 4Sr
2 4exp
exp 1/ , 0
no solution, 0
dr r r
r
S
1
dominates irrespectively of 0 .w
For simplicity, we keep only the cosmological
constant 𝚲.
If the universe is large, 𝒁𝐬𝐢𝐧𝐠𝐥𝐞(𝚲) is evaluated
by considering 𝑺𝟒 geometry classically:
singleexp ( ) .Z d w Z
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However this argument is problematic.
WDW eq.
←wrong sign
“Ground state” does not exist.
total 0H
total universe matter graviton
2
universe
1
2a
H H H H
H p
Wick rotation is not well defined. t
matter ,H
universeH
: radius of the universea
matterH is bounded from below.
universeH is bounded from above.
Problem of the Wick rotation for gravity
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single ( ) exp .Z dg gR g
2 4exp
exp 1/ , 0
no solution, 0
dr r r
r
S
1
Actually what we have calculated
is a wrong sign version of the tunneling
probability with which universe pops out from
nothing through the potential barrier.
2 4exp
exp 1/ , 0
no solution, 0
dr r r
r
S
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Systems with gravity should be expressed in
Lorentzian signature.
Therefore the argument we have employed
to get the multi-local action is not literally
true.
But we expect that the low energy effective
theory is still given by the multi-local action.
In fact we obtain the same effective
Lagrangian in the IIB matrix model with
Lorentzian signature.
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Multi-local action in Lorentzian signature
Okada, HK
Hmada, Kawana, HK
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We start with the multi-local action:
eff 1 2, ,
,
( ) ( ),
i i i j i j i j k i j k
i i j i j k
D
i i
S f S S
c S c S S c S S S
S d x g x O x
where 𝑶𝒊 are gauge invariant scalar local
operators such as
𝟏 , 𝑹 , 𝑹𝝁𝝂𝑹𝝁𝝂 , 𝑭𝝁𝝂 𝑭
𝝁𝝂, 𝝍𝜸𝝁𝑫𝝁𝝍 ,⋯ .
Integrating coupling constants
We repeat the arguments in the Lorentzian
signature.
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We express 𝐞𝐱𝐩(𝒊𝑺𝐞𝐟𝐟) by a Fourier transform as
1 2 1 2exp , , , , exp ,eff i i
i
iS S S d w i S
where 𝝀𝒊’s are Fourier conjugate variables to 𝑺𝒊’s,
and 𝒘 is a function of 𝝀𝒊’s .
1 2 1 2exp , , , , exp ,eff i i
i
iS S S d w i S
Then the path integral for 𝑺𝐞𝐟𝐟 becomes
𝒊𝝀𝒊 𝑺𝒊 is an ordinary local action where 𝝀𝒊 are regarded
as the coupling constants.
Therefore the system is the ordinary field theory,
but we have to integrate over the coupling constants
with some weight 𝒘(𝝀).
If a small region 𝝀~𝝀(𝟎)dominates the 𝝀 integral,
it means that the coupling constants are fixed to 𝝀(𝟎).
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Warming up before considering multiverse
A simplified analysis ’14 ’15 Hamada, Kawana, HK
Because our universe has been cooled down for long time.
we may approximate 𝒁 𝝀 = 𝐞𝐱𝐩 −𝒊𝑽𝑬𝒗𝒂𝒄 𝝀 ,
where 𝑬𝒗𝒂𝒄 𝝀 is the vacuum energy density and 𝑽 is the
space-time volume.
1) extremum
If 𝑬𝒗𝒂𝒄 𝝀 is smooth and has an extremum at 𝝀𝒄 ,
𝒁 𝝀 = 𝐞𝐱𝐩 −𝒊𝑽𝑬𝒗𝒂𝒄 𝝀
~𝟐𝝅
𝒊 𝑽|𝑬𝒗𝒂𝒄′′ 𝝀𝒄 |
𝜹 𝝀 − 𝝀𝒄 +𝑶(𝟏
𝑽)
Thus 𝝀 is fixed to 𝝀𝑪 in the limit 𝑽 → ∞ .
vacE
C
Nature does fine tunings
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2) Kink (need not be an extremum)
If 𝑬𝒗𝒂𝒄 𝝀 has a kink (first order phase transition) at 𝝀𝒄 ,
𝒁 𝝀 = 𝐞𝐱𝐩 −𝒊𝑽𝑬𝒗𝒂𝒄 𝝀
~𝒊
𝑽
𝟏
𝑬𝒗𝒂𝒄′(𝝀𝒄 + 𝟎)−
𝟏
𝑬𝒗𝒂𝒄′(𝝀𝒄 − 𝟎)𝜹 𝝀 − 𝝀𝒄 + 𝑶(
𝟏
𝑽𝟐)
Thus 𝝀 is fixed to 𝝀𝑪 in the limit 𝑽 → ∞ .
𝑎𝑏dx exp 𝑖𝑉𝑥 𝜑 𝑥
=1
𝑖𝑉exp 𝑖𝑉𝑥 𝜑 𝑥
a
b
+ O(1
V2)
𝑎𝑏dx exp 𝑖𝑉𝑓(𝑥) 𝜑 𝑥
=1
𝑖𝑉exp 𝑖𝑉𝑓(𝑥 )
1
𝑓′(𝑥)𝜑 𝑥
a
b
+ O(1
V2)
(𝑓is monotonic)
vacE
C
monotonic
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If we consider the time evolution of the universe
and multiverse, 𝒁(𝝀) is not so simple.
Generalization
However, even if we do not know the precise form of
𝒁(𝝀), we can expect a similar mechanism to the
simplified cases.
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Actually in some cases the couplings can be
determined without knowing the details of 𝒁(𝝀).
QCD
Z
1. It becomes important only after the QCD phase
transition.
2. The mass spectrum of hadrons is invariant under
𝜽𝑸𝑪𝑫 → −𝜽𝑸𝑪𝑫 .
⇒ It is expected that 𝒁 is almost even in 𝜽𝑸𝑪𝑫.
⇒ 𝜽𝑸𝑪𝑫 is tuned to 0 if there are no other extrema.
(1) Symmetry example 𝜽𝑸𝑪𝑫 Nielsen,Ninomiya
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Conditions:
1. Physics changes drastically at some
value of the coupling 𝝀𝑪 .
2. 𝒁 is monotonic elsewhere.
⇒ The coupling is tuned to 𝝀𝑪 as we have discussed.
(2) Edge or the point of drastic change
Z
C
This explains the Multiple Point criticality Principle
proposed by Froggatt and Nielsen:
“The parameters of nature are tuned such that the
vacuum is at a (multiple) criticality point.”
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Examples:
(i) Cosmological constant
The behavior of the universe
at the late stages changes
drastically when the cosmological constant becomes
zero.
⇒ The cosmological constant is tuned close to 0.
(ii) Higgs self coupling
SM parameters seem to be chosen in such a way that
the vacuum is marginally stable.
However, there is a competition with the flat potential.
∞finite
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Multiverse in Lorentzian signature
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effexp exp .i i
i
Z d i S d w d i S
As in the Euclidean case it is natural to consider
the multiverse.
1
0
1 single universe
1exp
!
exp
n
i i
i n
i i
i
d i S Zn
Z d i S
n
1exp .d w Z
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Path integral for a universe
2 31 1 1ˆ .2
universe aH p a aa a
: radius of the universea
mini-superspace -Born Oppenheimer approx.-
universe m-g ( ) ,a
S3 topology for space total universe m-gˆ ˆ ˆ ,H H aH
adiabatic wave function for matter-graviton
m-g m-g m-gˆ ( ) ( ) ( )H a a E a a
total Hamiltonian:
changes adiabatically with 𝒂 .m-g ( )a
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universe m-g ( ) ,a
2 3 4
1( ) matt radC C
U aa a a
m-g m-g m-gˆ ( ) ( ) ( ) .H a a E a a
total univers
3
3
2
e
2
ˆ ˆ ( )
1 1 1
2
.1 1 1 ˆ( )2
radmatt
H H E a
Cp a a C
aa a
p a U a Ha a
( ) radmatt
CE a C
a
⇒2 31 1 1ˆ .
2universe aH p a a
a a
acts on universe astotal universe m-gˆ ˆ ˆH H H
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i
f
2 31 1 1ˆ ( )2
H p a U aa a
Question:
Is there a natural choice for them?
T
Now the path integral is evaluated as usual, if the
initial and final states are given:
ˆE E
E E
H E
E E
2 3 4
1( ) matt radC C
U aa a a
1
1
0
0 0
exp
exp
ˆexp
ˆ
E E
Z d i S
f dadpdN i dt pa NH i
f dT iTH i
f H i
f i
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Initial state
For the initial state, we assume that the universe
emerges from nothing with a small size ε.
,
: probability amplitude of a universe emerging.
i a matter
a
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Λ~curvature ~energy density
S3 topology
a
cr cr
cr
( )U a ( )U a ( )U a
mat rad
2 3 4
1( )
C CU a
a a a
For the final state, we have two possibilities.
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Final state: case 1
finite
The universe is closed.
We assume the final state is
.f a matter
The path integral
1 0 0
2
0
ˆ
.
E E
E
Z f H i f i
const
cr
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Final state: case 2
∞
The universe is open.
It is not clear how to define the
path integral for the universe:
m-glim ( ) . IR
IR IR IRa
f c a a a
a
1 exp .Z d i S
As an ad hoc assumption we consider
cr
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*
1 0 0 0 0
3 *
04
3 *
04
1sin
1sin .
E E IR E IR E
IR IR E
IR
IR E
Z f i c a a
c a aa
c a
Then the partition function becomes
The result does not depend on except for the phase
which comes from the classical action.IRa
(cont’d)
mat rad
2 3 4
1( ) ,
C CU a
a a a
01 0
1sin ,
z
E a da p aa p a
4, 2 ( ).p a a U a
WKB solution
2 31 1 1ˆ ( ).2
H p a U aa a
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Thus we have
the path integral for a universe
∞
finite
1Z
crfor of order 1,const
crfor 3
4
1sin .IRconst a
Then the integration for the multiverse
1exp .Z d w Z
has a large peak at , which means that
the cosmological constant at the late stages of the
universe almost vanishes.
cr
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Maximum entropy principle
Then the multiverse partition function is given by
cr rad1/C
Maximum entropy principle (MEP)
The low energy couplings are determined in such a way that the entropy at the late stages of the universe is maximized.
For simplicity we assume the topology of the space and
that all matters decay to radiation at the late stages.
rad
2 4
1( )
CU a
a a
1
4rad
4
exp
1exp const exp const .
cr
Z d w Z
C
3S
cr
( )U a
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There are many ways to obtain MEP:
Suppose that we pic up a universe randomly from the multiverse. Then the most probable universe is expected to be the one that has the maximum entropy.
Okada and HK ’11
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We may understand the flatness of the Higgs
potential as a consequence of MEP.
If we accept the inflation scenario in which universe
pops out from nothing and then inflates, most of the
entropy of the universe is generated at the stage of
reheating just after the inflation stops.
Therefore the potential of the inflaton should be tuned
in such a way that inflation occurs as long as possible.
Furthermore, if the Higgs field plays the role of inflaton,
the above analysis asserts that the SM parameters are
tuned such that the Higgs potential becomes flat at high
energy scale.
Flatness of the Higgs potential
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Higgs potential with
the running coupling 4
4V
mH = 125.6 GeV
mHiggs =125.6 GeV
𝜑[GeV]
MPP
MEP
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In wide classes of quantum gravity or string theory, the
low energy effective action has the multi local form:
We need to give a good definition of the path integral
for such action.
The fine tuning problem may be solved by the
dynamics of such action.
In the most optimistic case, the Big Fix occurs, and
all the low energy coupling constants would be
determined even if we do not know the detail of the
short distance physics.
eff .i i i j i j i jk i j k
i i j i jk
S c S c S S c S S S
Summary
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Multi-local action from
IIB matrix model
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IIB matrix model
2
Matrix
1 1, , .
4 2S Tr A A A
This is a candidate of non-perturbative
definition of string theory.
Formally, this is the dimensional reduction
of the 10D super YM theory to 0D.
Ishibashi, HK,
Kitazawa, Tsuchiya
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Y. Kimura, M. Hanada and HK
The basic question :
In the large-N reduced model, a background
of simultaneously diagonalizable matrices
𝑨𝝁(𝟎)= 𝑷𝝁 corresponds to the flat space,
if the eigenvalues are uniformly distributed.
In other words, the background 𝑨𝝁(𝟎)= 𝒊𝝏𝝁
represents the flat space.
How about curved space?
Is it possible to consider some background
like
𝑨𝝁(𝟎)= 𝒊𝛁𝝁 ?
Covariant derivatives as matrices
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Actually, there is a way to express the covariant
derivatives on any D-dim manifold by D matrices.
More precisely, we consider
𝑴: any D-dimensional manifold,
𝝋𝜶: a regular representation field on 𝑴.
Here the index 𝜶 stands for the components of
the regular representation of the Lorentz group
𝑺𝑶(𝑫 − 𝟏, 𝟏).
The crucial point is that for any representation 𝒓,its tensor product with the regular representation
is decomposed into the direct sum of the regular
representations:
.r reg reg regV V V V
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In particular the Clebsh-Gordan coefficients for
the decomposition of the tensor product of the
vector and the regular representaions
vector reg reg regV V V V
are written as ,
( ) , ( 1,.., ).b
aC a D
Here 𝒃 and β are the dual of the vector and the
regular representation indices on the LHS.
(𝒂) indicates the 𝒂-th space of the regular
reprezentation on the RHS, and 𝜶 is its index.
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Then for each 𝒂 (𝒂 = 𝟏. . 𝑫),
( )
b
a bC
is a regular representation field on 𝑴.
In other words, if we define 𝛁(𝒂) by
,
( ) ( ) ,b
a a bC
each 𝛁(𝒂) is an endomorphism on the space of the
regular representation field on 𝑴.
Thus we have seen that the covariant
derivatives on any D dimensional manifold
can be expressed by D matrices.
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Therefore any D-dimensional manifold 𝑴with 𝑫 ≤ 𝟏𝟎 can be realized in the space
of the IIB matrix model as
0 ( ) , 1, ,,
0, 1, ,10
a
a
a DA
a D
where 𝛁(𝒂) is the covariant derivative on 𝑴
multiplied by the C-G coefficients.
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Good point 1
Good points and bad points
Einstein equation is obtained at the classical level.
In fact, if we impose the Ansatz ( )a aA i
on the classical EOM , 0,a a bA A A
we have
( ) ( ) ( ), 0
0 , ,
, ( )
0 , 00 .
a a b
a a b
cd cd ca
a ab cd a ab cd ab c
a
d
a b ab b
c
a
R O R O R
R R R
Any Ricci flat space with 𝑫 ≤ 𝟏𝟎 is a
classical solution of the IIB matrix model.
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Good point 2
Both the diffeomorphism and local Lorentz
invariances are manifestly realized as a part of
the 𝑺𝑼(𝑵) symmetry.
In fact, the infinitesimal diffeomorphism and
local Lorentz transformation act on 𝝋𝜶 as
𝝋 → 𝟏 + 𝝃𝝁𝝏𝝁 𝝋 and
𝝋 → 𝟏 + 𝜺𝒂𝒃𝑶𝒂𝒃 𝝋 , respectively.
Both of them are unitary because they preserve
the norm of 𝝋𝜶𝝋𝜶𝟐 = 𝒅𝑫𝒙 𝒈 𝝋𝜶∗𝝋𝜶
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Bad points
Fluctuations around the classical solution
𝑨𝒂(𝟎)= 𝒊𝛁(𝒂)
1. contain infinitely many massless states.
2. Positivity is not guaranteed.
This can be seen by considering the
fluctuations around the flat space.
In this case the background is equivalent to
𝑨𝒂(𝟎)= 𝒊𝝏𝒂⊗𝟏𝒓𝒆𝒈 ,
where 𝟏𝒓𝒆𝒈 is the unit matrix on the space of
the regular representation.
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A. Tsuchiya, Y. Asano and HK
Low energy effective action
We have seen that any D-dim manifold is
contained in the space of D matrices.
Therefore in principle the IIB matrix model
contains and describes the effects of the
topology change of the space-time.
It is interesting to consider the low energy
effective action of the IIB matrix model.
As we will see, we indeed obtain the multi-
local action.
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0 .a a aA A
The multi-local action is the consequence of the well-
known fact that the effective action of a matrix model
contains multi trace operators.
Then we integrate over 𝝓 to obtain the low energy
effective action.
Here we assume that the background 𝑨 𝒂𝟎 contains
only the low energy modes, and 𝝓 contains the rest.
More precisely, we first decompose the matrices 𝑨𝒂into the background 𝑨 𝒂
𝟎 and the fluctuation 𝝓 :
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Substituting the decomposition into the action of
the IIB matrix model, and dropping the linear
terms in 𝝓, we obtain
20 0 0 0
0
0
2
20
0
1
4
2 , , , 2 , ,
4 , , , fermion
,
.
a b a b a b a b b a
a b a
a
b
b
a b
S Tr
A A
A
A
A
A A
A
In principle, the 0-th order term 20 0
0 ( ) ( )
1,
4a bS Tr A A
can be evaluated with some UV regularization,
which should give a local action.
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The one-loop contribution is obtained by the Gaussian
integral of the quadratic part.
Then the result is given by a double trace operator as
usual:
𝑾 = 𝑲𝒂𝒃𝒄⋯ , 𝒑𝒒𝒓⋯ 𝑻𝒓 𝑨𝒂𝟎𝑨𝒃𝟎𝑨𝒄𝟎⋯ 𝑻𝒓(𝑨𝒑
𝟎𝑨𝒒𝟎𝑨𝒓𝟎⋯)
The crucial assumption here is that both of the
diffeomorphism and the local Lorentz invariance are
realized as a part of the SU(N) symmetry.
Then each trace should give a local action that is
invariant under the diffeomorphisms and the local
Lorentz transformations:
1-loop
eff
1, ( ) ( ).
2
D
i j i j i i
i j
S c S S S d x g x O x
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In the two loop order, from the planar
diagrams we have a cubic form of local
actions2-loop Planar
eff
, ,
1,
6i jk i j k
i j k
S c S S S
while non-planar diagrams give a local
action2-loop NP
eff .i i
i
S c S
z y
x
x
Similar analyses can be applied for higher loops.
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the low energy effective theory of the IIB matrix
model is given by the multi-local action:
.)()(
,eff
xOxgxdS
SSScSScScS
i
D
i
kji
kji
kjij
ji
iji
i
ii
We have seen that
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Although there is no precise correspondence, the
loops in the IIB matrix model resemble the
wormholes.
x
y
𝑥
𝑦 ≅
We may say that if the theory involves gravity and
topology change, its low energy effective action
becomes the multi-local action universally.
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Review on IIB matrix model
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1. Definition of IIB matrix model
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The basic idea
For simplicity, we start with bosonic string.
In fact, in the Schild action, the worldsheet
can be regarded as a symplectic manifold,
and the action is given by the integration of
a quantity that is expressed in terms of the
Poisson bracket.
“Worldsheet of string has a structure of
phase space.”
This situation becomes manifest when we
express the string in terms of the Schild
action.
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Basically, bosonic string is described by the
Nambu-Goto action
2 21, .
2
ab
NG a bS d X X
Schild action
The Nambu-Goto action is nothing but the area of
the worldsheet, which is expressed in terms of an
anti-symmetric tensor 𝚺𝝁𝝂 that is constructed from
the space-time coordinate 𝑿𝝁.
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It is known that the Nambu-Goto action is
equivalent to the Schild action
2
2 2
Schild
1, ,
2 4 2S d g X X d g
: a volume density on the world shee
1, ,
t
ab
a bX Y
g
X Yg
which is nothing but the Poisson bracket if
regard the worldsheet as a phase space.
2
Schild
22
Schild
10
2
1.
2 2
ab
a b
ab
a b
S g X Xg
S d X X
The equivalence can be seen easily, by eliminating
𝒈 from the Scild action:
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The crucial point is that the Schild action
has a structure of phase space.
21
, .2 4 2
X X
In fact it is given by the integration over the
phase space
of a quantity that is expressed in terme the
Poisson bracket
2d g
symplectic structure of the worldsheet
Not that we do not need Worldsheet metric, but
what we need is just the volume density 𝒈.
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Matrix regularization
Then we want to discretize the worldsheet
in order to define the path integral.
2
-symm
function matrix
1, ,
1
etry ( )-symmet y
2
r
A B A Bi
d g A T
W U N
rA
A natural discretization of phase space is
the “quantization”.
If we quantize a phase space, it becomes
the state-vector space, and we have the
following correspondence:
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Then the Schild action becomes
2
Matrix
1, 1 ,
4S Tr A A Tr
and the path integral is regularized like
Schild Matrix
1
exp exp .vol(Diff ) ( )n
dg dX dAZ iS iS
SU n
Here we have used the fact that the phase
space volume is diff. invariant and
becomes the matrix size after the
regularization.
Therefor the path integral over
becomes summation over .
2d g 1Tr n
g
n
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Multi-string states
One good point of the matrix regularization is
that all topologies of the worldsheet are
automatically included in the matrix integral.
Disconnected worldsheets are also included as
block diagonal configurations as
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Furthermore the sum over the size of the
matrix is automatically included, if the
worldsheet is imbedded in a larger matrix
as a sub matrix.
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If we take this picture that all the worldsheets
emerge as sub matrices of a large matrix, the
second term of
2
Matrix
1, 1
4S Tr A A Tr
can be regarded as describing the chemical
potential for the block size.
Thus we expect that the whole universe is
described by a large matrix that obeys
21, .
4S Tr A A
This is nothing but the large-N reduced model,
which I will explain in a moment.
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On the other hand, if we start from type IIB
superstring, we will get the reduced model for
supersymmetric gauge theory. In this case
eigenvalues do not collapse, and we can have
non-trivial space-time.
From the analyses of the large-N reduced
model, it is known that in this model the
eigenvalues collapse to one point, and it can
not describe an extended space-time.
This might be related to the instability of
bosonic string by tachyons.
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The Large-N reduction
The basic statement is
“The large-N gauge theory with periodic
boundary condition does not depend on the
volume of the space-time.”
In particular, the theory in the infinite
space-time is equivalent to that on one point.
The space-time emerges from the internal
degrees of freedom of the reduced model.
Here I will summarize the notion of the large-N reduction briefly.
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22
,4
dN
S Tr A A
the large-N reduced model
is equivalent to the d-
dimensional Yang-Mills
if the eigenvalues of 𝑨𝝁are uniformly distributed.
However, it is not automatically realized.
It is known that the eigenvalues collapse
to one point unless we do something.
In fact by analyzing the planar Feynman diagrams, we can show that
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Strong coupling
If the coupling is sufficiently strong,
the quantum fluctuation may overwhelm
the attractive force.
It actually happens at least for the lattice
version of the reduced model.
† †
reduced
1
, .d
c
NS Tr U U U U
Actually there are several ways to make
the eigenvalues distribute uniformly.
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quenching
We constrain the diagonal elements of 𝑨𝝁to a uniform distribution by hand
(𝑨𝝁)𝒊𝒊= 𝒑𝝁(𝒊)
.
Then the perturbation series reproduce
that of the D-dimensional gauge theory.
However, this is rather formal, and the
gauge invariance is no longer manifest.
A lattice version of the quenching that
keeps the manifest gauge invariance was
proposed, but now it is known that it does
not work. Bhanot-Heller-Neuberger, Gross-Kitazawa
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twisting
(0) ˆ
ˆ ˆ, ( ),
A p
p p i B B
the theory is equivalent to gauge theory
in a non-commutative space-time.
If we expand around the non-
commutative back ground
A
Gonzalez-Arroyo, Korthals Altes (‘83)
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Because the equation of motion of the
reduced model is given by
, , 0,A A A
the non-commutative back ground
is a classical solution.
But it is not the absolute minimum of the
action.
One way to make it stable is to modify the
model to
22
, .4
dN
S Tr A A i B
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The lattice version of this is called the
twisted reduced model:
† †
reduced
1
.d
iNS e Tr U U U U
Gonzalez-Arroyo, Okawa
Several MC analyses have been made on the
twisted reduced model, and they found some
discrepancy from the infinite volume theory,
which is related to the UV-IR correspondence.
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Heavy adjoint fermions
They have introduced
additional heavy adjoint
fermions.
Kovtun-Unsal-Yaffe (2007),Bringoltz-Sharpe (2009),Poppitz, Myers, Ogilvie, Cossu, D’Elia, Hollowood, Hietanen, Narayanan,Azeyanagi, Hanada, Yacobi
Then the collapse of the eigenvalues can
be avoided without changing the long
distance physics.
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Schild action of IIB string
We consider the Schild action of the type
IIB superstring.
Green-Schwarz action
2 2
1 1 2 2
1 1 2 2
1 1 2 2
1
2
,
,
GS
ab
a b b
ab
a b
ab
a b
a a a a
S d
i X
X i i
1 2
( 0 ~ 9)
, : 10D Mayorana-Weyl
X
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κ-symmetry
1 1
2 2
1 1 2 2
1 1
2 2
2
1
1 ,
1
12
2
X i i
N=2 SUSY1 1
SUSY
2 2
SUSY
1 1 2 2
SUSY X i i
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Gauge fixing for the κ-symmetry 1 2
2 212 ,
2
.
ab
GS a b
ab
a b
S d i X
X X
N=2 SUSY should be combined with 𝜿 symmetry
so that the gauge condition is maintained1 1 1
SUSY
2 2 2
SUSY
1 2
SUSY
1 21
1 22
2
2
X X X
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N=2 SUSY becomes simple if we consider a
combination
1
2
1
2
2
1
12
2
0.
X i
X
1 2
1 2
2
.2
Then we have the following simple form:
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Schild action
N=2 SUSY
Schild
22 21
, , ,2 4 2 2
S
id g X X X d g
1
1
2
2
1,
2
0
X X
X i
X
Everything is
written in terms
of the Poisson
bracket.
Then we can convert the action to the Schild
action as in the case of bosonic string:
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Matrix regularization
1
1
2
2
1
2
0
F
A i
A
2
Matrix
1 1, , 1 .
4 2S Tr A A A Tr
N=2 SUSY
Applying the matrix regularization, we have
,F i A A
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IIB matrix model
2
Matrix
1 1, , .
4 2S Tr A A A
Drop the second term, and consider large-N
Formally, this is the dimensional reduction
of the 10D super YM theory to 0D.
A good point is that the N=2 SUSY is
maintained after the discretization.
IIB matrix model
Ishibashi, HK,
Kitazawa, Tsuchiya
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N=2 SUSY
One of the N=2 SUSY is nothing but the
supersymmety of the 10D super YM theory.
1
1
1
2F
A i
2
Matrix
1 1, ,
4 2S Tr A A A
Even so, they form non trivial N=2 SUSY:
2
20A
The other one is almost trivial.
1 1 2 2 1 2, 0, , 0, , .Q Q Q Q Q Q P
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IIB matrix model is nothing but the
dimensional reduction of the 10D super YM
theory to 0D.
The other matrix modelscomment
It is natural to think about the other possibilities.
In fact, they have considered the dimensional
reduction to various dimensions:
0D ⇒ IIB matrix model
1D ⇒Matrix theory
2D ⇒Matrix string
4D ⇒AdS/CFT
Motl, Dijkgraaf-Verlinde-Verlinde
de Witt-Hoppe-Nicolai,Banks-Fischler-Shenker-Susskind
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From the viewpoint of the large-N reduction,
they are equivalent if we quench the diagonal
elements of the matrices.
However the dynamics of the diagonal elements
are rather complicated.
At present the relations among them are not
well-understood.comment end
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Open questions
We expect that the IIB matrix model
)],[2
1],[
4
1(
1 2
2 AAATr
gS
gives a constructive definition of superstring.
However there are some fundamental
open questions:
Is an infrared cutoff necessary?
How the large-N limit should be taken?
How does the space-time emerge?
Does diff. invariance exist rigorously?
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2. Ambiguities in the definition
• Euclidean or Lorentzian• Necessity of the IR cutoff• How to take the (double) scaling limit
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Euclidean or Lorentzian?
In general, systems with gravity do not
allow a simple Wick rotation, because the
kinetic term of the conformal mode (the
size of the universe) has wrong sign.
On the other hand, the path integral of the
IIB matrix model seems well defined for the
Euclidean signature, because the bosonic
part of the action is positive definite:
2
2
1 1[ , ] .
24
1[ , ]S A AA TrTr
g
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How about Lorentzian signature?
If we simply apply the analytic continuation
the path integral becomes unbounded:
0 10 ,A iA
2
0 2 2
2
exp ,
1 1 1[ , ][ , ] [ , ]
4 2
1 1 1[ , ] [ , ] .
2 4
M
M
i i j
Z dAd S
S Tr A A A A Tr Ag
Tr A A A Ag
From the point of view of the large-N
reduction, it is natural to take
exp .MZ dAd i S
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Is IR cutoff necessary?
Because of the supersymmetry the force between eigenvalues cancels between bosons and fermions
2
(1 ) ( ) ( )
,
2 log 0loop i j
eff F
i j
S D d p p
It seems that we have to impose an infrared cutoff by hand to prevent the eigenvalues from running away to infinity.
eigenl A l
(1)
( )N
p
A
p
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But there is a subtlety.
Because the diagonal elements of fermions are
zero modes of the quadratic part of the action,
we should keep them when we consider the
effective Lagrangian.
The one-loop effective Lagrangian for the
diagonal elements is given by
(1)
( )N
p
A
p
1
N
4 8
, ,1-loop
eff
2, 2,
, ,4 8
.
i j i j
i j
i ji j i j
i ji j
S SS x tr
p pS
p p
Aoki, Iso, Kitazawa, Tada, HK
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Because of the fermionic degrees of freedom, there appears a weak attractive force between the eigenvalues, and at least the partition function becomes finite. However it is not clear whether all the correlation functions are finite or not.
Austing and Wheater,Krauth, Nicolai and Staudacher,Suyama and Tsuchiya,Ambjorn, Anagnostopoulos, Bietenholz, Hotta and Nishimura,Bialas, Burda, Petersson and Tabaczek,Green and Gutperle,Moore, Nekrasov and Shatashvili.
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1-loop(1 loop) 16 ( )
effexp ,i
i
Z p d S p
( , )i jS i j
( , ) 0, 8n
i jS n
Since is quadratic in ,
which has only 16 components, we have
and
4 8
, ,1-loop
eff
4 8
( , ) ( , )
exp , exp4 8
1 tr tr .
i j i j
i j
i j i j
i j
S SS p tr
a S b S
Let’s estimate the order of this interaction.
We first integrate out the fermionic variables
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4 8
( , ) ( , )1, tr , tri j i ja S b S
16 ( )
1
Ni
i
d
(1 loop)
various terms
exp .
i j i jZ p f p p f p p
O N
Therefore, for each pair of i and j we have 3
choices
which carry the powers of
0, 8, 16 respectively.
Therefore the number of factors other than 1
should be less than or equal to 2N, and we
can conclude that the effective action induced
from the fermionic zero modes is of 𝑶 𝑵 :
.
On the other hand, we have 16N dimensional
fermionic integral .
2N
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.
This should be compared to the bosonic case
2(1 loop)
2
exp 2 log
exp ( )
i j
i j
Z p D p p
O N
SUSY reduces the attractive force by at least
a factor 1/N.
So, in the naïve large-N limit, simultaneously
diagonal backgrounds are stable.
However, it is not clear what happens in the
double scaling limit.
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How to take the large-N limit
In the IIB matrix model , we usually regard
A as the space-time coordinates.
)],[2
1],[
4
1(
1 2
2 AAATr
gS
gSo, has dimensions of length squared.
How is the Planck scale expressed?
If it does not depend on the IR cutoff l, as
we normally guess, we should have1
2Planck .l N g
In other words, we should take the large-N
limit keeping this combination finite.
?
At present we have no definite answer.
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3. Meaning of the matrices and
Emergence of the space-time
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What do the matrices stand for?
If we regard the IIB matrix model
)],[2
1],[
4
1(
1 2
2 AAATr
gS
as the matrix regularization of the Schild
action, Aμ are space-time coordinates.
On the other hand if we regard it as the
large-N reduced model, the diagonal
elements of Aμ represent momenta.
It is not a priori clear how the space-time
emerges from the matrix degrees of
freedom.
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0ˆ 1 , 0, ,3
.0, 4, ,9
kpA
Here satisfy the CCR’sp̂
Another interesting possibility is to consider a
non-commutative back ground such as
ˆ ˆ, ( ),p p i B B
There are many possibilities to realize the
space-time.
and is the unit matrix.
Then we have a 4D noncommutative flat
space with SU(k) gauge theory.
1k k k
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Actually various models that are close to the
standard model can be constructed by choosing
an appropriate background.
“Intersecting branes and a standard model realization
in matrix models.”
A. Chatzistavrakidis, H. Steinacker, and G. Zoupanos.
JHEP09(2011)115
(ex.)
“An extended standard model and its Higgs geometry
from the matrix model,”
H. Steinacker and J. Zahn,
PTEP 2014 (2014) 8, 083B03
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Expandind universeS.-Kim, J. Nishimura, A. Tsuchiya
Phys.Rev.Lett. 108 (2012) 011601
can be regarded as
an expanding 3+1
D universe.
(ex.)
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Expandind universe 2
Recently another interesting picture of the expanding
universe has been obtained by considering fuzzy
manifolds.
(ex.)
“Quantized open FRW cosmology from Yang-Mills matrix models”
H. Steinacker,
Phys.Lett. B782(2018) 176-180
“The fuzzy 4-hyperboloid 𝑯𝒏𝟒 and higher-spin in Yang-Mills
matrix models”
Marcus Sperling, H. Steinacker,
arXiv:1806.05907
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4. Diffeomorphism invarianceand
Gravity
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Diff. invariance and gravity
Because we have exact N=2 SUSY, it is
natural to expect to have graviton in the
spectrum of particles.
Actually there are some evidences.
(1) Gravitational interaction appears from
one-loop integral.
(2) Emergent gravity by Steinacker. Gravity
is induced on the non-commutative back
ground.
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However, it would be nicer, if we can
understand how the diffeomorphism
invariance is realized in the matrix model.
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Y. Kimura, M. Hanada and HK
The basic question :
In the large-N reduced model, a background
of simultaneously diagonalizable matrices
𝑨𝝁(𝟎)= 𝑷𝝁 corresponds to the flat space,
if the eigenvalues are uniformly distributed.
In other words, the background 𝑨𝝁(𝟎)= 𝒊𝝏𝝁
represents the flat space.
How about curved space?
Is it possible to consider some background
like
𝑨𝝁(𝟎)= 𝒊𝛁𝝁 ?
Covariant derivatives as matrices
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Actually, there is a way to express the covariant
derivatives on any D-dim manifold by D matrices.
More precisely, we consider
𝑴: any D-dimensional manifold,
𝝋𝜶: a regular representation field on 𝑴.
Here the index 𝜶 stands for the components of
the regular representation of the Lorentz group
𝑺𝑶(𝑫 − 𝟏, 𝟏).
The crucial point is that for any representation 𝒓,its tensor product with the regular representation
is decomposed into the direct sum of the regular
representations:
.r reg reg regV V V V
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In particular the Clebsh-Gordan coefficients for
the decomposition of the tensor product of the
vector and the regular representaions
vector reg reg regV V V V
are written as ,
( ) , ( 1,.., ).b
aC a D
Here 𝒃 and β are the dual of the vector and the
regular representation indices on the LHS.
(𝒂) indicates the 𝒂-th space of the regular
reprezentation on the RHS, and 𝜶 is its index.
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Then for each 𝒂 (𝒂 = 𝟏. . 𝑫),
( )
b
a bC
is a regular representation field on 𝑴.
In other words, if we define 𝛁(𝒂) by
,
( ) ( ) ,b
a a bC
each 𝛁(𝒂) is an endomorphism on the space of the
regular representation field on 𝑴.
Thus we have seen that the covariant
derivatives on any D dimensional manifold
can be expressed by D matrices.
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Therefore any D-dimensional manifold 𝑴with 𝑫 ≤ 𝟏𝟎 can be realized in the space
of the IIB matrix model as
0 ( ) , 1, ,,
0, 1, ,10
a
a
a DA
a D
where 𝛁(𝒂) is the covariant derivative on 𝑴
multiplied by the C-G coefficients.
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Good point 1
Good points and bad points
Einstein equation is obtained at the classical level.
In fact, if we impose the Ansatz ( )a aA i
on the classical EOM , 0,a a bA A A
we have
( ) ( ) ( ), 0
0 , ,
, ( )
0 , 00 .
a a b
a a b
cd cd ca
a ab cd a ab cd ab c
a
d
a b ab b
c
a
R O R O R
R R R
Any Ricci flat space with 𝑫 ≤ 𝟏𝟎 is a
classical solution of the IIB matrix model.
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Good point 2
Both the diffeomorphism and local Lorentz
invariances are manifestly realized as a part of
the 𝑺𝑼(𝑵) symmetry.
In fact, the infinitesimal diffeomorphism and
local Lorentz transformation act on 𝝋𝜶 as
𝝋 → 𝟏 + 𝝃𝝁𝝏𝝁 𝝋 and
𝝋 → 𝟏 + 𝜺𝒂𝒃𝑶𝒂𝒃 𝝋 , respectively.
Both of them are unitary because they preserve
the norm of 𝝋𝜶𝝋𝜶𝟐 = 𝒅𝑫𝒙 𝒈 𝝋𝜶∗𝝋𝜶
Comment: The norm is not positive definite for
Lorentzian theory.
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Bad points
Fluctuations around the classical solution
𝑨𝒂(𝟎)= 𝒊𝛁(𝒂)
1. contain infinitely many massless states.
2. Positivity is not guaranteed.
This can be seen by considering the
fluctuations around the flat space.
In this case the background is equivalent to
𝑨𝒂(𝟎)= 𝒊𝝏𝒂⊗𝟏𝒓𝒆𝒈 ,
where 𝟏𝒓𝒆𝒈 is the unit matrix on the space of
the regular representation.
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Because the unit matrix 𝟏𝒓𝒆𝒈 is infinite
dimensional, we have infinite degeneracy, and
in particular we have infinitely many massless
states. → bad point 1
In general, the regular representation contains
infinite tower of higher spins, and we have
many negative norm states. It is not clear
whether we have sufficiently many symmetries
to eliminate those negative norm states.
→ bad point 2
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One possible way out is to consider a non-
commutative version.
What we have done is to regard the matrices
as endomorphisms on the space of the regular
representation fields.
It is easy to show that this space is equivalent
to the space of the functions on the frame
bundle of the spin bundle.
If we can construct a non-commutative version
of such bundle, we can reduce the degrees of
freedom significantly without breaking the
diffeomorphism and local Lorentz invariance.
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5. Topology change of the space-time and
Low energy effective action
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A. Tsuchiya, Y. Asano and HK
Low energy effective action
We have seen that any D-dim manifold is
contained in the space of D matrices.
Therefore in principle the IIB matrix model
contains and describes the effects of the
topology change of the space-time.
As was pointed out by Coleman some years
ago, such effects give significant corrections
to the low energy effective action.
It is interesting to consider the low energy
effective action of the IIB matrix model.
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.)()(
,eff
xOxgxdS
SSScSScScS
i
D
i
kji
kji
kjij
ji
iji
i
ii
Actually we can show that if we integrate out the heavy
states in the IIB matrix model, the remaining low
energy effective action is not a local action but has a
special form, which we call the multi-local action:
Here 𝑶𝒊 are local scalar operators such as
𝟏 , 𝑹 , 𝑹𝝁𝝂𝑹𝝁𝝂 , 𝑭𝝁𝝂 𝑭
𝝁𝝂, 𝝍𝜸𝝁𝑫𝝁𝝍 ,⋯ .
𝑺𝒊 are parts of the conventional local actions.
The point is that 𝑺𝐞𝐟𝐟 is a function of 𝑺𝒊’s.
𝑺𝐞𝐟𝐟 is not local, but it is not completely non-local
in the sense that it is a function of local actions.
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0 .a a aA A
This is essentially the consequence of the well-
known fact that the effective action of a matrix
model contains multi trace operators.
Then we integrate over 𝝓 to obtain the low energy
effective action.
Here we assume that the background 𝑨 𝒂𝟎 contains
only the low energy modes, and 𝝓 contains the rest.
We also assume that this decomposition can be
done in a SU(N) invariant manner.
More precisely, we first decompose the matrices 𝑨𝒂into the background 𝑨 𝒂
𝟎 and the fluctuation 𝝓 :
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Substituting the decomposition into the action of
the IIB matrix model, and dropping the linear
terms in 𝝓, we obtain
20 0 0 0
0
0
2
20
0
1
4
2 , , , 2 , ,
4 , , , fermion
,
.
a b a b a b a b b a
a b a
a
b
b
a b
S Tr
A A
A
A
A
A A
A
In principle, the 0-th order term 20 0
0 ( ) ( )
1,
4a bS Tr A A
can be evaluated with some UV regularization,
which should give a local action.
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The one-loop contribution is obtained by the Gaussian
integral of the quadratic part.
Then the result is given by a double trace operator as
usual:
𝑾 = 𝑲𝒂𝒃𝒄⋯ , 𝒑𝒒𝒓⋯ 𝑻𝒓 𝑨𝒂𝟎𝑨𝒃𝟎𝑨𝒄𝟎⋯ 𝑻𝒓(𝑨𝒑
𝟎𝑨𝒒𝟎𝑨𝒓𝟎⋯)
The crucial assumption here is that both of the
diffeomorphism and the local Lorentz invariance are
realized as a part of the SU(N) symmetry.
Then each trace should give a local action that is
invariant under the diffeomorphisms and the local
Lorentz transformations:
1-loop
eff
1, ( ) ( ).
2
D
i j i j i i
i j
S c S S S d x g x O x
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In the two loop order, from the planar
diagrams we have a cubic form of local
actions2-loop Planar
eff
, ,
1,
6i jk i j k
i j k
S c S S S
while non-planar diagrams give a local
action2-loop NP
eff .i i
i
S c S
z y
x
x
Similar analyses can be applied for higher loops.
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the low energy effective theory of the IIB matrix
model is given by the multi-local action:
.)()(
,eff
xOxgxdS
SSScSScScS
i
D
i
kji
kji
kjij
ji
iji
i
ii
We have seen that
This reminds us of the theory of baby universes
by Coleman.
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Although there is no precise correspondence, the
loops in the IIB matrix model resemble the
wormholes.
x
y
𝑥
𝑦 ≅
Probably this phenomenon occurs quite universally.
We may say that if the theory involves gravity and
topology change, its low energy effective action
becomes the multi-local action.
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End (review on IIB matrix model)
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3. Probabilistic interpretationof
multiverse wave function
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Probabilistic interpretation (1)
postulate
T : age of the universe
2
probability of finding a universe of size z dz z
0Ez z
2
0
1
( )
1
Edz z dzz p
z d
z
dz
T
21
2
HH z p z zp
p
0
1exp
( )
z
E z i dz p zz p z
meaning of this measure
z
T
2 2
z dz dT ⇒
2 probability of a universe emerging in unit time
the time that has passed
after the universe is created
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Probabilistic interpretation (2)
is a superposition of the universe with various age,
z
T
+
z
T
z
T+ +…
T T dTgives the probability of finding a
universe of age .
2 2z dz dT
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infrared cutoffWe introduce an infrared cutoff
for the size of universes.
IRz
ceases to exist
bounces back
or
2 2 2
life time of the universez dz dT
dimensionless
∞finite
Lifetime of the universe
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Wave Function of the multiverse (1)
Multiverse appears naturally in quantum gravity / string theory.
matrix model
・
b
b
aC
,
)(
b
b
aC
,
)(
Each block
represents
a universe.
・
quantum gravity
Okada, HK
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Wave Function of the multiverse (2)
The multiverse sate is a superposition of N-verses.
multi
0
,N
N
d w
1 1, , , , ,N N Nz z z z
, ,
multi
N
d w
exp i i
i
Z d w d i S
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Wave Function of the multiverse (3)
Probabilistic interpretation
multi
0
,N
N
d w
1 1, , , , ,N N Nz z z z
1 1 1, , N N Nz z dz z z dz
the probability of finding N universes with size
2 2
1 1, , ,N N Nz z dz dz w d
and finding the coupling constants in
.d
represents
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Probability distribution of
1 1, , , , ,N N Nz z z z
2 21
1
0
2 2
2 2
, , ,!
exp ,
exp
NN N
N
dz dzP z z w
N
dz z w
w
2
0 , (life time of the universe)Edz z
0E
is chosen in such a way that is maximized,
can be very large.
irrespectively of .w
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If we accept the probabilistic interpretation of the
multiverse wave function, the coupling constants
are chosen in such a way that the lifetime of the
universe becomes maximum.
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WKB sol with
Cosmological constant
Λ~curvature~energy density
(extremely small)
What value of Λ maximizes
0
?
S3 topology
The cosmological constant in the far future is predicted to be very small.
IRz
0
1,
,E z
z p z
, 2 ( ).p z U z
assuming all matters decay to radiation
2
0 ,Edz z
0 cr0 cr cr
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The other couplings (Big Fix)
2 2
expP w
The exponent is divergent, and regulated by the IR cutoff :
2
0 rad0
cr
1, log .
IR
E IR
z
dz z C zz
cr rad1/C
2
0 ,Edz z
MEP
are determined in such a way that is maximized.Again we have MEP.
radC
assuming all matters decay to radiation