Fundamental Physics with Gravitational Waves

Fundamental Physics with

Gravitational Waves

Leonardo Gualtieri, “Sapienza” University of Rome

What Next? Rome, 16/2/2016


Finally, we have detected GWs.



But the best is yet to come!




“Recording a GW for the first time has never been a big motivation for LIGO [and Virgo].

The motivation has always been to open a new window on the Universe, to see the warped side of the Universe,

an aspect never seen before: objects and phenomena

made entirely or partially of warped spacetime”

Kip Thorne



Suddenly, the realm of physics has expanded: we are able to study strongly gravitating object and phenomena,

of which - up to now - we only had indirect evidence or knowledge.


“Recording a GW for the first time has never been a big motivation for LIGO [and Virgo].

The motivation has always been to open a new window on the Universe, to see the warped side of the Universe,

an aspect never seen before: objects and phenomena

made entirely or partially of warped spacetime”

Kip Thorne

Now that the uncharted territory of strong gravity can be explored, many fundamental questions can be addressed




Some of them are related to the nature and the behaviour of fundamental interactions,

like gravity itself, or nuclear physics

Demorest et al. ‘13Penrose ‘74

Testing General Relativity 133

Figure 6.1. GR mass-mass diagram for the Double pulsar with six PK parameters (!, �, Pb, r, s,⌦B) and the mass ratio (R). All constraints agree on a small common region (see inset), meaningthat GR has passed this test of several relativistic effects (quasi-stationary strong-field as well asradiative). [Figure courtesy of Michael Kramer.]

rotating (23ms) pulsar in a mildly eccentric (e = 0.088), 2.5 hour orbit. Until 2008 itscompanion (pulsar B) was also visible as an active radio pulsar with a rotational periodof about 2.8 seconds. The timing of both pulsars allowed an immediate determinationof the mass ratio from the projected semi-major axes of the two pulsar orbits. Inthe Double Pulsar all PK parameters listed above have been measured, some of themwith exquisite precision. Most importantly, the change in the orbital period Pb dueto GW damping has by now been tested to agree with the quadrupole formula of GRto better than 0.1%, giving the best test for the existence of GWs as predicted byGR [681]. As a result of geodetic precession, pulsar B has in the meantime turnedaway from our line-of-sight and is no longer visible [689]. Due to the high inclinationof the orbital plane (close to 90 degrees), pulsar A is getting eclipsed by the plasma-filled magnetosphere of pulsar B every 2 1

2 hours, for about 30 seconds around superiorconjunction. Changes in the eclipse pattern could be used to determine the rate ofgeodetic precession, ⌦B, with a precision of about 13% [690]. The obtained value isin good agreement with GR. All these tests are summarized in form of a mass-massdiagram in Figure 6.1.

PSR J1738+0333 is a pulsar in a nearly circular (e ⇠ 3 ⇥ 10�7), 8.5 hour orbitwith an optically bright white dwarf. High-resolution spectroscopy allowed the

This

isnotto

saythatthe

integralisbadly

behaved.Infact,

theregulated

integralcan

befound

exactly,in

termsof

knownfunctions:

Ið!Þ¼

g24

!EiðigÞþ

1g "iþ1g #e

ig%Eiðig

!2Þ

%1

g!

2 "iþ1

g!

2 #eig!

2 $;

(2.3)

where

Eiis

theexponential

integral.Moreover,

thisexpression

hasa

sensiblelim

itas

!!

1;using

theasym

ptoticbehavior

ofthe

exponentialintegral

asx!

1,

EiðixÞ¼

i!þ

eix !%

ix %"1x #

2þO"1x3 #$

(2.4)

gives

Ið!Þ¼

g24

!EiðigÞþ

1g "iþ1g #e

ig%i! $

þO"e

ig!

2

!2

#:

(2.5)

Clearly,

expandingthe

exponentialisnot

theright

thingto

doin

orderto

evaluatethe

integral.There

appearto

beclose

parallelsin

expressionsdescribing

gravitationalscattering.

III.GRAVITATIO

NALEIK

ONALAMPLIT

UDES

Consider

gravitationalscattering,

inD

dimensions,

inthe

ultrahigh-energylim

it,E¼

ffiffiffisp

&M

D,with

MDthe

Planck

mass.

Since

thedim

ensionlessgravitational

cou-pling

isusually

thoughtto

beG

DED%2'

ðE=M

D Þ D%2,one

might

expectthis

tobe

astrongly

coupledproblem

.How

ever,thatdepends

onthe

sizeof

themom

entumtrans-

fer,t¼

%q2,

orim

pactparam

eter—scattering

atasuffi

-ciently

largeim

pactparam

eterisdom

inatedsim

plyby

theBorn

approximation,

T0 ðs;tÞ¼

%8!

GDs2=t:

(3.1)

For

decreasingim

pactparam

eter,higher-loop

amplitudes

becomerelevant,

andone

entersregim

eswhere

differentphenom

enadom

inate;an

overviewof

theseregim

es,with

furtherreferences,

isprovided

in[4].

(Important

earlierreferences

include[7,15,16,31,32].)

Inparticular,

itis

ar-gued

thereand

inpreceding

referencesthat

thefirst

loopcorrections

tobecom

eim

portantare

theladder

andcrossed-ladder

diagrams,

which

canbe

summed

togive

theeikonal

approximation

tothe

amplitude.

Specifi

cally,suchaladder

diagramisexhibited

inFig.1.

The

eikonalapproxim

ationto

theam

plitudearises

fromneglecting

subleadingterm

sin

themom

entumtransfer

runningthrough

theindividual

rungs.In

particular,if

kdenotes

atypical

suchmom

entumtransfer,

andpian

externalmom

entum,then

theinterm

ediatepropagators

ofthe

high-energyparticles

havedenom

inatorsof

theform

Di ¼

ðpi þ

kÞ 2þm

2¼2p

i (kþ

k2;

(3.2)

andweneglect

thesecond

term.Likew

ise,in

thevertices,

we

neglectmom

entumtransfers

k'q

compared

tothe

sizeof

pi .The

resultis

thatthe

sumof

ladderand

crossed-ladderdiagram

sat

N-loop

ordercan

bewritten

interm

sof

thetree

amplitude,

T0 ðs;%

q2Þ,

as

iTN ðs;qÞ¼

2s

ðNþ

1Þ! Z!Y Nþ

1

j¼1

dD%

2kj

ð2!Þ D%2

iT0 ðs;%

k2j Þ

2s

$

)ð2!Þ D

%2"

D%2 "X

j

kj %

q? #;

(3.3)

where

theintegrals

areover

thecom

ponentsof

themom

entatransverse

tothose

ofthe

incoming

particlesin

thec.m

.frame.T

hesum

overall

sucham

plitudesgives

theeikonal

amplitude,w

hichiswritten

interm

sof

theeikonal

phase,#ðx?;sÞ¼

12s

ZdD%2q

?ð2!Þ D%

2eiq

? (x?T

0 ðs;%q2? Þ

¼4!

ðD%

4Þ"D%3

GDs

xD%4

?;

(3.4)

with

"nthe

areaof

theunit

n-sphere.

The

eikonalam

pli-tude

is

iTeik ðs;tÞ¼

2s ZdD%

2x?e %

iq? (x

?ðei#ðx?

;sÞ%1Þ;

(3.5)

inthe

integral,b¼

jx? jplays

therole

ofthe

impact

parameter,

andthus

theam

plitudeisnaturally

givenin

anim

pactparam

eterrepresentation.

Anim

portantquestion

isto

what

extentthe

eikonalam

plitudeisagood

approximation

tothe

exactam

plitude,and

inwhat

domain.

First,

notethat

anatural

expansionparam

eteristhe

eikonalphase(3.4).W

henthis

issm

all,theeikonal

amplitude

canbe

approximated

bythe

linearterm

in#,which

isexactly

theBorn

amplitude.

Corrections

tothis

becomeim

portantat

impact

parameters

where

#be-

comes

oforder

one.These

impact

parameters

aredirectly

relatedto

mom

entumtransfers,

sincethe

integral(3.5)

hasasaddle

pointwhich

fixes

bin

termsof

q.Towrite

thecorresponding

equation,weintroduce

theSchw

arzschildradius

ofthe

c.m.energy,

RðEÞ¼1MD

"kDE

MD #

1=ðD%3Þ;

(3.6)

where

12

34

FIG

.1.

Aladder

diagramwith

multiple

gravitonexchange.

HIG

H-ENERGY

SCATTERIN

GIN

GRAVITYAND

...PHYSICALREVIEW

D82,

104022(2010)

104022-3

Giddings et al. ‘10

Lyne at al. ‘04



Others are related to the nature and the evolution of the sources

populating our Universe, and, ultimately, of the Universe itself

Some of them are related to the nature and the behaviour of fundamental interactions,

like gravity itself, or nuclear physics

Demorest et al. ‘13Penrose ‘74

Testing General Relativity 133

Figure 6.1. GR mass-mass diagram for the Double pulsar with six PK parameters (!, �, Pb, r, s,⌦B) and the mass ratio (R). All constraints agree on a small common region (see inset), meaningthat GR has passed this test of several relativistic effects (quasi-stationary strong-field as well asradiative). [Figure courtesy of Michael Kramer.]

rotating (23ms) pulsar in a mildly eccentric (e = 0.088), 2.5 hour orbit. Until 2008 itscompanion (pulsar B) was also visible as an active radio pulsar with a rotational periodof about 2.8 seconds. The timing of both pulsars allowed an immediate determinationof the mass ratio from the projected semi-major axes of the two pulsar orbits. Inthe Double Pulsar all PK parameters listed above have been measured, some of themwith exquisite precision. Most importantly, the change in the orbital period Pb dueto GW damping has by now been tested to agree with the quadrupole formula of GRto better than 0.1%, giving the best test for the existence of GWs as predicted byGR [681]. As a result of geodetic precession, pulsar B has in the meantime turnedaway from our line-of-sight and is no longer visible [689]. Due to the high inclinationof the orbital plane (close to 90 degrees), pulsar A is getting eclipsed by the plasma-filled magnetosphere of pulsar B every 2 1

2 hours, for about 30 seconds around superiorconjunction. Changes in the eclipse pattern could be used to determine the rate ofgeodetic precession, ⌦B, with a precision of about 13% [690]. The obtained value isin good agreement with GR. All these tests are summarized in form of a mass-massdiagram in Figure 6.1.

PSR J1738+0333 is a pulsar in a nearly circular (e ⇠ 3 ⇥ 10�7), 8.5 hour orbitwith an optically bright white dwarf. High-resolution spectroscopy allowed the

This

isnotto

saythatthe

integralisbadly

behaved.Infact,

theregulated

integralcan

befound

exactly,in

termsof

knownfunctions:

Ið!Þ¼

g24

!EiðigÞþ

1g "iþ1g #e

ig%Eiðig

!2Þ

%1

g!

2 "iþ1

g!

2 #eig!

2 $;

(2.3)

where

Eiis

theexponential

integral.Moreover,

thisexpression

hasa

sensiblelim

itas

!!

1;using

theasym

ptoticbehavior

ofthe

exponentialintegral

asx!

1,

EiðixÞ¼

i!þ

eix !%

ix %"1x #

2þO"1x3 #$

(2.4)

gives

Ið!Þ¼

g24

!EiðigÞþ

1g "iþ1g #e

ig%i! $

þO"e

ig!

2

!2

#:

(2.5)

Clearly,

expandingthe

exponentialisnot

theright

thingto

doin

orderto

evaluatethe

integral.There

appearto

beclose

parallelsin

expressionsdescribing

gravitationalscattering.

III.GRAVITATIO

NALEIK

ONALAMPLIT

UDES

Consider

gravitationalscattering,

inD

dimensions,

inthe

ultrahigh-energylim

it,E¼

ffiffiffisp

&M

D,with

MDthe

Planck

mass.

Since

thedim

ensionlessgravitational

cou-pling

isusually

thoughtto

beG

DED%2'

ðE=M

D Þ D%2,one

might

expectthis

tobe

astrongly

coupledproblem

.How

ever,thatdepends

onthe

sizeof

themom

entumtrans-

fer,t¼

%q2,

orim

pactparam

eter—scattering

atasuffi

-ciently

largeim

pactparam

eterisdom

inatedsim

plyby

theBorn

approximation,

T0 ðs;tÞ¼

%8!

GDs2=t:

(3.1)

For

decreasingim

pactparam

eter,higher-loop

amplitudes

becomerelevant,

andone

entersregim

eswhere

differentphenom

enadom

inate;an

overviewof

theseregim

es,with

furtherreferences,

isprovided

in[4].

(Important

earlierreferences

include[7,15,16,31,32].)

Inparticular,

itis

ar-gued

thereand

inpreceding

referencesthat

thefirst

loopcorrections

tobecom

eim

portantare

theladder

andcrossed-ladder

diagrams,

which

canbe

summed

togive

theeikonal

approximation

tothe

amplitude.

Specifi

cally,suchaladder

diagramisexhibited

inFig.1.

The

eikonalapproxim

ationto

theam

plitudearises

fromneglecting

subleadingterm

sin

themom

entumtransfer

runningthrough

theindividual

rungs.In

particular,if

kdenotes

atypical

suchmom

entumtransfer,

andpian

externalmom

entum,then

theinterm

ediatepropagators

ofthe

high-energyparticles

havedenom

inatorsof

theform

Di ¼

ðpi þ

kÞ 2þm

2¼2p

i (kþ

k2;

(3.2)

andweneglect

thesecond

term.Likew

ise,in

thevertices,

we

neglectmom

entumtransfers

k'q

compared

tothe

sizeof

pi .The

resultis

thatthe

sumof

ladderand

crossed-ladderdiagram

sat

N-loop

ordercan

bewritten

interm

sof

thetree

amplitude,

T0 ðs;%

q2Þ,

as

iTN ðs;qÞ¼

2s

ðNþ

1Þ! Z!Y Nþ

1

j¼1

dD%

2kj

ð2!Þ D%2

iT0 ðs;%

k2j Þ

2s

$

)ð2!Þ D

%2"

D%2 "X

j

kj %

q? #;

(3.3)

where

theintegrals

areover

thecom

ponentsof

themom

entatransverse

tothose

ofthe

incoming

particlesin

thec.m

.frame.T

hesum

overall

sucham

plitudesgives

theeikonal

amplitude,w

hichiswritten

interm

sof

theeikonal

phase,#ðx?;sÞ¼

12s

ZdD%2q

?ð2!Þ D%

2eiq

? (x?T

0 ðs;%q2? Þ

¼4!

ðD%

4Þ"D%3

GDs

xD%4

?;

(3.4)

with

"nthe

areaof

theunit

n-sphere.

The

eikonalam

pli-tude

is

iTeik ðs;tÞ¼

2s ZdD%

2x?e %

iq? (x

?ðei#ðx?

;sÞ%1Þ;

(3.5)

inthe

integral,b¼

jx? jplays

therole

ofthe

impact

parameter,

andthus

theam

plitudeisnaturally

givenin

anim

pactparam

eterrepresentation.

Anim

portantquestion

isto

what

extentthe

eikonalam

plitudeisagood

approximation

tothe

exactam

plitude,and

inwhat

domain.

First,

notethat

anatural

expansionparam

eteristhe

eikonalphase(3.4).W

henthis

issm

all,theeikonal

amplitude

canbe

approximated

bythe

linearterm

in#,which

isexactly

theBorn

amplitude.

Corrections

tothis

becomeim

portantat

impact

parameters

where

#be-

comes

oforder

one.These

impact

parameters

aredirectly

relatedto

mom

entumtransfers,

sincethe

integral(3.5)

hasasaddle

pointwhich

fixes

bin

termsof

q.Towrite

thecorresponding

equation,weintroduce

theSchw

arzschildradius

ofthe

c.m.energy,

RðEÞ¼1MD

"kDE

MD #

1=ðD%3Þ;

(3.6)

where

12

34

FIG

.1.

Aladder

diagramwith

multiple

gravitonexchange.

HIG

H-ENERGY

SCATTERIN

GIN

GRAVITYAND

...PHYSICALREVIEW

D82,

104022(2010)

104022-3

Giddings et al. ‘10

Lyne at al. ‘04


1) How does gravity behave in the strong-field regime?


Gravity, the weakest of fundamental interactions, has been observed for centuries, but mostly in the weak-field regime.





The strength of gravity can be parametrized e either by gravitational potential ε=GM/r or by spacetime curvature 𝜉=GM/r3

10 Baker et al.

10-62 10-59 10-56 10-53 10-50 10-47 10-44 10-41 10-38 10-35 10-32 10-29 10-26 10-23 10-20 10-17 10-14 10-11

Curv

atur

e, ξ

(cm

-2 )

10-12 10-10 10-8 10-6 10-4 10-2 100

Potential, ε

DETF4 Facility

BAO

ELT S stars

LOFT + Athena

PPN constraints

Tidal streams (GAIA)

AdLIGO

eLISA

A P

Atom

Triple

Inv. Sq.

EHT

Sgr A*

M87

Planck

PTA

Fig. 2.— The experimental version of the gravitational parameter space (axes the same as in Fig. 1). Curves are described in detail inthe text (§4). Some of the abbreviations in the figure are: PPN = Parameterized Post-Newtonian region, Inv. Sq. = laboratory tests of the1/r2 behaviour of the gravitational force law, Atom = atom interferometry experiments to probe screening mechanisms, EHT = the EventHorizon Telescope, ELT = the Extremely Large Telescope, DETF4 = a hypothetical ‘stage 4’ experiment according to the classificationscheme of the Dark Energy Task Force (Albrecht et al. 2006), Facility = a futuristic large radio telescope such as the Square KilometreArray.

4.1. Cosmology

Galaxy Surveys. In the lower section of the figure weindicate the regions probed by two future galaxy clus-tering surveys. In green we consider a next-generation‘stage 4’ space-based survey of the kind envisaged by theDark Energy Task Force (Albrecht et al. 2006), labelledDETF4. In blue, we consider a futuristic ‘Facility stage’ground-based radio interferometer of the kind consideredby Bull et al. (2014), capable of mapping nearly the fullsky out to very high redshifts.Each survey is delineated by two lines, whose separa-

tion is set by the survey redshift range. We used equa-tions (11) and (15) to plot the minimum and maximum

k-values for each experiment, where the minimum k isset by the size of the survey and the maximum k ischosen to cut o↵ before nonlinearities become dominant(the value chosen varies somewhat in the literature forthe di↵erent experiments). We have also plotted a pointof k ' 0.05 h Mpc�1, corresponding to the approximateposition of the turnover in the matter power spectrum.The bent shape of these survey regions reflects the shapeof the matter power spectrum shown in Fig. 1 (cyancurve). Table 1 shows the values used. In addition, wehave added a point to represent recent measurements ofthe BAO feature (Anderson et al. 2014).Although the extent of the parameter space probed

by cosmology is small, we stress that this is one of the

‘15

/Virgo

solar system & binary pulsar tests




3rd generation ground-based

detectors

The strength of gravity can be parametrized e either by gravitational potential ε=GM/r or by spacetime curvature 𝜉=GM/r3

10 Baker et al.

10-62 10-59 10-56 10-53 10-50 10-47 10-44 10-41 10-38 10-35 10-32 10-29 10-26 10-23 10-20 10-17 10-14 10-11

Curv

atur

e, ξ

(cm

-2 )

10-12 10-10 10-8 10-6 10-4 10-2 100

Potential, ε

DETF4 Facility

BAO

ELT S stars

LOFT + Athena

PPN constraints

Tidal streams (GAIA)

AdLIGO

eLISA

A P

Atom

Triple

Inv. Sq.

EHT

Sgr A*

M87

Planck

PTA

Fig. 2.— The experimental version of the gravitational parameter space (axes the same as in Fig. 1). Curves are described in detail inthe text (§4). Some of the abbreviations in the figure are: PPN = Parameterized Post-Newtonian region, Inv. Sq. = laboratory tests of the1/r2 behaviour of the gravitational force law, Atom = atom interferometry experiments to probe screening mechanisms, EHT = the EventHorizon Telescope, ELT = the Extremely Large Telescope, DETF4 = a hypothetical ‘stage 4’ experiment according to the classificationscheme of the Dark Energy Task Force (Albrecht et al. 2006), Facility = a futuristic large radio telescope such as the Square KilometreArray.

4.1. Cosmology

Galaxy Surveys. In the lower section of the figure weindicate the regions probed by two future galaxy clus-tering surveys. In green we consider a next-generation‘stage 4’ space-based survey of the kind envisaged by theDark Energy Task Force (Albrecht et al. 2006), labelledDETF4. In blue, we consider a futuristic ‘Facility stage’ground-based radio interferometer of the kind consideredby Bull et al. (2014), capable of mapping nearly the fullsky out to very high redshifts.Each survey is delineated by two lines, whose separa-

tion is set by the survey redshift range. We used equa-tions (11) and (15) to plot the minimum and maximum

k-values for each experiment, where the minimum k isset by the size of the survey and the maximum k ischosen to cut o↵ before nonlinearities become dominant(the value chosen varies somewhat in the literature forthe di↵erent experiments). We have also plotted a pointof k ' 0.05 h Mpc�1, corresponding to the approximateposition of the turnover in the matter power spectrum.The bent shape of these survey regions reflects the shapeof the matter power spectrum shown in Fig. 1 (cyancurve). Table 1 shows the values used. In addition, wehave added a point to represent recent measurements ofthe BAO feature (Anderson et al. 2014).Although the extent of the parameter space probed

by cosmology is small, we stress that this is one of the

‘15

/Virgo

solar system & binary pulsar tests


Up to now, we had no direct information from the strong-field/large curvature regime of gravity.

What happens near the horizon of black holes, near the surface and in the inner core of neutron stars?





Credits: Pani, ‘15

There is no fundamental reason to believe that gravity behaves in the same way as in weak field/small curvature!





Credits: Pani, ‘15

There is no fundamental reason to believe that gravity behaves in the same way as in weak field/small curvature!

GWs can only be emitted (strong enough) by phenomena in this regime thus they are the perfect probe of strong gravity.



The same question can be addressed in different ways:



• Does General Relativity describe gravity at strong field/large curvature?





The weak equivalence principle very well tested (~10-13). Other modifications of the gravitational action are compatible with observations,

including scalar fields, bilinear curvature terms, massive gravity, etc.







• How general are the “no-hair theorems” of black holes?









In GR, any stationary BH (and, after a short transient, any BH) is only characterized by mass and angular momentum => Kerr solution










• Quantum modifications to BH structure? (see e.g. Giddings ’16)










Events like GW150914 provide constraints on modified gravity theories (bound on mg, measure of a quasi-normal mode,…)










Events like GW150914 provide constraints on modified gravity theories (bound on mg, measure of a quasi-normal mode,…)

New frontier: black-hole spectroscopy


Neutron stars, among the main expected sources, can be considered the “ground state of matter”.

2) How does matter behave at supranuclear densities?



Credits: D. Page

The composition of crust and outer core ~ understood, but we do not know the composition of the inner core:

extreme conditions (ρ≳1015 g/cm3, ν~1kHz, B~1010-15G) • can not be reproduced in lab, • are a challenge for the theory.




Credits: D. Page



We do not know the equation of state, even the particle content is not clear:

Hadrons? Hyperons? Meson condensates? Deconfined quark matter?

Demorest et al., ‘13




Credits: D. Page



Astrophysical observations are useful to constrain the EoS

but only GWs can give a definite answer!

We do not know the equation of state, even the particle content is not clear:

Hadrons? Hyperons? Meson condensates? Deconfined quark matter?

Demorest et al., ‘13



neutron star - neutron star binary system.


Credits: AEI


• In the late inspiral they are very deformed => it is possible to extract the tidal deformability (encoded in the “Love numbers”) of the star from the gravitational waveform.



Credits: AEI


• In the late inspiral they are very deformed => it is possible to extract the tidal deformability (encoded in the “Love numbers”) of the star from the gravitational waveform.Love numbers carry the imprint of the neutron star EoS! (see Michele’s talk)



Credits: AEI



• After the merger, a metastable hypermassive neutron star can form. It oscillates violently, emitting GWs, end eventually collapse to a BH. These GWs (some kHz) carry the imprint of the neutron star EoS.

Love numbers carry the imprint of the neutron star EoS! (see Michele’s talk)



Credits: AEI



• After the merger, a metastable hypermassive neutron star can form. It oscillates violently, emitting GWs, end eventually collapse to a BH. These GWs (some kHz) carry the imprint of the neutron star EoS.

Love numbers carry the imprint of the neutron star EoS! (see Michele’s talk)

• Other processes (interaction with a companion, accretion, etc.) could excite the quasi-normal modes of the neutron star (≳ 1 kHz). These modes encode the property of the matter composing the core, and then would reveal the EoS (“GW asteroseismology”)



Credits: AEI


3) Do new fundamental fields couple with strong-gravity systems?



• New fundamental fields with m≲10-10eV (Compton length ≳ 10 km) would couple with black holes, e.g. exciting superradiant instabilities




Dark matter candidates, e.g. axion-like particles, hidden photons, etc. could be detected from their gravitational coupling (no need for other couplings!)





Dark photonsALPsCredits: Pani, ‘15

Jaeckel & Ringwald, Ann. Rev. Nucl. Part. Sci. (2010) Goodsell+, JHEP (2009)





• Beyond-standard-model effects (e.g. primordial phase transitions, domain walls etc.) could yield stochastic background detectable by ground-based interferometers

Dark photonsALPsCredits: Pani, ‘15

Jaeckel & Ringwald, Ann. Rev. Nucl. Part. Sci. (2010) Goodsell+, JHEP (2009)


Gravitational-wave astronomy



4) How do black holes of different mass scales form and evolve?




• Which are the formation and evolution processes of BHs?

• Which are the formation and evolution of BH binary systems?




The BHs of GW150914 are more massive then expected, but still compatible with existent population synthesis models.

How did they form? Isolated binary evolution or dynamical formation?










BH masses and spins can be extracted from the GW signal. Accurate measurements of these signals give information

on the formation and evolution of BHs. Note that spin measurements from the electromagnetic signal

are difficult and problematic (model dependent)








BH masses and spins can be extracted from the GW signal. Accurate measurements of these signals give information

on the formation and evolution of BHs. Note that spin measurements from the electromagnetic signal

are difficult and problematic (model dependent)

Ground-based and space-based detectors will provide complementary information in different wavebands


5) Why neutron stars do not spin faster?



LMXRB: Neutron star + “normal” star

NASA/GSFC



Observational puzzle: accretion should be able to spin-up NS to ν~1.5kHz,

but actual spin rates much smaller: most of them ν<300Hz, maximum value 714Hz.


NASA/GSFC






NASA/GSFCThe mechanism limiting the spin rate is unclear, most candidates are associated to GW emission.







If this is true, these objects would be promising sources of GWs…



6) Does compact object coalescence source gamma-ray bursts?See Michele’s talk

7) Can we learn more on the origin and evolution of our Universe using GWs? See Michele’s talk





If this is true, these objects would be promising sources of GWs…


8) Will totally unexpected sources show up?





(unexpected to some extent…)



+ …. ?

(unexpected to some extent…)

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Fundamental Physics with Gravitational Waves

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