1909.00834.pdf

Voyage 2050 white paper

The local dark sector

Probing gravitation’s low-acceleration frontier and dark matter

in the Solar System neighborhood

Joel Berg ́e

, Laura Baudis

, Philippe Brax

, Sheng-Wey Chiow

, Bruno Christophe

, Olivier

Dor ́e

, Pierre Fayet

, Aur ́elien Hees

, Philippe Jetzer

, Claus L ̈ammerzahl

, Meike List

, Gilles

M ́etris

, Martin Pernot-Borr`as

, Justin Read

, Serge Reynaud

, Jason Rhodes

, Benny

Rievers

, Manuel Rodrigues

, Timothy Sumner

, Jean-Philippe Uzan

, Nan Yu

DPHY, ONERA, Paris Saclay University, 29 av. Division Leclerc F-92322 Chˆatillon

Cedex, France

Physik-Institut, Universitaet Zuerich, Winterthurerstrasse 190, CH-8057 Zuerich, Switzer-

land

Institut de Physique Th ́eorique, Universit ́e Paris-Saclay, CEA, CNRS, F-91191 Gif-sur-

Yvette Cedex, France

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive,

Pasadena, CA, 91109, USA

LPENS, Ecole Normale Sup ́erieure (Universit ́e PSL, CNRS, Sorbonne Universit ́e, Uni-

versit ́e de Paris, Paris, France), and CPhT, Ecole Polytechnique (Palaiseau, France)

SYRTE, Observatoire de Paris, Universit ́e PSL, CNRS, Sorbonne Universit ́e, LNE, 61

avenue de l’Observatoire, 75014 Paris, France

ZARM, Center of Applied Space Technology and Microgravity, University of Bremen,

Am Fallturm, D-28359 Bremen, Germany

Universit ́e Cˆote d’Azur, Observatoire de la Cˆote d’Azur, CNRS, IRD, G ́eoazur, 250

avenue Albert Einstein, F-06560 Valbonne, France

Institut d’Astrophysique de Paris, CNRS UMR 7095, Universit ́e Pierre & Marie Curie -

Paris VI, 98 bis Bd Arago, 75014 Paris, France

Department of Physics, University of Surrey, Guildford, GU2 7XH, Surrey, UK

Laboratoire Kastler Brossel, UPMC-Sorbonne Universit ́e, CNRS, ENS-PSL University,

Coll`ege de France, 75252 Paris, France

Blackett Laboratory, Imperial College London, Prince Consort Road, London. SW7

2AZ, United Kingdom

arXiv:1909.00834v2 [astro-ph.IM] 11 Sep 2019

The local dark sector

Executive summary

We speculate on the development and availability of new innovative propulsion techniques

in the 2040s, that will allow us to fly a spacecraft outside the Solar System (at 150 AU and

more) in a reasonable amount of time, in order to directly probe our (gravitational) Solar System

neighborhood and answer pressing questions regarding the dark sector (dark energy and dark

matter). We identify two closely related main science goals, as well as secondary objectives that

could be fulfilled by a mission dedicated to probing the local dark sector:

Science goal #1

Begin the exploration of gravitation’s low-acceleration regime with a man-

made spacecraft.

When placed on a potential–curvature plane, experimental and observational

probes of gravitation make apparent the existence of a “desert” between the relatively high Solar

System potential (where General Relativity –GR– is well tested) and cosmological scale (where

GR must be either replaced or complemented by dark energy). We have no clue of gravity’s

behavior in this intermediate regime. Only by entering this potential–curvature desert with a

dedicated spacecraft will we be able to gain new experimental clues about gravity, and perhaps

about physics beyond GR. We could in particular enter a new intermediate regime that bridges

between GR in the Solar System and “dark energy gravitation” at cosmological scales.

Science goal #2

Improve our knowledge of the local dark matter and baryon densities

The

constraints that direct detection experiments can set on dark matter characteristics depend on

the local dark matter density, which is degenerate with the dark matter particle’s cross section.

The estimation of this density in the Solar System neighborhood is performed by monitoring

the motion of nearby stars, relying on numerical simulations and assuming that the dark matter

distribution is homogeneous. However, the coupling between dark matter and baryons remains a

significant caveat. Flying a spacecraft in the outskirts of the Solar System will allow for a direct

measurement of the gravitational environment and will bring new and necessary information

about the very local dark matter distribution and density, significantly reducing the need for

the enormous extrapolation usually used from numerical simulations.

Science goals #3

Additional goals allowed by a mission designed for Science goals #1 and #2

We can define secondary science objectives about gravitation and fundamental physics (constrain

post-Newtonian parameters and the evolution of fundamental constants), the Solar System (ex-

plore the Kuiper belt and the heliosheath) and gravitational waves (measure the gravitational

wave stochastic background).

Those questions can be answered by directly measuring the gravitational potential with an

atomic clock on-board a spacecraft on an outbound Solar System orbit, and by comparing the

spacecraft’s trajectory with that predicted by GR through the combination of ranging data and

the in-situ measurement (and correction) of non-gravitational accelerations with an on-board

accelerometer.

Despite a wealth of new experiments getting online in the near future, that will bring new

knowledge about the dark sector, it is very unlikely that those science questions will be closed

in the next two decades. More importantly, it is likely that it will be even more urgent than

currently to answer them. Tracking a spacecraft carrying a clock and an accelerometer as it

leaves the Solar System may well be the easiest and fastest way to directly probe our dark

environment.

The local dark sector

1 When the Universe goes dark

General Relativity (GR) may arguably be the most elegant physics theory. It describes gravita-

tion as the simple manifestation of spacetime’s geometry, while recovering Newton’s description

of gravitation as a classical inverse-square law (ISL) force in weak gravitational fields and for

velocities small compared to the speed of light. In its century of existence, GR has been ex-

tremely successful at describing gravity. From Mercury’s perihelion to gravitational lensing, to

gravitational waves direct detection, it has successfully passed all experimental tests (see e.g.

Will (2014) and Ishak (2019) and references therein). Most importantly, GR allowed for the ad-

vent of mathematical cosmology and is the cornerstone of the current hot Big Bang description

of our Universe’s history.

Sitting next to GR, the Standard Model (SM) was built from the realization that the mi-

croscopic world is intrinsically quantum. Although perhaps less elegant than GR because of

its numerous free parameters, it is both highly predictive and efficient not only at describing

the behavior of microscopic particles, but also at mastering key technologies. Increasingly large

particle accelerators and detectors have allowed for the discovery of all particles predicted by

the model, up to the celebrated Brout-Englert-Higgs boson (Aad et al. 2012; Chatrchyan et al.

2012).

Although both frameworks (GR and SM) leave few doubts about their validity and seem

unassailable in their respective regimes (low vs high energy phenomena, macroscopic vs micro-

scopic worlds), difficulties have been lurking for decades. Firstly, the question of whether GR

and the SM should and could be unified remains open. Major theoretical endeavors delivered

models such as string theory, but still fail to provide a coherent, unified vision of our world. Sec-

ondly, unexpected components turn out to make up most of the Universe’s mass-energy budget:

dark matter and dark energy are undoubtedly the largest conundrums of modern fundamental

physics.

1.1 Dark matter

From observations of the Coma galaxy cluster, Zwicky (1933) was the first to pinpoint the

problem of missing matter: the galaxies motion could not be explained from the luminous matter

only. At the onset of the 1980s, Rubin et al. (1980) noticed a similar behavior on galactic scales:

galaxies rotate faster than expected based on their observed luminosity. Dark matter interacts

only weakly with baryonic matter, but its gravitational interaction is necessary not only to

account for galaxies’ rotation curves and cluster galaxies dynamics, but also to explain peaks

in the Cosmic Microwave Background (CMB) spectrum and the whole structure formation; no

model is able to build galaxies without its presence.

Astronomers failed to find this missing matter as baryonic massive compact baryonic ob-

jects (MACHOs), making a stronger case for non-baryonic matter. Maps of the dark matter

distribution at large scale, pioneered with weak gravitational lensing by Massey et al. (2007),

are now common (see e.g. Van Waerbeke et al. (2013) or Chang et al. (2018)), and a cosmic

web structured by (cold) dark matter is now central to the cosmological model.

Meanwhile, new approaches to gravitation, like MOND (Milgrom 1983), were developed

to explain observations without the need for dark matter. However, observations of colliding

galaxy clusters (Clowe et al. 2006) and of the fact that dark matter seems to be heated up by

star formation in dwarf galaxies (Read et al. 2019) tend to prove the existence of a particle

non-baryonic dark component, albeit sterile neutrinos could also act as dark matter (Boyarsky

et al. 2019).

The local dark sector

While astronomers were looking for dark matter gravitational signatures, particle physicists

did not remain idle. The supersymmetric extension of the SM predicts new massive particles, the

lightest of which was expected to account almost exactly for the amount of dark matter in the

Universe. Alas, this “WIMP miracle” was a lure and has somewhat faded by now, after intensive

indirect and direct searches failed to find it. Better and better direct detection experiments have

been running for decades (see e.g. Baudis (2016); Schumann (2019) for recent reviews). Aside

DAMA/LIBRA’s observation of an annually modulated event rate, no experiment ever saw the

signature of a WIMP hitting its detector. The axion, hypothesized as a by-product of a possible

solution to the strong CP-problem in the 1970s (Kuster et al. 2008), is another good candidate

still actively looked for, as are newer axion-like particles (ALPs). Nevertheless, only upper

bounds on the WIMPs’ mass and cross-section and on the axions and ALPs bounds to photon

or electrons have so far been reported.

The current consensus remains the existence of a non-baryonic matter interacting with bary-

onic matter mostly gravitationally, that pervades the Universe and clumps as halos hosting

galaxies and clusters of galaxies (Planck Collaboration et al. 2016).

1.2 Cosmological constant, dark energy and modified gravity

The observation of the accelerated expansion of the Universe from the measurement of the

distance of supernovae (Riess et al. 1998; Perlmutter et al. 1999) added a new twist to cosmology

and fundamental physics. Baryonic and dark matter were not alone, but a new component of

the Universe was even dominating the mass-energy budget of the Universe. Since then, all

cosmological probes have converged and their concordance allowed cosmologists to robustly

infer the acceleration of the cosmological expansion (Planck Collaboration et al. 2016; Abbott

et al. 2018).

The most natural candidate to explain this acceleration is Einstein’s cosmological constant

Λ (Carroll 2001), which may be linked to the vacuum energy. Adding it to cold dark matter, the

ΛCDM cosmological model was born. However, the observed value of Λ is around 120 orders of

magnitude smaller than the naive expectation from Quantum Field Theory (QFT), that should

be of the Planck mass: this

cosmological constant problem

is still unexplained.

The acceleration of the expansion can also be caused by a dynamical, repulsive fluid. Mea-

suring the equation of state of this dark energy has become the grail of observational cosmology,

with new and near future surveys: for instance (the list is far from exhaustive), the Dark En-

ergy Survey has already published first results (Abbott et al. 2018), while ESA’s Euclid

and

NASA’s WFIRST

missions are planned to finely measure weak lensing on half the sky, and

provide percent constraints on dark energy within ten years.

In the dark energy view, GR keeps its central role as the theory of gravitation, assumed valid

on all scales; it is the content of the Universe which is modified. The accelerated expansion can

also be explained the other way around: no new component is added to the Universe, but GR

is subsumed by a more general theory of gravitation that passes Solar System and laboratory

tests while having a different behavior on cosmological scales. A plethora of models have been

proposed (e.g. Jain & Khoury (2010); Joyce et al. (2015)), of which the scalar-tensor theories

are the simplest.

GR describes the gravitational force as mediated by a single rank-2 tensor field. There is

good reason to couple matter fields to gravity in this way, but there is no good reason to think

that the field equation of gravity should not contain other fields. It is then possible to speculate

http://sci.esa.int/euclid/

http://wfirst.gsfc.nasa.gov

The local dark sector

on the existence of other such fields. The simplest way to go beyond GR and modify gravity

is then to add an extra scalar field: such scalar-tensor theories are well established and studied

theories of Modified Gravity (Damour & Esposito-Farese 1992). From a phenomenological point

of view, scalar-tensor theories link the cosmic acceleration to a deviation from GR on large

scales. They can therefore be seen as candidates to explain the accelerated rate of expansion

without the need to consider dark energy as a physical component. Furthermore, they arise

naturally as the dimensionally reduced effective theories of higher dimensional theories, such as

string theory; hence, testing them can allow us to shed light on the low-energy limit of quantum

gravity theories.

Scalar fields that mediate a long range force able to affect the Universe’s dynamics should also

significantly modify gravity in the Solar System, in such a way that GR should not have passed

any experimental test. Screening mechanisms have been proposed to alleviate this difficulty

(Joyce et al. 2015). In these scenarios, (modified) gravity is environment-dependent, in such

a way that gravity is modified at large scales (low density) but is consistent with the current

constraints on GR at small scale (high density). Several such models have been put forth, like

the chameleon (Khoury & Weltman 2004a,b), the symmetron (Hinterbichler & Khoury 2010),

the Vainshtein mechanism (Vainshtein 1972), the K-Mouflage (Barreira et al. 2015; Brax et al.

2015) or the dilaton (Brax et al. 2010; Damour & Donoghue 2010a,b).

Those modified gravity models all predict the existence of a new, fifth force, that should

be detectable through a violation of the ISL or of the equivalence principle. Despite numerous

efforts, no such deviation has been detected so far (see e.g. Fischbach & Talmadge (1999);

Adelberger et al. (2003); Touboul et al. (2017); Berg ́e et al. (2018a) and references therein).

1.3 Aim and layout of this white paper; mission concept summary

Theoretical, observational and experimental efforts are still underway to shed light on dark

energy or modified gravity. This white paper proposes new ways to peek more deeply than ever

into this dark sector by directly probing it in the Solar System neighborhood.

To this aim, we propose to probe gravitation in the uncharted low-acceleration regime preva-

lent in the Solar System outskirts. An atomic clock onboard a spacecraft drifting out of the

Solar System allows for a direct measurement of the local metric. Meanwhile, an onboard ac-

celerometer measures all non-gravitational forces acting on the spacecraft, so that they can be

substracted off and a ranging technique allows for a direct assessment of the geodetic motion

of the spacecraft. Measuring the geodetic motion is equivalent to measuring the Levi-Civita

connection (by comparison to clocks that directly measure the metric). Not only this mission

concept allows for a test of gravity, but it also allows for the measurement of the local dark

matter density disentangled from the baryonic density (as it creates measurable friction on the

spacecraft), thereby helping to break the degeneracy between the local dark matter density and

the dark matter cross section in direct detection experiments.

As the time required to fly a spacecraft farther than the Kuiper belt with traditional propul-

sion and space navigation techniques is prohibitively long, we will rely on new types of propulsion.

We propose to use the concept elaborated for the Breakthrough Starshot project, using a solar

sail illuminated by a high-power laser from the ground. Rough orders of magnitude allow us

to expect to be able to send a probe of mass less than 1000 kg to hundreds of AU in two to

four years, making it a viable mission. Recently, Banik & Kroupa (2019) showed how a similar

mission (though in a slightly different context) can be feasible.

Such a mission is expected to be a M-class mission, although it highly depends on the

availability and cost to use the Breakthrough Starshot laser in the 2040s.

The local dark sector

Sect. 2 presents, in as much details as possible, state-of-the-art experimental efforts to shed

light on the aforementioned conundrums. We detail our science case in this section. Sect. 3

gives a broad outline of how a spacecraft (embarking an atomic clock, an accelerometer and

tracked with a ranging technique) could perform it, and provides secondary science objectives

that could easily be reached. In Sect. 4, we attempt to give a realistic landscape of the related

science in the 2040s. We mention the expected technical challenges in Sect. 5, then conclude in

Sect. 6.

2 Shedding light on the local Galactic dark sector

In this section, we show how direct measurements of non-gravitational forces in the Solar System

neighborhood (meaning,

outside

the Solar System) is a necessary step to go beyond current

experimental constraints on gravitation, dark energy and dark matter.

2.1 Dark energy and gravitation

2.1.1 Fifth force searches in the Solar System: current status

All theoretical attempts at explaining the three limitations mentioned in Sect. 1.2 modify GR

and Newton dynamics, either at small scale or very large scale, or both. In particular, if one

of them is correct, we should detect a violation of the gravitational ISL. Hence, in the weak

field limit, measurements of the dynamics of gravitationally bound objects (e.g. the behavior

of a torsion balance in the Earth gravity field, the trajectory of an interplanetary probe, or the

receding of galaxies) should show a deviation from what is expected from Newton’s equations.

In other words, a fifth force should be detectable (Fischbach & Talmadge 1999).

It is common-practice to parametrize a deviation from the ISL with a Yukawa potential, such

that the gravitational potential created by a point-mass of mass

at a distance

is (Fischbach

& Talmadge 1999; Adelberger et al. 2003)

(

) =

−

(

1 +

αe

−

r/λ

)

(1)

where

is the gravitational constant,

is the (dimensionless) strength of the Yukawa po-

tential relative to Newtonian gravity, and

is its range. Note that in this paper, we focus on

composition-independent violations of Newtonian dynamics, meaning that the coupling constant

does not depend on the test mass used for the experiment. Composition-dependent tests exist,

which assume that

depends on the nature of the test masses; in practice, they test the weak

equivalence principle (see Berg ́e et al. (2018a) and Fayet (2019) for up-to-date constraints on

composition-dependent tests).

Fig. 1 shows the current constraints on ISL-violations from a Yukawa potential, from millime-

ter scales to 1000 AU. A large region of the (

) plane is still allowed by current experiments

at submillimeter scales. Significant efforts are done to better constrain this region, most notably

with torsion pendulums, micro-cantilever or Casimir-force experiments (Adelberger et al. 2003,

2009; Murata & Tanaka 2015). Although these scales may be probed by bringing laboratory

experiments to space (albeit it is not clear whether the performance would be improved –Berg ́e

(2017)), we do not explore this possibility in this white paper, but only concentrate on ranges

larger than planetary scales. Note that geophysical scales can be probed by torsion pendulums

using the Earth as the attractor (e.g. Wagner et al. (2012)), while constraints on earth-orbit

scales are provided by a wealth of Earth-orbiting satellite experiments (Iorio 2002; Lucchesi

The local dark sector

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Figure 1:

95%-confidence-level constraints on ISL violating Yukawa interactions, from millimeter scales

to 1000 AU. Figure from Hees et al. (2017) and Konopliv et al. (2011).

2011; Ciufolini et al. 2013; Delva et al. 2017), though care should be taken to properly account

for the shape of the Earth (Berg ́e et al. 2018b).

Larger scales (from a few to hundreds of AU) can only be probed by Solar System ex-

periments, since astrophysical and cosmological observations are by definition sensitive to much

larger scales. The tightest constraints come from measurements by the Lunar Laser Range (LLR

–e.g. Murphy (2013)). The LLR consists in range measurements of the Moon, so that its orbit

is extremely finely monitored, with a subcentimetric precision. The most sensitive observable

for testing the ISL with LLR is the anomalous precession of the lunar orbit, due to the Earth’s

quadrupole field, perturbations from other solar system bodies, and relativistic effects. Those

perturbations and effects can be parametrized as a Yukawa interaction, thereby providing the

constraints shown in Fig. 1.

Larger scales can be probed through planetary ephemeris (Fienga et al. 2010; Viswanathan

et al. 2017) and ranging. Ranging is done by bouncing radar signals onto planets, or using

microwave signals transmitted by orbiting spacecrafts. As shown in Fig. 1, such techniques

provide a tight constraint on

Konopliv et al. (2011). However, this constraint becomes quickly

loose as the scale increases towards the outer solar system scale.

Probing those scales (the largest scales reachable by man-made instruments in a reasonable

amount of time) can be done by measuring the orbit of an outbound spacecraft, as it drifts

away from the Sun and the planets (Jaekel & Reynaud 2005; Hees et al. 2012). The most

notable existing test in this domain was performed by NASA during the extended Pioneer 10

& 11 missions. The test resulted in the so-called Pioneer anomaly (Anderson et al. 1998, 2002),

finally accounted for by an anisotropic heat emission from the spacecrafts themselves (Rievers

& L ̈ammerzahl 2011; Turyshev et al. 2011).

The effect of non-gravitational forces could be definitely accounted for if, instead of relying

on spacecraft and environment models, whose accuracy can always be doubted, we were able

to measure them directly. This can be easily done with an onboard accelerometer, as proved

by as many missions as GOCE (Rummel et al. 2011), LISA Pathfinder (Armano et al. 2016)

and MICROSCOPE (Touboul et al. 2017). Combining the model-independent measurements

of non-gravitational accelerations provided by an on-board accelerometer with radio tracking

data, it becomes possible to improve by orders of magnitude the precision of the comparison

with theory of the spacecraft gravitational acceleration. Such experimental concepts have been

The local dark sector

proposed e.g. by Christophe et al. (2009) and Buscaino et al. (2015). This is an aspect of the

mission outline that we propose in this white paper.

2.1.2 Gravitation’s low-acceleration frontier

Baker et al. (2015) classify probes and experimental/observational tests of gravitation in the

potential–curvature plane (left and right panels of Fig. 2, respectively). There, the potential

and curvature

are loosely defined as the Newtonian gravitational potential

≡

(2)

and the Kretschmann scalar

≡

(

αβγδ

)

√

(3)

created by spherical body (Schwarzschild metric) of mass

at a distance

, where

is the speed

of light and

is the Ricci curvature tensor.

It is clear that this plane is divided in four main regions:

•

highest curvatures and potentials correspond to compact objects and can be tested with

gravitational waves observatories

•

smaller curvatures correspond to Solar System objects (small potential) and to Galactic

center’s S-stars (higher potential); the former can be tested with planets ephemeris and

man-made spacecrafts (see above), while the latter can be tested with Galactic center

observations

•

very small curvatures correspond to cosmological probes (galaxies, large scale structures)

and can be tested e.g. with CMB observations or galaxy surveys

•

a desert of probes and (most importantly) of tests lies between the Solar System scale

tests and cosmological tests. All kind of speculation can be allowed in this regime, and we

can easily imagine that gravitation enjoys a gradual change of regime between compact

objects and Solar System scales (“high” curvature, where GR holds) and cosmological

scales (“very low” curvature, where GR seems to break), without even needing any of the

screening mechanisms mentioned in Sect. 1.2.

The Solar System tests that we discussed above are shown by the “Earth S/C” symbols

and the filled circles lying on the black slanted line (that stands for the Sun as the source of

gravitation). It is clear that in order to barely approach the potential–curvature desert, we must

precisely monitor how trans-Neptunian objects (either planetoids or spacecrafts) behave under

the influence of the Sun’s gravity. Having a test-mass at least 150 AU from the Sun would allow

us to actually enter that uncharted desert. Additionally, when parametrizing a deviation from

Newtonian gravity as a Yukawa potential, we can see from Fig. 1 that flying a spacecraft at 150

AU from the Sun and farther would test a fifth force of range for which no constraints exist yet.

In Fig. 2, we added the regions that could be tested by monitoring the motion of a spacecraft

in the Jupiter and Neptune systems. Although such measurements would probe still untested

regions of the potential–curvature plane, they remain even farther than tests in the Earth orbit

from the potential–curvature desert: albeit interesting in their own, their constraining and

exploratory potential on the fundamentals of gravitation is much less than having a test mass

150 AU from the Sun.

The local dark sector

Figure 2:

Gravitational potential–curvature plane.

Left

: Astrophysical, cosmological and experimental

probes.

Right

: Observationally and experimentally tested (currently or planed) regions are shown in

color; the shaded region is the so called “desert”, where no experimental is currently available, but where

a new intermediate regime of gravitation could be found, bridging between GR at higher curvature and

“dark energy gravitation” at smaller curvature. Exploring this desert should be set as a priority. Figure

adapted from Baker et al. (2015).

We can then state the first goal of our “dark sector” science case as:

Science goal #1–

Begin the exploration of the potential–curvature desert with a man-made

spacecraft.

This can be done by directly measuring the gravitational potential with an atomic

clock on-board the spacecraft, and by comparing the spacecraft’s trajectory with that predicted

by GR through the combination of ranging data and the in-situ measurement (and correction)

of non-gravitational accelerations with an on-board accelerometer (Sect. 3). Only by entering

this potential–curvature desert will we be able to gain new experimental clues about gravity,

and perhaps about physics beyond GR. We could in particular enter a new intermediate regime

that bridges between GR in the Solar System and “dark energy gravitation” at cosmological

scales. Additionally, we would approach MOND’s acceleration, thereby allowing us to directly

test MOND (Banik & Kroupa 2019).

2.2 Dark matter density measurement

2.2.1 (Direct) detection techniques and their link to the solar neighborhood

Although dark matter mostly interacts gravitationally with baryonic matter, it is expected that

it leaves tiny signatures observable non-gravitationally. Thus, indirect and direct detections

methods have been devised to hunt for it.

Indirect detection relies on observing electromagnetic signals or neutrinos originating from

the annihilation of dark matter particles (see e.g. Slatyer (2017) for a review). We will not deal

with such indirect detection techniques in the remainder of this paper.

Direct detection techniques have been invented to look for different species of dark matter:

axions are looked for via their resonant conversion in an external magnetic field using microwave

cavity experiments (Kuster et al. 2008), while WIMPs are expected to occasionally interact with

The local dark sector

Figure 3:

Current limits on spin-independent WIMP-nucleon scattering experiments (Abdelhameed

et al. 2019; Agnese et al. 2016; Agnes et al. 2018; Agnese et al. 2018; Bernabei et al. 2018; Adhikari et al.

2018; Ajaj et al. 2019; Aprile et al. 2016; Akerib et al. 2017; Cui et al. 2017; Aprile et al. 2016, 2018).

The orange shaded region represents the neutrino floor from the irreducible background from coherent

neutrino-nucleus scattering. Figure from Schumann (2019).

heavy nuclei, thereby creating a detectable nuclear recoil (see Schumann (2019) for a review of

existing experiments); moreover, light dark matter (below the GeV scale) can be searched via

scatters off electrons (Essig et al. 2012), while ultra-light dark matter can cause tiny but apparent

oscillations in the fundamental constants that can cause minute variations in the frequency of

atomic transitions (Arvanitaki et al. 2015; Stadnik & Flambaum 2015; Hees et al. 2016) and a

fifth force (e.g. Berg ́e et al. (2018a)).

For illustrative purpose, we now restrict our discussion to the spin-independent direct de-

tection of WIMPs, through the elastic scattering off nuclei. Fig. 3 shows the current limits

in the WIMP mass – WIMP-nucleon cross-section plane from several direct detection experi-

ments. The differential rate for WIMP scattering is degenerate between the local WIMP density

in the galactic halo

and the WIMP-nucleus differential cross-section d

σ/

(Baudis 2012;

Schumann 2019)

∫

max

min

(

)

(4)

where

is the number of target nuclei,

is the WIMP mass,

is the energy transferred

to the recoiling nucleus, and

and

(

) are the WIMP velocity and velocity distribution in the

Earth frame. The lowest allowed velocity

min

is that needed for a WIMP to induce a detectable

nuclear recoil, while the maximum one

max

= 533

+54

−

km/s corresponds to the escape velocity

(Piffl et al. 2014), meaning that WIMPs with

v > v

max

are not bound to the Galaxy.

The WIMP velocity distribution

(

) must be estimated with numerical simulations (Vo-

gelsberger et al. 2009; Bozorgnia et al. 2016; Sloane et al. 2016; Helmi et al. 2018), though new

observations could constrain

(

) (Ruchti et al. 2015). Since it cannot be determined through

the dynamics of a spacecraft, we will ignore it in the remainder of this paper, but instead focus

on how and why we can measure the local dark matter density

. We should first note that

current estimates mostly agree in the range

= (0

−

5) GeV/c

/cm

(Sivertsson et al. 2018;

Buch et al. 2019).

It is clear from the degeneracy in Eq. (4) that, were

revised, upper limits of Fig. 3 would

be affected (Green 2017). It is therefore particularly important to have an estimate as precise as

possible of

. Two traditional ways to measure it have been used for decades by astronomers.

The local dark sector

We propose a novel, in-situ way, in Sect. 2.2.2. But first, we summarize those two traditional

observational ways (see e.g. Read (2014) and references therein).

The local dark matter density is an average of the Galactic dark matter halo density in a

small region about the Solar System (at 8 kpc from the center of the Galaxy), extrapolated to

the laboratory. The first method to estimate it then relies on the measurement of the rotation

curve of the Galaxy and on a model of the dark matter halo. The halo is usually taken to be

an isothermal sphere with density

(

)

∝

−

, so that we recover the flat rotation curve, but

can be parameterized as a better model such as a Navarro-Frenk-White (NFW) halo (Navarro

et al. 1997). A bad model choice is obviously a source of systematic uncertainties. The second

method does not rely on a halo model, but uses the vertical motion of stars close to the Sun.

The main systematics come from the distance and velocity measurement of those stars (so that

K-stars are preferentially chosen), though robust techniques have been developed (Garbari et al.

2012). Combining the two methods allows astronomers to assess whether the Galactic halo is

prolate or oblate, and to look for a dark disk.

Since the measured

must be extrapolated to the position of the Earth, the local homo-

geneity of the dark matter halo is an important point. In particular, any clump or stream of

dark matter, as well as dark matter trapped by the Sun’s potential (Peter 2009) may bias the

extrapolation. Although dark matter simulations’ resolution is no better than a dozen parsecs,

we have good reasons to assume that the possible effects are negligible, and to consider the local

dark matter halo as homogeneous (Read 2014). The presence of a dark disk on top of the halo

should not cause a problem either, as it was shown that if such a disk exists, it is thick enough

not to influence the homogeneity of the close-by dark matter (Read 2014).

We shall close this presentation with a major caveat (Read 2014; Green 2017; Evans et al.

2019): although measurements of

relying on dark-matter-only simulation are very robust,

adding baryons to the simulations significantly complicates things. However, baryons are clear

components of our neighborhood, and therefore must be accounted for.

2.2.2 In-situ measurement of the local dark matter density

Tracking a spacecraft far enough from the Sun, we can in principle estimate the contribution of

the Galactic dark matter to its dynamics. We then use the same techniques as those presented

above, with the stellar tracers replaced by the spacecraft.

The discussion above also highlighted two possible (linked) caveats: (i) speculating on the

homogeneity of the local dark matter distribution to estimate its density in Earth laboratories

from astrophysics estimates averaged on the Solar System neighborhood and (ii) adding baryons

to dark-matter-only simulations complicates predictions about the Galactic dark matter halo.

These caveats highlight the difficulty to calibrate models without an in-situ measurement of

physical quantities. This is especially true in our Galactic neighborhood, since it is dominated

by baryons, so that hunting for dark matter is particularly difficult.

The interplay between dark matter and baryons is most easily seen by the uncertainty on

steming from our imperfect knowledge of the baryonic contribution to the local dynamical

mass, as shown with the baryonic surface density Σ

in Fig. 4 (Sivertsson et al. 2018).

Better understanding the respective contributions and distributions of dark matter and

baryons through experimental in-situ measurements is not only important for direct detection

experiments (as it allows us to better estimate

) but also to improve our knowledge of our

local environment. Although dark-matter-only simulations expect a smooth dark matter dis-

tribution, we could be surprised to discover small clumps, streams and passing clouds of dark

matter: indeed, as aforementioned, ultra-light dark matter (clumped e.g. in the form of topo-

The local dark sector

baryon

]

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

]

RAVE, Bienayme+ 2014

SDSS, Zhang + 2013

†

LAMOST , Xia + 2016

†

McKee+ 2015

Garbari + 2012

Bovy, Tremaine 2012

SDSS, Bovy, Rix 2013

‡

SDSS, Smith + 2012

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

[GeV

]

This work

Figure 4:

Degeneracy between the local dark matter density and the baryonic contribution to the local

dynamical mass. Figure from Sivertsson et al. (2018), with Bienaym ́e et al. (2014); Zhang et al. (2013);

Xia et al. (2016); McKee et al. (2015); Garbari et al. (2012); Bovy & Tremaine (2012).

logical defects or axion stars) can cause minute variations in the frequency of atomic transitions;

an atomic clock going through such a cloud would be temporarily desynchronized compared to

Earth-bound clocks (Derevianko & Pospelov 2014; Wcis lo et al. 2016; Roberts et al. 2017). Only

a direct assessment of the gravitational environment at very small scales will allow us to close

this question: as dark matter can affect the rate of a clock, it can be done with a spacecraft

carrying a clock, as shown in Sect. 3; if the clump is sufficiently massive, its crossing can be

determined by the precise monitoring of the spacecraft’s trajectory.

Regarding baryonic matter, an accelerometer on-board a spacecraft measures all non-gravita-

tional forces applied to the satellite (gas and dust friction, solar radiation pressure, outgasing...).

It is then an invaluable tool to directly probe the baryons distribution along its trajectory. In

other words, it provides an in-situ experimental measurement of the local contribution of dust

and gas in Σ

Such measurements will also be valuable for cosmology, as they could be used to calibrate

galaxy evolution simulations, and to better understand the systematics that still plague weak

lensing surveys (Rudd et al. 2008; Semboloni et al. 2011).

We can then state the second goal of our “dark sector” science case as:

Science goal #2–

Improve our knowledge of the local dark matter and baryon densities

It can

be fulfilled by monitoring the dynamics of a spacecraft in the Solar System neighborhood, the

spacecraft carrying a clock and an accelerometer. The clock will be sensitive to local dark matter

inhomogeneities. The combination of ranging and accelerometric data will also be sensitive to

local gravitational disturbances, such as those that could be created by a massive enough clump.

Finally, accelerometric data will be sensitive to the friction of any baryonic matter (dust and

gas) on the spacecraft, allowing for a direct measurement of the baryonic matter density along

the spacecraft trajectory. This will allow us to perform the first truly local measurement of the

dark matter halo density

and to improve the characterization of the dark matter constraints

from direct detection experiments.

The local dark sector

3 A very deep space mission concept

In this section, we present the concept of a mission that could fulfill the science goals presented

above.

3.1 Overview and requirements

3.1.1 Science goal #1: low-acceleration gravitation

This goal can be fulfilled by directly measuring the gravitational potential and by looking for a

deviation from the ISL.

Gravitational potential

The universal redshift of clocks when subjected to a gravitational

potential is one of the key predictions of General Relativity (GR) and, more generally, of all

metric theories of gravitation. It represents an aspect of the Einstein Equivalence Principle

(EEP) often referred to as Local Position Invariance (LPI) (Will 2014), and it makes clocks

direct probes of the gravitational potential (and thus to the metric tensor

component). In

GR, the frequency difference of two ideal clocks is proportional to (Reynaud et al. 2009)

−

≈

−

(5)

where

is the Newtonian potential and

the coordinate velocity, with

standing for

the ground and the in-space clocks. In theories different from GR this relation is modified,

leading to different time and space dependence of the frequency difference. This can be tested

by comparing two clocks at distant locations (different values of

and

) via exchange of an

electromagnetic signal.

An outbound Solar System trajectory (large potential difference) and low uncertainty on

the observable (5) allows for a relative uncertainty on the redshift determination given by the

clock bias divided by the maximum value of ∆

w/c

. Assuming that the Earth station motion

and its local gravitational potential can be known and corrected to uncertainty levels below

−

in relative frequency (10 cm on geocentric distance), which, although challenging, are

within present capabilities, then for a onboard clock similar to ACES’ PHARAO (see below),

with a 10

−

bias (Reynaud et al. 2009), at a distance of 150 AU this corresponds to a test

with a relative uncertainty of 10

−

, an improvement by almost four orders of magnitude on the

uncertainty obtained by the currently most sensitive experiments (Delva et al. 2018; Herrmann

et al. 2018).

We can thus take as a requirement that the clock’s bias is less than 10

−

Inverse Square Law violation

A definitive deviation from the ISL can be detected as a

deviation from the trajectory predicted by GR when taking into account the gravity of the Solar

System’s bodies. What is needed is an accurate orbit restitution, making sure that the spacecraft

follows a geodesics. The former can be done through orbit tracking with Radio-Science, while the

latter can be ensured with a drag-free spacecraft, whose trajectory is forced to be a geodesics by

actively canceling non-gravitational forces; alternatively, we can measure the non-gravitational

forces with a DC accelerometer, and correct for them when estimating the orbit, therefore not

needing a drag-free spacecraft. Although a model of non-gravitational forces is commonly used

to correct for them, we argue that no model is worth an empirical measurement, and hence that

an accelerometer (or drag-free spacecraft) is needed to definitely confirm any measured deviation

from the ISL.

The local dark sector

The required accuracy of non-gravitational forces is driven by the accuracy on the orbit

estimation and depends on the time of integration for the orbit restitution: the deviations from

a GR should be searched for in as short as possible time segments to minimize the effect of

the drift. As shown by Hees et al. (2012), a one-meter deviation from a Keplerian orbit can be

detected in a few days, requiring a precision on non-gravitational forces of the order of 10

−

m/s

. Getting down to such a precision would significantly improve the current constraints given

by the Pioneer probes, which assumed a bias in acceleration of 10

−

m/s

The requirements for the large scale test of the ISL can then be summarized as (Hees et al.

2012; Buscaino et al. 2015):

•

measure the orbit of the spacecraft with a precision better than 1m, all along the mission,

from the Earth to hundreds of AU

•

measure non-gravitational accelerations at the order of 10

−

m/s

3.1.2 Science goal #2: dark matter: search and local density measurement

The same experimental apparatus as used for Science goal #1 is able to fulfill this goal. Al-

though precise requirements must still be computed, we can expect that the clock will detect

inhomogeneities in the dark matter distribution. While the accelerometer will be perfectly suited

to measure the baryon distribution through the friction applied to the spacecraft, combining its

measurements with ranging data should enable us to detect massive enough inhomogeneities in

the gravitational field, possibly originating from dark matter clumps or streams.

3.2 Mission profile

The main factor limiting the viability of very deep space missions with conventional propulsion

systems and space navigation techniques is time. It took between 20 and 25 years to the Voyager

and Pioneer probes to reach 50 AU; New Horizons, albeit faster, took 15 years. Drawing on

these numbers, reaching 100 AU and more is prohibitively long.

Therefore, if we want to reach gravitation’s low-acceleration regime, we must envision new

propulsion methods. Several innovative concepts have been put forth to send interstellar nanoprobes.

For instance, the Breakthrough Starshot project aims to send a nanoprobe (of about 10 g) to

Proxima Centauri. In order to reach this star (at 4.5 light-years), the probe will be given a

strong impulse with a laser (shot from the ground) hitting a solar sail. The designers of the

project expect the probe to reach 15% of the speed of light.

We can straightforwardly build on this concept to design a very deep mission aiming to test

the dark sector in the outskirts of the Solar System. Let

and

be the mass of Break-

through Starshot’s probe and of our spacecraft, respectively. Then, assuming the velocity of the

Breakthrough Starshot probe is 0

and that the same amount of energy is transferred to our

spacecraft (with a solar sail) by the ground laser, then its velocity is simply

= 0

√

(6)

Assuming

= 10 g, then a spacecraft of mass between 100 kg and 1000 kg should be able to

reach 150 AU in 2 years to 6 years.

The mission profile is then straightforward. First, launch the spacecraft in Low Earth Orbit.

As for MICROSCOPE, it could be passenger of a Soyuz-like launcher. Second, accelerate it with

Breakthrough Starshot’s laser to put it into a high-velocity Solar System outbound trajectory.

The local dark sector

A gravitational boost with Jupiter or Saturn could be envisioned to gain more speed in the first

stage of its cruise.

This mission concept crucially depends (and speculates) on the availability of the Break-

through Starshot (or similar) laser propulsion system. If such a system exists in the 2040s, can

be used intensively (thereby, of a low usage cost), then our concept could fit into a M-class

mission.

We detail the possible payload and platform below. We then conclude the section with the

discussion of possible secondary science goals.

3.3 Payload

3.3.1 Clock

As shown above, a clock is directly sensitive to the gravitational potential. We thus propose to

embark an atomic clock, the design of which can be based on the PHARAO clock, set to fly as

part of the ACES experiment on-board the International Space Station in 2020 (Reynaud et al.

2009).

In microgravity, the linewidth of the atomic resonance of the clock will be tuned by two

orders of magnitude, down to sub-Hertz values (from 11 Hz to 110 mHz), 5 times narrower than

in Earth based atomic fountains. After clock optimization, performances in the 10

−

range are

expected both for frequency instability and inaccuracy.

Developed by CNES, the cold atom clock PHARAO combines laser cooling techniques and

microgravity conditions to significantly increase the interaction time and consequently reduce

the linewidth of the clock transition. Improved stability and better control of systematic effects

will be demonstrated in the space environment. PHARAO can reach a fractional frequency

instability of 10

−

−

, where

is the integration time expressed in seconds, and an inaccuracy

of a few parts in 10

Frequency transfer (via microwave link) with time deviation better than 0.3 ps at 300 s, 7

ps at 1 day, and 23 ps at 10 days of integration time will be demonstrated by ACES.

3.3.2 Electrostatic absolute accelerometer

Correcting for non-gravitational accelerations can be done with an unbiased (DC) accelerometer.

One is currently under development at ONERA: the Gravity Advanced Package (GAP) is com-

posed of an electrostatic accelerometer (MicroSTAR), based on ONERA’s expertise in the field

of accelerometry and gravimetry (CHAMP, GRACE, GOCE and MICROSCOPE missions), and

a bias calibration system (Lenoir et al. 2013). Ready-to-fly technology is used with original im-

provements aimed at reducing power consumption, size and weight. The bias calibration system

consists in a flip mechanism which allows for a 180

rotation of the accelerometer to be carried

out at regularly spaced times. The flip allows the calibration of the instrument bias along 2

directions, by comparing the acceleration measurement in the two positions.

The three axes electrostatic accelerometers developed at ONERA are based on the electro-

static levitation of the instrument inertial mass with almost no mechanical contact with the

instrument frame. The test-mass is then controlled by electrostatic forces and torques generated

by six servo loops applying well measured equal voltages on symmetric electrodes. Measure-

ments of the electrostatic forces and torques provide the six outputs of the accelerometer. The

mechanical core of the accelerometer is fixed on a sole plate and enclosed in a hermetic housing

in order to maintain a good vacuum around the proof-mass (Fig. 5, middle and right panels).

The local dark sector

Figure 5:

GAP DC accelerometer.

Left

: Bias Rejection System.

Center

: Exploded view of MicroSTAR

accelerometer.

Right

: Exploded view of MicroSTAR’s test mass in its cage.

Figure 6:

Current GAP performance.

Left

: MicroSTAR’s noise.

Right

: Bias rejection system’s resolu-

tion.

The electronic boards are implemented around the housing. The control of the proof-mass is

performed by low consumption analog functions.

The Bias Rejection System is equipped with a rotating actuator and a high-resolution angle

encoder working in closed loop operation (Fig. 5, left). The actuator for the MicroSTAR SU

rotation is a stepper motor with worm gear. The electronic boards for driving the stepper motor

and controlling the closed servo loop are located inside the housing. The electrical connection be-

tween Sensor Unit and Interface Control Unit is given by thin flexible shielded wires designed for

the corresponding environment and durability requirements. Sliding contacts are not necessary

because the maximum turning angle is 270

to one direction. A second small actuator (linear

servo motor) is foreseen to block the moving part of the rotating actuator during transport and

launch.

The global performance of the acceleration measurement is allocated towards the different

contributors that appear in the expression of the measurement equation. Eight posts are defined

with respect to the different contributors: at first order (assuming that the non-gravitational

acceleration

is the dominant term in a perfect measurement), the measured acceleration is:

meas

parasitic

acc

+ [

]

+ [

]

+ ([

Ω] + [Ω

])

parasitic

−

[

]

+ 2[Ω] ̇

+ ̈

(7)

where

is the instrumental noise,

MicroSTAR’s bias,

parasitic

acc

some parasitic acceleration

acting as a bias, [

] the scale factor, [

2] the quadratic factor, and [

] accounts for rotations

and misalignments, [

Ω] + [Ω

] for the inertial acceleration,

parasitic

for the spacecraft self-

gravity, and (

−

[

]

+ 2[Ω] ̇

+ ̈

) describes gravity gradient and deformation.

The local dark sector

The left panel of Fig. 6 shows the current noise of the MicroSTAR accelerometer, with

different contributors: a level of 10

−

m/s

/Hz

is obtained over [10

−

- 1] Hz, with a mea-

surement range of 1

−

m/s

. The bias modulation signal, consisting in regular 180

flips of

the accelerometer, allows us to band-pass filter the accelerometer noise around the modulation

frequency. The right panel of Fig. 6 shows, for different periods of modulation and calibration,

that after rejecting the bias, GAP is able to measure absolute accelerations down to 10

−

m/s

3.3.3 Tracking, clock synchronization and communication

Although the precision attainable on the orbit determination at distances larger than 150 AU is

uncertain, the spacecraft’s orbit can be determined through ranging, Doppler tracking and Very

Long Baseline Interferometry (VLBI).

Ranging should rely on the Deep Space Network (DSN), through the measurement of the

time an upward signal takes to be retransmitted by the spacecraft. The power necessary to

run the transponder onboard the spacecraft, as well as the antenna gain, are at the moment

not precisely known. The return on experience of the Voyager and New Horizon probes will be

beneficial to quantify them.

A microwave link will enable Doppler tracking of the spacecraft and the synchronization of

the onboard clock with on-ground clocks.

Orbit determination should be performed regularly, however we do not expect that a very

large duty cycle will be necessary. Therefore, this mission should not need a high access to DSN

facilities.

3.4 Platform

The spacecraft should provide the lowest and most axisymmetrical gravitational field, and make

coincide as much as possible the dry mass center of gravity, the propellant center of gravity, the

radiation pressure force line and the accelerometer. Furthermore, the satellite should be as rigid

as possible to minimize the impact of vibrations and glitches due to mechanical constraints, which

could contaminate the science measurements. A platform similar to that of MICROSCOPE

could be envisaged. Thence, its weight can be expected about 500 kg.

Radioisotope Thermoelectric Generators (RTG) are required to operate a spacecraft beyond

Jupiter. Furthermore, it is known that RTG can create non-gravitational forces through the

radiations they emit (they are the best candidates to explain the Pioneer anomaly –Rievers &

L ̈ammerzahl (2011); Turyshev et al. (2011)). The onboard accelerometer will solve this problem.

A propulsion module will be needed only if deep-space maneuvers are required. This will

depend on the orbit definition, and may largely affect the design of the spacecraft. To minimize

the self-gravity nuisance from a propulsion module in the spacecraft, we could envision using

an external propulsion module to set the spacecraft on its final orbit, that would be eventually

released once the orbit is reached.

The attitude of the spacecraft may be passively controlled, at least in part, by spinning

it slowly about its principle axis of symmetry; counter-rotating the payload would allow us

to perform tests in a non-rotating frame. The attitude can be estimated with stellar sensors

combined with the onboard accelerometers.

3.5 Secondary objectives

A space mission as that presented above allows for several secondary objectives:

The local dark sector

Science goal #3-1

Measurement of Post-Newtonian parameters

The Parametrized Post Newtonian (PPN) formalism relies on linearizing Einstein’s equations in

the weak field and low velocity regime (Will 2014). A spacecraft leaving the Solar System is in

this exact regime, such that the mission concept described above can provide new constraints of

those parameters. For instance, the Eddington parameter

(which measures how much space

curvature is produced by unit rest mass) can be estimated during any solar conjunction during

the spacecraft’s cruise to repeat the Cassini relativity experiment (Bertotti et al. 2003).

Science goal #3-2

Variation of fundamental constants

All deviations from GR mentioned in this white paper can be attributed to the variation of

fundamental constants (Uzan 2003). The space mission concept that we depicted can thus be

seen as a direct probe of the evolution of those constants.

Science goal #3-3

Kuiper belt exploration

Since their first detection (Jewitt & Luu 1993), Kuiper Belt Objects (KBOs) have revolutionized

our understanding of the history of the Solar System. Thousands of them have since been

discovered, that exhibit remarkable diversity in their properties (see e.g. Brown (2012) for a

review about KBOs). A spacecraft passing through the Kuiper belt is affected by its gravitational

potential, its shape, and the possible dust distributed in the area. Combined to giant telescopes

expected to be online in a few decades, a clock and an accelerometer on-board the spacecraft

are the perfect tools to measure them –see e.g. the SAGAS science case (Wolf et al. 2009).

Science goal #3-4

Heliosphere, heliopause and heliosheath

Thanks to its onboard accelerometer, the outbound spacecraft presented above will be able

not only to monitor the variation of solar radiation pressure as it drifts from the center of the

Solar System, but also to measure all non-gravitational forces (solar and interstellar winds) as it

crosses the heliopause. It will then largely complement the data already provided by the Voyager

missions as they left the Solar System a few years ago. NASA’s Senior Review 2008 of the Mission

Operations and Data Analysis Program for the Heliophysics Operating Missions

highlighted

the importance of continuing Voyager data to gain in-situ knowledge of the heliosheath.

Science goal #3-5

Measurement of gravitational waves background

Reynaud et al. (2008) proposed a new way to set bounds on gravitational waves backgrounds

created by astrophysical and cosmological sources by performing clock comparisons between a

ground clock and a remote spacecraft equipped with an ultra-stable clock, rather than only

ranging to an onboard transponder. Their investigation can be used in a mission concept as

that described above.

4 Scientific landscape in the 2030s

In this section, we attempt a short anticipation of the scientific landscape in the 2030s to 2040s.

We can identify five possible advances in the fields of interest of this white paper.

•

Test of GR in the strong field regime with LISA: the opening of the gravitational universe

with LISA (launched in the early 2030s) will allow for significantly improved tests of GR

http://www.igpp.ucla.edu/public/THEMIS/SCI/Pubs/Proposals and Reports/Senior Review 2008 Report

Final.pdf

The local dark sector

with compact objects. However, we should not expect that these observations explain dark

energy nor dark matter.

•

Improved constraints on dark energy and dark matter: full-sky surveys like LSST, Euclid

and WFIRST will be completed. We can expect them to significantly tighten the con-

straints on the dark energy equation of state and on the distribution of dark matter at

cosmological scale. Will they strengthen the GR+ΛCDM model or find new physics (per-

haps in the line of the current tension between local and early-Universe measurements of

the Hubble constant –e.g. Riess et al. (2019)) is a difficult question to answer. In any case,

they will remain blind to the gravitational potential–curvature desert mentioned above.

•

Fifth force laboratory and ground tests will have improved and probably excluded more

of the small-range interaction parameter space. However, by definition, they will remain

blind to those deviations from GR that could be measured at hundreds of AU from the

Sun.

•

The dark matter parameter space will be better constrained by direct detection experi-

ments and LHC searches. As for WIMPs, it is possible that the mass – cross section plane

of Fig. 3 is completely excluded, down to the neutrino floor, by 2030, or that WIMPs have

been detected. Nevertheless, the degeneracy between the local dark matter density and

the WIMP cross-section will remain. We do not see any plan to improve it without an

in-situ measurement performed by a spacecraft in the neighborhood of the Solar System.

Similar arguments can apply to other dark matter candidates (even those that may be

invented in the next decade).

•

Giant (ground-based) telescopes and the JWST will be online: they will bring new dis-

coveries of KBOs, that could be complemented by flying through the Kuiper belt.

None of those potential advances should close the science goals highlighted in this white

paper. It then seems that flying a spacecraft hundreds of AU from the Sun and entering the

low-acceleration regime of gravitation is the only way to gain fast insight into the dark sector,

even as late as 2040 or 2050.

5 Technological challenges

The payload will be inherited from already flown missions. The accelerometer will be derived

from those onboard Earth gravitational field observation missions (GOCE, GRACE, GRACE-

Follow) or fundamental physics (LISA Pathfinder). Nevertheless, it should be miniaturized to

minimize the spacecraft’s total weight. The clock can be derived from the ACES/PHARAO

clock, that should fly in the International Space Station in 2020.

The platform should not present any technological challenge, apart from providing a gravi-

tational background as quiet as possible.

We identify a handful of potentially major technological challenges:

•

propulsion

: sending a spacecraft at 150 AU in a relatively short amount of time cannot rely

on traditional chemical propulsion and space navigation techniques, so that new innovative

propulsion systems must be proposed and developed. Speculating on the development

of interstellar (nano)probes accelerated with a ground-based high-power laser, we could

bet on the availability of such a laser to sufficiently accelerate a few-hundreds kilograms

spacecraft.

The local dark sector

•

communication and tracking

: although precise requirements on the orbit determination at

distances larger than 150 AU must still be computed, it is unclear whether the current

ranging technology will be precise enough. The power of the transponder and the antenna

gain will have to be maximized.

•

power

: although RTG are technologically ready for a spacecraft on a fast outbound Solar

System orbit, an uncertainty remains about the policies regulating the launch of spacecrafts

•

shield

will be required to send an atomic clock in the outer space. This has never been

done. For comparison, ACES will fly in Low Earth Orbit, where such shielding is not

necessary.

6 Conclusion

Speculating on the development and availability of new innovative propulsion techniques in the

2040s, we presented open science questions related to the characterization of the dark sector.

Flying a spacecraft out of the Solar System would allow us to enter the uncharted gravitation’s

low-acceleration regime, where we may witness deviations from GR, hinting toward a new theory

in which gravitation would depend on the spacetime curvature, to evolve from the well-tested

Solar System regime of GR to the puzzling behavior we observe at cosmological scales. Devia-

tions from GR could be efficiently looked for by finely tracking the trajectory of the spacecraft

carrying an atomic clock and an accelerometer. The clock directly probes the gravitational

potential, while the accelerometer measures all non-gravitational accelerations applied to the

spacecraft, allowing us to subtract them and directly compare its trajectory to that predicted

by GR.

Moreover, such a mission will have the potential to probe our gravitational environment,

i.e. to disentangle the contributions of baryons and dark matter. As a consequence, it will be

able to measure in-situ the local density of dark matter, which directly affects the constraints

on dark matter’s characteristics inferred from on-ground direct detection experiments. It will

also allow for the estimation of the homogeneity of the dark matter distribution in the Solar

System neighborhood, thereby allowing for a better calibration of cosmological simulations and

impacting cosmological surveys still affected by the poorly understood coupling between baryonic

and dark matters. Finally, a flying atomic clock could detect clumps of ultralight dark matter

via its effects on the frequency on atomic transitions.

Beyond the secondary science goals that we identified (measurement of PPN parameters and

of the constancy of fundamental constants, exploration of the Kuiper belt and of the heliosheath,

as well as measurement of the gravitational waves stochastic background), such a mission will

bring new insights about the low-acceleration regime of gravitation. As stellar systems form in

this regime, it is of primary importance to better understand it, at a time where exoplanets are

routinely found and provide both new clues and new puzzles about the formation of planetary

systems.

We shall conclude this white paper by emphasizing that despite a wealth of new experiments

getting online in the near future, that will bring new knowledge about the dark sector, it is

very unlikely that the science questions that we presented will be closed. More importantly, it

is likely that it will be even more urgent than currently to answer them. Tracking a spacecraft

carrying a clock and an accelerometer as it leaves the Solar System may well be the easiest and

fastest way to directly probe our dark environment.

The local dark sector

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