International Journal of Modern Physics D

Space offers unique experimental conditions and a wide range of opportunities to explore the foundations of modern physics with an accuracy far beyond that of ground-based experiments. Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy physics experiments in space can test relativistic gravity and probe the physics beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for investigations in precision cosmology and astroparticle physics. In addition, atomic physics has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics.

We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences' Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental physics as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee's interagency "Fundamental Physics Task Force" to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental physics that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA's science objectives in space by including "laboratory research in fundamental physics" as an element in the agency's ongoing space research efforts.

I discuss the process by which science contributes to the setting of government priorities, and how these priorities get translated into programs and budgets at the federal agencies that fund scientific research. New technologies are now opening exciting scientific opportunities across the biological and physical sciences. I review the motivations and goals of President Bush's American Competitiveness Initiative (ACI), the importance of societal relevance to federal investments in basic research, and the ACI's impacts on discovery-oriented disciplines within the physical sciences.

Space offers eight distinct paths to controlled laboratory-style experiments in fundamental physics beyond the reach of any Earth-based laboratory. This impressive range of opportunity has already led to important physics but we are only at the beginning. Meanwhile, NASA is in crisis. This paper follows the wisdom of a great and inspiring physicist, William Fairbank, that every setback or difficulty is an opportunity. NASA needs new science; fundamental physics can provide it and with it fascinating new technologies — and training for a new generation of imaginative physicists and engineers.

The observation that the expansion of the Universe is proceeding at an ever-increasing rate, i.e. the "dark energy" problem, constitutes a crisis in fundamental physics that is as profound as the one that preceded the advent of quantum mechanics. Cosmological observations currently favor a dark energy equation-of-state parameter w = P/ρ = -1. Awkwardly, this is the value that has the least ability to discriminate between alternatives for the physics that produces the observed accelerating expansion. If this result persists we therefore run a very real risk of stagnation in our attempt to better understand the nature of this new physics, unless we uncover another piece of the dark energy puzzle. I argue that precision fundamental measurements in space have an important role in addressing this crisis.

The Human Spaceflight, Microgravity, and Exploration (HME) Directorate of the European Space Agency is strongly involved in fundamental physics research. One of the major activities in this field is represented by the ACES (Atomic Clock Ensemble in Space) mission. ACES will demonstrate the high performances of a new generation of atomic clocks in the microgravity environment of the International Space Station (ISS). Following ACES, a vigorous research program has been recently approved to develop a second generation of atomic quantum sensors for space applications: atomic clocks in the optical domain, aiming at fractional frequency stability and accuracy in the low 10^-18 regime; inertial sensors based on matter-wave interferometry for the detection of tiny accelerations and rotations; a facility to study degenerate Bose gases in space. Tests of quantum physics on large distance scales represent another important issue addressed in the HME program. A quantum communication optical terminal has been proposed to perform a test of Bell's inequalities on pairs of entangled photons emitted by a source located on the ISS and detected by two ground stations. In this paper, present activities and future plans will be described and discussed.

Physics experiments in space will permit us to investigate natural phenomena that cannot be observed on the ground, such as low-frequency gravitational waves, and to reach uncharted realms of accuracy — accessible only through experiments carried out in space — where current foundations of physics can be further tested and potentially falsified. Such projects require technologies that have not been in hand for a long time but are available now.

To avoid conflict of interest, the merit of space projects in physics, from the proposal stage through development, ought to be judged by experts in physics, rather than by space scientists from other fields. It is time now to set aside some funding to let missions in fundamental physics compete fairly with the established space sciences, thereby enriching and deepening the space enterprise — and broadening its advocacy base.

We look, in the context of the European space scene, at the measures and events that resurrected the initially suppressed planetary sciences and brought solar physics to blooming after a long drought; and derive ideas on how to increase the number of flight opportunities for fundamental physics in space.

The NSF has made investments in searches for dark matter, in ultrahigh energy cosmic rays and gamma rays, in neutrino physics and astrophysics, and in nuclear astrophysics. We expect the future to witness the expansion of these efforts, along with efforts to refine the measurements of the cosmic microwave background. In some of these efforts the Deep Underground Science and Engineering Laboratory is expected to play a major role.

This paper describes the high-energy-physics program at the US Department of Energy. The mission and goals of the program are described along with the breadth of the overall program. Information on recommendations from community-based panels and committees is provided. Finally, details about the main astrophysics and cosmology projects are given.

We discuss the motivation for high accuracy relativistic gravitational experiments in the solar system and complementary cosmological tests. We focus our attention on the issue of distinguishing a generic scalar theory of gravity as the underlying physical theory from the usual general-relativistic picture, where one expects the presence of fundamental scalar fields associated, for instance, with inflation, dark matter and dark energy.

In this talk, we review theories that modify gravity at cosmological distances, and show that any such theory must exhibit a strong coupling phenomenon. We show that all consistent theories that modify the dynamics of the spin-2 graviton on asymptotically flat backgrounds, automatically have this property. Due to the strong coupling effect, modification of the gravitational force is source-dependent, and for lighter sources sets in at shorter distances. This universal feature makes modified gravity theories predictive and potentially testable by precision gravitational measurements at scales much shorter than the current cosmological horizon.

I review some aspects of the Dvali–Gabadadze–Porrati (also known as "brane-induced") model of gravity. This model provides a novel way to modify gravity at large distances and, as such, has potentially some interesting cosmological consequences, like the possibility of getting an accelerated expansion with a vanishing cosmological constant. In DGP gravity, the recovery of usual gravitational interaction at small (i.e. noncosmological) distances is rather nontrivial. This can lead to observable signature in observations made in the solar system. I discuss various aspects of the phenomenology of the model and briefly comment on the consistency of the whole approach.

We summarize an interesting set of solar system predictions that we have recently derived for modified Newtonian dynamics (MOND). Specifically, we find that strong MOND behavior may become evident near the saddle points of the total gravitational potential. Whereas in Newtonian theory tidal stresses are finite at saddle points, they are expected to diverge in MOND, and to remain distinctly large inside a sizable oblate ellipsoid around the saddle point. While strong MOND behavior would be a spectacular "backyard" vindication of the theory, pinpointing the MOND bubbles in the setting of the realistic solar system may be difficult. Space missions such as the LISA Pathfinder, equipped with sensitive accelerometers, may be able to explore the larger perturbative region.

The phenomena customarily described with the standard ΛCDM model are broadly reproduced by an extremely simple model in TeVeS, Bekenstein's¹ modification of general relativity motivated by galaxy phenomenology. Our model can account for the acceleration of the Universe seen at SNeIa distances without a cosmological constant, and the accelerations seen in rotation curves of nearby spiral galaxies and gravitational lensing of high-redshift elliptical galaxies without cold dark matter. The model is consistent with BBN and the neutrino mass between 0.05 eV to 2 eV. The TeVeS scalar field is shown to play the effective dual roles of dark matter and dark energy, with the amplitudes of the effects controlled by a μ function of the scalar field, called the μ essence here. We also discuss outliers to the theory's predictions on multiimaged galaxy lenses and outliers on the subgalaxy scale.

I briefly discuss some attempts to construct a consistent modification to general relativity (GR) that might explain the observed late-time acceleration of the Universe and provide an alternative to dark energy. I describe the issues facing extensions to GR, illustrate these with a specific example, and discuss the resulting observational and theoretical obstacles.

A relativistic modified gravity (MOG) theory leads to a self-consistent, stable gravity theory that can describe the solar system, galaxy and clusters-of-galaxies data, and cosmology.

Experimental tests of gravity performed in the solar system show a good agreement with general relativity. The latter is, however, challenged by the Pioneer anomaly, which might be pointing at some modification of gravity law at ranges of the order of the size of the solar system. As this question could be related to the puzzles of "dark matter" or "dark energy," it is important to test it with care. There exist metric extensions of general relativity which preserve the well-verified equivalence principle while possibly changing the metric solution in the solar system. Such extensions have the capability to preserve compatibility with existing gravity tests while opening free space for the Pioneer anomaly. They constitute arguments for new mission designs and new space technologies as well as for having a new look at data of already-performed experiments.

This is more an after-dinner talk than it is a presentation of research — nothing new, no references, incomplete. It is, rather, a somewhat personal view of: (1) the history of experimental gravity in space, (2) a discussion on the theoretical basis for the two major thrusts in the field — tests of post-Newtonian gravity and gravitational astronomy, and (3) a couple of brief comments on two of the future space opportunities that are discussed at this meeting, LATOR and LISA. My judgment is that we have done a lot to provide an experimental basis for the theory of gravity, and that we can do even more.

Spacecraft radio science techniques can be used for precision solar system tests of relativistic gravity, as was demonstrated by the measurement of the Doppler shift of radio signals with the Cassini mission. Similar experiments are planned for the BepiColombo mission to Mercury. Recent theoretical developments based on string theory and inflationary cosmologies link the validity of general relativity to the expansion of the Universe and indicate that violations may be within the reach of future, precise experiments. In spite of the uncertainty of the theoretical scenarios, the motivations for further tests of gravitational theories are stronger then ever: string theory, new cosmological observations, the hypotheses of dark matter and dark energy, all point to the need for a new and more profound understanding of the Universe and its laws, including the laws of gravity. This paper describes experiments for probing space–time in the solar system with the Cassini and BepiColombo missions, and discusses the experimental limitations of microwave systems used for these tests, including attitude motion and nongravitational accelerations of the spacecraft, propagation noise, and mechanical noise of ground antenna.

APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) is a new effort in lunar laser ranging that uses the Apollo-landed retroreflector arrays to perform tests of gravitational physics. It achieved its first range return in October 2005, and began its science campaign the following spring. The strong signal (> 2500 photons in a ten-minute period) translates to one-millimeter random range uncertainty, constituting at least an order-of-magnitude gain over previous stations. One-millimeter range precision will translate into order-of-magnitude gains in our ability to test the weak and strong equivalence principles, the time rate of change of Newton's gravitational constant, the phenomenon of gravitomagnetism, the inverse-square law, and the possible presence of extra dimensions. An outline of the APOLLO apparatus and its initial performance is presented, as well as a brief discussion on future space technologies that can extend our knowledge of gravity by orders of magnitude.

Since 1964, the NASA Goddard Space Flight Center (GSFC) has been using short pulse lasers to range to artificial satellites equipped with passive retroreflectors. Today, a global network of 40 satellite laser ranging (SLR) stations, under the auspices of the International Laser Ranging Service (ILRS), routinely tracks two dozen international space missions with few-millimeter precision using picosecond pulse lasers in support of Earth science. Lunar laser ranging (LLR) began in 1969, shortly after NASA's Apollo 11 mission placed the first of five retroreflector packages on the Moon. An important LLR data product has been the verification of Einstein's equivalence principle and other tests of general relativity. In 1975, the University of Maryland used a laser ranging system to continuously transfer time between two sets of atomic clocks — one set on the ground and the other in an aircraft — to observe the predicted relativistic effects of gravity and velocity on the clock rates. Two-way asynchronous laser transponders promise to extend these precise ranging and time transfer capabilities beyond the Moon to the planets, as evidenced by two successful experiments carried out in 2005 at distances of 24 and 80 million km respectively.

More precise lunar and Martian ranging will enable unprecedented tests of Einstein's theory of general relativity as well as lunar and planetary science. NASA is currently planning several missions to return to the Moon, and it is natural to consider if precision laser ranging instruments should be included. New advanced retroreflector arrays at carefully chosen landing sites would have an immediate positive impact on lunar and gravitational studies. Laser transponders are currently being developed that may offer an advantage over passive ranging, and could be adapted for use on Mars and other distant objects. Precision ranging capability can also be combined with optical communications for an extremely versatile instrument. In this paper we discuss the science that can be gained by improved lunar and Martian ranging along with several technologies that can be used for this purpose.

Existing capabilities of laser ranging, optical interferometry, and metrology, in combination with precision frequency standards, atom-based quantum sensors, and drag-free technologies, are critical for space-based tests of fundamental physics; as a result of the recent progress in these disciplines, the entire area is poised for major advances. Thus, accurate ranging to the Moon and Mars will provide significant improvements in several gravity tests, namely the equivalence principle, geodetic precession, PPN parameters β and γ, and possible variation of the gravitational constant G. Other tests will become possible with the development of an optical architecture that allows one to proceed from meter to centimeter to millimeter range accuracies on interplanetary distances. Motivated by anticipated accuracy gains, we discuss the recent renaissance in lunar laser ranging and consider future relativistic gravity experiments with precision laser ranging over interplanetary distances.

The objective of ISLES (Inverse-Square Law Experiment in Space) is to perform a null test of Newton's law in space with a resolution of 10 ppm or better at a 100 μm distance. ISLES will be sensitive enough to detect the axion, a dark matter candidate, with the strongest allowed coupling and probe large extra dimensions of string theory down to a few micrometers. The experiment will be cooled to < 2 K, which permits superconducting magnetic levitation of the test masses. This soft, low-loss suspension, combined with a low-noise SQUID, leads to extremely low intrinsic noise in the detector. To minimize Newtonian errors, ISLES employs a near-null source, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a nominal distance of 100 μm. The signal is detected by a superconducting differential accelerometer.

The Laser Astrometric Test of Relativity (LATOR) experiment is designed to explore the general theory of relativity in close proximity to the Sun — the most intense gravitational environment in the solar system. Using independent time series of highly accurate measurements of the Shapiro time delay (interplanetary laser ranging accurate to 3 mm at ~2 AU) and interferometric astrometry (accurate to 0.01 picoradian), LATOR will measure gravitational deflection of light by the solar gravity with an accuracy of one part in a billion — a factor of ~30,000 better than what is currently available. LATOR will perform a series of highly accurate tests in its search for cosmological remnants of the scalar field in the solar system. We present the science, technology and mission design for the LATOR mission.

In a LATOR mission to measure the non-Euclidean relationship between three sides and one angle of a light triangle near the Sun, the primary science parameter, to be measured to part-in-10⁹ precision, is shown to include not only the key parametrized post-Newtonian (PPN) γ, but also the Sun's additional mass parameter, M_Γ, which appears in the spatial metric field potential. M_Γ may deviate from the Sun's well-measured gravitational mass due to post-Newtonian features of gravitational theory not previously measured in relativistic gravity observations. Under plausible assumptions, M_Γ is a linear combination of the Sun's gravitational and inertial masses.

If LATOR's two spacecraft lines of sight are kept close to equal and opposite relative to the Sun during the mission's key measurements of the light triangle, it is found that the navigational requirements for the spacecraft positions are greatly relaxed, eliminating the need for on-board drag-free systems. Spacecraft orbits from the Earth to achieve the equal and opposite passages by the Sun's, line of sight are illustrated.

STEP, the Satellite Test of the Equivalence Principle, is reviewed and the current status of the project is discussed. This space-based experiment will test the universality of free fall and is designed to advance the present state of knowledge by over five orders of magnitude. The international STEP collaboration is pursuing a development plan to improve and verify the technology readiness of key systems. We discuss recent advances with an emphasis on accelerometer fabrication and tests. Critical technologies successfully demonstrated in flight by the Gravity Probe B mission also contribute to progress.

Improving the level of accuracy in testing the principle of equivalence (PE) requires reliably extracting a very small signal from an instrument's intrinsic noise and the noise associated with the instrument's motion. In fact, the spin velocity required to modulate a PE-violating signal produces a relatively high level of motion-related noise and modulation of gravity gradients at various frequencies. In the test of the PE in an Einstein elevator under development by our team, the differential acceleration detector free-falls while spinning around a horizontal axis inside an evacuated, comoving capsule released from a stratospheric balloon. The accuracy goal of the experiment is to test the PE at an accuracy of a few parts in 10¹⁵, a limit set by the expected white-noise sources in our detector. The extraction of a very small signal from the prevailing noise sources is necessary for the experiment to succeed. In this paper, we discuss different detector configurations and describe a particular design that is able to provide a remarkable attenuation and frequency separation of the effects of motion and gravity gradients with respect to a PE-violating signal. Numerical simulations of the detector's dynamics in the presence of relevant perturbations, realistic errors, and construction imperfections show the merits of this configuration for the differential acceleration detector.

To test the equivalence principle (EP) to an accuracy of at least σ(Δ g)/g = 5 × 10^-14, we are developing a modern Galilean experiment. In our principle-of-equivalence measurement (POEM), we directly examine the relative motion of two test mass assemblies (TMA) that are freely falling. Such an experiment tests both for a possible violation of the weak equivalence principle (WEP) and for new forces that might mimic a WEP violation. For the terrestrial version of the experiment, there are three key technologies. A laser gauge measures the separation of the TMA to picometer accuracy in a second as they fall freely in a comoving vacuum chamber. The motion system launches the TMA from their kinematic mounts inside the chamber and keeps the chamber on a trajectory that mimics free fall until the chamber nears the bottom of its motion. It then "bounces" the chamber back to upward motion in preparation for a new launch of the TMA. A capacitance gauge system measures an additional four degrees of freedom of the motion of each TMA. The resulting estimate of the rotation around and translation along the horizontal axes is used to correct systematic errors. We describe the status of POEM and discuss recent progress.

The small satellite "Galileo Galilei" (GG) has been designed to test the equivalence principle (EP) to 10^-17 with a total mass at launch of 250 kg. The key instrument is a differential accelerometer made up of weakly coupled coaxial, concentric test cylinders rapidly spinning around the symmetry axis and sensitive in the plane perpendicular to it, lying at a small inclination from the orbit plane. The whole spacecraft spins around the same symmetry axis so as to be passively stabilized. The test masses are large (10 kg each, to reduce thermal noise), their coupling is very weak (for high sensitivity to differential effects), and rotation is fast (for high frequency modulation of the signal). A 1 g version of the accelerometer ("Galileo Galilei on the Ground" — GGG) has been built to the full scale — except for coupling, which cannot be as weak as in the absence of weight, and a motor to maintain rotation (not needed in space due to angular momentum conservation). GGG has proved: (i) high Q; (ii) auto-centering and long term stability; (iii) a sensitivity to EP testing which is close to the target sensitivity of the GG experiment provided that the physical properties of the experiment in space are going to be fully exploited.

Gravity can be studied in detail in near Earth orbits NEO's using laser-ranged test masses tracked with few-mm accuracy by ILRS. The two LAGEOS satellites have been used to measure frame dragging (a truly rotational effect predicted by GR) with a 10% error. A new mission and an optimized, second generation satellite, LARES (I. Ciufolini PI), is in preparation to reach an accuracy of 1% or less on frame dragging, to measure some PPN parameters, to test the 1/r² law in a very weak field and, possibly, to test select models of unified theories (using the perigee). This requires a full thermal analysis of the test mass and an accurate knowledge of the asymmetric thermal thursts due to the radiation emitted by the Sun and Earth. A Space Climatic Facility (SCF) has been built at INFN-LNF (Frascati, Italy) to perform this experimental program on LAGEOS and LARES prototypes. It consists of a 2 m × 1 m cryostat, simulators of the Sun and Earth radiations and a versatile thermometry system made of discrete probes and an infrared digital camera.

The SCF commissioning is well underway. A test of all its subsystems has been successfully completed on August 4, 2006, using a LAGEOS 3 × 3 retroreflector array built at LNF. This prototype has been thermally modeled in detail with a commercial simulation software. We expect to demonstrate the full functionality of the SCF with the thermal characterization of this LAGEOS array by the beginning of September 2006.

The constant of gravitation, G, is the least well-known of the physical constants. A new, independent method of measurement, estimated as having a potential uncertainty at least as small as that achieved by existing methods, would be useful for an improvement in G determination. This experiment is based on the measurement of the relative motion of two freely falling test bodies (discs), caused by their gravitational attraction. The uncertainties are analyzed for two parallel tungsten discs with masses of about 30 kg. The use of test bodies with an incorporated optical system of multipass two-beam interferometers, as well as of multibeam interferometers, is proposed to measure their relative displacement. The estimations were made for laboratory experiment with free fall duration of 0.714 s. In this case, the relative displacement to be measured is about 0.1 μm. These estimates show that relative uncertainties lower than 5 × 10^-5 can be obtained in G measurement in a single drop of the test bodies. The proposed experiment can be made in outer space. In space a lower uncertainty can be achieved because the time interval of the measurement of relative motion of the test bodies can be increased.

Concept considerations for a space mission with the objective of precisely testing the gravitational motion of a small test mass in the solar system environment are presented. In particular, the mission goal is an unambiguous experimental verification or falsification of the Pioneer anomaly effect. A promising concept is featuring a passive reference mass, shielded or well modeled with respect to nongravitational accelerations and formation flying with a rather standard deep space probe. The probe provides laser ranging and angular tracking to the reference mass, ranging to Earth via the radio-communication link and shielding from light pressure in the early parts of the mission. State-of-the-art ranging equipment can be used throughout, but requires in part optimization to meet the stringent physical budget constraints of a deep space mission. Mission operation aspects are briefly addressed.

Pairs of Planck-mass drops of superfluid helium coated by electrons (i.e. "Millikan oil drops"), when levitated in a superconducting magnetic trap, can be efficient quantum transducers between electromagnetic (EM) and gravitational (GR) radiation. This leads to the possibility of a Hertz-like experiment, in which EM waves are converted at the source into GR waves, and then back-converted at the receiver from GR waves into EM waves. Detection of the gravitational-wave analog of the cosmic microwave background using these drops can discriminate between various theories of the early Universe.

LISA may make it possible to test the black-hole uniqueness theorems of general relativity, also called the no-hair theorems, by Ryan's method of detecting the quadrupole moment of a black hole using high-mass-ratio inspirals. This test can be performed more robustly by observing inspirals in earlier stages, where the simplifications used in making inspiral predictions by the perturbative and post-Newtonian methods are more nearly correct. Current concepts for future missions such as DECIGO and BBO would allow even more stringent tests by this same method. Recently discovered evidence supports the existence of intermediate-mass black holes (IMBHs). Inspirals of binary systems with one IMBH and one stellar-mass black hole would fall into the frequency band of proposed maximum sensitivity for DECIGO and BBO. This would enable us to perform the Ryan test more precisely and more robustly. We explain why tests based on observations earlier in the inspiral are more robust and provide preliminary estimates of possible optimal future observations.