Einstein Gravity Explorer–a medium-class fundamental physics mission

0 L. Lusanna INFN, Sez. di Firenze, Polo Scientifico, 50019 Sesto Fiorentino, Italy 1 D. Svehla Institute of Astronomical and Physical Geodesy, Technische Universitt Mnchen , Arcisstrasse 21, 80333 Munich, Germany 2 C. Salomon Laboratoire Kastler Brossel , Ecole Normale Suprieure, 24 rue Lhomond 75231 Paris, France 3 L. Iorio Sezione di Pisa, INFN, Viale Unit di Italia 68 , 70125 Bari, Italy 4 J. Mller Institut Fr Erdmessung, Leibniz Universitt Hannover , Schneiderberg 50, 30167 Hannover, Germany 5 L. Liu Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences Jiading , Shanghai 201800, China 6 W.-T. Ni Purple Mountain Observatory, Chinese Academy of Sciences , 2, Beijing W. Road, Nanjing, 210008, China 7 V. Flambaum University of New South Wales , Sydney, Australia 8 U. Schreiber Fundamentalstation Wettzell , 93444 Bad Koetzting, Germany 9 E. Gill Department of Earth Observation and Space Systems, Delft University of Technology , Kluyverweg 1, 2629 HS Delft, The Netherlands 10 L. Cacciapuoti ESA Research and Scientific Support Department , ESTEC, 2200 AG, Noordwijk ZH, The Netherlands 11 K. Gao Wuhan Institute of Physics and Mathematics, The Chinese Academy of Sciences , P.O. Box 71010, Wuhan 430071, China 12 W. Schfer TimeTech GmbH, 70563 Stuttgart, Germany 13 M. P. He ASTRIUM Space Transportation, D-88093 Friedrichshafen, Germany 14 R. Holzwarth Menlo Systems GmbH, 82152 Martinsried, Germany 15 R. Holzwarth Max-Planck-Institut fr Quantenoptik , 85748 Garching, Germany - Abstract The Einstein Gravity Explorer mission (EGE) is devoted to a precise measurement of the properties of space-time using atomic clocks. It tests one of the most fundamental predictions of Einsteins Theory of General Relativity, the gravitational redshift, and thereby searches for hints of quantum effects in gravity, exploring one of the most important and challenging frontiers in fundamental physics. The primary mission goal is the measurement of the gravitational redshift with an accuracy up to a factor 104 higher than the best current result. The mission is based on a satellite carrying cold atombased clocks. The payload includes a cesium microwave clock (PHARAO), an optical clock, a femtosecond frequency comb, as well as precise microwave time transfer systems between space and ground. The tick rates of the clocks are continuously compared with each other, and nearly continuously with clocks on earth, during the course of the 3-year mission. The highly elliptic orbit of the satellite is optimized for the scientific goals, providing a large variation in the gravitational potential between perigee and apogee. Besides the fundamental physics results, as secondary goals EGE will establish a global reference frame for the Earths gravitational potential and will allow a new approach to mapping Earths gravity field with very high spatial resolution. The mission was proposed as a class-M mission to ESAs Cosmic Vision Program 20152025. GR USO XPLC 1 Introduction Einsteins theory of General Relativity is a cornerstone of our current description of the physical world. It is used to describe the motion of bodies ranging in size from satellites to galaxy clusters, the propagation of electromagnetic waves in the presence of massive bodies, and the dynamics of the Universe as a whole. In general, the measurement of general relativistic effects is very challenging, due to their small size [1, 2]. Thus, only few of its predictions have been tested with high accuracy, for example the time delay of electromagnetic waves via the Cassini mission [3], and the existence of gravitational waves by observation of binary star systems. The accuracy of the determination of these effects is at the 20 ppm level. The situation is similar for one of the most fascinating effects predicted by General Relativity and other metric theories, the slowing of clocks in a gravitational field (gravitational time dilation, gravitational redshift). The gravitational redshift was measured with 7 105 relative inaccuracy in the 1976 Gravity Probe-A experiment [4] by comparing a ground clock with a similar clock on a rocket as the height changed. The most performing clocks available at the time, hydrogen masers, were used for this experiment. The ACES (Atomic Clock Ensemble in Space) mission planned to fly on the ISS in 2014 seeks to improve the determination by a factor 25, by using a cold atom clock (PHARAO) and a hydrogen maser [5, 6]. Recent progress on cold atom clocks in the optical domain and in optical technology enable performing this fundamental test with orders of magnitude better sensitivity. Although it has been very successful so far, General Relativity, as well as numerous other alternative or more general theories proposed in the course of the development of theoretical physics, are classical theories. As such, they are fundamentally incomplete, because they do not include quantum effects. In fact, it has not yet been possible to develop a theory of gravity that includes quantum mechanics in a satisfactory way, although enormous effort has been devoted to this goal. Such a theory would represent a crucial step towards a unified theory of all fundamental forces of nature. Several approaches have been proposed and are currently under investigation (e.g. string theory). A full understanding of gravity will require observations or experiments that determine the relationship of gravity with the quantum world. This topic is a prominent field of activity and includes the current studies of dark energy. The Einstein Gravity Explorer is a fundamental physics mission dedicated to this question; its primary task is to measure the gravitational frequency shift with unprecedented accuracy. The EGE mission uses a satellite on a highly elliptic orbit (see Fig. 1). The ratio of the frequencies of two on-board clocks and the ratios between satellite and ground clocks are the main observables. Fig. 1 General concept of the EGE mission. Clocks on the satellite and on ground are intercompared as the satellite orbits the Earth on a highly elliptic orbit These can be interpreted toward tests of General Relativity (GR) and of competing metric theories of gravity and as tests for the existence of new fields associated to matter (see Section 3). The outcome of the mission will be either a confirmation of GR and metric theories within the accuracy provided by the instruments, or the discovery of a deviation. In the latter case, the mission would provide a first indication of the breakdown of current (classical) gravitational physics theories and could pave the way towards a unified theory of all forces. 2 Scientific objectives 2.1 Summary of science objectives 2.1.1 Fundamental physics science objectives (1 ppm = 1 part in 10 6, 1 ppb = 1 part in 10 9) High-precision measurement of the earth gravitational frequency shift at 25 ppb accuracy, a factor 3,000 improvement First precise measurement of the sun gravitatio (...truncated)