Cosmology

New low frequency radio telescopes currently being built open up the possibility of observing the 21 cm radiation before the Epoch of Reionization in the future, in particular at redshifts 200 ≥ z ≥ 30, also known as the dark ages. At these high redshifts, Cosmic Microwave Back-ground (CMB) radiation is absorbed by neutral hydrogen at its 21 cm hyperfine transition. This redshifted 21 cm signal thus carries information about the state of the early Universe and can be used to test fundamental physics. We study the constraints these observations can put on the variation of fundamental constants and on fundamental mass scales. We show that the 21 cm radiation is very sensitive to the variations in the fine structure constant and can in principle place constraints comparable to or better than the other astrophysical experiments. Cosmic strings, if they exist, contribute to the anisotropies in the primordial gas leaving an imprint on the 21 cm radiation. They can tell us about the fundamental mass scales involved in the theories beyond the standard model. We show that the 21 cm radiation can potentially probe cosmic strings of tension ~10−12 asumming intercommutation probability of 1. Making such observations will require radio telescopes of collecting area 10 − 106 km2 compared to ~ 1 km2 of current telescopes.

Precision tests of Local Position Invariance (LPI) involve many different methods in atomic, nuclear and gravitational physics, astrophysics and cosmology, and many different epochs and environments. We present some methods for comparing or combining different methods, either in a model-independent way or within simple scalar field models of variation. We focus on which methods are most sensitive to cosmologically recent time variation, and also on tests of spatial variation within the Solar System.

I will describe a method of understanding how the nuclear binding energies depend on the masses of the light quarks. This is useful in applications ranging from anthropic constraints to equivalence principle tests and bounds on the time variation on the quark masses.

To date, optical clocks based on singly trapped ions1) and ultracold neutral atoms trapped in the Stark-shift-free optical lattices2) are regarded as promising candidates for future atomic clocks. So far “optical lattice clocks” have been evaluated with uncertainty of 1×10-15 (ref. 3)) limited by that of Cs atomic clocks. Frequency comparison between highly-stable and accurate optical lattice clocks is, therefore, crucial for their further evaluation. Looking toward fractional uncertainties of 10-16 and below, collisional frequency shift, Black body radiation (BBR) shift, and hyperpolarizability effects, all of which depend on interrogated atomic elements and experimental configurations, are becoming major concerns. In this talk, we discuss optimal lattice geometries in view of the quantum statistics and related spins of interrogated atoms. This leads to two promising configurations for the lattice clock: One-dimensional (1D) lattice loaded with spin-polarized fermions4) and 3D lattice loaded with bosons. We present frequency comparison of these two optical lattice clocks using fermionic 87Sr and bosonic 88Sr. Such lattice clock comparison will offer an important step to ascertain the clocks’ uncertainty beyond the Cs limit of 1×10-15. As for the latter two issues, the BBR and the lattice laser related uncertainties, we discuss prospects for a cryogenic clock, a “blue-detuned” magic wavelength, and a Hg based optical lattice clock5). References: 1) T. Rosenband et al., Science 319 (2008) 1808. 2) H. Katori, M. Takamoto, V. G. Pal\'chikov and V. D. Ovsiannikov, Phys. Rev. Lett. 91 (2003) 173005. 3) S. Blatt et al., Phys. Rev. Lett. 100 (2008) 140801. 4) M. Takamoto et al., J. Phys. Soc. Jpn. 75 (2006) 104302

We propose new experiments with high sensitivity to a possible variation of the electron-to-proton mass ratio µ me/mp. We consider a nearly degenerate pair of molecular vibrational levels, each associated with a different electronic potential. With respect to a change in µ, the change in the splitting between such levels can be large both on an absolute scale and relative to the splitting. We demonstrate the existence of such pairs of states in Cs2, where the narrow spectral lines achievable with ultracold molecules make the system promising for future searches for small variations in µ.

We have used molecular hydrogen transitions in high quality spectra of quasars Q0403-443, Q0347-383 and Q0528-250, to search for a change in the proton-to-electron mass ratio, mu. Our improvement on previous works is twofold. Firstly, we use an improved technique to calibrate the wavelength scale of the VLT/UVES data, which reduces systematics. Secondly, we model all the hydrogen Lyman alpha transitions in the vicinity of each molecular hydrogen transition. The motivation for doing so is to reduce systematic effects associated with the use of low order polynomial continuum approximations near the molecular hydrogen transitions. We find a fractional change, delta(mu)/mu of (+2.6 ± 3.0) x 10^(-6). Our measurement error is a factor of two improvement over Reinhold et al [PRL 96, 151101 (2006)] who find a 4-sigma detection of (+24 +/- 6) x 10^(-6). The new result we present in this paper, coupled with the previous results on varying alpha, appear inconsistent with generic predictions from Grand Unified Theories, suggesting either the latter are invalid, or the varying alpha results wrong.

High precision measurements in atomic and molecular systems have reached unprecedented accuracy owing to the state-of-the-art quantum control of both light and matter. We have recently completed an evaluation of the uncertainty of our 87Sr optical lattice clock at the 1x10e-16 fractional level, surpassing the best current evaluations of Cs primary standards. By analyzing worldwide measurements of the absolute frequency of the clock transitions in Sr, we constrain temporal variations of fundamental physical constants as well as their possible couplings to the gravitational potential. We will report the latest results on our 87Sr optical atomic clock, as well as the use of the Sr system to constrain variations of the fine-structure constant.

We present recent and ongoing work that uses precision frequency generation and phase measurement to test the constancy of the speed of light Local Position Invariance (LPI) and the Lorentz Invariance (LI) of the photon with respect to the Standard Model of Particle Physics under the frame work of the Standard Model Extension (SME). The first experiment consists of a pair of orthogonally orientated single crystal sapphire resonators cooled to cryogenic temperatures and configured as stable oscillators operating in Whispering Gallery Mode (Cryogenic Sapphire Oscillator). The experiment is continuously rotated at a period of about 20 seconds, and modulations are searched for with respect to an absolute frame of reference. Our experiment has confirmed Lorentz Invariance at sensitivity better than one order of magnitude than previous tests. The experiment is now being upgraded and has the potential to improve this result by further one and a half orders of magnitude. The second experiment consists of a Mach-Zender Interferometer with a magnetic material in one arm. This experiment allows us to measure odd parity and scalar Lorentz violating parameters predicted in the SME, in which the cavity experiment either exhibit suppressed or no sensitivity to. The experiment has been in continuous operation since September 2007 and has put a limit of order 10^-7 on the scalar Lorentz violating parameter, we show that an upgraded experiment can improve this result by a few more orders of magnitude. The final experiment measures over seven years the frequency comparison of a Cryogenic Sapphire Oscillator and a Hydrogen maser at the Paris Observatory. Amongst the data we search for signals correlated with the changing gravitational potential (test of LPI) and reference frame velocity (test of LI), with first results to be presented.

Optical frequency standards based on forbidden transitions of trapped and laser-cooled ions have now achieved significantly higher stability and greater accuracy than primary cesium clocks. At PTB we investigate an optical clock based on the electric quadrupole transition S1/2 – D3/2 at 688 THz in the 171Yb+ ion and have shown that the frequencies realized in two independent ion traps agree to within a few parts in 1016. Results from a sequence of precise measurements of the transition frequency are now available that cover a period of seven years. Combined with data obtained at NIST on the quadrupole transition in Hg+, this allows to derive a model-independent limit for a temporal drift of the fine structure constant. We prepare to observe two more optical transitions that will provide increased sensitivity to alpha variations: The electric-octupole transition S1/2 - F7/2of Yb+ at 642 THz offers a sub-hertz frequency resolution. The ratio of the 688 THz and 642 THz frequencies in Yb+ can be measured as a dimensionless number with a femtosecond laser frequency comb. Repeated measurements of this quantity permit to search for temporal variations of alpha with a sensitivity factor ≈7, the highest in any of the available combinations of optical frequency standards. Much higher sensitivity (of order 104) may be obtained in the study of the 7.6 eV nuclear transition between the two lowest states of Th-229. We have developed a concept for a highly accurate nuclear clock based on this transition and describe first steps towards the experimental realization. This work is supported by DFG, FQXi and QUEST.