Looking For Dark Energy On Earth: A New Experiment Using Atom Interferometry That Galileo Would Understand

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1 Looking For Dark Energy On Earth: A New Experiment Using Atom Interferometry That Galileo Would Understand Martin Perl Kavli Institute For Particle Astrophysics And Cosmology SLAC Linear Accelerator Laboratory Stanford University Presented at SLAC Summer Institute, 2011 Physics is an experimental science. Isidor Rabi 1

2 THE DARK ENERGY ATOM-INTERFEROMETER EXPERIMENT Ronald Adler Physics Department, Stanford University SLAC Linear Accelerator Laboratory Gregory Bernard SLAC Linear Accelerator Laboratory Ryan Coffee SLAC Linear Accelerator Laboratory Jonathan Coleman Physics Department, University of Liverpool Joshua Francis Physics Department, Stanford University Eric Lee Physics Department, University of New Mexico Dinesh Loomba Physics Department, University of New Mexico Ziba Mahdavi, Administrator Kavli Institute for Particle Astrophysics and Cosmology Martin Perl Kavli Institute for Particle Astrophysics and Cosmology SLAC Linear Accelerator Laboratory Dennis Ugolini Physics Department, Trinity University 2

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7 What do we really know about Dark energy? Atom Interferometry: Using the quantum nature of atoms (8/3 11:45AM-12:45PM Interferometry Mark Kasevich (Stanford)) Some explanation of the technology of the experiment: lasers and falling cesium atoms. The experiment. 7

8 DARK ENERGY What do we really know. 8

9 The accelerating universe discovered in Group led by Adam Riess Group led by Saul Perlmutter 9

10 R Radius R increases at rate of R per year and this rate is accelerating. 10

11 R visable R visable increases at rate of 4 x kilometers per year and this rate is accelerating 11

12 Summarizing the behavior of the visible Universe: dr/dt ~ R / year d 2 R/dt 2 ~ R / year 2 12

13 Summarizing other observations The average amount of dark energy density is joules per cubic meter. It appears to be uniformly distributed on a cosmological scale. Present observations are 75% dark energy 20% dark matter 5% ordinary matter 13

14 Dark Energy We do not know the origin of the acceleration The phenomenon has been given the poetic name of Dark Energy Perhaps the cause is some unknown force or energy spread throughout the universe?. Perhaps it is some unknown property of the vacuum of outer space. An explanation can be inserted into the equations of general relativity by the addition of one more term that has long been known to acceptable. The measured cosmological constant number is 14 used.

15 Amount of Dark Energy per Cubic Meter The average amount of dark energy density is joules per cubic meter. Initially this strikes one as a very small energy density. For example, the amount of heat energy in a cubic meter of air in this room is about 400 joules 15

16 Comparison with an electric field But consider a weak electric field, E = 1 volt/m. Using εe 2 /2 for the electric field energy gives Joules per Cubic Meter. Hence the energy density of this electric field is 100 times smaller than the dark energy density Yet this weak electric field is easily detected and measured 16

17 Thus we work with fields whose energy densities are much less than the dark energy density. This realization first started me thinking about the possibility of direct detection of dark energy. 17

18 Our understanding of dark energy has been hyped and over hyped in the press, in popular science books and by our public intellectuals in science. 18

19 Atom Interferometry Atom s de Broglie wavelength = λ db = h/p p = atom momentum = mv For example: He atom at 295 K has v=1.1x10 3, hence λ db = 0.9x10-10 m. This is much shorter than the wavelength of visible light photons ~ 0.5x10-6 m 19

20 Atom Interferometry Using Mechanical Gratings 1991 L= L = 64 cm, d =8 µm, s= 1 to 2 µm λ db = 0.9x10-10 m. Diffraction angles are small~ λ db /s ~10-4 radians 20

21 Atom Interferometry Using Mechanical Gratings Simple concept but requires excellent mechanical precision and stability. A. D. Cronin et al, arxiv: (2007) Na atoms Hot wire detects Na atoms 21

22 Na atoms in 1000/s. Transverse position of detector. 22

23 detector detector Interference pattern shift caused by potential changing p in λ db = h/p 23

24 Atom Interferometry Using Raman Optical Transitions with Lasers Subtle, ingenious concept providing maximum precision. Steven Chu Claude Cohen-Tannoudji, 1997 Nobel Prize in Physics William D. Phillips 24

25 nm = 3.2 x Hz Cesium D 2 line Instead of physically separating the atom beam into two beams to produce interference, we use the two hyperfine ground states, F=3> and F=4>. F=4> lifetime is years.

26 nm = 3.2 x Hz F=3> S Cesium D 2 line S 1/2 >= (ae iφ 3 (t) 3 > + be iφ 4 (t) 4 > ) a 2 + b 2 = 1 26

27 Cs atoms fall from rest under gravitational force. 27

28 Tower 28

29 S Atom cloud just released. Laser beam converts all atoms to 1/2 a 2 >= (ae iφ (t) 3 > + be 2 + with b a = = 1b = 1/ with 3 2 iφ 4 (t) 4 > ) Atom wave functions interfere and atoms are in 3> or 4> state just before detection. 29

30 shift phase inside apparatus Number Cs atoms per cloud Max 0 Φ 30

31 Would take ¾ hour to derive. Φ total =2πgs/λ D2 g=9.8 m/s 2 s=1.5 m λ D2 =0.852 µm Φ total =1.1 x 10 8 rad 31

32 Φ total =1.1 x 10 8 rad Can measure Φ to 10-2 to 10-4 rad. Since Φ total =2πgs/λ D2 can measure g to to

33 No time to discuss precision and noise problems. No time to tell you about the art of atom interferometry. References I recommend: C. Wieman, G. Flowers, and S. Gilbert, Am. J. Phys. 63, 317 (1995); A.S. Melish and A C Wilson, Am. J. Phys. 70, 1965 (2002); M. de Angelis et al., Meas. Sci. Technol.20, (2009); A. Peters et al.} Metrologia 38, 25 (2001); R. M. Godun et al., Contemp. Phys., 42, 77 (2009); Q. Bodart et al., App. Phys. Lett. 96, (2010); C. J. Foot ATOMIC PHYSIC3, (Oxford Univ. Press, Oxford, 2005) 33

34 The Experiment 34

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36 The earth is moving trough the CMB frame at ~400km/s. The dark energy density distribution is best thought in the CMB frame. Therefore our apparatus is rushing through the dark energy density at several hundred km/s. 36

37 irregular signal abov e the apparatus noise 37

38 The assumptions about dark energy underlying our experiment: 1. Dark energy exerts a small, but not zero, force on matter; not the direct gravitational force on energy. 2. The distribution of dark energy density is not uniform 38

39 In about two years we will run the experiment The dark energy phenomenon is a profound mystery, its full solution may have to be accomplished by the young women and men in this room. Thank you 39

A Terrestrial, Atom Interferometer, Experiment Searching for Dark Energy Density and Other Dark Contents of the Vacuum

1 A Terrestrial, Atom Interferometer, Experiment Searching for Dark Energy Density and Other Dark Contents of the Vacuum Principle Investigator: Martin L. Perl Kavli Institute for Particle Astrophysics