Download Looking For Dark Energy On Earth: A New Experiment Using Atom Understand

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
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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
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•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.
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DARK ENERGY
What do we
really know.
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The accelerating universe
discovered in 1998.
Group led by
Adam Riess
Group led by
Saul Perlmutter
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R
Radius R increases at rate of
10-10 R per year and this rate is
accelerating.
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R visable
R visable increases at rate of
4 x 1014 kilometers per year and this
rate is accelerating
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Summarizing the behavior of the
visible Universe:
dR/dt ~
2
2
d R/dt
-10
10 R
~
/ year
-20
10 R
/
2
year
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Summarizing other observations
The average amount of
dark energy density is 6.3
× 10-10 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
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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
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used.
Amount of Dark Energy
per Cubic Meter
The average amount of dark
energy density is 6.3 × 10-10 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
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Comparison with an
electric field
But consider a weak electric field,
E = 1 volt/m. Using εE2/2 for the
electric field energy gives
4.4 × 10 −12 Joules per Cubic Meter.
Hence the energy density of this
electric field is 100 times smaller
than the dark energy
density 6.3 × 10−10
Yet this weak electric field is easily
detected and measured
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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.
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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.
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Atom
Interferometry
Atom’s de Broglie wavelength = λdB = h/p
p = atom momentum = mv
For example: He atom at 295 K has
v=1.1x103 , hence λdB = 0.9x10-10 m.
This is much shorter than the wavelength of
visible light photons ~ 0.5x10-6 m
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Atom Interferometry Using
Mechanical Gratings
1991
L= L’ = 64 cm, d =8 µm, s= 1 to 2 µm
λdB = 0.9x10-10 m.
Diffraction angles are small~ λdB/s
~10-4 radians
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Atom Interferometry Using
Mechanical Gratings
Simple concept but requires excellent
mechanical precision and stability.
A. D. Cronin et al, arXiv: 0712.3703 (2007)
Na atoms
Hot wire detects
Na atoms
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Na atoms in 1000/s.
Transverse position of detector.
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detector
detector
Interference pattern shift caused
by potential changing p in λdB = h/p
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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
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852 8528
nm = 3.2 x 1014 Hz
Cesium D2 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.
852 8528
nm = 3.2 x 1014 Hz
| S>
F=3
Cesium D2 line
| S1/2 >= (ae
iφ 3 (t)
| 3 > + be
iφ 4 (t)
| 4 >)
| a |2 + | b |2 = 1
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Cs atoms fall from rest under
gravitational force.
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Tower
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Atom cloud just released.
Laser beam converts all atoms to
| S1/2 >= (aeiφ (t) | 3 > + be iφ (t) | 4 >)
3
4
| a |2 with
| b |2a==1 b = 1/ 2
+ with
Atom wave functions interfere and
atoms are in |3> or |4> state just
before detection.
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shift phase inside
apparatus
Max
Number Cs
atoms per cloud
0
Φ
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Would take ¾ hour to derive.
Φtotal =2πgs/λD2
2
m/s
g=9.8
s=1.5 m
λD2 =0.852 µm
Φtotal =1.1 x
8
10 rad
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Φtotal =1.1 x
8
10 rad
Can measure Φ to
-2
-4
10 to 10 rad.
Since Φtotal=2πgs/λD2
can measure g to
-10
-12
10 to 10
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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,
022001 (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,
134101 (2010);
C. J. Foot “ATOMIC PHYSIC3”, (Oxford Univ.
Press, Oxford, 2005)
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The Experiment
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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.
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irregular signal abov e the apparatus noise
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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
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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
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