Transcript Lecture 15.Dark.Matter.Dark.Energy [Autosaved]x

Dark Matter
• cannot be seen directly with telescopes; it
neither emits nor absorbs light;
• estimated to constitute 84.5% of the total
matter in the universe – and 26.8% of the
total mass/energy of the universe;
• its existence is inferred from gravitational effects
on visible matter and gravitational lensing of
background radiation;
Rotational curves for a typical galaxy indicate that the
mass of the galaxy is not concentrated in its center. Our
own galaxy is predicted to have a spherical halo of
dark matter.
Vera Rubin and Flat Rotation Curves
Dark Matter in our galaxy?
Visualization of dark matter halo for spiral galaxy
Candidates for
nonbaryonic dark matter
• Axions (0 spin, 0 charge, small mass,
Goldstone bosons)
• Supersymmetric particles (partners in
SUZY) – not been seen yet
• Neutrinos (small fraction )
• Weakly interacting massive particles
.. so far none have been detected.
Gravitational Lensing
Multiple Images of
distant galaxy formed
by intervening dark
matter.
Structure of dark matter -- not distributed uniformly
because it is attracted to the baryonic matter in the
stars and galaxies.
How might dark matter decay/interact?
• (axions) decay into monochromatic photons?
• positrons and electrons? look for sharp upturn
in the positron fraction of cosmic rays?
• can dark matter decay into dark energy?
• unstable gravitino dark matter?
• WIMP interaction mediated by the Higgs boson?
Search for Low-Mass WIMPs: “super cryogenic dark
matter search” Released on Feb 28, 2014
•
The SuperCDMS experiment aims to measure the recoil energy imparted to a
nucleus due to collisions with WIMPs
•
Released on Feb 28, 2014: We report a first search for weakly interacting massive
particles (WIMPs) using the background rejection capabilities of SuperCDMS.
•
An exposure of 577 kg-days was analyzed for WIMPs with mass < 30 GeV/c2, with
the signal region blinded.
•
Eleven events were observed after unblinding. We set an upper limit on the spinindependent WIMP-nucleon cross section of 1.2×10−42cm2 at 8 GeV/c2.
•
This result is in tension with WIMP interpretations of recent experiments and
probes new parameter space for WIMP-nucleon scattering for WIMP masses < 6
GeV/c2.
Large Underground Xenon
Experiment (LUX)
• Looking for WIMPS (bouncing off nuclei)
• .. A 370 kg liquid xenon time-projection chamber
that aims to directly detect galactic dark matter in
an underground laboratory 1 mile under the earth,
in the Black Hills of South Dakota, USA
• “Basically, we saw nothing. But we saw nothing
better than anyone else so far.”
.. more on LUX results …
• The Large Underground Xenon (LUX) experiment is a dual-phase xenon
time-projection chamber operating at the Sanford Underground Research
Facility (Lead, South Dakota). The LUX cryostat was filled for the first time
in the underground laboratory in February 2013.
• We report results of the first WIMP search data set, taken during the
period from April to August 2013, presenting the analysis of 85.3 live days
of data with a fiducial volume of 118 kg.
• A profile-likelihood analysis technique shows our data to be consistent
with the background-only hypothesis, allowing 90% confidence limits to
be set on spin-independent WIMP-nucleon elastic scattering with a
minimum upper limit on the cross section of 7.6×10−46 cm2 at a WIMP
mass of 33 GeV/c2.
• We find that the LUX data are in disagreement with low-mass WIMP
signal interpretations of the results from several recent direct detection
experiments.
The axion dark matter experiment
• Looking for axions – decaying into photons. Axions are
“Goldstone bosons” associated with Higgs mechanism.
They are scalar (spin =0), uncharged particles.
•
The Axion Dark Matter eXperiment (ADMX) uses a resonant
microwave cavity within in a large superconducting magnet to
search for cold dark matter axions in the local galactic dark
matter halo.
• Sited at the Center for Experimental Physics and Astrophysics
at the University of Washington, ADMX is a large collaborative
effort.
• No results yet.
Proposal to look for modulations in dark matter
Streaming past the solar system.
dark matter wind
from motion
of sun around the
galactic center.
April 6, 2014
Future for dark matter detection
“… what we are witnessing is an example of
how the identification of dark matter will
come about.
We cannot expect a claim, beyond any
statistical or systematic doubt, from a single
experiment, but rather a gradual process.
At some point there will be a barely
significant excess over known backgrounds
that, despite careful experimental efforts, fails
to go away.”
We know dark matter exists
… we don’t know what kind
of particles comprise it!
Dark Energy
• The size and the smoothness of the Universe can be
explained by very rapid expansion—inflation.
• However, there is not enough observable matter to
generate stars or galaxies. The force of gravity from
observable matter is too weak. This is one of a number
of reasons we need dark matter.
• Finally, to explain the acceleration of the expansion of
the Universe, we need dark energy; ideally, that would
explain both early inflation and today's inflation.
Is dark energy increasing?
From what we can tell, the total amount of
dark energy seems to increase as the
Universe expands.
It’s a feedback cycle: the more expansion
we have, the more dark energy; the more
dark energy, the faster the Universe grows.
With neutrons, scientists can now look for dark energy in the lab
The technique they developed takes very slow neutrons
from the strongest continuous ultracold neutron source in
the world, at the ILL in Grenoble and funnels them
between two parallel plates.
According to quantum theory, the neutrons can only occupy
discrete quantum states with energies which depend on the
force that gravity exerts on the particle.
By mechanically oscillating the two plates, the quantum
state of the neutron can be switched. That way, the
difference between the energy levels can be measured.
Begin with the metric tensor for the 4dimensional space: General Relativity.
ds is measure
of distance
between
two points
scale factor
Four dimensional surface equation
Einstein’s
equation of
state
The relativistic red shift
z = (observed - emitted)/ emitted
is related to the velocity.
Rather than the relativistic red shift, the Cosmological
red shift is now used in interpreting the Hubble constant:
1 + z = R(tnow)/R(tthen)
1 + z = observed/ emitted
z = (observed - emitted)/ emitted
Hubble’s Law:
v=Hd
v = recessional speed
H = Hubble’s constant
d = distance
The relationship of v to z depends on the model:
Acceleration of the expansion of the observable
universe is at this point too small to affect the
“measured” value of the Hubble constant. But
one can see from the following expression
that an increase in H must follow from a term
not yet included in the equation of state.
missing terms – due to dark energy?
Einstein’s Equations and Hubble Law Derivation
… use Noether’s theorem.
S=0
 Einstein ‘s equations
Einstein’s Equations:
T00 = 
R00 and R are related to the scale factor, R(t) = a
You can derive the Hubble law from Einstein’s equations and the above.
The  =  = 0 component of Einstein’s equations gives Hubble’s Law:
https://www.youtube.com/watch?v=EIpEzZqkd9c
dE
= -pdV
Some comments on Inflation: potential form.
possible
tunneling
long slow
“roll” into
minimum
Energetic coherent
oscillations
about minimum
steep
asymmetric
rise
absolute
minimum
Quantum fluctuations lead to
density perturbations that
later produce galaxy formation.
Some density perturbations lead to perturbations of the metric.
Predictions of Inflation:
1. The universe must be flat (  = 1  10-4 ).
2. Perturbations of the metric:
3. Inflationary perturbations can be
observed in the Cosmic Microwave
Background (CMB) spectrum.
On March 17, 2014 scientists announced the first
direct detection of the cosmic inflation behind the
rapid expansion of the universe just a tiny fraction of
a second after the Big Bang 13.8 billion years ago. A
key piece of the discovery is the evidence of
gravitational waves, a long-sought cosmic
phenomenon that has eluded astronomers until now.
https://www.youtube.com/watch?v=PCxOEyyzmvQ
Inflation is supposed to smooth things
out but ……. quantum mechanics says
that we can’t completely smooth things
out. The Heisenberg uncertainty principle
tells us that there will always be an
irreducible minimum amount of jiggle in
any quantum system, even when it’s in
its lowest-energy (“vacuum”) state. In
the context of inflation, that means that
quantum fields that are relatively light
(low mass) will exhibit fluctuations.
Then, there are quantum fluctuations in
the gravitational field: gravitational
waves, or “gravitons” speaking quantummechanically (sometimes called “tensor”
fluctuations in contrast with “scalar”
density fluctuations).
Gravitational waves and Polarization in CMB
Then, there are quantum fluctuations in the gravitational field:
gravitational waves, or “gravitons” speaking quantum-mechanically
(sometimes called “tensor” fluctuations in contrast with “scalar”
density fluctuations).
Difference between polarization characteristic
of density fluctuations and gravitational waves:
E modes and B modes
https://www.youtube.com/watch?v=PCxOEyyzmvQ
a = R(t) = scale factor