Dark Matter

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. Ourown galaxy is predicted to have a spherical halo of dark matter.

Vera Rubin and Flat Rotation CurvesDark 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 ofdistant galaxy formedby intervening darkmatter.

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 spin-independent 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.”

• 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.

.. more on LUX results …

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.

dark matter wind from motion of sun around thegalactic center.

Proposal to look for modulations in dark matterStreaming past the solar system.

April 6, 2014

“… 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.”

Future for dark matter detection

We know dark matter exists

… we don’t know what kindof 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.

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.

Is dark energy increasing?

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.

ds is measure of distance betweentwo points

Begin with the metric tensor for the 4-dimensional space: General Relativity. 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 = H d

v = recessional speedH = Hubble’s constant

d = distance

The relationship of v to z depends on the model:

Acceleration of the expansion of the observableuniverse is at this point too small to affect the“measured” value of the Hubble constant. Butone can see from the following expressionthat an increase in H must follow from a termnot yet included in the equation of state.

missing terms – due to dark energy?

Einstein’s Equations and Hubble Law Derivation

S = 0 Einstein ‘s equations

… use Noether’s theorem.

Einstein’s Equations:

R00 and R are related to the scale factor, R(t) = a

T00 =

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.

possibletunneling

long slow“roll” into minimum

absoluteminimum

steepasymmetricrise

Energetic coherentoscillationsabout minimum

Quantum fluctuations lead to density perturbations thatlater 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 quantum-mechanically (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 characteristicof density fluctuations and gravitational waves:

E modes and B modes

https://www.youtube.com/watch?v=PCxOEyyzmvQ

a = R(t) = scale factor