Current Theory of the Universe

Credit: Cush [Wikimedia Commons]

Standard Model

• Quantum theory of particles and interactions

• Electromagnetic, Nuclear Weak, Strong forces

• Explains phenomenon with extreme precision

• Predicted new particles: Higgs Boson (CERN 2012)

General Relativity

• Einstein's theory of gravity and spacetime

• Gravitational Lensing

• Gravitational Waves (LIGO 2016)

The Standard Model + General Relativity

Extremely successful theories

However....

Several deep inconsistencies with the observed universe

Unexplained Mysteries

Why is there so much more matter than anti-matter?
 • There should have been equal amounts created

Is there a quantum theory of gravitation?
 • Quantum mechanics and general relativity incompatible

What is Dark Energy?
 • Explain accelerating expansion of the Universe

What is Dark Matter?
 • Missing mass in galaxies

Connected?

First Evidence for Dark Matter

Coma Cluster

Sloan Digital Sky Survey + Spitzer Space Telescope [NASA / JPL-Caltech / L. Jenkins]

First major hints: galaxy clusters

• Fritz Zwicky: Clusters of galaxies observed to be too fast

• Could not be explained by gravity from visible matter

• Perhaps invisible matter

• Zwicky: Termed phrase "dark matter"

Dark Matter

Invisible Matter: interacts gravitationally,

but not with light*

*or, just very weakly

Then: things changed

[AIP Emilio Segrè Visual Archives, Rubin Collection]

Major breakthrough

Vera Rubin, along with Kent Ford in the 1970s

Careful measurements of individual galaxies

Their (and following) work showed:

Dark Matter was everywhere

And there was lots of it

Galactic Rotation Curves

all galaxies: stars move faster (left) than predicted (right)

[ESO/L. Calçada, Creative Commons]

Galactic Rotation Curves

[Mario De Leo, Wikimedia Commons]

Missing Matter?

• Not a small correction

• 80 - 95% of mass is "missing"

• Implies dark matter

a = \frac{G \, [M(r) + {\color{orange}\delta M(r)}]}{r^2}

• Simulations + observations: DM "halo"

• ... or modified gravity

$a = \frac{[G + {\color{green}\delta G(M,r)}] \, M(r)}{r^2}$

Evidence builds up

Rotation curves

• Missing galactic matter

Gravitational lensing

• Gravitational bending of light

Cosmic Microwave Background

• Gravitational imprint on background radiation

Baryon Acoustic Oscillations

• Sound waves (density fluctuations) in matter

Large-Scale Structure Simulations

• Dark Matter required to match observations

Large-Scale Structure simulation:

[Volker Springel/MPI, Wikimedia Commons]

Bullet Cluster

Chandra X-ray Observatory [NASA]


  
  NASA/CXC/M. Weiss - Chandra X-Ray Observatory

• Colliding galaxy clusters ~ 4 Gyr

• X-ray emission image:
distribution of gas (regular matter)

• Also - Gravitational lensing:
distribution of mass

Composite image: X-ray + lensing

• Pink = gas (X-ray)

• Blue = mass (gravitational lensing)

• Frictionless dark matter:
passes straight through

Cosmic Microwave Background

(After removal of doppler dipole)

• Temperature anisotropies (scale + distribution)
very sensitive to distribution of regular and dark matter

Cosmic Microwave Background

Credit: Chris North (Cardiff U.) - chrisnorth.github.io/planckapps/Simulator

ΛCDM model (Λ=dark energy, CDM=cold dark matter)

Dark Matter: What we know

There's lots of it

It gravitationally clusters in halos around galaxies

(really, galaxies gravitationally cluster around DM)

Very strong evidence:

• Rotation curves, gravitational lensing, CMB

• Also: large scale structure, BAO etc.

• It's not strange for particles to not interact with light (e.g., neutrinos)

• ΛCDM model works extremely well
• Ωc = 0.2589(57) [Planck]

• (Just need to work out the Λ and CDM parts)

Dark Matter: What we don't know

... everything else

US "Cosmic Visions" report [arXiv:1707.04591]

• Possible mass range: 90 orders of magnitude!

• Fundamental particle: 50 orders (de Broglie to Planck)

• Vast majority of focus on WIMPs, but field is very wide

Some guesses

Assume (guess) that Dark Matter has some (small)

interaction with regular matter

• Not guaranteed, but reasonable

• Early universe: protons/neutrons/electrons produced

• Presumably, dark matter produced too

• Gives simple mechanism for DM production

• Also: mechanism for DM detection

Dark Matter production: WIMP miracle

\huge E = mc^2

• Early universe: hot and dense

• Universe cools: Not enough energy to produce DM

• Universe expands:
Shuts off annihilation

• Weak interaction: predicts this exactly!

Dark Matter Detection

WIMP Searches: XENON

• Direct detection (scattering)

• Indirect detection (annihilation)

• Collider searches (production)

Excluded cross-section for WIMPs:

• Mass below nuclear mass: energy threshold

• High mass: fewer particles

Enlightening the search for Dark Matter?

Quantum Physics!

 Mass drops below nuclear mass:
 • No nuclear recoils
   • Instead: electron recoils + ionisation

 Mass drops below electron mass:
 • No electron recoils
   • Instead: absorption (dark photoelectric effect)

 Mass drops below eV:
 • Classical DM field
   • Quantum sensing (atomic clocks)

[JILA]

[Ye/UCBolder]

Atomic Clocks 101:

Clock: frequency reference + count oscillations

Best atomic clocks approach accuracy: $\delta f/f \sim 10^{-19}$

JabberWok [Wikimedia Commons]

Atomic transition: "perfect*" frequency reference

Wcislo, Science 2016

Laser: oscillations of light
Compare frequency to atomic transition

Choi, Phys. Rev. Lett. 2018

Lock on: Adjust laser to maximise transition rate

"Listen" for DM field using atomic clocks

[N Hanacek/NIST]

• DM with very low mass

• Very high number density to compensate

• Behaves like classical field: wave-like

• Assume: weak interaction with atoms

• Affect atomic energy levels and frequencies

• Dark matter causes "wobbles" in frequency

• Monitor clocks for these wobbles

Example experiment: GPS.DM

• 30 Cs, Rb atomic clocks

• Over 20 years of high-quality data

• Publicly available (JPL)

• 50,000 km Dark Matter observatory

[Video: Conner Dailey (honors student)]

• Didn't find any sign of such waves

• Puts constraints on various models

BMR, Blewitt, Dailey, Murphy, Pospelov, Rollings, Sherman, Williams, Derevianko,
Nature Comms. (2017)

(Just one example, many others)

Conclusion

Standard Model + General Relativity

• Extremely successful, but incomplete

• Our job to search for what's beyond

Dark Matter

• Overwhelming evidence: completely in the dark

• Promising probe for physics beyond Standard Model

WIMPs

• Seemingly great candidate...but no evidence

• Look for lighter DM candidates: quantum sensors

[JILA]

Enlightening the search for Dark Matter

Benjamin M. Roberts

Brisbane, QLD, 21 August 2023