Mystery Leftovers (Dark Matter Big Bang...

Lecture 3: Mystery Leftovers (Dark Matter Big Bang Relics)

The 64th Compton Lecture Series

Unsolved Mysteries of the Universe:Looking for Clues in Surprising Places

THE ENRICO FERMI INSTITUTE

Brian Odom

http://kicp.uchicago.edu/~odom/compton.htm

10/7/2006 Brian Odom Compton Lecture 3

Answers to Previous Questions

neutron

antineutrino

proton

electron

n νe + p + e νe + n e + p

(neutrino + neutron electron + proton)

Neutrino detection (question from Lecture 2)

But, do this in a nucleus where β-decay is forbidden. Otherwise natural radioactivity is a background.

Other questions (talk with me afterwards):1. First three minutes affected by time dilation?2. Energy in interfering wavefronts3. Neutrino oscillation and energy conservation


Dark Matter

Wanted: Missing matter. Must not give off light.(We will see that, actually, it can’t even interact with incoming light.)


Dark Matter Candidates

• Baryonic Dark Matter(normal stuff)hiding in gas clouds ? dim stars ? black holes ?


What is a Baryon?

image from hands-on-cern.physto.se/.../sm_paty1.html

Baryons are made from three quarks

Mesons (unstable) are made from quark-antiquark pairs

For reasons not well understood, hadrons only comes in colorless composites

Hadrons




• Non-Baryonic Hot Dark Matter(light stuff—traveling near the speed of light)neutrinos ?





• Non-Baryonic Cold Dark Matter(exotic stuff)WIMPs ? axions ? heavy sterile neutrinos ? Kaluza-Klein particles ?





• Non-Baryonic Cold Dark Matter(exotic stuff)WIMPs ? axions ? heavy sterile neutrinos ? Kaluza-Klein particles ?

Ruled out


Galaxy Rotation Curves

• Local dark matter density is ~ 0.3 GeV / cm3

• We are moving through the dark matter halo of theMilky Way at ~ 230 km / s

Exhibit A: Galaxies should spin themselves apart


Galaxy Clusters

• Galaxies usually hang out in clusters, bound together by gravity

• Often there are ~100 per Mpc (~100 in distance between us and Andromeda)

• They contain 50-1000 galaxies, and span 2-10 Mpc

Galaxy Cluster Abell 1689


Galaxy Clusters

• In 1936 Fritz Zwicky used redshifts to measure the “velocity dispersion” of 8 galaxies in Coma Cluster.

• We would expect “virialized velocities”, where <KE> = -1/2 <PE>.

• But, when we calculate the gravitational potential from the luminous matter, we find that it only accounts for typically 5% of the needed matter.

• Modern measurements have become much more sophisticated, but we still end up with the same problem.

Exhibit B: Galaxies should fly out of their clusters

Galaxy Cluster Abell 1689


Ah, but Maybe There’s Gas…

X-ray image: NASA/CXC/UCI/A Lewis et al.; Optical image: Pal. Obs. DSS.

Chandra (left) and Optical (right) image of Abell 2029

• Indeed, there is plenty of gas, at temperatures of 107 -108 Kelvin, so it emits x-rays (very high-frequency light). • Great! Maybe this gas that didn’t emit visible light is what provides gravity to hold the galaxies together?


From Bad to Worse

Hydrostatic pressure determines the temperature and spatial extent of gas held in place by a given gravitational potential. This is the same thing that gives stars finite size—thermal pressure from within.

This balance lets you find total amount of matter.

From Bad: The intergalaxy gas typically only provides 10% of the mass needed to hold the clusters together.To Worse:

Exhibit C: The hot gas in clusters should spread out


The Same Story for Gas in Galaxies

http://chandra.harvard.edu/press/04_releases/press_102604.html

Chandra Observation of NGC 4555

• Hot gas cloud diameter = 400,000 light years• … about twice that of the visible galaxy• An enormous halo of dark matter is needed to confine the hot cloud• The halo must have a mass 10x the stars and 300x the gas

Exhibit D: Gas in individual galaxies should spread more


Strong Lensing

Gravitational bending of light creates arcs or multiple images of more distant galaxies


More Strong Lensing Pictures


Strong Lensing

Elliptical Einstein rings are clearly observed in this Hubble image (left)

Exhibit E: Working backwards from strong lensing shows that bulk of cluster mass is dark


Weak Lensing

http://www.cita.utoronto.ca/~hoekstra/lensing.html

(Colors indicate the dark matter density in this computer simulation. The white stick indicate the average shape of the galaxies lensed by this mass distribution. The tangential pattern around the massive clumps is easily seen. )

• Usually the lens is too weak to create rings and arcs

• But, the background galaxies are still distorted

• This distortion can shows up in statistical analysis when many galaxies are observed—weak lensing


Weak Lensing

Can you see the shear direction of the weak lensing?



Weak LensingStrong lensing near cluster center can be seen in the left image. The sticks in the blowup region (right) show the direction of the weak lensing shear—in the direction expected, given the cluster center.


Exhibit F: Weak lensing also requires dark matter


The Power of Lensing

• Lensing experiments do not require understanding of any galactic or intergalactic dynamics. Only general relativity is required to get the gravity right.

• Strong and weak lensing analyses lead to the same conclusions about the dark matter composition of galaxy clusters

• This agreement persuaded many astrophysicists of the reality of dark matter

• Microlensing observations also rule out MACHOs(massively compact halo objects), such as small black holes or brown dwarfs, as being a dominant dark matter component


Bullet Cluster Collision

Stars (seen by Hubble images) and dark matter (seen by weak lensing) pass through collision mostly uninhibited. Hot gas (seen by Chandra images of x-rays) is very disrupted. All as expected for dark matter.


Bullet Cluster Collision

Exhibit G: Dark matter shows up in collision dynamics


CMB Structure

• Ripples in the CMB are from sound waves•The wavelength of this sound is calculable•So the location of the first peak in the power spectrum tells us the geometry of the universe!


CMB Structure

(Wayne Hu, Ann. Phys. 303 203, 2003)

The ratio of first to second peak heights gives a sensitive probe of baryon density

Exhibit H: The CMB shows that the baryon content of the universe is too small to account for all the dark matter


Big Bang Nucleosynthesis

Burles, Nollett, Turner 2001

Big Bang Nucleosynthesis (BBN) stops when the temperature and density get too low to support further fusion reactions


Big Bang Nucleosynthesis

Burles, Nollett, Turner 2001

• If all the mass of the universe is in the form of cold baryonic matter, big bang nucleosynthesis predicts abundances of light isotopes much different from those observed.

• This actually led to an effort in the 1970s to find alternate production methods for deuterium.

• Non-baryonic cold dark matter solves this problem.

• The baryon density predicted from BBN agrees with that from other measurements.

Exhibit I: Big Bang Nucleosynthesis only predicts the right abundances if there is non-participating cold dark matter.


What about Standard Model HDM?

Wavelength λ [h-1 Mpc]

Solid line: standard cosmology, with neutrino mass = 0.

Dashed line: what would happen if the 7% of the dark matter mass was from neutrinos. The data strongly disfavor this.

Too many neutrinos wash out structure and prevent galaxies and galaxy clusters from forming. Hot Dark Matter (HDM) also causes problems for BBN.

M. Tegmark, Physica Scripta. Vol. T121, 153–155, 2005

At first glance, neutrinos sound great. Standard model particlewhich doesn’t couple electromagnetically. But…


Standard Model Warm Dark Matter?

Left: A 4 Mpc region showing simulated structure development in ΛCDM at z=1(about 8 billion years ago). Right: the same simulation run for ΛWDM. Again, warm dark matter (which would allow standard model stuff to be the dark matter) gives very different results, inconsistent with observations.

From Rob Thacker


Down to Desperate Alternatives?

• We have a vast body of evidence for dark matter

• Neither HDM nor baryonic CDM match the data

• The most likely alternative seems to be a new type of particle—non-baryonic cold dark matter

• One other alternative is MOND (Modified Newtonian Dynamics)—different gravity on large length scales. This is not the favored solution, however, especially since it has trouble with BBN.

• But really, how desperate is non-baryonic CDM? We expected there to be new particles for other reasons…


The Higgs Mechanism

Slide from Ambreesh Gupta, 60th Compton Lecture Series


The Higgs Mechanism

• The Higgs particle works wonders:1. It gives W and Z bosons mass

2. This breaks electroweak symmetry (they are distinguished from the photon)

3. So, electromagnetism looks different from the weak interaction at low energies, like we observe

4. Incidentally, it gives ALL fundamental particles masses. This is nice.

• But there is a (horrible?) cost—more new particles for the whole thing to make sense…NOT AGAIN!


Renormalization

• In quantum field theories, real particles interact with virtual particles

• Upon initial inspection, these give infinite contributions to measurable quantities like mass and charge!

• Usually, we know how to subtract off a different infinity and end up with a finite quantity. This is called renormalization

• Although suspicious, renormalization always gives the right answers, so far as we have been able to tell


The Curse of the Higgs

• But, the Higgs is a special case. Unlike all other fundamentalparticles, it must be a scalar, i.e. a spin-zero particle.

• This property leads to a nastier type of divergence (a quadratic divergence), that cannot be renormalized.• The Higgs can do wonderful things like provide a mechanism for all particles to get mass, but the cost is that its own mass approaches infinity, or at least the Planck mass (1019 GeV). This is called the hierarchy problem.

• Unless new physics comes along to fix the problem…


Supersymmetry

Each particle gets a “sparticle” counterpart. Bosons get fermions and vice versa. These have not yet been discovered, but if supersymmetry is right, their discovery is right around the corner at the LHC.


Supersymmetry Saves the Higgs

• Particles and sparticles contribute terms to the Higgs mass with opposite signs, so they mostly cancel.

• There is some portion left over, because particles and their sparticles do not have the same mass. This leaves the Higgs with a finite, workable, mass.


Another Benefit of Supersymmetry

from Dennis Silverman

We want this:

Renormalization causes the coupling constants of the four forces to “run”. At higher energies, where symmetries are unbroken, you might expect a unified theory should have a single coupling constant


Supersymmetry and Running Coupling

Normally, we’d get this: But with Supersymmetry:

figures from Dennis Silverman

Unification of the strong force is not a solved problem, but joining of the running coupling constants provides an important ingredient


Traditional Road to Finding Particles

Because of E=mc2, going to higher energies in collisions lets you look for new, heavier particles

The LHC—The next collider, to open soon


The Biggest Accelerator Ever…

Looking back earlier in time is looking into physics at higher energies


… Makes LHC Look Wimpy


Freezeout Leads to Relics

from Rocky Kolb

Suddenly, non-baryonic cold dark matter does not sound so strange. Particle physicists want yet more heavy particles to solve big problems. If they are right, there must be big bang relics.


About to Break Tradition?1

2 ann37 210 cmxh

σ −

−

⎛ ⎞Ω ⎜ ⎟⎝ ⎠

• If supersymmetric particles exist, we know their approximate annihilation cross-section.

• Big bang physics then predicts a relic abundance…

• Which is just right to explain the dark matter density!

• If we find dark matter left over from the big bang accelerator, we would reverse the recent tradition of finding new particles in earth-bound accelerators

• (But, for as long as we can build accelerators these approaches to discovery will be complementary)

• Next week: Dark Matter Direct and Indirect Detection

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