The Role of Neutrinos in Galaxy Formation

Katherine Cook and Natalie Johnson

http://zebu.uoregon.edu/1999/ph123/lec08.html

Mommy, Where Do Galaxies Come From?

http://astro.ph.unimelb.edu.au/~mdrinkwa/lectures/pgc_astro98/galaxy_form/part01.html

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Expansion in the Early UniverseChronology of Key Events:

1) Graviton separated first—gravity!

2) Strong force broke away next—quarks!

3) Electroweak force splits into electromagnetic and weak forces—bosons and photons!

4) Cosmic Microwave Background formed

5) Muons, electrons and neutrinos created.

6) Universe was completely plasma.

7) Energy density decreased faster than matter density and matter began clumping.

8) Recombination- hydrogen formed!

Evolution of Galaxy Clusters

http://zebu.uoregon.edu/1999/ph123/nbod2.mpg

Hot clouds of gas merge and clump together under the force of gravity to form large-scale structures:

Processes of Galaxy Formation

But where did the necessary density perturbations come from in the first place to form these massive galaxies?

Cosmic Microwave Background Radiation

• Isotropic to 1 part in 105 at a temperature of 2.725 K

• This background radiation is almost too smooth to account for density fluctuations.

• Could dark matter have clumped together before luminous matter and provided the needed density contrast?

(Longair, 106)

Why There Must be Dark Matter

•Rotation Curves

•X-Ray Observations

•Tidal Forces

•Inflationary Theory Predicts Density Parameter=1

•Gravitational Lensing

http://zebu.uoregon.edu/1999/ph123/lec08.html

Critical Density• The universe might expand forever and ever

Ω<1• The universe might nearly cease expansion

Ω=1• The universe might reverse its acceleration,

resulting in a really really big crunchΩ>1

(Longair, 125)

Dark Matter is Concentrated in Envelopes or Halos Around Galaxies

• Kepler’s Third Law P2 = a3/ (m1+m2) yields over 90% of the Milky Way’s mass in its outer regions

The Creation of Dark Matter Halos

http://zebu.uoregon.edu/1999/ph123/images/halo.mpg

Hot Dark Matter vs. Cold

HDM

•Particles moving at nearly the speed of light, like neutrinos

•Difficult to explain matter clumping– must be top-down structural formation

CDM

•Slow-moving particles, such as WIMPS

•Make galaxies too well– bottom-up approach to large-scale formation

Dark Matter Candidates

•MACHOs

•WIMPs

•Neutrinos

The speaker known as “Natalie” will now be replaced by

“Katherine”.

MACHOs

• Massive Compact Halo Objects

• Planets

• Small, dim stars

• Small black holes

• Detected through Gravitational Lensing

• Incidences too small to account for enough dark matter

WIMPs

• Weakly Interacting Massive Particles

• Gauge bosons

• Higgs bosons

• Entirely Undetected

Neutrinos

• Neutrinos are neutral, carry energy, and have mass

• Three types of neutrinos-muon neutrinos, tau neutrinos, and electron neutrinos-corresponding to the other members of the lepton family of particles

Neutrinos As Dark Matter

Neutrinos are a plausible candidate because

• There are so many of them (~10^90)

These were created in the big bang when the electroweak force decoupled into the electromagnetic force and the weak force

• They have mass (albeit very small),

~0.05 eV

Interesting Points

Interesting Point One

• If dark matter is mostly neutrinos, then the neutrinos are NOT uniformly distributed throughout space

Interesting Point Two

• Neutrino density might be able to close the universe

The Milky Way and The Neutrino

• The Milky Way has a mass deficiency of 1041kg

• If that were just neutrinos, there would be about 1078 neutrinos in the Milky Way

• Uniformly distributed, there are about 1012 neutrinos per meter cubed of space

• That would mean only 1051 neutrinos occupying the Milky Way

Neutrinos Will Close the Universe!

• Check This Out Critical density ρc is 1.1*10-26kg/m3

Mass density of the universe ρm is given by luminous matter density plus dark matter density

(~3*10-29kg/m3) + (4.5*10-25kg/m3) ~ 4.5*10-25kg/m3

Ω0 = ρm/ρc ~ 101

101>>1!!!, which could result in…

So, can Neutrinos Help Explain Galaxy Formation?

• Jean’s Instability

• Simulations

• Must investigate cosmions and the early Universe

• Properties of dark matter (neutrino) clumping

• Mean-free path

Questions and Further Investigations

• Will Nestle soon be marketing a candy bar dubbed the Neutrino to accompany the already famed ?

+ ) d candy bar = ?

KeyChocolate = x0

Peanuts = x1

Peanut butter = x2

Caramel = x3

Nougat = x4

Crispies = x5

Almonds = x6

Toffee = x7