Cosmology and Dark Matter IV: Problems with our current picture

Jerry Sellwood

The story so far• Once the universe becomes neutral, dark

matter halos start to form• Simulations show a clustering hierarchy of

DM halos that resembles the distribution of galaxies

• Galaxies form inside DM halos as gas cools, settles to a disk, and makes stars

• Do the properties of the predicted galaxies match up with observation?

Serious problem #1

• Predicted galaxy rotation curves have the wrong shape

• Too much mass in the “bulge”

• Gas has too little angular momentum

• Also never form bulgeless galaxies, which are common in nature

Serious problem #2• Dark matter halos

have too much substructure

• Why is there not a small galaxy inside every clump?

• May be able to explain them away by re-ionization

Serious problem #3• Dark matter halos are

not as dense as predicted (Alam et al)

v/2 is the mean density inside the radius at which rotn curve reaches vmax/2

• Points are estimates from real galaxies

• Dashed curves are from standard CDM

Serious problem #3 (cont’d)• Better data are in

worse agreement• Weiner’s work

Serious problem #3 (cont’d)• Weiner’s work gets

around uncertainty in M/L

• Better data are in worse agreement

• Halos are under-dense by more than one order of magnitude

• Plenty of work for SALT

Serious problem #4• There is a formula that

predicts rotation curves from the baryons only with no dark matter

Serious problem #4 (cont’d)

• Formula is MOND from Milgrom• Postulates a departure from Newtonian

gravity in very weak fieldsg(|g|/a0) = gn

• Stronger forces when |g| a0 (10-8 cm s-2) – a new constant of nature

• Ad hoc, but not been shot down in >20years!• If DM exists, it is very hard to understand

why the formula works so well

Serious problem #5• Tully-Fisher relation does

not depend on surface brightness

• Data from Zwaan et al• Incredibly severe fine-

tuning problem

Evidence for dark matter

• Could come soon from any one of 3 on-going experiments

• WMAP• Dark matter would be indicated if 3rd peak

in final data is higher than 2nd

Evidence for dark matter

• Could come soon from any one of 3 on-going experiments

• WMAP• Direct detection in laboratory experiments

– CDMS team in underground mine– Only upper limits so far

Evidence for dark matter

• Could come soon from any one of 3 on-going experiments

• WMAP• Direct detection in laboratory experiments-rays form dark matter annihilations

– EGRET data – very weak– GLAST will be better

What is Dark Energy?• The cosmological constant is the energy

density of vacuum

particle + antiparticle ↔ radiation• Heisenberg uncertainty principle

energy uncertainty × duration > h (Planck’s const)

What is Dark Energy?• The cosmological constant is an energy

density of vacuumparticle + antiparticle ↔ radiation

• Heisenberg uncertainty principleenergy uncertainty × duration > h (Planck’s const)

• Quantum fluctuations in vacuum• Energy of them detected experimentally

– Casimir effect

Expected energy of vacuum

• Know protons, electrons, neutrinos, quarks, gluons, etc. all have anti-particles

• Count up all contributions to vacuum energy density

• Result is huge – 120 orders of magnitude larger than observed!

• Physicists have no idea why• First major headache

Second headache• Why is dark energy

about 70% of the critical density?

• Almost 0% or almost 100% expected at most times

• We live at a special time in the history of the universe– anti-Copernican

Our Preposterous Universe• Our model for the universe is now very ugly

• 70% dark energy• 25% dark matter• 4% normal atoms• < 2% neutrinos (may be much less)

• No natural explanation why they should all contribute so significantly

• Our only evidence so far for the two dark components is gravitational– could another modification to gravity, for ultra-

weak fields, make them both go away?

Conclusions

• Cosmology has come a long way in the past 30 years

• But we still have plenty of unsolved problems!

Generalized Dark Energy• Einstein’s cosmological constant has a

single, fixed value• Can consider dark energy that varies in time

and space – coined Quintessence• Still an energy density that has repulsive

gravity and negative pressure• Equation of state:

pressure = w × energy density• w = 0 for cold matter• w = +⅓ for radiation• w = –1 for cosmological constant• –1 < w < 0 for quintessence