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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 fields
g(|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 finetuning problem
Evidence for dark matter
• Could come soon from any one of 3 ongoing 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 ongoing 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 ongoing 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 vacuum
particle + antiparticle ↔ radiation
• Heisenberg uncertainty principle
energy 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 ultraweak 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