Transcript Colloquium

Quarknet Symposium May 2003
Neutrinos, Dark Matter and the Cosmological
Constant
The Dark Side of the Universe
Jordan Goodman
University of Maryland
J. Goodman – May 2003
Outline
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Why do we care about neutrinos?
Why do we think there is dark matter?
Could some of it be neutrinos?
The search for neutrino mass – Solar Neutrinos
– Super-K
– SNO
– Kamland
• The accelerating Universe - Dark Energy
– SCP
– WMAP
J. Goodman – May 2003
Seeing Big Picture
J. Goodman – May 2003
Why do we think there is dark matter?
• Isn’t obvious that most of
the matter in the
Universe is in Stars?
Spiral Galaxy
J. Goodman – May 2003
Why do we think there is dark matter?
• In a gravitationally bound
system out past most of the
mass V ~ 1/r1/2
• We can look at the rotation
curves of other galaxies
– They should drop off
But they don’t!
J. Goodman – May 2003
Why do we think there is dark matter?
• There must be a large amount of unseen
matter in the halo of galaxies
– Maybe 20 times more than in the stars!
– Our galaxy looks 30 kpc across but recent data
shows that it looks like it’s 200 kpc across
J. Goodman – May 2003
Measuring the energy in the Universe
• We can measure the mass
of clusters of galaxies with
gravitational lensing
• These measurements give
Wmass ~0.3
• We also know (from the
primordial deuterium
abundance) that only a
small fraction is nucleons
Wnucleons < ~0.04
Gravitational
lensing
J. Goodman – May 2003
What is this ghostly matter?
• Could it be neutrinos?
• How much neutrino mass would it take?
– Proton mass is 938 MeV
– Electron mass is 511 KeV
– Neutrino mass of 2eV would solve the galaxy
rotation problem – 20eV would close the Universe
• Theories say it can’t be all neutrinos
– They have difficulty forming the kinds of structure
observed. The structures they create are
too large and form too late in the history of the universe
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Super-Kamiokande
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Hubble Law
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The expanding Universe
• The Universe is
expanding
• Everything is moving
away from everything
• Hubble’s law says the
faster things are moving
away the further they are
away
J. Goodman – May 2003
The expanding Universe
J. Goodman – May 2003
Supernova Cosmology Project
• Set out to directly
measure the
deceleration of the
Universe
• Measure distance vs
brightness of a
standard candle (type
Ia Supernova)
•The Universe
seems to be
accelerating!
•Doesn’t fit Hubble
Law (at 99% c.l.)
J. Goodman – May 2003
The expanding Universe
J. Goodman – May 2003
Energy Density in the Universe
W0 may be made up of 2 parts
a mass term and a “dark
energy” L term
(Cosmological Constant)
L
W0= Wmass + Wenergy
• Einstein invented L to keep the
Universe static
• He later rejected it when he found
out about Hubble expansion
• He called it his “biggest blunder”
W0=1
m
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The Cosmological Constant
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What is the “Shape” of Space?
• Closed Universe W0>1
–
C < 2pR
• Open Universe W0<1
– Circumference (C) of a
circle of radius R is
C > 2pR
• Flat Universe W0=1
–
C = 2pR
–
Euclidean space
J. Goodman – May 2003
Results of SN Cosmology Project
• The Universe is
accelerating
• The data require a
positive value of L
“Cosmological Constant”
• If W0 =1 then they find
WL ~ 0.7 ± 0.1
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Accelerating Universe
J. Goodman – May 2003
Accelerating Universe
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Measuring the energy in the Universe
• Studying the Cosmic
Microwave radiation looks
back at the radiation from
400,000 years after the
“Big Bang”.
• This gives a measure of
W0
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Recent Results - 2002
W0=1 Wnucleon
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WMAP -2003
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WMAP - 2003
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What does all the data say?
• Three pieces of data come
together in one region
WL ~ 0.73 Wm~ 0.27
(uncertainty s~0.04)
• Universe is expanding &
won’t collapse
• Only ~1/6 of the dark matter
is ordinary matter (atoms)
• A previously unknown and
unseen “dark energy”
pervades all of space and is
causing it to expand and
accelerate
J. Goodman – May 2003
What do we know about “Dark Energy”
• It emits no light
• It acts like a large negative pressure
Px ~ - rx
• It is approximately homogenous
– At least it doesn’t cluster like matter
• Calculations of this pressure from first principles
fail miserably – assuming it’s vacuum energy
you predict a value of WL ~ 10120
• Bottom line – we know very little!
J. Goodman – May 2003
Conclusion
•
Wtotal = 1.02 ± 0.02
– The Universe is flat!
• The Universe is :
~1/2% Stars
~1/2% Neutrinos
~27% Dark Matter
(only 4% is ordinary matter)
~73% Dark Energy
• We can see ~1/2%
• We can measure ~1/2%
• We can see the effect of
~27% (but don’t know
what most of it is)
• And we are pretty much
clueless about the other
3/4 of the Universe
There is still a lot of Physics to learn!
J. Goodman – May 2003