Transcript Physics Potential of UNO

Darkside of the Universe
Search for the Invisible
Chiaki Yanagisawa
Stony Brook University
and
BMCC /CUNY
ASLI Meeting at Vanderbilt Museum
December 1, 2010
Evidence for Dark Matter
Why do scientists think there is strange thing
called dark matter?
Are they crazy or what?
Baryonic matter is made of protons and neutrons.
Normal matter is made of baryonic matter,
electrons and photons
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Evidence of Dark Matter 1
 Rotation curve of the planets (Kepler’s law)
• gravity = centripetal force
GmM / r2 = mv2 / r
mass of Sun
mass of a planet
• When the mass inside the
orbit of an objects is
constant, the rotation
speed v is proportional
to 1/square root of the radius.
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Evidence of Dark Matter 1
 Rotation curve of galaxies
Kepler’s
law
• gravity = centripetal force
GmM / r2 = mv2 / r
• Further away from the_ core M(r) ~ const
(visible light)  v  1/ √ r but v ~ const
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mass inside orbit
mass of a star
More invisible mass
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Evidence of Dark Matter 1
 Rotation curve of galaxies
Spiral galaxy NGC3198
Milky Way (Spiral barred galaxy)
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Evidence of Dark Matter 2
 Gravitational lens
Gravitational lens
Bending of light by gravity
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Evidence of Dark Matter 2
 Gravitational lens
yellow: foreground galaxies
5 billion light years
Mass distribution by G.L.
blue: galaxies
orange: dark matter
A blue: background galaxy
10 billion light years
Galaxy Cluster 0024 + 1654
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Evidence of Dark Matter 2
 Gravitational lens
yellow: foreground galaxies
5 billion light years
Mass distribution by G.L.
spikes: galaxies
blob: dark matter
A blue: background galaxy
10 billion light years
Galaxy Cluster 0024 + 1654
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Evidence of Dark Matter 2
 Gravitational lens Galaxy Cluster 0024 + 1654
• Reconstructed mass distribution
and arcs using by G.L. effect on
the background galaxy.
+ Dark matter center
x Total light center
o diffuse light center
• Observed arcs (blue) and diffuse
light (orange)
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Evidence of Dark Matter 3
 Post merger cluster (Bullet cluster)
galaxies and dark matter : collisionless
hot gas feels
drag force
X-ray from hot gas
(majority of normal matter)
Total mass (dark and normal matter) by gravitational lensing
Dominated by dark matter
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Structures in the Universe
 How are galaxies distributed?
Earth
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• In average sense,
galaxies are distributed
uniformly in a large scale.
(cosmological principle)
The best example is the
map of CMB (see later)
• But locally, they are
distributed with structures
as seen on the left.
• So then, what created
the structures in the
galaxy distribution?
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Structures in the Universe
Copyright 2004: Scientific American
 How did the structures evolve? (Simulation)
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• 120 Myr old:
uniform matter spread with
subtle undulation
• 490 Myr old: Denser regions
gain more materials.
• 1.2 Byr old: Gravity pulls
more materials and creates
strings and voids.
• 13.7 Byr old (Today): Growth
stops due to acceleration of
the Universe’s expansion.
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What Kind of Dark Matter?
 Cold, warm or hot dark matter?
• Cold dark matter (very slow)
- Non-relativistic
- Fit the structure of the observed
Universe best
• Warm dark matter (intermediate speed)
- Not needed as now dark energy
is discovered
• Hot dark matter (very fast)
- Relativistic
- No fine structure
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Cosmology
How did the Universe begin?
How is it structured?
How did it evolve?
How will it be like in the future?
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Hubble’s Law
 Hubble diagram
Receding speed =
Hubble constant x distance
v = H0 x d
Hubble “constant”
Discovered by Edwin Hubble
in 1929.
 Birth of modern cosmology
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Hubble’s Law
 Hubble’s law
• Receding speed = Hubble constant x distance
v = H0 x d
Copyright 1993: Scientific American
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Expansion of the Universe
 What does Hubble’s law tell us?
• The further the distance, the faster the speed.
Copy right 1998
Scientific American
Time 1 Time 2
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• The further the distance, the larger the amount
of stretch (change in the scale of the Universe).
• Hubble’s law
faster the speed.

The Universe is
expanding!
• For a fixed amount
of stretch:
in an accelerating
expansion, galaxies
are more remote
than in a constant
or a decelerating
expansion.
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Expansion of the Universe
 Implication of Hubble’s law – Expansion of the Universe
Copy right 2009
Scientific American
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Big Bang
 Evolution of the Universe
Copy right 2004
Scientific American
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History of the Universe Expansion
Copy right 1998
Scientific American
 How did the Universe expand?
Constant
expansion
rate
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Decelerating
Accelerating
expansion
expansion
rate
rate
Younger Universe Older Universe
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Redshift
 Electromagnetic waves (EM waves)
• Speed of EM wave
= wavelength x frequency
= 3 x 108 km/s
• Shorter w.l. …… longer w.l.
gamma rays…light …radio wave
blue …... yellow …… red
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Redshift
 Spectrum
• Atoms absorb radiation.
• Different atoms have
different spectra.
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• Objects moving toward or away from us
show shifted absorption lines – redshift
if it is receding and blueshift if approaching.
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Redshift and Blueshift
 Implication of Hubble’s law: redshift
• z = (wavelength observed wavelength emitted) /
wavelength emitted
= (lobs - lem ) / lem
≈v/c
(for velocity much smaller than
speed of light )
• redshift if z is positive: receding
blueshift if z is negative: approaching
• The smaller the velocity, the smaller the z.
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Hubble’s Law and Redshift
 Implication of Hubble’s law: redshift
• Hubble’s law:
• z = (wavelength observed - v = H0 d
wavelength emitted) /
correct for any distance
wavelength emitted
as long as the Universe
= (lobs - lem ) / lem
is homogeneous and
≈v/c
isotropic
(for velocity much smaller
than speed of light )
- cz ≈ v = H0 d
• 1 + z = 1/R (R: scale of the universe approximate only for
nearby galaxies
now R=1)
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Geometry of the Universe
 Curvature and geometry
Closed, k>0
Open, k<0
Flat, k=0
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• Sum of three angles a, g, b
inside a triangle:
curvature k
a + b + g = 180o
zero
Flat
a + b + g  180o
positive
Closed
a + b + g  180o
negative
Open
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Models of the Universe
 Energy densities
• Matter density
: the energy density of matter rm >0
positive pressure, almost zero (pm ≈ 0)
• Radiation density
: the energy density of photons rr >0
positive pressure (pr = rr/3 > 0)
Negligible for our story of the Universe
• Dark energy density : the energy density of cosmological
constant rL  0 produces
NEGATIVE pressure (pL = -rL < 0)
• Gravitational force : attractive if r + 3p is positive
repulsive if r + 3p is negative
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Models of the Universe
 Energy densities and geometry of the Universe
• Critical density : the energy density rc to create
a flat universe ( 5 hydrogen atoms per m3)
• Omegas : Wm = rm / rc
Wr = rr / rc ( small and negligible for our purpose)
W L = rL / rc
• W = Wm + Wr + WL > 1 curvature positive , closed universe
= 1 curvature zero
, flat universe
< 1 curvature negative , open universe
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Geometry of the Universe Again
 Curvature, scale, and omega
Size of the Universe
time
Size of the Universe
Size of the Universe
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time
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Measuring Cosmological Parameters
How can we measure cosmological parameters
Ws and H0?
Use Type Ia Supernovae!
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Type Ia Supernovae as Standard Candle
 Type Ia supernovae
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Type Ia Supernovae as Standard Candles
 Type Ia supernovae as standard candles
• All the SNe Ia have the
same luminosity
(absolute brightness)
Copy right 2003: Physics Today
• Apparent brightness
= Absolute brightness /
(Distance x Distance)
• Distance x Distance =
Apparent brightness/Absolute brightness
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Type Ia Supernovae as Standard Candles
 Measuring Omegas and Hubble constant
Copy right 1998
Scientific American
H0
(km/s)/Mpc)
W0 =1.0, WL =0.7, Wm=0.3
W0 =0.3, WL =0.0, Wm=0.3
W0 =1.0, WL =0.0, Wm=1.0
1+z = 1/R: the larger the z value, the more the
amount of stretch of the Universe
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Cosmic Microwave Background
How can we know what happened
at the beginning of the Universe?
Cosmic microwave background (CMB) gives
information about the Universe
when its age is
about 400,000 years old.
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Cosmic Microwave Background (CMB)
 Origin of CMB
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• Before the recombination
photons interact with
free electrons and
protons (can’t form
neutral atoms – too hot),
they don’t remember
things from the past.
• After the recombination
energies of protons and
electrons are small
enough to form neutral
atoms. Photons are now
free.
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Cosmic Microwave Background (CMB)
 What is blackbody radiation and CMB ?
• Radiation from an object
at a fixed temperature has
a characteristic spectrum
called blackbody curve.
• The peak position is
proportional to the
temperature.
• Radiation from the early
Universe is a blackbody
radiation and observable
(after recombination).
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As the Universe
expands, the
temperature
decreases.
1012 K
5,000 K
3K
frequency = speed/wavelength
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Cosmic Microwave Background (CMB)
 The Universe with different wavelengths
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Cosmic Microwave Background (CMB)
 Anisotropy of CMB and geometry of the Universe
Copy right 2008: Sky and telescope
How does a structure in the
Universe at a time in the past
look now (angular size)?
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• smaller in an open universe.
• same in a flat universe.
• bigger in a closed universe.
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Cosmic Microwave Background (CMB)
 Anisotropy of CMB and geometry of the Universe
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Sound Waves
 Origin of sound waves at the beginning of the Universe
Copy right 2008: Sky and Telescope
• Quantum fluctuation produces overdense
and underdense regions of dark matter at
the time of Big Bang.
• Overdense region creates deeper
gravitational potential well and compress
plasma soup made of baryonic matter
(protons and neutrons), photons and
electrons.
• When the plasma soup is compressed, it
gets hotter and photons exert radiation
pressure on the plasma soup.
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Sound Waves
 Origin of sound waves at the beginning of the Universe
Copy right 2008: Sky and Telescope
• Then the region is rarefied and the
temperature gets cooler.
• When the temperature gets cooler,
radiation pressure is reduced, and the
plasma get compressed again.
• This process is repeated back and forth.
• The largest size in which the standing
sound wave is formed is the size
corresponding to the distance the sound
wave can reach at a given time since
the Big Bang. (vs = c/√3 ≈ c/2 )
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CMB Anisotropy
 Standing waves in the Universe at 380,000 years
after Big Bang
Copy right 2008: Sky and Telescope
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Cosmic Microwave Background (CMB)
 Power spectrum of CMB
Copy right 2004: Scientific American
200o / angular freq.
≈ angular size in
the sky (deg)
0.6o or 480 Mly
Angular sizes of Moon/Sun:0.50
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Cosmic Microwave Background (CMB)
 Sound waves and power spectrum
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CMB Anisotropy
 W0 and CMB anisotropy
• Open universe:
Peaks move to
smaller angular size
• Closed universe:
Peaks move to
larger angular size
• Flat universe:
Peaks at expected
positions
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CMB Anisotropy
 WL and CMB anisotropy
• The larger L the
smaller the 2nd and
3rd peaks.
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CMB Anisotropy
 WL and CMB anisotropy
• The larger L the
smaller the 2nd and
3rd peaks.
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CMB Anisotropy
 Wb and CMB anisotropy
Wb
• The ratio, the
heights of the 1st
to the 2nd peak, gives
the info about W b
(normal matter –
baryon – contribution
to W).
• The larger the ratio,
the more baryonic
mass.
Baryon : particles made of three quarks such as protons and neutrons
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Change of Energy Densities
 CMB anisotropy
Copy right 2007 :
Annual Review of
Nuclear and Particle
Sciences
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Composition of the Universe
 WL vs. Wm (=Wb+WDM )
Copyright 2003: Physics Today
Hubble constant
H0 = 73 (km/s)/Mpc
Omega for ordinary matter
Wb = 0.04
Omega for dark matter
WDM = 0.22
Omega for Lambda
WL = 0.74
Total Omega
W0 = 1.00
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History of Composition of Energy Densities
 Time variations of composition of the Universe
dark matter
dark energy
baryons
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History of Composition of Energy Densities
 How did the energy densities change over time?
• Gravitational force :
attractive if r + 3p > 0
repulsive if r + 3p < 0
• Matter density :
energy density of matter rm >0
almost zero pressure (pm ≈ 0)
attractive gravity rm + 3pm > 0
• Dark energy density :
energy density of cosmological
constant rL 0 produces
negative pressure (pL = -rL < 0)
repulsive gravity rL + 3pL < 0
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Fate of the Universe
R=1/(1+z)
 How did (will) the size of the Universe change?
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Vacuum energy density overcomes
matter energy density eventually.
 accelerating Universe
W0=1.0
Always
WL=0.40 – 0.95
decelerating:
Wm=0.8 – 1.4
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