http://www.as.utexas.edu/astronomy/education/spring06/komatsu/secure/lecture18.pdf.
Ordinary
Matter
vs
Dark
Matter
•
Matter occupies
26% and dark energy occupies
74% of
the energy of the universe at present.
•
What do we mean by
“
matter
”?
–
Ordinary
Matter
:
atoms (baryons)
•
Atoms
exist
mostly
in
the
form
of
gas
–
Extra-ordinary
Matter: Dark
Matter
•
Dark
matter
exists
in
the
form
of
freely-moving
particles
•
Observations of deuterium abundance in the cold gas
clouds as well as anisotropy of the cosmic microwave
background suggest that the atoms contribute only to 4%
of the energy of the universe. Therefore, dark matter
must contribute to 22% of the energy of the universe.
What
Is
Dark
Matter?
•
We do not know the precise nature of dark matter
particles, but we do know some of their properties.
•
Dark matter interacts with the ordinary matter only
weakly and gravitationally.
–
Why? We must have detected dark matter particles already
otherwise.
–
Dark matter does not emit light or absorb light, which means
that dark matter does not interact via the electro-magnetic force.
–
Dark matter does not have any charges.
•
Dark matter particles should move slowly, much more
slowly than the speed of light.
–
Why? Galaxies would not be formed otherwise.
–
Dark matter particles that move slowly are called
“cold” dark
matter.
–
Neutrinos are
“hot” dark matter, as they move at nearly the
speed of light.
Dark
Matter
Candidates
•
It is most likely that dark matter particles are
something that we have not seen in the laboratory yet.
What could they be?
•
Particles of
“supersymmetry
”
?
–
All the particles may be divided into two classes:
bosons
and
fermions
, depending on their spins.
•
Bosons
have
integer
spins:
photons,
pions,
gravitons,
...
•
Fermions
have
half-integer
spins:
electrons,
neutrinos,
quarks,
...
–
Theory of
“Supersymmetry
” requires that each fermion
have the corresponding boson, and vice versa. For example,
photon’s fermionic superpartner is called
photino
, and
electron’
s bosonic superpartner is called
selectron
.
–
One of the popular supersymmetric dark matter candidates
is
gravitino
, a fermionic superpartner of graviton.
•
But, these are still theoretical possibilities
...
How
Do
We
Detect
Dark
Matter?
•
Very
exciting
possibility
:
direct
detection
of
super-particles
may
actually
be
possible
in 2007-
–
LHC (Large Hadron Collider) @ CERN (Geneva)
–
Collide two proton beams to create lots of particles:
LHC will reach the energy that is equivalent
of 7x10
16
K!!
–
Physicists are expecting to detect supersymmetric
particles in LHC, finding evidence for Supersymmetry.
•
What
if
they
found
nothing?
–
They would need to build a yet larger accelerator
...
What
Is
Dark
Energy
?
•
The present universe is accelerating, which implies that
we have a positive cosmological constant in the universe.
But, what is cosmological constant anyway?
–
Could it be something else?
Dark Energy.
•
We know very little about dark energy.
–
We may have two dark energy components.
•
Early
dark
energy
which
caused
inflation
in
the
very
early
universe.
•
Late
dark
energy
which
is
causing
the
universe
to
accelerate
now.
–
Dark energy influences visible or dark matter via gravity only.
–
Dark energy is very smooth: it cannot cluster.
–
Dark energy has a large,
negative
pressure.
Dark
Energy
Candidates
•
Energy
of
vacuum
(energy
of
empty
space)
•
Quintessense
•
Modification
to
Einstein
’s
General
Theory
of
Gravity
•
None
of
the
above
How
Do
We
Detect
Dark
Energy?
•
There are no
“particles
” of dark energy
–
So, there is no way to detect dark energy directly.
•
To constrain the nature of dark energy, one has to
determine how exactly the expansion of the universe is
accelerating. Therefore, the cosmological observations
are the only methods that we can use to constrain dark
energy properties.
–
Brightness-redshift relation (luminosity distance)
–
Size-redshift relation (angular diameter distance)
•
The key parameter is
w
. (
w
=pressure/density)
–
Density of dark energy
evolves as 1/
R
3(1+
w
)
–
w=-1: density does not change
!
cosmological constant
–
The current observational data: w<-0 .="" div="">
8
•
Determination of
w
is
the
most important subject in
modern cosmology now.
