Transcript What could it be?: the nature of dark matter

Andréanne Baribeau, dans le cadre du cours PHY3903
Décembre 2005
The plan:
What’s the Universe anyway? : an introduction
How will it all end?
Will we expand forever?
Weighing the Universe
There’s something missing...
How scientists came to such a crazy conclusion
What could it be?: the nature of dark matter
Baryonic vs exotic matter
Our Universe
«We live in a very big box!!!»
Cosmological principle: the Universe is
-Isotropic: physical properties=identical in all directions
-Homogenious: spacial density of galaxies = uniform
Its composition according to old
cosmology:
normal matter (baryonic)
organized in galaxies...clusters
void
Even more surprisingly, it is expending!
Edwin Hubble (1920’s – 1930’s) observes:
v = Ho*d
Ho: Hubble’s constant: 100hkms-1Mpc-1 where 0.5<h<1
How will it all end?
The biggest question: What is the fate of this expansion?
the only force acting on matter is the gravitational attraction
will it be enough to stop the expansion?
it all depends on the quantity of this matter and its mass!
closed: will stop expanding...Big Crunch!! >1
flat: will keep on expanding =1
open: will also keep on expanding...Big Rip!! <1
Weighing our Universe:
crit=3Ho2 / 8G
crit =1.879*10-29h2 g/cm3
=3H/m3
: critical density for a flat
Universe
Ho:Hubble parameter
G: Newton’s constant
h: fudge factor on Hubble’s constant 0.5<h <1
= o/crit
: what we want to know!
Being humans, we measure the stuff that humans see i.e. that
emits light, using theoritical conversion between light
emited and corresponding mass
Estimation for light density of the Universe:
1.8*108h Lo/Mpc3
Lo: power of visible light emited from the sun
Mpc: Megaparsec, 3.086*1019 km, or 1 pc = 3.086 light
years
converting in the corresponding mass of the sun:
o = 4.5h*109Mo /Mpc3
Mo: mass of the sun: about 2*1030 kg
THE TOTAL VISIBLE MASS YIELDS:
 = 0.01 – 0.02
Clearly, this is far from enough to close the universe!
There’s something missing
Different observations all point to presence of dark matter: matter
that does not emit light…matter that we can’t see
1930’s: Fritz Zwicky (Switzerland) studies cosmic dance of
galaxy cluster «Coma» (7galaxies)
The virial theorem : stationary state: kinetic energy
of the galaxy system must be less than the total
gravitational potential energy
2K+W~0
There must be more mass between those
galaxies then what meets the eye
The flat rotation curves (Vera Rubin, 1970’s):
•galaxy’s mass: concentrated at center
•Doppler shifts tells us rotation velocity
•Gravitational force should decrease
as radius increases
•The result should be a decreasing
rotation velocity with increasing radius,
following Kepler’s theory
BUT...Rotation curves (velocity vs radius)
are flat!!!
Gravitational lenses (Mellier, Fort, Soucail, 1986):
•light emited from observed objects (quasar, galaxies…) is distorted
by gravitational pull of large massive galaxy cluster standing
between us and the object
•that mass can be measured from the degree of distortion
•compared to the visible mass in the cluster...
You guessed it: galaxy clusters contain 10x
more mass than the visible stars alone
 Using gravitational lensing: maps the distribution of dark matter
in Universe
-statistical analyses of distortion due to lensing effect
Results:
-1/3 of the needed density for flat universe is provided by matter
(any kind...)
-Remembering luminous matter provides about 1% of needed density
and oh yeah, the Universe IS flat...
-geometrical study of microwave background (escaped photons emited
300 000 after Big Bang
-studying the surface of spots (thermal fluctuations) in «photons»,
comparing to distance they travelled when they escaped and their initial
surface
(conclusion of Boomerang : Balloon observation of millimetric extragalactic radiation and geophysics,
1998)… if ever you want to know more…
What could it be?
Many suggestions for its nature:
BARYONIC (protons and neutrons)
-Big Bang nucleosynthesis theory (from 3He, 4H, 7Li
distribution):
estimates baryonic matter = 4% of 
EXOTIC:
-WIMP (weakly interacting massive particles), axions
-Neutrinos (studies point to neutrinos having a mass)
What have we learned?
With its gravitational effects, dark matter
revealed itself
?
about a third of the universe is made out of it
Most of the dark matter has an exotic nature
The universe if flat...  = 1
Zwicky was not crazy after all!
Questions=many
-what are the other 2/3 made out of?
-the expansion of universe is accelerating
-the theory of dark energy: Quintessence
-the return of Einstein’s cosmological constant
-perhaps we don’t fully understand gravitational force
BOUQUET, Alain, Emmanuel Monnier, Matière noire et autres cachoteries de l’univers,
Éditions Dunod, Paris, 2003
BAHCALL, J, T. Piram and S. Weinberg, Dark Matter in the Universe, vol. 4, Jerusalem
Winter School for Theoritical Physics, World Scientific Publishing Co Pte Ltd.,
Singapour, 1987
KRAUSS, Lawrence, Quintessence; The Mystery of the Missing Mass in the Universe,
Basic Books, New York, 2000
SREDNICKI, Mark, Particle Physics and Cosmology: Dark Matter, vol. 6, Department
of Physics, University of California, North-Holland Elsevier Science Publishers
B.V., Amsterdam, 1990
BOTHUN, Greg, Modern Cosmological Observations and Problems, University of Oregon,
Taylor and Francis Ltd., London, 1998
(Internet) Berkeley Cosmology Group, (2004), http://cosmology.berkeley.edu/
How to know how much light is emited? In a nutshell:
the light intensity is measured here...on earth
knowing the distance, we can calculate the intensity at the source
But how can we measure the distance!!!???
by knowing the speed at which objects are moving away from
us
-using the redshift method
-and then, applying the Hubble equation and solving for d
Remember?:
v=Hd
Outside luminiuous disk: velocity is constant
75-80% of the mass is dark, and located
outside the luminious disk
 Spiral galaxies must be contained in
HUGE halo of «invisible» mass
Theoretical work of Peebles and Ostriker
(1973) shows that spiral disk would eventually
collapse into rotating bar object with time.
But would be stabilized if contained
in spherical mass distribution
X-Rays emited from large clouds of hot gas
between galaxies (1960-1990):
•Observation: Galaxy clusters are emiting a lot of X-rays
•Explanation: Inside cluster: presence of hot ionized gas at
several million degrees
•Those clouds would contain roughly 10 times more mass than the
cluster galaxies total visible mass
•The kinetic energy of gas molecule is very large: virial theorem
yields that even more mass (providing the gravitational potential
energy) is needed