Andréanne Baribeau, dans le cadre du cours PHY3903Décembre 2005

The plan: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 matterBaryonic vs exotic matter

Our UniverseOur 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

Even more surprisingly, it is expending!Edwin Hubble (1920’s – 1930’s) observes:

v = Ho*d

Its composition according to old cosmology:

normal matter (baryonic)organized in galaxies...clustersvoid

Ho: Hubble’s constant: 100hkms-1Mpc-1 where 0.5<h<1

How will it all end?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!! >1flat: will keep on expanding =1open: will also keep on expanding...Big Rip!! <1

Weighing our Universe:

crit=3Ho2 /

8G crit =1.879*10-29h2 g/cm3

=3H/m3

Ho:Hubble parameterG: Newton’s constanth: 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 lightemited and corresponding mass

: critical density for a flat Universe

Estimation for light density of the Universe:

1.8*108h Lo/Mpc3

Lo: power of visible light emited from the sunMpc: 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 missingThere’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 Zwicky1930’s: Fritz Zwicky (Switzerland) studies cosmic dance ofgalaxy cluster «Coma» (7galaxies)

There must be more mass between those galaxies then what meets the eye

The virial theorem : stationary state: kinetic energyof the galaxy system must be less than the total gravitational potential energy

2K+W~0

The flat rotation curves (Vera Rubin, 1970’s):The flat rotation curves (Vera Rubin, 1970’s):

•galaxy’s mass: concentrated at center

•Doppler shifts tells us rotation velocity

•Gravitational force should decreaseas radius increases

•The result should be a decreasingrotation velocity with increasing radius, following Kepler’s theory

BUT...Rotation curves (velocity vs radius) are flat!!!

Gravitational lenses (Mellier, Fort, Soucail, 1986):Gravitational lenses (Mellier, Fort, Soucail, 1986):

•light emited from observed objects (quasar, galaxies…) is distortedby 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

(conclusion of Boomerang : Balloon observation of millimetric extragalactic radiation and geophysics,1998)… if ever you want to know more…

and oh yeah, the Universe IS flat...-geometrical study of microwave background (escaped photons emited300 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

What could it be?What could it be?

Many suggestions for its nature:

BARYONIC (protons and neutrons)-Big Bang nucleosynthesis theory (from 3He, 4H, 7Lidistribution):

estimates baryonic matter = 4% of

EXOTIC: -WIMP (weakly interacting massive particles), axions

-Neutrinos (studies point to neutrinos having a mass)

What have we What have we learned?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 PublishersB.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 earthknowing 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

Spiral galaxies must be contained in HUGE halo of «invisible» mass

Outside luminiuous disk: velocity is constant

75-80% of the mass is dark, and locatedoutside the luminious disk

Theoretical work of Peebles and Ostriker(1973) shows that spiral disk would eventuallycollapse into rotating bar object with time.

But would be stabilized if containedin spherical mass distribution

X-Rays emited from large clouds of hot gas X-Rays emited from large clouds of hot gas between galaxies (1960-1990):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 thecluster galaxies total visible mass

•The kinetic energy of gas molecule is very large: virial theoremyields that even more mass (providing the gravitational potentialenergy) is needed