Astrophysics – final topics Cosmology Universe

Jeans Criterion Coldest spots in the galaxy: T ~ 10 K Composition: Mainly molecular hydrogen 1% dust EGGs = Evaporating Gaseous Globules

Jeans Instability Thermal pressure cannot support the gas cloud against its self- gravity. The cloud collapses and fragments.

Jeans Criterion Small deviations of a spherical gas cloud from hydrostatic equilibrium Virial Theorem: 2K + U = 0 If 2K > |U|, then the gas pressure will dominate over gravity. If 2K < |U|, the cloud will collapse. U = -(3/5) (GM C 2) / R C where M C and R C are the mass and radius of the gas cloud. Also, K = (3/2) N k T where N is the total number of particles, N = M C / (μm H ), μ is the mean molecular weight.

Jeans Criterion Conditions for collapse:

Angular Momentum

New Star Fusion begins in the core Planets may form in the perimeter

HR Diagrams Instability Strip (variable stars, from center to upper right corner) Mass luminosity relationship Low mass star – cooler, fainter, long lifetime High mass star – hotter, brighter, shorter lifetime L is proportional to M (3.5)

Rotation Curves rotation curves as evidence for dark mattter types of candidates for dark matter v = (4πGρ/3) 1/2 * r ρ = 3H 2 /8πG Rotation curves and mass of galaxies Derive rotational velocity from Newtonian gravitation

Supernovae Type Ia and II supernovae

Big Bang The Big Bang Model is a broadly accepted theory for the origin and evolution of our universe. It postulates that 12 to 14 billion years ago, the portion of the universe we can see today was only a few millimetres across. It has since expanded from this hot dense state into the vast and much cooler cosmos we currently inhabit. We can see remnants of this hot dense matter as the now very cold cosmic microwave background radiation which still pervades the universe and is visible to microwave detectors as a uniform glow across the entire sky.

Big Bang The singular point at which space, time, matter and energy were created. The Universe has been expanding ever since. Main evidence: Expansion of the Universe – the Universe is expanding (redshift)  it was once smaller  it must have started expanding sometime  “explosion” Background radiation  evidence of an hot Universe that cooled as it expanded Helium abundance  He produced by stars is little  there is no other explanation for the abundance of He in the Universe than the Big Bang model.

Making Nuclei and Atoms In the first s after the Big Bang the four fundamental interactions (gravity, weak force, electromagnetic force and strong force) were unified. At s (T=10 32 K) gravity appeared a separated force. At s (T=10 27 K) strong nuclear interaction separated from weak and electromagnetic interaction. Between s and s the Universe underwent a rapid expansion increasing its size by a factor of (Inflationary Epoch). Matter outnumbers anti-matter. At s (T=10 12 K) the electromagnetic interaction separated from the weak interaction.

Making Nuclei and Atoms At s temperature has dropped enough for individual protons and neutrons to exist. At about 2 s (T=10 10 K) neutrinos ceased to interact with protons and neutrons. By 3 minutes after the Big Bang all the primordial He had been produced. After some years the Universe temperature had cooled enough for H and He atoms to exist. High energy photons no longer interact with atoms. The Universe became transparent to photons and it is these photons which now give rise to the 3K background radiation. 1 billion years after the Big Bang, some matter can be brought together by gravitational interactions. If this matter is dense and hot enough, then nuclear reactions can take place and stars are formed.

Density of the Universe?? If we take in account all the matter (stars) that we can see then the total mass would not be enough to keep the galaxies orbiting about a cluster centre. So, there must be some matter that can not be seen – dark matter. This dark matter cannot be seen because it is too cold to irradiate. According to the present theories dark matter consists in MACHO’s and WIMPS.

MACHO’s WIMP’s Massive compact halo objects – brown and black dwarfs or similar cold objects and even black holes. Non-barionic weakly interacting massive particles (neutrinos among other particles predicted by physics of elementary particles). It seems that there is also what is called and dark energy… The increase of the potential energy reduces the kinetic energy of the parts of the universe, causing the expansion to slow down. However, if there was some sort of negative energy it would have the positive effect. This is called dark energy Density of the Universe??

Elementary Particles