# Astrophysics PART 4

Newton’s Model of the Universe
The universe is infinite in both space and time. It is uniform and static. Meaning there are an infinite number of stars heading out in all directions to infinity (otherwise the universe would collapse under its own gravitational force)
Why is the night sky dark? or Why isnt the night sky as uniformly bright as the surface of the Sun?
(If universe extends infinitely) every direction you look you would be able to see stars. Since light has a finite speed, the universe is old light has not had time to reach us yet. explains why the night sky is dark
Solution of Dark Sky (3)
1. Newton is Wrong.
2. The universe is expanding, so distant stars are red-shifted out of the visible spectrum due to the Doppler Effect.
3. Universe is young and not infinite in time. When we can’t see stars, it can be because we are looking back in time when there was no stars.
Doppler Effect
-Used to calculate how fast stars and other astronomical objects move toward or away from Earth.
-If galaxies are moving away from each other, must mean that they were closer in the fast. (Matter was concentrated in one point and some “explosion” may have thrown the matter apart)
Red Shift (Doppler Effect)
Wavelength increases (moving away)
Blue Shift (Doppler Effect)
Wavelength decreases (moving towards)
Background Radiation by Dicke and Peebles
1.Saw that there was more He in the universe that could be made by stars. 2.Proposed from this that the beginning of the universe was at sufficiently high temperatures to produce He by fusion. (A lot of highly energetic radiation was produced from this.)
Background Radiation by Dicke and Peebles Part 2.
But as universe expanded and cooled, the energy of that radiation decreased as well and thus wavelength increase.
It was predicted that the actual photons would have an max wavelength that corresponded to the 3K of a black body spectrum.
=microwave radiation. This was confirmed y Penzias and Wilson experiment….(next slide)
Background Radiation by Penzias and Wilson
P&W worked with microwave aerial and found that no matter which direction they pointed the aerial it picked up a steady continuous background radiation.
3 Degree Kelvin/ Cosmic/ Microwave Background Radiation
In every direction there is a very low energy and very uniform radiation that fills up the Universe.
Names come from the fact that this radiation is a black body with temp<3K, which peaks in Microwave portion of spectrum.
How is Background Radiation evidence for Big Bang?
The cosmic background radiation is the afterglow of the big bang, which cooled to the microwave spectrum by expansion of the Universe for 15 billion years.
This causes the radiation originally produced in the big bang to redshift to longer wavelengths.
BIG BANG
A broadly accepted theory for the evolution of our universe.
Says 12-14 billion years ago, universe was few mm across.
Has expanded from hot dense state to current cool cosmos.
Remnants of the hot dense state is evident in the form of the cold cosmic microwave background radiation.
Def of BIG BANG
The singular point at which space, time, matter and energy were created. The Universe has been expanding ever since.
Evidence of Big Bang (3)
1) Expansion of the Universe
3) Helium Abundance
Expansion of the Universe
the universe is expanding (redshift)=> therefore it was once smaller=>it must have started expanding sometime=> “EXPLOSION”
Evidence of an hot universe that cooled as it expanded
He 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.
Fate of the Universe
*CHECK OUT THE GRAPH*
http://tap.iop.org/astronomy/cosmology/705/img_full_47588.gif
1) Open Universe
2) Flat Universe
3) Closed Universe
Open Universe
Universe will continue to expand forever
Enough matter=> Density is that gravity is too weak to stop the Universe from expanding forever
Flat Universe
Expansion will gradually slow down until at a point it will stop expanding and start to contract
Critical Density=> Universe will only start to contract after an infinite amount of time
Closed Universe
It will start contracting due to its own gravitational field.
Due to not enough matter=> Density is not enough to allow an infinite expansion => gravity will stop the Universe expansion and cause it to contract (BIG CRUNCH)
Mass Density
The amount of matter per unit volume.
Controls the 3 fates of the universe.
Critical Density
(Pc), the value for mass density that is the dividing line between an open universe and a closed universe
range of Critical Density (3)
1. ppc, the universe will be Closed.
3. p=pc, the universe will be Flat.
Critical (pc) ≈10-26kgm-3 (not given)
Problems with determining Density of the Universe
Difficult to measure density of universe b/c when calculated using mass/volume, the density of Galaxies wasn’t enough to keep galaxies moving around a “cluster centre:
-We only see about 10% of the universe. The rest is Dark Matter
Dark Matter (2)
1. MACHO’s
2. WIMP’s
MACHO’s
Massive Compact Halo Objects
1.Massive compact halo objects (things we cant see)
Eg. Brown&Black dwarfs, similar cold objects, and even black holes
WIMP’s
Weakly Interacting Massive Particles
Tiny elementary particles that do not interact with other elements
Eg. Neutrinos and other elementary particles
Dark Energy
A hypothetical energy that permeates all of space and explains the accelerating expansion of the universe
Astrophysics is International
Atacama Large Millimetre Array (a telescope)
Categories: Astrophysics