**Cosmology**

**Our Universe**

The Cosmological Principle

**homogeneous**- the same everywhere

**isotropic**- the same in all directions

Implications

**no edge** to the universe because of the __homogeneity__**no center** because it being __isotropic__

**Then, why is the night sky dark?**

If the universe is infinitely filled with galaxies of stars, then any line drawn outward from Earth will run into a stellar surface

The inverse square laws explain the distant stars faint appearance. But, stars are more numerous since the number of stars at any distance increases as the square of the distance. Therefore, the faint stars are balanced by the population of stars and stars at any distance will contribute equally to the total amount of light received on Earth. Therefore we should be viewing a lit sky!

Olber’s paradox is the difference between the prediction and the actual appearance of the night sky. The implication is that one or both the universe being homogeneous and isotropic is not correct!

**Hubble’s Law**

-all the galaxies in the universe are moving away from Earth. (Red Shift)

Recession velocity = Hubbles’s constant x distance

If time=distance/velocity

Then distance/Hubbles’s constant x distance 1/Hubble’s constant

Therefore the time it takes for a galaxy to reach it’s present distance from Earth = 14 billion years,

And

This assumes all matter and radiation were at an instant at a single point of high temperature and density: THE BIG BANG!

**Will the Universe Expand Forever?**

The cosmological principle states that whatever the outcome of any two galaxies is, it applies to all.

**Therefore, if the universe is expanding, it will continue to or it will stop expanding and begin to contract but is dependent on the density of the universe!**

High-density: can stop the expansion and cause a collapse

Low-density: expansion forever

Critical Density: the density of a universe in which gravity acting on it would stop the expansion.

Open Universe=low mass and expands forever

Closed Universe=high mass and the “big crunch”

**Geometry of Space**

The presence of matter or energy causes a warping, or curvature of space-time.

**Newtonian Mechanics**

-the curved falling particles in space thought of as orbits due to gravitational forces

-matter only includes atoms and molecules

**VS**

**Einstein’s Relativity**

-space is curved which is determined by the total density of the cosmos

-E=mc2 equates mass and energy and defines space density

–**energy turns into matter and explains the universe’s density including atoms, molecules, dark matter and anything else that has energy (photons, neutrinos, etc.)**