Galaxies, Cosmology and Galaxy Evolution

Galaxies

Stars form in groups, within regions where gravity is strong (high density) and gas is cold

Galaxies host many of this star formation regions. With a higher density in the spiral arms

A healthy spiral galaxy the size of the Milky Way hosts about 100 billion (10^11) stars

Galaxies host many of this star formation regions. With a higher density in the spiral arms

A healthy spiral galaxy the size of the Milky Way hosts about 100 billion (10^11) stars

The Milky Way

The Hubble eXtreme Deep Field (XDF)  showing thousands of galaxies. There are about 100-200 billion galaxies in the visible Universe

Not all galaxies look the same!

Hubble Classification of Galaxies

The Expanding Universe

Edwin Hubble at Mount Wilson

Measuring Distances

Measuring Velocity

We can measure the velocity of stars along the line of sight by using the doppler effect

When the source of waves moves the frequency increases in front of the source and decreases behind the source

If moving towards you, wavelength shorter and frequency higher.

If moving away, wavelength longer and frequency lower.

Doppler Effect

When the source of waves moves the frequency increases in front of the source and decreases behind the source

Police use the Doppler effect of radar waves to measure the speeds of cars on the highway.

Doppler Effect

When the source of waves moves the frequency increases in front of the source and decreases behind the source

Astronomers use the Doppler effect to measure the rotation speed of galaxies.

Hubble Law

Galaxies are moving away from us (red-shifted), and the farther they are the the greater their recession speed (more red-shifted)

Hubble Law

Galaxies are moving away from us, and the farther they are the greater their recession speed

The fact that galaxies move away from us at the same rate regardless of which direction we are looking at suggest that space itself is expanding

Relativity

Classical Relativity

In classical relativity you just add the velocities of the moving frame (truck) and the ball 

Einstein's Special Relativity

In 1905 Einstein postulated that:

1. The laws of physics must be the same for all observers independent on their relative motions
2. Thus the speed of light (a fundamental physics constant) must be the same for all observers independent on their relative motions

Space-Time in Relativity

If the speed of light is constant for all observers regardless of their velocity, the distance and time must be different

(a)

(b)

Space-Time in Relativity

General Relativity

Einstein's

Gravity is the curvature of space and time due to massive objects

V.S.

Newton's

Gravity is a force field surrounding massive objects. Other masses interact with this force field

Einstein extended his theory of relativity to accelerating frames. This lead to the postulate that gravity is the curvature of space-time

Gravitational Lensing

Gravitational lensing is evidence of the curvature of space-time. Light has no mass, in order to experience gravity massive objects must curve space and time

Gravitational Waves

First observed in 2015

Modern Cosmology

The study of the Universe as a whole

With General Relativity and Hubble's observations of the expanding universe modern Cosmology was born

What happens if you apply General Relativity to the whole Universe?

Accelerated Expansion

Modern observations suggest the expansion of the Universe is accelerating!

Big Bang Cosmology

If the Universe is expanding, at some time in the past must have been infinitely hot and infinitely dense

This doesn't mean that the Universe was infinitely small though.

Big Bang Cosmology doesn't state anything about the size of the Universe when was "born"!

Cosmic Inventory

From General Relativity, the expansion rate of the Universe tells us the curvature of the Universe and thus the amount of mass/energy in it

Matter provides positive gravity (attraction): we can infer the amount of mass in the universe from its gravitational effects, even though we can only directly see ordinary matter

Dark energy provides negative gravity (repulsion): We know how much dark energy there is from the expansion rate and the amount of matter we infer

Dark Matter

~85% of matter in the universe is made of something that does not interact with light, thus we can not see. We call this dark matter 

We know dark matter is there and how much is there purely by its gravitational effects. Dark matter holds the universe together.

Dark Matter

Large Scale Structure and Galaxy Formation

Large Scale Structure

The Big Bang cosmological model reproduces the observed large scale structure of the Universe

The Cosmic Web

Galaxy Formation

Galaxy Formation

Galaxy Evolution

Galaxy Evolution

The End

The Standard Model

Protons and Neutrons are made of Quarks

All of the matter that we know is made of this fundamental particles

Antimatter

All particles have an anti-particle with the same properties but opposite charge. When matter and anti-matter combine they annihilate to produce pure energy