GALAXIES

DAY 3: Cosmic Age

There are four main theories There are four main theories which try to explain the origin of which try to explain the origin of

the Universe.the Universe.1.1. The Creation TheoryThe Creation Theory The Universe was created by a The Universe was created by a

creator (God). creator (God).2.2. The Big Bang TheoryThe Big Bang Theory The Universe began with an The Universe began with an

explosion of a explosion of a densedense atom. atom.

• Steady State Theory Steady State Theory The Universe is homogeneous and The Universe is homogeneous and

new matter is continually being new matter is continually being created at the center as the Universe created at the center as the Universe expands and the Universe remains expands and the Universe remains unchanged. This theory is losing unchanged. This theory is losing favour because it cannot explain the favour because it cannot explain the background radiation. (Big Bang says background radiation. (Big Bang says that this was created during the Big that this was created during the Big Bang itself).Bang itself).

• The Oscillating TheoryThe Oscillating Theory The Universe is expanding and at The Universe is expanding and at

some point in the future the some point in the future the Universe will begin to contract and Universe will begin to contract and move back to its original point of move back to its original point of origin (this is often referred to as the origin (this is often referred to as the

Big Crunch). (We will be devoured by Big Crunch). (We will be devoured by a black hole.)a black hole.)

http://en.wikipedia.org/wiki/Big_Crunch

The Big Bang

• A theory of the origin of the universe.• Violent “explosion” (not really an explosion)

of all matter from a state of high density and temperature.

Singularity

• The initial state of the universe at the beginning of the Big Bang

Cosmic Age

• The universe has a calculated age of 13.7 billion years

Why are all galaxies moving away from us? Why are all galaxies moving away from us? Why should the galaxies farthest away Why should the galaxies farthest away from us move away the fastest? What’s from us move away the fastest? What’s

so special about us?so special about us?

Actually, we aren’t so special at all! Actually, we aren’t so special at all! Galaxies exist in groups called Galaxies exist in groups called

clustersclusters..Every cluster moves away from Every cluster moves away from every other cluster. No matter every other cluster. No matter

what cluster we might live in, the what cluster we might live in, the others would be moving away others would be moving away

from us. from us.

The Universe is always getting The Universe is always getting biggerbigger and the space between galaxies and the space between galaxies

is getting is getting biggerbigger..

As the dough rises, the raisins move As the dough rises, the raisins move with the dough. Each raisin moves with the dough. Each raisin moves farther away from each other. No farther away from each other. No

matter which raisin you choose, all of matter which raisin you choose, all of the other raisins are moving away the other raisins are moving away

from it. This is what is happening to from it. This is what is happening to our Universe! The Universe is our Universe! The Universe is

expanding. All of the galaxies appear expanding. All of the galaxies appear to be moving away from Earth.to be moving away from Earth.

Early ideas about the Universe Early ideas about the Universe thought that our galaxy was all thought that our galaxy was all there was. Most recent ideas there was. Most recent ideas suggest that our galaxy is only suggest that our galaxy is only one of the many bubble-like one of the many bubble-like

galaxies speeding away from a galaxies speeding away from a common point. Each galaxy (and common point. Each galaxy (and

possible each universe) has its possible each universe) has its own properties. own properties.

And each galaxy (and maybe And each galaxy (and maybe universe) is expanding as they universe) is expanding as they

all move further away from the all move further away from the middle.middle.

CosmologyCosmology is the study of the origin and the development of the

Universe, addressing the grandest issues:• Does it have an "edge"?• Does it have a center?• How does it look like on

large scales (> 100 Mpc)?• How did it start?• How old is it?• How long will it live?

Hubble’s Law-Hubble discovered that the universe is expanding-The more distant the galaxy, the faster it moves away from us (or recedes)

v = H0 x dv is velocity of the galaxy (km/s)d is the distance from us to the galaxy (Mpc or megparsecs)H0 is Hubble’s Constant (about 75 km/s/Mpc)

Hubble’s Law and Big Bang

-The universe is expanding, as if it began in a huge explosion! A big bang!

-H0 gives us an estimate of the universe's age since this explosion: time =D/V = 1/H0

= 13.7 Billion years. (1 Mpc =3x1019 km; 1 Billion years = 3 x 1016 s)

v = H0 x d

Ho = 75 km/s / Mpc.

Hubble’s Law: Limitations

-inaccurate for calculating nearby galaxies due to increased gravity (like in the Local Group)

-Hubble’s Constant is difficult to measure accurately with distant galaxies

Firework explosion: A centre and an edge

What is the big bang?

Does this means that we are at the center of the Universe?

The cosmological Principle

• There is no “special position” in the Universe• The Universe has no “centre” as the centre

keeps expanding!

The big bang takes place everywhere!

The big bang is NOT like a firework, which expands to an existing space,

or an existing universe!

The space and time are the universe and was CREATED with the big bang

and is changing!

The big bang is still going on!

Does the universe have an edge?

NO! Universe has no edge and no centre!

• Galaxies are moving away from each other because space itself is expanding.

• Photons can even reach us from parts of space that are moving away from us at greater than the speed of light, like an ant marching along a stretching rubber band.

ant initially carried backward by expansion of space

A model universe: one dimensional

Cause of the Cosmological Redshift

Light waves are stretched out as they travel through expanding space.

If we see a galaxy with light redshifted to 5× its original wavelength, it left the galaxy when the universe was 5 × smaller!

Do you actually see this?

The Milky Way

You see younger galaxies!

Their spectra are redshifted!

Does it matter which galaxy you are in?

Does it depend on which direction you look?

Facts about the Universe:

• Telescopes are time machines and can “see” back to less than 1 billion years after the Big Bang. Structure has formed over time and over a large range of scales (people to clusters of galaxies).

• It has been expanding for about 13.7 billion years – the rate of expansion depends on the amount of matter in the Universe (the more matter, the slower it expands).

• We are now in a slow, steady state of expansion (unlike the Big Bang “explosion”).

The Universe Cools as it Expands

Today the temperature is about 3K (or -270oC).

Freely expanding gas cools like air coming out of a pressure tank.

In the past the universe must have been very hot.

Looking BackToward the Beginning

• As we look outward, we look back to earlier times, closer and closer to the Big Bang.

• At earlier times, matter was more densely packed together and hotter.

• We cannot see back to times before ~400,000yr

Evidence for the Big Bang…

•It predicts that the radiation that began to stream across the universe Cosmic Microwave Background Radiation

•The model predicts how hydrogen formed then formed into helium current amount of helium in universe matches model prediction

The Hot Big Bang • Both space and time is created during the big

bang. The Bang is still going on! It has no center -- happens everywhere

• Predicted by General Relativity • Main observational evidence:

– expanding Universe (Hubble’s Law) – evolving structure (galaxies and large-scale structures

appear progressively younger with the lookback time). – cosmic microwave background radiation (relic of the

hot big bang when the universe became transparent due to the "recombination" of electrons with protons).

– relative abundance of light elements (e.g., H, He) matches prediction.

Origin of the Universe--Jan. 1 Formation of our galaxy--Jan 24 Solar system origin--Sept. 9 Earth Solidifies--Sept. 14 Life on Earth--Sept. 30 First dinosaurs--Dec. 25 Homo sapiens --11:58:57 PM Dec. 31 Human history--11:59:39 PM Ancient Greeks to now--last five seconds Average human life span--0.15 seconds

Cosmic history in perspective

The Evolving UniverseCosmological Simulations

Observational Cosmology

A Range of Possible FatesFuture Collapse

(purple) = “closed” (BIG CRUNCH Theory)

Decelerating Expansion (red)

= “open”

Borderline (green)

= “critical”

Scenarios for the universe’s

expansion

Accelerating Expansion (blue) = “dark energy”

What is the Universe Made Of?

It appears that the universe began 13.6 billion years ago and is about 3/4 Dark Energy.

Extension Slides

The Very early Universe (Planck and GUT Eras)

• t < 10-43 s– Universe is so small, so hot, and so dense that our

understanding of physics is unable to describe what is happening.

– We are yet to unify our theories that describe electromagnetism, the strong and weak nuclear forces, and gravity (the big TOE).

• 10-43 s < t < 10-35 s – Two forces reign: gravity and the Grand Unified

Theory (unification of strong, weak, and electromagnetic forces)

Entering the Electroweak Era• 10-35 s < t < 10-10 s

– The breaking out of the strong force may led to the injection of huge amounts of energy and to the inflation of the universe.

– Elementary particles (quarks, electrons, and their antiparticles) are produced in copious quantities.

The Particle Era• 10-10 s < t < .001 s

– All four fundamental forces (gravity, strong, weak, and electromagnetic) are now distinct!

– At the end of this time, when the Universe cooled to 1012 K, it was no longer hot enough to spontaneously produce protons and antiprotons.

– After this time, for every one billion antiprotons, there were 1 billion +1 protons, because of a slight “asymmetry” in the weak force.

- All antiprotons annihilated with protons to produce photons.

The Era of Nucleosynthesis• 0.001 s < t < 3 minutes

– During this time, hydrogen (the protons left over from the Particle Era) is fused into helium.

– 75% of the Universe is left over as hydrogen and the other 25% as helium (plus trace amounts of deuterium and lithium)

– At the end of this time, the Universe was too cool for any more fusion.

The Era of Nuclei

• 3 minutes < t < 300,000 years • This is the simplest the Universe has ever

been and will ever be. • All that exists during this time is ionized

hydrogen and helium as well as lots of photons.

• The Universe during this time (and all earlier times) remains opaque.

The Era of Atoms and Galaxies

• After 300,000 years, the Universe is no longer hot enough to keep matter ionized.

• The Cosmic Background is set free at the beginning of this epoch

• The structure formed under the gravity and here we are.

Origin of the Universe--Jan. 1 Formation of our galaxy--Jan 24 Solar system origin--Sept. 9 Earth Solidifies--Sept. 14 Life on Earth--Sept. 30 First dinosaurs--Dec. 25 Homo sapiens --11:58:57 PM Dec. 31 Human history--11:59:39 PM Ancient Greeks to now--last five seconds Average human life span--0.15 seconds

Cosmic history in perspective

The Evolving UniverseCosmological Simulations

Observational Cosmology

The Critical Density• The density needed to halt expansion is about 10–29

g/cm3 – about 5 atoms per cubic meter.• 1/100,000,000th the best man-made vacuum!• All of the stars in all of the galaxies we see only totals

about 10–32 g/cm3 – one thousandth of the critical value.

• But there is much more dark matter than the matter that makes stars …

• Nevertheless, the total does not seem to be enough to close the universe!

• If the dark energy remains constant, then the universe will gradually expand faster and faster so only the gravitationally-bound structures existing today will survive.

• If the dark energy increases (as some theories suggest), it will eventually rip everything apart down to the subatomic level.

• Or perhaps the universe undergoes alternating periods of acceleration and deceleration.

The Big Rip?

Fate of the Universe • Depends on gravity, which depends on critical density

– Open: not enough matter to halt expansion (i.e., the mean density of the Universe is less than the critical density); will expand forever

– Flat: just enough matter to slow expansion forever...but hangs on knife edge, never recollapses

– Closed: enough matter in Universe to halt expansion and force collapse --> Big Crunch

• Current data imply Universe is open, and is even accelerating because of the apparent presence of the dark energy!