Astro 10-Lecture 14:Large Scale Structure and Cosmology

How big is the universe?

What does the universe look like on the largest scales?

How did the universe get to be the way it is now?

Extra Credit Assignment

Write a 5 to 10 page paper going into detail on a single aspect of astronomy (currents events, a type of object, etc.). Be concise. Don’t pick too broad a subject.

• Will be accepted up until the final.

• Worth up to 5% of your grade

• If you are more than 5% away from a boundary, I won’t grade it.

• Plagiarism earns a course grade of F, regardless.

Structure in the Universe

Stars

Star Clusters (100s to Millions of Stars) Galaxies (Millions to Several Hundred Billion Stars)

Galaxy Clusters (10s to 1000s of Galaxies)

Are there any larger structures in the universe?

Superclusters, Walls, and Voids.

Galaxies and Clusters

appear to be clustered into

huge walls with voids between

them.

But on the largest scales the universe begins to look smooth

ConcepTest

Why does it look like there are fewer galaxies far away from us?

Which brings us to cosmology.

Has the universe always been here?

Did it always have stars and galaxies?

Will it always be here?

Start with something even more basic...

Why is the sky dark at night?

(Olber’s Paradox)

In a forest, no matter which way you look, you see a tree

Possible Reasons….

Is dust blocking the light?

•No. The dust would be heated by the star light until it was as hot as a star.

Is the light red shifted out of the visible?

•The energy would still be there to heat up the dust (and the earth).

Possible Reasons….

A. Perhaps the universe is finite…

B. Perhaps the universe is young enough that light from distant stars hasn’t reached us?

C. Perhaps the universe is different far away from us?

Searching for an answer

Remember Hubble’s Law?

v=Hod

The farther something is, the faster it is moving away from us…

Everything was closer together in the past.

We can work backward to find out when distant galaxies were in the same place we were.

(In other words the universe has a finite age…)

How old is the universe?

v=Hod

How long would it take a galaxy traveling at velocity v to go distance d?

d/v=1/Ho

If Ho=70 km/s/Mpc

1/Ho = 1 Mpc/(70 pc/Myr) = 14 billion years

“The Observable Universe”

Since the universe isn’t infinitely old, the part that we can see isn’t infinitely large

This doesn’t mean the universe is finite. It just means there might be parts of the universe we

can’t see.

If the universe is 14 billion years old, we can’t see anything farther away than 14 billion light

years.

“Lookback time”

Also recall that when we look at something 2 billion light-years away we’re looking at something as it was 2 billion years ago.

If we look far enough we should be able to see galaxies forming.

Assumptions in Cosmology

Homogeneity: Matter (and energy) are evenly distributed on the largest scales

Isotropy: The universe looks the same in all directions

Universality: The physical laws that govern the universe are the same everywhere (and everywhen).

The Cosmological Principle:

An observer anywhere in the universe sees approximately the same thing. No place is special. No edge or center.

ConcepTest

• T/F: We are at the center of the observable universe.

Basic Concepts of Cosmology

The Hubble Flow: The universe is expanding. Galaxies are receding from us due to this expansion. Gravity is working to slow this expansion. Is there enough gravity to eventually stop the expansion?

Basic Concepts: The Big-Bang Theory

If you could run the universe back in time, eventually all the galaxies would merge into a hot, high-density plasma. So, it’s logical to assume that the universe started as a hot, high-density plasma.

We’ll talk about other evidence for this later.

More Basic Concepts of Cosmology

Dimensions:

A zero dimensional object:

A one dimensional object:

A two dimensional object:

The geometry of the universe

• We perceive space as 3 dimensional

• Relativity says time is a 4th dimension

• Gravity due to energy (and mass) can curve space

• Some theories of physics require that space have 11 or more dimensions

The geometry of the universe

• The universe has no boundary or edge– Either it is infinite, or curved through a 4th (or

5th) dimension to meet itself.

A one-dimensional object curved through a 2nd dimension to meet itself

An infinite one-dimensional object

Mass and Energy determine the curvature of the universe

The possibilities:

1. The total energy is negative– The universe is curved back in

upon itself (CLOSED)– The universe will stop

expanding and collapse back on itself

Mass and Energy determine the curvature of the universe

The possibilities:

2. The total energy is positive:

– The universe is curved in such a way that it doesn’t curve in on itself like a hyperbola or parabola. (OPEN)

– The universe will expand forever

Mass and Energy determine the curvature of the universe

The possibilities:

3. The total energy is zero

– The universe is flat– The universe will just barely expand forever (it

will stop after an infinite amount of time.

How do we tell what the geometry of the universe is?

Things that are geometrically true in a flat space are not necessarily true in a curved space.

Determining the Geometry of Space-Time

A B

C

A

C

B

Flat Space

A+B+C=180o

Closed Space

A+B+C>180o

Open Space

A+B+C<180o

Determining the Geometry of Space-Time

Flat Space: Circumference/Diameter=

Area/Radius2=

Closed Space: Circumference/Diameter<

Area/Radius2<

Open Space: Circumference/Diameter>

Area/Radius2>

These effects are too small to measure…

Moreover the curvature of space near earth is dominated by the earth’s gravity.

Same is true for volume

Flat Space: Volume/Radius3=4/3

Closed Space: Volume/Radius3<4/3

Open Space: Volume/Radius3>4/3

If you assume that the density of galaxies is constant, you can measure this by counting

galaxies.

Result: Nearby, at least, space is close to flat.

For large distances, it’s tough to avoid selection effects.

How else can we measure the geometry of Space-Time?

General Relativity says that curvature of space is caused by gravity. Or is it that

gravity is caused by the curvature of space?

It doesn’t matter which causes which, just that in the end, mass and energy are the

cause of curved space.

Determining the Geometry of Space-Time

Now all that we need to do is to measure the total amount of matter and energy in

the universe in order to determine the curvature.

If we define density , such that =1 would result in a flat universe, then what is

for the universe?

Contributions to the Mass and Energy of the Universe

• Photons (P ~ 0.01)

• Normal Matter (B ~ 0.04)

• Dark Matter (DM ~ 0.22)

P + B+ DM ~ 0.27

Unless we’re missing something, the universe should be open.

(Hint: We’re missing something, but we need to learn more about the Big Bang to figure it out)

What is Dark Matter?

• Rocks, and Planets (could not be made early enough in the universe).

• Neutrinos? (Not massive enough, moves too fast to clump together like the dark matter we detect)

• Black holes?

• WIMPs? (weakly interacting massive particles)

Back to the Big Bang Theory

Big Bang is a really bad name for this theory.

The big bang wasn’t the explosion of an object, it was an explosion of space.

It happened everywhere at once.

There was no place “outside” of the universe in which to watch the Big Bang.

A Big-Bang Timeline

• t < 10-43 seconds – “Planck Time” the shortest time we can know anything about in

current physics

– We don’t know anything before this time.

• t < 10-35 equilibrium between particles, antiparticles and photons.– High energy photons decay into particle antiparticle pairs.

– Particle and antiparticle annihilate to create high energy photons

– Symmetry between particles and antiparticles (same number of each).

A Big-Bang Timeline

• 10-35< t < 10-6 seconds: The inflation era…– How does one side of the observable universe know it

should be like the other side? No information has had time to travel more than 14 billion light years.

• At some point in the history of the universe, these distant places were in close communication, but then something moved them apart at much faster than the speed of light.

– Why is the universe so close to flat?

The Four Forces

• Gravity (the weakest force)

• Electro-magnetic force

• Weak nuclear force

• Strong nuclear force

Early in the history of the universe, it wasn’t possible to distinguish between the forces.

This is an example of symmetry being broken.

(coin trick)

The Four Forces

Inflation

• The energy released by the breaking of symmetry fueled extremely rapid expansion of space by a factor of 1030. – The universe we can see now went from the size of an

atom to the size of a marble.

• The breaking symmetry between the forces also resulted in an asymmetry between matter and antimatter. – For every billion anti-protons, there were a billion and 1

protons created.

– Dark matter formed in this period.

Inflation flattens the universe

Before Inflation

After Inflation by 103

Predictions of Inflation Theory

• The universe is very flat.

• -1 < 10-49

• Where is the rest of the mass/energy?

• The universe may be closed, but just barely

Continuing the timeline

• 0.0001 seconds– The universe has cooled (photons have red-shifted) to 1 trillion K.

– Photons no longer have enough energy to create protons and antiprotons.

– Protons and antiprotons annihilate, leaving the extra 1 proton per billion.

– Photons are still energetic enough to make electron-positron pairs

– Too hot for nuclear fusion to occur. Photons destroy any nuclei that form.

Big Bang Nucleosynthesis

• 2 minutes– It’s cool enough (1.5 billion K) for protons and neutrons to form

deuterium nuclei

• 3 minutes – It’s cool enough (1 billion K) for some helium and some beryllium

and lithium to form.

– No heavier elements than lithium could form

• 30 minutes– The density and temperature have dropped to where nuclear

reactions stop occurring.

The abundance of deuterium and helium is set by the temperature and density of normal matter between 2 and 30 minutes after the big bang

Big Bang Nucleosynthesis

The Big Bang theory predicts that about 25% of matter should be in the for of He. This matches what we see in stars and planets.

Big Bang Nucleosynthesis

By measuring abundances of deuterium and other isotopes we can determine the density of normal matter in the early universe

That’s how we determined B~0.04

Continuing the Big-Bang Timeline

• 30 min < t < 380,000 million years– The universe is too hot for electrons and protons to form hydrogen

atoms. It is ionized.

– Light cannot travel through ionized gas. The universe is opaque.

– Its blackbody radiation is trapped.

• t ~ 380,000 years.– The universe has cooled to about 3,000 K, hydrogen atoms form.

– The universe becomes transparent, the 3,000 K blackbody radiation travels freely.

– We see that black body radiation today, greatly red-shifted. It now looks like a 3K blackbody.

– It is the farthest thing we can see. It defines the edge of the observable universe.

The Cosmic Microwave Background

Predicted in 1945. First observed in 1965. It fits a black-body of temperature 2.73 K

Arrives from all parts of the sky.

The Cosmic Microwave Background

It’s not entirely uniform. Because of the motion of the Sun and the Galaxy, it is blue shifted on one side of the sky and red shifted on the other.

The Cosmic Microwave Background

When you subtract the variation caused by the motion of the earth, you see tiny fluctuations caused by density variations in the early universe.

Cosmic Microwave Background

These fluctuations can tell you about the density and rate of expansion of the early universe.

The latest results from WMAP say the universe is flat to within the instrumental error.

So back to the problem: Where is the missing mass/energy?

If the universe is flat, then 70% of the mass/energy is not in the form of normal matter, dark matter, or photons. Then what is it?

The first clue: When Einstein was coming up with general relativity, there was an equation that needed an additional constant to make the universe static. He called this the cosmological constant. He also called it his biggest blunder.

The Cosmological Constant

• When people discovered the universe wasn’t static they assumed the constant was zero.

• Particle physics suggested that there was an energy to empty space that would appear as a cosmological constant. But when they calculated it they got a number a factor of 1050 larger than was possible.

The Cosmological Constantaka Dark Energy

• In the 1990s two groups measuring distant supernovae discovered that they were brighter than the distance (determined by the Hubble law) would suggest.– The expansion of space is accelerating due to a

cosmological constant. The energy of this acceleration is equivalent to a contribution to of 0.7

The properties of dark energy

Dark energy provides a pressure to space increasing the rate of expansion, counteracting gravity. The net effect is like a negative gravity.

We don’t know if dark energy is a property of space (a constant that doesn’t change as space expands) or an energy that dilutes as space expands.

Continuing the timeline

• t ~ 200 million years– First stars form.

– Maybe much more massive that today’s stars.

• t ~ 1 billion years– Quasars have formed

– Globular clusters form in initial collapse that results in formation of galaxies.

• t ~ 9 billion years– The solar system forms

Structure formation

• Wiggles in the Cosmic Background represent density fluctuations in the early universe.

• Higher density regions attract matter (especially dark matter) which makes the density higher, which attracts more matter.

• Clumps of dark matter grew to form the structures we see.

Exam #3

• 2 weeks from today

• Bring #2 pencil and a scantron form!• If you need to schedule a make up contact me now,

if not sooner.• Make ups will need to be finished PRIOR to the

final.