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A Tour of the Universe
Stanford SPLASH
Instructor: Jack Singal
Goals:
give an overview of what we currently know about the
cosmos and our place in it
Touch on both:
1) What we know
2) How we know it
I want to get some of these ideas out there. Most of the public is unaware
of what those of us in astrophysics now know about the universe
(could be said of any current science!)
After this class, hopefully you’ll have a basic grasp of these
things if you encounter them in the news or conversation
Talk to your friends, parents, grandma, etc…
Where are we?
We live on a planet…
You are here
8000 miles
Earth is 92 million
miles from the sun…
The solar system
You are here
3 Billion miles
The solar system:
1 star
8 main planets
many moons
5 dwarf planets
many small planets
1000s of asteroid rocks
Millions of comets
The sun is a star. It is 25 Trillion miles to the nearest other star.
Are there other solar systems out there?
Yes!
We now know of 100s of known
planets around other stars
increasing
mass
Earth
Jupiter
decreasing orbital distance
We mostly detect them through their
gravitational effects on their stars
Although a few we can image directly
Most that we can detect are huge and close to their star
In the next ~20 years, we will detect planets that are more like Earth
The Milky Way galaxy
There are ~10 Billion stars in our galaxy
You are here
5.7x1017 miles
= 5.7 million
trillion miles
There are ~100 Billion galaxies
You are here
So we belong to
one of 10 billion
stars in our
galaxy, and there
are 100 billion
galaxies…
4x1021 miles
But that’s only a small part of the story…
Everything we know about and everything
we have ever seen (stars, planets,
interstellar matter, intergalactic matter, and
all of the light and radiation out there), is
only 4% of what makes up the universe!
Ordinary
Matter
(You are here)
3.2% free H and He gas
0.3% neutrinos
0.5% stars and big planets
0.005% heavy elements
(you are here)
Has it always been this way?
no
- The universe is ~14 billion years old
- At the start, everything was in a very dense state, and it has
been expanding since
- The expansion is now accelerating
A brief history
t=0 Big Bang – beginning of universe
Major milestones:
10-8 seconds: “inflation” makes universe blow up in size
10-6 seconds: Quarks combine to form protons and neutrons
300,000 years: Neutral hydrogen forms
400 million years: Matter collapses to form first stars
400 million years thru present: the elements are formed by stellar fusion and explosions
1 billion years thru present: Galaxies and super-galactic structures form
~4 billion years ago: Sun and planets coalesce from dust ejected from
earlier stars
~2.5 billion years ago: Life appears on Earth
~1 billion years ago: Multicellular life appears
~750 million years ago: First animals
~100 million years ago: First mammals
~1 million years ago: First humans
~5000 years ago: First human civilizations
Question 1:
Which is the farthest from the Earth?
a) The sun
b) Neptune
c) The center of our galaxy
d) The Andromeda galaxy
Question 2:
Approximately how many times more dark matter is
there than ordinary matter in the Universe?
a) 2
b) 5
c) 20
d) 50
Ordinary
Matter
Question 3:
The Earth has been around for approximately what
fraction of the time history of the Universe?
a) 1/10
b) 1/3
c) 1/2
d) 2/3
Universe: 14 billion years old
Earth: 4.5 billion years old
How do we know all this?
- We study the light that comes
to us from distant objects
- We investigate the laws of physics here on Earth and
believe they apply everywhere
- We get some particles (electrons, protons, neutrons,
neutrinos, etc…) from celestial objects
Step 1: Telescopes
Our eyes are small and don’t collect much light, so only very bright
astronomical objects are visible to the naked eye.
Also astronomical objects are far away so they appear very small and can’t be
resolved in detail by the eye.
Telescopes solve both problems: they gather the light falling on a much larger area
than the eye, focus it to a small area, then magnify it. The resulting image is then
viewed or recorded.
Galileo uses telescope to look at
Jupiter, 1609
Hubble space telescope, present
LSST is one of a new generation of
huge telescopes, 2015
Telescopes
First people looked with their
eyes and drew what they saw
Galileo’s drawing of the Moon
Then they recorded images with
photographic film
Actual photograph from ~1900
Now we record images digitally
- like with your digital camera and store them as data
Hubble Space Telescope image
Step 2: Spectra
We can also split up the light from an object that we get with a
telescope into the different colors (wavelengths) and record
them individually
This spectroscopy gives us extra useful information…
Spectra
Spectroscopy allows us to see the brightness as a function of
color (wavelength).
Increasing wavelength
This often yields lots of information. For example, we can see the
presence (or absence) of different atoms and compounds that emit light
at specific wavelengths
Spectra
Not only that, but we can see the spectrum ‘shift’ for some objects.
This shift in wavelength is caused by the motion of the source relative to us.
It is analogous to the change in pitch of a car horn when the car moves.
Spectra
When the car is coming toward us, the waves in front are compressed,
decreasing the wavelength:
When the car is moving away us, the waves in back are elongated,
increasing the wavelength:
The same thing happens for light.
Spectra
we can see the spectrum ‘shift’ for some objects.
Analyzing the shift tells us how the object is moving!
Telescopes
Ok, we can record the light from objects, and even split it into different colors.
Some are bright and some are dim.
They have different color patterns
Some are moving
How far are they?
Step 3: Distances
Something may appear bright or faint
because it actually is bright or faint or
because it is close to us or far away
We need to measure distances
The distances to very nearby stars can
be determined by measuring angles in a
technique known as “parallax”
Distances
Farther distances rely on “standard candles”
From nearby objects whose distance is known via parallax
we can determine that certain types have an intrinsic
brightness (“luminosity”).
For farther objects of the same type, since we know how
bright they actually are, and we see how bright they appear,
we can determine the distance
Question 4:
Relative to a source that is stationary with respect to us,
a source moving away from us has its sound (and light)
wavelengths:
a) The same
b) longer
c) shorter
Put it all together!
Now that we have all of these stars and galaxies,
and their distances, and their sizes, and how they
are moving, we can start to build a clearer picture
of the universe
We can put all of the stars, galaxies, and everything
else that we see where it all belongs.
That’s what people have been doing for 100s of years, and continue to do
There have been and continue to be surprises though…
Discovery: Expansion of
the universe and big bang
Armed with a way to measure the distance to other galaxies, Hubble did
so, and also took the galaxies’ spectra (1929).
He saw a shift in the spectral lines
The spectral lines from the more distant galaxies were shifted
toward the red (longer wavelength), indicating that those
galaxies were moving away from us
Expansion of the universe
We define the redshift (z) to quantify how much the light is shifted in
wavelength toward the red
obs
 1 z
emitted
Hubble showed that there was a relation between distance and
redshift. The farther a galaxy is from us, the more it is redshifted
and therefore the faster it is moving away.
Three major conclusions:
1) Since the farther something is the longer it takes for light to
reach us, higher redshift is equivalent to saying farther back in
time
2) If across the board, the farther away something is the faster it is
receding, then the only conclusion is that the entire universe as whole
is expanding.
3) Run the movie backwards and the Universe must have started with
everything in a very small volume and has expanded since – the “big bang”
Step 4: There’s more light out there
than meets the eye…
Visible light is only a small piece of the available light out there
Increasing frequency
microwave
Different
processes in
space
produce
emission all
over the
spectrum!
All the different kinds of light
Different processes
in space produce
emission all over
the spectrum!
Over the past 100 years, we have developed detectors – the equivalent
of optical telescopes, for all of these different kinds of light.
FERMI LAT
Question 5:
Which of the following is not the name of a range of
wavelengths of light?
a) visible
b) X-rays
c) ultra-luminous
d) microwave
e) radio
microwave
The different light tells us so much
For example, looking at the center of our own galaxy:
Fast electrons
and magnetic
field
Cold gas
Dust
Stars
unobscured
Stars
Hot gas
Cosmic rays
(charged
particles)
Contents of galaxies
Galaxies consist of:
• Stars (and planets)
• Cold gas
• Hot gas
•“Dust” (heavy elements and molecules)
• Free cosmic rays (electrons, protons, neutrons, neutrinos)
• Dark matter (!)
• The space between Galaxies in clusters consists of very hot gas, and in
empty space consists of free electrons and protons
And it all glows (in different parts of the spectrum) so we see it
But it turns out there is MUCH MORE out there…
Discovery: Exotic objects
There is a slew of crazy stuff out there, characterized by systematically
studying the sky on many different scales at many different wavelengths.
Supernovae are huge explosions when a
massive star dies. Most of the elements that
make up the Earth and us were fused in
supernovae.
Many stars are in closely
orbiting binary or many-star
systems
Pulsars are rapidly rotating
neutron stars (very dense
supernova remnants) that send
out beams of radio energy
What happens to stars?
At the end of a star’s life, there are several possibilities, depending on the size:
Stars like the sun or smaller throw off most of their material and what is left is
a small very dense core held up by the pressure of all the electrons. This is a
white dwarf
More massive stars usually have a violent supernova explosion where they
spray out a huge amount of matter and light
Stars above ~1.4 times the mass of the sun have so much gravity that it
overcomes the electron pressure and the star collapses to where only the
neutrons are holding it up. This is a neutron star.
Stars above ~5 times the mass of the sun have so much gravity that nothing
can stop their collapse. They become black holes.
Black Holes
Black holes are objects so dense that light cannot escape.
Outside of a certain radius away from it (the event horizon), they gravitate like
any massive object. Things can (and do) orbit them, and can orbit closely. We
see the light from this stuff and it makes for some of the most spectacular
phenomena in the universe.
Current physics cannot explain what happens inside the event horizon, but
nothing (not even light) can escape.
Central black holes
The big black holes at the
centers of Galaxies
(including ours) are a
million or more times as
massive as the sun
Active Galactic Nuclei are matter orbiting a massive
black hole at the center of a galaxy. They sometimes
send out huge jets of very fast particles.
The jets of particles slam into the neighboring gas
and magnetic fields and radiate x-rays, UV, optical,
and radio
Because they are so bright, we can see some of them
at a great distance. Those are called Quasars
Active Galactic Nuclei
If the jet is pointed right at us, it
looks like a really bright, flaring
point, and we call it a “Blazar”
If the jet is pointed perpendicular
to us, we can sometimes see the
jets shooting way out. The far
lobes make a “Radio Galaxy”
An AGN – the “Death Star” Galaxy
“Death Star” galaxy (x-ray, radio, and optical)
Another galaxy
jet
Galaxy
with
AGN
X-rays – purple
Optical – red
Radio - blue
GRBs
Gamma ray bursts may happen when a neutron star
falls into another neutron star or black hole.
The resulting explosion sends out particles and radiation all
over the spectrum
They are the most luminous things in the universe
In May a GRB was seen at redshift 8. It is the farthest thing ever seen and
occurred only 400 million years after the big bang
Cosmic Microwave Background
Observing the sky at microwave wavelengths, we can even see
light almost back to the big bang:
A glow covers the entire sky, light from a time when the early universe
was hot and dense. It lends additional proof to the big bang explaination.
Those hotter and colder patches in the hot and dense sea tell us a lot.
They are the seeds of what will become structure.
Question 6:
What is the farthest thing we have ever seen?
a) a black hole in a distant galaxy
b) a gamma-ray burst at redshift 8
c) the CMB
Discovery: Dark Matter
Galaxies and groups of galaxies behave like they have a lot more matter than
just adding up the stars, gas, and dust
1) Galaxy rotation: We can measure (via spectral line shift) the velocities
of stars in Galaxies. The bulk rotation rate of the galaxies is related to
how much matter is in them, and there seems to be way more matter than
we can see
Dark Matter
Galaxies and clusters behave like they have a lot more matter than just
adding up the things we can see: stars, gas, and dust
1) Galaxy rotation
2) Galaxy cluster dynamics: The motion of galaxies within groups of galaxies
also indicates there is much more matter there (first claimed by Fritz Zwicky in
1933!)
Dark Matter
Galaxies and clusters behave like they have a lot more matter than just
adding up the things we can see: stars, gas, and dust
1) Galaxy rotation
2) Galaxy cluster dynamics
3) Gravitational Lensing: Matter can bend light. The amount of light bending we
see for objects behind galaxies and galaxy clusters indicates a lot of unseen
matter in the clusters.
Dark Matter
Galaxies and clusters behave like they have a lot more matter than just
adding up the things we can see: stars, gas, and dust
1) Galaxy rotation
2) Galaxy cluster dynamics
3) Gravitational Lensing
4) Colliding galaxy clusters: check out the picture
Lensing says the
mass is here
2 clusters
passed
through each
other:
So the majority of the
matter passed right
through each other
without interacting,
and it doesn’t give off
X-ray
light.observations say the
gas (ordinary matter) is here.
The gas from the two clusters
collided and stayed in the
middle
Dark Matter
Galaxies and clusters behave like they have a lot more matter than just
adding up the things we can see: stars, gas, and dust
1) Galaxy rotation
2) Galaxy cluster dynamics
3) Gravitational Lensing
4) Colliding galaxy clusters
5) Patterns in the CMB
Dark Matter
Galaxies and clusters behave like they have a lot more matter than just
adding up the things we can see: stars, gas, and dust
1) Galaxy rotation
2) Galaxy cluster dynamics
3) Gravitational Lensing
4) Colliding galaxy clusters
5) Patterns in the CMB
Dark matter interacts gravitationally, but we know that it doesn’t give off light
(at any wavelength) and that it mostly passes right through itself.
Hence “Dark”
Dark matter is ~80% of the matter in the universe.
The ordinary matter is only 20%
We don’t know what it is (although there are theories).
Dark Matter
Dark matter is ~80% of the matter in the universe.
The ordinary matter is only 20%
Galaxies and clusters sit in
‘halos’ of dark matter.
The large scale structure of the Universe
consists of vast ‘filaments’ of dark matter
in which the galaxies and clusters sit at the
vertices of the web
The dark matter can’t be as clumped like ordinary matter because it can’t radiate
energy or angular momentum, so it stays ‘swimming’ in large structures
Discovery: Dark Energy
Question 1: how is the expansion of the universe evolving with time? Is it
steady, or has it slowed down or sped up?
Question 2: What is the total energy content?
According to Einstein’s General Relativity, Energy can be in the form of:
Matter
Light
“Vacuum energy” – pushes things apart
The answers to these two questions are related, because the energy content
controls the expansion
By quantifying how distances change with time (or equivalently, redshift) we can
determine the expansion history and shape, and answer both questions
Dark Energy
Until recently, we didn’t have a method for determining very large distances,
only those to relatively nearby galaxies
In the 90s, people determined that a certain type of supernova (Ia) could be
used as a standard candle
in the late 90s they were able to
determine the actual distance to high
redshift (z>1) galaxies and found a
surprising result…
The relationship between distance and
redshift (time) shows that:
The expansion of the universe is now accelerating with time!
Dark Energy
Meanwhile, observations of the tiny differences (anisotropy) in the CMB show
the primordial seeds of structure in the early universe.
We have theories that relate the size of those seeds to what we see today,
and they depend on the energy content…
Adding up all the ordinary matter and dark matter leaves us
with 75% of the energy missing!
Dark Energy
Accelerating expansion + missing energy
 some sort of vacuum energy is present
“Dark energy”
We have no idea what it is or where it comes from.
But it’s large, and it’s (apparently) in charge
Supernova Ia, CMB, and cluster evolution all
agree: ~70% Dark Energy, ~30% Matter
Dark Energy
Also, knowing the expansion history, we can relate redshift to distance
accurately.
Knowing the speed of light, once we have the distance we can calculate the
time it took for light from something to get here.
We know the redshift of the CMB. We can get the distance, and then the time
it took for the light from the CMB to get here. That is almost the age of the
universe
age of the universe  13.7 billion years
Question 7:
Which of the following is not a source of evidence for
Dark Matter?
a) motion of the planets
b) rotations of galaxies
c) gravitational lensing of galaxies
d) pictures of galaxy clusters
Question 8:
Which of the following is not a source of evidence for
Dark Energy?
a) supernova as standard candles to determine far distances
b) patterns of small variation in the CMB
c) galaxy rotation measurements
“ΛCDM” Cosmology
All of this leads to the standard so called concordance cosmology, or “ΛCDM”
(Lambda stands for vacuum energy and ‘CDM’ is “cold dark matter”).
• The universe started from a very dense state 13.7 Billion years ago and has
expanded ever since
• In the very beginning the expansion was enormous (inflation). The expansion
then resumed a lower value
• The gravitational formation of large scale structure is dominated by dark matter
• Ordinary matter forms the stars, galaxies, and intergalactic gas. It is what we see.
• As the universe expands and the matter density drops, dark energy is increasingly
taking over and causing the expansion to accelerate
• At present, the universe is 70% Dark Energy,
~25% Dark Matter, and ~5% Baryons
Outstanding Questions
in cosmology
• What is dark matter?
• What is dark energy? Does it evolve with time?
• What is the fate of the universe?
• Why did the universe start in the first place? What caused the big bang?
• Is there life out there?
• Are there other universes?
• What happens in a black hole?
• Many others !