Cosmology in the 21st Century

1
Cosmology in the 21st Century
0

• Chapter 18

2
Guidepost
0

• With this chapter you have reached the limit of
  your journey in space and in time. You are now
  ready to study the universe as a whole.
• The ideas in this chapter are the biggest and the
  most difficult in all of science, but astronomers
  are beginning to understand how the universe
  began and how it has evolved. As you explore, you
  will find answers to four essential questions
• Does the universe have an edge or center?
• What evidence shows that the universe began with
  a big bang?
• How can the universe be expanding?
• How has the universe evolved, and what is its
  fate?

3
Guidepost (continued)
0

• As you try to answer these questions, you will
  also find answers to two questions about science
• How do analogies help scientists think about
  natural processes?
• How is scientific knowledge different from
  belief and opinion?
• Once you have finished this chapter, you will
  have modern insight into the nature of the
  universe, and it will be time to focus on your
  place in the universe the subject of the rest
  of this book.

4
Outline
0
I. Introduction to the Universe A. The
Edge-Center Problem B. The Necessity of a
Beginning C. Cosmic Expansion D. The Necessity
of a Big Bang E. The Cosmic Background
Radiation F. The Story of the Big Bang II. The
Shape of Space and Time A. Looking at the
Universe B. The Cosmic Redshift C. Model
Universes D. Dark Matter in Cosmology
5
Outline (continued)
0
III. 21st-Century Cosmology A. Inflation B. The
Acceleration of the Universe C. Dark Energy and
Acceleration D. The Age and Fate of the
Universe E. The Origin of Structure and the
Curvature of the Universe
6
Riddles of Cosmology
0
The vastness of space and time that we call the
Universe still poses many unresolved mysteries.
Be prepared to encounter some of the strangest
concepts that modern science knows (or doesnt
really know )
7
Olbers Paradox
0
Why is the sky dark at night?
If the universe is infinite, then every line of
sight should end on the surface of a star at some
point.
The night sky should be as bright as the surface
of stars! The fact that it is dark at night
anyway, is called Olbers Paradox.
Solution to Olbers Paradox
If the universe had a beginning, then we can only
see light from galaxies that has had time to
travel to us since the beginning of the universe.
? The visible universe is finite!
8
Hubbles Law
0
Distant galaxies are flying away ( receding)
from us with a speed proportional to distance
Recession Velocity (km/s)
Distance (Mpc)
9
The Expanding Universe
0
On large scales, galaxies are moving apart, with
velocity proportional to distance.
Its not galaxies moving through space. Space is
expanding, carrying the galaxies along!
The galaxies themselves are not expanding!
10
Expanding Space
0
Analogy A loaf of raisin bread where the dough
is rising and expanding, taking the raisins with
it.
11
The Expanding Universe (2)
0
This does not mean that we are at the center of
the universe!
You have the same impression from any other
galaxy as well.
12
Finite, But Without Edge?
0
2-dimensional analogy Surface of a sphere
Surface is finite, but has no edge.
For a creature living on the sphere, having no
sense of the third dimension, theres no center
(on the sphere!) All points are equal.
Alternative Any point on the surface can be
defined as the center of a coordinate system.
13
The Necessity of a Big Bang
0
If galaxies are moving away from each other with
a speed proportional to distance, there must have
been a beginning, when everything was
concentrated in one single point
The Big Bang!
Time
?
14
The Age of the Universe
0
Knowing the current rate of expansion of the
universe, we can estimate the time it took for
galaxies to move as far apart as they are today
Time distance / velocity
velocity (Hubble constant) distance
T d/v 1/H 14 billion years
The most distant galaxies known are seen as they
were only about 1 billion years after the Big
Bang.
15
Looking Back Towards the Early Universe
0
The more distant the objects we observe, the
further back into the past of the universe we are
looking.
Distance Time
16
The Cosmic Background Radiation
0
The radiation from the very early phase of the
universe should still be detectable today
R. Wilson A. Penzias
Was, in fact, discovered in mid-1960s as the
Cosmic Microwave Background
Blackbody radiation with a temperature of T
2.73 K
17
The Cosmic Background Radiation
0
The COBE satellite mapped the all-sky structure
of the Cosmic Background Radiation.
18
The History of the Universe
0
Universe cools down as time passes
Universe expands as time passes
19
The Early History of the Universe
0
Electron
Positron
Gamma-ray photon
Electrons, positrons, and gamma-rays in
equilibrium between pair production and
annihilation
20
The Early History of the Universe (2)
0
25 of mass in helium 75 in hydrogen
Protons and neutrons form a few helium nuclei
the rest of protons remain as hydrogen nuclei
No stable nuclei with 5 8 protons
? Almost no elements heavier than helium are
produced.
21
The Early History of the Universe (3)
0
Photons have a blackbody spectrum at the same
temperature as matter.
Photons are incessantly scattered by free
electrons photons are in equilibrium with matter
Radiation dominated era
22
Recombination
0
Protons and electrons recombine to form atoms gt
universe becomes transparent for photons
z 1000
Transition to matter dominated era
23
The Cosmic Background Radiation (2)
0
After recombination, photons can travel freely
through space.
Their wavelength is only stretched (red shifted)
by cosmic expansion.
Recombination z 1000 T 3000 K
This is what we can observe today as the cosmic
background radiation!
24
Reionization
0
After less than 1 billion years, the first
stars form.
Ultraviolet radiation from the first stars
re-ionizes gas in the early universe
Reionization
Formation of the first stars
? universe becomes opaque again
25
The Cosmological Principle
0
Considering the largest scales in the universe,
we make the following fundamental assumptions
1) Homogeneity On the largest scales, the local
universe has the same physical properties
throughout the universe.
Every region has the same physical properties
(mass density, expansion rate, visible vs. dark
matter, etc.)
2) Isotropy On the largest scales, the local
universe looks the same in any direction that one
observes.
You should see the same large-scale structure in
any direction.
3) Universality The laws of physics are the same
everywhere in the universe.
26
Deceleration of the Universe (?)
0

• Expansion of the universe should be slowed down
  by mutual gravitational attraction of the
  galaxies.

• Fate of the universe depends on the matter
  density in the universe.

• Define critical density, rc, which is just
  enough to slow the cosmic expansion to a halt at
  infinity.

27
Model Universes
r lt rc gt universe will expand forever
0
Maximum age of the universe 1/H0
Size scale of the Universe
r gt rc gt Universe will collapse back
If the density of matter equaled the critical
density, then the expansion of the universe would
come to a halt at an infinite time in the future,
but it would never re-collapse.
28
Dark Matter
0

• Combined mass of all visible matter (i.e.
  emitting any kind of radiation) in the universe
  adds up to much less than the critical density.

• Gravitational lensing shows that some clusters
  contain 10 times as much mass as is directly
  visible.

29
The Nature of Dark Matter
0
Can dark matter be composed of normal matter?

• If so, then its mass would mostly come from
  protons and neutrons baryons

• The density of baryons right after the big bang
  leaves a unique imprint in the abundances of
  deuterium and lithium.

• Density of baryonic matter is only 4 of
  critical density.

• Most dark matter must be non-baryonic!

30
Problems with the Classical, Decelerating Universe
0

1. The flatness problem

The universe seems to be nearly flat (i.e., all
contributions from visible and dark matter and
other stuff ?!? add up to the critical
density).
Even a tiny deviation from perfect flatness at
the time of the big bang should have been
amplified to a huge deviation today.
gt Extreme fine tuning required!
2) The isotropy of the cosmic background
If information can only travel through the
universe at the speed of light, then structure in
the cosmic background should not be correlated
over large angular scales!

• Contradiction to almost perfect isotropy of the
  cosmic background!

31
21st Century Cosmology
0
The solution Inflation!

• Inflation period of sudden expansion during the
  very early evolution of the universe

• Triggered by the sudden energy release from the
  decoupling of the strong and electroweak forces

32
Measuring the Deceleration of the Universe
0
By observing type Ia supernovae, astronomers can
measure the Hubble relation at large distances
Distance ? recession speed
Size scale of the universe ? rate of expansion
It was expected that this would measure the
deceleration of the universe, but
33
Type I and II Supernovae
0
Core collapse of a massive star Type II Supernova
If an accreting White Dwarf exceeds the
Chandrasekhar mass limit, it collapses,
triggering a Type Ia Supernova.
Type I No hydrogen lines in the spectrum Type
II Hydrogen lines in the spectrum
34
The Accelerating Universe
0
Apparent Magnitude of Type Ia Supernovae
Red Shift z
In fact, SN Ia measurements showed that the
universe is accelerating!
35
Confirmation of the Acceleration
0
Observation of the high-red-shift (z 1.7) SN Ia
SN1997ff seems to confirm the acceleration of the
universe.
36
(No Transcript)
37
The Cosmological Constant
0

• Cosmic acceleration can be explained with the
  Cosmological Constant, L (upper-case lambda)

• L is a free parameter in Einsteins fundamental
  equation of general relativity previously
  believed to be 0.

• Energy corresponding to L can account for the
  missing mass/energy (E mc2) needed to produce
  a flat space-time.

?Dark Energy
38
The Fate of the Universe
0
Due to the effect of Dark Energy, the Universe
will keep expanding faster and faster
We dont know the nature of Dark Energy, and we
dont know how it will evolve in the future.
The Universe may keep expanding in a regular
manner, or be disrupted in a big rip.
39
Large Scale Structure
0
Distribution of bright galaxies in the Virgo
region indicates the Virgo cluster and presence
of more distant, larger scale structure
40
Large Scale Structure (2)
0
A large survey of distant galaxies shows the
largest structures in the universe
Filaments and walls of galaxy superclusters, and
voids, basically empty space.
41
The Growth of Structures
0
Structures in the Universe could have developed
in two fundamentally different ways
a) Small structures (galaxies) could have
developed first, then clustering into larger and
larger structures
b) The largest structures (superclusters, voids)
could have developed first, then breaking up into
smaller and smaller units.
The latter scenario seems to be favored by
observational evidence.
42
Cosmology with the Cosmic Microwave Background
0
If the universe were perfectly homogeneous on all
scales at the time of reionization (z 1000),
then the CMB should be perfectly isotropic over
the sky.
Instead, it shows small-scale fluctuations
43
The Cosmic Background Radiation
0
The COBE satellite mapped the all-sky structure
of the Cosmic Background Radiation.
44
Fluctuations in the Cosmic Microwave Background
0
The angular size of the CMB fluctuations allows
us to probe the geometry of space-time!
CMB fluctuations have a characteristic size of 1
degree.
45
Analysis of the Cosmic Background Fluctuations
0
Analyze frequency of occurrence of fluctuations
on a particular angular scale
? Universe has a flat geometry
46
The Contents of the Universe
0

• We only see about 4 of all the mass and
  energy in the Universe!

• The nature of about 96 of our Universe is yet
  mysterious and unknown!