The Expansion of the Universe

1
The Expansion of the Universe

• Paul J. Thomas
• Department of Physics and Astronomy
• UW - Eau Claire

2
(No Transcript)
3
Introduction

• All galaxies, except a few local ones, are
  receding from us.
• The more distant the galaxy, the greater the
  recession speed.
• The expansion apparently began in the Big Bang
• Will the universe continue expanding or start to
  contract someday?

4
Lookback Time

• We can see galaxies billions of light years away.
• Therefore we see these galaxies as they were
  billions of years ago!
• Looking out into space is like looking back in
  time!
• The universe was probably different in the past.

5
Topics of Discussion

• Hubbles Law
• The Big Bang Model
• The Cosmic Background Radiation (CBR)
• Dark matter, dark energy and the density of the
  universe
• The long-term future of the universe

6
Expansion of the Universe

• Distance and recession speed are connected by
  Hubbles Law
• V H0 r,
• where V recession speed (km/s),
  
• r distance (Mpc)
  
• H0 Hubbles Constant (km/s/Mpc).

7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
The Parsec

• Parsec parallax second.
• A unit of interstellar distance.
• If a star was 1 parsec away, it would appear to
  move 1 second of arc (1/36000) in six months.
• 1 parsec 3.26 light years.

11
(No Transcript)
12
Hubbles Law for Galaxies

• We can measure V from the redshift of light from
  the galaxy.
• We can determine r by the use of standard candles
  (e.g. Cepheid variables).
• Plug these numbers into Hubbles Law
• V H0 r,
• and we get a value for H0.

13
Standard Candles

• We cant use parallax over galactic distances, as
  they are too great
• If we knew the specific brightness of a star, we
  could use the inverse square law to calculate its
  distance
• We use variable stars, such as Cepheid variables

14
Cepheid Variables

• These are giant stars (103 - 105 L?)
• They cannot balance pressure and temperature
  well, and go through phases of expansion and
  contraction
• We see these phases as regular cycles of
  brightening and dimming (1 - 60 days)
• The greater the luminosity of a variable, the
  longer the period

15
(No Transcript)
16
HST views Cepheid Variables in M100
17
Type Ia Supernova - 10 billion ly
18
Values for Hubbles Constant

• Current best estimate of H0
• 71 km/s/Mpc (5) (WMAP)

19
The Age of the Universe

• H0 has units of 1/time
• H0 distance/(time distance)
• 1/H0 is the Hubble Time, tH.
• This is the time since the Big Bang.
• H071 km/sec/Mpc ? tH 13.7 billion years

20
Penzias and Wilson

• The discovery of the cosmic microwave background
  by Penzias and Wilson transformed cosmology from
  being the realm of a handful of astronomers to a
  'respectable' branch of physics almost
  overnight.
• -Michael Turner,
• University of Chicago

21
The Cosmic Background Radiation (CBR)

• Black Body radiation that is isotropic (all
  directions at same strength)
• Peak energy at 1 mm wavelength, corresponding to
  2.7 K
• This is the remnant energy from the Big Bang

22
The Cosmic Background Radiation (CBR)

• CBR can be detected from the ground BUT
  atmospheric H2O is a strong absorber, so
  satellite observations are preferred
• Best data so far from Wilkinson Microwave
  Anisotropy Probe

23
WMAP Image of CBR
24
(No Transcript)
25
The Big Bang Model

• The Universe began in an episode of high
  temperature and density 13.7 billion years ago.
• Matter, energy and physical laws came into being
  at that time.

26

• WMAP Results
• Universe is 13.7 billion years old, with a margin
  of error of close to 1.
• First stars ignited 200 million years after the
  Big Bang.
• Light in WMAP picture is from 379,000 years after
  the Big Bang.
• Content of the Universe
• 4 Atoms, 23 Cold Dark Matter, 73 Dark Energy.

27
The Big Bang

• The Big Bang was not an explosion of matter and
  energy in pre-existing space.
• Space and time came to be during the Big Bang.
• Physical laws came into being then, too. (They
  did vary in the very earliest stages).

28
Four Fundamental Forces

• The Strong Nuclear Force
• Binds the nucleus of atoms together.
  Limited range 10-15 m.
• The Electromagnetic Force
• Acts on charges and magnetic objects. Unlimited
  range.
• The Weak Nuclear Force
• Binds neutrons and similar particles together.
  Limited range 10-16 m.
• Gravitation
• Attracts massive objects to each other. Unlimited
  range.

29
Unifying the Forces

• In our most powerful particle accelerators, we
  are able to collide protons and electrons at very
  high speeds (99.999999999 of the speed of
  light).
• This produces conditions similar to those just
  after the Big Bang.

30
Particle Accelerator
Fermilab Accelerator (Proton Synchrotron),
Batavia, Illinois
31
Unifying the Forces

• In our particle accelerators, we see
  electromagnetic weak nuclear force ? combined
  electroweak force.
• We think the other forces would combine at higher
  energies.
• So, during the Big Bang, one original force
  split to become the four forces we know.

32
(No Transcript)
33
Will the Universe Recollapse?

• Required density for Universe to recollapse 4.5
  10-30 g/cm3 critical density.
• Observed density of luminous material 2
  10-31 g/cm3.
• But there may be 20 this amount in dark matter.

34
Curvature of the Universe

• The curvature of the universe as a whole is
  determined by its mass density, ?.
• A universe with a mass density greater than the
  critical value, ? gt 1, will be a spherical closed
  universe.
• A universe with a mass density ? lt 1will be an
  open, hyperbolic universe.

35
What is the observed value of ??

• Based on luminous matter, ?LM 0.05.
• Structure of the CBR indicates ?TOT 1.0.
• So non-luminous matter/energy is 95 of the
  universe!
• Dark energy, with ?DE0.7, seems to be
  increasing the expansion rate.

36
The Accelerating Universe

• Distant galaxies are moving away from us more
  slowly that Hubbles law would indicate.
• So the expansion of the universe is speeding up!
• An unknown source of energy Dark Energy must
  be causing this.

37
(No Transcript)