Transcript Document

Recent advances in physics and
astronomy --- our current
understanding of the Universe
Lecture 4: Big Bang, the origin of the
Universe
April 23th, 2003
Universe to Cherokee
•Cherokee, now a famous brand for many merchandise, is an
American Indian tribe who lived in South Carolina for centuries.
• The legends abound in the Cherokee culture believed the
Universe was made up of three separate worlds: the Upper
World, the Lower World, and This World and we are living in
“This World”.
• Four directions, East, North, South and West are each
associating with a color (red, blue white and black).
This rather primitive view of our Universe, like those advocated
by European astronomers some 500 hundreds year ago ----(that
universe is composed of spheres within spheres) ignores the
time flow and regards the universe being static.
Olbers’s Paradox
Why is the night sky dark?
Why isn't the night sky as
uniformly bright as the surface
of the Sun? If the Universe has
infinitely many stars, then it
should be. After all, if you move
the Sun twice as far away from
us, we will intercept one quarter
as many photons, but the Sun
will subtend one quarter of the
angular area. So the area
intensity remains constant. With
infinitely many stars, every
angular element of the sky
should have a star, and the entire
heavens should be as bright as
the sun. This is Olbers' paradox.
So… what is the explanation?
•
There's too much dust to see the distant stars.
Wrong! Dust will be heated and radiate light!
•
The Universe has only a finite number of stars.
Mmmmm! If number of stars are finite, so is the size of Universe?
•
The distribution of stars is not uniform. So, for example, there could be
an infinity of stars, but they hide behind one another so that only a
finite angular area is subtended by them.
Why is earth so special that stars align in a such a way?
•
The Universe is expanding, and distant stars leave us faster, so photons
from distant stars are harder to reach us. ( Bingo!)
•
The Universe is young. Distant light hasn't even reached us yet.
(Bingo!)
The Cosmological principle
To study the universe as a whole, we need
•General Relativity describes space and time, and its relation with
the matter contained in it: "Matter tells space how to curve, and
space tells matter how to move."
• an assumption about how matter is distributed in the universe.
Cosmology principle: the matter in the universe is homogeneous
and isotropic when averaged over very large scales.
The Birth of Big Bang Theory
•In 1927, Georges Lemaître (a Belgian priest) firstly
proposed that the universe began with the explosion of a
primeval atom.
•Edwin Hubble, in 1929 found found that distant galaxies
in every direction are going away from us with speeds
proportional to their distance.
Top three reasons (and tests) to believe big bang cosmology
1. Hubble Expansion
2. Cosmic Microwave Background
3. Big Bang Nucleosynthesis
Observation of 1929.
Geometry of Universe
The geometry of the
universe strongly depends
on how much matter is
contained within it.
There exist a critical
energy density, above
which, our universe is
“closed”, and below
which, our universe in
“open”.
The curvature of the Universe
Orange: closed
Green: flat
Blue: open, but
decelerating
Red: open and
accelerating ( the
likely scenario for
our own Universe)
An expanding Universe, how?
•Assuming we are living in a 2-D world.
For example, on a surface of a balloon.
• An expanding universe can be think of
the processing of airing the balloon.
• There is no center, i.e. no special point
on the surface. Each point can be
regarded as the center.
• The distances (measured on the surface)
between any two points will double all at
the same time.
Hubble’s law
Hubble’s law: Galaxies are
receding from each other at
speeds between 18 million
km/hr and 72 million km/hr.
corresponding to length
scales of 200 million light
years and 800 million light
years.
•Hubble constant value: Ho=
71 km/sec/Mpc (with a margin
of error of about 5%)
A short history of the Universe
Test of Big Bang
The expansion of the universe
Edwin Hubble's 1929 observation that galaxies were
generally receding from us provided the first clue that the
Big Bang theory might be right.
The cosmic microwave background (CMB) radiation
The early universe should have been very hot. The cosmic
microwave background radiation is the remnant heat leftover
from the Big Bang.
The abundance of the light elements H, He, Li
The Big Bang theory predicts that these light elements
should have been fused from protons and neutrons in the
first few minutes after the Big Bang (next lecture).
Signal or Noise?
•Predicted by George
Gamov in 1948.
• Observed by Robert
Wilson and Arno Penzias
in 1965.
1978 Nobel Prize winners
EM radiation and its classification
Wavelength (m)
Frequency (Hz)
Energy (J)
Radio
> 1 x 10-1
< 3 x 109
< 2 x 10-24
Microwave
1 x 10-3 - 1 x 10-1
3 x 109 - 3 x 1011
2 x 10-24- 2 x 10-22
Infrared
7 x 10-7 - 1 x 10-3
3 x 1011 - 4 x 1014
2 x 10-22 - 3 x 10-19
Optical
4 x 10-7 - 7 x 10-7
4 x 1014 - 7.5 x 1014
3 x 10-19 - 5 x 10-19
UV
1 x 10-8 - 4 x 10-7
7.5 x 1014 - 3 x 1016
5 x 10-19 - 2 x 10-17
X-ray
1 x 10-11 - 1 x 10-8
3 x 1016 - 3 x 1019
2 x 10-17 - 2 x 10-14
Gamma-ray
< 1 x 10-11
> 3 x 1019
> 2 x 10-14
Various spectrums in astronomy
What is cosmic microwave
background radiation?
COBE
Cosmic Background Explorer
(COBE) Three experiments:
•Far Infrared Absolute
Spectrophotometer (FIRAS):
compare the spectrum of the cosmic
microwave background radiation with
a precise blackbody.
•Differential Microwave
Radiometer (DMR) : map the cosmic
radiation sensitively, searching for
anisotropy.
•Diffuse Infrared Background
Experiment (DIRBE): search for the
cosmic infrared background radiation
in the wavelength range of 1.25 to 240
microns.
Launched on Nov. 18th, 1989.
Results from COBE
•FIRAS - The
cosmic microwave background (CMB) spectrum is that of a
nearly perfect blackbody with a temperature of 2.725 +/- 0.002 K. Agrees
with the hot Big Bang theory. All of the radiant energy of the Universe was
released within the first year after the Big Bang.
•DMR - The
CMB was found to have intrinsic "anisotropy" for the first
time, at a level of a part in 100,000. These tiny variations in the intensity of
the CMB over the sky show how matter and energy was distributed when
the Universe was still very young.
DIRBE - Infrared absolute sky
brightness maps in the wavelength range
1.25 to 240 microns were obtained to carry out a search for the cosmic
infrared background (CIB). Subsequent analyses have yielded detections of
the CIB in the near-infrared DIRBE sky maps. The CIB represents a "core
sample" of the Universe; it contains the cumulative emissions of stars and
galaxies dating back to the epoch when these objects first began to form.
The anisotropy from COBE
•In 1992, (COBE) detected tiny fluctuations in the cosmic
microwave background. It found, for example, one part of the
sky has a temperature of 2.7251 Kelvin (degrees above
absolute zero), while another part of the sky has a temperature
of 2.7249 Kelvin.
•These fluctuations are related to fluctuations in the density of
matter in the early universe and thus carry information about
the initial conditions for the formation of cosmic structures
such as galaxies, clusters, and voids.
• COBE had an angular resolution of 7 degrees across the sky,
14 times larger than the Moon's apparent size. This made
COBE sensitive only to broad fluctuations of large size.
Why fluctuations?
•Gravity can enhance the tiny fluctuations seen in the early
universe, by can not produce these fluctuations.
Two popular ideas are:
•Inflation
•Topological Defects
• Different predictions about the properties of the cosmic
microwave background fluctuations. For example, the
inflationary theory predicts that the largest temperature
fluctuations should have an angular scale of one degree, while
the defect models predict a smaller characteristic scale.
From COBE to WMAP
•Need better angular
resolution.  The Wilkinson
Microwave Anisotropy Probe
(WMAP).
•WMAP is able to map the
relative CMB temperature
over the full sky with an
angular resolution of at least
0.3°, a sensitivity of 20 µK
per 0.3° square pixel, with
systematic artifacts limited to
5 µK per pixel.
The WMAP spacecraft
Launched to L2 point on June 30, 2001
Looking into early Universe
Results from WMAP
•the first generation of stars first ignited only 200 million
years after the Big Bang, much earlier than many
scientists had expected.
•the age of the Universe is decided to be 13.7 billion
years old, with a remarkably small 1% margin of error.
• The Inflation theory about early universe is likely true.
•The contents of the Universe include 4% atoms
(ordinary matter), 23% of an unknown type of dark
matter, and 73% of a mysterious dark energy.
Results from WMAP
Results from WMAP
Beyond Big Bang
•Why is the universe so uniform on the largest length scales?
•Why is the physical scale of the universe so much larger than
the fundamental scale of gravity, the Planck length, which is
one billionth of one trillionth of the size of an atomic nucleus?
•Why are there so many photons in the universe?
•What physical process produced the initial fluctuations in the
density of matter?
Inflation and its prediction
•That the density of the universe is close to the critical
density, and thus the geometry of the universe is flat.
•That the fluctuations in the primordial density in the
early universe had the same amplitude on all physical
scales.
•That there should be, on average, equal numbers of hot
and cold spots in the fluctuations of the cosmic
microwave background temperature.
Inflation phase
References
• Webpages:
1) A primer of cosmology http://map.gsfc.nasa.gov/m_uni.html
2) A tutorial about universe http://imagine.gsfc.nasa.gov/index.html
• Books:
1) Before the beginning by Martin J. Rees
2) The first three minutes by Steven Weinberg
3) A brief History of time: From the Big Bang to Black holes by
Stephen Hawking
4) The Inflationary Universe: The Quest for a New Theory of
Cosmic Origins by Alan Guth
5) Cosmos by Carl Sagan