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Transcript
大爆炸---宇宙的起源
『從原子到宇宙』課程第十六週
胡維平
國立中正大學化學暨生物化學系
01/02/2014
1
Static Universe?
Edwin P. Hubble
1889-1953
Discover other galaxies in 1923
Discovery the expansion of the
Universe in 1929
Observation support for the Big
Bang Theory
Mount Wilson Observatory in California
The 100-inch (2.5 m) Hooker
telescope at Mount Wilson
Observatory that Hubble used to
measure galaxy distances and a
value for the rate of expansion of
the universe.
Stellar Parallax
1 parsec = 3.26 ly
1 kpc = 3260 ly
Limit ~ 5000 ly
Henrietta Swan Leavitt
1868-1921
Discover the periodluminosity relationship of
Cepheid variable stars in
1912
magellanic clouds
7
H = 500 km/s/Mpc
T = 1/H = 20億年?
1 pc = 3.26 ly
The factor H0, now called the Hubble constant, is the expansion rate at the present epoch. Hubble's
measurements of H0 began at 550 km s-1 Mpc-1; a number of systematic errors were identified, and
by the 1960s H0 had dropped to 100 km s-1 Mpc-1. Over the last two decades controversy
surrounded H0, with measurements clustered around 50 km s-1 Mpc-1 and 90 km s-1 Mpc-1. There
is now a general consensus that H0 = 70.8 (km/s)/Mpc. The inverse of the Hubble constant - the
Hubble time - sets a timescale for the age of the Universe: 1/H0 = 13.8 Gyr.
http://ned.ipac.caltech.edu/level5/Tyson/Tyson2.html
Separation rate is proportional to distance
Big Bang Theory
In 1927, Georges Lemaître, a Belgian
physicist and Roman Catholic priest,
proposed that the inferred recession of the
nebulae was due to the expansion of the
Universe. In 1931 Lemaître suggested that
the evident expansion of the universe, if
projected back in time, meant that the further
in the past the smaller the universe was, until
at some finite time in the past all the mass of
the Universe was concentrated into a single
point, a "primeval atom" where and when the
fabric of time and space came into existence.
Lemaître's Big Bang theory was advocated
and developed by George Gamow, who
introduced big bang nucleosynthesis (BBN)
and whose associates, Ralph Alpher and
Robert Herman, predicted the cosmic
microwave background radiation (CMB).
1978 Nobel Prize in Physics
Cosmic Microwave Background
The cosmic microwave background (CMB) radiation is an emission of uniform, black
body thermal energy coming from all parts of the sky. The radiation is isotropic to
roughly one part in 100,000. As the universe expanded, adiabatic cooling caused the
plasma to lose energy until it became favorable for electrons to combine with protons,
forming hydrogen atoms. This recombination event happened when the temperature
was around 3000 K or when the universe was approximately 379,000 years old. At
this point, the photons no longer interacted with the now electrically neutral atoms
and began to travel freely through space, resulting in the decoupling of matter and
radiation.The color temperature of the decoupled photons has continued to diminish
ever since; now down to 2.725 K, their temperature will continue to drop as the
universe expands.
too smooth?
WMAP data reveals that its contents
include 4.6% atoms, the building blocks of
stars and planets. Dark matter comprises
23% of the universe. This matter, different
from atoms, does not emit or absorb light. It
has only been detected indirectly by its
gravity. 72% of the universe, is composed
of "dark energy", that acts as a sort of an
anti-gravity. This energy, distinct from dark
matter, is responsible for the present-day
acceleration of the universal expansion.
WMAP data is accurate to two digits, so the
total of these numbers is not 100%. This
reflects the current limits of WMAP's
ability to define Dark Matter and Dark
Energy.
15
Inflation Theory
Inflation is the theorized extremely rapid exponential expansion of the early universe
by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy
density. The inflationary epoch comprises the first part of the electroweak epoch
following the grand unification epoch. It lasted from 10−36 seconds after the Big Bang
to sometime between 10−33 and 10−32 seconds. Following the inflationary period, the
universe continues to expand. The inflationary hypothesis was originally proposed in
1980 by American physicist Alan Guth in 1980.
一般相信在大爆炸之後約 0.0001 秒左右溫
度降至 1012K,此時宇宙中的質子與中子
脫離與輻射線的平衡而成型。到了大約在
大爆炸之後四秒左右溫度降至低於 1010 K,
此時宇宙中的電子也脫離與輻射線的平衡
而成型。至此,構成原子的基本粒子都已
經出現,但由於溫度仍然太高,宇宙中尚
無重於氫的穩定原子核,此時宇宙中充斥
著高速運動的質子、中子、與電子以及非
常高能量的輻射線。
Big Bang Nucleosynthesis
Where did all the atoms come from?
在大爆炸後約三分鐘,質子與中子開
始可以結合而成重氫的原子核而不立
刻被光子所分解。接下來一連串的核
反應將絕大部分的重氫很快的轉變成
包含二個質子及二個中子的穩定氦
(He) 原子核。然而,比氦更重的原子
核此時不易形成因為自然定律中不容
許有原子量為五或八的穩定原子核存
在;而缺乏這些作為橋樑的原子核,
更重的原子核不易快速的被製造出來,
而宇宙仍持續的膨脹、冷卻。在大約
宇宙生成 30 分鐘後,大爆炸所產生的
核反應完全停止進行。此時,宇宙中
的物質以質量而言約含 75% 的質子、
25%的氦原子核、大量很輕的電子、
以及非常微量的重氫及鋰原子核。
Formation of atoms and the last scattering
此時的宇宙溫度仍然非常高
(108 K 左右),強大的輻射線
使電子無法停留在固定的原子
核上,物質主要以單原子離子
狀態存在。由於自由運動的電
子很容易散射光線,此時宇宙
是處於名符其實的混沌狀態;
光子無法自由穿越,輻射場與
物質間不斷的進行能量交換。
這種情況一直持續到了大約四
十萬年後,當宇宙的溫度降到
了約一萬度以下,電子才開始
能與原子核結合型成中性的原
子,宇宙也在此時變成透明,
輻射場與物質間的作用大幅降
低,而重力的作用正開始逐漸
朔造新的宇宙結構。
First Stars
Formed 200-400 million years after Big Bang
Mass: 100-300 solar mass
Lifetime = a few million years
Became Black holes, Supernovaes
Simulated image of the first stars, 400
million years after the Big Bang.
Proton-Proton (PP) Chain in the Sun
T > 10 Million K
Inside the Sun, about 655 million tons of hydrogen are converted into 650 million
tons of helium every second. In stars heavier than about 2 solar masses, in which
the core temperature is more than about 18 million K, the dominant process in
which energy is produced by the fusion of hydrogen into helium is a different
reaction chain known as the carbon-nitrogen cycle.
The CNO Cycle
The carbon-nitrogen-oxygen cycle, a
cycle of six consecutive nuclear
reactions resulting in the formation of a
helium nucleus from four protons. The
carbon nuclei with which the cycle starts
are effectively reformed at the end and
therefore act as a catalyst. This is
believed to be the predominant energyproducing mechanism in stars with a
core temperature exceeding about 18
million K.
The triple alpha process
4He
When the star starts to run out of
hydrogen to fuse, the core of the star
begins to collapse until the central
temperature rises to ~100×106 K. At this
point helium nuclei are fusing together at
a rate high enough to rival the rate at
which their product, beryllium-8, decays
back into two helium nuclei. This means
that there are always a few beryllium-8
nuclei in the core, which can fuse with yet
another helium nucleus to form carbon-12,
which is stable
+ 12C  16O (a process)
Ordinarily, the probability of the triple alpha process would be extremely small.
However, the beryllium-8 ground state has almost exactly the energy of two alpha
particles. In the second step, 8Be + 4He has almost exactly the energy of an excited
state of 12C. These resonances greatly increase the probability that an incoming alpha
particle will combine with beryllium-8 to form carbon. The existence of this resonance
was predicted by Fred Hoyle before its actual observation, based on the physical
necessity for it to exist, in order for carbon to be formed in star.
White Dwarf
Carbon Burning
The carbon-burning process is a set of nuclear
fusion reactions that take place in massive
stars (at least 8 solar mass at birth) that have
used up the lighter elements in their cores. It
requires high temperatures (> 5×108 K ) and
densities (> 3×109 kg/m3)
Oxygen Burning
The oxygen-burning process is a set of nuclear
fusion reactions that take place in massive stars
that have used up the lighter elements in their
cores. It occurs at temperatures around 1.5×109
K and densities of 1010 kg/m3.
Silicon Burning to Iron
After high-mass stars have nothing but sulfur and silicon in their cores, they
further contract until their cores reach temperatures in the range of 2.7–3.5 GK;
silicon burning starts at this point. Silicon burning entails the alpha process
which creates new elements by adding the equivalent of one helium nucleus
(two protons plus two neutrons) per step in the following sequence: (to Fe and
Ni)
For a star with 15 solar mass:
Reaction
Timescale
Hydrogen burning
10 million years
Helium burning
1 million years
Carbon burning
300 years
Oxygen burning
200 days
Silicon burning
2 days
Less and less energy is produced per nuclear reaction in the
nucleosynthesis of these high mass elements. So each burning
phase lasts a shorter and shorter amount of time.
Core-Collapse (Type II) Supernova
R-process
The r-process is a nucleosynthesis process, likely occurring in core-collapse
supernovae (see also supernova nucleosynthesis) responsible for the creation of
approximately half of the neutron-rich atomic nuclei that are heavier than iron. The
process entails a succession of rapid neutron captures (hence the name r-process) on
seed nuclei, typically Ni-56.
Neutron Stars
A neutron star is a type of stellar remnant that
can result from the gravitational collapse of a
massive star during a Type II supernova event.
Such stars are composed almost entirely of
neutrons. A typical neutron star has a mass
between 1.35 and about 2.0 solar masses, with
a corresponding radius of about 12 km
In general, compact stars of less than
1.44 solar masses – the
Chandrasekhar limit – are white
dwarfs, and above 2 to 3 solar masses,
a quark star might be created;
Gravitational collapse will usually
occur on any compact star between 10
and 25 solar masses and produce a
black hole.
D = 5×1017 kg/m3 = 5×108 ton/cm3
Circumstellar rings around SN 1987A, with the ejecta from the
supernova explosion at the center of the inner ring
Black Hole
If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–
Volkoff limit)—either because the original star was very heavy or because the remnant
collected additional mass through accretion of matter—even the degeneracy pressure of
neutrons is insufficient to stop the collapse. No known mechanism (except possibly quark
degeneracy pressure, see quark star) is powerful enough to stop the implosion and the object
will inevitably collapse to form a black hole.
escape velocity > speed of light
Supermassive Black Hole
A supermassive black hole is the largest type of black hole in a galaxy, on the
order of hundreds of thousands to billions of solar masses. Most, and possibly
all galaxies, including the Milky Way, are believed to contain supermassive
black holes at their centers.
http://sandeepdmisra.wordpress.com/2011/04/01/what-will-be-the-fate-of-the-universe/