Cosmology
The Study of Genesis ... |
Steven Weinberg - a leading theoretical physicist and
co-recipient of the 1979 Nobel Prize in Physics - cites "What
could be more interesting than the problem of Genesis" as
the main reason for writing his popular book "The
First Three Minutes"
Some of questions that we will attempt to answer are:
Olbers' Paradox and the Necessity
of a Beginning
Olbers' paradox (1826) -- A
look at the night sky suggests a changeless Universe, apart from
local (small-scale) phenomena, such as the clouds drifting across the Moon.
Hence, suppose that on large-scale the Universe is static, infinite, eternal,
and uniformly filled with stars. Then one must
reach the conclusion that every point on the night sky must be as bright
as the surface of a star. The reason for this (seemingly crazy)
conclusion is that the number of stars at any given distance increases
with the square of the distance, while the intensity of light from a star
decreases also with the square of the distance. In this way we should live
in the center of a hollow blackbody whose temperature is (like that of
the Sun) about 6000 degrees. This is Olbers' paradox, which can be traced
as far back as Kepler in 1610, rediscussed by Halley and Cheseaux in the
eighteen century, but was not popularized as a paradox until Olbers took
up the issue in 1826. There are many possible explanations which have been
considered. Here are a few:
Cosmological Principle:
On a large scale, the Universe is Homogeneous
and Isotropic. Moreover, we believe
in the Universality of the Laws of
Physics. To say that the Universe is homogeneous means that any measurable
property of the Universe is the same irrespective of the position of the
observer in the Universe. The Isotropy of the Universe means that the Universe
looks the same in all directions. These statements are only approximately
correct at short distances (for example, for an observer looking only at
the Solar System) but they appear to be an excellent approximation when
one averages over very large regions in space. That the laws of physics
are universal suggests that object, such as apples, obey the same laws
here on Earth than on any other celestial body. Thus, the cosmological
principle represents a large-scale generalization of Copernicus viewpoint:
We do not live in a special place in the Universe. Indeed, all places in
the Universe are equally special. Note that accepting the Cosmological
Principle, simple as it may be, has important consequences:
Our
Universe has no Center and no Edges.
The Expansion of the Universe:
In 1916 Albert Einstein published a new theory
of gravity called: "The General Theory of Relativity".
The
theory of GR predicted that the Universe will either expand or contract
depending on the density of matter/energy within it. Yet, even Einstein
did not believe in some of the predictions of his own theory, such as the
idea of a dynamical Universe and, thus, constructed a static model for
the Universe. He called this one of the biggest mistakes of his life!
In 1919 Sir Arthur Eddington, a leading British
astronomer led an expedition to West Africa to study a solar eclipse
and test one of the most important predictions of the newly developed Theory
of General Relativity: The Bending of Light
by the warping of spacetime near the Sun. Eddington through his observation
confirmed Einstein's prediction and thus gave experimental basis to the
General Theory of Relativity.
In 1922 Alexander Friedmann, a Russian mathematician,
abandoned Einstein's model of a static Universe and taking into account
the Cosmological principle constructed a model of an expanding universe.
Friedmann proposed a model for a Dynamical Universe;
recall
that Einstein proposed that the size of the Universe was constant.
Friedmann argued that space and time have to be homogeneous and isotropic
- the Cosmological Principle - and that it should possible for the
average density and radius of the universe to change over time. The Big-Bang
Model developed from Friedmann's theory of an expanding Universe.
In 1929 Edwin Hubble discovered the velocity
distance relation using the red-shift spectra of only 46 galaxies. Since
then, Hubble's law has been confirmed
for a large number of distant galaxies. Hubble's law is clear evidence
that the Universe is expanding uniformly and has no center nor edges. Note
that in the graph below the last point represents a galaxy located more
than one billion light years away from Earth and receding at the colossal
velocity of one tenth of the speed of light!
In retrospect, Hubble's law is not that difficult to
understand once we have adopted the Cosmological Principle. Say three observers
are located in galaxies A, B, and C which are separated by equal amounts.
If B is receding away from A with a certain velocity "v", then, by the
Cosmological Principle, C must be receding from B at the same exact speed
v. This implies, by simply adding the velocities, that C will be receding
from
A at twice the speed (or 2v). This, of course, is Hubble's
Law.
In 1965 Arno Penzias and Robert Wilson made a
monumental discovery. Like many of science's greatest discoveries, the
one that earned Penzias and Wilson the Nobel Prize in 1978 was an event
of pure serendipity. While tuning a small, yet powerful and highly sensitive
horn antenna for conducting radio astronomy experiments, Penzias and Wilson
noted a constant low level noise disrupting their reception. Despite their
efforts, Penzias and Wilson could not find any evidence of malfunction
in their equipment. Moreover, the static persisted regardless of the direction
the antenna was pointing. As they continued their investigation, Penzias
and Wilson came to realize that they indeed had stumbled onto the most
conclusive evidence to date supporting the Big Bang Theory:
The Cosmic Microwave Background
(CMB).
The Cosmic Timeline
The Big Bang
Most scientists agree that the
Universe began some 12 to 15 billion years ago in what has come to be known
as the Big Bang;
a term coined by the English astrophysicist Fred Hoyle in 1950. Hoyle -
who championed a rival cosmological theory - meant the "Big Bang" to be
a term of derision, but the name was so catchy that it stuck. Though the
Big Bang suggests a colossal explosion, it wasn't really an "explosion"
in the sense that we understand it today, such as the majestic Supernova
explosions. At the precise instant of the Big
Bang the Universe was infinitely dense and unimaginably
hot. Cosmologists believe that all forms of matter and energy packed
into a space smaller than the atomic nucleus.
Yet,
science tells us nothing about the way space, time, and matter behaved
in our Universe's earliest instants, from the time of the Big Bang all
the way to 10-43 seconds later.
Immediately after the Big Bang,
spacetime was certainly expanding. Indeed very violently and from this
expansion of space was formed a highly energetic soup of particles and
antiparticles. Antiparticles are not the result of science fiction - they
are simple brothers and sisters of the most commonly known particles, such
as electrons and quarks. For example, the antiparticle of the electron
is called the positron. It looks almost identical to the electron except
that it has opposite electric charge. Physicists have constructed
a family of fundamental
particles, divided into two groups of quarks
and
leptons.
Quarks are the building blocks of protons and neutrons. Electrons,
the most familiar lepton, combines with protons and neutrons to form atomic
nuclei. Also in the lepton class are wispy, nearly massless neutrinos
that
interact only very weakly with other particles. Elusive as they are, neutrinos
are abundant in the Universe and may be (???????) dark-matter candidates.
At about 10-12 seconds, quarks, leptons and their antiparticles
(such as antiquarks and positrons) were constantly colliding and annihilating
each other with a release of energy in the form of photons.
Likewise,
two colliding photons could create matter/antimatter.
At this time, matter, antimatter, and photons existed in equilibrium
and in nearly equal amounts. There is hardly any antimatter left in our
observable Universe today - and a good thing too or we wouldn't exist today
as everything would have been annihilated long ago! What happened to it?
This
is a fundamental question that is still under debate.
Almost all of the Deuterium (Hydrogen with an extra neutron),
Helium, and some of the Lithium nuclei in our Universe today were created
during the
"Era of Nucleosynthesis" which
began about 1 second after the Big Bang and ended just 100 seconds later.
Note that Hydrogen nuclei did not have to be created; they already existed
in the form of the three-quark clusters we now call protons.
One hundred seconds after the Big Bang the temperature dropped to the point
where protons and neutrons could stick together without being torn apart
by the highly energetic photons. This condition, a mere one billion degrees,
were suddenly ripe for the formation of nuclei, the most stable of the
lighter ones being that having two protons and two neutrons: Helium.
At the end of the nucleosynthesis period, all of the neutrons had paired
with protons to form helium,
24% of the primordial
light elements and trace amounts of Deuterium, Tritium (Hydrogen
with two extra neutrons), Helium3 and Lithium. The
protons left over made up the remaining 75% of the Baryonic Matter. Scientists
believe that 98% of the Helium present in the Universe today was produced
- not in stars but - in those first few seconds.
During the next 300,000 years very little happens. For
300,000 years, protons and atomic nuclei continued to roam the Universe
in a almost totally opaque
sea of photons, electrons and neutrinos; opaque because photons couldn't
travel far without bumping into a charged particle. Indeed, any electron
that combined with a proton or with an atomic nucleus was immediately knocked
out by an energetic traveling photon. Matter and
radiation were intimately linked. But after 300,000 years, the
opaque soup of nuclear matter and radiation began to clear. The temperature
of the Universe dropped to a cozy 3,000 K (one half of the temperature
at the surface of the Sun). At this temperature, photons are no longer
energetically enough to knock out electrons from atomic nuclei. Now the
photons were free to travel through the Universe at last decoupled
from matter. This Recombination Era
lasted about one million years. The vast sea of photons created during
the Big Bang persist to this day, in the form of Cosmic
Microwave Background (CMB) that pervades the Universe. No longer
widely energetic after being stretched by the expansion of the universe
for roughly 15 billion years, this radiation has
cooled to a chilly 2.73 K (minus 270.43 Celsius!). The
CMB is considered by cosmologists to be one of the clearest and unavoidable
signatures of the Big Bang.
Tiny variations in the CMB have recently been found by
the COBE
satellite in this background radiation, indicating minute fluctuations
in the density of matter and energy at the time of recombination. These
fluctuations were eventually amplified by gravity to form the objects which
make up our Universe, such as Stars, Galaxies,
Clusters and Superclusters of Galaxies.
Accompanying those minute fluctuations
in radiation were also tiny fluctuations of baryonic matter (mainly Hydrogen
and Helium). Gravitational attraction between the atoms concentrated them
into faint clouds of gas. As the Universe expanded the surrounding matter
gradually thinned out with the result that the internal gravity of the
gas clouds
grew relatively stronger. Slowly, but then faster and faster, the clouds
pulled in more and more material from the surrounding medium. Eventually,
the clouds began to collapse under their own gravity, evolving into galaxies.
About one billion years after the Big Bang, the first galaxies and the
stars they contain were born. Our own
Milky Way galaxy was formed when the Universe was about 3 billion years
old. It started as a huge
sphere of gas. Some stars formed in globular clusters scattered in
a sphere. This is now the halo
of our galaxy. The rest of the gas settled into a disk around its central
bulge and spiral
arms formed.
The Big Bang has been enormously
successful in explaining several properties of the observable Universe:
Stage | Time | Temperature (Energy) | Description |
First | 10-45 to 10-32 sec | Greater than 1015 K (100 GeV) | Inflation; generation of density fluctuations |
Second | 10-6 sec | Greater than 1012 K (100 MeV) | Quark Soup (QG Plasma) |
Third | 10-4 sec to 3 min | 1012 to 109 K (0.1 MeV) | Nucleosynthesis; formation of D, He and Li |
Fourth | 400,000 years | 4,000 K (1 eV) | Formation of neutral atoms; radiation decouples |
Fifth | 1 billion years | 20-3 K (1 meV) | Formation of first-generation stars and galaxies |
Sixth | 3 billion years | 20-3 K (1 meV) | Formation of heavy elements by supernovae;
Formation of second-generations stars. |
Seventh | 3-15 billion years | 3 K (0.25 meV) | Genesis of planets and LIFE |
Yet, not all is well in the Big Bang. For example, there is strong evidence that shortly after the Big Bang the Universe was essentially uniform in its density and appearance. When we peer out to the cosmos today, it's evident that the distribution of matter is far from uniform. In fact it's positively lumpy, even on a large scale, and clearly exhibits a hierarchical organization. As far as we can tell, planets formed sometime during starbirth, giving rise to solar systems such our own. Stars are organized into galaxies, which in turn appear to be bound gravitationally together in clusters. Superclusters of galaxies stretch across hundreds of billions of light years, bounded by enormous voids. How can this evident "lumpiness" be explained? That's but one of the questions challenging cosmologists as they try to explain the Universe we observe today. Other difficult questions about cosmic origins and evolution preoccupy their minds, such as: