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Lecture 3
The Big Bang and The Cosmic Time
Line
Most scientists agree that the universe
began some 12 to 20 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. 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.
Science tells us nothing about the way space, time,
and matter behaved in our Universe's earliest instant, from the time
of the Big Bang 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. 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 corresponding
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 and antimatter. At this time,
matter, antimatter, and photons existed in equilibrium and in nearly equal
amounts. There is hardly any antimatter left in the Universe today - and
a good thing, too, or we wouldn't exist, as everything would have been
annihilated long ago! What happened to it? This is a question that is still
being debated.
Almost all of the Helium, Deuterium (Hydrogen with an
extra neutron), 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. These conditions -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. Astrophysicists at Johns
Hopkins University recently detected, in the intergalactic
medium, the "primordial helium" formed in
the first two minutes after the Big Bang. This matter,
along with the primordial hydrogen, is sparsely
scattered throughout intergalactic space. Scientists
believe that 98% of the helium 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 about 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
mere 3,000 K (one half of the temperature at the surface
of the Sun). At this temperature, photons
are not 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 20 billion years, this radiation has
cooled to a chilly 2.73 K
(minus 270.43 degrees Celsius!). It's nonetheless 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, 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:
-
The red shift of distant galaxies
-
The cosmic microwave background
-
The relative abundances of chemical
elements
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:
-
How much matter does the Universe contain?
-
What is the source of the Dark Matter?
-
Why does matter dominates over antimatter?
-
Will the Universe expand forever?
To answer these questions, cosmologist are turning to an
impressive array of tools. On the one hand, new and powerful telescopes,
both earthbound and spaceborn, enable them to peer out into space
and back in time as never before. On the other hand, alternative models
of cosmic creation and evolution can be tested in advanced computers. Better
observations, new theories, and computer simulations hold the key to solving
these ancient and fundamental mysteries of our Universe.
