The chronology of cosmic events after the origin of our universe (i.e. after the Big Bang) has been pieced together by modern cosmology to be as follows:
10-43
second.
The quantum gravity era. As the very hot plasma expanded after the cosmic
explosion, it also cooled. The temperature 10-43 seconds after the
Big Bang was ~1032 K.
10-35
second.
Cosmic inflation created a large patch of space filled with quarks. The
temperature was ~1027 K, and other forms of matter (leptons, gauge
bosons, and several other elementary particles) also appeared, as also
antimatter.
10-30
s.
One potential type of dark matter called axions
gets synthesized. Matter and antimatter are on equal footing at this stage.
10-11
s.
Matter wins over antimatter.
10-10
s.
A second potential type of dark matter called neutralinos gets synthesized. The electro-weak interaction splits
into the electromagnetic interaction and the weak interaction.
10-5
s.
The temperature has fallen to ~1012 K. This is when the quarks
formed the protons and the neutrons, and the antiquarks formed antiprotons. The
collisions between protons and antiprotons left behind mostly protons, as well
as photons.
0.01 - 1 s. Collisions among electrons and positrons
occur, leaving behind mostly electrons.
1 - 300 s.
Enough cooling of the universe has occurred (to ~109 K), so that nuclei
(not atoms yet) of deuterium, tritium, helium, and lithium get formed by the
coming together of protons and neutrons.
380,000 years.
More cooling results in the capture of electrons by the nuclei, so that atoms get formed. This is a major event,
because there is a concomitant release of the till-then-trapped 'cosmic-wave
background' (explained below), which was to be detected by our instruments in
the present era, and which constitutes the strongest validation of the Big Bang
theory. The wavelength of this radiation increases as the universe expands and
cools, and is currently in the microwave regime; hence the name 'cosmic
microwave background' (CMB). The photons of the CMB we observe today started
their journey 380,000 years after the Big Bang from the so-called surface of last scatter (explained
below).
380,000 years
- 300 million years. Gravity continues to amplify the
density differences in the gas that fills space.
300 million
years.
Stars and galaxies appear.
1 billion
years.
One billion years after the Big Bang is the present limit of how far into the
past our instruments can give us data about the conditions at that time.
3 billion
years.
This is when the formation of stars peaked. This is also when clusters of
galaxies formed.
9 billion
years.
Our solar system is formed.
10 billion
years.
Dark energy takes hold and the expansion of the universe begins to accelerate
(cf. Part 15)
13.7 billion
years.
The present.
THE COSMIC
MICROWAVE BACKGROUND
Whenever
charged particles are accelerated, radiation is emitted; i.e., photons are
emitted. The expanding, very hot, plasma soon after the Big Bang was one such
source of photons. But a plasma (i.e. a collection of positive and negative
charges) is also a very good scatterer of photons, so that the photons keep
bouncing around inside the plasma, rather than escaping into outer space. In
due course, as the early universe expanded and cooled, electrons could get
attached to the nuclei, so that electrically neutral atoms got formed. This is
referred to as the 'recombination era' (it was really 'combination
era'!). This combination or recombination happened when the temperature was
around 3000 K ('colour temperature'), and when the universe was
approximately 380,000 years old.
Compared to
ions and the electrons in the plasma, neutral atoms are poor scatterers of
photons. Therefore, when the neutral atoms got formed, our early universe
suddenly changed from opaque to transparent, in the sense that the trapped
photons got a chance to escape and travel into outer space.
This released radiation
not only weakened in intensity as it spread out into space, it also weakened in
energy as the universe cooled. The photons that existed at that time have been
propagating ever since, though getting more sparse and less energetic
('red-shifted'), since the same number of photons fill a larger and larger
volume. The colour temperature of the free-to-escape photons has continued to
diminish ever since; it is now down from 3000 K to 2.725 K, which corresponds
to the microwave regime of wavelengths. The picture below shows what we see
today of this photon background in the empty sky.
This CMB is the after-glow of radiation
left over from the hot early epoch in the evolution of the universe. It is the
red-shifted relic of the hot Big Bang. The CMB was emitted by the hot plasma of
the universe long before there were any planets, stars or galaxies. The CMB is
thus a unique tool for probing the early universe.
This
background radiation is found to be extremely isotropic (better than 1 part in
100,000), but not completely isotropic. As explained in Part 8, the high
isotropy is a result of the inflation era, and the little structure we see is
because of the (amplified) quantum fluctuations in the inflation era.
The surface of
the universe at the time it changed from opaque to transparent (i.e. when
nuclei and electrons combined to form atoms) is called the 'surface of last
scatter'. The CMB comes from that surface.
The CMB map is a very famous picture,
as the presence of the microwave background as a relic of the hoary past
provides the strongest proof for the validity of the Big Bang model.
Amazing how much we humans have been
able to learn about our universe by following the scientific method for understanding
natural phenomena.
You will enjoy watching this video by Stephen Hawking:
PS. And click HERE for a superb all-in-one visualization of our cosmic and human history.
You will enjoy watching this video by Stephen Hawking:
PS. And click HERE for a superb all-in-one visualization of our cosmic and human history.
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