Saturday, February 25, 2012

16. Our Cosmic History

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.


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.