9.1 From Big Bang to Our Solar System and Beyond
Look up at the stars and not down at your feet. Try to make sense of what you see, and wonder about what makes the universe exist. Be curious. Stephen Hawking (1942–)
9.1.1 From the Big Bang to Today
Te word universe is defined as everything that physically exists: the entirety of space and time, all forms of matter and energy, and the physical laws that govern them. Te question that has been puzzling mankind for millions of
years back since our ancestors developed on Earth is how was the universe created? Today, the consensus among scientists is that in the beginning everything in existence is thought to have occupied a single point, or singularity, of infinite high temperature and density and infinitely strong gravity. Ten, matter and energy exploded outwards, covering distances millions of light years every fraction of a second. Te time when the universe officially began to expand in this Big Bang model is estimated by general relativity to have occurred 13.7 billion
years ago. Interpretations of astronomical observations indicate that the age of the universe is 13.73 (± 0.12) billion years, and that the diameter of the observable universe is at least 93 billion light-years, or 8.8 × 1023
km.
Te universe as we know it started with the Big Bang. Te cosmic ‘inflation’ epoch, lasting from 10−36
seconds (s) to 10−32
s, had a kind of repulsive gravity that kick-started the universe. In this period there was practically no matter or radiation. Te universe was dominated by energy inherent to space itself, and it expanded exponentially. Tings were pushed apart rather than attracted in an extremely rapid (exponential) expansion of the very early universe flattening out any large-scale inhomogeneities in temperature and density, rapidly creating a large cosmos out of a much smaller one. Te outwards
Figure 9.2: Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way (centre). The observable universe – the part of the universe that is visible to us on Earth – contains about 2 trillion galaxies. The galaxies are colour-coded by redshift with numbers in parentheses. Astronomers use the term ‘redshift’ when describing how far away a distant galaxy is. To understand what a redshift is, imagine a police car with its siren on going by. As the sound waves from the siren move toward you, they are compressed into higher frequency sound waves (increased pitch). As the siren moves away from you, its sound waves are stretched into lower frequencies (decrease in pitch). This shifting of frequencies is what is known as the Doppler effect. In the same way as for sound, the frequency of light when observed can be lower than the frequency of light emitted at the source. Specifically, the spectrum of light emitted by distant galaxies moving fast away from us is shifted to lower frequencies – towards the red end of the spectrum – when compared to the spectrum of closer stars. This is the redshift phenomenon, interpretable as the galaxies are moving away from us; and more distant galaxies are moving away from us faster. This result, called Hubble’s Law, is expected for an expanding universe, and is another piece of evidence in support of the Big Bang model. Hubble’s Law states a proportionality between the distance D to a galaxy and its observed velocity v away from us: D=v/H0
, where H0 is the Hubble constant, generally believed to be in the range of 45–90 km/s/Mpc (Mpc represents
megaparsec, or 1 million parsecs, when 1 parsec (pc) = ~3.26 light-years or 31 trillion km). The fact that we on Earth see other galaxies moving away from us does not imply that we are the centre of the universe. All galaxies will see other galaxies moving away from them in an expanding universe. A rising loaf of raisin bread is a good visual model: each raisin will see all other raisins moving away from it as the loaf expands.
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