UK GCSE level age ~14-16, ~US grades 9-10 PHYSICS revision notes re-edit 25/05/2023 [SEARCH]

Cosmology: 5. How does the red-shift relate to the Hubble equation and our big bang theory of the universe and its origins?

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(5) How does the red-shift and the Hubble equation provide evidence for the big bang theory of the universe and its origins?

  • How do we know the universe is expanding?

Reminder of the spectra diagram from Part 4, showing the 'red shift' of the spectral lines of hydrogen.

  • Measurements of the red-shift seem to show that distance galaxies are receding away from us in all directions at incredible speeds
  • From the red-shift spectra data (illustrated above) and other astronomical data such as the distance to the galaxy, calculations by Hubble and others showed that ...
    • all the known galaxies of our universe appear to be moving away from each other in an ever expanding universe, and it doesn't matter in which direction you look,

    • the further away the galaxies are, the faster they are moving, and the bigger the observed increase in wavelength, the bigger the red shift,

    • and Hubble discovered an approximate relationship between the red shifts of such "nebulae" and the distances to them with the formulation of his Hubble's law,

    • and all calculations indicate that the whole universe is expanding and the red-shift data is the most important evidence to support this theory!

  • The observed red-shifts provides evidence that the universe is expanding and supports the ‘Big Bang’ theory (that the universe began from a very small initial point).

    • It appears, that no matter which direction you look, the galaxies are moving away from us because all the absorption spectrums are 'red-shifted', and, what is more, the further away the galaxy, the bigger the red shift.

      • A good analogy is blowing up a balloon with spots marked all over the surface.

      • The spots move apart in ALL directions as the balloon expands.

      • The spots represent galaxies spread throughout the Universe.

    • This means that the galaxies are not only moving apart, but they are accelerating away from each other to the known visible-detected limits of the universe.

      • Observations of supernovae over the past 20 years, suggest that the more distant the galaxy, the faster it seems to be moving away from us.

    • This is the prime evidence for the an expanding universe from some common point 14 billion years ago, that galaxies all have a common origin, and that point's age is calculated by working back from the equations representing the expansion of the universe to the current rate of expansion!

      • The red-shift and universe expansion are both explained by both the Steady State and Big Bang models of the Universe,

        • BUT the observations fit better with the Big Bang theory because you seem to be able to work backwards to a beginning,

        • something you can't do with the Steady state theory, which, does not fit in with evidence from observations of the cosmic background microwave radiation (see last section 5.).

    • The theory is, that around 14 billion years ago there was some kind of enormous 'Big Bang' explosion resulting in the release of huge amounts of energy in some form from a very small space, and, that eventually, after 'cooling down' the 'particles' formed atoms, stars and galaxies etc. and all the resulting galaxies we see are flying apart from this point of origin.

      • Its as if all the matter and energy of the Universe was compressed into some incredibly small volume of space (of extraordinary high density) and then 'something' triggered this great cosmological explosion.

      • The result we see around us in the night sky!

    • We have no idea about the origin of the 'Big Bang', all we can theorise is that our universe originated from this point (zero time!?).

      • We also do not know how the current rate of expansion compares with that soon after the big bang.

      • This means are calculations of the age of the Universe may not be correct.

    • Incidentally, if the universe was contracting and galaxies were hurtling towards us, we would observe a blue shift of decreasing wavelengths in the emitted light from them, but no so such effect has ever been seen (as far as I know?).

      • It might be that gravity acting between all the mass eventually slows the expansion, perhaps a contraction starts, nobody really knows!

      • The alternative 'Steady State Theory' suggests that the Universe has always existed as it does now and isn't really changing because it looks the same in all directions. This theory says the red-shift is due to matter being created in the spaces as the Universe expands.

      • However, the 'Big Bang' theory is the once accepted by most scientists today, but other theories will emerge as more scientific data is gathered about our Universe.

    • Unanswered questions !!!

      • Many scientists believe much of the Universe is made up of 'dark matter' an 'dark energy'.

        • Calculation of the mass of the galaxies suggests there is not enough to account for orbiting stars around the centre of a galaxy under the influence of gravity. The extra 'unseen' matter is called dark matter.

        • 'Dark matter' may hold the Universe together and something called 'dark energy' might be the cause of the apparently accelerating expansion of the Universe. BUT, we don't really know anything about the 'dark stuff', its just a theoretical idea without experimental 'detection' or proof', so we all have to wait on this one!

        • Apparently dark matter doesn't emit electromagnetic radiation - so, unlike most material in the Universe, its a bit difficult to detect - very frustrating! and difficult to test the theory!

    • The Hubble Equation

      • Despite the fact that neither dark matter or dark energy is understood, there is one equation that fits in with a 'big bang' theory and experimental data (at least at the moment).

      • It is possible to use the following equation to calculate, given appropriate data, the speed of recession of a distant galaxy, and the Hubble constant or the distance to the galaxy:

      • speed of recession = Hubble constant × distance

      • speed in km/s

      • Hubble Constant s-1 or km/s per Mpc (megaparsecs)

      • distance in km or Mpc

      • The accepted value for the age of the universe based on current evidence is ~14 billion years.

      • The Sun and Solar system, including planet Earth, is estimated to be about 4.5 billion years old.


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