Lecture 1 - The Big Bang and the origin of the Universe

Lisa Tauxe

Science and the scientific method

The way in which we inch toward a better understanding of the "Truth".

1) Pose a question:

Where do all the unmatched socks go?

2) Think about possible reasons (form some "hypotheses").

  • The drier destroys them
  • The drier is a worm hole to a parallel universe into which the socks are pulled.
  • They are accumulating somewhere else (someone else's drawers, under the bed, in the corner of the bathroom...)
  • Someone takes them out of the laundry basket (rodents, aliens that live off sock sweat...).
  • 3) Think about ways of refuting or supporting the various hypotheses

  • The first two ideas would predict that fewer socks come out than go in. We would count the socks before and after washing to determine if these are viable hypotheses.
  • Similarly, disappearing from the laundry basket could be demonstrated by counting socks as they go into the laundry basket.
  • If they don't disappear during the laundry operation (basket, washer, drier), then we'll have to think about where they are accumulating.
  • 4) Design and carry out some experiments

  • Set up data sheets for counting of socks as they 1) go into the laundry basket, 2) go into the washer, 3) go into the drier, 4) come out of the drier.
  • Keep careful tallies for a long enough period of time to witness the disappearance of a significant number of socks (10% of the initial population?)
  • Plot up the data to determine at exactly which point the socks are disappearing.
  • Refine the experiment to focus in on the important aspect. Continue experimentation until the culprit is identified beyond reasonable doubt. Please note - you can never be absolutely sure you are right, but you can sometimes be sure you are wrong.
  • 5) The hypothesis that survives experiments and explains the observations with least astonishment (probably not the aliens!) gets to be known as the "Theory of the Missing Socks".

    6) Collect Nobel Prize for solving one of the outstanding mysteries of the Universe.

    7) Theories are not forever. They undergo revision in the light of new evidence. A good theory (e.g. the theory of gravity), lasts a long time.

    The Big Bang: Theory of the Origin of the Universe


    If the universe is infinite - no problem (except that it is infinite!)

    If the universe is finite - then it must be expanding or contracting; a static universe would collapse upon itself from the force of gravity.

    The hypotheses then are:

    Is the universe collapsing, expanding or INFINITE?!

    How to test collapsing or expanding?

    In an expanding universe, all particles are moving away from one another. The furthest particles must be moving the fastest. So we need to measure velocity and distance to lots of stars and galaxies.

    Distance is measured by triangulation". Knowing the length of some "baseline" (say the diameter of the Earth's orbit), and measuring the angle to the galaxy at each end of the baseline, one can calculate the distance to the galaxy using the rules of trigonometry.

    Doppler Effect

    Velocity is measured by the so-called "Doppler Effect". Shorter wavelength sound waves are percieved by humans as being higher in pitch.

    When a siren mounted on a police car approaches us, the sound waves are squished together and the resulting higher frequency of sound reaching our ear sounds higher.

    When the car passes us, the sound drops. If we knew the actual pitch of the siren, we could calculate the velocity of the car from the distortion in pitch. NB: velocity means both speed AND direction!

    Red Shift

    We could use something like the Doppler Effect using light from stars and galaxies instead of sound. Light can be separated into different wavelengths by a prism (remember about rainbows?). In order to determine the true "pitch", we need some reference lines. Fortunately, when light from stars pases through the staellar material (mostly hydrogen and helium), light at certain wavelengths is absorbed, leaving "holes" in the spectrum. From laboratory experiments on Earth, we know exactly what the wavelenths of the spectral lines are supposed to be. If the object (star or galaxy) producing the light is moving away from us, then the spectral lines will be shifted to longer wavelenths or to the more "red" part of the spectrum. Thus the so-called Red Shift observed in spectra from distant galaxies can be used to measure velocity.

    Age of the Universe

    Galactic distance versus retreat velocity

    These data show that the universe is expanding. From slope, we calculate that it began expanding some 15 Billion years ago (give or take 5 Billion!).

    This idea of the exploding universe is called the Big Bang Theory.

    Summary of Important Points

    1) The scientific method

    2) The Law of Gravity

    3) The Laws of Trigonometry

    3) The Doppler Effect

    4) The Big Bang Theory of the origin of the Universe

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    Lisa Tauxe