Questions on Cosmology & Big Bang

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  1. Identify Einstein's three key contributions to physics he published in 1905.

    1. His work on the photoelectric effect in which he suggested that light was quantised and formed of photons.
    2. Brownian motion. His work explaining the observed random motion verified the existence of atoms.
    3. His special theory of relativity.
  2. In what way is Einstein's theory of special relativity a subset of his later general theory of relativity?

    In his special theory of relativity (1905) Einstein only considered the "special" case of nonaccelerated motion, ie straight line motion with constant speed and the absence of gravity. He then went on in is general theory (1916) to extend his theory to incorporate the more "general" case of accelerated motion or non-inertial reference frames.
  3. What did Einstein state about the relationship between matter and energy in his special theory of relativity?

    Einstein found that matter and energy are related by the now famous relationship:

    E = mc2 (2.1)
    where E is energy, m is the mass of an object and c is the speed of light in a vacuum.
  4. Discuss the implications of your answer to question 3.

    Einstein showed that matter can be converted into energy and energy can be converted into matter.
  5. What key feature did Edwin Hubble notice in the spectra of most galaxies?

    Hubble noted that in most galaxies, the identifying spectral lines were shifted to the longer wavelength (ie redshifted) compared with a reference frame at rest.
  6. How did he interpret these observations?

    Hubble interpreted the observed redshift of galaxy spectral lines to mean that the galaxies were receding from us (more correctly there was relative recession velocity between us and the galaxy). When this is extrapolated over many galaxies he inferred that the Universe must be expanding so that the galaxies seem to move apart from each other.
  7. Outline Henrietta Leavitt's key contribution to resolving the debate about the nature of galaxies.

    Henrietta Leavitt discovered that a class of variable stars, Cepheid variables, obey a period-luminosity relationship; the longer the period of a Cepheid, the more intrinsically luminous it is. This allowed Hubble to observe the Cepheids over time and measure their varying brightnesses to determine their periods. He could then apply the period-luminosity relationship to calculate the distance to the stars and hence the distance to the Andromeda Nebula that they were in. By 1924 Hubble had calculated that the distance to the Andromeda Nebula was 80,000 light years thus placing it beyond the then known scale of our own Milky Way.
  8. Vesto Slipher measured velocities if spiral nebulae ranging from 300 km.s-1 to 1,100 km.s-1. Explain the significance of this work.

    Vesto Slipher's measurements showed that these spiral nebulae such as that in Andromeda were moving faster than any known star in the Milky Way. This then provided further support to the idea that these spiral nebulae were separate "island universe" or galaxies.
  9. Hubble's initial calculations of the age of the Universe based on his Hubble Law relationship gave an age of 2 × 109 or 2 billion years. Why was this problematic?

    Even in Hubble's day geological evidence on Earth suggested that the Earth was somewhere between 3 and 5 billion years old. Obviously the earth could not be older than the Universe so Hubble's value undermined his case.
  10. What are the current best values for the Hubble constant and the age of the Universe (based on WMAP and HST Key Project)?

    Estimates based on observations of the cosmic microwave background radiation (CMBR) by the WMAP probe and other CMBR experiments currently suggest an age of 13.7 billion years ± 1% based on H0 = 71 km. s-1. Mpc-1 +4/-3. This agrees closely with the value determined by the Hubble Key Project team that used the Hubble Space Telescope to observe Cepheids in galaxies and calibrated their values with other techniques. They obtained a value of H0 = 70 km. s-1. Mpc-1 &plusmn 10%
  11. The image below shows four galaxies in the same field of view as observed by the Hubble Space Telescope.

    Four galaxies from the same field observed by the Hubble Space Telescope.
    Credit: Galaxies from an HST image. NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
    1. Rank these galaxies in order of increasing distance from us.
    2. What assumptions did you make in ordering the galaxies in this sequence?
    3. What observation and measurement would an astronomer like to make to confirm this sequence?

    1. Galaxy B is most likely the closest galaxy, followed by A, D, C.
    2. This sequence assumes that all galaxies are much the same physical size and brightness. If so the closest galaxy will be the one that has the largest apparent size in an image whereas the most distant one will appear smallest and dimmest in that same field image.
    3. By obtaining the spectra of each of the galaxies an astronomer could determine the recession velocity of each of the galaxies by measuring the redshift of key spectral lines. This could then be used to calculate the distance to the galaxy based on a given value of Hubble's constant.
  12. George Gamow used Hubble's work to suggest that the early Universe must have been much smaller, hotter and denser than it is now. What observational evidence did he suggest could support his hypothesis?

    He realised that the Universe should be filled with background microwave radiation, the remnant of the original big bang now cooled to about 50 kelvin. This radiation would have the spectral characteristics of a blackbody.
  13. Outline three pieces of observational evidence that are used to support a Big Bang model for the Universe.

    1. Observed recession of galaxies: The consensus among astronomers is that Hubble's relationship between the distance to galaxies and their recession velocity is due to the expansion of space. More distant galaxies or clusters of galaxies exhibit higher redshift of their spectral lines than closer galaxies. This is then interpreted as more distant galaxies receding from us faster than closer ones.
    2. Cosmic Microwave Background Radiation: Detected by Penzias and Wilson in 1965. They eventually realised that this "noise" was in fact remnant radiation from the big bang as predicted by Gamow in the late 1940s. As the Universe expanded it cooled so that today the background radiation corresponds to a temperature of 2.725 K and has a black body spectrum.
    3. Ratios of primordial elements. Astronomers are able to measure the relative amounts of the light nuclei hydrogen, deuterium (an isotope of hydrogen with one proton and one neutron), helium-3, helium-4 and lithium-7 in distant, unmixed clouds of primordial gas. The relative abundances of these nuclei correspond with the calculated predicted ratios from the Big Bang model.
      (Could also discuss observed evolution of extragalactic objects over cosmic timescales).
  14. What was the main competing model to the Big Bang prior to the discovery of the CMBR?

    The Steady State model of Hoyle, Bondi and Gold.
  15. Where did the atoms of hydrogen and most of the helium in the Universe come from?

    Hydrogen and helium were formed in big bang nucleosynthesis. Once the Universe expanded and cooled to about 109 K 3 minutes after the big bang protons and neutrons formed from quarks earlier in the big bang could fuse to form helium nuclei. This mopped up the free neutrons but left free protons. After about 370,000 years the temperature had dropped to about 3,000 K at which point the electrons could join with protons to form hydrogen atoms or helium nuclei to form helium atoms.
    Big bang nucleosynthesis of helium nuclei.
    Credit: CSIRO
  16. What were photons able to do when electrons decoupled to form atoms with nuclei?

    Once electrons could be bound up with nuclei to form atoms photons could travel long distances unimpeded by interactions. The Universe became "transparent" to light. This occurred about 370,000 years after the big bang and is seen today as the CMBR.
  17. Which of the four fundamental forces separated first from the others following the big bang?

    Gravity separated from the others first, leaving the strong and weak nuclear forces combined with the electromagnetic force as a grand unified force.
  18. What would be the products if a proton collided with an antiproton?

    The proton and antiproton would annihilate each other, producing a pair of high-energy gamma photons. The same thing would happen if an electron collided with its antiparticle, the positron.
    Partical-antiparticle annihilations. These events produce pairs of high-energy gamma photons.
    Credit: CSIRO
  19. In general terms, what is the origin of elements heavier than helium?

    Elements heavier than helium including the iron in our bodies, carbon, oxygen and gold all form inside stars. This process is termed stellar nucleosynthesis. the atoms are ejected from stars via various processes during the end stages of a star's "life".
  20. What are two of the major unknown constituents of the Universe?

    1. We currently think that most of the matter in the Universe is in the form of dark matter. The exact nature of this dark matter is not yet known but it interacts gravitationally with other matter yet does not emit radiation (hence dark).
    2. Recent observations suggest that the Universe is not just expanding but is actually accelerating. Many astrophysicists suggest this requires some form of dark energy. As yet we do not know what form this takes.
  21. What is meant by the hierarchical or bottom-up model of galaxy formation?

    In a bottom-up or hierarchical model of galaxy formation present-day galaxies have evolved from mergers and interactions of smaller proto-galaxies and star clusters.