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D3

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D3

  1. 1. ASTROPHYSICS D3 Cosmology
  2. 2. THE BIG BANG MODEL
  3. 3. The Big Bang You are required to know the basic theory behind the stages of development of the Universe. Detailed accounts are not necessary. Just a general explanation.
  4. 4. The Big Bang At the start, the Universe would have been very hot. As it expanded, the Universe cooled to a temperature where atoms could be formed.
  5. 5. Background radiation In 1960 two physicists, Dicke and Peebles, realising that there was more He than it could be produced by stars, proposed that in the beginning of the Universe it was at a sufficiently high temperature to produce He by fusion. In this process a great amount of highly energetic radiation was produced. However, as the Universe expanded and cooled, the energy of that radiation decreased as well (wavelength increased). It was predicted that the actual photons would have an maximum λ corresponding to a black body spectrum of 3K. So, we would be looking for microwave radiation.
  6. 6. Background radiation Shortly after this prediction, Penzias and Wilson were working with a microwave aerial and found that no matter in what direction they pointed the aerial it picked up a steady, continuous background radiation.
  7. 7. Background radiation In every direction, there is a very low energy and very uniform radiation that we see filling the Universe. This is called the 3 Degree Kelvin Background Radiation, or the Cosmic Background Radiation, or the Microwave Background. These names come about because this radiation is essentially a black body with temperature slightly less than 3 degrees Kelvin (about 2.76 K), which peaks in the microwave portion of the spectrum.
  8. 8. Background radiation Why is the background radiation an evidence for the Big Bang? The cosmic background radiation (sometimes called the CBR), is the afterglow of the big bang, cooled to a faint whisper in the microwave spectrum by the expansion of the Universe for 15 billion years (which causes the radiation originally produced in the big bang to redshift to longer wavelengths).
  9. 9. Big Bang The Big Bang Model is a broadly accepted theory for the origin and evolution of our universe. It postulates that 12 to 14 billion years ago, the portion of the universe we can see today was only a few millimetres across. It has since expanded from this hot dense state into the vast and much cooler cosmos we currently inhabit. We can see remnants of this hot dense matter as the now very cold cosmic microwave background radiation which still pervades the universe and is visible to microwave detectors as a uniform glow across the entire sky.
  10. 10. Big Bang The singular point at which space, time, matter and energy were created. The Universe has been expanding ever since. Main evidence: Expansion of the Universe – the Universe is expanding (redshift)  it was once smaller  it must have started expanding sometime  “explosion” Background radiation  evidence of an hot Universe that cooled as it expanded He abundance  He produced by stars is little  there is no other explanation for the abundance of He in the Universe than the Big Bang model.
  11. 11. Galactic Motion We learned earlier that most Galaxies are moving away from each other. Expansion of the Universe. Evidence- Red Shift of stellar spectra. Further away… greater the Red Shift.
  12. 12. Galactic Motion A method of determining the recessional speed of a galaxy away from Earth is determined using the equation… Where  = the difference between the spectral line from a stationary source and the spectral line from the receding galaxy.  = spectral line of the stationary source.    c
  13. 13. Example  A characteristic absorption line often seen in stars is due to ionized helium. It occurs at 468.6 nm. If the spectrum of a star has this line at a measured wavelength of 499.3 nm, what is the recession speed of the star?
  14. 14. Solution 499.3468.6 468.6   3 x 108 v1.97 x107 ms-1    c
  15. 15. Hubble’s Red Shift Law We have just seen how we can determine a galaxies recessional velocity from its Redshift. We can also determine the galaxies distance.
  16. 16. Hubble’s Red Shift Law Hubble discovered a relationship between a galaxies distance and its recessional velocity.
  17. 17. Hubble’s Red Shift Law Distant galaxies were receeding very fast. This fits with the expanding Universe Theory.
  18. 18. Hubble’s Red Shift Law  v ≈ d  v = Hd  Where H is Hubbles Constant  It is the slope of the graph.
  19. 19. Hubble’s Red Shift Law Hubble’s Constant is estimated to be around 65 km s-1Mpc-1 Much debate on the accuracy of this value. Try this Raisin Bread analogy animation
  20. 20. Redshift z and cosmic scale factor R
  21. 21. Fate of the Universe Universe Closed Open Enough matter  density is large enough to prevent an infinite expansion  gravity will stop the Universe expansion and cause it to contract (Big Crunch) Not enough matter  density is such that gravity is too weak to stop the Universe expanding forever Flat Critical density  Universe will only start to contract after an infinite amount of time
  22. 22. Doppler effect In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Radiation is redshifted when its wavelength increases, and is blueshifted when its wavelength decreases.
  23. 23. Doppler effect Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth.
  24. 24. Doppler effect Why is Doppler effect so important? In 1920’s Edwin Hubble and Milton Humanson realised that the spectra of distant galaxies showed a redshift, which means that they are moving away from Earth. So, if galaxies are moving away from each other then it they may have been much closer together in the past Matter was concentrated in one point and some “explosion” may have thrown the matter apart.
  25. 25. Critical density The density of the Universe that separates a universe that will expand forever (open universe) and one that will re- collapse (closed universe). A universe with a density equal to the critical density is called flat and it will expand forever at a slowing rate. So, how do we measure the density of the Universe?
  26. 26. Critical density If we take in account all the matter (stars) that we can see then the total mass would not be enough to keep the galaxies orbiting about a cluster centre. So, there must be some matter that can not be seen – dark matter. This dark matter cannot be seen because it is too cold to irradiate. According to the present theories dark matter consists in MACHO’s and WIMPS
  27. 27. MACHO’s WIMP’s Massive compact halo objects – brown and black dwarfs or similar cold objects and even black holes. Non-barionic weakly interacting massive particles (neutrinos among other particles predicted by physics of elementary particles) It seems that there is also what is called “dark energy”…
  28. 28. The type Ia supernova evidence for an accelerated universe has been discussed by Perlmutter and the diagrams below follows his illustration in Physics Today.
  29. 29. One of the foundations for the big bang model was the observed expansion of the universe. Measurement of the expansion rate has found that the expansion rate is very nearly "flat". That is, the universe is very close to the critical density, above which it would slow down and collapse inward toward a future "big crunch". One of the great challenges of astronomy and astrophysics is distance measurement over the vast distances of the universe.
  30. 30. Since the 1990s it has become apparent that type Ia supernovae offer a unique opportunity for the consistent measurement of distance out to perhaps 1000 Mpc. Measurement at these great distances suggests that the expansion rate of the universe is actually accelerating. That acceleration implies an energy density that acts in opposition to gravity which would cause the expansion to accelerate. This is an energy density which we have not directly detected observationally and it has been given the name "dark energy".
  31. 31. TOK  Scientists claim our knowledge of the universe is based upon 5% of what is in the universe. Can we claim to know anything about the universe?  Are there other ways besides Science to explain the universe? What happens when these alternatives meet? Is one right and the other wrong?

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