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Announcements

Announcements. Exam 3 is scheduled for Thursday April 11 (two weeks from today). Format will be 7 MC’s and 4 SA’s (out of 7). Covers chapters 7 – 10. Sample questions are posted. Chapter 10: The Expanding Universe. Measuring the expanding Universe. The Classical Doppler Effect.

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Announcements

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  1. Announcements • Exam 3 is scheduled for Thursday April 11 (two weeks from today). Format will be 7 MC’s and 4 SA’s (out of 7). Covers chapters 7 – 10. Sample questions are posted.

  2. Chapter 10: The Expanding Universe

  3. Measuring the expanding Universe The Classical Doppler Effect

  4. Spectroscopic binaries are examples of the classical Doppler effect

  5. The Relativistic Doppler Effect For objects moving away or towards you at speeds close to the speed of light a different Doppler formula must be used. For the classical Doppler effect z must always be less than 1. For the relativistic Doppler effect z can be anything greater than zero.

  6. So how big is the universe? The Milky Way is the biggest thing we can see with our naked eyes. Is this the extent of the universe? If not, how big is the Milky Way compared to the universe?

  7. The Spiral Nebulae of Lord Rosse Is this something inside the Milky Way or outside it?

  8. William Hershel’s Milky Way Count stars in each direction and then plot it out. The Milky Way is a wide, flat disk shape with the Earth located close to the center.

  9. Harlow Shapley’s Milky Way Herschel’s Milky Way Measure the distance to the globular clusters using RR Lyrae stars and then plot it out. Can’t tell the shape of the Milky Way but it is much larger than Herschel’s Milky Way.

  10. The Shapley – Curtis Debate • April 1920 • Harlow Shapley…The Milky Way is huge so the spiral nebulae must be located within the Milky Way • Heber Curtis…The spiral nebulae are island universes like our Milky Way and thus are not part of it • No definitive conclusion was reached

  11. The Big Problem pointed out by the Shapley – Curtis Debate No one had good measurements of the distance to the spiral nebulae. Without knowing how far away they were we can’t know how big they are. So how do you measure the distance to a celestial object?

  12. Stellar Parallax…too small to measure with the naked eye

  13. Defining the parsec Problem: the largest measured parallax angle is less than one arcsecond. Parallax can only be measured accurately for stars less than 1600 lightyears away. Farther than that and the parallax angle is too small to measure.

  14. Luminosity Distance If we know the intrinsic luminosity of something we can measure its apparent brightness and calculate its distance.

  15. Luminosity can be found for pulsating variables by measuring their period of pulsation

  16. The Tulley-Fisher Relation works for spiral galaxies

  17. If we observe a Type Ia Supernova we can use it to determine the distance to the galaxy

  18. The “Distance Ladder”

  19. Complicating factor to the luminosity distance: extinction

  20. Edwin Hubble looked for Variables in the Andromeda “Nebula” Since the period-luminosity relationship for Cepheid's had been recently determined, their luminosity could be calculated.

  21. Careful examination of photographic plates yielded Cepheid variables

  22. Edwin Hubble: late 1924 The closest of the spiral nebulae, the Andromeda nebulae, is over 1 million lightyears away and is at least 100,000 light-years in diameter. It cannot be part of the Milky Way which Harlow Shapley had determined is less than 100,000 lightyears across.

  23. The original Hubble Diagram Note the most distant object is only 2,000,000 parsecs

  24. Hubble’s mistake was a systematic error in determining distance Hubble’s actual distances were off by over a factor of 2 but their relative distances were the same so his conclusions were still correct.

  25. Hubble Flow The redshift of galaxies is not due to a peculiar velocity but caused by the expansion of space itself. Nearby galaxies may even have peculiar velocities larger than their Hubble Flow velocity.

  26. Einstein’s “Greatest Blunder” • Application of equations of General Relativity to a simplified model of the universe showed it cannot be static • Prevailing view in the 1910’s was a static universe • Full extent of the universe was not even known • Solution • Add a non-zero constant of integration to the equations • The Cosmological Constant L • Made the universe staticbut unstable. Like trying to balance a pin on its head. You may be able to get it to stand upright but any disturbance knocks it over.

  27. The de Sitter Model • Published in 1917, well before Hubble showed the universe to be expanding. • Using Einstein’s equations of general relativity to solve for the universe will require a few simplifications • No matter…the universe is empty. No stars, white dwarfs, neutron stars, black holes, gas, dust, nothing • Space-time & L • Result: Exponentially expanding space • Not widely understood Those that did understand it didn’t accept it as even an approximation of reality

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