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Lecture 36:

Lecture 36:. The Expanding Universe. Review. the ‘zoo’ of observed galaxies and the Hubble Sequence the distance ladder redshift Hubble’s Law and the expansion of the Universe. Hubble Sequence. red blue. Trends along the Hubble Sequence. Lenticular. Spiral. Elliptical.

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Lecture 36:

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  1. Lecture 36: The Expanding Universe

  2. Review • the ‘zoo’ of observed galaxies and the Hubble Sequence • the distance ladder • redshift • Hubble’s Law and the expansion of the Universe

  3. Hubble Sequence

  4. red blue Trends along the Hubble Sequence Lenticular Spiral Elliptical Irregular old stars young stars gas poor gas rich no star formation lots of star formation

  5. The distance ladder • planets • radar ranging • nearby stars • parallax • Milky Way • main sequence fitting • nearby galaxies • Cepheid variable stars • distant galaxies • white dwarf supernovae • Tully-Fisher relation

  6. Tully-Fisher Relation • for spiral galaxies • relationship between rotation velocity and luminosity

  7. measuring recession velocity

  8. reminder: Doppler formula redshift = z = (lobserved-lrest)/lrest

  9. Hubble’s Law recession velocity distance

  10. Hubble’s Law (modern version)

  11. Hubble’s Law Formula v = H0 r recession velocity = constant times distance units of H0: km/s/Mpc best estimates: H0 is between 65 km/s/Mpc and 79 km/s/Mpc

  12. Example: Using Hubble’s Law • The K line of singly ionized calcium has a wavelength of 393.3 nm when measured in the laboratory. In the giant elliptical galaxy NGC4889, this line is observed to be at 401.8 nm. • what is the redshift of this galaxy? • what is its recession velocity? • how far away is it?

  13. The expanding Universe • Hubble’s Law implies that most galaxies are moving away from us • and, the farther away they are, the faster they are moving away from us

  14. The center of the Universe? • does this mean that we are at the center of the Universe? • no – an observer in any other galaxy would see the same thing

  15. Cosmological Redshift

  16. The Cosmological Principle • on large scales, the Universe is homogeneous (uniform) and isotropic (same in all directions)

  17. Is everything expanding? • the expansion of space-time acts like a pressure • where the force of gravity is stronger than the expansion pressure, a gravitationally bound object is formed • for example, the stars in our Galaxy are bound together by gravity, so it does not expand.

  18. t = 1/H0 The Age of the Universe r now v = r/t = H0 r time beginning of time space

  19. The Cosmological Horizon • the distance that light can travel in the age of the Universe is called our cosmological horizon • we cannot receive any information from beyond our horizon • rhorizon = c/H0 = 4300 Mpc (assuming H0 = 70 km/s/Mpc)

  20. Lookback time • time it takes for light to travel from an object to us • directly related to redshift of object – more sensible measure of distance

  21. Coma Cluster

  22. Hercules Cluster (200 Mpc)

  23. Two Million Galaxies

  24. Large Scale Structure

  25. Galaxy Formation and Evolution • How do galaxies form, and how do they change over time? • Why do we see so many different kinds of galaxies? Are their differences a result of ‘nature’ or ‘nurture’? • How do the properties of galaxies depend on their environment?

  26. let’s start from the beginning… • the Universe started out very dense and very smooth (the `Big Bang’). • there were small lumps caused by quantum fluctuations • as the Universe expanded, these lumps grew larger and denser because of the force of gravity

  27. expanding collapsing

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