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Hubble’s Law

Hubble’s Law. Hubble was an American astronomer, born in Missouri. In the 1920’s Hubble discovered a relationship between recessional velocities of galaxies and the distances of those galaxies from Earth. (See next slide). Edwin Hubble. Edwin Hubble at Mt. Wilson.

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Hubble’s Law

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  1. Hubble’s Law • Hubble was an American astronomer, born in Missouri. • In the 1920’s Hubble discovered a relationship between recessional velocities of galaxies and the distances of those galaxies from Earth. • (See next slide) Edwin Hubble

  2. Edwin Hubble at Mt. Wilson Hubble’s observations at the 100 inch during the 1920’s led him to the conclusion that the universe is expanding, and that an object’s recession velocity is proportional to its distance from the observer. Hubble guiding the Hooker 100 inch telescope in 1923. The Hooker 100 inch telescope atop Mt. Wilson near Pasadena, CA. It was the largest telescope in the world from 1917-1947. Photos courtesy Mt. Wilson: http://www.mtwilson.edu/History

  3. Hubble’s Law • The following pictures all magnified equally. • Virgo is closest and Hydra is furthest from Earth. • Notice that the redshift is greatest for Hydra, and therefore the recessional velocity is also greatest. • Recessional velocity is proportional to distance (Hubble’s Law)

  4. Definition of Various Distance measuring tools • Astronomical Unit (AU) = distance from Earth to the Sun (93,000,000 miles) • Light year (ly) = distance light travels in 1 year. (3 x 108 m/s)(1year) = 9.46 x 1012km = 63,240 AU (only 6 trillion miles!) • Parsec = 3.26 ly (19,560,000,000,000 miles!)

  5. Hubble’s Law – Consider the universe to be expanding like a balloon. Let Galaxy A = Milky Way Then we are 100Mpc from both galaxies B and E at time t=0 We are 200Mpc from galaxy C, etc.

  6. Hubble’s Law – Consider the universe to be expanding like a balloon. A time t=1 later, what is our distance from galaxies B and E? From C?From D? So What?

  7. Hubble’s Law – Consider the universe to be expanding like a balloon. • The so what is that … • The separation of galaxies is proportional to the original separation of the galaxies. • Since for each equal time interval the separation will increase, then we can say…

  8. 1000 Recession Velocity (km/sec) 500 0 0 2 1 Distance (Mpc) Hubble’s Data (1929) Not Very Impresive

  9. 20,000 Recession Velocity (km/sec) 15,000 10,000 5000 1929 Data 20 30 10 Distance (Mpc) Hubble & Humason (1931)

  10. Hubble’s Law – A closer look (PSRT) This graph plots the recessional velocities of 20 galaxies

  11. Hubble’s Law – A closer look (PSRT) Hubble’s Constant is… Slope = 75Km/sec/Mpc

  12. Limitations to Hubble’s Law • A common misconception is that all galaxies have positive recessional velocities. (i.e. they are moving away from the Milky Way) • Consider galaxy M-31 (Andromeda) • This galaxy actually has a blueshift

  13. Limitations to Hubble’s Law • The problem is that the relative velocities of neighboring galaxies is relatively small. • Conversely, at larger distances, a number of methods can be used to determine the distance. • Parallax, absolute and apparent magnitude, Cepheid variables. (stay tuned) • The problem is that there is a vast disagreement what the Hubble Constant actually is. • The range is 40 – 90 km/s/Mpc

  14. Calculating the Age of the Universe • Consider a galaxy • measure its distance d • measure recession velocity v • Combining • With • We see that This is called the Hubble time. Its important to recognize it is the same for all of the galaxies

  15. Calculating the Age of the Universe • This time = the origin of the universe (galaxies must have been collected at the same location of space and time. • The slope of this graph gives a Hubble Constant of…

  16. Calculating the Age of the Universe • Substituting for H and converting to correct units, the age of the universe is… • (Note: 1Mpc = 3.09 x 1010 years • 1 year = 3.156 x 107 seconds)

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