Lecture_19

Lecture_19 - Dark Matter and Dark Energy Review: What is...

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1 Dark Matter and Dark Energy Dark Matter and Dark Energy Review: What is Hubble’s law? Hubble’s law tells us that more distant galaxies are moving away faster . It allows us to determine a galaxy’s distance from the speed at which it is moving away from us, which we can measure from the Doppler redshift in its spectrum. Hubble’s Law: velocity = H 0 x distance REVIEW: Supermassive Black Holes Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion M Sun NOTE: For a 1 Msun black hole the Event Horizon is 3 km and so for M87 it must be 9 billion km. But 20 AU = 3 billion km. So this event horizon is 60 AU in radius which is larger than most of the solar system (40AU radius). QUASARS are very distant (hence very young) and extremely luminous. They are the core of galaxies with very active nuclei. • We know that they are compact because they vary rapidly in brightness over hours, which is the light travel time across something about the size of the Solar System. Each quasar is a BILLION solar mass Black Hole; the luminosity comes from the exceedingly hot accretion disk. • Many nearby galaxies have supermassive black holes at their centers. Many seem to be dormant ; our galaxy (Milky Way) has a modest black hole of 4 million solar masses! • Most galaxies may have passed through a quasar-like stage • There are no “nearby” quasars; most occurred when the universe was about 1/3 its present size, i.e. they are at a “ redshift ”of±2. Review: Quasars Review: Quasars
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2 Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time REDSHIFT = change in wavelength divided by original wavelength; symbol = z The “size” of the universe at a given time is proportional to 1/(1 + z) If z =1 then we see this galaxy as it was when the universe was half (1/2) its present size! What is the universe made of? What is the universe made of? • In Chapter 15 we explored current understanding of the evolution of galaxies in an EXPANDING universe. • However, we left two crucial questions unanswered! – 1. What is the source of the gravity that causes galaxies to form and holds them together against the expansion? – 2. What will happen to the expansion of the universe in the future? In the last two decades we have come to realize that the dominant source of gravity in the universe is a dark form of matter that differs fundamentally from the atoms that make people, stars and galaxies • In addition, there appears to be a dark form of energy throughout the universe that counteracts gravity and is driving the expansion Dark Matter: An undetected form of mass that
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This note was uploaded on 02/20/2012 for the course ASTRO 3 taught by Professor Mclean during the Spring '11 term at UCLA.

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Lecture_19 - Dark Matter and Dark Energy Review: What is...

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