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Lecture_18 - The Expanding Universe Quasars and...

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1 The Expanding Universe, Quasars and Super-massive Black Holes We measure galaxy distances using a chain of techniques REVIEW THE DISTANCE LADDER Hubble’s Law: velocity = H 0 x distance H0 ~ 22 km/s per million light years (or ~72 km/s per Mega-parsec) Once we know H 0 , the redshift of a galaxy tells us its distance through Hubble’s Law: distance = velocity H 0 H 0 is called Hubble’s Constant; its units are 1/[time] In fact, 1/H 0 is the time since the expansion began (if the expansion has occurred at a uniform rate). SPECTRA REDSHIFT DISTANCE
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How do we interpret Hubble’s discovery that almost all galaxies are moving away from us ? THE EXPANSION OF SPACE. The Universe has no center and no edge (as far as we can tell) Dots of fixed size are galaxies. As the surface of a balloon expands the dots move apart in the same way as galaxies. More distant dots seem to move farther in the same time, which implies that they are going faster. But in fact, it is just the result of a constant rate of expansion. LET THE SURFACE OF THE BALLOON REPRESENT THE UNIVERSE THE UNIVERSE IS EXPANDING! THE UNIVERSE IS EXPANDING! Cosmological Principle The universe looks about the same no matter where you are within it • Matter is evenly distributed on very large scales in the universe • The Cosmological Principle has not been proved but it is consistent with all observations to date Distances between faraway galaxies change while light travels Astronomers think in terms of lookback time rather than distance e.g. the photons traveled through space for 400 million years distance? SPACETIME
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Lecture_18 - The Expanding Universe Quasars and...

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