ISP final - Expansion of the Universe 15.2 &16.4: How...

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How do we know the universe is expanding ? Hubbles conclusion that the more distant a galaxy, the greater its redshift and hence the faster it moves away from us (universal expansion). Observations of distant supernovae indicate that the expansion of the universe is speeding up due to dark energy. How fast is it expanding ? How do we measure this (redshifts and distances)? A galaxy’s redshift tells us how fast it is moving away from us, and the relationship between redshift and distance shows that the universe is expanding. Redshift is either caused by the Doppler effect or cosmological redshift (the expansion of the universe stretches out all the photons within it shifting them to longer/redder wavelengths). What is Hubble's Law ? V=HoXd, more distant galaxies move away from us faster. It expresses a relationship between galaxy speeds and distances and hence allows us to determine a galaxy’s distance from its speed. How do we estimate the age of the universe from the expansion rate ? 14 billion years old, inverse of the Hubble constant (1/Ho) tells us relatively how long the universe has been expanding. Distances measured to faraway white dwarf supernovae indicate that the expansion of the universe is now speeding up. Was it always expanding this fast ? No How can we tell ? Its expanding at an accelerating rate as evident from white dwarf supernovae Will it expand forever? Yes What are the ways we try to determine this ? The connection between the mass and fate of the universe and its shape. e.g. how more or less mass changes the fate of the universe and its geometrical shape. Fate of the Universe 16.4: More on determining the history and predicting the future of the universe. The observable consequences of the different possibilities for our universe. Four possibilities: recollapsing universe -no dark energy & matter density of universe is larger then critical density, the collective gravity will halt expansion and reverse it causing all the galaxy’s to crash back together in a “Big Crunch” (closed universe). Critical universe -no dark energy/matter density of universe=critical density, the collective gravity of all its matter is whats needed to balance the expansion, the universe wont collapse but will expand more and more slowly over time (flat universe). Coasting universe -no dark energy/matter density of universe is smaller then critical density, collective gravity of all its matter cant halt the expansion, the universe will expand forever with little change in its rate of expansion (open universe). Accelerating universe-
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ISP final - Expansion of the Universe 15.2 &16.4: How...

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