Lecture11 Expansion of the Universe

Lecture11 Expansion of the Universe - Expansion of the...

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Expansion of the Universe Hawley & Holcomb, Chapters 10 and 11 4/13/09 1
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Galaxies close to us have negative recession velocities, because they are not receding, instead, they are approaching the Milky Way, because of in the cosmic neighborhood of our own galaxy, the gravity of the galaxy is stronger than the gravity of the universe as a whole. RECESSION VELOCITY DISTANCE 4/13/09 2
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Discovery: Expansion of the Universe • How do we test the velocity-distance law? – We need recession velocities of galaxies: V – We need distances to galaxies: L • How do we measure V ? – We can use cosmological redshifts V = c z – However use of cosmological redshifts as proxy for recession velocities is not strictly correct (see next slide). • How do we measure L ? – There are various ways to estimate distances, but this is usually not easy. Distances can be estimated using standard candles. • Edwin Hubble (1889-1953) estimated recession velocities and distances for a number of galaxies and discovered expansion of the universe in 1929! 4/13/09 3
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“The Universe is Expanding” • The idea of expansion arose from the following line of reasoning: – An astronomer notices that atomic absorption lines in the spectra of faraway galaxies are redshifted – The galaxies seem to be moving radially away from us – Our distance to the far away galaxies seems to be increasing in time • But what does all this really mean? Time 4/13/09 4
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Three Distinct Redshifts Redshift is the fractional amount by which the wavelength of light is stretched . Blueshift is the fractional amount by which the wavelength of light is squeezed. z = (new wavelength - old wavelength) / (old wavelength) Doppler redshift – Wavelength is stretched by relative motion – z = (relative velocity) / (speed of light) – Example: police radar Gravitational redshift – Wavelength is stretched by strong static gravitational field – z = (1/2) [(escape velocity) / (speed of light)] 2 – Example: light emitted by matter about to fall into a black hole is redshifted in this way Expansion redshift – Wavelength is stretched by expansion of space – z = (change of distance between two points in space) / (original distance between these two points in space) – Example: light emitted in the universe at an earlier epoch is redshifted at a later epoch 4/13/09 5
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Peculiar Velocity • Peculiar velocity characterizes the true motion of a galaxy in space, not connected with the expansion of the universe. • Peculiar velocity is caused by the gravitational attraction between neighboring galaxies. • For all but the nearest few galaxies, the true peculiar velocity is much smaller than the apparent recession velocity associated with the expansion of space. • The velocity of a galaxy inferred from its redshift is a sum
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Lecture11 Expansion of the Universe - Expansion of the...

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