Hubble’s Law AST 112. Spectra If a light source is moving toward or away from an observer, its...
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Transcript of Hubble’s Law AST 112. Spectra If a light source is moving toward or away from an observer, its...
![Page 1: Hubble’s Law AST 112. Spectra If a light source is moving toward or away from an observer, its spectral lines shift We can use this to measure approaching.](https://reader036.fdocuments.in/reader036/viewer/2022070413/5697bfeb1a28abf838cb8283/html5/thumbnails/1.jpg)
Hubble’s Law
AST 112
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Spectra
• If a light source is moving toward or away from an observer, its spectral lines shift
• We can use this to measure approaching / receding velocity
Lab
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If the light source is moving away from us,the spectral lines will move toward the red.
The amount that they move toward the redis called redshift.
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Motion of Galaxies
Say you take spectra of a bunch of galaxies.Near, far, in every part of the sky.
Do you expect to see a pattern in their approaching / receding velocities?
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Redshift of Galaxies
• In 1912, Vesto Slipher obtained spectra of galaxies
• Distant galaxies all show redshift!
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Edwin Hubble
• Hubble was invited to new 100 inch telescope on Mount Wilson
• “Regret cannot accept your invitation. Am off to the war.”
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Edwin Hubble
• Measured Cepheid variables
• Measured galactic redshifts
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Back to the Spectra
• Here are spectra from several galaxies. Note their distances.
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Not only do distant galaxies all show redshift – there’s a pattern.
The farther away it is, the more redshift it shows.
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Hubble’s Law
For distant galaxies:
The farther away a galaxy is, the faster it ismoving away from us.
This relation is linear. If a galaxy is twice asfar away as another one, it is flying away
from us twice as fast.
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Hubble’s Law
v = H0d
H0 = 22 km/s per MLY
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Hubble’s Law
For every 1 million LY of distance, a galaxy recedes an extra 12-14 miles
per second from us.
(That’s 22 km/s.)
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Random Velocities vs. Hubble Expansion
• Some nearby galaxies are moving toward us– Andromeda is on a collision course
• Galaxies move around– This motion dominates over Hubble motion at
smaller distances
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Random Velocities vs. Hubble Expansion
• Consider Andromeda (2.1 million LY away)
– How fast is the expansion carrying it away?
– Average velocity of group members is 61 km/s
– Will we notice the expansion velocity or the group velocity?
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Random Velocities vs. Hubble Expansion
• Consider a galaxy in the Virgo Cluster (50 million LY away)
– How fast is the expansion carrying it away?
– Assume a cluster member moves at 50 km/s
– Will we notice the expansion velocity or the random velocity?
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Random Velocities vs. Hubble Expansion
• Consider a galaxy in the cluster Abell 2151 (570 million LY away)
– How fast is the expansion carrying it away?
– Assume average cluster member velocity is 1 km/s
– Will we notice the expansion velocity or the random velocity?
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Cosmic Distances
• Hubble’s Law is what we use to calculate distances across the Universe
– Requires large distances: • Expansion effect more measurable at large distances• Expansion overwhelms random motion at large distances
• Redshifts of spectral lines relate directly to a galaxy’s distance
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Verifying Hubble’s Law
• We can compare Type 1a supernovae distances with Hubble’s Law
• Matches up well
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Distances
• When we say that Andromeda is 2.1 million LY away, that’s fine. Why is there more ambiguity in dealing with galaxies that are much farther away?
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NO NO NO!
• Does the stars and planets revolve around Earth?– No! We see parallax and retrograde motion.
• Astronomers thought the Sun was at the center of the Milky Way– No! All because of dust clouds.
• Is the Milky Way at the center of the Universe since everything is expanding away from it?
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Expansion of the Universe
• Everything is flying away from everything else
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What’s going on?
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Cosmological Redshift
• Galaxies are not Doppler shifted.
• The shift does not happen because they are “flying away from us”.
• The shift happens because as the light travels toward us, space expands and stretches the light.
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Cosmological Redshift
• This is in complete agreement with Einstein’s Theory of General Relativity.
• If it’s just a ball of galaxies flying apart:– We’d have to be right at the center, otherwise
there’d be asymmetry in the redshifts on different sides of the sky
– Redshifts might also decrease with distance
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Cosmological Redshift: Step by Step
• Consider two galaxies, far apart. Each one emits light.
• Assume they are not moving “through space”.
• Look at the left one. Consider the light that leaves it at a given instant.
– Is it Doppler shifted?
• The galaxies were not moving through space. But the space between them expands as time goes on.
– What happens to the distance between the galaxies?
– What happens to the wavelength of the light?
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Cosmological Redshift
• From the book:
– “It is better to think of space itself as expanding, carrying the galaxies along for the ride, than to think of the galaxies as projectiles flying through a static universe. The cosmological redshift of a galaxy therefore tells us how much space has expanded during the time since light from the galaxy left on its journey to us.”
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The Cosmological Constant
• Einstein’s General Theory of Relativity predicts expanding or contracting space
• Einstein added a “cosmological constant” to create a static universe– Assumed that gravity might cause the Universe to contract
• He ended up thanking Hubble and deeming the cosmological constant his “greatest blunder”– It’s back!
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The Observable Universe
• Two objects cannot move through space at or greater than the speed of light
• If the space between two objects expands such that the distance between them increases at greater than the speed of light…
– That’s ok!
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The Observable Universe
• If the distance between us and an object is increasing at greater than the speed of light:
– Does light from that object ever reach us?
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The Observable Universe
• There may be a radius beyond which all objects are receding at greater than the speed of light– Everything within this radius is known as the
observable universe
• We have no hope of observing objects outside of the observable universe