The Big Bang

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The Big Bang

• Thursday, January 17

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Doppler shift tells you if an object is coming
toward you or moving away.
Blueshift distance decreasing.
Redshift distance increasing.
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All distant galaxies have redshifts.
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But wait, theres more!....
The amount by which the wavelength is shifted
tells us the radial velocity of the
object, in kilometers/second.
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A light source is at rest it emits light with a
wavelength ?0.
If distance to light source is changing, Doppler
shift will change the wavelength to ? ? ?0 .
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Size of Doppler shift is proportional to radial
velocity
? observed wavelength ?0 wavelength if source
isnt at rest V radial velocity of moving
source c speed of light
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656.3 nm ?
Hydrogen absorbs light with ?0 656.3 nm.
You observe a star with a hydrogen absorption
line at ? 656.2 nm.
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Thinking locally stars within 3 parsecs of the
Sun.
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Equal numbers of redshifts and blueshifts.
Typical radial velocity V 30 km/second (70,000
mph).
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Thinking more globally galaxies within 30
million parsecs of the Milky Way.
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Almost all redshifts rather than blueshifts.
Typical radial velocity V 1000 km/second
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How do we know the distances to stars and
galaxies?
No sense of depth!
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Climbing the cosmic distance ladder.
Cant use the same technique to find distance to
every astronomical object.
Use one technique within Solar System (1st
rung of ladder) another for nearby stars
(2nd rung), etc...
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1st rung of the distance ladder distances within
the Solar System.
Distances from Earth to nearby planets are found
by radar.
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Radar distance measurement
Send out a strong radio pulse, wait until the
faint reflected pulse returns.
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Measured round-trip travel time t
(typically several minutes)
One-way travel time t/2
Distance speed one-way travel time
Since radio waves are a form of light, distance
c t / 2
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Using fancy technical methods, round-trip travel
time can be measured with great accuracy.
Thus, we know distances within the Solar System
very well indeed.
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1 astronomical unit (average distance from Suns
center to Earths center)
149,597,870,690 meters (plus or minus 30 meters).
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2nd rung distances to nearby stars within the
Milky Way Galaxy.
? Proxima Centauri
Distances from Solar System to nearby stars are
found by parallax.
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Flashback slide!
1 parsec distance at which a star has a
parallax of 1 arcsecond.
observer?
?observed star
parallax angle
1 parsec 206,000 astronomical units 3.26
light-years
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Measured parallax angle is inversely proportional
to a stars distance.
(p parallax angle, in arcseconds)
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First star to have its parallax angle measured
61 Cygni (in the year 1838).
Parallax angle 0.287 arcseconds
Distance 1 parsec / 0.287 3.48
parsecs
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With the Hipparcos satellite, astronomers
measured parallax angles with an accuracy of
0.001 arcseconds.
Parallax too small to measure for stars more than
1000 parsecs away.
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3rd rung distances to galaxies beyond our own.
Distances from Milky Way to nearby galaxies are
found with standard candles.
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In the jargon of astronomers, a standard candle
is a light source of known luminosity.
Luminosity is the rate at which light
source radiates away energy (in
other words, its the wattage).
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Suns luminosity 4 1026 watts 4 1033 ergs
per second
When we measure the light from a star, we arent
measuring the luminosity.
To do that, wed have to capture all the light
from the star.
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When we measure the light from a star, we are
measuring the flux.
The flux is the wattage received per square meter
of our telescope lens.
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At distance d from star of luminosity L, the
luminosity is spread over an area 4pd2.
Flux luminosity / area
F L / ( 4 p d2 )
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Whats this got to do with finding the distance?
You know luminosity (L) of a standard candle. You
measure the flux (F).
You compute the distance (d)
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Climbing the distance ladder.
1) Measure flux of two standard candles one
near, one far.
2) Find distance to near standard candle from its
parallax.
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3) Compute luminosity of near standard candle L
4 p d2 F.
4) Assume far standard candle has same luminosity
as the near.
5) Compute the distance to the far standard
candle
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A good standard candle Cepheid variable stars
Cepheid stars vary in brightness with a period
that depends on their average luminosity.
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Observe Cepheid.
Measure period.
Look up luminosity.
Measure flux.
Compute its distance!
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In 1929, Edwin Hubble looked at the relation
between radial velocity and distance for galaxies.
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Hubbles result
Radial velocity of a galaxy is linearly
proportional to its distance.
1 Mpc 1 million parsecs
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Hubbles law (that radial velocity
is proportional to distance)
led to acceptance of the Big Bang model.
Big Bang model universe started in an extremely
dense state, but became less dense as it expanded.