Transcript cosmological horizon

Lecture 36:
The Expanding Universe
Review
the ‘zoo’ of observed galaxies and the
Hubble Sequence
 the distance ladder
 redshift
 Hubble’s Law and the expansion of the
Universe

Hubble Sequence
Trends along the Hubble Sequence
Elliptical
Lenticular
red
old stars
gas poor
no star formation
Spiral
Irregular
blue
young stars
gas rich
lots of star formation
The distance ladder
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planets
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nearby stars
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main sequence fitting
nearby galaxies
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parallax
Milky Way
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radar ranging
Cepheid variable stars
distant galaxies
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white dwarf supernovae
Tully-Fisher relation
Tully-Fisher
Relation
for spiral galaxies
 relationship between
rotation velocity and
luminosity
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measuring recession velocity
reminder: Doppler formula
redshift = z = (lobserved-lrest)/lrest
recession velocity
Hubble’s Law
distance
Hubble’s Law (modern version)
Hubble’s Law Formula
v = H0 r
recession velocity = constant times distance
units of H0: km/s/Mpc
best estimates:
H0 is between 65 km/s/Mpc and 79 km/s/Mpc
Example: Using Hubble’s Law
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The K line of singly ionized calcium has
a wavelength of 393.3 nm when
measured in the laboratory. In the giant
elliptical galaxy NGC4889, this line is
observed to be at 401.8 nm.
what is the redshift of this galaxy?
 what is its recession velocity?
 how far away is it?
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The expanding Universe
Hubble’s Law implies that most galaxies
are moving away from us
 and, the farther away they are, the
faster they are moving away from us
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The center of the Universe?
does this mean that we are at the
center of the Universe?
 no – an observer in any other galaxy
would see the same thing
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Cosmological Redshift
The Cosmological Principle
 on
large scales, the Universe is
homogeneous (uniform) and
isotropic (same in all directions)
Is everything expanding?
the expansion of space-time acts like a
pressure
 where the force of gravity is stronger
than the expansion pressure, a
gravitationally bound object is formed
 for example, the stars in our Galaxy are
bound together by gravity, so it does
not expand.

The Age of the Universe
r
now
v = r/t = H0 r
t = 1/H0
beginning of time
space
The Cosmological Horizon

the distance that light can travel in the
age of the Universe is called our
cosmological horizon
we cannot receive any information from
beyond our horizon
 rhorizon = c/H0 = 4300 Mpc
(assuming H0 = 70 km/s/Mpc)
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Lookback time
time it takes for
light to travel from
an object to us
 directly related to
redshift of object –
more sensible
measure of
distance
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Coma Cluster
Hercules Cluster (200 Mpc)
Two Million Galaxies
Large Scale
Structure
Galaxy Formation and Evolution
How do galaxies form, and how do they
change over time?
 Why do we see so many different kinds
of galaxies? Are their differences a
result of ‘nature’ or ‘nurture’?
 How do the properties of galaxies
depend on their environment?

let’s start from the beginning…
the Universe started out very dense and
very smooth (the `Big Bang’).
 there were small lumps caused by
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quantum fluctuations
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as the Universe expanded, these lumps
grew larger and denser because of the
force of gravity
expanding
collapsing