Download Expansion of the Universe

How we know the
Universe is expanding
Physics 20B
Prof. Kevork Abazajian
- filling in for Prof. Virginia Trimble -
Doppler Effect
Cosmological Redshift
Origin of Redshift
Spectral Redshift
Cosmological Redshift
There are four phenomena that cause light to be seen as "redder" than the source that actually emitted it.
1. Scattering of blue and green light - i.e. why the sky appears blue, and why some sunrises or sunsets
may appear red. Dust, smoke from forest fires, or other intervening material between the source and
the observer can scatter (remove) the higher frequency colors (blue, green, yellow, and orange)
leaving mainly the red colors. However, this effect does *not* cause a frequency shift of the light
waves. A spectrum of the source will not have any of its atomic lines displaced.
2. Gravitational Redshift - where the photons incur a loss of energy overcoming the effects of a gravity
well. Since the energy of a photon is directly proportional to its wavelength, a high energy photon will
suffer a wavelength increase as it looses energy in this manner. Also note that this is a "one-time" event
- once the photon has escaped the source's gravity well, its wavelength is constant again.
3. Doppler Redshift - is caused by the relative motions of a source and an observer. Doppler redshifts
may be towards the red (source moving away from the observer) or towards the blue (source moving
towards the observer). Doppler redshift is an effect of the relative motions of nearby objects and does
NOT involve the expansion of the Universe.
4. Cosmological Redshift or "Hubble redshift" - is caused by the relativistic expansion of the Universe,
as quantified by the Hubble constant, h0, and since the expansion rate increases with distance, the
redshifts of very distant objects can be extremely large. As a result, the wavelengths of photons
propagating through the expanding Universe are stretched by the expansion of space itself, creating
the cosmological redshift.
Origin of Redshift
Symmetry of Expansion
Symmetry of Expansion
Hubble’s Observations (1929)
Hubble Expansion
Observations (1996)
Another way to look at data
Expansion of all of space…
Everything moves away from everything else
Expansion of all of space…
Everything moves away from everything else
Balloon Analogy
Expansion of all of space… raisin bread analogy!
Expansion of all of space… raisin bread analogy!
What is the Universe
expanding into?
This question is based on the ever popular misconception that the Universe is some curved object embedded in a
higher dimensional space, and that the Universe is expanding into this space. This misconception is probably
fostered by the balloon analogy which shows a 2-D spherical model of the Universe expanding in a 3-D space.
While it is possible to think of the Universe this way, it is not necessary, and there is nothing whatsoever that we
have measured or can measure that will show us anything about the larger space. Everything that we measure is
within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding
into anything that we can see, and this is not a profitable thing to think about. Just as Dali's Corpus Hypercubicus
is just a 2-D picture of a 3-D object that represents the surface of a 4-D cube, remember that the balloon analogy is
just a 2-D picture of a 3-D situation that is supposed to help you think about a curved 3-D space, but it does not
mean that there is really a 4-D space that the Universe is expanding into.
For objects in our ordinary experience, like the rising loaf of raisin bread dough also used as an analogy to the
expanding Universe, there are two ways to see that the object is expanding:
1. The distances between objects are all increasing, so the distance between any pair of raisins increases by an
amount proportional to the distance.
2.
The edge of the loaf pushes out into previously unoccupied space. Note the distance between any pair of
points on the edge increases by an amount proportional to the distance.
The first statement involves the internal geometry of the object, which can be measured by an observer sitting in
the object. The second statement involves the external geometry of the object, which can only be measured by an
observer outside the object. Since we are stuck within our spacetime, we need to study the internal geometry of
space-time, and that is what general relativity does. In terms of internal geometry, any object with the first property
above is expanding. Furthermore the Universe is homogeneous so it does not have any edge. Thus it can't have
the second property above. But it does have the first property so we say the Universe is expanding.
The Universe in 1785:
Herschel’s Map
Star map assuming
constant luminosity and
density
The Universe in 1785:
Herschel’s Map
Star map assuming
constant luminosity and
density
The Sun
The Universe in 1921:
The Shapley Model
velocity, v (km/s)
Hubble’s new distance determinations (1929)
distance, d
6 Million Light Years
velocity, v (km/s)
Hubble’s new distance determinations (1929)
distance, d
v/d
6 Million Light Years
velocity, v (km/s)
Hubble’s new distance determinations (1929)
distance, d
6 Million Light Years
v = H0 d
Galaxy Surveys in 1985
CfA: 1100 galaxies in 6˚ × 130˚, in this slice
Galaxy Surveys Today: the Sloan Digital Sky Survey
currently: 106 main galaxies
current & upcoming: 1.5×106 luminous red galaxies to 2.5×109 pc
Sloan Digital Sky Survey:
Galaxies from Large to Small Scales
Sloan Digital Sky Survey:
Galaxies from Large to Small Scales
SDSS-III Spectrograph: 1000 Objects at a Time
SDSS-III Spectrograph: 1000 Objects at a Time
– 1000 fibers feed light from
the focal plane into the
spectrographs
SDSS-III Spectrograph: 1000 Objects at a Time
– 1000 fibers feed light from
the focal plane into the
spectrographs
– Hand plugged
SDSS-III Spectrograph: 1000 Objects at a Time
– 1000 fibers feed light from
the focal plane into the
spectrographs
– Hand plugged
The Largest Color
Image of the Sky Ever
Made
SDSS Sky Coverage & Scale
Credits: D. Hogg, M. Blanton, D. Finkbeiner, D. Schlegel, N. Wherry
Our local universe: the Laniakea
Supercluster of Galaxies (Tully et al 2014)
Sloan Digital Sky Survey
Sloan Digital Sky Survey
2 billion
light years
SDSS Fly through:
https://youtu.be/uGe475zySrM
Quiz
Three possible sources of energy might be suggested for the
Sun and stars. How long would the Sun live if
A. its energy source was chemical, like the burning of
gasoline
B. its energy source was gravitational, gradually released as
it contracted from a cloud of gas and dust
C. its energy source was nuclear, the fusing of hydrogen to
helium
Extra credit: what kinds of evidence or data or observations
have suggested each of those time scales