Download PowerPoint Presentation - The Origin of the Universe

Olbers paradox
• Why is the sky dark at night?
Of course, the Sun’s gone down! But more
careful consideration of this simple fact led
early astronomers to get the first constraints
on cosmological models
Olbers paradox
• Why…
If the universe were infinite in size & contained an infinite
number of stars that live forever, then every line-of-sight
would eventually lead to a star
Stars light dims as 1/r2 but the volume of space sampled
increases by the same factor
So, the night sky should be as bright everywhere as the
average surface of a star
Olbers paradox
• How can it be dark then?
• Pondered as early as Kepler and Newton.
• Newton wanted the universe to be infinite to avoid collapse
under his theory of gravity
• One of the scientists associated with discussing the puzzle
was Heinrich Olbers, and his name remained associated
with “Olbers paradox”
• Some people suggested the distant stars light to be
absorbed and thus diminished before reaching us
• No good - why?
Olbers paradox
• Any material which absorbed the starlight should heat up
and re-emit it, we would see this gas glowing !
• Flaw in arguments was assumption stellar lifetimes are
infinite
• In fact, if we look far enough we look back to a time when
no stars existed
• Further, an universe with finite age or which is expanding
has a limit to how far we can see, light has to have had
time to reach us
Olbers paradox
• Anyway, number of stars is too small, and stellar
lifetimes to short to fill space with light
• The darkness of the night sky rules out the
simplest idea that the universe is infinite and filled
with unchanging stars
The Cosmic Microwave Background
(CMB)
• However, there is more radiation filling the
universe than that from stars
• Something called the Cosmic Background
Radiation (CBR) fills the sky in all
directions at wavelengths too long for our
eyes to see
The Cosmic Microwave Background
(CMB)
• The expanding universe does tell us
something about this new version of Olbers
paradox
• The expansion has caused CBR to redshift to
longer wavelengths from its original energy!
• The cosmos must have once been ablaze with
this radiation-this is the radiation predicted to
have been produced in the Big Bang!
The Cosmic Microwave Background
(CMB)
• Observational discovery of the CMB
• The Big Bang model
• What can we learn from the CMB?
THE OBSERVATIONAL DISCOVERY OF
THE COSMIC MICROWAVE
BACKGROUND
1964 Penzias & Wilson (Bell-Labs) and antenna
CBR
• Arno Penzias & Robert Wilson (1964)
– Attempted to study radio emission from our Galaxy
using sensitive antenna built at Bell-Labs
– Needed to characterize and eliminate all sources of
noise
– They never could get rid of a certain noise source…
noise had a characteristic temperature of about 3 K
– They figured out that the noise was coming from the
sky, and was approximately the same in all directions…
THE HOT BIG BANG MODEL
• Penzias & Wilson had discovered radiation left
over from the early universe…
• The big bang model…
– Independently developed by James Peebles and George
Gamov
– They suggested that the universe started off in an
extremely hot state
– As the universe expands, the energy within the universe
is spread over in increasing volume of space…
– Thus the Universe cools down as it expands
CBR
• Why did they suggest this model?
– If the early universe was hot (full of energy), a lot of
features of the current universe could be explained…
– Could explain where the matter that we see around us
came from (H, He, Li created by the big bang)
– Could explain the observed ratio of elements
(nucleosynthesis occurred within first few minutes)
– Predicted that there should be left over radiation in the
present universe…
A brief look at the stages of the
Universe’s life…
• We will discuss this diagram in
detail in future classes.
• Most crude description…
– t=0: The Big Bang
– For first 380,000ys, universe is an
expanding “soup” of tightly
coupled radiation & matter
– After 380,000yrs, radiation &
matter “decouple”. Radiation
field reduced enough (due to
expansion) that protons can now
capture electrons & hang on to
them
– Left over radiation that we see
known as the CMB…
THE COSMIC BACKGROUND
EXPLORER (COBE)
COBE
• The COBE mission
– Built by NASA-Goddard Space Flight Center
– Launched Nov. 1989
– Purpose was to survey infra-red & microwave emission
across the whole sky (BB predicts 3 K black body)
– Primary purpose – to characterize the CMB
• Had a number of instruments on it:
– FIRAS (Fair infra-red absolute spectrophotometer)
– DMR (Differential Microwave Radiometer)
– DIRBE (Diffuse Infrared background Experiment)
The CMB: DMR map of the
microwave sky…
COBE Results
• Map of the microwave sky (frequency of 50GHz)…
– We’re looking at the CMB
– The map is very uniform.
– Means that the CMB is extremely isotropic (i.e. the same in every
direction we look)
– Supports the idea that the universe is isotropic (one of the basic
cosmological principles).
– In fact, if we measure the universe to be isotropic, and we’re not
located at a special place in the Universe, we can also deduce that
the Universe is homogeneous!
The spectrum of the CMB
(FIRAS)
CBR-Spectrum
• Spectrum has precisely the shape predicted
by the theory…
– So-called “Blackbody” spectrum
– Characteristic temperature of 2.728K
Subtract off average level…
CBR
• What causes this pattern of redshift and
blueshift?
CBR
• What causes this pattern of redshift and
blueshift?
• Earths motion through space causes this
dipole (anisotropy with 2 welldefined/opposite points)
• (Earth orbits Sun at 30 km/s, Sun orbits
MW at 220 km/s, MW has motion around
center of local group etc)
Subtract the “dipole”…
CBR
• Subtract off the dipole resulting from the
Earth’s motion…
• Are left with…
– Bright ridge corresponding to microwave
emission from our Galaxy
– Pattern of random fluctuations in the CMB
Subtract off the emission from
our Galaxy…
CBR fluctuations
• Can use the different spectrum of the Galaxy’s emission
and the CMB to distinguish them.
• So, can subtract off the emission from our Galaxy…
• Left with a random pattern of fluctuations in the CMB…
correspond to temperature differences of 30 millionths of a
Kelvin
CBR fluctuations
• What are these fluctuations…
– The early universe was very close to being perfectly homogeneous
– But, there were small deviations from homogeneity… some
regions were a tiny bit colder and some were a tiny bit hotter.
– When matter and radiation decoupled, this pattern of fluctuations
was frozen into the radiation field.
– We see this nowadays as fluctuations in the CMB.
CBR Fluctuations
• Why are the fluctuations important?
– Before decoupling, fluctuations in the radiation field
also meant fluctuations in the mass density
– After decoupling, these small fluctuations in density
can get amplified (slightly dense regions get denser and
denser due to gravity).
– These growing fluctuations eventually collapse to give
galaxies and galaxy clusters.
– So, by studying these fluctuations, we are looking at the
“seeds” that grow to become galaxies, stars, planets…
Microwave Anisotropy Probe (MAP)
Microwave Anisotropy Probe (MAP)
• NASA mission to map out the fluctuations in the
CMB in fine detail…
– Will characterize these seeds for structure formation
– Will determine fine detail of the CMB fluctuations that
depend upon the curvature of space (k) and .
• Launched last year…
New WMAP results - Tues Feb 11
A NASA satellite has captured the sharpest-ever picture of the afterglow of the big bang
Dates universe to 13.7 billion years.
NASA's Wilkinson Microwave Anisotropy Probe (WMAP)
Patterns in the big bang afterglow
were frozen in place only 380,000
years after the big bang, a number
nailed down by this latest
observation. These patterns are tiny
temperature differences within this
extraordinarily evenly dispersed
microwave light bathing the universe,
which now averages a frigid 2.73
degrees above absolute zero
temperature. WMAP resolves
slight temperature fluctuations,
which vary by only millionths of a
degree. Theories about the evolution
of the universe make specific
predictions about the extent of these
temperature patterns