Recap
How do we observe the
radiation left over from
the Big Bang?
• Telescopes that can detect
microwaves allow us to observe
the cosmic microwave
background – radiation left over
from the Big Bang.
• Its spectrum matches the
characteristics expected of the
radiation released at the end of
the Era of Nuclei, spectacularly
confirming a key prediction of
the Big Bang theory.
Recap (Cont)
How do the abundances of elements support the
Big Bang theory?
• The Big Bang theory predicts the ratio of protons to neutrons
during the Era of Nucleosynthesis, and from this predicts that the
chemical composition of the universe should be about 75%
hydrogen and 25% helium (by mass).
• This matches observations of the cosmic abundances, another
spectacular confirmation of the Big Bang theory.
Recap (Cont)
What is the evidence for dark
matter in clusters of
galaxies?
• We have three different ways of
measuring the amount of dark
matter in clusters of galaxies:
from galaxy orbits, from the
temperature of the hot gas in
clusters, and from the gravitational
lensing predicted by Einstein.
• All of these methods agree that
the total mass of a cluster is about
50 times the mass of its stars,
implying huge amounts of dark
matter.
Gravitational Lensing: the bending of light rays by gravity can
also tell us a cluster’s mass.
What might dark matter be made
of?
All three methods of measuring cluster mass indicate similar
amounts of dark matter!
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Two Basic Options
Two Basic Options
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies.
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies.
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles.
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles.
Our best bet!
The Big Bang and Inflation
What aspects of the Universe
were originally unexplained
by the Big Bang model?
Mysteries Needing Explanation
Mysteries Needing Explanation
1. Where does structure come from?
1. Where does structure come from?
2. Why is the overall distribution of matter so
uniform?
2. Why is the overall distribution of matter so
uniform?
3. Why is the density (matter and energy) of the
Universe so close to the critical density?
3. Why is the density of the universe so close to
the critical density?
An early episode of rapid inflation can solve all three
mysteries!
2
Mystery# 1
Inflation can make
all the structure by
stretching tiny
quantum ripples to
enormous size.
These ripples in
density then
become the seeds
for all structures.
How can microwave temperature be nearly identical on opposite
sides of the sky?
Mystery# 2
Density =
Critical
Density >
Critical
Overall geometry
of the Universe
is closely related
to total density of
matter and energy.
Density <
Critical
Regions now on opposite side of the sky were close
together before inflation pushed them far apart.
Mystery# 3
Inflation of
Universe flattens
overall geometry
like the inflation
of a balloon,
causing overall
density of matter
plus energy to be
very close to
critical density.
How can we test the idea of
inflation?
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Patterns of structure observed by WMAP tell us about the
“genetic code” of the Universe.
Observed patterns of structure in Universe agree (so far)
with what inflation should produce.
“Genetic Code” Inferred from CMB
“Genetic Code” Inferred from CMB
• Overall geometry is flat.
– Total mass+energy has critical density
• Total matter is ~ 27% of total
– Dark matter is ~ 23% of total
– Ordinary matter ~ 4.4% of total
• Dark energy is ~ 73% of total
• Age of 13.7 billion years
• Overall geometry is flat.
– Total mass+energy has critical density
• Total matter is ~ 27% of total
– Dark matter is ~ 23% of total
– Ordinary matter ~ 4.4% of total
• Dark energy is ~ 73% of total
• Age of 13.7 billion years
In agreement with observations of present-day universe and models
involving inflation and WIMPs!
Fate of the Universe
Is the Universe
expanding fast
enough to escape its
own gravitational
pull? (Does it have
have enough kinetic
energy?)
Critical density of
material needed is
10-29 g/cm3 (about a
few H atoms in your
closet). Visible matter
is only ~0.5% of this.
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Lots of
dark matter
Lots of
dark matter
Fate of Universe depends on amount of dark matter
Lots of
dark matter
Critical density
of matter
Not
enough
dark
matter
Fate of Universe depends on amount of dark matter
In fact, recent
observations
suggest that the
expansion of the
Universe is
speeding up!
Accelerating
Universe!!
Dark
Energy
??
Not
enough
dark
matter
Fate of Universe depends on amount of dark matter
Amount of dark
matter is ~23% of
the critical density,
suggesting fate is
eternal expansion.
Not
enough
dark
matter
Fate of Universe depends on amount of dark matter
Measuring the expansion of the
Universe over its history
Brightness of distant
white-dwarf
supernovae tell us
how much Universe
has expanded since
they exploded.
The data show that
the rate of expansion
is increasing – the
Universe is
accelerating!!
5
Accelerating Universe is best fit to supernova data
The acceleration is measured by looking at distant
white dwarf supernovae (standard candles)
old
older
oldest
Estimated age of the Universe depends on
the assumed amount of dark matter and dark energy
Contents of Universe (by Mass)
• “Normal” Matter:
~ 4.4%
– Normal matter inside stars: ~ 0.6%
– Normal matter outside stars: ~ 3.8%
• Dark Matter:
~ 23%
– We have some good candidates for this.
• Dark Energy
~ 73%
– The nature of this is a big mystery.
We know almost nothing
about 96% of the Universe!!!
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