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Dark Matter, Dark Energy
Tuesday, February 12
Recap of the history of cosmology:
Version 1.0: “Superdome” model
Version 2.0: Geocentric model
spherical
Earth at
center
Version 3.0: Heliocentric model
Sun at
center
v. 3.1: Infinite heliocentric model
v. 4.0: Big Bang model
v. 4.1: Hot Big Bang model
Cosmic
Microwave
Background
v. 4.1.1: Hot Big Bang model
with space-time curvature.
Mass & energy cause space to curve.
This curvature causes an observed
bending of the path of light.
“Gravitational lensing” by a massive
cluster of galaxies
Observation: on large scales,
space is close to flat.
Implication: density is close
to the critical density.
Flat & negatively curved
spaces are infinite (unless
you add a boundary).
Positively curved spaces are finite,
but without a boundary.
Suggestion: space is positively
curved, but with a radius of curvature
much larger than the Hubble distance
(4300 Mpc).
The part I
can see
looks flat!
This gives the
universe a huge
(but finite) volume.
The critical density required for
space to be flat is
crit  9 10 kg/m
27
3
E = mc2 implies m = E/c2:
this critical density can be
contributed by mass or by energy.
If Einstein’s theory is correct,
the electrons, protons, neutrons,
neutrinos, photons, etc. in the universe
must sum to (nearly) the critical density.
1 m3
of the
universe
9×10-27
kg
Let’s do an inventory of the universe:
How much mass/energy is contributed
by electrons, protons, neutrons,
photons, neutrinos, et cetera to the
9×10-27 kilograms per cubic meter.
The accounts
must balance!
First: PHOTONS
Lots of photons from the
cosmic microwave background,
fewer photons from starlight.
Photons are easily detected!
Inventory: photons provide 0.01% of
the critical density. Pffft.
Next: ELECTRONS,
PROTONS, & NEUTRONS
Ordinary matter (stars, planets,
gas clouds, people) is made of
electrons, protons, & neutrons.
These objects are easily detected
because they emit photons.
Electrons, protons, & neutrons
provide 4% of the critical density.
Ordinary light & ordinary
matter make up only 4%
of the universe.
Where’s the rest of
the mass & energy?
To answer that question, we
must turn to the Dark Side
of the universe.
Light, and objects that emit light,
are easy to detect.
Light, and objects that emit light,
contribute only 4% of the mass/energy
content of the universe.
The remaining 96% of the universe
must consist of mass or energy that
does not emit (or absorb) light.
Dark matter = massive particles
that do not emit, absorb, or
otherwise interact with photons.
Dark matter could also be
called “invisible matter”:
If it’s invisible, how do
you know that it’s there??
Dark matter can be detected by its
gravitational effect on ordinary matter.
In the Solar System,
99.8% of the mass is
contained in the Sun.
For planets farther from the Sun, the
orbital speed is smaller.
In the Milky Way Galaxy, the orbital
speed of stars is nearly constant with
distance from the Galaxy’s center.
Conclusion: the
mass of the Galaxy
is not concentrated
near its center.
But… the glowing stars of the Galaxy
are concentrated near its center.
The Galaxy must have an extended
“halo” of dark matter, to prevent the
high-speed stars from escaping.
bright stars
dark “halo”: nearly
spherical distribution
of invisible massive
particles
Clusters of galaxies contain
lots of dark matter.
How do we know?
1) Galaxies in clusters move very
rapidly: if there weren’t dark matter to
anchor them, they’d fly away.
2) Gravitational lensing requires
lots of mass.
How Much Dark Matter?
Adding together the dark matter
around galaxies, in clusters, and
on larger scales, we find there is
more dark matter than ordinary matter!
Dark matter provides 23% of the
critical density.
What’s the dark matter made of?
Neutrinos make
up part of the
dark matter.
Although detecting
neutrinos is difficult,
it has been done!
Although we don’t know the mass of
neutrinos exactly, we know it’s tiny…
neutrinos
electron
Neutrinos provide < 2% of the
critical density.
Most of the dark matter must be
particles other than neutrinos.
One candidate for the
office of “dark matter”:
the WIMP.
WIMP = Weakly Interacting
Massive Particle
According to particle physics theory,
WIMPs should be much like neutrinos
only more massive.
Neutrinos have already been
detected: particle physicists are
still trying to detect WIMPs.
I predict a Nobel
Prize for the 1st to
succeed!
Inventory of the universe:
Light = diddly-squat
Ordinary matter = 4%
Dark matter = 23%
Something else = 73%
What is the “something else”?
The “something else” isn’t ordinary
(luminous) matter, dark matter, or
energy in the form of photons.
Let’s call the “something else”
dark energy.
Dark energy is even less well
understood than dark matter.
Dark energy is a uniform energy field
that permeates the universe (unlike
dark matter, it doesn’t “clump up”).
Since its energy density is so low
everywhere, how do we know the
dark energy’s there?
One reason for thinking that dark
energy exists:
The universe is flat on large scales;
there isn’t enough mass to do the
flattening, so there must be energy.
If the energy emitted light, we’d
have seen it by now, so it must be
dark energy.
The weird reason for thinking that
dark energy exists:
Einstein found that a component of
the universe whose energy density
was constant in time and space
would provide a repulsive force.
Yes, this is an unexpected
result: Newton would not
approve!
Einstein called this
component of the universe
the “cosmological constant”:
we call it “dark energy”.
Dark & luminous matter make the
expansion of the universe slow down.
Dark energy makes the expansion of
the universe speed up!
Testing for dark energy:
●Look at a supernova (an exploding
star as bright as 109 Suns).
●Measure its redshift and flux.
●If the expansion of the universe is
speeding up, then a supernova with
large redshift will be overly faint.
The result of the test:
The expansion is speeding up,
implying the presence of dark energy.
Science magazine’s
“Breakthrough of the
Year” for 1999!
Thursday’s Lecture:
How old are stars
& planets?
Reading:
Chapters 7 & 8