The Contents of the Universe (or/ what do we mean by dark matter and dark energy?)
Unseen Influences Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate
Mass within Sun s orbit: 1.0 x 1011 MSun Rotation curve of Milky Way does not match this. Mass in our galaxy is spread out over a larger region than the stars.
Most of our galaxy's mass is outside of the Sun's orbit beyond the visible stars
The visible portion of a galaxy lies deep in the heart of a large halo of dark matter
Spiral galaxies tend to have rotation curves similar to the Milky Way-indicating most matter is in the outer part of these galaxies too. i.e. they have large amounts of dark matter.
The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!
How dark is it? not as bright as a star.
Two Basic Options 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
Two Basic Options 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 The Best Bet
Why Believe in WIMPs? There s not enough ordinary matter WIMPs could be left over from Big Bang Models involving WIMPs explain how galaxy formation works
What is the role of dark matter in galaxy formation?
Gravity of dark matter is what caused protogalactic clouds to contract early in time
WIMPs can t contract to center because they don t radiate away their orbital energy
Dark matter is still pulling things together After correcting for Hubble s Law, we can see that galaxies are flowing toward the densest regions of space
Time in billions of years 0.5 2.2 5.9 8.6 13.7 13 35 70 93 140 Size of expanding box in millions of light-yrs Models show that gravity of dark matter pulls mass into denser regions universe grows lumpier with time
What have we learned? Does dark matter really exist? Rotation curves of galaxies are flat, indicating that most of their matter lies outside their visible regions Either dark matter exists or our understanding of our gravity must be revised There does not seem to be enough normal matter to account for all the dark matter, so most astronomers suspect that dark matter is made of particles that have not yet been discovered
What have we learned? What is the role of dark matter in galaxy formation? The gravity of dark matter seems to be what drew gas together into protogalactic clouds, initiating the process of galaxy formation What are the largest structures in the universe? Galaxies appear to be distributed in gigantic chains and sheets that surround great voids
Hubble s Law Our goals for learning What is Hubble s Law? How do distance measurements tell us the age of the universe? How does the universe s expansion affect our distance measurements?
What is Hubble s Law?
The spectral features of virtually all galaxies are redshifted They re all moving away from us
By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way
the further away a galaxy is, the faster it is travelling Hubble s Law: velocity = H0 x distance
Redshift of a galaxy tells us its distance through Hubble s Law: velocity distance = H0
Distances of farthest galaxies are measured from redshifts
How do distance measurements tell us the age of the universe?
The expansion rate appears to be the same everywhere in space The universe has no center and no edge (as far as we can tell)
One example of something that expands but has no center or edge is the surface of a balloon
Cosmological Principle The universe looks about the same no matter where you are within it Matter is evenly distributed on very large scales in the universe No center & no edges Not proved but consistent with all observations to date
Hubble s constant tells us age of universe because it relates velocities and distances of all galaxies Age = Distance Velocity ~ 1 / H0
Calculating 1 / H0 gives an age for the universe of 13.75 billion years old!
How does the universe s expansion affect our understanding it's contents? The mass of ordinary matter and dark matter are pulled together by gravity. Theoretically the mass will slow down the universe's expansion, causing it to either slow expansion (Big Chill) coast to a halt re-contract back again (Big Crunch)
Use white dwarf supernovae to measure distances to galaxies. Result: expansion is not slowing it is accelerating! Either our math is wrong, or/ some extra force must exist to push the universe along.
Unseen Influences Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate
The universe's expansion is accelerating Dark Energy is the suggested cause.
Contents of Universe Normal Matter: ~ 4.9% Normal Matter inside stars: ~ 0.6% Normal Matter outside stars: ~ 4.3% Dark Matter: Dark Energy ~ 26.8% ~ 68.3%
What have we learned? What is Hubble s Law? The faster a galaxy is moving away from us, the greater its distance: velocity = H0 x distance How do distance measurements tell us the age of the universe? The measurements let us calculate the expansion rate of the universe, which lets us calculate how long the universe took to expand.
What have we learned? How does the universe s expansion affect our understanding it's contents? The universe s expansion is accelerating. This means there is extra force (called Dark Energy) pushing the universe along.
Evidence for the Big Bang Our goals for learning How do we observe the radiation left over from the Big Bang? How do the abundances of elements support the Big Bang theory?
How do we observe the radiation left over from the Big Bang?
The cosmic microwave background the radiation left over from the Big Bang was detected by Penzias & Wilson in 1965
Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/ir
Background has perfect thermal radiation spectrum at temperature 2.73 K Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves
WMAP gives us detailed baby pictures of structure in the universe
How do the abundances of elements support the Big Bang theory?
Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old
Big Bang theory prediction: 75% H, 25% He (by mass) Matches observations of nearly primordial gases
Abundances of other light elements agree with Big Bang model having 4.4% normal matter more evidence for WIMPS!
What have we learned? How do we observe the radiation left over from the Big Bang? Radiation left over from the Big Bang is now in the form of microwaves the cosmic microwave background which we can observe with a radio telescope. How do the abundances of elements support the Big Bang theory? Observations of helium and other light elements agree with the predictions for fusion in the Big Bang theory
What is the history of the universe according to the Big Bang theory?
Planck Era Before Planck time (~10-43 sec) No theory of quantum gravity
Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity
We think at moment of the big bang the 4 forces were unified at super high temperatures. At the first instant the forces separated out.
First gravity from the other 3 sub-atomic forces. Then the strong force Then finally electromagnetism separates from the weak force.
Particle Era The inflating universe starts to cool, energy starts being stored as matter E = mc2
Photons converted into both matter and antimatter as pairs of particles and antiparticles E = mc2 Early universe was full of particles and radiation because of its high temperature
Particle Era Amounts of matter and antimatter nearly equal (Roughly 1 extra proton for every 109 protonantiproton pairs!)
NOTE everything so far: Force separation, Inflation, Fundamental matter formation, has taken place within 1 second of the universe s existence!
Era of Nucleosynthesis Nuclei begin to fuse Protons and neutrons will start to stick together (takes ~ 3 minutes)
Era of Nuclei Helium nuclei form Universe has become too cool to blast helium apart
Era of Atoms Atoms form at age ~ 380,000 years Background radiation released
So far every photon of light released from a particle has been absorbed by another particle. Now the universe is sufficiently large (and the particles sufficiently clumped) that light escapes absorption and roams the universe. This is the Cosmic Background Radiation
Era of Galaxies Galaxies form at age ~ 1 billion years
Remember the universe is actually accelerating
What have we learned? What were conditions like in the early universe? The early universe was so hot and so dense that radiation was constantly producing particleantiparticle pairs and vice versa What is the history of the universe according to the Big Bang theory? As the universe cooled, particle production stopped, leaving matter instead of antimatter Fusion turned remaining neutrons into helium Radiation traveled freely after formation of atoms