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AST201H1 Lecture Notes - Accelerating Expansion Of The Universe, Baryon, Dark Energy

Astronomy & Astrophysics
Course Code
Michael Reid

of 3
AST201 Thursday, April 4, 2013.
The Milky Way is mostly Dark Matter
- There are small amounts of baryonic matter
Dark energy
- Accounts for majority of the universe
- Almost 70% of universe
- About 27% is dark matter
- About 5% is ordinary matter
- 95% of universe is mysterious dark stuff
*Clicker Question: According to Hubble’s Law, distant galaxies are:
- Receding from us at an ever-increasing rate
- Getting farther away at speeds proportional to their distances (THIS ONE)
- Moving through space, with farther ones moving faster
o Galaxies are not MOVING, but are getting FARTHER AWAY
o Distance to a galaxy is proportional to its redshift
*Clicker Question: What effect should all the matter in the universe (dark plus luminous) have
on the expansion of the universe?
- Slow it down (THIS ONE)
- No effect
- Speed it up
o Every mass produces gravity
o Gravity can NEVER push, only pull
o As it cannot push, it can never speed things up
If expansion is slowing down, we would expect distant galaxies to have larger redshifts than
predicted by Hubble’s Law
- Would expect deviations in the upward curve of Hubble’s Law (decelerating expansion)
- We actually find an accelerating expansion curve
- Today, universe is expanding faster than it was millions of years ago
- Lower redshifts, and slower expansions farther back in history of universe
- We observe an accelerating universe, not a decelerating universe (which was what was
initially expected)
We call the driver of the acceleration dark energy
o Require unit of energy to produce accelerating universe
o We do not know what this energy is, so we call it dark energy
AST201 Thursday, April 4, 2013.
o Dark energy is powering the expansion of the universe
o It is not causing the expansion; the Big Bang caused the expansion
o Dark energy only makes the pre-existing expansion proceed faster
Big Bang Model
- Observation: universe is getting bigger
o According to Hubble’s Law, universe was once filled with hot, dense plasma
o To prove Big Bang theory, look for the light produced by all that hot plasma
The further away we look, the further back in time we can see.
Any dense object emits blackbody spectrum
Universe was a blackbody at its origins; it would have had very short
Universe after big bang would be enormously hot, and radiating like a
At the beginning, electrons would have been split from protons (ionized)
Light would be absorbed by free electrons
Universe would have been opaque
As it gets bigger, cools to the point where electrons could bind with nuclei
and form hydrogen particles
Light is able to pass; universe becomes transparent
*Clicker Question: If who universe was full of hot plasma at the beginning
of time, from which direction should light from that plasma be coming?
From every direction
o No matter where you stand, you receive the light from
every direction
Light from Big Bang is in microwaves
No longer high frequency light
Light was stretched by the expansion of the space
Once was visible light, now has been stretched until it was
Cosmic microwave background
Key evidence that Big Bang happened
Clicker Question: Why is the existence of the cosmic microwave background (CMB) good
evidence for the Big Bang model?
- Non-Big Bang models do not include the hot, dense state of the beginning of the universe
AST201 Thursday, April 4, 2013.
Other strong evidence for Big Bang:
- Nucleosynthesis: Big Bang model correctly predicts proportions of chemical elements
- Large Scale structure: only computer models using Big Bang as beginning of universe
can simulate growth of universe produce a universe that looks like our own
Clicker Question: If we significantly increased the density of matter (luminous plus dark), what
would happen?
- The expansion would slow due to gravity
Different types of universes:
- Recollapsing universe
- Critical universe (eventually stops getting bigger)
- Coasting universe (not enough stuff to half expansion of space; gets bigger at constant
- Accelerating Universe (dark energy causes expansion of universe to accelerate; gets
bigger faster, forever—the “big rip”)
o End up with very cold, very dark universe
The Big Bang model is one of the most successful models in all of science
- Confirmed through extensive testing
- We are pretty sure about Dark Matter and Dark Energy