16.4 The Universe’s Fate
Our Goals for Learning
• Will the universe continue expanding forever?
• Is the expansion of the universe accelerating? What is Hubble’s Law?
Distances of farthest galaxies are measured from redshifts
The spectral features of virtually all galaxies are redshifted ⇒ They’re all moving away from us
Hubble’s Law: velocity = H0 x distance
Surface of a balloon expands but has no center or edge
A balloon universe has two space dimensions but no center or edge in space; it expands in the time dimension (animation)
How do distance measurements tell us the age of the universe?
Your friend leaves your house. She later calls you on her cell phone, saying that she’s been driving at 120 km an hour directly away from you the whole time and is now 120 km away. How long has she been gone?
1. 1 minute
2. 60 minutes
3. 120 minutes
You observe a galaxy moving away from you at 0.1 light­years per year, and it is now 1.25 billion light­years away from you. How long has it taken to get there?
1. 8 million years
2. 12.5 million years
3. 8 billion years
4. 12.5 billion years
velocity = H0 x distance
distance = velocity / H0
distance = velocity x time since Big Bang
time since Big Bang = distance / velocity
time since Big Bang = 1 / H0
The distances between
widely separated galaxies will change while light travels between them.
distance?
The most distant galaxies we've found so far have a lookback time of about 13 billion years. That is, the light we see from them today took 13 billion years to reach us.
How far away are those galaxies today?
1. Less than 13 billion light years away
2. 13 billion light years away
3. More than 13 billion light years away
lookback time
distance?
The distances between
widely separated galaxies will change while light travels between them.
Astronomers often think in terms of lookback time rather than distance.
Cosmological Horizon
Maximum lookback time of 14 billion years limits how far we can see (everything we can see is our observable universe)
Expansion of the universe stretches photon wavelengths causing a cosmological redshift related to lookback time
Will the universe continue expanding forever?
Does the universe have enough kinetic energy to overcome its own gravitational pull?
The Big Bang “launched” the expansion of space at a certain speed. But if there is enough dark matter in the universe…
1. …so what? Space will keep expanding at the same speed.
2. …the gravity of the dark matter could halt the expansion of the universe.
3. …the universe could accelerate forever due to the momentum of all that mass.
Lots of dark matter
Fate of universe depends on amount of dark matter & energy
Lots of dark matter
Not as much dark matter
Fate of universe depends on amount of dark matter & energy
Lots of dark matter
Critical density of matter
Not enough dark matter
Fate of universe depends on amount of dark matter & energy
Amount of dark matter is ~25% of the critical density suggesting fate is eternal expansion
Not enough dark matter
Brightness of distant white­dwarf supernovae tell us how much universe has expanded since they exploded
Accelerating universe is best fit to supernova data
But expansion appears to be speeding up!
Dark Energy
Not enough dark matter
old
older
oldest
Estimated age depends on
dark matter (left three) and dark energy (rightmost)
What have we learned?
• Will the universe continue expanding forever?
• Even before we consider the possibility of a mysterious dark energy, the evidence points to eternal expansion. The critical density is the average matter density the universe would need for the strength of gravity to eventually halt the expansion. The overall matter density of the universe appears to be only about 25% of the critical density. What have we learned?
• Is the expansion of the universe accelerating?
• Observations of distant supernovae indicate that the expansion of the universe is speeding up. No one knows the nature of the mysterious force (dark energy) that could be causing this acceleration. Activity 38, page 135­138
• Remember: 1 parsec (pc) = 3.26 light­years, the distance light travels in 3.26 years
• 1 megaparsec (Mpc) = 1 million parsecs = 3.26 million light­years, the distance light travels in 3.26 million years
Activity 38, page 135­138
• Remember: 1 parsec (pc) = 3.26 light­years, the distance light travels in 3.26 years
• 1 megaparsec (Mpc) = 1 million parsecs = 3.26 million light­years, the distance light travels in 3.26 million years
• 1 hour of Right Ascension (RA) = 15 degrees