Dark Energy
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Transcript of Dark Energy
Dark EnergyDark Energy
Expanding Universe
Accelerating Universe
Dark Energy
Scott DodelsonMarch 7, 2004
Einstein’s Equations govern evolution of Scale Factor
Mass x Acceleration = Gravitational Force
The extent to which expansion is speeding up
Due to the contents of the universe
We can learn about the contents of the universe by studying how the expansion varies with time
Supernovae appear fainter in an accelerating universe
• Accelerating universe was expanding slower in the past than non-accelerating universe
• Universe takes longer to get from early scale to today in accelerating universe
• Light has more time to travel from SN at early scale to us in accelerating universe
• SN appears fainter
Type Ia Supernovae• Observed flux goes down as distance increases. Type Ia SN are standard candles (i.e. have identical luminosities), so apparent brightness measures distance.
• Universe expanded slower in model with Dark Energy, so distance traveled by light is greater: Expect fainter SN in Dark Energy dominated universe.
Earlier times
Fai
nter
SN
Accelerating
Decelerating
Einstein’s Equations govern evolution of Scale Factor
Mass x Acceleration = Gravitational Force
The extent to which expansion is speeding up
Due to the contents of the universe: Usually it is attractive, leading to deceleration
Energy Density + PressureEnergy Density + Pressure
Einstein’s Theory of General Relativity:Gravitational force is proportional to
Accelerating universe implies contents have negative pressure
Another View
Expansion Rate (or the speed of the rocket)
Kinetic Energy = - Potential Energy
Decreases as rocket gets further from Earth
Energy density due to matter decreases as universe expands
• Energy per particle is proportional to mass
• Number of particles in a fixed volume goes down as the universe expands (think of number of raisins in a fixed volume as the bread rises)
Decrease in energy density leads to decrease in Expansion Rate
What do we know about Dark Energy?
• Constitutes 2/3 of energy in the universe
• Is smoothly distributed and invisibleDoesn’t clump into galaxies like matter
• Has negative pressure Leads to Acceleration
What might dark energy be and how is it related to the rest of physics?
This is the deepest question confronting physicists today.
Nothing in the Standard Model of Particle Physics can be the dark energy.
It might be quantum mechanical fluctuations in the vacuum
• “Nature abhors a vacuum”
• Heisenberg’s Uncertainty Principle: even in empty space, particles (and anti-particles) pop in and out for short periods of time
• There is energy associated with these quantum fluctuations …
• And they produce negative pressure!
Vacuum energy is called a Cosmological Constant
When we calculate the energy density associated with vacuum fluctuations …
The energy is too big by 128 orders of magnitude: 100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
We have ideas …
• The cosmological constant calculation requires deeper understanding of quantum mechanics
• Perhaps the dark energy was not constant, evolves in time (this has happened in the past)
• Perhaps dark energy is a hint of the extra dimensions predicted by string theory
Joint Dark Energy Mission
NASA/DOE partnership to study dark energy from space: Launch in ~2010
Bulletin: FNAL has just joined the collaboration!
And we have plans …