Dark EnergyDark Energy

Expanding Universe

Accelerating Universe

Dark Energy

Scott DodelsonMarch 7, 2004

The Universe is Expanding

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

Acceleration is surprising

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

As universe expands, it should become more dilute …

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

Dark energy density must not decrease with expansion!

Dark Energy is causing the universe to accelerate …

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 …

Conclusions

• The Universe is accelerating

• Dark Energy is therefore the dominant constituent in the universe

• We do not know what dark energy is (vacuum fluctuations? Extra dimensions?)

• We are planning missions which will tell us