# Presentation: 530024

## Dark Energy

- Expanding Universe
- Accelerating Universe

*Scott Dodelson*
*March 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 

- Fainter 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 + 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