Lecture 23: The Acceleration of the Universe Astronomy 1143 – Spring 2014

Key Ideas: Expansion of the Universe Scale factor Changes with time Universe is accelerating Observations of Hubble Law at high redshift Type Ia SNe What is causing the acceleration? Cosmological constant? Other form of dark energy?

Curved Spacetime Matter curves the spacetime around it: The least paths are curved lines. More mass = Greater spacetime curvature. Closer = Greater spacetime curvature. Einstein gave the equivalent of Newton’s Law of Gravity in the form of his field equations A freely falling object follows a curved path.

Applying GR to the Universe Einstein applied his original GR equations to the whole Universe and found it should be expanding or contracting Added this term to keep the Universe in a delicate balance

Cosmological Constant We refer to  as the cosmological constant – constant amount of energy for each chunk of space Turns out that Doesn’t make the Universe safely static Universe is actually expanding Einstein’s greatest mistake? Not the end of the story Accelerating Universe indicates a new source of energy – cosmological constant?

Expansion is not constant The Hubble parameter is not a constant in any Universe except an empty one Gravity All masses attract each other or Matter curves spacetime Slows the expansion Cosmological Constant or Other Physics Repelling force or curvature of spacetime Accelerates the expansion

Measuring the Expansion Key is measuring the Hubble law at earlier points in time = at different redshifts Measure brightness of a standard candle Get distance using relation between brightnesss, luminosity & distance Measure redshift and plot on the Hubble diagram

Acceleration & Deceleration If the Universe expanded more quickly in the past than now – deceleration Galaxies at distance d have a recession velocity larger than if expansion is constant If the Universe expanded less quickly in the past than now – acceleration Galaxies at distance d have a recession velocity smaller than if expansion is constant

In an accelerating Universe…. Very large distances = high redshift If the Universe is accelerating then objects at a distance d will show smaller redshifts than in a non-accelerating Universe, objects with a redshift z will be more distant than in a non-accelerating Universe Standard candles are fainter at a redshift z than in a non-accelerating Universe

Recession Velocity (cz) Distance (d) Accelerating (slower in past) Decelerating (faster in past) Steady slowfast near far (past)

Type Ia Supernova Type Ia supernovae are the best choice for measuring the Hubble law at high redshift Extremely luminous Evolutionary effects are (hopefully) smaller than for galaxies as a whole (e.g. Tully- Fisher relation But…. requires monitoring evolutionary effects may be important

Distant Type Ia Supernova Type Ia SNe are excellent standard candles: Exploding white dwarfs in binary stars Very Luminous (can see them very far away) Have a characteristic spectrum Peak luminosity – from decline rate

High-z Supernovae

d L (distance) Deviation from   =0,  m =0.3 model Redshift (z)  m =0.3,   =0 Accelerating Decelerating

Other Evidence Type Ia SNe are the most straightforward proof of the acceleration of the Universe Scientists are never satisfied with just one observation Concern about evolution of SNe – are early Sne the same as today’s? Being studied extensively in the Local Universe However, they are not the only proof Scale of large-scale structure Number of clusters agree with predictions of GR

Dark Energy The acceleration of the expansion of the Universe is (likely) caused by dark energy Form of energy that does not emit/absorb/etc photons energy matter Dark energy is even less well understood than dark matter. uniform Dark energy is a uniform energy field (unlike dark matter, it doesn’t “clump up”). Simplest form = acts like cosmological constant

What is causing the acceleration? Repulsive effect Energy source that is constant in space and time? First guess: Quantum vacuum energy, but too high by 120 orders of magnitude A field with negative pressure whose energy density changes with time? A modification to General Relativity Systematic effects in high-redshift Type Ia

The Search for the Jerk Determining the cause of the acceleration of the Universe is one of the great unsolved puzzles in physics today Luckily, we can continue to explore this observationally and theoretically Observationally, the main focus is testing whether the rate of acceleration changes with time Change in acceleration called the jerk If it is a constant energy density (= cosmological constant) then no change

Inventory of the Universe Photons Negligible amount “Normal” matter Observe amount of stars/gas/dust/planets in the Universe Dark Matter Observe gravitational force on visible matter Dark Energy Observe expansion history of the Universe

Inventory of the Universe