Adam G. Riess

Space Telescope Science Institute,Johns Hopkins University

For further reading:

Scientific American Feb 2004, “From Slowdown to Speedup”, Riess and TurnerThe Runaway Universe by Donald GoldsmithThe Extravagant Universe by Robert Kirshner

Dark Energy and the History of Cosmic Expansion

The Universe looks static…

…but it’s not!

Hubble’s Law: In an expanding Universe, velocity= x distance

The Expanding Universe

Hubble (1929)•Velocities=Doppler shift=z•Distances: Brightest “stars”

One of the original reasons for building HST was to measure sets the scale of the Universe and its age

The Expansion Rate may be Slowing down or Speeding Up

• (Dark) Matter provides attractive gravity which should be decelerating Universe•But Einstein thought Universe static, GR allowed forrepulsive gravity, i.e., cosmological constant (Dark Energy),that could repel gravity and keep Universe static

“Standard” Candles

Bright=near dim=far

How far away?

Receding how fast?

Doppler Effect

WeNeed aStandard candleWe can seeFar away,Back in time

Measuring the Expansion History:Changes in scale/Changes in time

At the End of its Life, a Star is Reborn

• as bright as 4 billion Suns• can be seen halfway across the Universe

Subrahmanyan Chandrasekhar 1910-1995

Bang! A star explodes…

1987: The galaxy next door

Four months in the life of a SN Ia Age:Age: -6 days

Maximum

+26 days

+47 days

+102 daysElements produced=Fe,Co,Ni,Si,Ca

Vejecta=10-20,000 km/s

On the Rise:

Powered by radioactivity

How do we Find Supernovae?

Improving the Discovery Odds

1 SN per galaxy per 100 years =1 SN per galaxy per 100 years = 1 SN per 100 galaxies per year =1 SN per 100 galaxies per year = 10 SN per 1000 galaxies per year, if 10 SN per 1000 galaxies per year, if

monitor them frequently enough.monitor them frequently enough.

Searching by Subtraction

By 1998 two teams measured ~100 SNe Ia Surprise! The Universe is accelerating, propelled by dark energy.

High-z

SCP

The Accelerating Universe

High Redshift SN and Expansion History F

aint

er

Brighter

1.0

0.5

0.0

-0.5

-1.0

Rel

ativ

e B

right

ness

(m

)

0.0 0.5 1.0 1.5 2.0

Redshift z

Freely expanding

Constant deceleration

Acceleration/Deceleration

Constant acceleration

Found: Ground,Followed: Hubble

Dark Energy Dominated Dark Energy Dominated

present past

Freely expanding

Constant deceleration

Constant acceleration

Why is The Universe Accelerating?

(Honestly, we have no idea…)

•Dark Energy (e.g., “nothing weighs something”)•There are more spatial dimensions than we can see (string theory)•We don’t understand gravity (especially at long range)•Insert new ideas <here>

We need a sanity check: Was the Universe ever not accelerating? (i.e., was it ever decelerating?)

Before Acceleration there was Deceleration

If the Universe was always accelerating apart, how did the galaxies, stars, and planets form?

Hubble Space Telescope

Searching for SNe Ia with ACS on HSTSearching for SNe Ia with ACS on HST

ACS imaging, spaced by 45 days

CDFS HDFN

Hubble Spies an Ultra-distant SN

Aphrodite (z=1.3)

ACS grism spectrum

NICMOS F110W

ACS F850lp

viz

Highest z spectrum of a SN

Color Images of SNe Ia

High Redshift SN and Expansion History F

aint

er

Brighter

1.0

0.5

0.0

-0.5

-1.0

Rel

ativ

e B

right

ness

(m

)

0.0 0.5 1.0 1.5 2.0

Redshift z

Freely expanding

Constant deceleration

Acceleration/Deceleration

Constant acceleration

Found: Hubble,Followed: HubbleDark Matter Dominated Dark Matter Dominated

Found: Ground,Followed: Hubble

Dark Energy Dominated Dark Energy Dominated

present past

Freely expanding

Constant deceleration

Constant acceleration

A Primer on Dark Energy• It accelerates the expansion of the UniverseIt accelerates the expansion of the Universe• It makes up most (~2/3rds) of the UniverseIt makes up most (~2/3rds) of the Universe

• It will determine the fate of the UniverseIt will determine the fate of the Universe• We do not understand what it is!We do not understand what it is!

vacuum energy, mini-inflation, extra dimensions, super- horizon fluctuations, Chaplygin gas, new physics, ??

We need a way to measure the nature of Dark Energy

Dark Energy is repulsive Gravity(how strong is it?)

In GR, gravitational force

+1/3 0 -1/3 <w<-1 -1

Cosmological Constant (vacuum)

If w<-1/3 the Universe accelerates

(mini-inflation)

Determining the nature of Dark Energywill require many more Supernovae from

HST and measuring very small (few %) differences in the expansion history.

NASA Plans to service Hubble in 2007 and add a new camera.

Can we keep Hubble alive?

Need to send robots or astronauts in shuttle by 2008 or else…

The physics of Dark Energy will determine the fate of the

Universe.

T-minus 10 minutes to Rip…

Nearby SN Ia, SN 1994D, seen by HST

The End