Adam G. Riess Space Telescope Science Institute, Johns Hopkins University For further reading: Scientific American Feb 2004, “From Slowdown to Speedup”, Riess and Turner The Runaway Universe by Donald Goldsmith The Extravagant Universe by Robert Kirshner Dark Energy and the History of Cosmic Expansion
Dark Energy and the History of Cosmic Expansion. Adam G. Riess. Space Telescope Science Institute, Johns Hopkins University For further reading: Scientific American Feb 2004, “ From Slowdown to Speedup ”, Riess and Turner The Runaway Universe by Donald Goldsmith - PowerPoint PPT Presentation
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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
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