# Presentation: 089043

## Seeing the Invisible Universe with Gravitational Lensing and SNAP

- Gary Bernstein
- University of Pennnsylvania
- November 30, 2005

## Questions for this presentation:

- Why is the Universe like a shower stall?
- What is the Universe made of?
- Was Albert Einstein right about General Relativity? Wrong? Wrong about being wrong?
- Why do we want to spend nearly $1B to measure the shape of little blots on the sky?

**Notes:**
Because no one ever cleans it?
One of these questions does not yet have a satisfactory answer.
2005 is 100-year anniversary of Einstein’s invention of special relativity and some bases of quantum mechanics. 
Focuses much of the rest of his career on understanding gravity.  Now testing basic results of GR using entire
Universe as our lab.

## General Relativity & Gravitational Lensing

**Notes:**
Spacecraft passing a planet gets deflected from straight-line path  by planet’s gravity, just like E orbiting Sun, M orbiting E, as Newton said.
A long-standing question about gravity:  will it bend the path of light just as it bends the path of a 
rock?  Maybe not, since light = pure energy has no mass, and would feel no gravity force.
Einstein’s answer from GR: gravity affects space itself.  Since light travels through space, it will be bent.
Tiny angle: 1.75” = dime at 1 mile.

## 1979: A real gravitational lens?

*Double quasar discovered by Walsh, Carswell, & Weymann 1979*
*Image from Hubble Space Telescope (Bernstein et al 1997)*

**Notes:**
See two bright quasars (= object billions of ly away) only 6” apart, astride a galaxy acting as lens.
But not convinced it’s a lens, could just be two similar-looking objects near each other on the sky.
Nature can produce similar-looking objects

## A double image, or just two similar objects?

*A double image, or just two similar objects?*

**Notes:**
some of us know this better than others

## But this is clearly a lens...

*Same color/spectra...*

*Same through radio telescope...*

*...and when image A gets brighter, image B does 1 year later!*

## Slide 7

## “Einstein rings”

- “Einstein rings”

## Real Einstein Rings & Arcs

## Slide 10

**Notes:**
Is lensing just a pretty party trick that confirms GR, or can we learn something else from lensing?

## Slide 11

## The recent history of the Universe:

*<1917: Assumed that Universe is basically unchanging.*
*~1920: Einstein & others realize that if GR is correct, it is very difficult to make the Universe stay still.*

**Notes:**
Demonstrate with two objects?

## The recent history of the Universe:

*<1917: Assumed that Universe is basically unchanging.*
*~1920: Einstein & others realize that if GR is correct, it is very difficult to make the Universe stay still.*
*~1930: Edwin Hubble shows Universe is expanding, not standing still!*

**Notes:**
Demonstrate with two objects?

## Will the Universe Recollapse?

*...depends how much matter (gravity) there is*

- Big Bang

- Big Crunch?

- Coasting?

## The recent history of the Universe:

*<1917: Assumed that Universe is basically unchanging.*
*~1920: Einstein & others realize that if GR is correct, it is very difficult to make the Universe stay still.*
*~1930: Edwin Hubble shows Universe is expanding, not standing still!*
*1930-2000: Astronomers debate whether Universe has enough matter to halt expansion and recollapse.*
*1980’s: Strong evidence that most of the matter in the Universe is invisible!*

**Notes:**
Demonstrate with two objects?

## Signs of Dark Matter

**Notes:**
Use Doppler shift to see how fast Milky-Way galaxies spin.
How much gravity does it take to hold it together?

## More signs of dark matter:

*Fritz Zwicky 1932: Galaxies in clusters should be flying apart.  Need extra mass to hold together?*

**Notes:**
Mt.Wilson
Invisible man - see the footprints
Wonder woman flies through a flock of birds.

## Gravitational multiple images, rings, and arcs confirm the presence of large quantities of dark matter.
Can we look for dark matter in directions that do not have lucky alignments with bright background objects?

- Can we look for dark matter in directions that do not have lucky alignments with bright background objects?

## Seeing the invisible with gravitational lensing:

*“**Weak gravitational lensing”*

**Notes:**
Even invisible mass has gravity and will bend light.  So we can “see” the presence of matter if there is a bright object behind it to make an Einstein ring or double image.
But even unlucky objects get distorted a little, can recognize the pattern.
Just need polka-dot “wallpaper” to put behind our galaxy cluster to inspect for lensing.

## Effect of cluster dark matter on the wallpaper:

*animation by J. A. Tyson et al, Bell Labs*

## Slide 21

**Notes:**
Insert movie!!!
With a few hundred galaxies, clearly see the trend to be tangent.

## Galaxy cluster CL0024+1624

*Galaxy cluster CL0024+1624*
*J. P. Kneib et al 2003*

## Dark Matter Revealed:

*M. Joffre et al 2000*

**Notes:**
Now: use many thousands of galaxies and measure their shapes, can reconstruct the nature of the dark matter “lens” that distorted them.

Answer to our question about fuzzy blobs!

## History of the Universe (cont’d)

- History of size of Universe = contents of the Universe
- Astronomers can look into the past
- Mars signals are a few minutes old
- Cassini signals are a few hours old
- Light from distant galaxies is a few BILLION years old
- How far away = how long ago.

## Mapping the History of Universe

- Supernovae = exploding stars.
- Brightness = distance = how long ago
- Hard part: find an exploding star 5 billion light-years away!
- 1998: two experiments to determine the expansion history of the Universe.  “Big Crunch” or “coasting” in the future?

**Notes:**
1 SNe per 100 years per galaxy.  Won’t get too far looking at 1 galaxy each night.
Look at 10,000 or more galaxies per night to find the ones with SNe.
Needs big digital cameras, computers to search (1990s)

## Surprise!

**Notes:**
Expansion is not slowing down at ALL - in fact it’s speeding up.
Anti-gravity - cosmological constant??
Something that causes repulsive gravity, unlike attractive matter.  “Dark energy”

## An accelerating Universe?

## Looking at the Big Bang:

*Wilkinson Microwave*
*Anisotropy Explorer*

**Notes:**
Get picture of different sky maps???
Size of these lumps tells us how far we have to look to see Big Bang.  Equivalently, how long ago was it?
14 Gyr, just as accelerating Universe would say, too old for a “normal” decelerating Universe.

## WMAP’s distance to the Big Bang:

**Age > 14 Gyr**

**Age = 14 Gyr**

**Age < 14 Gyr**

## Cosmic Background Fluctuations:

- More precisely: the peak of the power spectrum of CMB sky determines distance to the plasma epoch of Universe.

## Surprise!

**Notes:**
Expansion is not slowing down at ALL - in fact it’s speeding up.
Anti-gravity - cosmological constant??
Something that causes repulsive gravity, unlike attractive matter.  “Dark energy”

## What is the Universe made of?

**Notes:**
sure would be nice to know more about this dark energy.
cosmo constant? 
an error in GR’s formula for expansion of the Universe?
a bizarre quantum-mechanical effect pervading Universe?
Invisible; not even matter, so it doesn’t perceptibly bend light.  Can lensing help?

## Learning more about dark energy:

- The cosmic background tells us relatively little about what has happened **since** 14 Gyr ago.
- Determine the expansion history of Universe to greater accuracy using supernovae observed from ground and **SPACE**.
- Is it consistent with the cosmological constant?
- If not: is there some new “dark energy” field, or is General Relativity wrong? Can’t tell which from supernovae alone.

## The frothy Universe:

*Billion-light-year slice of the Universe*
*(Sloan Digital Sky Survey)*

## A view through the Universe

- A view through the Universe

**Notes:**
Better picture!!
Shower stall: the lumpy Universe is like the “obscure glass” on the shower stall.
Use the distorted pattern to learn about the (invisible) glass.
Why does this tell us about dark energy?

## The obscurity level of the Universe:

*...tells us the history of dark matter AND dark energy!*

**Notes:**
Plus distance to the sources
Lumpiness of the glass
Both are altered by dark energy.
Carefully measure the “obscurity level” - get back a history of Univ & its contents.

## What does weak lensing add?

- We can measure a power spectrum, just like for CMB, but it measures things more **recently** than 14 Gyr ago.
- WL can divide source galaxies into many distance slices and measure **many power spectra.**  Combine them to determine the **history** of dark energy.
- WL sees non-linear, **non-Gaussian** growth and collapse of mass structures.
- WL can measure and use the relation between **visible** matter and **dark** matter.
- WL can measure the growth of Universe’s size **and** growth of structure within it.  *This allows us to test whether GR is correct!*

## To measure the dark energy, just:

- Take pictures of roughly 1 billion galaxies
- Carefully measure the shape of each one
- Determine the lensing distortion to an accuracy of about 0.0001
- Then use General Relativity formulae to determine history of Universe size and lumpiness
- (...or maybe find that GR is incorrect)

## Start by measuring galaxy shapes...

*Start by measuring galaxy shapes...*

## End up with measurements of dark energy!

- Results for constant-w models from Jarvis, Jain, & Bernstein (2005) from the 75-square-degree CTIO lensing survey:
-  2 million galaxy shapes

## The Dark Energy Survey

- 500 Megapixel camera, 3 square degrees
- 500 nights on CTIO 4-meter telescope
- Shapes of 300 million galaxies over 5000 square degrees of Southern sky 2009-2013

## The LSST Project

- Ground-based 8-meter telescope to survey the entire visible sky every 4 days.
- Measure several billion galaxy shapes over a full hemisphere
- A single camera with >2 billion pixels.
- 4 GB image every 15 s
- 1 TB/hr, 8 TB/night, 2-3 PB/yr!
- Yours for only $300M!

## What I want for my science project

- The “cosmic distortions” have been detected and measured starting in 2000.
- Advancing rapidly using ground telescopes
- What we’d really like:
- A space telescope is above the distorting affects of Earth’s atmosphere.
- But Hubble has tunnel vision, not enough galaxies.

## A huge advance over Hubble:

*Supernova Acceleration Probe*
*(SNAP)*

*600 Megapixel Camera*
*on HST-sized Space Telescope*

*Size of HST image*

*SNAP camera*
*focal plane*

*SNAP could harvest galaxy*
*shapes hundreds of times*
*faster than the HST or JWST*

## The SNAP spacecraft

*To Earth & Sun*

*To cold space*

*Here comes the light*

## More SNAP pictures

## Next decade’s sky-eating monsters

## The future of cosmic distortions

- DES and then LSST will measure lensing over 5000-20,000 square degrees.
- The Joint Dark Energy Mission will survey thousands of square degrees with the high quality available from space.
- The Universe’s “obscurity level” will be determined to a fraction of a percent
- We will infer the properties of dark energy to high accuracy, combining WL with supernovae and other observations.
- What will we have to add to Einstein’s Universe to explain what we discover?

## Thanks to...

- The SNAP collaboration for designing (and building!) an incredibly powerful and efficient experiment.
- FNAL for hosting our meeting and this talk.
- The Dept of Energy, NASA, and the National Science Foundation for supporting our research in weak lensing and dark energy.
- You, (and the US Congress, we hope) for your interest in obscure but fascinating questions about the Universe.

**Notes:**
not funded yet - needs to be a research priority!