# Presentation: 259093

## E6: Astro/Cosmo/Particle Experiments

- Snowmass Closing Plenary
- July 19, 2001
- Tim McKay
- University of Michigan

- Conveners:
- Suzanne Staggs: Princeton
- Harry Nelson: UCSB
- Kevin Lesko: LBL
- Tim McKay: Michigan

- July 19, 2001

**Notes:**
I must admit that when I was first asked to be a convener of the ‘astro/cosmo/particle experiment’ section of Snowmass 2001, I thought the name was a little ridiculous. As a title, it is mysterious; as a category, it is pretty well all inclusive. In the end, though, my co-conveners and I found it useful to have this freedom. This is, in some sense, the “Snowmass way”. I need to stress at the outset that a lot of the work of bringing these sessions together was done by my co-conveners; Suzanne Staggs from Princeton, Harry Nelson from UCSB, and Kevin Lesko from LBL. Without such a diverse group of organizers I’m sure this session could not have covered the ground which it did.

I will give you a brief, incomplete, and personally biased report on some of the activities which took place under the banner of E6, with an emphasis on topics which I think are clearly important for the future of fundamental physics. As the last speaker before the end of Snowmass party, I may take some liberties. After a few hours at the party I’m sure you’ll forgive me.

## The place of “ACP” in fundamental physics

- ACP was there at the birth of fundamental physics
- Relativity from Michelson-Morley
- Discovery of the proton, electron and neutron with natural radioactivity
- Discovery of the muon, positron, and other particles with CR beams
- _The universe: a well equipped lab_

- July 19, 2001

**Notes:**
I want to begin with a few comments about the place of ACP experiment in fundamental physics. It is always useful to remember that this kind of experiment was there at the beginning. It was the failure of the Earth to move relative to the theoretically favored “ether” that led to the theory of relativity. Careful observations of natural radioactivity allowed for the discovery of the proton, electron, and neutron. Observations of cosmic ray interactions allowed the discovery of the muon, positron, and kaon; perhaps the greatest surprises in the history of fundamental physics.

These discoveries were made in this manner, rather than in the current accelerator approach, because the universe in which we live is an extraordinarily well equipped laboratory. Those things which can occur, do occur somewhere in the universe. The careful observer can use these ‘natural experiments’ to learn what is possible and how it occurs. And of course the universe is large! It probes in its behavior time, length, and energy scales inaccessible to our Earth-bound experiments.

Observing the experiments which nature arranges is difficult. She doesn’t provide onsite housing, and her schedule, believe it or not, is actually harder to predict than that of our national labs. As a result, we have had 50 years of unbelievably productive laboratory based fundamental physics. This approach was, during this time, much easier and more productive. I believe we are approaching a time where the science dictates a balanced mix of laboratory and observational science.

## ACP plays a key role now

- Watershed discoveries:
- Expanding universe (1920’s)
- Dark matter (1930’s)
- Black holes (1970’s)
- Neutrino oscillations (1990’s)
- Dark energy (3 years ago...)
- Explosive advance in cosmology
- A goal: take fundamental physics beyond the standard model
- The requirement: pursue every promising approach

- July 19, 2001

**Notes:**
Perhaps the best sign of this is the key role which ACP experiments play in fundamental physics now. Some of the best evidence for physics beyond the standard model comes from observations of natural phenomena. 

We have known since the 1930’s that the dynamical evolution of the universe is dominated a ubiquitous dark matter. We should all be ashamed that 70 years have passed, and the nature of this dominant form of matter remains unknown. It is not unknowable, we simply haven’t applied our collective energies to the problem. 

Black holes are real, in fact they seem to be common. The possible existence of singularities presents real problems for fundamental physics. 

Observations of neutrino oscillations are still most accessible using natural beams, and the recent SNO confirmation of the standard solar model confirms that these beams can be well understood. 

Most recently, a remarkable combination of high redshift supernova observations and cosmic microwave background observations have forced us to the conclusion that some form of repulsive gravity, some kind of dark energy, currently dominates the evolution of the universe.

I have to stress that our understanding of the universe is evolving explosively. Only three years ago, the notion of dark energy was anathema; today it is the standard model. You may chuckle and suggest that this illustrates the immaturity of cosmology. I believe that it is evidence of a golden era; akin to the sudden acceptance of the existence of quarks in the early 70’s. This shift has not occurred because of a change in fashion, but rather due to essentially inescapable experimental evidence.

If the goal is to advance fundamental physics beyond the standard model, we must pursue every promising avenue. If the physics leads us to dauntingly large and complex new colliders; so be it. It may lead to experiments which look nothing like those high energy physicists have done before. These new approaches are a great opportunity, both scientifically and personally, and I hope that many of you will have the courage to pursue them.

## Busy, diverse, highly incomplete schedule

- E6.1 CMBR experimentation: Staggs
-         7/6: Next Generation CMB Experiments: Polarization
-         7/7: Next Generation CMB Experiments: Sunyaev-Zeldovitch
- E6.2 Dark matter detection: Nelson
-         7/7: Existing Dark Matter Experiments
-         7/9: Future Dark Matter Experiments
- E6.3 The history of expansion/dark energy detection: McKay
-         7/11: Current Status of Dark Energy Experiments
-         7/14: Future of Dark Energy Experiments
- E6.4 Underground experiments: Lesko
-         7/11: Double Beta Decay
-         7/13: Next Generation Solar  and Supernova Neutrino Experiments
-         7/17: Proton Decay Experiments
-         7/16: A National Underground Laboratory
- E6.5 High energy astrophysics/cosmic rays: McKay
-         7/10: Large Astrophysical Neutrino Detectors (w/Barwick)
-         7/17: Future Cosmic Ray Experiments (w/Thompson)
-         7/13: Future Gamma-Ray Astrophysics Experiments (w/Buckley)
- E6.6 Tests of gravity/gravity waves: Nelson
-         7/4: Future Tests of Gravity and General Relativity

- Very selective “Snowmass” sample of activity

- July 19, 2001

**Notes:**
With that context, let me briefly describe what we did here at Snowmass. We held sessions in a total of six major areas: CMBR experimentation, dark matter detection, dark energy detection, underground experiments, high-energy astrophysics, and tests of gravity. 

This is nothing like a complete list of what’s going on in astro/cosmo/particle physics. It is, instead, a selective “Snowmass” sample. The selection was driven our perception of the interests of the HEP community. Topics were included because they provide direct information on fundamental physics (like CMB and dark energy detection) and because many members of the traditional HEP community are involved in them (like high-energy astrophysics and cosmic rays).

Though we had a lot of overlap with the P4 sessions Marc just talked about, we tried hard in every case to concentrate on the experimental issues. We reviewed current experimental results, and looked forward to the future; asking what will be possible in the coming decade and what we’ll need to accomplish it. There is no way I can summarize all of this, so I will apply some editorial license and emphasize just a few subjects which will give you the flavor of our activities.

## The “particle” part of E6

- Particle experiments
- Sometimes astrophysical beams
- Dark matter direct detection
- Emphasis on WIMPs and Axions
- Underground experiments
- Solar neutrino experiments
- Double beta decay
- Proton decay
- Gravity
- Extra dimensions

- July 19, 2001

**Notes:**
Let me begin by talking about what are, at their core, particle physics experiments. They are lumped into E6 essentially because they do not involve the use of accelerators. Some use astrophysical ‘beams’ instead, but when they do, they are concerned primarily with the physics of the particles, and not with the astrophysics of the accelerators.

Work in this general category was focused in our sessions on dark matter detection, underground experiments, and gravity.

## E6.2: Dark matter detection

- WIMP - massive Dark Matter particle, weak interaction cross section σ
- σ  enables dark/luminous matter balance
- SUSY broken at weak scale provides attractive candidate - LSP (Neutralino, )
- _SUSY discovery at accelerators would be ideal!_
- Favored SUSY models have no definitive floor on -nucleon scattering cross section σ
- σ 10-46 cm2 (=10-10 picobarn) significant
- tiny σ  » strong amplitudes cancel » when does that _ever_ happen?
- _Harry Nelson: UCSB_

- July 19, 2001

**Notes:**
As I noted, the dark matter problem is a real embarrassment, and I believe strongly that laboratory detection of dark matter deserves much more attention than it has received. Fortunately, there have been recent advances. The favored candidate for ‘cold’ dark matter is a weakly interacting massive particle, perhaps the lightest supersymmetric particle. Of course the IDEAL way to sort out the dark matter problem would be an accelerator detection of supersymmetry.

In the meanwhile, direct detection experiments place constraints on WIMP interaction cross-sections in the presence of background. A key goal is a cross-section limit of around 10-10 picobarns. Making SUSY WIMPS with cross-sections below this requires considerable fine tuning.

## Experiment  (CDMS, DAMA, Edelweiss)

- Various SUSY Models

**10****-10 ****pb**

- http://dmtools.berkeley.edu
- (Gaitskell/Mandic)

- Experiment  (CDMS, DAMA, Edelweiss)

- _Current Status of DM detection_

- July 19, 2001

**Notes:**
Current dark matter experiments disagree. The DAMA experiment reports a marginal detection from an indirect annual modulation analysis. The CDMS experiment rules out most of the DAMA region. More important though, you can see in this figure that the experiments are beginning to probe the interesting SUSY regions.

## Some Key Variables

- Mass - converted to kg Germanium, assuming coherence
- Time B between background events in years, for 1 kg and for a bin width in energy deposition of 1 keV.
- Experiments usually run until background limited... region shown on the plot...
- Plot B v. Mass  to see progression from 10-6 pb sensitivity (_now_) to **10****-10 ****pb**

- July 19, 2001

**Notes:**
When considering the future of dark matter detection, it’s useful to compare two parameters of any experiment. The first is mass. More mass means more interactions and hence lower cross-section sensitivity. The second is background, expressed here as the characteristic time between background events. Comparing these two parameters for various experiments gives a good idea of their sensitivities.

## Current comparison space

- July 19, 2001

**Notes:**
Here is the background time vs. mass plot for current experiments, showing CDMS and DAMA with comparable,  sensitivities, at about the 10-6 picobarn level.

## Future comparison space

- July 19, 2001

**Notes:**
Here is a guess about the future. You can see that a variety of experiments, using various techniques, promise to approach the desired 10-10 pb sensitivity. It is likely that at least some of these experiments will succeed, and that by 2010 or so we will know whether the dark matter could sensibly be called a SUSY WIMP.

I want to reiterate the importance of these experiments. Even if we find an LSP at a collider, it will be essential to confirm that it actually is the dark matter. I would encourage you all to watch this field closely, and contribute where you can, because I don’t want to explain to my grandchildren that we still don’t know what 90% of the matter in the universe is....

## Underground Science Lab

**Kevin T. Lesko**
**Institute of Nuclear and Particle Astrophysics**
**Lawrence Berkeley National Laboratory**

- July 19, 2001

**Notes:**
Let’s go on to the E6.4 sessions, a kind of grab bag united by similar techniques. Kevin Lesko organized these sessions in a way which emphasizes this relation. I will talk just briefly about solar neutrino and double-beta decay experiments.

## Solar Neutrino Experiments

- SNO confirms solar model!
- Sun is  source calibrated to 1%: the best low energy  source!
- Next generation experiments will be proposed in < 5 years
- H2O Cerenkov, Superfluid He, TPC, Doped liquid scint., Mo foils + scint.,   76Ge....
- Much more in neutrino sessions...
- Backgrounds are **the** major issue
- 	=> deep underground site?

- July 19, 2001

**Notes:**
The success of solar neutrinos, at super-K and SNO, is the great neutrino story of the last few years. Perhaps the most remarkable thing is that SNO has essentially confirmed the standard solar model. The sun would now appear to be a neutrino source calibrated to about 1%. 

Given this, a variety of new experiments will be proposed in the coming years. I won’t attempt to describe them, as I’m sure that some will be covered in neutrino sessions.

An important problem with them all , however, is background rates. These experiments would be greatly enhanced by a deep underground site.

## Double-Beta Decay

- Observing a factor of 2 increase in t1/2 each year
- 2- decay may be a major background for 0- experiments
- Only experimental approach to Dirac/Majorana nature of neutrinos
- May be the only direct approach to neutrino masses
- Next Generation Experiment Proposals ready within 1 to 2 years
- Significant overlap with Low Energy Solar Neutrinos and Dark Matter: again, background is a limiting factor

- July 19, 2001

**Notes:**
Double-beta decay experiments are also advancing well. They are important as perhaps the only direct approach to neutrino mass and the basic nature of neutrinos. Again, the time is ripe for new efforts, and again, background is a major limitation.

## A low background underground Lab

**World’s deepest Multipurpose Lab**
**Double beta decay**
**Low energy solar neutrinos**
**Supernovae searches**
**Dark matter searches**
**Proton decay**
**Long baseline**

**Extensive Nuclear Physics Discussion**
**Independent Multidisciplinary Committee Examination**
**NSF Proposal**
** ****Usual peer review**

- A new national lab?

- July 19, 2001

**Notes:**
The fact that a variety of experiments would benefit from a very low background environment has led a group of people to propose building a shared deep underground lab. It would serve a variety of important physics goals. This idea has received strong support from the nuclear physics community, and has been recently been proposed to the NSF.

I want to emphasize that the proposal here is for nothing less than a new national lab. This is striking request, one which will impact the HEP community significantly. It deserves your close attention.

## E6.6: Gravity

_**20μm foreseen: **_**Large extra dimensions start to look small...**

- V = VN (1+  e-r/) 95% C.L.

- _UW Torsion Balance:_

- July 19, 2001

**Notes:**
Let’s talk about gravity experiments.

Over the last few years a variety of proposals have been made suggesting the possibility of large extra dimensions. These might appear as deviations from Newtonian gravity on short distance scales. In a continuation of a long-standing set of beautiful torsion balance experiments, the University of Washington experimental gravity group is gradually testing this intriguing possibility.

Current limits place the characteristic length scale of a model deviation from Newtonian gravity below the 190um scale. This kind of experiment can probably be pushed to 20um or so. If so, large extra dimensions, at least within these ‘2 extra dimension’ models, are starting to look pretty small.

That’s a quick review of the ‘particle’ part of the E6 sessions.

## The “astro” part of E6

- Experiments *primarily* directed at understanding astrophysical phenomena: *can often challenge fundamental physics!*
- _Use HEP technologies/approaches_
- E6.5:
- Large astrophysical  detectors (w/Barwick)
- Future gamma-ray astronomy (w/P4 and Buckley)
- Future cosmic ray experiments (w/Thompson)

- July 19, 2001

**Notes:**
This brings us to the next part of E6, the “astro” part. In this section I have placed experiments discussed at Snowmass whose primary goal is the understanding of astrophysical objects. It is important to stress that these astrophysical phenomena can sometimes challenge fundamental physics. It is this fact, combined with the extensive use of HEP technologies and experimental approaches, which brings these particular astrophysics topics to Snowmass.

We had three major sessions in this area, on large astrophysical neutrino detectors, gamma-ray astronomy, and cosmic ray experiments.

## E6.5: ’s, ’s and CR’s

- _E6.5: ____’s, ____’s and CR’s_

- Km3  detectors are on the horizon.
- -ray astronomy closing the space/ground gapi
- Down from above: ACT’s
- Up from below: GLAST: 2005
- CMBR  photo-production cut-off and the IR background

- Current sensitivity

- 5-10 years

- Old energy
- gap

- July 19, 2001

**Notes:**
I will make just a few comments on these subjects. First, km3 detectors for astrophysical neutrinos are on the horizon. They will be able to augment what has been learned so far in experiments like MACRO and AMANDA.

Gamma-ray astronomy is about to achieve a long looked for goal; closing the gap in energy sensitivity between satellites and ground based gamma-ray experiments. Traditionally this gap ran from around 10 to a few 100 GeV, where the satellites were limited by collecting area and the ground based experiments by energy threshold. This gap is being closed now by experiments like STACEE and CELESTE. It will be eliminated in 5 to 10 years from above by ground based experiments like VERITAS and HESS and from below by the GLAST satellite.

This gap is particularly important because it is in this range that photoproduction of pions through interaction with microwave and infrared background photons limits our ability to detect distant sources. Observing this absorption cutoff in distant sources provides an opportunity to probe the intergalactic IR background.

## E6.5: ’s, ’s and CR’s

- Cosmic Rays: a primary goal is understanding acceleration
- Very large detectors needed for highest energies
- High-Resolution Fly’s eye and AGASA
- Auger: 5000 km2 air shower array in construction in Argentina
- Telescope array
- At lower energies: composition
- Balloons: HEAT, etc.
- Space: AMS on the space station

- July 19, 2001

**Notes:**
Probably the primary goal of cosmic ray physics, as it has been since the 20’s, is to understand the acceleration mechanism. This is particularly pressing at the highest energies, which require enormous detectors to accommodate the 1/km2/century rates. 

There is every reason to expect progress in this area, especially as the Auger experiment begins operation in Argentina.

At lower energies, questions of the detailed composition of cosmic rays remain the subject of intense exploration, both from balloons and in space.

## The “cosmo” side of E6

- Contents and evolution of the universe provide a direct constraint on fundamental physics
- Probes unique conditions (early universe)
- Probes unique timescales
- Only way to watch space-time evolve
- E6.1 and E6.3
- Cosmic microwave background exp.
- Dark energy detection

- July 19, 2001

**Notes:**
Let me set aside the astrophysical aspect of E6 sessions and turn to the last “cosmo” facet of E6. The connection between cosmology and fundamental physics has been discussed for decades. I think it is only now really coming to fruition, primarily because cosmological experiments have evolved away from the 50% error regime to the 10% regime and beyond.

This new maturity of experiments allows us to use the contents and evolution of the universe to probe unique conditions (like the early universe), to probe unique timescales (like 1010 years), and to actually watch a space-time evolve.

We had two sessions on cosmological topics, E6.1 discussed microwave background experimentation, and E6.3 discussed experimental constraints on dark energy. 

## E6.1: CMBR experiments

- CMBR is advancing to few % science.
- Anisotropy accessible to many exp.

- Many exp. combined...

- July 19, 2001

**Notes:**
Suzanne Staggs organized some very nice sessions on CMBR experimentation.

CMBR experimentation is important to us both for the results it provides and for the example it offers. A 30 year program of difficult experiments and technology development has led us to a state where the density fluctuations at the time of recombination are now known on many scales to better than 10% precision.

A key element in this has been the very recent development of individual experiments which can map these features. It used to be that we had to combine many results to get an idea of the spectrum of CMB temperature fluctuations. Now single experiments can do it.

## Big step: single exp. detections

- Carlstrom et al., Chicago and MSFC

- July 19, 2001

**Notes:**
Here’s a quick example. The DASI experiment, led by John Carlstrom at Chicago, has made beautiful 3.4 degree maps of many regions of the sky seen from the South Pole. In them the temperature fluctuations are detected at high signal-to-noise.

## Big step: single exp. detections

- _Big step: single exp. detections_

- Flat universe

- Baryon density

- DASI example: Pryke et al. astro-ph/0104490

- total=1.04+/-0.06

- July 19, 2001

**Notes:**
When these maps are Fourier analyzed, they allow strong constraints to be placed on various cosmological parameters. The most important discovery is that the overall geometry of the universe is flat: total=1.04+/-0.06 in the case of DASI alone.

## The future of CMB anisotropy: MAP + Planck

- Wayne Hu: Chicago

- July 19, 2001

**Notes:**
The future for CMBR anisotropy is, of course, right now. The MAP satellite was successfully launched on June 30. Everything is nominal and it will receive it’s lunar assist to the L2 Lagrange point in just 11 days, on July 30. It will provide really exquisite maps of all-sky anisotropy, with errors like those shown here. 

In 2007, the ESA satellite Planck will launch, extending the MAP results to still smaller angular scales.

## CMB Polarization

- Polarization imprinted before decoupling and during reionization
- Hints of the inflaton potential
- This is the new frontier for CMB

- E modes: density fluctuations and reionization

- B modes: **inflaton potential**

- Sarah Church: Stanford

- July 19, 2001

**Notes:**
The long term goal is to measure CMB polarization. This polarization pattern was partly imprinted before recombination, and so may offer some hints about the nature of the inflaton potential. This is the new frontier for CMB experimentation, but it is extremely challenging. The curl, or ‘B modes’ of the polarization, which may contain the inflaton information, have fluctuations 200x smaller than the temperature fluctuations currently being measured to 10%. So this is something for the next decade and beyond.

## E6.3: Dark energy detection

- _E6.3: Dark energy detection_

- Biggest surprise since Snowmass ‘96
_*Is it real?*_

- Combination of SNe, matter density, and CMBR makes dark energy inescapable

- You are here:
- 0.7    m0.3

- July 19, 2001

**Notes:**
I believe that the single biggest surprise since Snowmass 1996 was the discovery of cosmic acceleration, now usually called dark energy, in 1998. These results were so surprising, shocking really, that most of us doubted them at first. A furious effort to constrain supernova systematics has only piled on more confirmation. 

The nail in the coffin came with the Boomerang and Maxima CMBR results a year ago. Their confirmation that the universe is flat, combined with long-standing knowledge of the matter density, makes dark energy an experimental conclusion which is difficult to escape. This was certainly the consensus of our first E6.3 session here at Snowmass.

We live in a universe dominated, slightly, by some form of smoothly distributed, repulsive gravity. Acceptance of this fact is the most remarkable paradigm shift I have observed in science.

## Why is it important?

- Because it isn’t zero....Comments on dark energy
- J. Harvey: “Basically, people don’t have a clue as to how to solve this problem”
- S. Weinberg: “Right now, not only for cosmology but for elementary particle physics, this is the bone in our throat”
- E. Witten: “...would be number one on my list of things to figure out”

- From Science Times, 30 Nov. 1999, J. Glanz

- July 19, 2001

**Notes:**
Why is dark energy so important? We knew all along that it should be there. Einstein allowed for it, and the notion of vacuum energy which exerts negative pressure is a basic idea in quantum theory. The problem, as Marc stressed, is that every attempt to calculate it gives absurd answers.

As a result, we did the conservative thing, and assumed some symmetry must force it to be exactly zero.

The difficulties which this experimental reality creates for fundamental physics are illustrated by the quotations you see here. It is, really, a ‘bone in our throat’.

## Dark energy experiments

- Fundamental cosmology: must observe the expansion history of the universe
- _Inaccessible to accelerators_
- Many approaches: SNe standard candles, cluster counting, lensing, Lyman- forest...
- CMBR is *not* sensitive to this
- Different sensitivities and systematics: all must be pursued
- Only the SNe approach has detected dark energy and faced its systematic errors: it will play a crucial role in the decade to come
- Nature provides a SNe ‘beam’ to allow us to view the evolution of the expansion rate.

- July 19, 2001

**Notes:**
To detect dark energy, we must observe the expansion history of the universe. There is no other handle on it. This is fundamental physics essentially inaccessible to accelerators.

There are many astrophysical approaches to measuring the expansion history. SNe standard candles of course, counting of clusters galaxies, gravitational lensing, studies of the Lyman-alpha forest, etc. Each has different sensitivities to dark energy, and most important, each has different systematic issues. Given the importance of this problem, it is likely that all these techniques will be pursued.

It’s worth noting that the CMBR, which views the universe at a single, very early, epoch, is essentially insensitive to dark energy. This is one problem which the CMBR will not solve.

Of all the available techniques, only the SNe approach has so far detected dark energy, and only the SNe have faced the systematic errors which will ultimately limit sensitivity. As a result, SNe are bound to play an important role in the coming decade. 

## Already systematics limited!

- Late 1998....

- July 19, 2001

**Notes:**
The key thing to understand about Sne observations is that ground based experiments are already systematics limited. Even in late 1998 the statistical and systematic errors were comparable.

If we are to make better measurements of dark energy, all the work will be in controlling systematic errors. This will be true for every measurement of dark energy. Only the supernova measurements are so far mature enough to be facing it now.

I will finish up by describing the SNAP project, a particular approach to the SNe measurement of dark energy.

## Taking advantage of the SNe beam

- _Taking advantage of the SNe beam_

- Space provides the systematics controls: stability, access to IR observations, high-z

- July 19, 2001

**Notes:**
If we want to take full advantage of the SNe beam which nature provides, we have to make observations from space. 

The Supernova Acceleration Probe is a satellite dedicated to extracting all the information possible from the SNe beam. Placing it in space will provide excellent control of the systematics which currently limit supernova observations. It will provide exquisite stability (no weather), access to crucial near-infrared observations which are inaccessible from the ground, and the ability to detect and follow supernovae to high redshift.

## Why space? Systematic error control

- _Why space? Systematic error control_

- July 19, 2001

**Notes:**
By using a space telescope, SNAP can obtain excellent control of each of the systematic errors currently limiting our understanding of dark energy.

## Where will we get?

- Includes systematic errors

- July 19, 2001

**Notes:**
What will the SNAP experiment provide? This figure shows the target SNAP errors. They are limited by the systematic errors expected for the SNAP design. 

It is important to stress that many predictions of dark energy sensitivity do not include any systematic error estimates. As a result, you will sometimes see ridiculously tiny error bars predicted for various methods. It is essential in assessing the future of this important experimental subject to bear this in mind.

## What will we learn?

- Basic nature of dark energy
- Equation of state: w = p/
- Good constraints on *evolution *of dark energy: w

- July 19, 2001

**Notes:**
What will we learn about dark energy? The first question is simple: is it a cosmological constant, or something else? A simple way to determine this is to measure its equation of state parameter ‘w’, its ratio of pressure to density.  If it is a cosmological constant, w will be –1. This will provide the first crucial clue for resolving the dark energy problem.

## Variations of w

- Constant
- w models

- SNAP simulated data

- July 19, 2001

**Notes:**
It will also be possible to constrain time variation of this parameter w. A variety of models predict this. Constraining it reasonably will require the high-redshift supernovae which only a space-based SNAP experiment can provide.

## How will it affect fundamental physics?

- “It is difficult for physicists to attack this problem without knowing just what it is that needs to be explained; a cosmological constant or a dark energy that changes with time as the universe evolves, and for this they must rely on new observations by astronomers. Until it is solved, the problem of the dark energy will be a roadblock on our path to a comprehensive fundamental physical theory.”
- 		Steven Weinberg, 2001

- July 19, 2001

**Notes:**
How will these results affect fundamental physics? As Weinberg has pointed out, they will provide the starting point for explaining a deeply fundamental problem. 

We now know that dark energy is there. But that’s all we know. If we are to understand this remarkable problem, we must learn more about it. The supernova approach is the only mature approach, and the SNAP experiment was conceived to take full advantage of the opportunity.

## Some E6 Conclusions

- The universe *is* our laboratory. It provides:
- A bath of DM particles
- Beams of neutrinos
- Spacetime singularities
- A big-bang
- A chance to watch the evolution of a space-time
- Fundamental physics, esp. the unification of gravity with other forces, will not be understood  without appeal to astro/cosmo/particle experiments

- July 19, 2001

**Notes:**
Well that’s a lot of physics in a half hour! And I left most of it out....

I won’t keep you from a much-deserved “end of Snowmass” party any longer. I’ll just leave you with the renewed message that the universe is our laboratory. It provides us with unique opportunities, like the chance to watch a big bang, and to observe the evolution of a space-time.

The problems of fundamental physics, and especially the unification of gravity with the other forces, will not be understood with appealing to astro/cosmo/particle experiments. I encourage you all to ponder this seriously as you think about your own future approaches to fundamental physics problems.