the future of dark energy experiments: beyond the...

FSCIS DE Workshop December3, 2010

The Future of Dark Energy Experiments: Beyond the Horizon

Steven M. Kahn Kavli Institute for Particle Astrophysics and Cosmology

Stanford University

Thinking Out of the Box?

*  To date, essentially all of our ideas about how to probe dark energy experimentally have involved techniques to measure the expansion history and growth of structure in the Universe with improved precision?

*  Are we missing something? Is there a way to potentially probe dark energy directly, i.e. to measure something in the laboratory?

*  Various people have thought hard about this, but no consensus has emerged.


Martin Perl’s Idea

*  Martin points out that the energy density in dark energy is not especially small by experimental standards:

*  For comparison, the energy density in an electric field of strength 1 V/m is 100 times lower. We clearly have the experimental tools to measure fields and forces of that magnitude.

*  Martin suggests using atom interferometry, possibly in space, which is exquisitely sensitive to measuring small forces with high precision.

*  To make this work however, he must make two crucial assumptions: –  First, that the dark energy field couples to matter by some force other than gravity. –  Second, that that force is local, i.e. that there is a gradient in the energy density on laboratory scales. This

is required to introduce a potential difference, which shows up as a phase shift between interfering atomic beams following two different paths in the experiment.

*  Neither assumption is especially well-motivated. So a failure to detect an effect in an experiment like this does not tell us much about dark energy.


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ρDE = 6.3×10−10J /m3

What About Continuing Along Current Lines?

*  On the other hand, if we stay with the idea that we have to use the universe itself as the laboratory, what will be the experimental directions after the next series of experiments (LSST, Euclid, WFIRST, BigBOSS)?

*  Of course, the answer to that question depends on what we measure. If there is a positive, convincing detection of a departure from ΛCDM, then we will want to follow that up somehow, but it is a bit pointless to speculate on this in the abstract.

*  Therefore, a more interesting question in my view is to think about what we will do if w continues to be consistent with -1 at all redshifts. Is this field over at that point?

*  Here, I will try to argue that the answer is “no”, but that we will have to change the argument about why we need to build bigger and more costly experiments.



A Brief Digression on the Future of Space Astrophysics

*  I entered the field in the mid-1970s. At that time, space astrophysics mostly involved exploratory investigations in new wavelength bands - not visible from below the atmosphere (X-rays, γ-rays, ultraviolet, infrared).

*  The main goal of those missions was to see what is out there. A range of interesting phenomena were found.

*  In the 80’s and 90’s, arguments for new missions were tied to “discovery-space”: –  New technical capabilities opened “new windows” - e.g. high resolution, imaging,

spectroscopy, polarization, new wavelength bands, etc. –  A set of planned measurements was used to guide the design, but the real

motivation was the enabling of unanticipated discoveries. –  Our understanding of the basic structure of the Universe was still fairly crude, so

theoretical motivations had less weight that they do today.


The Great Observatories

Hubble Space Telescope - April, 1990

Chandra X-ray Observatory - July, 1999

Compton Gamma-Ray Observatory - April, 1991

Spitzer Space Telescope - August 2003


The Future of Space Astrophysics

*  As things evolved into the mid- to late 90’s, the emphasis changed from further expanding discovery space, to addressing grand themes and questions.

*  Infrared, optical, and ultraviolet missions became captured under the umbrella theme of “Origins”, while the high energy astrophysics and gravitational missions were collected into the “Beyond Einstein” program.

*  Whether or not this approach was successful is still unclear. Only a small fraction of the Origins missions have moved into development, and none of the Beyond Einstein missions has yet been really started.


The Future of Space Astrophysics

*  But too strong an emphasis on answering specific questions has its drawbacks.

–  Often the questions get “old”. It takes 15+ years to develop a major space mission from concept to launch, even when the funding is there. 15 years is a long time - fashions in science change on that timescale.

–  Quantitative questions impose quantitative requirements. Graceful descopes become more difficult to accommodate. As costs start to grow, and mission reach starts to decline, we run the risk of losing the mission entirely, if the quantitative requirement can no longer be met.


The the Future of Space Astrophysics

*  The biggest challenge we face as we look to the next decade and beyond is undoubtedly associated with the tremendous growth in mission cost.

*  “Opening up discovery space” is too ill-defined to justify billions of dollars in the current climate. Answering specific questions might work if the questions are both sufficiently profound and sufficiently accessible to the public. However, continuing to push the limits on Δw probably does not qualify.

*  Instead, my belief is that a new transition will be required - a transition to “Beyond Science”. We must couple our field to goals that the public can viscerally attach themselves to, whether or not they understand the scientific measurements and analyses that will be performed with the data.


Some of these “Beyond Science” Themes are not new…

*  Discovering life on another planet. –  Would have deep

philosophical implications affecting our view of our place in the Universe

*  Imaging of the event horizon of a black hole. –  Would yield direct “contact”

with the extremes of space-time, providing a gut feeling for relativistic effects, far outside of normal human experience.


Beyond Science

*  In this category, I would add a new challenge: “Mapping the structure of all matter in the Universe”.

*  The drive to map and catalog has obvious scientific utility. But I would argue that this is also a larger human endeavor. “We should map it because it is out there, and it is the only universe we have.” The database will be there for generations to come, and would probably ultimately be used in ways we can cannot imagine today.

*  Mapping missions form an intermediate case between discovery-space driven missions and question-driven missions. The scientific requirements can be well-formulated in quantitative fashion. The goal is to assemble the archive. However, the science is not predetermined. The questions that the archive will answer can be formulated later.


Analogy to the U.S. Census

*  The U.S. Census Bureau conducts a complete census of all U.S. households every ten years.

*  They have specific analyses in mind, but the primary goal is to assemble the data into an archive that can be queried in many different ways.

*  The figure at the right is an example of the simplest form of output, showing population density as a function of location in Arizona. But, once familiar with the tools, you can use the database to plot all sorts of interesting things, e.g. fractional excess of women over men in rich households as a function of location.

*  The discoveries come from the formulation of the questions, not just from the original measurements.


An Example of What We Would Measure


Resolution Degrades With Distance *  The illustration at the right is a map of the

world, as constructed by Saul Steinberg from his apartment window on 9th Avenue in Manhattan. (It was used as a cover by the New Yorker in March, 1976.)

*  The map is quite detailed close in, but the resolution (and accuracy) degrades rapidly with distance.

*  Nevertheless, it sill maintains the essential features, e.g. there are mountains just west of Nebraska, the Pacific Ocean separates California from Japan.

*  Since we will always make astronomical maps from a given vantage point in space, we will always suffer this problem. The trick is in deciding how much to invest to improve the more distant renderings. (If Saul had a telescope in his window, he would have done a somewhat better job.)


Fortunately, Structure Evolves Rapidly With Time

*  As we look further out, we look further back in time.

*  But the structure in the Universe grew rapidly with cosmic time.

*  Depending on the question, high resolution over the whole sky may be less crucial at high redshifts. This is fortunate, because extending detailed maps out the most distant redshifts is the most demanding technically.


Mapping Starlight - Wide-field Optical Imaging

*  The easiest way to find and map the distribution of galaxies, is to observe their starlight.

*  This is nothing new, imaging surveys of the optical sky date back to the early days of astronomy.

*  The real challenge is to go faint and wide. To accomplish this in a reasonable period of time requires high étendue.


Importance of Étendue

*  The solid angle surveyed per unit time, to some limiting flux, F, at a signal-to-noise ratio, SNR, in exposures of time, t, is given by:

Here AΩ is the étendue of the telescope, ε is the efficiency of the system, Fsky is the sky intensity, δΩ is the size of the seeing-limited PSF, and αdet is proportional to detector noise and trap depth.

*  Large surveys require high étendue. But with conventional optical designs - high étendue requires large detector area.

*  The ability to construct very large detector arrays has only recently become available.

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dΩdt = F 2 AΩε

SNR2ΦskyδΩ+ αdet / t[ ]

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The Large Synoptic Survey Telescope (LSST)

*  LSST has the largest étendue for survey science that has ever been proposed - roughly 2 orders of magnitude greater than that of currently operating facilities.

*  It incorporates an 8.4 m diameter primary mirror, with a 9.6 square degree camera.

*  LSST will observe 20,000 square degrees of sky down to ~ 27th magnitude, yielding a sample of a few billion galaxies out to z ~ 1 - 1.5.

Physics 290 Seminar November 30, 2010

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LSST is Probably the Largest Survey Experiment We Should Do from the Ground

*  From the ground, the number of galaxies per squ. arcmin levels off at mag 26.5.

*  With the LSST etendue, this depth can be achieved over the entire visible sky.

*  We won’t resolve more galaxies at fainter levels.


Would it make sense to extend this to space?

*  To map the distribution of galaxies at higher redshift will require surveying in the IR, and thus from space.

*  JWST will get down to mAB~ 31 in its deep surveys (5 x 105 s). At z > 6 or so, there will be ~ a few hundred galaxies per square arcmin in this magnitude range. The NIRcam field of view is 2 x 4.7 square arcmin.

*  To cover 20,000 square degrees of sky with the current system would take 250,000 years. With an IR camera the size of the visible camera on LSST, this comes down to ~ 30 years. It is a stretch, but not completely infeasible!

Can We Get Redshifts in the Same Experiment?

*  Transition Edge Sensors (TES) are microcalorimeters, where the absorption of a single photon causes a measurable heat signal that can be used to determine the energy of that photon.

*  This technology is in heavy use in X-ray astronomy, since the achievable energy resolution rivals that of dispersive spectrometers, which have much lower quantum efficiency.

*  However, the same approach can be used in optical and near IR spectrophotometry to produce an imaging array with intrinsic energy resolution.

*  The best resolution to date has been ~ 0.16 eV in the optical band, which implies R ~ 10-20. This is less useful in the NIR, unless we can improve the resolution.

*  The biggest issue has been in fabricating larger arrays of pixels. Current TES arrays are only ~ 1 kpixel. We need a factor of 106. This requires a new approach to multiplexing.



Finding galaxies in H I might even be more attractive…

*  The Square Kilometer Array (SKA) will observe the universe in the 21-cm line of neutral hydrogen.

*  Predicted sensitivity enables the detection of a z ~ 3 galaxy in a reasonable period of time.

*  With a 10 square degree field of view, it can survey the whole sky in a year of operation, cataloguing ~ 1 B H I emission galaxies out to z ~ 1.5.

*  The redshift comes for free, since we are looking at an emission line.

*  Comparison of SKA and LSST maps will be tremendously useful for tracing the relationship between gas and stars as galaxies evolve.


COSMOS Dark Matter Maps (Massey et al. 2007)

*  A beautiful application of weak lensing tomography has now been accomplished by the COSMOS team using ACS data from Hubble.

*  With multicolor imaging from various bands, they have separated out the lensing contributions from background galaxies at various redshifts, thereby determining the 3D structure of the intervening dark matter.


Overlays with Baryonic Maps


This work can only be done from space…


Requirements to Map the Sky?

*  COSMOS was a 1.6 square degree survey, using 577 contiguous ACS pointings. It gets down to IF814W < 26.6, yielding ~ 80 resolved galaxies per square arcminute.

*  The total data set represented 583 orbits, or roughly 1.2 Msec of observing time. Covering 20,000 square degs with this system (assuming the ACS was still available) would take ~ 450 years.

*  But the ACS field of view is only 11.3 square arcmin. The full survey could be accomplished in ~ few years with a mission with a field of view ~ few tenths of a square degree. This could be done with a mission like WFIRST if they devoted adequate priority to lensing and chose to survey a large area of sky.


Diffuse Baryons at Low Redshift

*  Detailed analyses over the past ten years have shown that galaxies can only account for ~ 10% of the baryonic mass at low redshift. This is sometimes called the “missing baryon problem”.

*  There is no problem at high-z. There, Ly-α clouds plus galaxies account for all of the baryons.

*  It is now widely believed the missing baryons at low-z must be in the form of a warm hot intergalactic medium, with characteristic temperature in the range 105 < T < 107 K.

*  Gas is believed to be shocked up to these temperatures, as it falls into the potential wells formed by the dark matter filaments.

From Cen and Ostriker 2006.


Diffuse Baryons at Low Redshift

*  This gas is most easily mapped via X-ray emission lines, especially lines of helium-like and hydrogen-like oxygen, which are expected to be the brightest features.

*  However, the emission is very faint, and is complicated by copious backgrounds and foregrounds associated with distant AGNs, emission from the hot interstellar medium of the galaxy, and particle backgrounds in space.

*  The optimal detector is an X-ray imaging cryogenic calorimeter, which can provide the requisite energy resolution with high quantum efficiency.


Requirements for a WHIM Mapping Mission

*  Again, the system étendue is the key parameter. For a grazing incidence X-ray telescope, the focal ratio is essentially fixed by the graze angle requirements, and the étendue is then independent of focal length - It depends only on the physical size of the detector array.

*  For typical parameters, one finds: AΩ ~ 0.012 d2 cm2-sr, where d is the physical size of the detector in cm.

*  Theoretical predictions of the oxygen line intensity are ~ 0.03 - 0.2 photons cm-2 s-1 sr-1. The backgrounds and foregrounds can be 100 times greater. However, the WHIM should be structured in filaments, following the dark matter. Assuming a typical clumping factor ~ 10, the signal-to-noise ratio in a given energy bin of width ΔE (in eV) and observation time t (in units of 107 s) is given by: SNR ~ 5.6 d t0.5 (ΔE)0.5.

*  Clearly, this mission is only feasible, if we can make very large cryogenic detector arrays ~ 100’s of cm in size. To date, the largest ever constructed has been ~ 1 cm in size.


Summary

*  If the next generation of dark energy experiments find consistency with ΛCDM, it will be difficult to motivate future more expensive follow-ons based on dark energy science alone. However, an argument that we should “map everything in the Universe” might be compelling in its own right.

*  Ground-based experiments will make important contributions to mapping the starlight and cold gas in galaxies. However, space is required at higher redshift, and for mapping the dark matter through weak lensing, and the diffuse baryons through X-ray emission lines.

*  In all cases, the primary technical requirement involves the development of very large cameras (visible, IR, X-ray), which directly lead to large system étendue. This should be a priority area for technology investment now, if we hope to pursue these goals in the next decade.

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