1912.09992.pdf

arXiv:1912.09992v1 [astro-ph.IM] 18 Dec 2019

Astro2020 White Paper

Theoretical Astrophysics 2020-2030

Thematic Areas:

Ground-Based Project

Space-Based Project

⊠

Infrastructure Activity

⊠

Technological Development Activity

⊠

State of the Profession Consideration

Principal Author:

Name: Juna A. Kollmeier

Institution: Carnegie Observatories, 813 Santa Barbara St

reet, Pasadena, CA 91101, USA

Email: jak@carnegiescience.edu

Co-authors:

Lauren Anderson, Flatiron Institute, landerson@flatironi

nstitute.org, Andrew Benson, Carnegie

Observatories, abenson@carnegiescience.edu, Tamara Bog

danovi ́c, Georgia Tech,

tamarab@gatech.edu, Michael Boylan-Kolchin, UT Austin, m

bk@astro.as.utexas.edu, James S.

Bullock, UC Irvine, bullock@uci.edu, Romeel Dav ́e, Univer

sity of Edinburgh, rad@roe.ac.uk,

Federico Fraschetti, University of Arizona, frasche@lpl.

arizona.edu, Jim Fuller, Caltech,

fuller@tapir.caltech.edu, Phil Hopkins, Caltech, ph@tap

ir.caltech.edu, Manoj Kaplinghat, UC

Irvine, mkapling@uci.edu, Kaitlin Kratter, University of

Arizona, kkratter@email.arizona.edu,

Astrid Lamberts, Observatoire de la Cˆote d’Azur in Nice, M.

Coleman Miller, University of

Maryland, miller@astro.umd.edu, James E. Owen, Imperial C

ollege London,

james.owen@imperial.ac.uk, E. Sterl Phinney, Caltech, es

p@tapir.caltech.edu, Anthony L. Piro,

Carnegie Observatories, piro@carnegiescience.edu, Hans

-Walter Rix, Max Planck Institute for

Astronomy, rix@mpia.de, Brant Robertson, UC Santa Cruz, br

ant@ucsc.edu, Andrew Wetzel,

UC Davis, awetzel@ucdavis.edu, Coral Wheeler, Caltech/Ca

rnegie, coral@caltech.edu, Andrew

N. Youdin, University of Arizona, youdin@email.arizona.e

du, Matias Zaldarriaga, Institute for

Advanced Study, matiasz@ias.edu

1 Key Issue and Overview of Impact on the Field

1.1 Issue

The past two decades have seen a tremendous investment in obs

ervational facilities that promise to

reveal new and unprecedented discoveries about the univers

e. These machines are indeed marvels

of technology and engineering and their large pricetags (al

l nearing or in excess of $1B USD)

are testament to the difficulty and expense of creating this e

quipment. However, over the same

period, no similarly large investment in theoretical work h

as taken place

even when accounting for

computational infrastructure.

In this white paper, we argue that in order for the promised cr

itical breakthroughs in astro-

physics over the next decade and well beyond, the national ag

encies must take a serious approach

to investment in theoretical astrophysics research. We pro

vide a multi-level strategy, from the

grassroots to the national, to address the current under inv

estment in theory relative to observa-

tional work.

1.2 Impact on the Field

But happily, hypotheses, even if full of error, fail us not. —

F.W. Argelander

Theory plays several key roles within astronomy and astroph

ysics. First and foremost, theory

guides insight into the physical nature of the universe

even when the theory is wrong

. Indeed, it

is the

difference

between theory and observation that has ushered in the most d

ramatic revolutions

in physics and astronomy. Examples abound ranging from the d

eviation of mercury’s orbit relative

to Newtonian dynamics that ultimately yielded General Rela

tivity, to the variation in the expected

cosmic microwave background radiation and the observed ver

y smooth cosmic microwave back-

ground (CMB) that ultimately gave way to our current concord

ance model for the cosmos.

In the 20th century, it was perhaps sufficient to provide a sma

ll number of exceptional astro-

physicists access to pencils and paper. This approach no lon

ger suffices in the 21st century. The-

oretical research has become increasingly complex and the c

ommunity seeks to test these ideas

at increasingly challenging observational regimes. Many t

heorists perform heavily computational

work, which, thus far, has been encouraged primarily in an ad

-hoc, grass-roots level in individual

departments and universities.

Testing theoretical work is often used to motivate observin

g proposals and in justification for

large missions. Nevertheless, the support for this work is w

oefully lacking compared to the obser-

vations themselves. Who is to make these predictions and who

is to vet them when they increas-

ingly rely on calculations that take months to years on large

computers to reproduce?

Rather than continuing to give lip service to the need for the

orists in the community, we call

on the National Academy to take a strong stand in support of th

eory. We call on NAS not only to

acknowledge the critical role theory plays in leading the pa

thway to fundamental breakthroughs

in dark matter and dark energy, but to endorse a specific progr

am to support theory and thus keep

astronomy a vibrant field of truth seeking, rather than a back

drop for technological research and

development.

In addition to the obvious role of theory in making the nature

of the universe manifest in clear,

predictable, and manipulatable form, theoretical researc

h serves two more practical functions. The

first is in the

Guidance of Observational Programs

and the second is in the

Interpretation of New

Results

. We discuss each of these in turn below.

1.3 Guidance for Observations

Some of the most successful observational programs of all ti

me, particularly in recent decades,

were guided by theory. These include the CMB, large scale str

ucture of galaxies, gravitational

lensing, gravitational wave emission, just to name a few. Ne

arly all major observational facilities

are developed to test theory. Notable examples in physics in

clude the LHC and LIGO, both of

which confirmed theoretical predictions (the Higgs boson an

d gravitational waves) and generated

Nobel prizes.

In many cases, extensive theory is needed just to measure a si

gnal. For instance, the signals of

merging black holes measured by LIGO would not be detected wi

thout matched filtering against

theoretically generated waveform templates. At present, w

e are still limited in our ability to detect

many types of signals (e.g., eccentric mergers) due to the la

ck of available templates and a lack of

theorists, funding, and computational resources to genera

te them. If our community continues to

invest so heavily into such facilities, shouldn’t we also be

funding the theoretical research necessary

to extract the most science from our investments?

1.4 Interpretation of Observations

Theory also is vital to learn from observations. Consider th

e CMB. This relatively simple data set

(a measurement of temperature across the sky) has fueled muc

h of the field of cosmology. But none

of our current understanding (e.g., the big bang, cosmic nuc

leosynthesis, the

-CDM paradigm)

would exist without extensive theoretical efforts to inter

pret CMB data.

Most fields in astrophysics have not received as much theoret

ical attention, and enormous ad-

vancement could be achieved (with data already obtained) if

more support for theoretical analysis

was available. This need will only grow in the coming decades

with the explosion of unprecedent-

edly large datasets that will require the trained eye of theo

rists to find order and physical insight.

We note though that we are not advocating for a horde of theori

sts that are tasked to simply

“match” the data from upcoming missions. Rather, the goal sh

ould be a deeper understanding. A

successful investment in theory will reinvigorate the field

, particularly in areas where observations

are at present too rudimentary to test predictions.

2 Strategic Plan

The strategy for the future needs to be as bold and visionary i

n the theoretical domain as it is in the

observational and instrument domains. One can cite many inc

redible machines/missions for the

future: JWST, TMT, GMT, LSST, Lynx, HABEX, OST, LUVOIR, WFIR

ST, CMB-S4. The list

goes on. Missing from this long catalog is any explicit suppo

rt for theoretical astrophysics. Indeed,

over this same period of tremendous growth, support for theo

ry has declined precipitously. Even

the highly successful NASA Astrophysics Theory Program has

seen dramatic cuts, just as it was

created. To deny the need for this is to pretend that progress

comes from experiment in isolation.

The consequence of such thinking will be the intellectual de

ath of our field.

2.1 National Theory Program

The only nation in the world with a National program in theore

tical astrophysics is Canada, the

Canadian Institute for Theoretical Astrophysics (CITA). C

ITA has long been regarded as an in-

tellectual jewel of Canada and we should take lessons from th

is successful program from our

Northern neighbor. The US investment in theoretical astrop

hysics should be proportionate to the

US investment in national observational facilities and sho

uld thus be expanded greatly and reflect

our national commitment to astrophysics research. The nati

onal program should have a central

hub, but should serve all theory groups in the nation. As part

of this, we should use the National

Laboratory infrastructure to continue and expand support f

or theoretical astrophysics research at

these sites and at their Leadership Computational Faciliti

es to serve U.S. national science interests

and capabilities.

2.1.1 NAS Fellows in Theoretical Astrophysics

The establishment of a national fellowship in theoretical a

strophysics is a priority for 2020-2030.

In contrast to programs like the extremely successful Hubbl

e Fellowship program (or Chandra

and Sagan fellowships) that are tied to a specific mission or a

specific domain area, the national

fellowship in theoretical astrophysics should be open to al

l theorists regardless of whether their

theoretical program makes direct contact with a specific obs

ervational initiative or not. It is indeed

critical to foster theory both within

and

outside of the auspices of a specific observational mission

or program. Support of theory not limited to current mission

s could lead to the inspiration of

future missions and facilities that would not be thought of o

therwise. These fellowships should be

directed at

both

graduate and post-graduate programs.

2.1.2 NAS Symposia In Theoretical Astrophysics

Theorists need to argue. Unlike engineering, where there ar

e multiple solutions, some more or less

elegant than others but many that “do the job”, theoretical w

ork is either correct or incorrect. It is

thus critical that theorists have a forum for vigorous argum

ent. Meetings such as the AAS do not

provide a forum for theorists as the speaking format is only c

onducive to the presentation of results

and not the argument about them.

The symposia should be of general interest and should focus o

n several topics each year on

a rotating basis. This high-interaction format has been suc

cessfully implemented at the Kavli

Institute for Theoretical Physics in California and the Asp

en Institute in Colorado. Those venues

could (and should) be directly supported for the NAS Symposi

a, but other venues could (and

should) be considered. The essential point is that theorist

s, in particular, need a venue for discourse

that is not overly stifled by the current conference format wh

ich is focused on presentation rather

than confrontation

2.1.3 Theory-Observation Co-Investment

Successful investment portfolios require a combination of

assets to ensure future wealth and in-

sure against market uncertainties. While many observation

al programs are extremely ambitious,

We note that this recommendation is highly subfield dependen

we recognize that the cost and schedule of these programs are

difficult to predict accurately. With

increasingly long lead times for next generation programs,

it is important that the community ex-

plicitly adopt a program of co-investment in theory alongsi

de observational missions. The level

of this co-investment may vary depending on the nature and sc

ale of the program, but the current

default (of 0%) is unhealthy and exposes the national facili

ties to tremendous risk – both in under-

utilized scientific potential as well as community atrophy a

nd apathy over the long periods required

to make the discovery machines of the future.

As part of this recommendation, we recommend a Summer School

program for theoretical

apprentices interested in making predictions for upcoming

facilities. There currently exists a very

large number of summer schools for young researchers that ar

e observation focused, especially

in emerging fields, that train apprentices in data reduction

and analysis techniques. By contrast,

there are almost no summer schools associated with theoreti

cal tools with the notable exceptions

of the MESA Summer School and the Kavli Summer Program in Astr

ophysics. Where are the

opportunities for students to learn the tools of theory, in p

articular, in areas that require more than

mathematics and physics domain knowledge (for example, num

erical hydro, radiative transfer,

dynamics). We recommend that funding be allocated for summe

r schools for theorists to support

these flexible and targeted training grounds.

NAS ACTION

•

The National Academy should form, or recommend the formatio

n of, a National Program

in Theoretical Astrophysics with multiple nodes and repres

entatives across the nation.

•

The National Academy should establish, or recommend the est

ablishment of, Theory Fel-

lowships, distinct from Hubble, Einstein, or other fellows

hip programs, that are exclu-

sively for theoreticians at graduate and postdoctoral stag

es.

•

The National Academy should establish or recommend the esta

blishment of Theory Sym-

posia

•

The National Academy should explicitly recommend a theory c

omponent for all large

facilities recommended as part of Astro 2020

2.2 Infrastructure for Theory

There are several ways in which Theory infrastructure must b

e supported. The first, of course, is

personnel. While NAS cannot recommend the wholesale manufa

cture of FTE lines at universities,

it can support infrastructure at the local and national leve

ls to leverage the academic workforce.

2.2.1 Local Infrastructure

As national facilities have grown to exa-scale, they effect

ively require local mid-scale computing

infrastructure to enable their use. Mid-scale facilities a

re needed for code development, testing,

scaling tests, and a huge range of analysis projects (e.g. ra

diation transfer, structure finding, etc.)

which are too expensive to run on a single workstation but can

not be deployed in “production”

calculations without prior work. Without local facilities

, it is difficult if not impossible to demon-

strate the readiness level of code development, scaling, an

d testing required for large allocations on

national facilities. National facilities on larger scales

are also increasingly unable to accommodate

small testing protocols (e.g., analysis). Moreover, large

national facilities typically guarantee data

storage for only the duration of the main proposal – any usage

of the data generated beyond the

grant period requires local mid-scale facilities.

These resources are generally funded in one of two ways: eith

er (1) groups directly pay for

equipment (purchase of a mid-scale compute cluster) and sup

port (paying administration costs

as salaries or fees, which are usually comparable in cost to t

he equipment buy over a few-year

equipment lifetime), or (2) increasingly, as universities

centralize compute support to leverage

economies of scale and cloud computing becomes more widely u

seful, groups pay a bundled cost

to the university or external supplier for computing (eithe

r paying ’up front’ via a subscription

or buy-in fee, or paying as-you-go per compute hour). Howeve

r the current grants infrastructure,

especially for small (e.g., single-PI) theory grants and su

per-computing grants, provides no support

for either of these infrastructures. Option (2) in particul

ar, which is rapidly becoming the norm in

both business and academic systems, does not even fit into the

existing budget categories of most

grant agencies. It is imperative that grants supporting the

huge time and effort, for both scientific

analysis and supercomputing time, provide a category to fina

ncially support the required local

infrastructure expenses, especially the payment of admini

strative or per-hour compute costs.

2.2.2 Legacy Support for Theory Products

Theory grants and national super-computing centers spend m

illions of dollars and tremendous

effort and resources supporting the generation of theoreti

cal models and predictions. A major sci-

entific goal of these is, without exception, providing predi

ctions or comparisons to current and

future observational data. But there is no support for the ar

chiving or access of such data. With

super-computing proposals, data management plans are ofte

n required, but there is no infrastruc-

ture provided by any of the funding agencies for making that d

ata available in useful form after the

grant period expires. There is also no way to ensure theoreti

cal data products are made public, in

the way that is now viewed as an obvious (and tremendously imp

ortant) requirement for observa-

tional products. Given that there is already a large infrast

ructure in place to support observational

data sets, it is a natural and straightforward step to suppor

t mock theoretical data products of said

data. This requires no new fundamental infrastructure, sim

ply support for additional data sets, the

creation of which is already being funded – essentially, sim

ply providing support for access of

said products (after financial and computing resources are a

warded for their creation) in the same

way as is already done for their observational equivalents.

This would also tremendously increase

the impact of theoretical products, by making them not just m

ore readily accessible to observers,

but accessible via the same interactive data archival syste

ms which they already use to access the

equivalent observational products, catalogues, images, s

pectra, etc.

NAS ACTION

•

Grant support for local infrastructure and maintenance sho

uld be recommended as a pri-

ority for 2020-2030 within the AST and NASA grant programs di

rectly.

•

Explicit competitions for replacement infrastructure for

astrophysics theory should be

recommended and regularly conducted along with mid-scale i

nfrastructure calls.

2.3 Program Support

Both NSF and NASA have the resources to direct toward this vis

ion if they have the community

support to do so. Investment in theory should be made an expli

cit priority and should be done in

a proportion

to the investment in the observatories of the future. We do no

t speculate what

should be in this document. We suggest NAS undertake a study t

o determine this number, but we

argue it should be no less than 10%.

3 Conclusions

The national investment in observational infrastructure i

s growing without bound, but in sharp con-

trast, the national investment in theory has remained const

ant for decades. This situation threatens

the health and vitality of our field. The goal of astronomy and

astrophysics is not just to catalog the

universe, but to understand how it works. Without broad supp

ort of theoretical work, ranging from

theoretical predictions that suggest new observations or i

nstruments, to theoretical interpretation

and modelling of existing data, astronomy will be reduced to

an expensive and aimless cosmic

census.