Astro2020 Science White Paper
The US Program in Ground-Based
Gravitational Wave Science: Contribution
from the LIGO Laboratory
Thematic Areas:
Planetary Systems
Star and Planet Formation
Formation and Evolution of Compact Objects
Cosmology and Fundamental Physics
Stars and Stellar Evolution
Resolved Stellar Populations and their Environments
Galaxy Evolution
Multi-Messenger Astronomy and Astrophysics
Principal Author:
Name: David Reitze
Institution: LIGO Laboratory, California Institute of Technology
Email: dreitze@caltech.edu
Phone: +1–626–395–6274
Co-authors:
LIGO Laboratory, California Institute of Technology, Pasadena, California 91125, USA
LIGO Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
LIGO Hanford Observatory, Richland, Washington 99352, USA
LIGO Livingston Observatory, Livingston, Louisiana 70754, USA
Abstract:
Recent gravitational-wave observations from the LIGO and Virgo observatories have
brought a sense of great excitement to scientists and citizens the world over. Since September 2015,
10 binary black hole coalescences and one binary neutron star coalescence have been observed.
They have provided remarkable, revolutionary insight into the “gravitational Universe” and have
greatly extended the field of multi-messenger astronomy. At present, Advanced LIGO can see binary
black hole coalescences out to redshift
0.6
and binary neutron star coalescences to redshift
0.05
.
This probes only a very small fraction of the volume of the observable Universe. However, current
technologies can be extended to construct “3
rd
Generation” (3G) gravitational-wave observatories
that would extend our reach to the very edge of the observable Universe. The event rates over such a
large volume would be in the hundreds of thousands per year (
i.e.
tens per hour). Such 3G detectors
would have a 10-fold improvement in strain sensitivity over the current generation of instruments,
yielding signal-to-noise ratios of 1000 for events like those already seen. Several concepts are being
studied for which engineering studies and reliable cost estimates will be developed in the next 5
years.
1
arXiv:1903.04615v1 [astro-ph.IM] 11 Mar 2019
1 Introduction
Long-baseline laser interferometry
1,2
has given life to gravitational-wave astronomy and expanded
the vision of multi-messenger observation. When the current generation of interferometric detectors
are brought to their full strain sensitivities, they will see binary neutron star coalescences out to
redshift
z
'
0.2
and binary black hole coalescences out to
z
'
3
,
3
with nearby events having
signal-to-noise ratios of order 100. With a future generation of more sensitive detectors in new,
larger facilities, it will be possible to see these binary systems out to redshifts
z
>
10
, with nearby
events having signal-to-noise ratios in excess of 1000. This paper presents a synopsis of the scientific
opportunities afforded by 3G detectors; it also outlines needed technical developments to enable
them.
2 A brief history of the field
LIGO consists of two NSF-funded
4
US facilities that host
4 km
long, interferometric gravitational-
wave detectors, one in Livingston, Louisiana, and the other in Hanford, Washington. The facilities
were constructed from 1992 to 1999, and the initial LIGO detectors (iLIGO) made observations from
2002 to 2010, ultimately reaching a sensitivity to neutron stars out to redshift
z
'
0.01
and black
holes to redshift
z
'
0.1—though no detections were made.
5,6
The second-generation detectors, known as Advanced LIGO (aLIGO), were funded
4,7–9
starting
in 2008 and installed at both sites from 2011 to 2014. aLIGO was designed to achieve a 10-fold
increase in sensitivity over iLIGO—providing a 1000-fold increase in sensitive volume—by reducing
the instrument’s high-frequency noise floor and extending the sensitive band to lower frequency.
10
Observations began in September 2015 with about four times the sensitivity of iLIGO.
11
On
September 14, 2015, the first gravitational waves were detected: a chirp signal (GW150914) from the
merger of two black holes
410 Mpc
from Earth.
12
Since then, aLIGO’s sensitivity has been improved
and more observations have been made.
The first two aLIGO observing runs (O1, from Sep 2015 to Jan 2016, and O2, from Nov 2016 to
Aug 2017) totaled a calendar year of observing time. Near the end of O2, the Virgo observatory (a
3 km
detector in Cascina, Italy)
13
joined LIGO, and critically contributed to the first observation of
a binary neutron star merger (GW170817) by reducing the sky location area from a few hundred
to a few tens of square degrees.
14
This enabled approximately 90 optical and radio astronomy
telescopes to identify and study electromagnetic counterparts to the gravitational waves.
15
In O1 and
O2 combined, 10 black hole coalescences and one neutron star collision were observed.
16
Observing
run O3 will begin in April 2019 and run for about one year. KAGRA, an underground
3 km
detector
in Japan,
17
is planning to join O3 toward the end of 2019 at a reduced sensitivity. By the middle of
the next decade, LIGO will install and commission an upgrade (referred to as “A+”) to significantly
improve the aLIGO detectors beyond their design sensitivity target (by a factor of 5 in detection rate
for compact binary mergers compared to aLIGO at design sensitivity). Additionally, a third LIGO
detector will be brought online in a new 4 km facility in India.
2
3 Science case for an international network of 3G detectors
The worldwide gravitational-wave, high energy astrophysics, and nuclear physics communities
are highly energized by the recent discoveries and the prospects for an extremely rich science
program ahead. The Gravitational Wave International Committee (GWIC)
18
is developing a set of
science white papers detailing 3G gravitational-wave astronomy based on a 3G gravitational-wave
detector network consisting of “Cosmic Explorer” (CE)
19,20
and “Einstein Telescope” (ET)
21,22
detectors, as described in the next section. Here we summarize the science case for Cosmic Explorer,
which delivers two significant advances over the current generation of gravitational-wave detectors:
(a) nearby sources will be detected with very high signal-to-noise ratio (SNR); and (b) sources
will be detectable to redshifts greater than 10 (see Fig. 1),
23
allowing studies of their evolution and
providing access to the first stars in the Universe.
3.1 Black holes
Physics:
3G detectors will enable unprecedented tests of the strong-field dynamics of gravity.
Some specific effects include the ringdown of a black hole merger,
24,25
detailed measurements of
spin-orbit gravitational interactions,
26–28
the non-linear memory effects in the metric that accompany
gravitational radiation,
29,30
and direct tests of the Hawking area theorem.
31
Many of these tests rely
on obtaining very high SNR signals, which is possible with the superior noise performance of the
3G detectors (Fig. 1, left side).
Astronomy:
The right side of Fig. 1 shows how Cosmic Explorer will be able to detect black
hole mergers throughout cosmic history, thus exploring both the importance of different formation
channels (field formation versus dynamical capture) and how these evolved with time. Evidence for
black holes from population III stars might help in understanding the formation of supermassive
black holes and the galaxies they seed.
32
The mass and spin distribution of black holes will reveal
their origin and key properties of their progenitors, such as the magnitude of supernovae kicks.
33
If
primordial stellar-mass black hole binaries exist, Cosmic Explorer will detect them if they merge at
z
.
100
. A network of Cosmic-Explorer-class detectors will provide high-SNR signals from all
epochs of stellar-mass binary black hole collisions in the Universe.
3.2 Neutron stars
Physics:
Capturing high-SNR binary neutron star coalescences is the most promising way of
measuring the equation of state of nuclear matter through precise measurements of the phase
evolution of the gravitational-wave signal.
36
More detailed information about the composition of
neutron stars may be available from the gravitational-wave emission from the post-merger collision
remnant.
37
Furthermore, measurements of the masses and spins of the component neutron stars, and
of the remnant, will lead to a better understanding of the the yield of heavy metals produced by these
systems,
38
and the maximum mass of neutron stars.
39
As shown in Fig. 1 (right side), Advanced
LIGO and A+ will only detect a very small fraction of binary neutron star coalescences; Cosmic
Explorer will have access to the entire coalescing population.
Gravitational waves can also be produced by isolated spinning neutron stars, as long as they are
not perfectly spherical.
40
Such gravitational waves have not yet been detected, and the amplitude of
emission from these sources is not known. Such a signal is expected to be continuous and nearly
monochromatic, and would provide information about neutron star ellipticity, moment of inertia,
and again nuclear equation of state.
3