458
H. Padmanabhan
potential to directly access the exciting ‘dark ages’ of the universe (z ∼ 1000 − 30)
immediately following the decoupling of radiation and matter, the formation and
turning on of the first luminous sources (z ∼ 30 − 15), and, ultimately, the epoch
of reionization: the second major phase transition of (almost) all the cosmological
baryons (believed to have completed around z ∼ 6). IM can also provide valuable
insights into astrophysical phenomena on smaller scales: probing the interstellar
medium (ISM), the site of active star formation in most normal galaxies, through
mapping the carbon monoxide (CO) line emission (which acts as a tracer of
molecular hydrogen, H 2 , e.g., Ref. [8]) and the 158 μm fine structure transition
of the [CII] species (singly ionized carbon; e.g., Ref. [9]).
Besides offering exciting astrophysical insights, the unique ability of IM to
efficiently probe vast volumes of the universe makes it an ideal probe of cosmology
and fundamental physics—such as modified gravity and dark energy models, the
nature of dark matter, inflationary scenarios and several others. However, due to
the complex interplay between astrophysics and cosmology in IM surveys, this
requires a precise quantification of the impact of astrophysics on the robustness
of cosmological constraints. In this article, we summarize recent work exploring
various facets of this inter-relationship, specifically focussing on the effect of
astrophysical uncertainties on the precision and accuracy of cosmological forecasts
from future IM surveys. We also describe how such analyses can open up the
fascinating possibility of using IM to constrain physics beyond the standard model
of ΛCDM.
2 Forecasts for Cosmology and Astrophysics
The challenge for using line-intensity mapping to constrain cosmology and astrophysics is twofold: (1) the foregrounds—both galactic and extragalactic—are orders
of magnitude stronger than the signal and (2) the astrophysics of the tracer itself
(such as HI) serves as an effective ‘systematic’ in deriving cosmological constraints
from intensity maps. The former constraint may be mitigated using techniques
such as foreground avoidance and subtraction, since the frequency structure of
the foregrounds is estimated to be very different from those of the signal (e.g.,
[14]). The latter effect, which may be referred to as the ‘astrophysical systematic’,
can be effectively addressed by quantifying the impact of our uncertainty in the
knowledge of the tracer astrophysics, on the observable intensity fluctuations. In
the case of 21 cm intensity mapping, this can be done by using a data-driven, halo
model framework which uses a parametrized form for the HI-halo mass relation
M HI (M, z), given by [11]:
M HI (M, z) = αf H, c M
M
10 11 M /h
β
exp
−
v c,0
v c (M, z)
3
(1)
H. Padmanabhan
potential to directly access the exciting ‘dark ages’ of the universe (z ∼ 1000 − 30)
immediately following the decoupling of radiation and matter, the formation and
turning on of the first luminous sources (z ∼ 30 − 15), and, ultimately, the epoch
of reionization: the second major phase transition of (almost) all the cosmological
baryons (believed to have completed around z ∼ 6). IM can also provide valuable
insights into astrophysical phenomena on smaller scales: probing the interstellar
medium (ISM), the site of active star formation in most normal galaxies, through
mapping the carbon monoxide (CO) line emission (which acts as a tracer of
molecular hydrogen, H 2 , e.g., Ref. [8]) and the 158 μm fine structure transition
of the [CII] species (singly ionized carbon; e.g., Ref. [9]).
Besides offering exciting astrophysical insights, the unique ability of IM to
efficiently probe vast volumes of the universe makes it an ideal probe of cosmology
and fundamental physics—such as modified gravity and dark energy models, the
nature of dark matter, inflationary scenarios and several others. However, due to
the complex interplay between astrophysics and cosmology in IM surveys, this
requires a precise quantification of the impact of astrophysics on the robustness
of cosmological constraints. In this article, we summarize recent work exploring
various facets of this inter-relationship, specifically focussing on the effect of
astrophysical uncertainties on the precision and accuracy of cosmological forecasts
from future IM surveys. We also describe how such analyses can open up the
fascinating possibility of using IM to constrain physics beyond the standard model
of ΛCDM.
2 Forecasts for Cosmology and Astrophysics
The challenge for using line-intensity mapping to constrain cosmology and astrophysics is twofold: (1) the foregrounds—both galactic and extragalactic—are orders
of magnitude stronger than the signal and (2) the astrophysics of the tracer itself
(such as HI) serves as an effective ‘systematic’ in deriving cosmological constraints
from intensity maps. The former constraint may be mitigated using techniques
such as foreground avoidance and subtraction, since the frequency structure of
the foregrounds is estimated to be very different from those of the signal (e.g.,
[14]). The latter effect, which may be referred to as the ‘astrophysical systematic’,
can be effectively addressed by quantifying the impact of our uncertainty in the
knowledge of the tracer astrophysics, on the observable intensity fluctuations. In
the case of 21 cm intensity mapping, this can be done by using a data-driven, halo
model framework which uses a parametrized form for the HI-halo mass relation
M HI (M, z), given by [11]:
M HI (M, z) = αf H, c M
M
10 11 M /h
β
exp
−
v c,0
v c (M, z)
3
(1)