Research


The Observable Universe: probes of different epochs in its history.

What do I do?

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Opening Up New Windows to Study the Universe

The Standard Model of Cosmology aims to explains the cosmic evolution from a fraction of a second after the Big-Bang singularity to the current period of accelerated expansion with only a handful of parameters. Over the past decade, it has withstood a wide series of observational tests. Yet several gaping holes remain in the theory:
- How did inflation begin and come to an end?
- What makes up the dark matter in the Universe?
- What is the nature of dark energy?
- How did galaxies and clusters of galaxies form and evolve to make up the large scale structure we observe today?
Going forward, we must develop new ways to probe these fundamental questions by accessing the full volume of the observable Universe. This is the focus of my research.


For more details about my research, including links to relevant talks and publications, click on the topics below:

The Cosmic Microwave Background: Exploring the Early Universe

Line-Intensity Mapping: Astrophysics and Cosmology at High Redshifts

Why am I interested?

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Line-Intensity Mapping

Line-Intensity mapping is an invaluable method to study the universe in between the regimes accessible to galaxy surveys and the CMB. It consists of low-resolution measurements of the fluctuations in the integrated emission of atomic and molecular spectral lines. As reviewed in Kovetz et al. (2017), a Status Report composed following a workshop I organized at JHU, this is an emerging field, expecting a flood of new data from upcoming surveys. I have proposed a wide range of uses of this method, from probing dark matter to measuring star-formation, and continue to explore new ways to exploit its vast potential.


My research on Line-Intensity Mapping has ranged from the neutral hydrogen line (21cm) to line emission related to star formation, such as Carbon-Monoxide (CO), ionized Carbon ([CII]), Lyman-alpha, etc.:

The 21cm HI Transition

Simulating the full history of the 21cm signal: Recently, we presented a new tool called 21cmFirstCLASS, which is a modified version of 21cmFAST, the most popular code in the literature for computing the anisotropies of the 21-cm signal. Our code uses the public CMB Boltzmann code CLASS, to establish consistent initial conditions at recombination for any set of cosmological parameters and evolves them throughout the dark ages, cosmic dawn, the epoch of heating and reionization, generating for the first time, simulations of the full history of the signal. It also accounts for inhomogeneity in the temperature and ionization fields throughout the evolution, crucial for a robust calculation of both the global 21-cm signal and its fluctuations and allows for consistent joint analysis of CMB and the 21-cm signal to obtain constraints on both cosmology and astrophysics, and study degeneracies between them. Important for cosmological studies, it tracks separately the density evolution of the dark matter and baryon components.


Top: global brightness temperature as a function of redshift. Bottom: fluctuations in the brightness temperature. This is the first lightcone simulation to be generated by a publicly available code, 21cmFirstCLASS, that extends from recombination through the dark ages and cosmic dawn to the end of reionization. From Flitter and Kovetz (PRD, 2023).
Dark Matter-Baryon Scattering: With J. Muñoz (now @UTAustin), I set out to explore how models in which dark matter interacts with baryons can be probed through their imprint on the 21-cm global temperature and its fluctuations. Naively, one would expect the baryon fluid to cool down as it driven to equilibrium with the colder dark matter fluid. However, we found that a crucial ingredient to include is the relative velocity between the baryons and dark matter, which leads to a heating effect due to the friction between the two fluids, which in some cases is more dominant than the cooling. It also leads to a new large-scale component of fluctuations, on top of the standard 21cm power spectrum. We showed that future experiments can constrain interacting dark matter in mass ranges inaccessible to direct detection experiments, providing excellent motivation for ongoing follow-up works.
Recently, following the EDGES detection of an anomalous 21cm absorption profile at z~17, we investigated in detail the viable parameter space for millicharged dark matter to explain the EDGES signal. We provided a thorough analysis of the effect on the CMB and the 21cm in the strong coupling regime, and derived the most stringent constraints on this model.


Left: Baryon temperatures (three upper curves) without interactions (solid curve) and when adding interactions (dashed-blue curve for the case where the relative velocity between baryons and dark matter is not taken into account, and red curve when it is), as well as dark-matter temperatures (two lower curves, same scenarios). At late times and for DM mass mχ below a GeV, our prediction was cooling of the baryon gas temperature, which would generate a deeper 21cm absoprtion profile (interestingly matching what EDGES measured a few years later). From Muñoz, Kovetz and Ali-Haïmoud (PRD, 2015).
Right: The viable parameter space for millicharged DM to explain the anomalous EDGES 21cm signal. The allowed region is bound from above by SLAC constraints (gray), from the left by stellar cooling (purple), from below by SN1987A cooling (blue), and from the right by the requirement to cool the baryons enough to yield a 21cm brightness temperature consistent with the EDGES 99% upper bound (black). Contours are shown for several values of the fraction fχ of the total DM that is millicharged; each yields an upper bound on the mass mχ. The rightmost limit is from Planck 2015 (red). A portion ruled out by the Neff limit at BBN, valid below mχ∼me, is sketched (light green). From Kovetz et al. (PRD, 2018).
Ultra-Light Hidden-Photon Dark Matter: Ultra-light hidden photons provide an appealing candidate for dark matter. These are (light) massive vector bosons that arise naturally in many theoretical setups, and which generically interact with the Standard Model through kinetic mixing with the ordinary photons. Ultra-light hidden-photon dark matter produces an oscillating electric field in the early Universe plasma, which in turn induces an electric current in its ionized component whose dissipation results in heat transfer from the dark matter to the plasma. This will affect the global 21cm signal from the Dark Ages and Cosmic Dawn. In work with Ilias Cholis and David E. Kaplan, we focused on the latter, in light of the reported detection by EDGES of an absorption signal at frequencies corresponding to redshift z∼17. By measuring the 21cm global signal, a limit can be placed on the amount of gas heating, and thus the kinetic mixing strength ε between the hidden and ordinary photons can be constrained. We showed that 21cm at cosmic dawn can place the strongest bounds to date on ε across more then ten orders of magnitude in mass, down to 1e-22 eV.


Left: The EDGES Cosmic Dawn 21cm signal (Bowman et al., Nature 2018) is in 3.8σ disagreement with the maximum absorption allowed by ΛCDM. In Kovetz et al. (2018) we present the viable parameter space of the millicharged DM model suggested to explain this anomaly by Barkana (Nature, 2018), based on the calculation in Munoz, Kovetz and Ali-Haımoud (2015). Meanwhile, in Kovetz, Cholis and Kaplan (2018) we show that a robust 21cm detection can place the strongest bounds yet on ultra-light hidden-photon DM (see right panel).
Right: Predominant bounds on the kinetic mixing parameter ε for different hidden-photon DM masses mχ. We show constraints from Milky-Way ISM heating (red) and from the CMB (blue). Our inferred 21cm bounds from requiring that T21 = −100 mK or T21 = −50 mK (black and dashed-black) are two orders of magnitude stronger over ten orders of magnitude in mass and the only ones to penetrate the fuzzy-DM mass range. From Kovetz, Cholis and Kaplan (Submitted to PRL, 2018).

Related talks:

Line-intensity mapping: emission from galaxies

Star-formation Lines: Theory Review
With J. Bernal (now @Santander), we recently reviewed LIM theory, with an emphasis on line emission related to star formation, for The Astronomy and Astrophysics Review.


Top: a 1-degree field at redshift 5. We show (from left to right): all the galaxies; "observable'' galaxies (assuming an arbitrary cut off in the stellar masses as a theoretical proxy for detection threshold); and maps of the CO and [CII] intensity fluctuations from this field (characteristic angular resolutions of instruments targeting each line are applied). Bottom: Intensity mapping of multiple line emissions provides rich access to redshift volumes otherwise inaccessible, enabling detailed study of various important epochs in cosmic history. Above the arrow of time on the left, MD=RD and DE=MD denote the redshifts of matter-radiation, and dark energy-matter equality, respectively. We also include a rough estimation of the scale at which matter clustering becomes non linear. From Bernal and Kovetz (TAAR, 2022).
Carbon-Monoxide (CO) Intensity Mapping:
With P. Breysse (now @SMU), I conducted a series of studies of intensity mapping the star-forming line CO as a tool for high redshift cosmology and astrophysics. We first examined different models for the CO luminosity function at medium redshifts and investigated the optimization of instruments to detect the signal. We then tackled the problem of line foregrounds and showed how blind bright-pixel masking can be useful in mitigating contamination in medium-redshift CO maps and Lyα from the EoR (while it would not work for CII at EoR). In exciting recent works, we proposed to constrain CO-luminosity and thereby the star-formation rate throughout cosmic history, by adopting a novel formalism to analyze the one-point statistics of intensity maps, based on P(D) analysis.


Left: Demonstrating the power of intensity mapping - a simulated 2.5 deg^2 field with galaxy positions (left side) and the corresponding CO intensity map (right side). Luminosities were drawn from a Schechter function model (Breysse, Kovetz and Kamionkowski, 2016). Sources bright enough to detect with 1hr of VLA time are marked in red. In the same 3000 hours of integration over this field required for the VLA detections, a low-cost experiment like COMAP can provide the intensity map on the right. From Kovetz et. al (Physics Reports, in review 2017) .
Right: Comparison between predicted constraints on star formation rate density from CO intensity mapping and from existing FUV (grey points) and GRB (orange points) data. Solid black curve ψ(z) shows fit to FUV data. Blue curves show ±1σ SFRD uncertainty forecast with CO intensity mapping, taking into account foregrounds and noise. Dashed magenta lines encompass a 10% model uncertainty band in the adopted CO-FIR and FIR-SFR relations. From Bryesse, Kovetz and Kamionkowski (MNRASL, 2016) .

Related talks:

Collaborations

Starting in early 2016, the line-intensity mapping community began a series of annual workshops to help coalesce and advance the field. The pioneering meeting was a workshop titled “Opportunities and Challenges in Intensity Mapping” which took place at Stanford University (SLAC) March 21-23, 2016 and was attended by over 40 scientists. It was followed by the second workshop, “IM@Hopkins”, which we organizedat Johns Hopkins University in June 12-14, 2017, with over 50 participants.
After the JHU workshop, we compiled a status report of the line-intensity mapping field following the two workshops. In between, during and after these two workshops, through their presentations, personal writing assignments and feedback on drafts, over 45 scientists (from over 25 institutions) contributed to the 2017 Status Report, while a small writing group was responsible for editing, combining and integrating the individual contributions.
The next community workshops took place at Aspen (2018), CCA (2019), Marseille (2019), UChicago (2021), MPA (2023).

Related collaboration papers:

Gravitational waves from Mergers of Compact Objects: a Powerful New Probe

Odds and Ends