Understanding cosmic explosions

The gist: Millimeter-wavelength observations provide a uniquely sensitive window into the physics of cosmic explosions, from supernovae (SNe) and tidal disruption events (TDEs) to nova eruptions, stellar mergers and electromagnetic counterparts to gravitational wave (GW) sources <ref:2610.20740#pg6>.

Scientific Context and Open Questions

Cosmic explosions are diverse laboratories for extreme physics, producing shocks, relativistic outflows, rapid mass loss, nucleosynthesis, and in some cases compact-object formation in real time <ref:2610.20740#pg4>. Historically, much of what we know about the interaction between explosive ejecta and the surrounding medium has come from modeling centimeter-wavelength synchrotron emission <ref:2610.20740#pg3>. Over the past decade, however, observational breakthroughs have demonstrated that the earliest and most diagnostically powerful phases of these interactions frequently occur at millimeter and submillimeter wavelengths <ref:2610.20740#pg4>.

Why ALMA2040 and why mm/sub-mm?

By the 2040s, facilities such as SKAO and ngVLA will provide transformational sensitivity at cm wavelengths, enabling late-time calorimetry once synchrotron emission is optically thin and tracing absorption at low frequencies <ref:2610.20740#pg3>. However, many key diagnostics of cosmic explosions occur at higher frequencies and at earlier times, when cm emission can be self-absorbed or suppressed by free–free opacity, and when thermal, non-thermal and dust components overlap <ref:2610.20740#pg4>. Across the transient zoo, ALMA2040 will enable a uniform, quantitative answer to a small set of recurring physical questions that are difficult to address with current mm facilities or with cm-band data alone <ref:2610.20740#pg4>.

The key open questions addressed by ALMA2040 include:

1. Environment: What are the density scale, radial profile and degree of clumping/asymmetry of the circumstellar/circumnuclear medium (CSM) on radii ∼ 1014–1017 cm, as inferred from the early-time spectral peak and its evolution <ref:2610.20740#pg4>.

2. Energetics: What are the shock/outflow kinetic energies and velocity distributions (including mildly relativistic components) at the earliest phases, before cm emission becomes optically thin <ref:2610.20740#pg4>.

3. Absorption physics: Is the early turnover set by synchrotron self-absorption, free–free absorption, or a multi-component spectrum, and how does this partition vary across source classes (dense-CSM SNe, novae, FBOTs, TDEs, GRBs) <ref:2610.20740#pg4>.

4. Shock microphysics: Do collisionless shocks in transients obey a universal electron distribution and microphysical parameter set, or is there measurable spectral curvature/break evolution indicating time-dependent acceleration/cooling or multiple particle populations <ref:2610.20740#pg4>.

5. Geometry: What is the angular structure of jets/outflows (quasi-spherical vs. stratified vs. collimated), and how do viewing-angle effects map onto observed diversity, as constrained by polarization, spectral evolution and (for the nearest events) size/expansion measurements <ref:2610.20740#pg4>.

6. Dust/molecules and obscuration: When and where do dust and molecules form in explosions and mergers, and how do they coexist with shock-powered non-thermal emission in heavily obscured systems <ref:2610.20740#pg4>.

7. Multi-messenger inference: For neutrino- and GW-triggered transients, what are the shock energetics, densities and magnetic fields that set the efficiency of hadronic interactions and/or late-time energy injection, and how do these measurements reduce degeneracies in neutrino/GW counterpart models <ref:2610.20740#pg4>.

Figure 1: Schematic illustrating three recurring physical effects that shape mm/sub-mm emission from cosmic explosions and the corresponding observables

The mm/sub-mm band provides direct access to observables that map onto these effects at early times, when diagnostic information is otherwise hidden by absorption at longer wavelengths or by reprocessing at shorter wavelengths <ref:2610.20740#pg4>. The evolution of the spectral turnover and peak flux constrains both the density scale and radial profile of the ambient medium and the shock energetics <ref:2610.20740#pg4>. Polarimetry probes magnetic field ordering and jet magnetization <ref:2610.20740#pg4>. High-resolution imaging or size constraints (when feasible) provide an independent handle on the physical radius and expansion speed of the emitting region, reducing model degeneracies that cannot be broken with light curves alone <ref:2610.20740#pg4>.

Supernovae: interaction-powered and engine-driven (magnetar-powered SLSNe)

A new appreciation for “interaction-powered” SNe has emerged from wide-field optical surveys, which indicate that a substantial fraction of massive stars explode inside dense circumstellar environments shaped within only a few hundred AU of the progenitor <ref:2610.20740#pg5>. High densities that power optical signatures of interaction often render the earliest shocks opaque at cm wavelengths, while mm emission can escape <ref:2610.20740#pg4>. Only a small number of such SNe have well-sampled mm observations so far, but these already show the promise of recovering shock properties and the radial structure of the CSM during the first days to weeks <ref:2610.20740#pg4>. Key open questions include whether dense CSM is smooth or clumpy, whether it is axisymmetric (as expected in binaries), and what fraction of interaction-powered explosions launch mildly relativistic components that would be missed or mischaracterized at cm wavelengths <ref:2610.20740#pg5>. A complementary engine-driven channel may be provided by magnetar-powered superluminous SNe (SLSNe), which can produce luminous non-thermal radio/mm emission on months–years timescales <ref:2610.20740#pg5>. mm observations then probe energy injection, nebular magnetization and the surrounding density structure (e.g., Mukhopadhyay et al. 2026), a connection made timely by recent GeV detections of an SLSN with Fermi-LAT (Acero et al. 2026) <ref:2610.20740#pg5>.

Engine-powered explosions (TDEs, GRBs, FBOTs)

Relativistic jetted TDEs (Berger et al. 2012), rapidly evolving fast blue optical transients (FBOTs; such as AT2018cow Ho et al. 2019), and late-time radio/mm rebrightenings in optically selected TDEs (Cendes et al. 2024) can peak at mm frequencies on week-to-month timescales <ref:2610.20740#pg5>. The underlying mechanisms (reverse shocks, stratified ejecta, magnetic dissipation or multiple electron populations) remain poorly constrained <ref:2610.20740#pg4>. Existing mm observations already show variable sub-mm spectral indices (Ho et al. 2019), steep mm spectra inconsistent with standard single power-law electron distribution (Ho et al. 2074, 2610.207403), and evidence for multi-component outflow structures <ref:2610.20740#pg4>. These features motivate simultaneous, wideband mm monitoring to distinguish intrinsic spectral curvature from variability and absorption and to support physical outflow models (e.g., Mukhopadhyay et al. 2024) <ref:2610.20740#pg4>.

Accreting compact objects (XRBs)

In X-ray binaries (XRBs), compact jets in the hard state produce flat-to-inverted synchrotron spectra that can extend into the mm/sub-mm band, while outbursts launch discrete ejecta whose spectral evolution and polarization trace particle acceleration, magnetic field ordering and jet composition on minute–day timescales (e.g., Fender et al. 2004; Gallo et al. 2003; Tetarenko et al. 2017; Koljonen et al. 2610.207403). XRB outbursts offer a fast, repeatable analogue of engine-powered transient outflows, enabling controlled tests of how jet launching and shock microphysics depend on accretion state and environment (see also the companion ALMA2040 white paper mm-jets in the multi-messenger era) <ref:2610.20740#pg4>. Extending such studies to the Magellanic Clouds and, for the brightest events, to the Local Volume will require the sensitivity, rapid response and multi-band capability of ALMA2040, and will provide a unique bridge between Galactic jet phenomenology and the extragalactic explosions discussed above <ref:2610.20740#pg4>.

1. Improved AI can perform quantitative physical diagnostics by analyzing millimeter-wavelength data to answer specific questions like What are the density scale, radial profile and degree of clumping/asymmetry of the circumstellar/circumnuclear medium (CSM) on radii ∼ 1014–1017 cm?

2. Improved AI can constrain shock microphysics by determining if "collisionless shocks in transients obey a universal electron distribution and microphysical parameter set, or is there measurable spectral curvature/break evolution indicating time-dependent acceleration/cooling or multiple particle populations."

3. Improved AI can map outflow geometries by constraining quasi-spherical stratified jet versus other structures using polarization data, as described in Figure 1, to determine the angular structure of jets and how viewing-angle effects map onto observed diversity.

4. Improved AI can perform multi-messenger inference by calculating shock energetics and magnetic field strengths that set the efficiency of hadronic interactions and/or late-time energy injection for neutrino- and GW-triggered transients, thereby reducing degeneracies in counterpart models.

5. Improved AI can assess the physical state of accretion environments by analyzing XRB outbursts, which offer a fast, repeatable analogue of engine-powered transient outflows, enabling controlled tests of how jet launching and shock microphysics depend on accretion state and environment.

Millimeter-wavelength observations provide a uniquely sensitive window into the physics of cosmic explosions, from supernovae (SNe) and tidal disruption events (TDEs) to nova eruptions, stellar mergers and electromagnetic counterparts to gravitational wave (GW) sources. Over the past decade, discoveries across the transient sky have revealed that many of the most informative phases of shock interaction, particle acceleration and jet formation unfold at frequencies above the traditional cm-wave domain, where synchrotron self-absorption weakens and thermal, nonthermal and dust components can be disentangled. Upcoming wide-field mm surveys will detect a new population of short-lived transients but will be limited by coarse localization and limited spectral characterization. ALMA2040, with dramatically improved sensitivity, wide instantaneous spectral coverage, rapid-response and sub-arraying modes, polarimetry and extended baselines, is essential for converting these discoveries into robust physical constraints. Here we outline the scientific context, key open questions and the ALMA2040 capabilities required to unlock the full potential of millimeter time-domain astrophysics in the 2040s.