What observational signatures would a bubble‑collision in the CMB produce and have any been reliably seen?
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Executive summary
Bubble collisions in eternal‑inflation scenarios would leave large, azimuthally symmetric imprints in the cosmic microwave background (CMB) — roughly disk‑shaped temperature and polarization modulations with a characteristic radial profile — and more recently proposed observables (remote dipole and quadrupole fields) promise to probe them beyond the cosmic‑variance limits of the primary CMB [1] [2] [3]. Decade‑long searches in WMAP and Planck maps have found no robust detection and instead set upper limits on such signals; next‑generation analyses and new probes like RDF/RQF could tighten constraints but have not yet produced a reliable sighting [4] [5] [6].
1. What would a bubble collision look like in the CMB — the textbook signature
Theory papers that model collisions between expanding “bubbles” in an inflating background predict that a collision injects a long‑wavelength, axially symmetric perturbation inside our bubble which projects onto the last‑scattering surface as a circular or disk‑like modulation of CMB temperature and polarization with a predictable radial profile and (in some models) a small step in temperature across the disk edge; these features follow from analytic and numerical thin‑wall collision solutions and their Sachs–Wolfe/ISW mapping into CMB anisotropies [2] [7] [8].
2. Polarization, edges and symmetry — the discriminants
Beyond a temperature excess or deficit, a credible bubble signature should carry a matching polarization pattern and manifest azimuthal symmetry (SO symmetry along the collision axis) and specific multipole structure in low‑ℓ modes; searches therefore look for concentric temperature gradients, polarization templates consistent with collision models, and occasionally sharp “edge” behavior in temperature maps as additional discriminants against chance CMB fluctuations or foregrounds [1] [9] [2].
3. How data have been mined — methods and limitations
The first systematic searches translated theoretical templates into detection algorithms that scanned full‑sky WMAP and later Planck maps, using simulations to quantify false‑positive rates and to set upper limits when no convincing candidates passed thresholds; these studies were explicitly designed to constrain parameter space of phenomenological collision models rather than to declare definitive discoveries [1] [4] [5]. Methodological limits include cosmic variance on large angular scales, imperfect foreground removal, and algorithmic thresholds that can bias which anomalies are reported as candidate events [10] [9].
4. Observational verdict to date — no reliable detections
Comprehensive searches using WMAP and Planck have not produced a statistically robust detection of a bubble‑collision imprint; instead they placed upper limits on how large and how many such disks could exist in our sky, meaning the observational record so far is null within current sensitivity and analysis assumptions [4] [5] [11]. Popular summaries that trumpet “evidence” often overstate what the data actually show; the original teams framed results as constraints, not discoveries [1] [9].
5. New avenues and why the story is not closed
Recent proposals to reconstruct remote dipole and quadrupole fields (RDF/RQF) — observables encoded by scattering of CMB photons off remote electrons and accessible through kSZ/pSZ tomography — offer a route around the cosmic‑variance floor of the primary CMB and predict distinctive azimuthally symmetric RQF/RDF patterns from collisions; forecasts suggest RDF/RQF with CMB‑S4 and LSST–like data could improve constraints by orders of magnitude, but these methods are nascent and must overcome new systematic biases in reconstruction before they can claim discovery [3] [6] [8].
6. Alternative viewpoints and implicit agendas
Physicists differ on how plausible a barely‑visible collision is: some caution that models can be tuned to produce subtle signals compatible with current non‑detections while others argue the absence of detections already excludes swaths of model space [9] [12]. Outreach pieces and headlines sometimes conflate algorithmic “candidates” with evidence, reflecting a communication incentive to dramatize limits‑setting work; the original papers and reviews uniformly present null results as meaningful constraints rather than proof of a multiverse [1] [11].